There exist few sources of tunable electromagnetic radiation at frequencies between 0.1–10 THz. Of the sources based on nonlinear optical processes which do exist, the conversion eﬃciencies are on the order of 10e−6 for systems which ﬁt on an optical table. With the advent of quasi-phase-matched (QPM) nonlinear-optical frequency conversion processes, coherent sources of frequency-tunable radiation in the visible and infrared were developed. QPM can be extended to generating tunable THz-frequency radiation via parametric frequency down-conversion where energy is transferred from radiation in the near-infrared (λ ∼ 2 µm) to the far-infrared (λ ∼ 300 µm). GaAs is an excellent nonlinear-optical material for QPM THz generation due to its large nonlinear-optical coeﬃcient, small absorption coeﬃcient at THz frequencies (∼1 order of magnitude smaller than other nonlinear-optical materials), and small mismatch between the optical and THz refractive indices. Three types of micro-structured GaAs have been used to generate THz radiation by parametric frequency down-conversion: (i) orientation-patterned GaAs, OP-GaAs (ii) optically contacted GaAs wafers, OC-GaAs, and (iii) diﬀusion-bonded GaAs plates, DB-GaAs. THz frequencies between 0.5–3.5 THz were generated using the various GaAs samples.
THz average powers as large as 1 mW generated from a pump power of 8.5 W, corresponding to an optical-to-THz power conversion eﬃciency of 1.2 × 10e−4 were observed by placing the GaAs inside a doubly resonant synchronously pumped optical parametric oscillator. The quantum conversion eﬃciencies were as large as 1.2%. The parametric conversion eﬃciency for THz generation is inherently small since the ratio of the THz and optical frequencies is small. Diﬀerence-frequency generation (DFG) between the intracavity signal and idler waves generated the THz radiation. The doubly resonant optical parametric oscillator (DRO) resonated the signal and idler pulses, with picosecond-scale pulse widths and greater than 50 W of average power in each wave at λ ≈ 2 µm. The frequency splitting between the signal and idler waves was tuned by adjusting the temperature of the DRO gain material, periodically poled LiNbO3 (PPLN). The bandwidths of the resonant signal and idler waves were between 100–200 GHz since the OPO process used Type-II QPM where the signal and idler ﬁelds were orthogonally polarized. Designs for maximizing the THz power for both the singly and doubly resonant OPOs were described yielding expressions for the THz, signal, idler, and pump powers in terms of crystal length, optical beam size, and optical absorption coeﬃcient.
A THz-cascading process was observed during which the THz wave was ampliﬁed in the GaAs crystal by multiple pairs of infrared waves. Quantum-mechanically, THz cascading corresponds to the generation of multiple THz photons from a single infrared photon. For proper designs of the OPO-cavity losses and compensation of the dispersion of the intracavity PPLN and GaAs crystals, quantum conversion eﬃciencies far greater than 100% can be achieved.
An electronic feedback system was developed to stabilize the intracavity power of the DRO as well as the generated THz power. Locked operation lasted as long as 30 minutes limited only by the thermal expansion of the optical table and the ﬁnite expansion of the PZT element. A passive thermo-optic feedback eﬀect also stabilized the DRO power, where absorbed optical power in the GaAs deposited heat leading to a rise in the refractive index of the GaAs. A characterization of this thermo-optic eﬀect in terms of a negative feedback system has been described.
Independently varying the signal and idler cavity lengths in the DRO led to the discovery of certain cavity-length regimes where oscillation may not occur as well as cavity-length regimes where the temporal overlap of the signal and idler pulses is maximized. A numerical simulation was developed modeling the temporal features of the DRO. The results of the numerical simulations agreed well with experimental measurements. The temporal overlap of the pulses was calculated for several values of parametric gain and DRO round-trip loss, and operating regimes where the pulses were symmetric and the temporal overlap was nearly maximized were identiﬁed. An approach to re-time the pulses using a pair of intracavity birefringent crystals such that the temporal overlap is maximized is described.
Fluctuations of the intracavity power of the synchronously pumped optical parametric oscillator were measured. Over certain cavity-length detunings, the ﬂuctuations were aperiodic with microsecond-scale transients. At longer cavity-length detunings, the ﬂuctuations were periodic (and nearly sinusoidal) with fundamental frequencies between 200–700 kHz. The numerical simulations reproduced the ﬂuctuations and showed that the minimum set of physical eﬀects necessary to produce the ﬂuctuations are three-wave mixing, group-velocity mismatch, and self-phase-modulation of the resonant wave in the case of a singly resonant oscillator. Operating regimes that evade the appearance of these oscillations were identiﬁed.