We demonstrate generation of 0.2 mJ terahertz (THz) pulses in lithium niobate driven by Ti:sapphire laser pulses at room temperature. Employing tilted pulse front technique, the 800 nm-to-THz energy conversion efficiency has been optimized to 0.3% through chirping the sub-50 fs pump laser pulses to overcome multi-photon absorption and to extend effective interaction length for phase matching. Our approach paves the way for mJ-level THz generation via optical rectification using existing Ti:sapphire laser systems which can deliver Joule-level pulse energy with sub-50 fs pulse duration.
The ultrafast optoelectronic response in topological insulators (TIs) has been recognized as one of the keys for applications on quantum computing and high-speed devices, which thus has attracted great attention recently. In this work, we systematically investigate the ultrafast transient terahertz emission excited by femtosecond laser pulses in Bi2Te3 with terahertz emission spectroscopy serving as an ultrafast and contactless detector. The nonlinear terahertz emission surpasses the terahertz emission from the sum of the drift and diffusion current contributions even at oblique incidence with an incident angle up to 70°, manifesting remarkable surface nonlinear effects on TIs. Quantitatively comprehensive microscopic analysis of the nonlinear terahertz emission origins indicates the 120°-periodic azimuth-angle dependence, which reveals a microscopic picture that the nonlinear current flows along the Bi-Te bonds. Our exploration not only enhances the microscopic understanding of the nonlinear responses in TIs on a femtosecond timescale but also lays a foundation for their applications on high-speed and low-power-consumption devices and systems.
We study theoretically the response of group III-V compound semiconductors (AlAs, AlP, GaAs, GaP, GaSb) to free-electron laser irradiation, identifying their damage thresholds. The employed hybrid code XTANT is capable of modeling both thermal and nonthermal effects under ultrafast electronic excitation. It allowed us to reveal common trends in the studied materials: all but the AlAs III-V compounds studied here exhibit a phase transition into a metallic disordered state of lower density than the solid phase via a thermal phase transition. This transition is instigated by electron-ion coupling at doses below the nonthermal melting. Irradiated AlAs showed two possible phases produced: low-density and high-density liquid. We demonstrate that the transferrable tightbinding method within the Born-Oppenheimer approximation significantly overestimates the damage threshold predicting only nonthermal melting in comparison to a non-Born-Oppenheimer scheme, which accounts for both effects and their interplay.
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