We demonstrate record ∼0.9% efficiencies for optical conversion to narrowband (<1% relative bandwidth) terahertz (THz) radiation by strongly cascaded difference frequency generation. These results are achieved using a novel, to the best of our knowledge, laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (≥250 ps). THz radiation generation in 5% MgO-doped periodically poled lithium niobate (PPLN) crystals of varying poling period was explored at cryogenic and room temperature operation as well as with different crystal lengths. This work addresses an increasing demand for high-field THz radiation pulses which has, up to now, been largely limited by low optical-to-THz radiation conversion efficiencies.
We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to
900
m
J
/
c
m
2
, showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.
We generate temporally modulated optical pulses with a beat frequency of 255 GHz, a duration of 360 ps, and a repetition rate of 2 MHz. The temporal envelope, beat frequency, and repetition rate are computer-programmable. A frequency comb serves as a phase and frequency reference for the locking of two laser lines. The system enables beat frequencies that are adjustable in steps of the frequency comb's repetition rate and exhibit Hzlevel precision and accuracy. We expect the optical beat pulses to be well suited for versatile multi-cycle terahertz-wave generation with controllable carrier-envelope phase. We demonstrate that the inherent synchronization of the frequency comb's ultra-short pulse train and the synthesized optical beat (or later the multi-cycle terahertz) pulses enables rapid and phase-sensitive sampling of such pulses.
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