Optical rectification of intense near-infrared pulses at 100 kHz repetition rate in the organic crystal MNA

S. Mansourzadeh1, A. Omar1, T. Vogel1, M. F. Nielson2, D. J. Michaelis2, M. Havenith3, J. A. Johnson2, C. J. Saraceno1

1Photonics and Ultrafast Laser Science (PULS), Ruhr-University Bochum, Germany
2Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT, USA
3Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, Germany

We report on high power and broadband THz generation using the organic crystal MNA (2-amino-5-nitrotoluene) by optical rectification of intense, 1036 nm pulses from a commercial Yb-based amplifier. The THz source reaches high average power of 9.2 mW at a repetition rate of 100 kHz and an ultra-broad bandwidth of >10 THz. We explore in detail the interaction of the pulses and the crystal to explain the observed trends in the generated THz pulses.

Optical rectification (OR) in organic crystals is now a commonly used technique to generate high power, broadband THz radiation with a high optical-to-THz conversion efficiency. Compared to inorganic materials such as gallium phosphide or lithium niobate, organic crystals exhibit collinear phase matching over a much broader THz bandwidth [1]. The organic crystal MNA has been previously identified as an excellent candidate for nonlinear applications, however, until recently could not be produced in significantly large sizes for power and energy scaling [2]. In this context, 1030 nm lasers have recently shown great potential for generation of high power and broad bandwidth THz radiation in other organic crystals (for example BNA and HMQ-TMS) at high repetition rates [3], however MNA has remained widely unexplored at this wavelength.
Here, we demonstrate a high average power and high bandwidth THz source based on OR in MNA, driven by an Yb-based laser. Our THz source reaches 9.2 mW of THz average power with a broad bandwidth >10 THz and a corresponding conversion efficiency of 0.27%. To the best of our knowledge, this is the highest average power achieved so far using any organic crystal, and shows remarkable broad bandwidth for the available average power. Additionally, we present an exploration of the nonlinear effects resulting from the interaction of the intense 1030 nm pulses in the crystal showing that a strong blue-shift and spectral broadening. This exploration and detailed understanding on the nonlinear THz generation process will be critical in taking these results to even higher average powers and high bandwidth in the near future.

Figure 1: Full experimental setup of the pump laser, MPC and THz-TDS. TFP: thin film polarizer.

The experimental setup is shown in Figure 1. The laser system is an industrial, Yb-based amplifier system providing up to 400 μJ pulse energy, but 70 μJ only is used in the current experiment. A home-built, Herriott-type multi-pass cell (MPC) compressor is used to compress the pulse duration down to 50 fs. After the MPC, the laser beam splits in two parts: 99% is used to generate THz radiation. The MNA crystal available at the time of the experiment has a thickness of 1 mm and is directly fused on a sapphire substrate for improved thermal dissipation. The crystal is mounted in such a way that the pump passes through the sapphire before reaching the MNA crystal. The collimated pump beam has a 1/e2 diameter of 2.2 mm at the position of the MNA. The beam diameter is optimized to get the highest THz power for the available pump pulse energy. The generated THz radiation is collimated and refocused using two off-axis parabolic mirrors (OAP). In order to reduce the thermal load on MNA, an optical chopper is placed before the crystal. The remaining 1% of the total laser beam is used to probe the THz radiation in an electro-optic sampling (EOS) setup.

Figure 2a) shows the THz power versus pump power on the crystal measured by a calibrated THz power meter placed in the focus of the second OAP. The crystal is pumped without any irreversible damage up to 3.4 W of laser average power after chopper with chopping frequency of 18 Hz, which results in a maximum THz average power of 9.2 mW. The corresponding conversion efficiency is shown in the right axis which has the value of 0.27% at the maximum THz power of 9.2 mW.
In order to detect the THz electric field using the EOS setup, the THz power meter is replaced with a 0.65 mm thick MNA crystal. Figure 2b) shows the THz trace in time domain, which is averaged over 78 traces and recorded in 78 s. The corresponding power spectrum, on the logarithmic scale, is obtained by Fourier transformation from the measured THz trace and is shown in Figure 2c). The spectrum has a wide bandwidth which spans more than 10 THz with a dynamic range of 60 dB. The dips present in the spectrum are due to phase matching due to the rather long crystal available at the time of the experiments. In future experiments, thinner crystals will be tested to reach a smoother spectral response.

Figure 2: THz characterization: a) THz average power. b) THz trace in time domain c) Corresponding THz spectrum.

In order to understand the limitations observed, the pump laser spectrum is measured before and after the MNA generation crystal. In Figure 3a), the shaded area shows the spectrum of the pump laser before propagation through the crystal. The spectra after the crystal propagation are shown in different colors for different pump pulse energy. An energy-dependent blue-shift is revealed in the spectra after the crystal propagation. This could be explained by sum frequency generation (SFG) of the THz photons and pump photons. The SFG process could have a detrimental effect on the optical-to-THz conversion efficiency, since some THz photons are consumed for the blue-shift [4]. However, further investigation is needed to understand the nonlinear effects at play, which we are currently carrying out together with numerical simulations.
Furthermore, we use the obtained spectra to calculate the transmission of the pump beam through the crystal at different pump energies shown in Figure 3b). The transmission reduces by 18% when the pump pulse energy increases from 12 μJ to 64 μJ, indicating a strong influence of nonlinear absorption, which is known to limit the conversion efficiency. In future experiments, the influence of temperature on linear and nonlinear absorption will be studied in more detail to disentangle the relevant effects.

Figure 3: Pump laser characterization. a) spectrum before and after crystal for different pulse energies. The spectra before the crystal did not change, only the grey area is shown as a representative spectrum. b) Dividing the spectra before and after crystal results in a monotonic decreasing transmission of MNA vs. pump pulse energy.

In conclusion, the demonstrated THz source has a high average power of 9.2 mW and a broad bandwidth spanning >10 THz at a high repetition rate of 100 kHz. To gain further insight into the THz generation process, detailed of the interaction of the laser pulses with the crystal are presented. In future studies, we will compliment this investigation with temperature-resolved studies and numerical simulations. Furthermore, we will explore the influence of the crystal thickness on the generated bandwidth and efficiency. In the long term, we believe even higher bandwidth and average power should be feasible in optimal conditions.

[1] M. Jazbinsek, U. Puc, A. Abina, and A. Zidansek, “Organic Crystals for THz Photonics,” Applied Sciences, 9(5) 5 (2019), doi: 10.3390/app9050882
[2] B. W. H. Palmer et al., “Large Crystal Growth and THz Generation Properties of 2-Amino-5-Nitrotoluene (MNA),” ACS Appl. Electron. Mater. 4(9) 4316–4321 (2022), doi: 10.1021/acsaelm.2c00592
[3] S. Mansourzadeh et al., “Towards intense ultra-broadband high repetition rate terahertz sources based on organic crystals [Invited],” Opt. Mater. Express 13(11) 3287 (2023), doi: 10.1364/OME.502209
[4] K. Ravi et al., “Cascaded parametric amplification for highly efficient terahertz generation,” Opt. Lett. 41(16) 3806–3809 (2016), doi: 10.1364/OL.41.003806

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