Terahertz Optical Rectification And Optical-to-Terahertz Conversion Efficiency

Photonics Terahertz sources are one of two main terahertz radiation sources. Photonics Terahertz sources may generate terahertz radiation with high power, coherent, narrow bandwidth, frequency tunable, and may be compactable, room-temperature operation etc. Among those photonic Terahertz sources the terahertz optical rectification in extremely wide spectrum range have been widely used in Terahertz imaging and Terahertz time domain system, which can be widely used in researches of terahertz interaction with materials. But the optical-to-terahertz conversion efficiency in terahertz optical rectification usually is limited. If we use a femtosecond laser with 1 watt output as a pump sources the generated Terahertz power is usually in order of micro-watt. The optical-to terahertz conversion efficiency of terahertz optical rectification is usually in order of 10-5. So, to further improve the optical-to terahertz conversion efficiency of terahertz optical rectification and to study the mechanism of Terahertz generation via second-order nonlinearity is still a substantial project.This dissertation presents a description of the second order susceptibility using classical enharmonic oscillator model and density matrix respectively, address an analysis of the mechanism of terahertz optical rectification. We present an analysis of second order nonlinearity of the zincblende crystal as terahertz emitter and detector, obtain the optimal geometry of (1,1,0) and (1,1,1) oriented zincblende crystal as terahertz detector respectively. When the incident light is in the form of Gaussian pulse, we present an analysis of terahertz radiation via the second order susceptibility; obtain the relationship between optical-to-terahertz conversion efficiency and pulse duration, crystal length. In the condition of phase-matched in periodically inverted crystal, the affection of absorption coefficient on the choice of crystal length, the spectrum bandwidth dependence on the crystal length is obtained. We theoretically demonstrate the possibility of frequency difference Terahertz pulse shaping based on the theory of nonlinear different frequency generation, propose a new approach for terahertz pulse shaping, which therefore not only can control the polarity and polarization of induced electric field but also can manipulate the shape of THz pulse.Innovation points;1) We introduce the Terahertz wave absorption coefficient in crystals to the wave equation for the first time, which result in the relationship between effective length and the coherent length with different absorption coefficients respectively. We made a simulation of optical-to-terahertz conversion efficiency of crystal ZnTe,DAST,LiNbO3 which can provide theoretical reference for fabrication of photonic terahertz sources.2) We propose a new idea of terahertz pulse shaping via frequency difference for the first time, and demonstrate the possibility with the help of the theory of nonlinear different frequency generation. This new method of pulse shaping, which not only can manipulate the pulse shape but also can control the polarity and polarization of induced terahertz radiation, have advantages over present pulse shaping methods based on optical rectification and photoconductor.3) We propose a new experimental setup for terahertz pulse shaping for the first time, which can provide with us a new approach of terahertz pulse shaping.