Thermal signature control structures
Subwavelength conducting particles can be arranged on conducting surfaces to provide arbitrary thermal emissivity spectra. For example, a thermal emissivity spectrum can be tailored to suppress a thermal signature of an object without sacrificing radiative cooling efficiency.
1. An apparatus, comprising:
a conducting surface;
a plurality of conducting particles arranged on the conducting surface, each particle having a flat surface and forming a planar gap region between the conducting surface and the conducting particle;
wherein the plurality of conducting particles is arranged according to a specific arrangement selected to provide a predetermined thermal emissivity spectrum for the apparatus with respect to a specific radiative cooling efficiency for the apparatus and either or both a size and a shape of each of the plurality of conducting particles are selected to provide the predetermined thermal emissivity spectrum for the apparatus.
2. The apparatus of claim 1 , wherein the predetermined thermal emissivity spectrum is a thermal emissivity spectrum that reduces a thermal signature of the apparatus by a first factor and reduces a radiative cooling efficiency of the apparatus by a second factor that is substantially smaller than the first factor.
3. The apparatus of claim 2 , wherein the thermal signature corresponds to thermal radiance integrated over a selected spectral range of infrared wavelengths.
4. The apparatus of claim 2 , wherein the radiative cooling efficiency corresponds to thermal radiance integrated over all infrared wavelengths.
5. The apparatus of claim 1 , wherein the predetermined thermal emissivity spectrum provides:
an apparent temperature of the apparatus that is substantially less than an actual temperature of the apparatus; and
an actual radiative cooling rate that is substantially greater than an apparent radiative cooling rate.
6. The apparatus of claim 5 , wherein the apparent temperature corresponds to a temperature of a blackbody having a blackbody thermal radiance in a selected spectral range equivalent to an actual thermal radiance of the apparatus in the selected spectral range.
7. The apparatus of claim 1 , wherein each of the conducting particles has a resonant wavelength selected from a set of resonant wavelengths, the set of resonant wavelengths corresponding to a set of sizes of the conducting particles.
8. The apparatus of claim 7 , wherein the set of sizes of the conducting particles is a set of lengths of planar gap regions between the plurality of conducting particles and the conducting surface.
9. The apparatus of claim 7 , wherein the selected thermal emissivity spectrum includes:
one or more spectral ranges of enhanced thermal emissivity that include the set of resonant wavelengths;
one or more spectral ranges of suppressed thermal emissivity that exclude the set of resonant wavelengths.
10. The apparatus of claim 9 , wherein the one or more spectral ranges of suppressed thermal emissivity include a selected spectral range, and the set of resonant wavelengths includes one or more resonant wavelengths below a lower wavelength limit of the selected spectral range or above an upper wavelength limit of the selected spectral range.
11. The apparatus of claim 1 , wherein the plurality of conducting particles is a colloidal assembly of conducting particles on the conducting surface.
12. The apparatus of claim 1 , wherein the plurality of conducting particles is a lithographically-defined arrangement of conducting particles on the conducting surface.
13. The apparatus of claim 3 , wherein the selected spectral range is a range of atmospheric transmission of thermal infrared radiation.
14. The apparatus of claim 3 , wherein the selected spectral range is a range of detector response for a thermal infrared detector.
15. The apparatus of claim 1 , further comprising:
a layer of infrared-transparent material covering the conducting surface and the conducting particles.
16. The apparatus of claim 15 , wherein the layer of infrared-transparent material includes ZnO or FeO particles.
17. A method of fabricating an apparatus, comprising:
arranging a plurality of conducting particles on a conducting surface according to a specific arrangement, each particle having a flat surface and forming a planar gap region between the conducting surface and the conducting particle, wherein the specific arrangement and either or both a size and a shape of each of the plurality of conducting particles are selected to provide a predetermined thermal emissivity spectrum for the apparatus with respect to a specific radiative cooling efficiency for the apparatus.
18. The method of claim 17 , further comprising:
placing a flexible layer on a substrate; and
depositing the conducting surface as a metal layer on the flexible layer.
19. The method of claim 18 , further comprising:
after arranging the plurality of conducting particles, peeling the flexible layer off of the substrate.
20. The method of claim 18 , further comprising:
depositing a spacer layer on the conducting surface.
21. The method of claim 17 , wherein the arranging of the plurality of conducting particles includes:
colloidally assembling the conducting particles on the conducting surface.
22. The method of claim 17 , wherein the arranging of the plurality of conducting particles includes:
photolithographically arranging the plurality of conducting particles on the conducting surface.
23. The method of claim 22 , wherein the photolithographic arranging is a photolithographic arranging by a lift-off process.
24. The method of claim 17 , further comprising:
covering the arranged plurality of conducting particles with an infrared-transparent material.
25. The apparatus of claim 6 , wherein the selected spectral range is a range of atmospheric transmission of thermal infrared radiation.
26. The apparatus of claim 6 , wherein the selected spectral range is a range of detector response for a thermal infrared detector.
27. The apparatus of claim 10 , wherein the selected spectral range is a range of atmospheric transmission of thermal infrared radiation.
28. The apparatus of claim 10 , wherein the selected spectral range is a range of detector response for a thermal infrared detector.
29. The apparatus of claim 1 , wherein the specific radiative cooling efficiency is about 50% of a radiative cooling efficiency of the apparatus absent the plurality of conducting particles arranged on the conducting surface.