Fresnel zone plate

1. A Fresnel zone plate for encountering incident light having a wavelength, comprising:

a substrate defining a generally planar surface;

a first set of rings radially spaced about a central axis along the surface of the substrate, the first set of rings being transparent;

a second set of rings radially spaced about the central axis along the surface of the substrate, each ring of the second set of rings including a surface lying in a plane perpendicular to the central axis and being opaque; and

a plurality of silicon nanowires extending into at least one of the surfaces of the second set of rings, the plurality of silicon nanowires;

having lengths in the range of 1 micrometer to 6 micrometers; and

being randomly distributed in the at least one of the surfaces of the second set of rings;

wherein each of the plurality of silicon nanowires extending along a corresponding axis generally perpendicular to the plane and being spaced from an adjacent silicon nanowire by a distance less than the wavelength of the incident light.

2. The Fresnel zone plate of claim 1 wherein at least one ring of the first and second sets of rings includes a mounting surface spaced from and generally parallel to the surfaces of the second set of rings and wherein the substrate includes a tuning structure mounted to the mounting surface.

3. The Fresnel zone plate of claim 2 wherein the tuning structure includes a plate wherein mechanically stretching of the plate adjusts a focal length of the Fresnel zone plate.

4. The Fresnel zone plate of claim 3 wherein the plate is fabricated from an elastomer.

5. The Fresnel zone plate of claim 3 wherein the plate is transparent.

6. A Fresnel zone plate for encountering incident light having a wavelength, the Fresnel zone plate having a focal length, comprising:

a wafer including first and second sides and alternating transparent and opaque zones, the second side of the wafer including:

a first set of rings radially spaced about a central axis, the first set of rings being transparent; and

a second set of rings radially spaced about the central axis, each ring of the second set of rings including a surface lying in a plane perpendicular to the central axis and being opaque;

a plurality of silicon nanowires extending into at least one of the surfaces of the second set of rings, the plurality of silicon nanowires being randomly distributed in the at least one of the surfaces of the second set of rings and each of the plurality of silicon nanowires:

extending along a corresponding axis generally perpendicular to the plane;

having lengths in the range of 1 micrometer to 6 micrometers; and

being spaced from an adjacent silicon nanowire by a distance less than the wavelength of the incident light; and

a tuning structure mounted to the first side of the wafer;

wherein actuation of the tuning structure adjusts the focal length.

7. The Fresnel zone plate of claim 6 wherein the tuning structure is actuated by mechanical stretching.

8. The Fresnel zone plate of claim 6 wherein the tuning structure includes a plate.

9. The Fresnel zone plate of claim 8 wherein the plate is fabricated from an elastomer.

10. The Fresnel zone plate of claim 8 wherein the plate is transparent.

11. A Fresnel zone plate for encountering incident light having a wavelength, the Fresnel zone plate having a focal length, comprising:

a wafer including alternating transparent and opaque zones, and a mounting surface, the opaque zone including an outer surface lying in a first plane generally parallel to the mounting surface;

a plurality of silicon nanowires extending into the outer surface of the opaque zone of the wafer along corresponding axes which are generally perpendicular to the outer surface, the plurality of silicon nanowires being randomly distributed in the opaque zone and spaced from an adjacent silicon nanowire by a distance less than the wavelength of the incident light; and

a mechanically stretchable tuning structure mounted to the mounting surface;

wherein stretching of the tuning structure varies the focal length of the Fresnel zone plate.

12. The Fresnel zone plate of claim 11 wherein the mounting surface is on a first side of the wafer and wherein the second side of the wafer includes:

a first set of rings radially spaced about a central axis, the first set of rings being transparent; and

a second set of rings radially spaced about the central axis, each ring of the second set of rings including the outer surface which is perpendicular to the central axis.

13. The Fresnel zone plate of claim 11 wherein the plurality of silicon nanowires have lengths, the lengths of the plurality of silicon nanowires are in the range of 1 micrometer to 6 micrometers.

14. The Fresnel zone plate of claim 11 wherein the tuning structure includes a plate, the plate being fabricated from an elastomer.

15. The Fresnel zone plate of claim 11 wherein the tuning structure includes a plate, the plate being transparent.