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

我们提出了一种新型的可调谐太赫兹振荡器，该振荡器基于块体GaSe晶体，块体GaAs晶体和键合多个GaAs板，每个GaAs板都由亚皮秒激光脉冲同步泵浦。这种类型的太赫兹振子的机制在于这些结构中的相位匹配或准相位匹配的光学参量振荡。通过使用超短激光脉冲，我们可以利用高峰值激光强度。除了传统的空转波在空腔中谐振的OPO外，我们还将使用泵浦波和空转波在空腔中谐振的配置。这种结构非常稳定，不像双谐振OPO。为了实现我们的目标，我们建议研究太赫兹参数过程，包括相位匹配光学整流，差频产生，参数振荡和放大。对于GaSe晶体，我们可以基于晶体的双折射实现相位匹配。另一方面，对于块体GaAs材料，为了实现相位匹配，泵浦波长应小于双光子吸收开始时对应的波长。因此，双光子吸收（TPA）可以成为一个重要的问题。我们的估计表明，TPA可能增加振荡的阈值强度。然而，它不会造成光学损伤或相当大的热效应。我们计划研究TPA如何影响所提议的太赫兹振荡器。除了我们提出的实验活动外，我们还计划从理论上探讨opo在TPA存在下的性能。这一理论组成部分将用于预测TPA如何影响opo，并支持我们的实验发现。我们还将在交替旋转的扩散键合GaAs板上实现所有这些参数化器件。然而，在这种情况下，通过适当选择GaAs板的厚度，我们可以在带隙的一半以下使用泵浦光子能量来完全避免TPA。随着我们最近在GaSe中高效太赫兹产生的成功，我们将尝试在基于近简并光参量振荡器内腔内差频产生的集成、高效和紧凑的脉冲太赫兹系统上实现我们的新想法，该系统由倍频Nd:YAG或Nd:YVO4激光器泵浦。我们的可调谐太赫兹振荡器可用于遥感，化学鉴定，生物医学成像，目标识别和安全筛查。它的时间和空间相干性远远优于太赫兹天线和其他类似技术的太赫兹源。

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.