Characterizing intense broadband THz pulses is necessary for their use in spectroscopic studies

The use of terahertz waves has led to advances in fields from materials science to biological sensing, but the ability to characterize intense broadband THz pulses is lacking. In a new study, Mou et al. demonstrated coherent hyper-Raman spectroscopy in the terahertz regime in a gallium selenide crystal and studied the signal as a function of wavelength and time delay.

“Gallium selenide crystals have a relatively large electro-optic coefficient, high damage threshold and suitable transparent range in the THz range,” said author Domenico Paparo. This high damage threshold together with a large nonlinear coefficient has allowed the researchers to generate very high THz field amplitudes.

By scanning through different wavelengths and time delays, the group found an interference effect in the central part of the THz hyper-Raman intensity signal. This indicates a suitable regime for coherent detection of the THz optical field at high voltage, allowing efficient access to spectroscopic regions that standard linear technologies may not span.

“One important feature of the THz hyper-Raman is that the information on the THz material resonances is encoded in the output visible THz hyper-Raman beam, and hence, detection schemes, which are difficult or even impossible with THz radiation, such as photon counting or scanning probing microscopy, become feasible,” Paparo said.

The authors hope to apply this spectroscopic technique to investigate low-energy excitations in a range of materials, such as in 2D systems. “There is still a lot of work to be done to fully exploit all the potentialities of this technique,” said Paparo.

Source: “Terahertz hyper-Raman time-domain spectroscopy of Gallium selenide and its application in terahertz detection,” by Sen Mou, Andrea Rubano, and Domenico Paparo, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5115986 .