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 Ropt, 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.
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