High-Power Broadband THz Source in Organic Crystal BNA at MHz Repetition Rates

Abstract

Optical rectification in organic crystals have proven to be an excellent material to achieve ultra-broad bandwidth THz radiation with high optical-to-THz conversion efficiency up to the percent level in simple collinear setups. However, so far, most results using organic crystals were restricted to low repetition rates, because of the generally accepted idea that these crystals have poor thermal properties. Only very recently the organic crystal HMQ-TMS was explored at MHz repetition rate showing mW-level THz average power [1] , [2] . Another promising crystal is BNA (N-benzyl2-methyl-4-nitroaniline), which has recently been used with a conversion efficiency of 0.8 % albeit at moderate repetition rate of 100 kHz [3] . Here, we demonstrate mW-class, high dynamic range, broadband THz generation in BNA crystal (Swiss Terahertz GmbH), driven by an ultrafast laser with MHz repetition rate. We obtain 490 µW of THz average power with a conversion efficiency of 5×10 -4 . To the best of our knowledge, this is the first time this material is investigated at high (MHz) repetition rate. To reach this efficiency, we use bursts of MHz-repetition rate driving pulses with variable duty cycle which can decouple the average power scaling from the pulse energy and allows us to disentangle the influence of average power, thermal effects, and peak intensity.