Coherent, broadband midinfrared terahertz beam sensors

The generation of midinfrared pulses through the optical rectification extended the radiation frequency from a few THz to several tens of THz. The coherent measurement of a detection frequency greater than 5 THz was demonstrated using a GaP and a ZnTe electro-optic sensor. This optoelectronic technique opens the door to the midinfrared ~midIR! region for coherent emission spectroscopy, time-domain spectroscopy, and ultrafast pump-probe spectroscopy. In this letter, we report the recent study of coherent, broadband midinfrared THz beam sensors. We focus on freespace electro-optic sampling of the THz pulse in the frequency range higher than the optical phonon levels of the sensor crystals. Experimental results of midinfrared THz pulse measurements indicate that the thickness of the electrooptic crystals ~both emitter and sensor! is a crucial factor influencing the temporal wave form and spectral bandwidth of the measured THz pulse. With proper selection of the emitter and sensor, taking proper consideration of their phonon bands, it is feasible to achieve a useable frequency bandwidth ranging from 100 GHz to 30 THz. We present the experimental results of the coherent generation and detection of femtosecond THz midinfrared pulses. The experimental setup of the electro-optic mid-IR detection system is depicted in Fig. 1. A 12 fs Ti:sapphire laser delivers an average power of nearly 500 mW at a center wavelength of 800 nm. Electro-optic crystals ~including GaAs, InP, ZnTe, CdTe, GaP, BBO, and LiTaO3) with different thickness were used as the emitters and sensors. Three hundred and fifty milliwatts of the laser power were focused on the emitter by a gold-coated off-axis parabolic mirror with a 5 cm effective focal length. The broadband THz radiation generated from the emitter by optical rectification was collimated and then focused on the sensor by a pair of f /0.6 off-axis parabolic mirrors. The laser probe beam was combined to collinearly travel with the THz beam through a 2-mm-thick pellicle, which had a negligible effect on the laser pulse width and THz beam. If the emitter crystal is transparent to the optical excitation beam, such as for ZnTe, GaP, or LiTaO3, a silicon wafer is placed after the emitter crystal to block the optical beam but transmit the THz beam. The electro-optic modulation induced by the ultrafast Pockels effect was detected by using a pair of balanced photodiodes. By varying the time delay between the optical pump and probe pulses, the temporal wave form of the midinfrared transient was sampled.