The claimed invention of ultra-small cavity via placing phase-fitting metasurfaces coupled to cavity mirrors, relates to the field of nanophotonics, impacting areas including, but not limited to, nanolasers, threshold-less lasing, applications resulting from the Purcell effect, e.g., spontaneous emission enhancement, single photon sources, quantum computation devices, and nanometer scale cavity-based optical devices such as optical parametric oscillators, interferometers, laser phase and frequency stabilizers, laser spatial and temporal filters, adaptive beam, and pulse shaping devices.
Light confinement in conventional cavities like Fabry-Perot, made of parallel mirrors, should satisfy a resonant condition of having a round trip phase shift of an integer multiple of 2 π. This implies that the cavity should have a length equal to an integer number of half the wavelength, setting a lower limit to the length of the cavity of λ/2 and a minimum modal volume of λ3/8.
Minimizing the cavity size below the aforementioned limit has numerous applications including nano-lasers, and a set of applications coming from the Purcell effect of enhancement of spontaneous emission rate which is inversely proportional to the volume of the cavity. This effect is useful in single photon sources, quantum computation devices, and in threshold-less lasing. Additionally, it is aimed to scale down the size of optical components to be compatible with the size of nano-electronic devices, and there are numerous cavity-based optical device applications including, but not limited to, optical parametric oscillators, interferometers, laser phase and frequency stabilizers, and laser spatial and temporal filters.
Different techniques have been proposed to scale-down the size of cavities including using metamaterials with negative permittivity and permeability, photonic crystal cavities, cavities confining surface plasmon waves, and cavities having meta-film layers in the middle of the cavity between two dielectric layers. The technique of using negative permittivity and permeability is complicated in practice, e.g., to build a bulk meta-material with such properties, and it would include losses due to a large amount of metal used in the meta-material. Photonic crystal cavities can provide very high Quality factors (Q), but their size is limited by the wavelength order and they do not reach deep sub-wavelength. Cavities operating on surface plasmon polariton (SPP) waves can reach very small sizes, but SPP waves have resonant modes that are dissimilar to modes existing in conventional optic devices, making it harder to couple waves to and from SPP cavities, or to integrate them efficiently with external devices. Solutions provided using meta-films embedded between dielectric layers have limited effects of reducing the size of the cavity by a factor of only 2 or 3. This limited effect is due to the relatively week coupling of a wave to one layer of metafilm (compare with the strong coupling of a wave to a metasurface coupled to mirrors with the strong physical effect of gap plasmon resonance).
Using a parallel plate cavity to replace the conventional mirrors introduces a phase shift of π for each mirror with a reflecting metasurface that introduces an arbitrary phase shift to the reflected wave, which will change the roundtrip phase condition, and the length of the cavity will no longer keep the lower bound of λ/2.
It is important that the surface replacing the mirror does not change the phase shift at the expense of light confinement inside the cavity. For example, using metals with high losses as mirrors can result in a reflecting phase shift different from IC due to a significant portion of the wave penetrating through the metal during reflection. Such a structure can have a distance between mirrors of less than λ/2 but the mode will not truly be confined between minors, as part of the mode will penetrate the metal, causing high losses and a very unreliable quality factor.
The present invention provides a new approach for subwavelength cavity solutions.
Employment of a reflecting metasurface based on plasmonic nanostructure elements changes the cavity resonance condition that currently causes restrictions on minimum length. This also does not occur at the expense of light confinement inside the cavity because plasmon elements couple electromagnetic waves into free electron plasma. Hence, the localized energy not only exists in the form of electromagnetic energy but also exists in the form of mechanical energy of oscillating electrons over very short-length scales near the metasurface. Physically speaking, the short length of wave propagation between the cavity walls is compensated by strong localization of electromechanical energy near the metasurface walls, which experience considerable phase shifts over a very small distance. All energy forms remain highly confined between the cavity walls.
The abstract design of the invention is illustrated by
Adding top layer of metal over extra-deposited dielectric polymer will result in a cavity structure. The cross-section of such a structure is shown in
In the preferred embodiment the multilayer structure of
It is possible also to use a metasurface with both mirrors, applicable to cases such as the use of graphene disks. Graphene has a remarkable feature of having a plasmonic resonance in the infrared region (6-7 μm wavelength) using very thin disks with a thickness on the order of 1 nm, but the phase shift obtained from this resonance doesn't span the entire range from 0 to 2 π. Using metasurfaces with both mirrors, as shown in
The structures currently disclosed are easily implemented using well-established fabrication techniques. Metal thin films and alumina spacers are obtained using Physical Vapor Deposition (PVD). Spacers can be also fabricated using Atomic Layer Deposition (ALD). Spin coating or PVD can be used to produce the polymer filling. Electron Beam Lithography (EBL) is used to compose the array of metallic disks. For the case of ciraphene array, Chemical Vapor Deposition (CVD) is used. Standard nanofabrication techniques and conditions are used.
For the graphene structure
In another embodiment, the structure can be implemented with curved mirrors, e.g., spherical or parabolic mirrors, as shown in
The design can be adjusted during fabrication to obtain resonance at various wavelength ranges. Changing dimensions can tune the resonant wavelength across the visible and near-IR range. Changing materials (e.g., switching from silver to graphene) can move the resonant wavelength to far IR ranges.
The structure can also be used to make a device that is tunable after fabrication. It can be tuned, for example, using light, bias voltage, or temperature. To tune using a control light source, the metallic disks can be replaced with amorphous silicon. A control light signal can be used to excite electron-hole pairs and the hence change the free carrier concentration. Also these disks can be replaced with highly doped semiconductors where the carrier concentration varies based on the applied voltage. Temperature-based tuning is possible if the spacer or the polymer are replaced with liquid crystal, which varies in refractive index based on temperature.
The structure of the present invention can be used to build an array of cascading cavities, as shown in
Arrays of cavities can be used in many applications including, but not limited to, quantum electrodynamics applications, where single photon emitters are coupled to each other to generate entangled photons, or to generate other forms of entangled qubits, such as entangling 2 level atoms in adjacent cavities. Other applications of cavity arrays include pulse shaping and imaging applications through spatial variation of cavities. They can also be used to obtain compact arrays of laser sources for optical signal processing.
It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed in the scope of the claims.
Moreover, the words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form.
This patent application claims priority to U.S. provisional application No. 61/933,342 filed on Jan. 30, 2014, which fully incorporated herein by reference.
This invention was made with government support under FA9550-12-1-0024 awarded by United States Air Force. The government has certain rights in the invention.
Number | Date | Country | |
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61933342 | Jan 2014 | US |