Single-cycle, MHz repetition rate THz source with 66 mW of average power

Abstract

We demonstrate THz generation using the tilted pulse front method in Lithium Niobate, driven at unprecedented high average power of more than 100 W and at 13.3 MHz repetition rate, provided by a compact amplifier-free modelocked thin-disk oscillator. The conversion efficiency was optimized with respect to pump spot size and pump pulse duration, enabling us to generate a maximum THz average power of 66 mW, which is the highest reported to date from a laser-driven, few-cycle THz source. Furthermore, we identify beam walk-off as the main obstacle that currently limits the conversion efficiency in this excitation regime (with moderate pulse energies and small spot sizes). Further upscaling to the watt level and beyond is within reach, paving the way for linear and nonlinear high-average power THz spectroscopy experiments with exceptional signal-to-noise ratio at MHz repetition rates.Terahertz time domain spectroscopy (THz-TDS) has been one of the cornerstones of THz science for several decades, leading to a vast number of advances in science and technology. One example in scientific research where THz-TDS continues to enable major breakthroughs is in condensed matter physics, where timeresolved spectroscopic techniques (pump-probe, multidimensional spectroscopy) in the THz region have become ubiquitous [1]. However, in this and many other fields, new challenges keep arising, as THz-TDS is performed to study increasingly difficult samples, such as water with large THz absorption or at long distances, such as in imaging applications and/or sensing. These and other applications would benefit from greater signal-to-noise ratios and shorter measurement times, enabled by high repetition rates, while maintaining sufficient THz pulse energy -i.e. from sources with high average power. Experiments requiring high average powers are thus typically exclusively performed in accelerator facilities, at the expense of extremely high cost, very restrictive accessibility and difficulties in achieving phase stability required for TDS [2]. Meanwhile, labbased few-cycle THz sources for TDS have immensely progressed in terms of pulse energy [3,4], bandwidth [5] and tunability [6] in recent years, but this progress is typically made at the expense of the repetition rate of the source (<1 kHz), therefore average power levels remain very low, typically in the few to hundreds of µW range. This is mostly due to the limited average power of commonly used energetic Ti:Sapphire amplifiers as drivers, which are typically limited to <10 W of average power.One promising yet widely unexplored path to increase the average power of current THz sources is therefore to adopt novel ultrafast solid-state lasers based on Yb-doped gain media as driving sources, which nowadays reach kilowatt average powers [7][8][9], possibly in combination with efficient THz generation schemes. Among these novel ultrafast technologies, we focus our attention on modelocked thin-disk lasers (TDLs), which are capable of providing femtosecond pulses with tens of microjoules of pu...

High-Power Lensless THz Imaging of Hidden Objects

“…It is operating in the soliton modelocking regime [28], [29], using a semiconductor saturable absorber mirror (SESAM) [31] for starting and stabilization of the soliton-shaped pulses. The laser system as used in this specific experiment has the same layout as the laser in [20]. It delivers up to 100 W of average power at 13.4-MHz repetition rate, corresponding to 7.5-µJ pulse energy.…”

“…Further, nonpolar and nonmetallic materials usually have high transmission loss, making imaging through typical obstacles, i.e., wood, glass, and stone, challenging. For this and other applications, many groups worldwide have recently started to explore the possibilities opened by novel high-average power ultrafast lasers with hundreds of watts of average power to increase the average power of THz-TDS, resulting in significant advances [20]- [22]; reaching nowadays a performance level which was traditionally only achieved with accelerator-based sources [23], see Fig. 1.…”

“…In this respect, one high-average power ultrafast laser technology stands out as particularly promising, namely modelocked thin-disk laser (TDL) oscillators, to drive THz generation [24]. These onebox modelocked oscillators operate at MHz repetition rates but outperform other oscillator technologies by several orders of magnitude in terms of average power [20], [25], providing the same output power levels as complex multi-chain amplifiers with additional advantages such as potentially lownoise levels close to the quantum limit of high-Q resonator [26] and transform-limited femtosecond pulses. The state-of-the-art of these oscillators is 350 W average power with pulse duration of 940 fs, and a repetition rate of 8.88 MHz [27], and 80-µJ pulse energy at 242 W of average power [28].…”

“…The state-of-the-art of these oscillators is 350 W average power with pulse duration of 940 fs, and a repetition rate of 8.88 MHz [27], and 80-µJ pulse energy at 242 W of average power [28]. Based on this technology, we recently achieved a THz-TDS with 66 mW of average power, based on optical rectification in the tilted pulse front regime in Lithium Niobate (LN), which is the highest average power reported so far from a single-cycle THz source at multi-MHz repetition rates [20]. However, so far, the potential of this and other unique highaverage power THz-TDS sources for high-dynamic range applications had not been conclusively demonstrated.…”

Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths

“…OR typically allows us to generate intense THz radiation below 5 THz (sketched in light green and light red in Figure 1c), while DFG covers higher frequencies (dark green and dark red in Figure 1c). Both inorganic (LiNbO 3 [66][67][68][69][70][71][72][73][74][75][76][77], GaSe [78][79][80], ZnTe [81,82], LiGaS 2 [83], GaP [84][85][86][87][88][89]) as well as organic (DSTMS [89][90][91][92], OH1 [93][94][95][96][97], DAST [98,99], DPFO [100], HMQ-TMS [101], OHQ-N2S [102]) crystals are phase-matched in the NIR and can emit pulsed THz fields with peak amplitudes in excess of 1 MV/cm. The approximate spectrum of the strongest THz pulses generated to date by non-linear optical methods in inorganic [62,64] and organic [63,65] crystals are shown in green and in red in Figure 1c, respectively.…”