Research On Terahertz Radiation Based On Nonlinear Optical Difference Frequency Generation

Terahertz is in the special position of spectrum. It has the characteristic that is widelyused in physics, astronomy, spectral imaging, security testing, biological sciences andother areas. It begins to attract more and more attention. THz source is the basis in theTHz field.At the same time, it is also the emphasis. The research methods of THzsource include electronics methods and photonics methods. Nonlinear optical basedon difference frequency generation can generate continuous THz wave, whose linewidth can reach MHz. Then it can get higher resolution. In this paper, we use thephotonics methods to study the nonlinear optical based on difference frequencygeneration to generate THz-wave.Its main contents and innovations are as follows:1. Applying for the classical electromagnetic theory, we deduce optical differencefrequency generation coupled wave equations from the Maxwell equations. Byanalyzing the absorption coefficient, the coherence length, crystal thickness oforganic DAST crystal theoretically, we can get the impact on the differencefrequency generation to generate THz-wave. What’s more,we analysis collinearphase-matching conditions, the allowable angle, width and other allowableparameters.2. Introducing the molecular structure, preparation methods, the near-infrared andTHz dispersion characteristics of DAST crystal, we also introduce thephase-matching angle and allowable parameters of GaSe crystal. It can providetheoretical basis for designing experiments by using nonlinear optical based ondifference frequency generation to generate THz.3. We design small experiental device to generate THz based on difference frequencygeneration. We use KTP crystal (KTP-OPO) to produce1300-1500nm dualwavelengthes. By pumping organic DAST crystals, the THz whose tuning range is1.37-19.34T is generated. Then we make KTP-OPO generating2μm dualwavelengthes pump the GaSe crystal to generate THz whose energy is μJ magnitudeby difference frequency generation.4. Introducing the basic properties of MgO: LiNbO3crystal, such as the optimal spot size, near-infrared and THz range dispersion characteristics, we design the bestgeometry of MgO:LN.Then we set up a synchronously pumped parametric oscillatormodel without considering the pulse walk-off and group velocity dispersion. Weanalysis the amplitude, phase, gain equation and its physical meaning of the cavityelectric field. We discuss the ps-TPO cavity factors influencing the stability bysolving the equations. Then we design external cavity parametric oscillationgenerating picosecond pulses THz-wave (ps-TPO) experimental apparatus based ontheoretical analysis.