Mid-infrared THz beam sensors: exploration and application for phonon spectroscopy

We present an investigation of coherent mid-infrared pulse measurement system using free-space electro-optic sampling technique. A series of nonlinear materials are investigated for the nonresonant optical rectification and electro-optic sampling for the generation and detection, respectively. A sampling bandwidth up to 40 THz is achieved. For spectroscopic application we present THz field radiation by optically excited coherent phonons in the mid-infrared frequency range. We compare the THz radiation under extremely different excitation conditions. The comparison shows the THz radiation property of the resonantly driven phonons in the surface field layer. Furthermore we investigate the coherent reststrahl band reflectivity in the time domain. Unexpectedly strong oscillations are observed near the longitudinal-optical phonon frequency. This THz waveform is discussed with the reststrahl band dispersion of the reflectivity.