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Abstract

Optical rectification of ultrashort near-IR laser pulses with tilted
pulse fronts and pulse energies of a few muJ in Mg-doped stoichiometric
LiNbO3 cooled to low temperature is a powerful technique for efficient
generation of THz pulses. The pulse energy critically depends on the Mg
doping (necessary for preventing photorefractive damage) and can be
easily increased by a factor of three if the MgO content is reduced.
Pulse energies up to 400 pJ at repetition rates of 200 kHz and 3.4%
quantum conversion efficiency are achieved at 77 K. At 10 K, changing
the tilt angle of the pump pulse front results in continuous tuning of
the frequency across the 1.0-4.4 THz range. The temporal pulse shapes
measured by electro-optic sampling are in good agreement with the
signal calculated by a simple theory. This model predicts tunability on
a considerably broader range and narrower spectra even at room
temperature if GaSe is used instead of LiNbO3. The advantages of the
velocity matching technique utilizing tilted pulse fronts are analyzed
in comparison with quasi-phase-matching in periodically poled LiNbO3
crystals. The first method provides a ten times higher pulse energy
conversion efficiency.