Metallic dielectric photonic crystals and methods of fabrication
A metallic-dielectric photonic crystal is formed with a periodic structure defining a plurality of resonant cavities to selectively absorb incident radiation. A metal layer is deposited on the inner surfaces of the resonant cavities and a dielectric material fills inside the resonant cavities. This photonic crystal can be used to selectively absorb broadband solar radiation and then reemit absorbed radiation in a wavelength band that matches the absorption band of a photovoltaic cell. The photonic crystal can be fabricated by patterning a sacrificial layer with a plurality of holes, into which is deposited a supporting material. Removing the rest of the sacrificial layer creates a supporting structure, on which a layer of metal is deposited to define resonant cavities. A dielectric material then fills the cavities to form the photonic crystal.
1. A photonic crystal for absorbing sunlight, comprising:
a periodic structure defining a two-dimensional array of resonant cavities, the periodic structure having a period less than a predetermined cut-off wavelength;
a layer of metal disposed on an inner surface of at least one resonant cavity in the plurality of resonant cavities, the at least one resonant cavity supporting at least one optical mode; and
a dielectric material, deposited within the at least one resonant cavity comprising a cylindrical waveguide, having a dielectric constant selected such that the at least one optical mode has a wavelength substantially equal to the predetermined cut-off wavelength so as to cause the photonic crystal to convert the sunlight at wavelengths below the pre-determined cut-off wavelength into heat energy.
2. The photonic crystal of claim 1 , wherein the photonic crystal is configured to absorb the sunlight over a wavelength range of about 0.5 μm to 3 μm and over an angular range of about 0° to about 70°.
3. The photonic crystal of claim 1 , wherein the layer of metal has a thickness equal to or greater than a skin depth of the layer of metal at the predetermined cut-off wavelength.
4. The photonic crystal of claim 1 , further comprising:
an anti-reflection coating disposed on top of the periodic structure, the anti-reflection coating having a thickness of about 1 nm to about 200 nm.
5. The photonic crystal of claim 1 , wherein the layer of metal comprises at least one of tungsten, ruthenium, platinum, silver, gold, copper, titanium, or tantalum.
6. The photonic crystal of claim 1 , wherein:
the layer of metal has a first thermal expansion coefficient; and
the dielectric material has a second thermal expansion coefficient less than the first thermal expansion coefficient.
7. The photonic crystal of claim 1 , wherein the dielectric material comprises at least one of SiO 2 , TiO 2 , Al 2 O 3 , or HfO 2 .
8. The photonic crystal of claim 1 , wherein the period is equal to or less than half of the predetermined cut-off wavelength.
9. The photonic crystal of claim 1 , wherein the cavity has a depth that is at least half the period of the periodic structure.
10. A photonic crystal of for absorbing incident radiation, comprising:
a periodic structure defining a plurality of resonant cavities, the periodic structure having a period less than a predetermined cut-off wavelength;
a layer of metal disposed on an inner surface of at least one resonant cavity in the plurality of resonant cavities, the at least one resonant cavity supporting at least one optical mode;
a dielectric material, deposited within the at least one resonant cavity, having a dielectric constant selected such that the at least one optical mode has a wavelength substantially equal to the predetermined cut-off wavelength; and
an anti-reflection coating disposed on top of the periodic structure, the anti-reflection coating having a thickness of about 1 nm to about 200 nm,
wherein the anti-reflection coating comprises a layer of the dielectric material.
11. A photonic crystal for incident radiation, comprising:
a periodic structure defining a plurality of resonant cavities, the periodic structure having a period less than a predetermined cut-off wavelength;
a layer of metal disposed on an inner surface of at least one resonant cavity in the plurality of resonant cavities, the at least one resonant cavity supporting at least one optical mode;
a dielectric material, deposited within the at least one resonant cavity, having a dielectric constant selected such that the at least one optical mode has a wavelength substantially equal to the predetermined cut-off wavelength, and
a diffusion barrier layer, disposed below the periodic structure, to prevent diffusion or mixing of the layer of metal and a substrate.
12. A method of fabricating a photonic crystal for absorbing sunlight, the method comprising:
A) providing a substrate coated with a sacrificial layer;
B) patterning the sacrificial layer with a plurality of holes arrayed at a period equal to or less than a predetermined cut-off wavelength;
C) depositing a supporting material over an inner wall of at least one hole in the plurality of holes;
D) removing a remaining portion of the sacrificial layer to form at least one supporting structure defined by the supporting material;
E) depositing a layer of metal on the at least one supporting structure to form a periodic structure defining a two-dimensional array of resonant cavities, the periodic structure having a second period less than the predetermined cut-off wavelength and at least one resonant cavity in the two-dimensional array of resonant cavities comprising a cylindrical waveguide; and
F) depositing a dielectric material in the at least one resonant cavity, the dielectric material having a dielectric constant selected such that at least one optical mode supported by the at least one resonant cavity has a wavelength substantially equal to the predetermined cut-off wavelength so as to cause the photonic crystal to convert the sunlight at wavelengths below the pre-determined cut-off wavelength into heat energy.
13. The method of claim 12 , wherein B) comprises patterning the sacrificial layer using a reactive ion etching process.
14. The method of claim 13 , wherein B) comprises:
B1) disposing a photo-mask having a checkerboard pattern in optical communication with a photoresist disposed on the sacrificial layer;
B2) irradiating the photo-mask at an irradiation wavelength so as to define a plurality of holes in the photoresist, the plurality of holes having a minimum inter-hole spacing less than the irradiation wavelength; and
B3) etching the plurality of holes into the sacrificial layer via the photoresist.
15. The method of claim 12 , wherein C) comprises depositing the supporting layer using an atomic layer deposition process.
16. The method of claim 12 , wherein D) comprises a XeF 2 gas phased etching process.
17. The method of claim 12 , wherein E) comprises depositing the layer of metal using at least one of atomic layer deposition, sputtering, or chemical vapor deposition.
18. The method of claim 12 , wherein F) comprises depositing the dielectric material using an atomic layer deposition process.
19. The method of claim 12 , further comprising:
G) depositing a layer of the dielectric material on the resonant cavity so as to form an anti-reflection coating on the photonic crystal.
20. The method of claim 12 , further comprising;
H) annealing the resonant cavity so as to remove undesired gap modes and/or increase absorption.
21. The method of claim 12 , wherein:
the substrate is further coated with a diffusion barrier layer beneath the sacrificial layer, the diffusion barrier layer comprising at least one of SiN or SiO 2 .
22. The method of claim 12 , wherein the sacrificial layer comprises polysilicon.
23. The method of claim 12 , wherein the supporting material comprises Al 2 O 3 .
24. The method of claim 12 , wherein the layer of metal comprises at least one of tungsten, ruthenium, platinum, silver, gold, copper, titanium, and tantalum.