Highly efficient scalable monolithic semiconductor terahertz pulse source

Abstract

Intense pulses at low terahertz (THz) frequencies of 0.1-2 THz are an enabling tool for constructing compact particle accelerators and for strong-field control of matter. Optical rectification in lithium niobate provided sub-mJ THz pulse energies, but it is challenging to increase it further. Semiconductor sources suffered from low efficiency. Here, a semiconductor (ZnTe) THz source is demonstrated, collinearly pumped at an infrared wavelength beyond the three-photon absorption edge and utilizing a contact grating for tilting the pump-pulse front. Suppression of free-carrier absorption at THz frequencies in this way resulted in 0.3% THz generation efficiency, two orders of magnitude higher than reported previously from ZnTe. Scaling the THz energy to the mJ level is possible simply by increasing the pumped area. This unique THz source with excellent focusability, pumped by novel, efficient infrared sources, opens up new perspectives for THz high-field applications. Terahertz (THz) pulses with high energy and field strength are enabling novel applications [1-4], including resonant control over ionic motion, bound and free electrons, as well as nonresonant and strong-field interactions [3]. Intense THz pulses hold promise for the development of a new generation of compact particle and x-ray sources [1,2]. Laser-and THz-driven particle accelerators with unprecedented flexibility can be important for free-electron lasers [2,5] and materials science and could revolutionize medical therapy with x-ray, electron, or proton beams [1,2].Single-cycle or nearly single-cycle THz pulses with high energy can be generated by optical rectification of femtosecond laser pulses. The highest so far THz pulse energy reported from such a source, utilizing the novel organic nonlinear material DSTMS, was 0.9 mJ [6]. The spectrum obtained from organic materials is typically centered in the 2 to 10 THz range, well suited for nonlinear spectroscopic studies. THz sources with lower frequencies are optimally fitted to the requirements of particle acceleration [1,2,7]. The frequency range below 2 THz can be better accessed with another nonlinear material, lithium niobate, utilizing pump pulses with a tilted intensity front for non-collinear phase matching [8]. THz pulses with more than 0.4 mJ energy were generated with 0.77% efficiency using this technique [7]. However, increasing the THz energy further turned out to be very challenging because of the large pulse-front tilt angle (63°) and the associated large angular dispersion of the pump [9,10]. The effect of a strong THz field on the pump pulse, owing to their nonlinear interaction, leads to additional difficulties involving the reduction of the THz generation efficiency [11] and the distortion of the THz beam [12].Semiconductor nonlinear materials have been extensively used to access the low-frequency part of the THz spectrum. The most popular material is ZnTe, where collinear phase matching is possible at the commonly used 0.8 μm pump wavelength of Ti:sapphire lasers. The highest THz pu...

Laser‐Driven Strong‐Field Terahertz Sources

“…A plane-parallel contact-grating THz source with uniform interaction length could be successfully demonstrated in ZnTe [45] (see also Section 2.3), but the realization of a practical (hybrid) contact-grating source in LN so far turned out to be very challenging. Recently, a modified hybrid approach was proposed to provide uniform interaction length across large pump and THz beams.…”

“…[12] THz pulse energies on the order of 1 µJ were demonstrated both with OR [44,75] and OPA. [45,46] For this reason, OR in semiconductors is discussed here in more detail. [77] Recent developments indicate that OR will expectedly be scalable to substantially higher, mJ-level energies.…”

High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge

“…The significantly smaller effective nonlinear coefficient of semiconductors, as compared to that of LN, can be compensated for by a larger effective interaction length, enabled by the much smaller PFT angle. Furthermore, a small tilt angle is very advantageous for the realization of a semiconductor contact-grating THz source with exceptionally favorable energy scaling properties [26,27,31,32]. …”

“…An estimation of ( ) THz pulses with as high as 14 μJ energy were generated, 9 × higher than the highest previously reported value for semiconductors [13]. Further increase of the THz pulse energy to the mJ level can be expected by increasing the pump spot size to a few cm and scaling the pump pulse energy to the 100-mJ level, in combination with the scalable monolithic contactgrating technology [26,27,31,32]. Such new THz sources, in combination with novel efficient infrared pump sources in the 1.7 to 2.5 μm wavelength range based, for example, on Holmium laser technology [34][35][36][37], opens new perspectives for THz high-field applications.…”

110-mJ 225-fs cryogenically cooled Yb:CaF_2 multipass amplifier

“…Ultrafast strong-field science, a rapidly expanding research field, steadily demands higher peak and average power lasers for pumping secondary radiation sources based on non-linear frequency conversion, such as (i) parametric amplifiers based on χ <2> and χ <3> interactions [1,2]; (ii) powerful THz radiation sources relying on optical rectification [3][4][5][6][7] and generation of laser plasma microcurrents [8]; (iii) coherent soft EUV/X-ray generation via higher-order harmonic generation [9]; and (iv) incoherent hard X-ray radiation emerging from K α and K β transitions and bremsstrahlung in strongly excited solid-state targets [10].…”