Terahertz generation through optical rectification in reflection

Abstract

In this paper, we study terahertz generation through optical rectification in reflection at normal incidence in a dielectric nonlinear crystal. We first analyze, with a nonlinear optical model, the sample parameters (thickness, absorption at both laser and terahertz wavelengths, etc.) for which a terahertz optical rectification reflection scheme is preferable to the common transmission scheme. Then, we report our experimental observations of a reflected terahertz signal generated at the surface of a ZnTe crystal. The reflected terahertz signal shares all the characteristics of a signal generated in transmission but is not limited by absorption losses in the crystal, thereby providing a broader bandwidth. At high pump laser power, the signal exhibits saturation, which is caused by the decrease of the nonlinear susceptibility due to photocarriers generated by two-photon absorption. This reflection scheme could be of great importance for terahertz microscopy of opaque materials like, e.g., humid samples or samples exhibiting strong absorption bands or to study samples for which the transmitted signal cannot be recorded.

Aspects of Terahertz Reflection Spectroscopy

“…As concluded in the published paper [208] (appended Publication D), "we experimentally demonstrated the generation of a reflected THz signal at normal incidence through OR in a ZnTe crystal. The reflected signal originates in the boundary conditions for the nonlinear fields at the crystal surface.…”

“…The excellent fit with our experimental data is obtained for η = 0.259, meaning that roughly a quarter of the photoexcited carriers interact with the THz pulse generated in reflection. As mentioned in our paper [208] (appended Publication D) "The reason could be that they both recombine and diffuse inside the sample (Dember effect, both longitudinal and transversal 33,34 ) in between two successive laser pulses." It should be emphasized that this model is crude and the explanations must be validated by an exhaustive study as mentioned above.…”

“…Only a very limited number of experiments regarding OR in reflection can be found in the literature, and none in view of microscopy. A small review is given in our recent paper [208] (appended Publication D): "Generation of THz pulses through OR in reflection has only been studied to a lesser extent. In 2005, Reid et al 9 have reported OR THz generation from a semi-conductor recorded in reflection at 45 • incidence.…”