Patent Application
20080089642
Furthermore, the invention provides embodiments and other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings.
Exemplary embodiments in accordance with the invention provide a photonic crystal apparatus and a method for fabricating a photonic crystal apparatus.
Two-dimensional photonic crystal slab apparatus 100 comprises a periodic lattice having a rectangular array of holes 112. This is intended to be exemplary only, as holes 112 can also be arranged in other configurations, for example, a square-shaped array or a triangular-shaped array, without departing from the scope of the present invention.
Although not illustrated in
Two-dimensional photonic crystal slab apparatus 100 has a photonic bandgap that is a function of the design of the apparatus. For example, apparatus 100 can be constructed to have a photonic bandgap between about 1300 nm and about 1600 nm by etching holes 112 having a diameter of about 244 nm to define a triangular-shaped lattice having a lattice constant of about 44 nm in a Si slab material about 260 nm thick.
Additional functionality is engineered into a photonic crystal by introducing one or more defects into the otherwise periodic variation of the index of refraction of the photonic crystal. In two-dimensional photonic crystal slab apparatus 100, a single defect 116 is introduced into the periodic lattice structure defined by the array of holes 112. In particular, defect 116 is created by forming one hole of the array of holes 112 to be of a reduced diameter, for example, about 176 nm. It should be understood, however, that defect 116 can also be formed in other ways, for example, by increasing the diameter of one or more holes 112 or by changing the shape of one or more holes 112, and it is not intended to limit the invention to a defect having any particular configuration.
Defect 116 defines a resonance chamber having a resonance frequency within the photonic bandgap of two-dimensional photonic crystal slab apparatus 100 that is localized in the vicinity of the defect. Light is coupled into and out of two-dimensional photonic crystal slab apparatus 100 by light guiding structure such as ridge waveguides 118. Light at the resonance frequency can be detected in the vicinity of defect 116 using a suitable light detecting apparatus such as an InGaAs photodetector or other suitable photodetector (not shown in
Two-dimensional photonic crystal slab apparatus 100 functioning as a resonator can be used as an optical sensor in the field of small volume sensing wherein the apparatus is used to detect the presence of nanoparticles, for example, biomolecules such as proteins, antibodies and viruses.
Slab body 210 is optically coupled to a pair of waveguides, not shown in
Two-dimensional photonic crystal slab sensor apparatus 200 can be used to detect the presence of nanoparticles in or passing through resonance chamber (defect hole) 216. Typically, the nanoparticles are suspended in a carrier liquid such as, for example, water, and are caused to flow through the apparatus from above the apparatus to below the apparatus as indicated by the “fluid in” and “fluid out” designations 236 and 238, respectively, in
The responsivity of two-dimensional photonic crystal slab sensor apparatus 200 is defined as a change in wavelength Δλ with respect to a change in refractive index Δn. For a two-dimensional photonic crystal slab sensor apparatus comprising a photonic crystal slab formed of silicon on insulator, (SOI) material, the responsivity Δλ/Δn typically ranges from about 150 nm to about 300 nm. When the refractive index changes only in resonance chamber 216 and not in the array of holes 212, the responsivity typically ranges from about 75 nm to about 150 nm. Typical dimensions for an exemplary embodiment of two-dimensional photonic crystal slab sensor apparatus 200 in accordance with the invention includes a lattice constant of about 400, a radius for holes 212 of about 0.25 a to about 0.4 a, a radius for resonance chamber 216 of about 0.15 a to about 0.25 a and a slab body thickness of about 0.6 a.
A typical volume for resonance chamber 216 is thus about 6×106 nm3. Hence, a 10 nm diameter nanoparticle, such as a biomolecule, within resonance chamber 216 occupies a fractional volume of about 10−4. Most common organic molecules such as proteins, antibodies or viruses have a refractive index of about 1.5 while the refractive index of water is about 1.3. Accordingly, the presence of a single 10 nm diameter molecule in resonance chamber 216 provides a refractive index change of about 2×10−5 resulting in a shift in operating wavelength of light input into two-dimensional photonic crystal slab sensor apparatus 200 of about 0.003 nm. By detecting this change in wavelength, the presence of nanoparticles in the resonance chamber can be detected.
Individual molecules can be delivered to resonance chamber 216 using microfluidic channels or other delivery mechanisms that are well-known in the art.
Typical dimensions for biomolecules are about 2-4 nm for proteins, 4-10 nm for antibodies and 40-200 nm for viruses. Two-dimensional photonic crystal slab sensor apparatus 200 can be tuned to maximize responsivity to single nanoparticles of a particular size by varying the radii of holes 212 and resonance chamber 216 with respect to the lattice constant of the periodic lattice in slab body 210, and by determining the change in operating frequency for refractive index changes in resonant chamber 216 normalized to the volume of the defect resonance chamber.
Photodetector 304 measures the relative power transmitted at the two different wavelengths. An error signal from bandpass filter 306 centered at f0 tunes the lower frequency or wavelength such that the current from photodetector 304 is equal for both wavelengths. The operating wavelength is then at the midpoint between the lower and upper wavelength; and, as indicated above, by measuring the operating wavelength, a nanoparticle in defect hole 216 of two-dimensional photonic crystal slab sensor apparatus 200 can be readily detected. Detection circuits such as illustrated in
As described above, it is desirable that the index of refraction of the material in the array of holes 212 in two-dimensional photonic crystal slab sensor apparatus 200 be as low as possible to provide a relatively large contrast between the index of refraction of the slab body and the index of refraction of the periodic lattice formed by the holes, and that the index of refraction of the material in the array of holes not change during a sensing operation so that a change in the index of refraction of the material in resonance chamber (defect hole) 216 caused by the presence of a nanoparticle can be accurately detected to identify the presence of the nanoparticle in resonance chamber 216. When using two-dimensional photonic crystal slab sensor apparatus 200 to detect the presence of nanoparticles, however, it is necessary to cause a fluid within which nanoparticles are suspended to flow through resonance chamber 216, and it is difficult to do so while, at the same time, preventing fluid and particles from flowing into and through holes 212.
According to an exemplary embodiment in accordance with the invention, nanoparticles are prevented from flowing through holes 212 and the index of refraction of the material in holes 212 is maintained at a low, constant value by filling the holes with a material referred to herein as “solid air” as illustrated at 220 in
According to an exemplary embodiment in accordance with the invention, solid air material 220 comprises a solid-state dielectric material, either organic or inorganic, having a dielectric coefficient that is substantially lower than the dielectric coefficient of the dielectric material forming slab body 210. For example, a two-dimensional photonic crystal that includes a slab body formed of silicon dioxide, a commonly used dielectric material, has a dielectric coefficient K of about 4.0. When used with a silicon dioxide slab body, a suitable solid air material has a dielectric coefficient of about 2.7 or lower. The term “solid state” refers to one of the three phase of matter (solid, liquid, gas) and relates to physical properties of solid materials. A solid state material is characterized by being resistant to deformation and to change of volume.
Suitable dielectric materials include spin-on organosilicates that are used as a low-K material in back end of the line (BEOL) interconnects in semiconductor chips. The low-index organosilicates can contain intrinsic micropores or mesopores generated by porgen. The mesopores can be generated by selectively removing organic porogen molecules from phase separated organosilicate and porogen nanohybrids. The nanohybrids can be generated by thermal crosslinking of the organosilicate in a mixture of porogen and organosilicate. The amount of porogen determines porosity, hence the dielectric constant of the solid-state dielectric material. The dielectric constant of porous organosilicate generated by this method ranges from 1.2 to 2.7, and ensures that a satisfactory refractive index contrast be maintained between the material of slab body 210 and the material in holes 212 at all times.
“Solid air” material 220 can be used to fill all of holes 212 leaving only resonance chamber 216 open to receive nanoparticles. Alternatively, inasmuch as surrounding holes in the vicinity of resonance chamber 216 are also sensitive to the nanoparticles, these holes can also be left unfilled as shown at 218 in
Both hydrophilic and hydrophobic dielectric materials can be used depending on how it is desired to selectively detect the nanoparticles. By tuning the surface to be either hydrophilic or hydrophobic, the binding of the nanoparticles to the porous dielectric surface can be suppressed or enhanced. This will also depend on the affinity of the nanoparticles, as well.
While what has been described constitute exemplary embodiments in accordance with the invention, it should be recognized that the invention can be varied in numerous ways without departing from the scope thereof. Because exemplary embodiments in accordance with the invention can be varied in numerous ways, it should be understood that the invention should be limited only insofar as is required by the scope of the following claims.
