Influence of emission angle of THz beam from nonlinear quantum cascade laser on spectroscopic imaging

The terahertz (THz) nonlinear quantum cascade laser (THz NL-QCL) is a small and easy to handle semiconductor source. The THz NL-QCL is capable of ultra-broadband emission and can be operated at room temperature. The THz NL-QCL is based on Cherenkov intra-cavity difference frequency generation. However, the THz emission angle can be slightly different depending on the terahertz frequency owing to the refractive index dispersion of the substrate. To evaluate the influence of the emission angle on THz imaging, we performed a geometrical optics ray tracing method simulation and found the change of the focusing position caused by the difference of the emission angle in the imaging system was approximately within +/- 0.2 mm. We confirmed that the difference of focusing position did not affect spectroscopic imaging.