Methods and apparatus for generating terahertz radiation

1. A method for generating terahertz radiation, comprising:

forming an orientation patterned nonlinear semiconductor crystal on a substrate having a surface, wherein a first domain of the orientation patterned nonlinear semiconductor crystal is formed over a first region of the surface of the substrate by forming the first domain on an intermediate layer, said intermediate layer on the first region of the surface of the substrate, and a second domain of the nonlinear semiconductor crystal is formed over a second region of the surface of the substrate, said second region comprised of an exposed portion of the surface of the substrate, said first domain has a first crystal orientation and said second domain has a second crystal orientation that is opposite to the first crystal orientation, wherein the intermediate layer is etched to provide the exposed portion of the surface of the substrate, and wherein widths of the first and second domains achieve quasi-phase-matching between an optical pulse having a wavelength greater than a threshold for two-photon absorption in a material that comprises the orientation patterned nonlinear semiconductor crystal and the terahertz radiation;

providing an optical pulse having a wavelength less than approximately 1.5 μm; and

illuminating the orientation-patterned nonlinear semiconductor crystal with the optical pulse.

2. The method of claim 1 , wherein the intermediate layer is at least one of GaN, AlN, ZrN, ZnN, AlO, Al x Ga 1−x N, In x Ga 1−x N, Al x In y Ga 1−x−y N, AlO x , and Zn x N y , where x and y are any fractional number between 0 and 1.

3. The method of claim 1 , wherein the intermediate layer is formed by at least one of sputtering, ion beam deposition, electron-beam evaporation and atomic layer deposition.

4. The method of claim 1 , wherein the widths of the first and second domains are determined by widths of the portion of the surface of the substrate exposed by etching.

5. The method of claim 1 , wherein the orientation-patterned nonlinear semiconductor crystal is a wurtzite crystal.

6. The method of claim 1 , wherein the orientation-patterned nonlinear semiconductor crystal is one of: Al x Ga 1−x N, In x Ga 1−x N and Al x In y Ga 1−x−y N.

7. The method of claim 1 , wherein the orientation-patterned nonlinear semiconductor crystal is at least one of GaN and AlN.

8. An apparatus for generating terahertz radiation, comprising:

a nonlinear semiconductor crystal, wherein an orientation-patterned nonlinear semiconductor crystal has been formed on a substrate having a surface, wherein a first domain of the orientation patterned nonlinear semiconductor crystal is formed over a first region of the surface of the substrate by forming the first domain on an intermediate layer, said intermediate layer on the first region of the surface of the substrate, and a second domain of the nonlinear semiconductor crystal is formed over a second region of the surface of the substrate, said second region comprised of an exposed portion of the surface of the substrate, said first domain has a first crystal orientation and said second domain has a second crystal orientation that is opposite to the first crystal orientation, wherein the intermediate layer is etched to provide the exposed portion of the surface of the substrate, and wherein widths of the first and second domains achieve quasi-phase-matching between an optical pulse having a wavelength greater than a threshold for two-photon absorption in a material that comprises the orientation patterned nonlinear semiconductor crystal and the terahertz radiation;

and an optical pulse generator configured to illuminate the orientation-patterned nonlinear semiconductor crystal with an optical pulse having a wavelength less than approximately 1.5 μm.

9. The apparatus of claim 8 , wherein at least a portion of the terahertz radiation is produced by optical down conversion.

10. The apparatus of claim 8 , wherein the orientation-patterned nonlinear semiconductor crystal has a hexagonal crystal symmetry.

11. The apparatus of claim 8 , wherein the orientation-patterned nonlinear semiconductor crystal is a wurtzite crystal.

12. The apparatus of claim 8 , wherein the orientation-patterned nonlinear semiconductor crystal is one of: Al x Ga 1−x N, In x Ga 1−x N and Al x In y Ga 1−x−y N.

13. The apparatus of claim 8 , wherein the orientation-patterned nonlinear semiconductor crystal is at least one of GaN and AlN.

14. The apparatus of claim 8 , wherein the wavelength of the optical pulse is in a range between approximately 0.4 and 1.5 μm.

15. The apparatus of claim 13 , wherein the orientation-patterned nonlinear semiconductor crystal is GaN, and wherein the optical pulse has a wavelength greater than a threshold for two-photon absorption in the orientation-patterned nonlinear GaN crystal.

16. The apparatus of claim 15 , wherein the optical pulse has a wavelength greater than approximately 0.75 μm.

17. The apparatus of claim 13 , wherein the orientation-patterned nonlinear semiconductor crystal is AlN and wherein optical pulse has a wavelength greater than a threshold for two-photon absorption in the orientation-patterned nonlinear AlN crystal.

18. The apparatus of claim 17 , wherein the optical pulse has a wavelength greater than approximately 0.4 μm.

19. The apparatus of claim 8 wherein the intermediate layer is at least one of GaN, AlN, ZrN, ZnN, AlO, Al x Ga 1−x N, In x Ga 1−x N, Al x In y Ga 1−x−y N, AlO x , and Zn x N y , where x and y are any fractional number between 0 and 1.

20. The apparatus of claim 8 , wherein the intermediate layer is formed by at least one of sputtering, ion beam deposition, electron-beam evaporation and atomic layer deposition.

21. The apparatus of claim 8 , wherein the widths of the first and second domains are determined by widths of the portion of the surface of the substrate exposed by etching.

22. A method for generating terahertz radiation, comprising:

forming an orientation patterned nonlinear semiconductor crystal on a substrate having a surface, wherein a first domain of a nonlinear semiconductor crystal is formed over a first region of the surface of the substrate, wherein the first region has been treated to modify a chemical structure of the first region surface of the substrate, and a second domain of the nonlinear semiconductor crystal is formed over a second region of the surface of the substrate, wherein the second region of the nonlinear semiconductor crystal is formed over an untreated section of the surface of the substrate, said second region adjacent to the first region, said first domain has a first crystal orientation and said second domain has a second crystal orientation that is opposite to the first crystal orientation, wherein widths of the first and second domains achieve quasi-phase-matching between an optical pulse having a wavelength greater than a threshold for two-photon absorption in a material that comprises the orientation patterned nonlinear semiconductor crystal and the terahertz radiation;

providing an optical pulse having a wavelength less than approximately 1.5 μm; and

illuminating the orientation-patterned nonlinear semiconductor crystal with the optical pulse.

23. The method of claim 22 , wherein treating the first region of the surface of the substrate further comprises exposing the first region with an ion beam, wherein the ion beam modifies the chemical structure of the substrate in the first region.

24. The method of claim 22 , wherein the widths of the first and second domains are determined by widths of the first and second regions.

25. An apparatus for generating terahertz radiation, comprising:

a nonlinear semiconductor crystal, wherein an orientation-patterned nonlinear semiconductor crystal has been formed on a substrate having a surface, wherein a first domain of the nonlinear semiconductor crystal is formed over a first region of the surface of the substrate, wherein the first region has been treated to modify a chemical structure of the first region surface of the substrate, and a second domain of the nonlinear semiconductor crystal is formed over a second region of the surface of the substrate, and wherein the second region of the nonlinear semiconductor crystal is formed over an untreated section of the surface of the substrate, said second region adjacent to the first region, said first domain has a first crystal orientation and said second domain has a second crystal orientation that is opposite to the first crystal orientation, wherein widths of the first and second domains achieve quasi-phase-matching between an optical pulse having a wavelength greater than a threshold for two-photon absorption in a material that comprises the orientation patterned nonlinear semiconductor crystal and the terahertz radiation; and

and an optical pulse generator configured to illuminate the orientation-patterned nonlinear semiconductor crystal with an optical pulse having a wavelength less than approximately 1.5 μm.

26. The apparatus of claim 25 , wherein treating a first region of the surface of the substrate further comprises exposing the first region with an ion beam, wherein the ion beam modifies a chemical structure of the substrate in the first region.

27. The apparatus of claim 25 , wherein the widths of the first and second domains are determined by widths of the first and second regions.