Patent Application
20140332077
This invention relates generally to sub-wavelength periodic structures for enhanced transmission of incident optical radiation and, more particularly, to sub-wavelength aperture array structures and grating structures with polarization tunability and enhanced transmission, having geometries adapted to support coupled mode resonances for enhanced transmission, fight channeling, circulating, and weaving. This invention further relates to devices that include such structures.
There has been much interest in the phenomenon of enhanced transmission in periodically patterned metal structures, both in two-dimensionally periodic hole-arrays and in one-dimensionally periodic transmission grating structures. Referring to
Enhanced transmission occurs when incident light 16 is transmitted with a transmittance (7) greater than a ratio of the area (Agroove) of grooves 14 that separate the contacts 12 to the total area of the structure 10 on which incident light 16 impinges (Atotal), as described by Equation 1 below:
T>A
groove
/A
total. (I)
Hence, the incident light 16 is channeled around the metal contacts 12 and through the grooves 14 of the grating structure 10 to transmit radiation 18. Structures with grooves having an area of only a few percent of the total area of the film have been found to transmit close to 100% of the incident light at particular wavelengths, polarization states and angles of incidence.
Enhanced optical transmission is an extremely useful property that can be exploited for use in a variety of optical devices, if it can be accurately modeled for different applications. Until fairly recently, this phenomenon was attributed to horizontally oriented surface plasmons (HSPs), surface plasmons that are oriented parallel to the surface, for both one-dimensional periodic grating structures and two dimensionally periodic hole arrays. Accordingly, these prior art enhanced transmission gratings have been limited to specific configurations designed to optimize HSP coupling.
For example, U.S. Pat. No. 5,973,316 to Ebbesen et al. (“Ebbesen”) discloses an array of low profile sub-wavelength apertures in a thin metallic film or thin metal plate for enhanced light transmission by coupling to an HSP mode, where the period of the array is chosen to enhance transmission within a particular wavelength range. Ebbesen further discloses that the array can be used to filter and collect light for photolithographic applications.
In another example, U.S. Pat. No. 5,625,729 to Brown discloses an optoelectronic device for resonantly coupling incident radiation to a local surface plasmon wave. The device, e.g., a metal-semiconductor-metal (“MSM”) detector, includes a multiplicity of substantially planar and regularly spaced low-profile electrodes on a semiconductor substrate to resonantly couple an HSP mode propagating along the grating and the substrate.
Those of ordinary skill in the art will appreciate that only incident transverse-magnetic (TM) radiation (defined as electromagnetic radiation with the magnetic field oriented parallel to the grating elements (wires, e.g.)) will couple to HSPs. Accordingly, these and other prior art sub-wavelength enhanced transmission gratings are limited to specific configurations designed to optimize HSP coupling and, consequently, to gratings which enhance transmission of TM radiation.
The present invention relates to polarization-tunable enhanced transmission sub-wavelength grating and aperture array structures that can be tuned to selectively transmit a predetermined polarization state or to simultaneously enhance transmission of both TM and transverse-electric (TE) radiation. The present invention also relates to enhanced transmission sub-wavelength structures that support cavity modes (“CMs”), including hybrid cavity modes to produce light-circulating or light-weaving structures, depending on the angle of incident radiation. The sub-wavelength structures of the present invention are easy to fabricate and, consequently, are easy to integrate into devices requiring polarization-tunable transmission. Accordingly, the present invention further relates to devices that include any of the sub-wavelength structures of the present invention.
A device for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength includes a structure comprising an array of apertures in a thin film. The structure is adapted to preferentially support cavity modes for coupling to and enhancing transmission of a predetermined polarization state at the predetermined wavelength. The structure is adapted to induce light circulation or weaving of the transmitted predetermined polarization state at the predetermined wavelength. The array of apertures is arranged with a periodicity that is on the order of or less than the predetermined wavelength.
A device for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength includes a structure comprising an array of apertures in a thin film. The structure includes a repeating unit cell having at least a first aperture and a second aperture, wherein a parameter of the first aperture differs from that of the second aperture. The unit cell repeats with a periodicity on the order of or less than the predetermined wavelength. The structure is adapted to preferentially support cavity modes for coupling to and enhancing transmission of a predetermined polarization state at the predetermined wavelength.
These structures can be stacked with spacer layers of air between them, or with spacer layers comprised of any material which will enhance light circulating, channeling, weaving, or any other enhanced transmission effect described herein.
A device for enhancing transmission of incident electromagnetic radiation within more than one predetermined wavelength band includes a structure comprising an array of apertures in a thin film. The structure includes a repeating unit cell having more than one aperture including a first aperture and a second aperture. A parameter of the first aperture differs from that of the second aperture. The unit cell repeats with a periodicity on the order of or less than the more than one predetermined wavelength. The structure is adapted to preferentially support cavity modes for coupling to and enhancing transmission of unpolarized light within the predetermined wavelength bands. The structure is further adapted to channel light within a first predetermined wavelength band into the first aperture of each unit cell and to channel light within a second predetermined wavelength band into the second aperture of each unit cell.
Such device includes a solar cell, wherein a first aperture is filled with a semiconductor material that strongly absorbs light within the first wavelength band and the second aperture is filled with a semiconductor material that strongly absorbs fight within the second wavelength band. The unit cell can include more apertures optimized to channel and absorb other predetermined wavelength bands.
Any of these aperture array structures or grating structures described below can be further adapted for light circulation or weaving. Devices that can be formed from such aperture array structures include polarizers, wavelength filters, wavelength sensitive channeling devices, light storage, memory, or controlling devices, wavelength and/or polarization sensitive photodetectors and polarization sensors.
The present invention also relates to polarization-tunable enhanced transmission sub-wavelength (PETS) gratings that can be tuned to selectively transmit a predetermined polarization state or to simultaneously enhance transmission of both TM and transverse-electric (TE) radiation. The present invention also relates to enhanced transmission sub-wavelength gratings that include structure that supports CMs, including hybrid cavity modes to produce light-circulating or light-weaving structures, depending on the angle of incident radiation. The gratings of the present invention advantageously have a small form factor, are easy to fabricate, and, consequently, are easy to integrate into devices requiring polarization-tunable transmission. Accordingly, the present invention further relates to devices that include any of the sub-wavelength gratings of the present invention.
A grating for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength of the present invention includes a grating structure adapted to preferentially support cavity modes for coupling to and enhancing transmission of a transverse-electric (TE) polarization state of incident electromagnetic radiation. The grating structure includes a plurality of wires arranged with a periodicity that is equal to or less than the predetermined wavelength; and a groove between each adjacent pair of the plurality of wires. The groove includes a width between the wires and a height, wherein the groove is filled with a dielectric material having a dielectric constant equal to or greater than 1.
In one embodiment of any of the grating structures of the present invention, the dielectric constant is greater than or equal to 1.2. In another embodiment, the dielectric constant is greater than or equal to 2.0. In yet another embodiment, the dielectric constant is greater than or equal to 10, preferably greater than or equal to 14.
Any of the grating structures of the present invention can include an aspect ratio of the groove width to the periodicity in a range of at least 1 to less than or equal to 10.
Any of the grating structures of the present invention can include wires that are formed from any highly conductive material, including one or more of aluminum, silver, gold, copper and tungsten.
Any of the grating structures of the present invention can be superposed on a substrate, which can include a plurality of layers, preferably where at least two layers are of different materials. Any of the substrates in the gratings of the present invention can include one or more of silica, silicon, silicon dioxide, Ge, GaAs, InP, InAs, AlAs, GaN, InN, GaInN, GaAlAs, lnSb, fused silica, sapphire, quartz, glass, and BK7.
The dielectric material in the grooves of any of the grating structures of the present invention can include at least one of silica, silicon, silicon dioxide, silicon nitride, alumina, an elastomer, a crystalline powder, a semiconductive material, crystalline ditantalum pentoxide, polycrystalline ditantalum pentoxide, crystalline hafnium oxide and polycrystalline hafnium oxide.
The present invention further includes a grating for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength including a grating structure adapted to preferentially support cavity modes for simultaneously coupling to and enhancing transmission of a transverse-electric (TE) polarization state and a transverse-magnetic (TM) polarization state of incident electromagnetic radiation at the predetermined wavelength. The grating structure includes a plurality of wires arranged with a periodicity that is equal to or less than the predetermined wavelength; and a groove between each adjacent pair of the plurality of wires, the groove including a width between the wires and a height, and wherein the groove is filled with a dielectric material having a dielectric constant equal to or greater than 1.
One embodiment of the grating has a transmission efficiency of each of the TE and TM polarization state of at least 80%.
The present invention further provides a grating including a grating structure adapted to preferentially support TE-excitable cavity modes at a first predetermined wavelength for coupling to and enhancing transmission of a transverse-electric (TE) polarization state of incident electromagnetic radiation at the first predetermined wavelength and to preferentially support TM-excitable cavity modes at a second predetermined wavelength for coupling to and enhancing transmission of a transverse-magnetic (TM) polarization state of incident electromagnetic radiation at the second predetermined wavelength. The grating structure includes: a plurality of wires arranged with a periodicity that is equal to or less than the predetermined wavelength; and a groove between each adjacent pair of the plurality of wires, the groove including a width between the wires and a height. The grating structure is further adapted to reflect the TM polarization state at the first predetermined wavelength and to reflect the TE polarization state at the second predetermined wavelength.
The present invention still further provides a grating for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength that includes a grating structure adapted to preferentially support cavity modes for coupling to and simultaneously enhancing transmission of a TE-polarization state and a TM-polarization state at the predetermined wavelength. The grating structure includes a grating period that extends from a leading edge of a first wire in one of the sets to a leading edge of a first wire in the next set, so that a set of at least two wires and two grooves occurs within the grating period; i.e., the grating period includes two grooves per period. A first groove is between an adjacent pair of wires within each the set. Each first groove is associated with a first set of grating parameters including a first groove width, a first groove dielectric constant, and a first groove height. A second groove is between each repeating set of wires. The second groove is also associated with a second set of grating parameters including a second groove width, a second groove dielectric constant, and a second groove height.
In one embodiment, at least one of the first grating parameters differs from the corresponding second grating parameter by an amount that is sufficient to prevent the production of cavity modes in adjacent grooves that have overlapping transmission spectra.
In another embodiment, either the first width differs from the second width or the first dielectric constant differs from the second dielectric constant or both width and dielectric constant differ by a combined amount sufficient to prevent the production of cavity modes in adjacent grooves that have overlapping transmission spectra.
A metal-semiconductor-metal detector device of the present invention includes a sensor for measuring an intensity of a transmitted TM and TE polarization state respectively at a predetermined wavelength and a grating for enhancing transmission of incident electromagnetic radiation at the predetermined wavelength that includes a grating structure adapted to preferentially support cavity modes for coupling to and simultaneously enhancing transmission of the TE-polarization state and the TM-polarization state at the predetermined wavelength and to preferentially transmit the TE-polarization state through the first grooves, and the TM-polarization state through the second grooves. The grating structure includes a grating period that extends from a leading edge of a first wire in one of the sets to a leading edge of a first wire in the next set, so that a set of at least two wires and two grooves occurs within the grating period; i.e., the grating period includes two grooves per period. A first groove is between an adjacent pair of wires within each the set. Each first groove is associated with a first set of grating parameters including a first groove width, a first groove dielectric constant, and a first groove height. A second groove is between each repeating set of wires. The second groove is also associated with a second set of grating parameters including a second groove width, a second groove dielectric constant, and a second groove height.
The present invention further includes a grating for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength including a grating structure adapted to preferentially support cavity modes for coupling to and enhancing transmission of a predetermined polarization state at the predetermined wavelength, and for inducing light circulation or light weaving of the transmitted predetermined polarization state at the predetermined wavelength. The grating structure includes: a grating period having at least two grooves per grating period, a set of at least two wires occurring within each period, the grating period extending from a leading edge of a first wire in one of the sets to a leading edge of a first wire in the next one of the sets. The grating structure includes a first groove between an adjacent pair of wires within each set, where each first groove is associated with a first set of grating parameters including a first groove width, a first groove material having a first dielectric constant, and a first groove height. A second groove is between each adjacent set of wires, where the second groove is associated with a second set of grating parameters including a second groove width, a second groove material having a second dielectric constant, and a second groove height.
In one embodiment, one or more of the first grating parameters differs from the corresponding one or more of the second grating parameters by an amount that is sufficient to produce cavity modes in adjacent grooves that have overlapping transmission spectra.
In another embodiment, the first groove dielectric constant differs from the second groove dielectric constant and the first groove width differs from the second groove width.
A light storage device of the present invention includes an embodiment of the light circulating grating of the present invention.
The present invention further provides a method of fabricating a waveband filter, the waveband filter including a grating structure adapted to enhance transmission of both transverse magnetic (TM) and transverse electric (TE) polarized incident electromagnetic radiation within a waveband that includes a predetermined wavelength, and a substrate on which the grating structure is superposed. The grating structure includes a groove dielectric constant ∈groove, a grating period Λ, a groove width, and a groove height. The method includes the following steps:
As a result, the present invention provides polarization-tunable enhanced transmission sub-wavelength (PETS) gratings that can be tuned to selectively transmit a predetermined polarization state or to simultaneously enhance transmission of both TM and transverse-electric (TE) radiation. In some embodiments, these PETS gratings are further adapted for light circulation or weaving. The present invention also provides enhanced transmission sub-wavelength gratings that include structure that supports cavity modes, including hybrid cavity modes, and devices that include any of the sub-wavelength gratings of the present invention. Such devices include polarizers, wavelength filters, light storage, memory, or controlling devices, and metal-semiconductor-metal photodetectors and polarization sensors.
Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.
Referring to
In one embodiment, as shown in
The grating structure 22, which is preferably superposed on a substrate 36, but may optionally be encased within a substrate material, is structured to support cavity modes (“CMs”) at a particular predetermined wavelength.
The grating structures of the present invention are optimized to support cavity modes at a particular predetermined wavelength, preferably within a particular band that includes the predetermined wavelength. One of ordinary skill in the art will recognize that the particular examples of grating structures provided herein can have dimensions sealed appropriately to a particular wavelength range of interest and include the corresponding appropriate materials for the wires and grooves and substrate material.
In particular, in various embodiments, any of the grating structures of the present invention can be adapted to support resonant modes at a predetermined wavelength between: 1 nm and 400 nm; 400 nm and 700 nm; 0.7 microns and 100 microns; 100 microns and 1 mm; and 1 mm and 400 mm.
The substrate in any of the gratings of the present invention can be composed of any dielectric suitable for the particular application, including any one or more of glass such as BK7, silica, fused silica, silicon dioxide (SiO2), silicon (Si), (including crystalline, poly-crystalline or amorphous), air, sapphire, quartz, or any or more semiconductor material, including III-IV and ternary compound semiconductors, including Ge (Germanium), Gallium Arsenide (GaAs), Indium Phosphide (InP), Indium Arsenide (InAs), Aluminum Arsenide (AlAs), Gallium Nitride (GaN), Indium Nitride (InN), Indium Antimonide (In Sb), Gallium Indium Arsenide (GaInAs), Gallium Indium Nitride (GaInN), Gallium Aluminum Arsenide (GaAlAs), and mercury cadmium telluride (HgCdTe).
The substrate can include more than one layer. Each of the multiple layers can be composed of a different material.
In one embodiment, the substrate includes an anti-reflective material.
Cavity modes (CMs), as referred to herein, are resonant modes produced within the grooves of a grating structure that satisfy the well-known Fabry-Perot resonance condition within the grooves. CMs include resonant modes produced by waveguide modes (WGs) of incident transverse-electric (TE) polarized radiation; and resonant modes produced by either WGs or vertically-oriented surface plasmons (VSPs) on the walls of the grooves of incident transverse-magnetic (TM) polarized radiation. The term “cavity mode”, in referring to the light circulating structures of the present invention also includes hybrid cavity modes that induce phase resonances.
TM-polarized (p-polarized) radiation is defined as electromagnetic radiation oriented so that its magnetic field is parallel to the grating wires. TE-polarized (s-polarized) radiation is electromagnetic radiation oriented so that its electric field is parallel to the grating wires.
The enhanced transmission gratings of the present invention are “sub-wavelength” gratings for enhancing transmission of incident electromagnetic radiation at a predetermined wavelength. “Sub-wavelength,” as referred to herein, means that a periodicity of the wires of the grating is equal to, or on the order of, or less than the predetermined wavelength, so that the spacing between the wires is less than the predetermined wavelength.
The grating structures and gratings formed in accordance with the present invention, which enhance transmission of one or more polarization states to produce grating devices for various applications, are collectively referred to herein for convenience as “polarization-tunable enhanced transmission sub-wavelength” (“PETS”) grating structures and gratings. Use of this acronym is not to be construed in any way as limiting the grating structures of the present invention.
The wires of the present invention, which are also referred to as contacts, can be of any shape, size and of any material and arranged in any geometrical pattern to form a grating structure that preferentially supports CMs for enhancing transmission of a predetermined polarization state at a predetermined incident wavelength to form an embodiment of a grating structure of the present invention. For example, depending on the predetermined polarization state, predetermined wavelength, and desired application, the wires can be of a width that is 1%-95% relative to the period of the particular grating structure and of a height that is 1%-1000% relative to the period of a particular grating structure. The grooves in the grating structure preferably have widths of 1% 1000% relative to the period.
The height “h” as referred to herein refers to a groove height, which is preferably equivalent to an adjacent wire height. However, it is contemplated to be within the scope of the invention to arrange the wires within recesses of a substrate, so that a wire height could be greater than an adjacent groove height. In such cases, the height h referred to herein is the groove height. It is also contemplated to provide different wires having different heights in a multiple-groove-per-period structure. In such cases, the height h referred to herein is a groove height corresponding to one of the adjacent wires.
Alternatively, the grating structures of the present invention can be formed from arrays of holes in thin (metallic) films.
Preferably, the wires in any of the grating structures can include any highly conducting metals, for example, any one or more of gold (Au), silver (Ag), aluminum (Al), copper (Cu), and tungsten.
In one embodiment, each wire has a quadrilateral cross-section such as rectangular, square, or trapezoidal. The intersection between the wires and the substrate is preferably formed of straight edges, but a curved or sloped interface can occur in the manufacturing process. This slight curvature of the interface does not affect the excitation of CMs, but it can shift the energy at which resonance occurs. Such shifts are preferably accounted for in the optimization of the grating structure parameters.
Referring to
In a preferred embodiment, the grooves 24 are filled with a dielectric material having dielectric constant ∈groove, of at least 1.2, most preferably at least 2. In one embodiment, the dielectric constant ∈groove of the material ranges from 2-20.
In another embodiment, the dielectric constant ∈groove of the material in the grooves is at least 10, preferably at least 14. For example, the material in the grooves can be crystalline or polycrystalline ditantalum pentoxide or crystalline or polycrystalline hafnium oxide. These “high-K” materials, i.e., materials having a high dielectric constant, are particularly advantageous for TE-transmitting radiation, as described herein.
The grooves can be filled with air or with any material useful to the particular application. In one embodiment, the grooves 24 are filled with semiconductor materials, including one or more of silicon (Si), germanium, (Ge) and other III-V semiconductor compounds. The grooves can also be filled with at least one of silica, silicon, silicon dioxide, silicon nitride, alumina, an elastomer, and a crystalline powder.
Any of the grating structures or gratings of the present invention can also be adapted to localize a predetermined polarization state of incident electromagnetic radiation at a predetermined wavelength, and within a particular waveband, within the grating structure or grating.
The present invention is, in part, a result of the Applicants' efforts to accurately model the modes responsible for enhanced transmission in a known one-dimensional (1-D) sub-wavelength grating. Contrary to prior teachings on the subject that reported HSPs as primarily responsible for enhanced optical transmission (BOT), Applicants Crouse and Keshavareddy found and reported in a publication entitled “The role of optical and surface plasmon modes in enhanced transmission and applications, Optics Express, Vol. 13: Iss. 20, pp. 7760-7771 (Oct. 3, 2005) (“Crouse 2005”), the entirety of which is incorporated herein by reference thereto, that HSPs can both strongly inhibit and weakly enhance transmission in such sub-wavelength gratings. Applicants further reported that the predominant effect appeared to be a strong inhibition of transmission and interference with the transmission-enhancing properties of other resonant modes that could contribute to the phenomenon of enhanced transmission.
More recently, Applicants were able to theoretically show that cavity modes (CMs) in a lamellar grating structure can produce enhancements in transmission selectively for one or all polarizations of incident light. In addition, Applicants discovered that the properties of such CM-coupled grating structures (e.g., bandwidth, electromagnetic field profiles) and their dependencies on wavelength, angle of incidence, and structural geometries differ significantly from those of prior-art gratings optimized for HSP-induced enhanced transmission.
The formulation of the dependence of the parameters of a sub-wavelength grating on enhanced transmission was reported in Crouse and Keshavareddy, “Polarization independent enhanced optical transmission in one-dimensional gratings and device applications,” Oplies Express, Vol. 15, No. 4, pp. 1415-127 (Feb. 19, 2007) (“Crouse 2007”), the entirety of which is incorporated herein in reference thereto.
In particular, Applicants have found that it is the cavity modes (CMs) defined herein, e.g., the resonant modes produced by WGs or the cavity mode component of a hybrid mode (consisting of both cavity resonance and surface plasmons resonance) that play the primary role in EOT of TE radiation, i.e., radiation polarized parallel to the metal wires.
Applicants have similarly found that similar cavity resonances can be found for TM radiation, i.e., for radiation polarized perpendicular to the wires, and that these resonances can help channel light through the grooves of the grating structure of the present invention to achieve enhanced optical transmission for this polarization state.
In other words, Applicants found that grating structures can be tailored to selectively support cavity modes corresponding to those modes that satisfy the Fabry-Perot condition inside the grooves, which can be preferentially excited by one or both of TM and TE-polarized radiation. Applicants further found that excitation of these cavity modes at a particular predetermined energy or wavelength can predictably provide enhanced transmission of one or both of TM and TE radiation through the grooves. It has also been found that the energy location of the peak transmission shifts to lower energies as the groove height or the dielectric constant of the groove is increased.
In optimizing the grating structure of the present invention to provide such polarization-tunable enhanced transmission, Applicants have surprisingly found that an essential design parameter in tuning the peaks of enhanced transmission for both TE and TM polarization states, not reported in the prior art, is the spacing between the wires, or the groove width c 26, referring to
For TM-polarized light CMs produced in very narrow groove openings, the resonantly enhanced electromagnetic fields is relatively uniform throughout the groove and as the groove width is increased, the field redistributes with high intensity electromagnetic fields remaining close to the groove walls for wide openings. On the other hand, for TE polarization, the electromagnetic fields inside the grooves are concentrated more at the center of the groove, with very little fields on the side walls. As the groove width is increased, more resonance modes start occurring, redistributing the fields into lobes of high field intensities.
These characteristics and dependencies of CMs on the parameters of a grating structure are exploited, as described in more detail later in this specification, to form polarization-tunable enhanced transmission sub-wavelength (PETS) grating structures in accordance with the present invention by adapting and optimizing the grating structure parameters to selectively support the cavity modes that will couple to the predetermined polarization state (TE, TM or both, e.g.) at the predetermined wavelength.
Referring to
Referring still to
Yet another embodiment 56 of a PETS grating of the present invention shown schematically in
Each grating structure of the PETS gratings shown in
The grating 70 of
In one embodiment of the grating 70, the periodicity Λ 76 is 1.75 microns, height h 82 is 1 micron, and silicon, a material having a dielectric constant ∈groove of 11.9, fills the grooves 74. Using the methods of the present invention for modeling PETS gratings, one can generate a plot, as shown in
As can be seen from
In one embodiment of the grating structure of the present invention, the dielectric material filling the grooves has a dielectric constant ∈groove of at least 10, preferably at least 14. Applicants have determined that for grooves having a high dielectric constant, the grating structure of the present invention: provides TE-polarization enhanced transmission at lower energies than is possible without their use; inhibits TM-polarization transmission in gratings when there is not a TE-polarized CM excited; and allows alignment of TE-polarized and TM-polarized CMs at lower energies. Accordingly, a preferred embodiment of the grating structure that is tuned for simultaneous TE and TM transmission includes a dielectric constant ∈groove of at least 10, preferably at least 14.
Referring to
Starting again with an embodiment of the grating structure 78 shown in
In one particular embodiment, a groove width of 0.45 microns is chosen.
Accordingly, grating structure 78 having parameters c of 0.45 microns, Λ 76 of 1.75 microns, height h 82 of 1 micron, and c of 11.9, also represents a grating structure that provides enhanced transmission of TM-polarized light at a first predetermined wavelength (0.45 microns in this example) and enhanced transmission of TE-polarized light at a second predetermined wavelength (3.729 microns in this example).
Referring to
In a preferred embodiment, the aspect ratio is in a range of at least about 1 to less than about 10.
The PETS grating structures of the present invention can be used for many device applications including polarizers and wavelength filters. A preferred embodiment of a polarizer or wavelength filter firmed in accordance with the present invention includes a PETS grating structure having only one groove per period, as described in reference to
Examples of embodiments of narrow band filters formed from the PETS grating structures of the present invention optimized to simultaneously transmit both TE and TM incident radiation, as described in reference to
In particular,
The PETS grating structures of the present invention adapted to support CMs produce within the grooves as described herein have a high degree of wavelength, bandwidth and polarization tunability and can, with the use of wires composed of low loss metals, and grooves and substrate materials composed of low-loss dielectrics, transmit close to 100% of the desired polarization component of the incident light.
In particular, in one embodiment of a PETS grating structure for enhanced transmission of either TE or TM polarization at a predetermined wavelength, at least 60% of incident TE or TM radiation respectively at the predetermined wavelength is transmitted.
In another embodiment of a PETS grating structure for enhanced transmission of either TE or TM polarization at a predetermined wavelength, at least 80% of incident TE or TM radiation respectively at the predetermined wavelength is transmitted.
In yet another embodiment of a PETS grating structure for enhanced transmission of either TE or TM polarization at a predetermined wavelength, at least 90% of incident TE or TM radiation respectively at the predetermined wavelength is transmitted.
In still another embodiment of a PETS grating structure for enhanced transmission or either TE or TM polarization at a predetermined wavelength, at least 95% of incident TE or TM radiation respectively at the predetermined wavelength is transmitted.
In one embodiment of a PETS grating structure for simultaneous enhanced transmission of TE and TM polarization at a predetermined wavelength, at least 60% of incident TE and TM radiation at the predetermined wavelength is transmitted.
In another embodiment of a PETS grating structure for simultaneous enhanced transmission of TE and TM polarization at a predetermined wavelength, at least 80% of incident TE and TM radiation at the predetermined wavelength is transmitted.
In yet another embodiment of a PETS grating structure for simultaneous enhanced transmission of TE and TM polarization at a predetermined wavelength, at least 90% of incident TE and TM radiation at the predetermined wavelength is transmitted.
In still another embodiment of a PETS grating structure for simultaneous enhanced transmission of TE and TM polarization at a predetermined wavelength, at least 95% of incident TE and TM radiation at the predetermined wavelength is transmitted.
The grating structures of the present invention described above in reference to
The set 144 of wires can be composed of a pattern of wires of different materials, heights, and or shapes. In one embodiment, the grooves are composed of the same material. In other embodiments, the grooves are filled with different materials.
In one preferred embodiment, the grating structure 140 is adapted to preferentially support cavity modes for coupling to and simultaneously enhancing transmission of the TE-polarization state and the TM-polarization state at the same predetermined wavelength.
Preferably, the grating structure is further adapted to preferentially transmit the TM-polarization state through one set of grooves, for example, the first 152 narrower grooves, at the predetermined wavelength, and to preferentially transmit the TE-polarization state through the other set of grooves, for example, the second 156 wider grooves.
In one such embodiment, which is desirable for simple separation of polarized components of incident radiation, one or more of the groove parameters (e.g., groove width, dielectric constant) of the first groove differ(s) from that of the second groove by an amount sufficient to prevent the production of neighboring CMs that have overlapping shoulders within their transmission spectra. In one embodiment, only the groove widths differ, e.g., the first groove width 154 and second groove width 158 in
With reference to
This TE-polarized mode corresponds to the n=m=1 mode found according to the formula for 100% confined (within the cavity) CMs provided by Equation (2) below:
where n and m are integers and ngroove (=√{square root over (∈groove)}) is the index of refraction of the dielectric material in the grooves.
Referring to
Referring to
Additional multiple-groove-per-period gratings for enhanced transmission and separation of predetermined polarization states are contemplated as being within the scope of the present invention. Such embodiments include a grating structure including a plurality of single-groove-per-period sub-grating structures, where each sub-grating structure is associated with grating parameters (including wire compositions, substrate material, periodicity, groove width, groove dielectric, period, wire height and shape, and so on), wherein at least one sub-grating structure differs sufficiently from another sub-grating structure to produce enhanced transmission without substantial phase interactions occurring between their associated CMs.
Referring to
Referring to
In this embodiment, the grating structure includes a plurality of grooves per period. Each groove within the period can be considered to be associated with a sub-grating structure that includes grating parameters (including wire compositions, substrate material, periodicity, groove width, groove dielectric, period, wire height and shape, and so on). At least one sub-grating structure differs sufficiently from another sub-grating structure to produce enhanced transmission and light-circulation, but not enough to prevent phase interactions occurring between their associated CMs.
Though TM-polarized it modes have been reported in the prior art, TE-polarized it modes and the light circulation effect have not. Referring to
The light circulating grating structures of the present invention include those that enhance transmission of and produce light-circulation of one or both of TM- and TE-polarized incident light.
In Example 3, one embodiment 230 of the grating structure adapted for enhanced transmission and light circulation of TE-polarized light formed in accordance with the present invention has two grooves per period 232, with a first groove width 240 c1=0.755 microns and a second groove width 242 c2=0.735 microns, and ∈1groove equaling ∈2groove=23. The wires are gold. This structure is a light-circulating structure for the TE-mode at a normal angle of incidence of the incident light.
In another embodiment of the grating structure described in Example 3, if the groove dielectric are also changed so that ∈1groove does not equal ∈2groove but rather ∈1groove=25 and ∈2groove=21, then enhanced transmission and light circulation of TM-polarized light occurs for light at a normal angle of incidence. Accordingly, the light-circulating grating structures of the present invention can be adapted to produce hybrid CM or π modes for light-circulation of any predetermined polarization state at a predetermined wavelength.
Referring to
“Light weaving” occurs when incident electromagnetic radiation 262 with a nonzero in-plane momentum (i.e., momentum in the direction parallel to the surface of the wire) is woven through alternating grooves 264, localizing light near the wires as it travels parallel to them. The light weaving grating structures of the present invention can be useful for photodetectors or for the propagation of signals or data.
Referring to
In yet another embodiment of the grating structure shown in
Hole arrays in thin, preferably metal, films that are adapted and arranged to produce the light circulating modes described herein are also considered within the scope of this invention.
One embodiment of a method for tailoring any of the PETS grating structures of the present invention includes applying a coupled mode algorithm that uses the well-known surface impedance boundary conditions (SIBC) as described in Example 1 provided below in the “Examples” section.
Example 1 assumes normal incident radiation, but the grating structures of the present invention also include those optimized for enhanced transmission at any predetermined angle of incidence, depending on the particular application and desired result.
Various parameters, including wire compositions, refractive index of a groove material, substrate material, periodicity, groove width and height can be varied, as described in Example 1, to optimize parameters for the grating structure having enhanced transmission of the desired polarization state(s) at the desired predetermined wavelength and for a predetermined bandwidth.
The present invention, therefore, includes a method of optimizing the spacing between the wires, pitch, and orientation to exploit the optical and surface plasmon resonances effect, to achieve polarization independent enhanced optical transmission. These parameters can be optimized in accordance with the preferred wavelength, polarization, and angle of incidence in accordance with the present invention. The height defined by the metal wires can be further optimized to achieve different line widths for the transmission peaks.
In particular, one embodiment of the method of the present invention assumes, as an approximation, that the CMs are perfectly confined to the grooves. For CMs perfectly confined to the grooves, their wavelengths are given by Equation (3):
where n and in are integers and ngroove(=√{square root over (∈groove)}) is the index of refraction of the dielectric material 88 in the grooves 74.
Even though CMs are not perfectly confined to the grooves, Equation (3) is still approximately true for CMs produced by waveguide modes and even for CMs produced by TM-polarized VSPs. More importantly, inherent in Equation (3) are the dependencies of the CMs on the structural parameters ngroove, h, and c of the grating structures of the present invention, with the lowest in value allowable for TM-polarization (also referred to as “p-polarization”) and TE-polarization (also referred to as “s-polarization”) being m=0 and n=1 respectively. Because of this last fact, the lowest order TE-polarized CM occurs at a higher energy than the lowest energy p-polarized CM. Depending on the ratio of h/c, there can be many TM-polarized CMs with lower energies than the lowest energy TE-polarized CM, resulting in an undesirably large wavelength separation between the lowest order CMs for the different polarizations.
A more thorough description of all the dependencies of the TE-polarized and TM-polarized CMs on structural parameters (e.g., groove width, height and groove dielectric constant) is given in Crouse 2007, and also in Example 1 in the Examples section below.
Summarizing these dependencies, the TM-polarized CMs have strong dependencies on h and ∈groove and a weak dependence on c if the m=0 mode is used. Also, TM-polarized CMs can have a strong dependence on Λ, especially when Λ is such to produce a Wood-Rayleigh anomaly (WR) or a HSP at a wavelength close in value to that of the CM. The TE-polarized CMs have strong dependencies on h, c, and ∈groove and a weak dependence on Λ. With these basic characteristics and structural dependencies of the CMs in mind, one embodiment of a method for tuning (with respect to wavelength) the lower order TE-polarized CMs and TM-polarized CMs is provided as follows.
The method and gratings of the present invention allow for the use of a high-index (or high-k) dielectric material in the grooves, which has the following advantages. To achieve the highest degree of transmission of an incident beam of radiation into the 0th order (“straight-through”) transmitted beam, the transmission enhancing CMs for both TM and TE polarizations should occur at a lower energy than the onset of 1st order diffraction. For the grating structure of the present invention superposed on a substrate (e.g., glass, semiconductors, and so on) with a dielectric constant of nsubstrate=(√{square root over (∈substrate)}), the onset of 1st order diffraction occurs for a wavelength λ1st order=Λ/nsubsrate. For substrates other than air, realistic aspect ratios (height/width of the grooves), and with h small enough to produce TM-polarized CM transmission peaks that do not crowd together (i.e., the bandwidth of the transmission peaks is at least twice the wavelength separation of adjacent peaks), a material within the grooves with a dielectric constant at least as large as the substrate's is typically desirable to lower the energy of the TE-polarized CMs below the onset of 1st order diffraction. Also, high-index dielectrics (e.g., high-k dielectrics, such as hafnium oxide or ditantalum pentoxide) inhibit TM-polarization transmission through the relatively wide TE-polarization transmitting grooves (relative to the width of TM-polarization transmitting grooves) when a TM-polarized CM is not excited.
Accordingly, with reference to
An example of a grating structure formed according to this method is provided as Example 2 in the Examples section below.
The optimized parameters determined in accordance with any of the methods of the present invention can be used to fabricate any of the grating structures of the present invention using any appropriate method of fabrication known to those of ordinary skill in the art for fabricating sub-wavelength gratings.
For example, for the grating structures optimized to enhance radiation at predetermined wavelengths in the ultraviolet, visible and near infrared, mid-infrared long wavelength infrared and very long wavelength infrared, standard microfabrication technologies can be used. Such fabrication methods can include physical deposition of the wires and groove and substrate materials such as metals, oxides and semiconductors by thermal evaporation, electron beam evaporation, sputtering, or chemical vapor deposition.
The grating structures of the present invention can be generated using photolithography or electron beam lithography along with wet chemical etching and/or reactive on etching or ion beam milling. For structures that operate in wavelength regions longer than the very long wavelength infrared, such as the terahertz and microwave regions, less expensive fabrication techniques can be used, including computer numerical control (CNC) micro milling; machines.
The optical and electromagnetic characteristics of lamellar gratings, such as those of the present invention, are modeled in this example using a coupled mode algorithm that uses the surface impedance boundary condition (SIBC) approximation. This method is described in detail in D. Crouse, “Numerical Modeling and Electromagnetic Resonant Modes in Complex Grating Structures and Optoelectronic Device Applications,” IEEE Trans. Electron Devices 52: 2365-2373 (2005), the entirety of which is incorporated herein by reference, and are only summarized here. Referring to
E
II
=Z{circumflex over (n)}×H
II (A1)
where Z=1/nmetal, with nmetal being the complex index of refraction of the metal. This approximation is valid if the dielectric constant of the metal is much larger than the neighboring dielectric (which is largely true in the infrared and visible spectral regions).
The electromagnetic fields are expressed as a linear combination of orthogonal modes as follows:
where f1(x, y) is the {circumflex over (z)} component of the magnetic field or the {circumflex over (z)} component of the electric field depending on if the TM polarization or TE polarization is being modeled respectively. The other electric and magnetic field components can be obtained using relations derived from Maxwell's equations. Also, αn=ko sin θincident+nK, K=2π/d, βn=√{square root over (kn2−αn2)}, {tilde over (β)}n√{square root over (∈substrateko2−αn2)} with n is an integer, d being the period of the structure θincident the angle of incidence, λ the wavelength, and ∈i the dielectric constant of the ith region. In Eqs. (A1) and (A3), the orthogonal modes used in the modal expansion are plane waves in the air and substrate layers and the following orthogonal modes Φn (x, y) are used in the grooves:
Φn(x,y)=Xn(x)Yn(y) (A5)
X
n(x)=d sin(μnx)+cos(μnx) (A6)
Y
n(y)=anexp(iξny)+bnexp(−iξny) (A7)
where the terms μn and ξn obey the relation:
μn2+ξn2=∈grooveko2 (A8)
Applying the SIBC condition to the left-hand and right hand sides of the grooves results in the following equations (respectively):
where c is the width of the groove and ηgroove=kn∈grooveZ/i for TM polarization and ηgroove=ka/iZ for TE polarization. The most essential step in the above method is the solution to Eqn. (A10). In this method the roots of Eqn. (A10) are found by integration starting from an initial value. We have performed the integration using the Runge-Kutta method.
Applying boundary conditions equating the tangential field components and the SIBC conditions at the metal/dielectric interfaces at y=h/2 and y=−h/2 yields the following equations.
where
for the TM polarization and
for the TE polarization and
Then multiplying Eqs. (A11) and (A13) by Xin (x) and integrating over the region 0≦x≦c and multiplying Eqs. (A12) and (A14) by
and integrating over the region 0≦x≦d yields the following matrix equations that are used to determine the unknown coefficients Rn, Tn, an and bn:
where the matrices φ, β, ξ are square matrices with nonzero components along the main diagonal given by φ, βn, ξ
, that have been previously defined; G, K are matrices with components given by:
The number of modes used in the electromagnetic field expansions were large and the solutions were convergent. The results obtained using the above approach were checked using another method that assumes that the walls of the grooves are perfectly conducting. These results yield practically identical results indicating that even though the convergence of TE polarization solutions using the SIBC approximation is worse than the convergence of TM polarization solutions, the main results showing EOT for both TM and TE polarizations will hold true when more accurate methods are used for the calculations.
Once Eq. (A15) is used to find all of the unknown coefficients, the reflectance (i=air in Eq. A23), transmittance and diffraction efficiencies (i=substrate in Eq. A23) can be calculated as the ratio of the ŷ-component of the Poynting vector for an outward propagating mode and the ŷ-component of the incident beam (assuming a normalized incident beam and a top layer being air):
where Ωis either Rn or Tn and θ
is the angle of the outward propagating mode.
Referring to
Two numerical modeling methods were used and their results compared to ensure agreement and accuracy. One method uses the surface impedance boundary condition (SIBC) approximation and allows for very quick calculation of all optical characteristics of a wide range of grating structures. The other method is the finite-element method solver HFSS™ commercially available from Ansoft Corp. Note that CMs, HSPS, VSPs, WRs, diffraction and all other optical effects occur in the microwave as they do in the infrared (IR) and visible spectral regions but the CMs and diffraction features occur at wavelengths that scale with groove height and width and grating pitch or period. The transmittance (
The fabricated device was formed in accordance with the methods presented herein to produce cavity modes that simultaneously couple to TM- and TE-polarized radiation at the predetermined wavelength of 11.91 mm. Such millimeter-scale structures are far cheaper and quicker to fabricate than their nanoscale counterparts, and they can provide just-as-good experimental verification of the pertinent theoretical constructs, since the effects and wavelengths of the WRs and CM modes responsible for the device performance all scale with the device dimensions. In the case of periodic features on a millimeter scale, for example, theory predicts that enhanced transmission will be observed in the microwave spectral region. In moving from the IR to microwave spectral region, the only difference between the reflectivity and transmittance curves is the slightly higher energies and intensities for the HSP and CM resonances, as metals in the microwave behave as almost perfect conductors; the dielectric constant of A1 that is used is ∈A1=104+i·107 for the microwave to λ=31 μm and tabulated data26 from λ=31 μm to λ=600 nm. Additionally, unlike studies undertaken in the visible or even the IR, we need not worry about variation in the permittivity of the materials used; essentially the metal is perfectly conducting, and the dielectric filling the grooves is virtually non-dispersive at these wavelengths. It is therefore a very sensible approach to undertake these proof-of-principle studies at longer wavelengths.
The experimental sample was constructed by machining a set of identical grooves, each or width c=3.82 mm, spaced with a periodicity of Λ=10.34 mm and milled all the way through an aluminum alloy plate of thickness h=6.05 mm to cover an area of approximately 400 mm×400 mm. The voids were then carefully filled with an elastomer (Dow Corning® Sylgard® 184 silicone encapsulant) that had been mixed and left to rest under vacuum until completely evacuated. The real part of the permittivity of the elastomer is ˜2.8 in the GHz regime. Linearly polarized microwave radiation from a standard gain horn was collimated using a spherical mirror to impinge upon the sample at normal incidence. A continuous wave source sweeps the frequency in bands 18≦v≦26.5 GHz and 26.5≦v≦40 GHz (i.e., 7.5≦λ≦16.7 mm) and feeds the fixed position antenna. Before striking the sample, the incident beam was passed through an aperture of a broadband microwave absorbing material in order to restrict the incident beam spot to the useful sample area. Furthermore, in order to obtain averaging of the transmitted signal over a large number of grating periods, the transmitted beam is collected using another spherical mirror before being focused into a second horn antenna and detector. The polarization of both the incident beam and that detected can be altered in this configuration via simple rotation of each horn antennae about its central axis.
The experimental transmissivity data, setting both the incident and detected polarizations to either TM-polarization 400 or TE-polarization 402, normalized to a spectrum in the absence of the sample, are shown in
It is known that phase resonances for TM-polarized incident light can arise in gratings that have multiple grooves per period that differ with respect to composition, geometry or orientation. In these types of structures, TM-polarized VSP-CMs in neighboring grooves can couple, producing field profiles of equal magnitude but with a π radians phase difference; such modes have come to be called it modes or resonances, as described, for example, in Alastair P. Hibbins, et al., Physics Review Letters 96 257402 (2006). However, light-circulation has not been previously reported for any polarization.
For TE-polarized light, there is no component of the electric field that is normal to any metal/dielectric interface, and hence SPs and VSP-CMs cannot be excited, However, Applicants have found that WG-CMs do occur, and along with Rayleigh anomalies, are responsible for a large number of the enhanced or anomalous optical effects, including TE-polarized π modes with properties similar to the properties of TM-polarized π modes. The light circulation and weaving effects of the multiple-groove-per-grating structures formed in accordance with the present invention have been found by Applicants to occur for both s-polarized and p-polarized incident light.
To demonstrate a grating structure adapted to support hybrid CMs for inducing light circulation in accordance with the present invention, two grating structures are discussed in reference to
If the widths of the grooves are perturbed so that every other groove has a width of c1=0.755 μm and the rest of the grooves have widths of c2=0.735 μm while keeping all the other parameters unchanged, the resulting structure is the two-groove-per period Grating 2 of
Many similarities and several important differences between s-polarized and p-polarized π modes exist. The Poynting vector representation of
The incident beam cannot directly couple to the π radian out-of-phase field in every other groove. Because of this fact, the it resonances will always be located on the shoulders of the broad transmission peak. Applicants have observed in numerous two-groove-per-period gratings that the s-polarized π modes tend to be closer to the center of the transmission peak than the p-polarized π modes. This property arises because of the different components that make up the s-polarized and p-polarized π modes. Applicants have found that the components of the s-polarized π mode are two very similar, inherently radiative, WG-CMs that have slightly different resonant frequencies. The alternating groove width perturbation simply splits the original WG-CM band into two bands that are slightly asymmetric bands because the it resonance still has to occur on the shoulder of the original WG-CM transmission peak, but typically more symmetric than the two transmission peaks one either side of a p-polarization π mode. This greater symmetry affects the light circulation produced by π mode.
By examining the power flow, Applicants found that at or around the transmission minimum produced by the π modes, light is transmitted with high transmissivity through the two sets of grooves but then circles around, and is transmitted with high transmissivity through the neighboring grooves, resulting in a reflection maximum. It is clear that π modes are hybrid modes, composed of two coupled s-polarized WG-CMs. Furthermore, at the transmission minimum, these two transmission channels, created by the two coupled CMs, are equal in magnitude but produce counter propagating circulations of light resulting in high field intensities in the grooves but a net zero power flow in the grooves as equal amounts of power flow up and down each groove.
However, the weaker transmission channel is still strong enough to present to the now transmitted light on the substrate side, a strong and viable transmission channel back through the grating. This weaker transmission channel is the only channel possible because the transmitted light will not curve 180° and go back through the same groove through which it was initially transmitted. The net result of this process is a high reflectance. For energies progressively further from the transmission minimum, the weaker transmission channel re-transmits progressively lesser amounts of light which had been transmitted to the substrate via the stronger transmission channel, resulting in decreasing light circulation and increasing transmissivity.
Referring to
Though specific examples of PETS gratings for enhanced TM, TE or simultaneous enhanced TM- and TE-transmission and also those optimized for light circulation and weaving are described herein, one of ordinary skill in the art will recognize that various known methods can be used to iteratively vary one or more parameters of the grating structure to optimize the design of any grating structure adapted to support CMs as described herein. As a result, it is understood that the scope of the present invention includes any sub-wavelength grating structure adapted to support CMs at a predetermined wavelength as described herein, including any grating structure formed in accordance with any embodiment of the method of the present invention for optimizing and tuning the grating structures, described herein including in the “Examples” section.
The present invention also includes sub-wavelength aperture array structures formed in thin films in such a way to produce high-efficiency polarization and wavelength dependent structures, transmitting and/or reflecting polarized light incident at one or more predetermined wavelengths. These multiple-apertures-per-unit-cell arrays can be structured according to the methods described herein to perform the following functions: polarizing, wavelength filtering, light channeling, localizing light, light weaving and circulation.
Referring to
A high-efficiency polarizing beam-splitter of the present invention includes the aperture array structure 418 deposited on a substrate 444, which can include a single layer or multiple layers of any suitable substrate materials as described herein. The apertures of each unit cell 420 can be dimensioned and positioned and the periodicity of the array structure 418 chosen to form a beam-splitter that can operate at one or more predetermined wavelengths. The number of wavelengths that can be filtered depends on the number and relative orientations and dimensions of the apertures within one unit cell 420. In addition, different polarization states can be selected for transmission or reflection by adjusting the shapes of the apertures. This is a property of these sub-wavelength aperture array structures that is useful for a number of different applications.
Accordingly, the aperture array structure of the present invention can be configured as a wavelength filter and as any one or combination of: a transverse electric pass polarizer (either absorbing polarizer or beam-splitting polarizer); a transverse magnetic pass polarizer (either absorbing polarizer or beam-splitting polarize); an intensity detector; and a phase detector for individual or multiple wavelengths or ranges of wavelengths.
Similarly, a wavelength and/or polarization sensitive photodetector can include embodiments of the aperture array structure 418 of the present invention. The photodetector can be capable of single or multiple wavelength filtering and polarization selection. The construction of the unit cell and selection of the periodicity of the array structure will determine the wavelength(s) and polarization state(s) detected. Apertures of the same dimension, shape and dielectric composition within a unit cell can transmit a wavelength and/or polarization different than that transmitted through a differently shaped or composed aperture within the same unit cell.
The inventors have discovered the unexpected result that light-circulation can be produced and enhanced transmission maintained in such aperture array structures having more than one aperture per unit cell, if at least one aperture within a unit cell differs sufficiently from the others within the unit cell, but not enough to prevent phase interactions occurring between the CMs associated with each aperture.
Light circulation due to excited and coupled CMs has not been shown in the prior art for any periodic structure. The inventors have discovered that CMs in aperture arrays are produced by waveguide modes or VSPs within the apertures. In addition, they found that incident light at a predetermined angle of incidence, wavelength and polarization can be transmitted through one set of identical apertures (a first sub-array) by excited CMs and then re-transmitted through a properly configured second set (a second sub-array) of identical apertures that are appropriately shaped, positioned, composed, or otherwise configured differently than the first set of apertures to result in a high net reflectivity for light at a predetermined wavelength, polarization and angle of incidence according to the methods described herein. Some of the re-transmitted light can also be redirected through the first sub-array apertures. The same aperture array structures can also exhibit light weaving at a properly selected angle of incidence, which as described above, is useful inter cilia for photodetectors and for the propagation of signals or data.
Any one or more of the following parameters of the apertures of one sub-array can differ from those of another sub-array of the aperture array structure: dimension, dielectric constant of materials filling the apertures, height of the apertures (thickness of thin film), shape, and orientation. Any other parameter that can be varied to affect the coupling of excited CM modes according to the present invention is also considered to be within the scope of this invention.
The apertures forming any sub-array can be of any suitable shape, including circular, elliptical, square, bowtie or figure eight.
The dimensions of each aperture within one repeating unit cell of an aperture array structure of the present invention are preferably at least 0.25% of the period Λ of the aperture array structure. The cumulative dimensions of the apertures can be as high as 95%. Dimension refers to diameter of a circular aperture, length and width of a rectangle, major and minor axes of an ellipse and so on.
The height 438 of any aperture, or thickness of the thin film surrounding the aperture, can be 0.05%-1000% of the period Λ.
The magnitude of the displacement vectors, or the distance between two neighboring apertures in a unit cell can be in the range of 1% of the shortest wavelength for which enhanced optical transmission or light circulation is desired through 95% of the largest wavelength for which enhanced optical transmission is desired.
The period Λ of the aperture array structure is on the order of 1 mm-400 mm and on the order or less than the wavelength of the incident radiation, where the operating wavelength or wavelengths is in the range of 1 mm-400 mm.
One of ordinary skill in the art will recognize that the array dimensions can be scaled by an appropriate factor such that the wavelengths at which resonant cavity modes occur in the resulting aperture arrays are centered in any part of a predetermined operating wavelength regime: for example, the period Λ is on the order of 1 nm to 400 nm for operating in the deep ultra-violet and ultra-violet region of the electromagnetic spectrum of 1 nm-400 nm and so on.
The thin film of an aperture array structure of the present invention is preferably metallic, for example, any one or more of gold, silver, aluminum, copper, platinum, tungsten, titanium, hafnium, tantalum, lanthanum, lead, tin, iron or any alloy of these metals.
An aperture array structure of the present invention can be superposed on any suitable substrate including silicon (including polycrystalline or amorphous), germanium, silica, fused silica, silicon dioxide, quartz, gallium arsenide, indium phosphide, indium arsenide, gallium nitride, indium nitride, gallium indium nitride, gallium aluminum arsenide, indium antimonide, mercury cadmium telluride, mercury telluride, sapphire cadmium telluride, cadmium sulfide, cadmium selenide, glass, elastomer, polymer, crystalline powder, or any other suitable dielectric, oxide or semiconductor material.
The dielectric material filling the apertures in a sub-array can include any suitable material (or with nothing other than air) including: silica, silicon oxide, silicon dioxide, polycrystalline silicon, hafnium oxide, or any other suitable material including those listed herein as substrate and thin film materials and alloys thereof.
An aperture array structure of the present invention can also include a passivation or protective layer superposed thereon. The protective layer can include any suitable material including one or more of a polymer, plastic, oxide, or glass.
Referring to
The layered structures 452, 454, 456 can be oriented in any way relative to each other. They can be separated by any combination of air, patterned or unpatterned spacer layers 458 and 460, which can include any dielectric material suitable to produce light circulation, light channeling, or enhanced transmission at predetermined wavelength(s), polarization state(s) and angle(s) of incidence. The dielectric material can include crystalline silicon, polycrystalline silicon, amorphous silicon, silicon oxide, silicon nitride, gallium arsenide, aluminum arsenide, gallium aluminum arsenide, indium phosphide, indium antimonide, indium phosphide antimonide, gallium nitride, indium nitride, gallium indium nitride, silica, borosilicate glass, mercury cadmium telluride, cadmium sulfide, cadmium telluride, or some other semiconductor, oxide, polymer or plastic material.
As discussed herein, the structures formed in accordance with the present invention can be adapted to preferentially channel incident light at one predetermined wavelength and/or polarization into one sub-array of apertures (or gratings), and to preferentially channel incident light at another predetermined wavelength into a second sub-array of apertures. It will be appreciated that this spatial separation and localized concentration of wavelength and/or polarization is particularly useful for applications such as focal plane arrays or any other application that would benefit from efficient separation of incident radiation by wavelength range.
Referring to
In correspondence with the wavelength bands supported by each sub-array, the solar cells within the apertures of each sub-array are conventional single-junction solar cells composed of material that efficiently absorbs solar radiation within that same wavelength band.
The solar cells can be composed of any suitable material including silicon (both p-type, n-type and intrinsic), III-V semiconductors and their alloys (i.e., ternary and quaternary compound III-V semiconductors), II-VI semiconductors and their alloys (i.e., ternary and quaternary compound II-VI semiconductors) or other materials.
The solar cell device of the present invention comprise multiple different single-junction solar cells distributed horizontally over one single layer rather than vertically stacked like the tandem solar cells of the prior art. One significant advantage of this device is that it allows the use of electrochemical deposition techniques, such as chemical bath deposition, to fabricate the sets of semiconductor solar cells. These electrochemical techniques can be substantially cheaper than other fabrication techniques, such as molecular beam epitaxy and metal-organic chemical vapor deposition.
Referring again to
The columns of metal 520 surrounding the cavities are preferably separated from each other by open or filled spaces 560. These metallic columns can include any suitable conductive material, including aluminum, gold, silver, copper, titanium, tungsten, tin, or lead.
Each column can have a cross-sectional length of between 50 nanometers to 100 centimeters and a width of 50 nanometers to 10 micrometers.
The entire array of cavities containing solar cells can be superposed on a substrate 530. The substrate can be any material that is either rigid or flexible, e.g., glass, quartz, fused silica, silicon, plastic or other polymer material or any other material. The substrate can have thicknesses of 50 nanometers to 10 centimeters.
Each cavity can also be superposed on one or more layers 540 that serve afferent purposes (including adhesion promotion and electrical contact). For example, such layers may be added as adhesion promoters, electrical contacts, to eliminate deleterious reactions or intermixing of materials in the structure or other purposes. These layers can be of thicknesses between 0.1 nanometers to 1 centimeter and can be composed of platinum, titanium, tantalum, aluminum, chrome, silicon dioxide, polycrystalline silicon, silicon nitride, copper or any other conductive or insulating materials.
The columnar cavities 512, 514, 516, 518 defining the apertures can be of any suitable shape, including cylindrical, elliptical, rectangular, or square, which can be adapted for channeling and enhancing transmission within the particular wavelength range.
The cavities can have widths (i.e., diameters in the case of cylindrical cavities, or major and minor axes lengths in case of elliptical cavities, or widths and breadths in the case of rectangular or square cavities, or width in case of grooves) ranging from 50 nanometers to 5 micrometers. The dimensions of these cavities are chosen to produce surface plasmons on the walls of the cavity or an optical cavity mode within the cavity that acts as a tight whirlpool, sucking light (of a certain wavelength band) from distant areas into the cavity. These dimensions can vary depending on if surface plasmons or optical cavity modes are used to produce this effect, what material is in the cavity, and what material is at the base of the cavity, in accordance with the methods described herein. Both radii and depths for the cylindrical cavities, for example, can be in the range of 50 nanometers to 5 micrometers.
In another embodiment, a solar cell device of the present invention can be configured with a grating structure as described herein, wherein the grooves are filled with solar cells composed of the appropriate wavelength sensitive material.
Referring to
Additional layers 612 of metal, insulator, polymer or other materials can be placed on the walls of the cavity in for various purposes. For example, layers of oxide, polymers, metals, insulators or other materials may be on the walls of the cavity, either entirely or partially, to serve different purposes including electrical insulation, chemical precursor for electrochemical deposition, provide a conductive layer to aid in the support of cavity modes or other purposes. This layer may have thickness of 0.1 nanometers to 5 micrometers.
The light channeling or whirlpool effect induced by the cavity and surface Plasmon mode coupling produces strong light concentration in the absorbing wells and can result in 30%-100% of the light of separate wavelength bands to be channeled into and absorbed within a small volume of solar cell material.
Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.
This application claims priority to international patent application PCT/US07/25351 filed Dec. 10, 2007, which claims priority to U.S. provisional application Ser. No. 60/874,037, filed Dec. 8, 2006, and also claims priority to pending U.S. patent application Ser. No. 61/191,292, entitled “Horizontally Distributed, Tandem Solar Cells Using Surface Plasmons and Resonant Cavity Mode,” filed Sep. 8, 2008, the entireties of which are incorporated herein by reference.
The U.S. Government may have certain rights in this invention including the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of a Phase I Small Business Innovative Research (SBIR) Contract No. 0539541, entitled “Advanced Silicon-based Photodetectors Using Light Localization and Channeling” awarded by the National Science Foundation.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12746897 | Sep 2010 | US |
| Child | 14446766 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2007/025351 | Oct 2007 | US |
| Child | 12746897 | US |
