The Arbitrary Angular Radiation Efficiency And Tuning Analysis Of THz Wave

THz waves is the electromagnetic radiation with frequency range of 0.1～10 THz (1 THz= 1012 Hz), Its wavelength range is between micro-wave and infrared(3000-30μm). The wavelength of this range of electromagnetic radiation has a very unique nature. In recent years, THz technology in either the basic research or the application research, has achieved certain progress and development. For example, as in medical imaging, medical diagnostics, environmental testing, security check, radio astronomy, and broadband mobile communications and so on, THz technology has a wide application prospect. THz waves as the last research band of electromagnetic wave, its research includes THz waves generation, THz waves detection, THz waves transportation and THz waves application, (THz spectrum, THz imaging, THz wireless communication and so on), and the research of the THz source is a top priority. The developed countries such as America and Japan are in a great deal of manpower and resources dedicated to the research. The positive and comprehensive research of THz science and technology also has an important strategic significance to China.There are many methods to get THz radiation, mainly divided into electronics and photonics. This article is based on the current situation of domestic and foreign research, focus on the method of photonics to get THz radiation, and in detail, the nonlinear optical methods which including nonlinear optical difference frequency generation, THz wave parametric oscillation, THz wave parametric generation and optical Cherenkov radiation. This article is based on the method of quasi-phase matching nonlinear optical difference frequency generation. The advantage of this approach can be realized the high peak power and narrow-band THz radiation. And the experiment equipment not only has a compact structure but also can operate in room temperature.There are mainly three parts in this paper, as follows:In the first part, based on the theory of quasi-phase matching difference frequency generation, we deduced a arbitrary angle three-wave coupling equation and the radiation efficiency in two situations. The first one is we neglect the absorption and the pump and single wave depletion, and the second one is we neglect the absorption and the pump wave depletion. We find that the coupling equation and the radiation efficiency that we neglect the absorption and the pump and single wave depletion are the same as collinear radiation formally. But the radiation efficiency that we neglect the absorption and the pump wave depletion is not the same as collinear radiation formally, there is a radiation angle 9 in the formula, and because of the existence ofθ, we have concluded that the radiation efficiency will reach to the maximum when we take collinear or close to collinear radiation.The second part we based on the theory of many radiation mechanism of THz that radiated from a PPLN crystal quasi-phase matching nonlinear optical difference frequency generation, I have given some concrete tuning characteristic analysis and summary. We listed some common THz radiation mechanism, including 1-D PPLN collinear radiation,1-D PPLN non-collinear radiation (angular radiation),2-D PPLN non-collinear radiation (angular radiation). In summary we take only two major mechanisms, and we analyze the tuning characteristics of the radiation mechanism. We conclude that we can get a large tuning range when we take the approach of collinear radiation and the big grating period; the fourth mechanism has a larger tuning range in the situation of the same emission angle; and the first mechanism has a larger tuning range in the situation of the same grating period.The third part in this paper, based on the theory of various polarization vector compensating various quasi-phase matching progress respectively in two dimensional PPLN, we can generate various wave length THz radiation, and give the relationships between polarization cycles. In addition, the difference between 1-D PPLN and 2-D PPLN lies in:The polarization vector of 1-D PPLN in x direction has a projection in any direction, so is the continuous change. More over, the polarization vectors in other directions are smaller than x direction; the polarization vector of 2-D PPLN in x direction can't project to continuous direction, so is not the continuous change. More over, the polarization vectors in other directions may be bigger than x direction.