Tunable THz Oscillators Based on Synchronously-Pumped Optical Parametric Oscillation in GaSe and GaAs Crystals

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We propose to investigate a new type of tunable THz oscillators based on a bulk GaSe crystal, a bulk GaAs crystal, and bonded multiple GaAs plates, each of which is synchronously pumped by subpicosecond laser pulses. The mechanism for this type of the THz oscillators lies in phase-matched or quasi-phase-matched optical parametric oscillation in these structures. By using ultrashort laser pulses, we can take advantage of high peak laser intensities. Besides the conventional OPO in which the idler is resonant in a cavity, we will also use a configuration in which both pump and idler waves resonant the cavity. Such a configuration is very stable, unlike a doubly-resonant OPO. To accomplish our objectives, we propose to investigate THz parametric processes including phase-matched optical rectification, difference-frequency generation, parametric oscillation and amplification. For GaSe crystals, we can achieve phase-matching based on birefringence of the crystals. On the other hand, for a bulk GaAs material in order to achieve phase matching, the pump wavelength should be less than that corresponding to the onset of two-photon absorption. Therefore, two-photon absorption (TPA) can be an important issue. Our estimate shows that TPA may increase the threshold intensity for the oscillation. However, it would not cause the optical damage or sizeable thermal effect. We plan to investigate how TPA affects the proposed THz oscillator. We also plan to theoretically explore performance of OPOs in the presence of TPA in addition to our proposed experimental activities. This theoretical component will be used to predict how TPA affects OPOs and to support our experimental findings. We will also implement all these parametric devices in alternatively-rotated diffusion-bonded GaAs plates. In this case, however, by properly choosing the thicknesses of the GaAs plates, we can use the pump photon energy below the half of the bandgap to completely avoid TPA. Following our recent success in efficient THz generation in GaSe, we will attempt to implement our new idea on an integrated, efficient, and compact pulsed THz system based on intracavity difference-frequency generation inside a near-degenerate optical parametric oscillator, pumped by a frequency-doubled Nd:YAG or Nd:YVO4 laser. Our tunable THz oscillator can be used for remote sensing, chemical identification, biomedical imaging, target recognition, and security screening. It has temporal and spatial coherence far more superior than the THz sources from THz antennas and other similar techniques.