  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 4e-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.
