Intracavity Terahertz Generation in a Synchronously
Pumped Optical Parametric Oscillator using Quasi-
Joseph E. Schaar, Konstantin L. Vodopyanov, and Martin M. Fejer
E. L. Ginzton Laboratory, 450 Via Palou, Stanford University, CA 94305
Abstract: We generated 1 mW of average power at 2.9 THz (540 GHz bandwidth) in a nearly-
diffraction-limited beam by placing a room-temperature quasi-phasematched GaAs crystal inside
the cavity of a synchronously pumped optical parametric oscillator.
2006 Optical Society of America
OCIS codes: (190.2620) Frequency conversion; (190.4970) Parametric oscillators and amplifiers
Parametric frequency down-conversion of optical pulses is a known method of generating terahertz (THz) radiation.
This technique can generate either narrow bandwidth THz radiation using difference frequency generation (DFG)
 or broadband THz transients using optical rectification (OR) . Using quasi-phasematched (QPM) electro-
optic (EO) crystals such as PPLN has potential for improving optical-to-THz conversion efficiencies for both DFG
 and OR . GaAs is attractive for THz generation because of its small THz absorption coefficient (<5 cm-1 for
0-3 THz) and its large coherence length due to a small mismatch between the optical group and THz phase
velocities. Additionally, techniques to fabricate GaAs with periodically rotated orientation were developed in the
last decade, namely diffusion-bonded GaAs (DB-GaAs)  and orientation-patterned GaAs (OP-GaAs) . The
difference frequency mixing of two picosecond (ps) pulses, where the pulsewidths are larger than the THz period,
can achieve the same conversion efficiency as OR of fs pulses while reducing the effects of higher order
nonlinearities such as multi-photon absorption and nonlinear refractive index . For these reasons, we generated
THz radiation using difference frequency mixing.
2. Setup and Results
We generated THz radiation by mixing the orthogonally polarized ps signal and idler pulses inside a QPM-GaAs
crystal placed in a doubly resonant synchronously pumped optical parametric oscillator (DRO) (Fig. 1). The pump
was a Nd:YVO4 modelocked laser with a 50-MHz pulse repetition rate, 7-ps pulsewidth, 1064-nm wavelength, and
10-W average power. The linear OPO cavity was ~3 m in length with a round-trip time matching the repetition rate
of the pump pulses. The gain medium for the OPO’s signal and idler waves was a 5-mm-long MgO:PPLN crystal
with a type-II (o-oe) QPM period of 14.1 µm. The THz radiation was generated in a 6-mm-long DB-GaAs sample
with a QPM period of 504 µm.
Fig. 1. DRO with “offset cavity” design. M1-M8 were cavity mirrors and M9 was a parabolic mirror for THz out-
The pump waist in the center of the PPLN crystal was 30 µm. Mirrors M1-M8 were AR-coated OPO cavity Download full-text
mirrors, and M9 was a gold-coated 90°-off-axis parabolic mirror for THz outcoupling. Mirrors M1 and M2 were
end mirrors for the p-polarized wave, and mirrors M3 and M4 were end mirrors for the s-polarized wave. M9
outcoupled >90% of the THz wave and fully transmitted the optical waves. The thin-film plate polarizers, TFP1 and
TFP2, separated and directed the orthogonally polarized waves to their respective end mirrors. Back-conversion in
the DRO was avoided by separating the signal and idler pulses in time on the return trip using TFP1, M1, and M3,
and then overlapping the pulses again in the next forward pass through the PPLN using TFP2, M2, and M4. The
wavelengths of the s- and p-polarized optical waves (with respect to the polarizers) were tuned by the PPLN crystal
temperature. The best THz performance occurred with signal and idler wavelengths of 2107 nm and 2150 nm (T =
82.7 °C), respectively, corresponding to a generated wave at the difference frequency of 2.9 THz inside the room-
temperature DB-GaAs crystal. The FWHM signal and idler bandwidths at this temperature were ~390 GHz (Fig.
2a), and the bandwidths varied with temperature (smaller bandwidths approaching degeneracy).
With a pump average power of 8.5 W, the resonated signal and idler average powers were 10.2 W and 17 W,
respectively. The OPO threshold was 2.4 W, and the maximum pump depletion was 67%. After focusing by a
Picarin lens, the waist of the THz beam was ∼200 µm measured by a room-temperature pyroelectric camera (Fig.
2b). The focused THz beam, after passing through a set of polyethylene filters for the visible and mid-infrared, was
measured by a room-temperature DLaTGS detector. The THz average power at 2.9 THz was 1 mW (after M9).
This was an OPO optical-to-THz conversion efficiency of 1.2×10-4 and a quantum efficiency of 1.2%. The
calculated THz beam bandwidth was ∼540 GHz (convolution of signal and idler spectra).
Fig. 2. (a) Orthogonal OPO signal and idler waves (p-pol and s-pol, respectively), at a PPLN temperature of 83 °C, with
FWHM bandwidths ~6 nm (389 GHz). (b) Focused THz beam with 1/e2 radius waist of ∼200 µm measured by a room-
temperature pyroelectric camera (Spiricon Pyrocam-III). One pixel = 100 µm × 100 µm.
We demonstrated a novel room-temperature THz source with ~1 mW average power at 2.9 THz, based on
intracavity difference frequency mixing, between the two orthogonally-polarized resonating optical waves near
2 µm (signal and idler), in a quasi-phasematched GaAs crystal. Using samples with variable QPM periods, the
whole range of 0.5—4 THz can be accessed. Achieving higher average power was primarily limited in our
experiment by the poor quality of the DB-GaAs sample and lossy polarizers. With reduced intracavity loss, scaling
of the THz average power to 10—100 mW is possible.
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