A Compact High Average Power Ultrafast THz Radiation Source

Ultrafast terahertz techniques are finding growing applications in fundamental research, territorial safety and bio-medical imaging. All these applications call for the development of stable, compact and efficient sources of high-power THz pulses.The work of this thesis focuses on the study of a compact, low cost and stable high average power ultrafast THz source. The factors that affect the conversion efficiency of THz radiation via optical rectification were analyzed, and a femtosecond PCF amplifier was used as the optical pump. By optimizing the experimental setup, we achieved a compact high average power THz radiation source. Meanwhile, we studied the nonlinear processes that accompany THz generation, and designed new emitters with better performance. The main contents are described below:1. The key factors affecting the THz conversion efficiency were analyzed. A femtosecond PCF amplifier with one stage amplification in a stretcher-free configuration was built to realize a stable, compact optical pump with a short pulse width.2. We systematically investigated the nonlinear characteristics of undoped GaP, including measurements of the nonlinear parameters with a Z-scan technique, measurements of the crystal surface damage threshold and a study of the second harmonic and third harmonic generation processes . It was found that three photon absorption dominated when the crystal was pumped by 1040 nm fs pulses.3. A compact THz-TDS system with a 3 mm GaP emitter was built. After optimizing the key parameters such as the crystal azimuthal angle, the pump pulse duration and the pump spot size, we obtained an average output power of 163μW measured by a Golay-cell detector, an output 25 times higher than the one from a conventional fiber amplifier.4. We designed a THz emitter with micro-pyramid structured anti-reflection layers based on an equivalent graded refractive index distribution principle. Experiments confirmed that the output coupling could be enhanced and the transmitted pump beam could be scattered.5. THz waveguide emitters were designed. THz frequency tuning was achieved by changing the pump pulse duration. We considered a waveguide array emitter to achieve a wider tuning range. A GaP waveguide emitter in photonic crystal fiber form was proposed to optimize the phase matching condition and realize spectral tuning by parameter tailoring.