Coherent Terahertz Hyper‐Raman Spectroscopy

In this paper, we review our recent discovery of a novel nonlinear effect in the terahertz frequency range, which we have termed terahertz hyper-Raman (THYR) spectroscopy. This technique shares many characteristics with conventional hyper-Raman spectroscopy but is applied within the THz spectrum. By utilizing intense terahertz pulses, THYR allows for the investigation of low-energy excitations in materials. Using crystalline quartz as a model, we demonstrate that THYR can simultaneously detect phonon modes, including those outside the Brillouin zone, as well as infrared-active vibrations and polaritons, offering capabilities beyond traditional Raman and infrared spectroscopy. Additionally, we discuss our recent progress toward the generation of circularly polarized THz pulses, enhancing the potential of THYR to study chiral excitations. These developments position THYR as a powerful tool for advancing nonlinear optics and material analysis. As we approach the centennial anniversary of the discovery of the Raman effect, we will show that our technique can bring a new wave of innovation to the field by merging the core principles of Raman spectroscopy with the rapidly advancing field of THz optics and technology.