  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.
