Far-infrared non-phasematched SHG pulse duration measurements in DAST

The free electron laser for infrared experiments (FELIX) emits tunable radiation between 5 and 100 ym. Unfortunately, it is very difficult to diagnose pulses in this wavelength region because conventional nonlinear optical materials have strong absorption bands that attenuate the laser pulses at the fundamental and/or second harmonic wavelength. Here, we report on the use of the organic salt dimethylamino-4-N-methylstilbazolium tosylate (DAST) to generate the second harmonics of radiation at a fundamental wavelength between 30 μm and 55 μm. We demonstrate that with this technique we can measure the duration of FELIX pulses in a second harmonic autocorrelation setup