Abstract- Terahertz Electro-Optic Sampling in Thick ZnTe Crystals Below the Reststrahlen Band With a Broadband Femtosecond Laser

Bang Wu,  Lei Cao,   Zhe Zhang,   Qiang Fu,   Yongqian Xiong

https://ieeexplore.ieee.org/document/8319525/

The method of electro-optic (EO) sampling of terahertz (THz) pulses in thick EO crystals leads to waveform distortions due to effects of phase mismatch, dispersive propagation, and absorption. In this paper, we demonstrate theoretically and experimentally that EO sampling with a broadband femtosecond (fs) laser could significantly eliminate these distortions below the Reststrahlen band. Our simulation results show that the oscillations and dips in the signals of EO response function of thick crystals are smoothed out by the broadband spectrum of the femtosecond laser. In the experiment, we use a laser with a bandwidth of 100 nm and a low temperature GaAs photoconductive antenna to generate typical THz pulses (0.1-3 THz). The measurement results confirm that a {110}-oriented 3-mm-thick ZnTe crystal is an attractive EO material for sampling THz pulse below 3 THz and agree with simulation results. This technique is particularly useful in areas where a large time window is needed, for example in the THz time-domain spectroscopy system.

Spotlight on OPTICS:Optimized terahertz generation via optical rectification in ZnTe crystals

http://www.opticsinfobase.org/spotlight/summary.cfm?URI=josab-31-1-149

Published in JOSA B, Vol. 31 Issue 1, pp.149-153 (2014)
by S. Vidal, J. Degert, M. Tondusson, E. Freysz, and J. Oberlé

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Spotlight summary: Terahertz light falls in the range between infrared and microwaves, and until about 25 years ago was a niche area in optical science and technology. Since then, there has been an explosion of research and applications with the advent of new and efficient methods for generation and detection of THz pulses. Solids, liquids, gases, and plasmas are probed via THz spectroscopy. Nanomaterials, metamaterials, insulators, conductors, and superconductors are probed via THz spectroscopy. THz imaging for both security applications and nondestructive evaluation is being actively pursued. 

As a general rule, any optical system benefits from more sensitive detectors and/or brighter light sources, other things being equal. Oberlé et al. report a factor of 2.4 enhancement in THz pulse generation in a 1-mm thick ZnTe crystal using shaped pump pulses from a Ti:Sapphire amplifier and an optimization algorithm. This is noteworthy because attempts to significantly increase the THz pulse energy by simply increasing the pump fluence will fail. 

THz pulses are generated in ZnTe by optical rectification, which is a second order process. Thus, the natural inclination is to simply increase the pump beam fluence since the THz pulse energy has a quadratic dependence on it. Unfortunately, two-photon absorption (TPA), which depletes the pump beam, is also a second order process. In addition, free carrier absorption (FCA) from conduction band electrons generated via TPA will attenuate the THz pulse concurrently with its generation.

Instead, Vidal et al. chose to use pulse shaping techniques with a 640-pixel programmable spatial light modulator to vary the amount of group delay dispersion, as well as third, fourth, and fifth order dispersion of the pump pulse. An evolutionary or “genetic” algorithm was used for optimization of these four parameters. They found that the THz pulse energy from the shaped pump pulse relative to that from a transform limited pulse of the same fluence, denoted R_opt, rapidly increased to 90% of its converged value within the first 5 iterations, and then required another 10 iterations of the algorithm to achieve final convergence.

Once they determined the optimal spectral characteristics of the pump pulse, they numerically solved the 3D nonlinear Schrödinger equation to analyze and understand the propagation of the pump pulse through the crystal since the optical properties of ZnTe are well known. For low fluences, where TPA is negligible, the pump pulse intensity of the transform-limited pulse is diminished by dispersion in the ZnTe crystal as it propagates. Thus, a negative prechirp allows it to have maximum intensity at the center of the crystal and thereby produce about 40% higher THz pulse energy. On the other hand, at high pump fluences where TPA and FCA play a prominent role in limiting the THz pulse energy, they found that nearly twice as much negative prechirp maximized the THz output. In this case, the pump pulse duration is minimized near the back face of the crystal, and an impressive enhancement factor of 2.4 is obtained.

This work has shown that the use of a pulse shaper and optimization algorithm can lead to a quantitative understanding of the interplay of beneficial and detrimental processes during THz pulse generation in ZnTe. 

--Charles Schmuttenmaer