Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. These resonant nonlinearities continually attract attention, particularly in newly discovered materials. However, they are frequently not as heightened as currently predicted, limiting their exploitation in nanostructured nonlinear optics. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive, as well as constructive, interference effects in complex systems. Using terahertz quantum-cascade-lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an unexpected interplay of the second-order nonlinear contributions of multiple light and heavy hole states.
Many applications such as toxic gas detection or H2S monitoring in natural gas require operation in the THz spectral region, where gas species show distinct spectral “fingerprints” that can be easily discriminated by the gas matrix background absorption features.
So far, continuous-wave THz quantum cascade lasers employed in quartz-enhanced photoacoustic (QEPAS) sensors required liquid helium-cooling systems. In this work, we demonstrated the first liquid nitrogen-cooled THz QEPAS sensor for H2S detection operated in pulsed mode and mounting a spectrophone based on a quartz tuning fork with 1.5 mm prong spacing. A sensitivity level in the part-per-billion concentration range was achieved.