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High-repetition-rate accumulation effects in air-plasma THz sources
Robin Löscher1, Malte C. Schroeder1, Tim Vogel1, Alan Omar1, Claudius Hoberg2, Martina Havenith2, and Clara J. Saraceno1
1Photonics and Ultrafast Laser Science, Ruhr-Universität Bochum, 44801 Bochum, Germany
2Department of Physical Chemistry II, Ruhr-Universität Bochum, 44780 Bochum, Germany
Air-based two-color filament plasmas are popular sources of ultra-broadband THz radiation for a wide range of applications, including linear and non-linear spectroscopy, plasma diagnostics and material identification [1]. This method of THz generation relies on laser-induced plasma channels, yielding low-frequency contributions from the plasma and high-frequency contributions by four-wave mixing [2]. One major drawback of these sources is the inherent requirement for high driving peak power, required to reach a large ionization fraction. As such, THz-air-plasma sources realized so far have mostly been based on mJ-pulse-energy Ti:Sapphire regenerative amplifier systems, typically running at a repetition rate of 1 kHz or less. The increasing availability of short-pulse high-average-power Ytterbium-based laser systems opens the door for air-photonics THz-source development at high repetition rates of 10s to 100s of kHz [3], and beyond towards the MHz range. These developments are promising to significantly reduce the typically long measurement times, which are particularly severe with ultra-broad bandwidths due to the low sensitivity of suitable broadband detection methods. However, the effect of higher repetition rates on the THz conversion efficiency is not well understood. In [4], experiments showed that the THz-pulse-energy decreased by 50 % while increasing the laser repetition rate from 6 Hz to 6 kHz. Recently, we demonstrated that cumulative hydrodynamic effects occur at laser repetition rates exceeding 10 kHz, affecting the air dynamics in high-average-power laser-induced filament plasmas [5], showing an increasingly pronounced stationary gas-density depletion at repetition rates of 40 kHz and 100 kHz, which is expected to significantly affect THz generation.
Figure 1: (a) Experimental setup showing the temporal multi-pass cell pulse compression, the hydrodynamics characterization based on folded-wavefront interferometry, and the air-photonics THz generation with electro-optic sampling (EOS). FS: fused silica, GDD: group-delay dispersion, CAM: CMOS camera, BBO: β-barium borate, TFP: thin-film polarizer, Ge: germanium window, GaP: gallium phosphide (500 μm). Raw image of the density hole at (b) Δτ=0 μs, and (c) Δτ=2 μs, frep=100 kHz. The images show the typical bending of the interference fringes and the laser-induced air-pressure wave circularly propagating from the plasma channel. (d) 30-waveform averaged electric field of the THz-pulses measured via EOS, exemplarily at a laser repetition rate of 100 kHz.
In this work, we present the first steps in our detailed study of time-resolved gas-density depletion from a two-color plasma and investigate its influence on the generated THz-average- power, and spectrum. We believe this fundamental work will have a significant impact on the THz-spectroscopy community, enabling average power scaling of broadband THz-waveforms and ultimately allowing accelerating measurement times by several orders of magnitude.
The experimental setup is shown in Fig. 1a. We use a commercial 300-μJ, 220-fs, 1036-nm laser at repetition rates up to 100 kHz, temporally compressed by a home-built multi-pass cell compressor to reach a peak power of approximately 7 GW. In this way, we exceed the critical peak power for self-focusing in air (P\sub>cr~ 6 GW), allowing for a short filament plasma to form. We performed two experiments at four repetition rates: THz generation and characterization, and time-resolved hydrodynamics. In both experiments, we generated a two-color plasma channel by focusing the 3-mm diameter pump beam with a 100-mm focal length lens through a 100-μm thick β-barium borate (BBO) crystal for second-harmonic generation.
For the THz generation, an output coupler with 1% transmission splits the beam into pump and probe arms, and a 4-f imaging of the THz radiation is performed via gold-coated off-axis parabolic mirrors (OAP). A Ge window is used for the filtering of the residual 1036-nm and 518-nm pump radiation. The THz radiation is focused into a 500-μm thick GaP crystal for EOS of the THz waveforms. For characterization of the hydrodynamics, we used a ns-pulsed 450-nm diode laser for probing, which is triggered by a digital delay generator referenced to the pump laser. The probe beam is collinearly counter-propagating through the plasma channel and experiencing a phase shift, corresponding to a local change of the air refractive index. By f-2f imaging of the probe through a folded-wavefront interferometer, the interference-fringe-encoded phase-shift can be recorded by a camera [5,6], exemplarily shown in Figures 1b, and 1c.
In Fig. 1d, we present the measured electric field and spectrum for a laser repetition rate of 100 kHz. We reached a maximum dynamic range of 40 dB and a bandwidth of up to 6 THz by averaging over 30 electric-field waveforms. The influence of water vapor absorption is visible in both the electric field and the spectrum, as the measurements were performed in ambient air. The total bandwidth detected by our setup was mainly limited by the 500-μm thick GaP crystal in the EOS and absorption of the Ge wafer. Based on our probe-delay scan frequency of 0.5 Hz, the results in Fig. 2 were achieved within a timeframe of 30 s.
For all repetition rates, the time-resolved characterization of the hydrodynamics shows the typical radially localized and axially extended heating of the air for probe delays of a few μs [6]. At repetition rates above 10 kHz, a residual gas density depression can be seen at all probe delays before and after the arrival of the succeeding pulse, confirming previous results with continuous-wave probing [5]. However, this implies that each pump pulse, when considering high-repetition-rate filamentation, encounters a channel of depressed gas density. The measured power and conversion efficiency as a function of repetition rate at constant pulse energy are presented in Fig. 2a, and 2b. We measured an unexpected increase in conversion efficiency at increased laser repetition rate. We believe this is due to the decreased turbulences when a stationary density depletion is reached at higher repetition rates.
In the near future, the THz generation and detection schemes can be significantly improved to increase the THz average power and measurable bandwidth. For example, improved polarization management of the pump beam by separate control of the fundamental and second-harmonic wavelengths is expected to enhance the THz generation efficiency; furthermore, ABCD-type detection techniques will allow us to measure the full bandwidth of the generated transients [1], and thus fully characterize the source and repetition rate dependences. To the best of our knowledge, we demonstrate the first experiments for correlating local laser-induced hydrodynamics with THz-generation from two-color filament plasmas driven at high repetition rates up to 100 kHz. With improved polarization management of the pump and THz beam, higher THz average powers, and an increased conversion efficiency is expected. Broadband gas-based THz-detection schemes will be utilized to extend our investigations toward the high-frequency components above 10 THz. Our work forms the basis for high-repetition-rate air-photonics THz-spectroscopy with short measurement runtimes.
Figure 2: Measured THz average power (a) and optical-to-optical efficiency (b) for measurements at repetition rates of 1 kHz, 10 kHz, 50 kHz, and 100 kHz. The error bars indicate the standard deviation after 2 min measurement. (c) 30-waveform averaged spectrum of the THz-pulses measured via EOS, exemplarily at a laser repetition rate of 100 kHz.
To the best of our knowledge, we demonstrate the first experiments for correlating local laser-induced hydrodynamics with THz-generation from two-color filament plasmas driven at high repetition rates up to 100 kHz. With improved polarization management of the pump and THz beam, higher THz average powers, and an increased conversion efficiency is expected. Broadband gas-based THz-detection schemes will be utilized to extend our investigations toward the high-frequency components above 10 THz. Our work forms the basis for high-repetition-rate air-photonics THz-spectroscopy with short measurement runtimes.
References
[1] B. Clough, J. Dai, and X.-C. Zhang, Mater. Today 15, 50 (2012)
[2] V. A. Andreeva et al., Phys. Rev. Lett. 116, 063902 (2016)
[3] D. K. Kesim et al., in CLEO Europe 2021 (2021)
[4] A. D. Koulouklidis et al., Opt. Lett. 45, 6835 (2020)
[5] R. Löscher et al., APL Photonics 8, 111303 (2023)
[6] Y.-H. Cheng et al., Opt. Express 21, 4740 (2013)