Methods and apparatus for generating terahertz radiation

Abstract

Apparatus and methods for generating radiation via difference frequency generation (DFG). In one exemplary implementation, a quantum cascade laser (QCL) has a significant second-order nonlinear susceptibility (χ (2) ) integrated in an active region of the QCL. The QCL is configured to generate first radiation at a first frequency ω 1 , second radiation at a second frequency ω 2 , and third radiation at a third frequency ω 3 =ω 1 −ω 2 based on difference frequency generation (DFG) arising from the nonlinear susceptibility. In one aspect, the QCL may be configured to generate appreciable THz radiation at room temperature.

1 . An apparatus, comprising:
 a quantum cascade laser (QCL) having a significant second-order nonlinear susceptibility (χ (2) ) integrated in an active region of the QCL, the QCL configured to generate first radiation at a first frequency ω 1 , second radiation at a second frequency ω 2 , and third radiation at a third frequency ω 3 =ω 1 −ω 2  based on difference frequency generation (DFG) arising from the nonlinear susceptibility.   
     
     
         2 . The apparatus of  claim 1 , wherein the QCL comprises:
 a first quantum cascade (QC) structure configured to generate the first radiation; and   a second QC structure configured to generate the second radiation,   wherein the nonlinear susceptibility is integrated in at least one of the first QC structure and the second QC structure.   
     
     
         3 . The apparatus of  claim 2 , wherein the nonlinear susceptibility is integrated in both of the first QC structure and the second QC structure. 
     
     
         4 . The apparatus of  claim 2 , wherein:
 the first QC structure includes at least one two-phonon structure; and   the second QC structure includes at least one bound-to-continuum structure.   
     
     
         5 . The apparatus of  claim 1 , wherein the QCL comprises a single quantum cascade (QC) structure to generate the first, second and third radiation. 
     
     
         6 . The apparatus of  claim 1  wherein the first radiation and the second radiation have respective first and second wavelengths in the mid-infrared band of the electromagnetic spectrum, and the third radiation has a wavelength in the terahertz (THz) band of the electromagnetic spectrum. 
     
     
         7 . The apparatus of  claim 1 , wherein the apparatus is configured to operate at room temperature. 
     
     
         8 . The apparatus of  claim 1  wherein the apparatus is configured to variably tune the first, second and third frequencies based on controlled variations of operating temperature, and wherein the apparatus further comprises a thermoelectric cooler coupled to the QCL to control the operating temperature of the QCL. 
     
     
         9 . The apparatus of  claim 1 , wherein the QCL includes a surface plasmon waveguide structure comprising:
 first and second conducting planes, wherein the active region is disposed between the first and second conducting planes; and   a low-doped buffer material disposed with the active region between the first and second conducting planes.   
     
     
         10 . The apparatus of any of  claim 1 , further comprising a grating disposed on the surface plasmon waveguide structure and configured to extract the third radiation along a length of the surface plasmon waveguide structure. 
     
     
         11 . A method, comprising:
 A) integrating a significant second-order nonlinear susceptibility (χ (2) ) in an active region of a quantum cascade laser (QCL) so as to facilitate generation of first radiation at a first frequency ω 1 , second radiation at a second frequency ω 2 , and third radiation at a third frequency ω 3 =ω 1 −ω 2  based on difference frequency generation (DFG) arising from the nonlinear susceptibility.   
     
     
         12 . The method of  claim 11 , wherein the QCL comprises a first quantum cascade (QC) structure configured to generate the first radiation and a second QC structure configured to generate the second radiation, and wherein A) comprises:
 integrating the nonlinear susceptibility in at least one of the first QC structure and the second QC structure.   
     
     
         13 . The method of  claim 12 , wherein A) comprises:
 integrating the nonlinear susceptibility in both of the first QC structure and the second QC structure.   
     
     
         14 . The method of  claim 12 , wherein:
 the first QC structure includes at least one two-phonon structure; and   the second QC structure includes at least one bound-to-continuum structure.   
     
     
         15 . The method of  claim 11 , wherein the QCL comprises a single quantum cascade (QC) structure to generate the first, second and third radiation. 
     
     
         16 . The method of  claim 11 , wherein the first radiation and the second radiation have respective first and second wavelengths in the mid-infrared band of the electromagnetic spectrum, and the third radiation has a wavelength in the terahertz (THz) band of the electromagnetic spectrum. 
     
     
         17 . The method of  claim 11 , further comprising:
 operating the QCL at room temperature.   
     
     
         18 . The method of  claim 11 , further comprising:
 controllably varying an operating temperature of the QCL so as to variably tune the first, second and third frequencies.   
     
     
         19 . The method of  claim 11 , further comprising:
 applying a continuous electric current to the QCL so as to operate the QCL in a continuous-wave (CW) mode.   
     
     
         20 . The method of  claim 11 , further comprising:
 applying a pulsed electric current to the QCL so as to operate the QCL in a pulsed mode.