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.
Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant
potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique
utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will
discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the
confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin
diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of
discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented
using a transgenic mouse model.
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