Terahertz Sources Based On Intracavity Parametric Frequency Down-Conversion Using Quasi-Phase-Matched Gallium Arsenide


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.


June, 2009