THz Generation By Optical Rectification And Competition With Other Nonlinear Processes

Abstract

We present a study of the terahertz (THz) generation by optical rectification in a 2 mm thick <110> ZnTe crystal. The conditions for the most efficient THz generation are described. We investigate the competition between optical rectification (OR), second harmonic generation (SHG), twophoton absorption (TPA) and free-carrier absorption in the diffraction limit, for excitation spot sizes smaller than the THz wavelength. We find that while free-carrier absorption contributes significantly to reduce the THz emission, this contribution does not depend on the excitation spot size. On the contrary, two-photon absorption induces a strong dependence of the THz emitted power for decreasing excitation sizes.

Optimized terahertz generation via optical rectification in ZnTe crystals

“…However, for pump pulses with high peak intensity, saturation effects mainly due to free carriers generated by two-photon absorption (TPA) limit the conversion efficiency. Indeed, TPA not only depletes the pump pulse, but also leads to a reshaping of the THz waveform that can be attributed to the influence of the photoinduced free carriers on the complex refractive index of the crystal in the THz range [2][3][4][5]. Interestingly, it has been shown that, due to free carriers saturation, increasing further the pump pulse fluence leads to a recovering of the quadratic dependence of the THz power with respect to the pumping fluence usually observed at low intensity [5].…”

Influence of free-carrier absorption on terahertz generation from ZnTe(110)

“…TPA of the excitation beam and FCA of terahertz radiation have been identified as processes that compete with optical rectification. [8][9][10][11][12] We find that FCA attenuates the latter part of the terahertz pulse, while TPA depletes the power available for terahertz generation.…”

“…More recently, studies have shown that processes such as second harmonic generation ͑SHG͒, twophoton absorption ͑TPA͒, and free-carrier absorption ͑FCA͒ can all compete with optical rectification. [8][9][10][11][12] SHG is the sum-frequency analog of optical rectification and removes photons from the excitation pulse that would normally produce terahertz radiation. TPA also depletes the excitation beam intended for optical rectification and more importantly, produces free carriers, which can then absorb the generated terahertz radiation through FCA.…”

“…Although many papers which analyze these processes have been published, the available literature is often incomplete and contradictory. For example, Gaivoronskii et al 9 conclude that TPA and FCA are not responsible for attenuating terahertz radiation at high powers, whereas Zhao and Tignon 11 identify a strong contribution from FCA of terahertz radiation. Others admit that TPA decreases the excitation power available for terahertz generation but do not analyze the effects of FCA after terahertz generation.…”

Optimized Optical Rectification and Electro-optic Sampling in ZnTe Crystals with Chirped Femtosecond Laser Pulses

“…1. The influence of TPA on generation of THz radiation by below-bandgap excitation in electro-optic crystals has been investigated by several authors [20][21][22][23][24]. The effect of above-bandgap TPA on carrier dynamics and free-carrier absorption of THz radiation has also been investigated [25].…”