[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Enhancing THz Emission using a Shallow-Bounce Configuration

摘要： THz radiation promises to revolutionise applications in fields from security and counter terrorism, to medicine and agriculture. In the drive to develop robust, powerful and frequency-tunable THz laser sources, systems based on the stimulated polariton scattering (SPS) process, derived from solid state architectures, are arguably at the forefront. Within the realm of THz-SPS lasers, there are significant opportunities to enhance the performance of these systems, through careful consideration of the system architecture. It is well known that SPS-active crystals are inherently highly absorbing in the THz frequency range. This presents a dilemma in that while these crystals are capable of optically generating THz radiation, they are also the greatest cause of THz loss within these systems. This has led to system architectures/designs such as the surface-emitting configuration, which place the THz generation region as close to the emitting surface as possible. While this design has proven effective, it is but one consideration in maximising the possible THz power emitted from these systems. Of great relevance is the overall generation volume, its proximity to the emitting surface, and how this evolves with THz frequency tuning. Here, we report both computational modelling and experimental results contrasting the use of a new, novel, shallow-bounce architecture, in comparison to conventional linear and surface-emitting designs; the architecture of each is shown in Fig. 1. The shallow bounce system is shown schematically in Fig. 2. The system comprises a diode-end-pumped 0.3 at. % Nd:YVO4 laser crystal generating a fundamental field at 1342 nm. A flat end-mirror (M1), an x-cut 5 at. % MgO:LiNbO3 SPS crystal (HC Photonics Corp.) and a concave high reflectivity (HR) mirror (M2, radius of curvature (ROC) = 1000 mm) defined the ends of the fundamental resonator. The MgO:LiNbO3 crystal had dimensions of 5×5×20 mm3, and was AR coated on both end surfaces (R < 0.2 % at 1340 – 1380 nm at normal incidence). The system was Q-switched to achieve pulsed operation at a repetition rate of 5 kHz. A separate pair of HR mirrors (M3 and M4, ROC = 1000 mm; R = 99.9 % from 1340 – 1380 nm) were used to construct a 95 mm-long resonator which oscillated and bounced the Stokes field at the same TIR surface position as the fundamental field. Two high-resistivity Si-prisms with angles of 33°/33°/114° (dimensions 6×6×10 mm3) were placed in close proximity to the Teflon-coated polished x-z surface of the MgO:LiNbO3 crystal. These prisms extracted two THz beams at angles of ~ ± 30°relative to the TIR surface. The bounce angle of the fundamental field is 84.5 o (to the crystal normal), in comparison to 65 o which is typical in surface-emitting configurations. The shallow-bounce system generated dual-beam THz emission (with similar output powers and beam profiles) with a total, maximum average power of 67.5 (cid:541)(cid:58) for an incident CW pump power of 15.1 W. Notably, the frequency-tuning range was broad, covering the range 1.05 – 2.89 THz. High emission is maintained throughout this tuning range, especially in comparison to that achieved from linear and surface-emitting geometries, the details of which will be elucidated and discussed in this paper.