Low-contrast metasurfaces
Disclosed herein are metasurfaces formed on a substrate from a plurality of posts. The metasurfaces are configured to be optically active at one or more wavelengths and in certain embodiments are configured to form lenses having unexpectedly strong focusing power. In particular, the metasurfaces are formed from “low-contrast” materials, including CMOS-compatible materials such as silicon dioxide or silicon nitride. Accordingly, the disclosed metasurfaces are generally CMOS compatible and therefore embody a new paradigm in metasurface design and manufacturing.
1. An Alvarez lens, comprising:
a first lens having a first low-contrast metasurface having optical activity at a first wavelength, the first low-contrast metasurface comprising;
a plurality of cylindrical posts formed from a first material and arranged on a substrate in a square pattern, wherein the plurality of cylindrical post are formed from a material having a first refractive index of 2.1 or less;
interstices between individual posts of the plurality of cylindrical post comprising an interstitial substance with a second refractive index that is 0.6 to 1.1 less than the first refractive index;
wherein the individual posts of the plurality of cylindrical posts have a diameter in a range of ⅛ of the first wavelength to ⅔ of the first wavelength;
wherein the plurality of cylindrical posts have a periodicity in a range of 0.4 times the first wavelength to 1.0 times the first wavelength; and
wherein the plurality of cylindrical posts have a thickness in a range of 0.5 times the first wavelength to 1.0 times the first wavelength; and
a second lens;
wherein the first lens and the second lens are arranged along an optical axis and configured such that displacement between the first lens and the second lens along a transverse axis in relation to the optical axis will produce a non-linear change in focus.
2. The Alverez lens of claim 1 , wherein the first material is silicon nitride.
3. The Alvarez lens of claim 1 , wherein the interstitial substance is a gas or a solid.
4. The Alvarez lens of claim 1 , wherein the interstitial substance is air or a polymer.
5. The Alvarez lens of claim 1 , wherein at least a portion of the first low-contrast metasurface defines the first lens, comprising a first metasurface portion having a first optical activity and a second metasurface portion having a second optical activity, different than the first optical activity.
6. The Alvarez lens of claim 5 , wherein the first lens is defined by patterning the first metasurface portion and the second metasurface portion to be shaped so as to produce a lens effect based on a difference in diffraction at the first wavelength between the first metasurface portion and the second metasurface portion.
7. The Alvarez lens of claim 6 , wherein the first lens has a numerical aperture of 0.1 to 0.7.
8. The Alvarez lens of claim 6 , wherein the first lens has a transmission efficiency of at least 90% as measured for a light source with a bandwidth of 25 nm centered at 625 nm.
9. The Alvarez lens of claim 6 , wherein the first lens has a focusing efficiency of at least 40% as measured for a light source with a bandwidth of 25 nm centered at 625 nm.
10. The Alvarez lens of claim 6 , wherein the first lens has a transmission efficiency of 90% and a focusing efficiency of 30% within at the first wavelength and an NA of 0.7.
11. The Alvarez lens of claim 6 , wherein the first lens has a transmission efficiency of 80% and a focusing efficiency of 10%.
12. The Alvarez lens of claim 1 , wherein first low-contrast metasurface defines a freeform optical element.
13. The Alvarez lens of claim 1 , wherein the substrate is substantially flat.
14. The Alvarez lens of claim 1 , wherein the optical activity is diffraction or reflection.
15. The Alvarez lens of claim 1 , wherein the second lens comprises a second low-contrast metasurface.
16. The Alvarez lens of claim 15 , wherein the first low-contrast metasurface is the optical inverse of the second low-contrast metasurface.