We have demonstrated a high-average-power, high-repetition-rate optical terahertz (THz) source based on difference frequency generation (DFG) in the GaSe crystal by using a near-degenerate 2 μm intracavity KTP optical parametric oscillator as the pump source. The power of the 2 μm dual-wavelength laser was up to 12.33 W with continuous tuning ranges of 1988.0–2196.2 nm/2278.4–2065.6 nm for two waves. Different GaSe cystal lengths have been experimentally investigated for the DFG THz source in order to optimize the THz output power, which was in good agreement with the theoretical analysis. Based on an 8 mm long GaSe crystal, the THz wave was continuously tuned from 0.21 to 3 THz. The maximum THz average power of 1.66 μW was obtained at repetition rate of 10 kHz under 1.48 THz. The single pulse energy amounted to 166 pJ and the conversion efficiency from 2 μm laser to THz output was $1.68\times {10}^{-6}$. The signal-to-noise ratio of the detected THz voltage was 23 dB. The acceptance angle of DFG in the GaSe crystal was measured to be 0.16°.

Terahertz (THz) sources, ranging from 0.1 THz to 10 THz, between microwaves and infrared waves, have important applications in spectral detection, medical imaging, communication, etc. Difference frequency generation (DFG) is an effective method for generating terahertz with the characteristics of low cost, simple structure, widely tunable range, no threshold, and room-temperature operation. This paper reviews various optical terahertz sources of difference frequency generation based on nonlinear crystals, including DFG with inorganic crystals, DFG with organic crystals, DFG with quasi-phase-matching (QPM) crystals, DFG in waveguides, cavity-enhanced DFG, and cascaded DFG. Their recent advances, as well as their advantages and disadvantages, are fully present and discussed. This review is expected to provide a comprehensive reference for researchers in this field and a quick understanding of optical THz sources of difference frequency generation with nonlinear crystals.

We demonstrate an optical parametric oscillator (OPO) based on random phase matching in polycrystalline ZnSe. The OPO was pumped by Cr:ZnS laser pulses (2.35 μm, 62 fs, 79 MHz), had a pump threshold of 90 mW, and produced an ultrabroadband spectrum spanning 3–7.5 μm.

A high-energy, tunable, long-wave mid-infrared optical parametric oscillator (OPO) based on the B a G a 4 S e 7 crystal was demonstrated in this Letter with 1064 nm laser pumping. The mid-infrared OPO was designed as a double-pass single resonant oscillator (DP-SRO) to reduce the threshold and improve the outputs. Further optimization on the cavity length was theoretically and experimentally studied. With a short cavity length of 30 mm, the output energy of over 1 mJ/pulse at 11 µm was obtained with the pump energy of 39.5 mJ/pulse. In addition, a wide tuning range of 8–14 µm was experimentally achieved by rotating the B a G a 4 S e 7 crystal.

We demonstrate a dual-wavelength laser based on a new laser material—Nd, Gd:YSGG, or Nd:GYSGG for short—for the first time to our knowledge. Besides its attractive properties such as antiradiation, high segregation coefficient, etc., this kind of laser crystal also shows excellent laser performance. For continuous-wave operation, the maximum output power is $10.1\mathrm{W}$ with the absorbed power of $18.45\mathrm{W}$ at $808\mathrm{nm}$, corresponding to the slope efficiency of nearly 60%. The maximum single pulse energy and peak power reach $277\mathrm{\mu J}$ and $4.6\mathrm{kW}$ ($60\mathrm{ns}$) when the absorbed pump power is $11.4\mathrm{W}$ for acousto-optic Q-switched operation.

Research on an efficient continuous-wave Nd:GYSGG laser at 1053 nm with excellent stability is demonstrated. The maximum output power is 4.17 W, corresponding to the conversion efficiency of 33.9% and the slope efficiency of 42.92%. Using a Cr:YAG absorber, pulsed dual-wavelength operation at 1053 and 1058.4 nm is obtained, of which the maximum single pulse energy and peak power are 172.1 μJ and 26.1 kW, respectively, when the pulse width is 6.6 ns and the repetition rate is 4.3 kHz. A polarization property is observed, owing to the anisotropy of Cr:YAG. This stably Q-switched dual-wavelength laser is a good pump source for the generation of a terahertz wave at 1.53 THz.