Photonic THz wave generation

Optical terahertz wave generation using periodic GaAs structures

Photonic generation of THz waves via frequency down conversion in electro-optic crystals is an attractive (thanks to the progress in developing compact near-IR optical sources) but inefficient (because of the Ω-cubed scaling factor, where Ω is THz frequency) process. We develop photonic THz sources for imaging and spectroscopic application with much higher than state-of-the-art optical-to-THz conversion efficiency. These sources are based on (i) optical rectification of femtosecond pulses in a new material: periodically-inverted GaAs, where we produce monochromatic THz output tunable in the 0.5 – 4.5 THz range [1-3] and (ii) resonantly-enhanced difference frequency mixing in periodically-inverted GaAs with picosecond [4,5] and continuous-wave [6] pumping. Here we take advantage of the enhancement of the optical field inside the cavity of a doubly-resonant optical parametric oscillator (OPO) (Fig. 2). We observed red-shifted satellites in the optical spectrum, suggesting that overcoming of Manley-Rowe conversion limit is possible due to cascaded THz generation (similar to cascaded Raman effect). This approach allowed generating of 1 mW of average THz power, potentially scalable to 10 – 100 mW.

Fig.1.  Periodically-inverted GaAs crystal for THz generation. Vertical dimension ~ 0.5 mm.

Fig.2. Quasi-monochromatic THz generation via optical rectification of femtosecond pump pulses in periodically-inverted crystal. Lower trace – E-field of the optical pulse. Upper trace – E-field of the THz pulse.

Fig. 3.  Conceptual scheme for resonantly-enhanced THz-wave generation. Near-degenerate type-II doubly-resonant PPLN optical parametric oscillator produces two closely-spaced in frequency domain and orthogonally-polarized ‘signal’ and ‘idler’ waves near λ=2 µm. THz output is produced at the beat frequency between these two waves, via frequency mixing in periodically-inverted quasi-phase-matched GaAs.

Fig.4. Schematic of a THz imaging setup using up-conversion to near-IR in GaP crystal.

Fig.5. Real-time THz imaging via up-conversion.

[1] K. L.Vodopyanov, M. M. Fejer, X. Yu, J. S. Harris, Y.-S. Lee, W. C. Hurlbut, V. G. Kozlov, D. Bliss, C. Lynch, Terahertz wave generation in quasi-phase-matched GaAs, Appl. Phys. Lett. 89141119 (2006)

[2] K. L. Vodopyanov, Optical generation of narrow-band terahertz packets in periodically-inverted electro-optic crystals: conversion efficiency and optimal laser pulse format, Opt. Express 14, 2263-76 (2006)

[3] G. Imeshev and M. E. Fermann, K. L. Vodopyanov and M. M. Fejer, X. Yu and J. S. Harris, D. Bliss and C. Lynch, High-power source of THz radiation based on orientation-patterned GaAs pumped by a fiber laser, Opt. Express 14, 4439-44 (2006)

[4] J.E. Schaar, K.L. Vodopyanov, M.M. Fejer, Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs, Opt. Lett. 32,1284-87 (2007)

[5] K.L. Vodopyanov, Optical THz-wave generation with periodically-inverted GaAs, Laser & Phot. Reviews 2, 11-25 (2008)

[6] J. Kiessling, I. Breunig, P. G. Schunemann, K. Buse and K. L. Vodopyanov, High power and spectral purity continuous-wave photonic THz source tunable from 1 to 4.5 THz for nonlinear molecular spectroscopy, New Journal of Physics 15, 105014 (2013)