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

TL;DR

This paper demonstrates the use of the organic crystal PNPA for broadband THz wave generation and detection at MHz repetition rates, enabling portable spectroscopy with high dynamic range and compatibility with telecommunication lasers.

Contribution

It introduces the first MHz-rate operation of PNPA, a recently discovered organic NLO crystal, for broadband THz generation and detection using low-energy pulses.

Findings

Achieved THz generation and detection with 40 dB dynamic range over 3.6 seconds.

Extended organic crystal applications to telecommunication wavelengths beyond 5 THz.

Demonstrated compatibility with a 50 MHz, nanojoule pulse laser system.

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.