Terahertz wave up-conversion detection based on organic nonlinear optical crystals

Abstract

Laser pumped terahertz (THz) wave up-conversion detection with high sensitivity, fast responsivity and wide frequency band is achieved at room temperature, based on home-made organic nonlinear crystals 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST). Green laser pulses pumped KTiOPO₄ optical parametric oscillators are utilized as the sources of dual-wavelength near-infrared (NIR) beams (1.3–1.6 μm, for THz-wave difference frequency generation (DFG)) and a single NIR beam (1.2–1.4 μm, for up-conversion detection). The nonlinear medium for both THz-DFG and detection is DAST (grown by CETC-46). A nanosecond-time-resolved THz pulse is obtained with an InGaAs p-i-n photo-diode. The spectrum of the up-converted NIR light is acquired, which allows us to measure the THz frequency indirectly. The sensitivity (also at room temperature) is 4 orders better at 19 THz than the sensitivity of a commercial thermal detector (Golay Cell). The wide frequency band operation is realized with different sets of band-pass filters, which cover the entire range from 3.15 to 29.82 THz except 8.4 THz of the strong absorption peak of DAST. The dynamic range of a THz source based on DFG can be commonly improved by 2–3 orders, by changing the traditional thermal detector with the up-conversion detection. The presented technology can promote the applications of DFG THz source in the fields of high-resolution spectroscopy and imaging.