Abstract
We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near λ = 2 μm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2 μm was attained with GaAs inside cavity. This allows dramatic improvement in terms of generated THz power, as compared to the state-of-the art CW methods. We achieved >25 μW of single-frequency tunable CW THz output power scalable to >1 mW with proper choice of pump laser wavelength.
Highlights
Experimental setupA DR-optical parametric oscillator (OPO) (figure 1) is pumped by a single-frequency (linewidth
16 kHz at 230 GHz, 25 kHz at 461 GHz and 32 kHz at 691 GHz
Using χ (2) process, continuous wave (CW) narrow-bandwidth THz output with spectral width ∼2 MHz was produced via difference frequency generation in a LiNbO3 waveguide using two 760 mW CW lasers operating near λ = 1.5 μm [8]
Summary
A DR-OPO (figure 1) is pumped by a single-frequency (linewidth 99%) at the pump wavelength and highly reflecting (>99.9%) around 2060 nm, the OPO degeneracy wavelength, except for the outcoupler mirror transmitting 0.9 ± 0.1% for intracavity power monitoring. A 2.5 cm long periodically poled lithium niobate (PPLN) crystal, serving as the OPO gain element, is placed in one focal point of the resonator (beam waist, 1/e2 intensity radius, w = 77 μm). With the 13.5 μm domain reversal period of PPLN, we achieve type II quasi-phase matching with signal and idler waves close to degeneracy with crossed polarizations. These two waves are mixed in the OP-GaAs crystal placed in the second focal point of the cavity (beam waist w = 90 μm) to produce THz output.
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