| Terahertz wave is the electromagnetic radiation whose frequency ranges from millimeter waves to the far infrared. It has many pretty unique properties and has wide applications in nondestructive testing, imaging, environment monitoring, communications, national defense and security, physics, bioscience, and chemistry. Terahertz sources play a very significant role in the application and development of terahertz technologies. Terahertz parametric sources based on the stimulated polariton scattering have many advantages such as high peak power, continuous tunability, good temporal and spatial coherence, room temperature operation, compactness, and ease of use. They have attracted much attention since 1990’s. There are three types of terahertz parametric sources, including terahertz parametric generator (TPG), terahertz parametric oscillator (TPO), and injection-seed terahertz parametric generator (is-TPG). TPG can generate terahertz waves with a wide spectral range and high energy. Terahertz waves generated by TPO have narrow linewidth and continuous tunability. Is-TPG can narrow the linewidth of the terahertz wave to the Fourier transform limit by injecting the Stokes seed with a narrow linewidth, improving the spectral resolution in the spectral imaging and analysis.However, some problems exist in the terahertz parametric sources. The conversion efficiency from the pump to the terahertz waves and the coupling efficiency are relatively low. Only LiNbO3 or MgO:LiNbO3, whose damage threshold is low, are practically used in the past more than 20 years. The main contributions in this dissertation include four aspects. First, a theoretical model for the terahertz parametric oscillator based on the laser rate equations and coupling wave equations is constructed. Second, we design and demonstrate a TPO with a slab MgO:LiNbO3 crystal and a surface-emitted configuration, which could generate multiple terahertz beams. Third, the KTiOPO4 (KTP) and KTiOAsO4 (KTA) crystals are used in TPOs. Fourth, the theoretical model for thermal dynamic process of the Golay Cell is build, and based on this model, the energy responsivity to the terahertz pulses is analyzed. The concrete contents are as follows.1. A theoretical model for terahertz parametric oscillators is deduced by combining the rate equations describing the laser operation and the coupling wave equations of the nonlinear optical frequency conversion. Due to the noncollinear phase matching condition, which is different from that in other nonlinear processes like OPO and Raman laser, the spatial intensity distributions of the pumping, Stokes and terahertz beams are considered. The theoretical output energies of the terahertz and Stokes waves with regard to the pumping energy were analyzed. A TPO based on MgO:LiNbO3 crystal is realized and used to verify the theoretical model. By changing the phase matching angle between the pump and Stokes waves, the terahertz wave can tune from 1.30 to 2.61 THz. The output-input characteristics are studied by change the output mirrors with different transmittances (approximately 20%,40%,60% and 80%). The experimental results are well in accordance with the theoretical results. Under the pump energy of 87.5 mJ and the transmittance of 20%, the terahertz wave energy is 746 nJ.2. A novel MgO:LiNbO3 slab crystal configuration is designed for the surface-emitted terahertz parametric oscillator. In this slab MgO:LiNbO3 crystal, the pump and the oscillating Stokes beams are totally reflected at the side surface of the crystal and propagate in a zigzag way. Up to five terahertz beams are emitted perpendicularly to the crystal surface. The total output energy of the five THz-wave beams is 3.56 times as large as that obtained from the conventional surface-emitted TPO at the same experimental conditions. The characteristics of the THz wave were analyzed qualitatively. The intensity distribution of the terahertz wave is symmetrical in the vertical direction, while unsymmetrical in the horizontal direction.3. The crystal characteristics of the KTP crystal in the terahertz spectral range are analyzed. A surface-emitted terahertz-wave parametric oscillator with KTP crystal as the nonlinear medium is designed. The oscillating Stokes beam propagates along the x-axis of the KTP crystal, the pumping beam propagates with a small incident angle θext to the x-axis, and the polarizations of the pumping beam, the Stokes beam and the terahertz wave are along the z-axis. When θext is changed from 1.250° to 6.000°, the THz wave is intermittently tuned from 3.17 THz to 6.13 THz. The maximum output of the THz wave is 336 nJ, obtained at 5.72 THz with a pumping energy of 80 mJ. The two frequency gaps, from 3.44 THz to 4.19 THz and from 5.19 THz to 5.55 THz, are located in the vicinities of the A1 modes of 134 cm-1 and 178.7 cm-1, which are strongly infrared absorbing.4. The crystal characteristics of the KTA crystal in the terahertz spectral range are analyzed. The phase matching of the stimulated polariton scattering in the KTA crystal is obtained in the experiment and a surface-emitted terahertz-wave parametric oscillator with KTA crystal as the nonlinear medium is designed. The oscillating Stokes beam propagates along the x-axis of the KTP crystal, the pumping beam propagates with a small incident angle θext to the x-axis, and the polarizations of the pumping beam, the Stokes beam and the terahertz wave are along the z-axis. When the incident angle θext of the pump wave is changed from 1.875° to 6.500°, the THz wave is intermittently tuned from 3.59 to 6.43 THz. The obtained maximum THz wave energy is 627 nJ at 4.30 THz with a pump energy of 100 mJ. Four much weaker transverse A1 modes of 132.9 cm-1,156.3 cm-1, 175.1 cm-1, and 188.4 cm-1 cause four frequency gaps, from 3.97 THz to 4.20 THz, from 4.51 to 4.89 THz, from 5.17 to 5.61 THz and from 5.67 to 5.91 THz, respectively.5. The Frequency Transfer Function (FTF) of the Golay Cell detector is constructed based on its thermal dynamic process. It well connects the input light signal to the output waveform displayed in the digital oscilloscope. It can be expressed as the function of a wavelength-dependent parameter K and two time parameters τ1 and t2. By using the experimental results of an incident 1064 nm step signal, three parameters are determined.6. According to the verified theoretical expression for the FTF, the energy responsivity Rp to the light pulses is obtained. For pulses whose duration is much smaller than the Golay Cell response time, if the response voltage u is defined as the voltage difference between the beginning of the pulse and the maximum value in the waveform, the energy responsivity Rp (the ratio of u to the pulse energy E) is (1/τ1-1/τ2)K, independent of the pulse duration and repetition frequency. The main innovations of this thesis are summarized as follows:1. A theoretical model for terahertz parametric oscillators is deduced based on the rate equations describing the laser operation and the coupling wave equations of the nonlinear optical frequency conversion. And the spatial intensity distributions of the pumping, Stokes and terahertz beams are considered for the first time.2. A novel MgO:LiNbO3 slab configuration is designed for the surface-emitted terahertz parametric oscillator. Up to five Terahertz beams are emitted perpendicularly to the surface of the crystal. The total output energy of the five THz-wave beams is 3.56 times as large as that obtained from the conventional surface-emitted TPO at the same experimental conditions.3. For the first time, KTP crystal is used as the nonlinear medium in a surface-emitted terahertz-wave parametric oscillator. The THz wave is intermittently tuned from 3.17 THz to 3.44 THz, from 4.19 THz to 5.19 THz and from 5.55 THz to 6.13 THz. The maximum output of the THz wave is 336 nJ, obtained at 5.72 THz with a pumping energy of 80 mJ.4. A terahertz parametric oscillator based on KTA crystal is demonstrated for the first time. The THz wave is intermittently tuned from 3.59 to 3.96 THz, from 4.21 to 4.50 THz, from 4.90 to 5.16 THz, from 5.62 to 5.66 THz and from 5.92 to 6.43 THz. The obtained maximum THz wave energy is 627 nJ at 4.30 THz with a pump energy of 100 mJ.5. The Frequency Transfer Function (FTF) of the Golay Cell detector is constructed based on its thermal dynamic process for the first time. Based on the verified theoretical expression for the FTF, the energy responsivity Rp to the light pulses is obtained. It is independent on the pulse duration and repetition frequency for the short pulses whose pulse duration times are much less than the Golay Cell response time. |
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