Broadband THz wave generation and detection in organic crystal PNPA at MHz repetition rates

Abstract: Organic nonlinear optical (NLO) crystals have emerged as efficient room-temperature emitters of broadband THz radiation with high electric field strengths. Although initially confined to high-pulse-energy excitation in the millijoule range with kHz rates, recent efforts have focused on tailoring the physical properties of organic NLO materials for compatibility with popular MHz-rate telecommunication wavelength lasers emitting nanojoule energy pulses. This is motivated by the large potential of such crystals for more portable and user-safe spectroscopic systems, additionally complemented by their room-temperature field-sensitive THz detection capabilities. In this work, we demonstrate the MHz-rate operation of the organic crystal PNPA ((E)-4-((4-nitrobenzylidene)amino)-$N$-phenylaniline), which was recently discovered through data mining algorithms and reported to surpass the conversion efficiency of rivaling NLO crystals at 1 kHz repetition rate. Using a compact 200 mW Er:fiber laser producing 18 fs pulses at 50 MHz repetition rate, we demonstrate the THz field generation and detection capabilities of PNPA via performing gas phase spectroscopy in the 1.3-8.5 THz range. We obtain a dynamic range of 40 dB over 3.6 s and 70 dB over 2 hours. Our work extends the family of organic crystals compatible with telecommunication-wavelength excitation using nJ pulses for spectroscopy beyond 5 THz.