Temperature-dependent THz properties and emission of organic crystal BNA.

As high-average power ultrafast lasers become increasingly available for nonlinear conversion, the temperature dependence of the material properties of nonlinear crystals becomes increasingly relevant. Here, we present temperature-dependent THz complex refractive index measurements of the organic crystal BNA over a wide range of temperatures from 300 K down to 80 K for THz frequencies up to 4 THz for the first time. Our measurements show that whereas the temperature-dependent refractive index has only minor deviation from room temperature values, the temperature-dependent absorption coefficient decreases at low temperature (-24% from 300 K to 80 K). We additionally compare these measurements with conversion efficiency and spectra observed during THz generation experiments using the same crystal actively cooled in the same temperature range, using an ultrafast Yb-laser for excitation. Surprisingly, the damage threshold of the material does not improve significantly upon active cooling, pointing to a nonlinear absorption mechanism being responsible for damage. However, we observe a significant increase in THz yield (+23%) at lower temperatures, which is most likely due to the reduced THz absorption. These first findings will be useful for future designs of high-average power pumped organic-crystal based THz-TDS systems.

Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry.

Metallic spintronic terahertz (THz) emitters have become well-established for offering ultra-broadband, gapless THz emission in a variety of excitation regimes, in combination with reliable fabrication and excellent scalability. However, so far, their potential for high-average-power excitation to reach strong THz fields at high repetition rates has not been thoroughly investigated. In this article, we explore the power scaling behavior of tri-layer spintronic emitters using an Yb-fiber excitation source, delivering an average power of 18.5 W (7 W incident on the emitter after chopping) at 400 kHz repetition rate, temporally compressed to a pulse duration of 27 fs. We confirm that a reflection geometry with back-side cooling is ideally suited for these emitters in the high-average-power excitation regime. In order to understand limiting mechanisms, we disentangle the effects on THz power generation by average power and pulse energy by varying the repetition rate of the laser. Our results show that the conversion efficiency is predominantly determined by the incident fluence in this high-average-power, high-repetition-rate excitation regime if the emitters are efficiently cooled. Using these findings, we optimize the conversion efficiency and reach highest excitation powers in the back-cooled reflection geometry. Our findings provide guidelines for scaling the power of THz radiation emitted by spintronic emitters to the milliwatt-level by using state-of-the-art femtosecond sources with multi-hundred-Watt average power to reach ultra-broadband, strong-field THz sources with high repetition rate.

Milliwatt average power, MHz-repetition rate, broadband THz generation in organic crystal BNA with diamond substrate.

We demonstrate a 13.3 MHz repetition rate, broadband THz source with milliwatt- average power, obtained by collinear optical rectification of a high-power Yb-doped thin-disk laser in the organic crystal BNA (N-benzyl-2-methyl-4-nitroaniline). Our source reaches a maximum THz average power of 0.95 mW with an optical-to-THz efficiency of 4×10-4 and a spectral bandwidth spanning up to 6 THz at -50 dB, driven by 2.4 W average power (after an optical chopper with duty cycle of 10%), 85 fs-pulses. This high average power excitation was possible without damaging the crystal by using a diamond-heatsinked crystal with significantly improved thermal properties. To the best of our knowledge, this result represents the highest THz average power reported so far using the commercially available organic crystal BNA, showing the potential of these crystals for high average power, high repetition rate femtosecond excitation. The combination of high power, high dynamic range, high repetition rate and broadband spectrum makes the demonstrated THz source highly attractive to improve various time-domain spectroscopy applications. Furthermore, we present a first exploration of the thermal behavior of BNA in this excitation regime, showing that thermal effects are the main limitation in average power scaling in these crystals.