Frequency Process In Efficient Energy Conversion And Adiabatic And Diathermic Process

Nonlinear frequency conversion via three wave mixing process is a fundamental concept in the field of nonlinear optics. The generation of tunable optical radiation typically relies on nonlinear frequency conversion in crystals. In this process, light of two frequencies is mixed in a nonlinear crystal,resulting in the generation of a third color at the sum or difference frequency. These three-wave mixing processes are typically very sensitive to the incoming frequencies, due to the requirement of phase matching. Thus angle, temperature or other tuning mechanisms are needed to support efficient frequency conversion.In practice, the broad bandwidth response is in particular important for frequency conversion of ultrashort pulses.However, simultaneous phase matching of a broad frequency range is hard to achieve.Therefor, how to achieve both high efficiency and large bandwidth becomes a very important scientific problem.The demonstration is experimentally realized using an adiabatically varying aperiodically poled potassium titanyl phosphate (APPKTP) in a quasiphased matched (QPM) design., but at a cost of a significantly reduced efficiency. Chirped QPM gratings have been utilized to manipulate short pulses both in second harmonic generation (SHG), difference frequency generation (DFG), and in parametric amplification. Recently,Haim Suchowski proposes a technique for achieving both high efficiency and large bandwidth in sum frequency conversion using the adiabatic inversion scheme and present a physical intuition of the process of sum frequency generation in the Bloch sphere. The detailed study of the properties of this converter is done using the Landau-Zener theory dealing with adiabatic transitions in two level systems.But,there is no detailed physical conversion mechanism and theoretical bandwidth response range in the scheme.Thus, it is necessary to understand the relation between the problem of energy conversion in the nonlinearity parameter process and adiabatic or diathermanous process.Based on adiabatic population transfer mechanism of two-level system and rotation transformation,the paper achieved adiabatic condition which concered with energy conversion between two superposition field composed of signal and idler field in the process of sum frequency generation. The physical mechanism of adiabatic or diathermanous process and the process of sum frequency generation are discussed in a novel insight. It is showed that the physical mechanism of high efficiency energy conversion and the key position of energy conversion are determined by the position of energy conversion of two superposition fields or diathermanous condition.The paper is mainly divided into three parts. In the first part, we obtain coupled equations of superposition field with rotation transformation and its analytical solution when the phase mismatch is a constant.In addtion,we explain the physical meaning of analytical solution with superposition field. In the second part, we present an idea for achieving both high efficiency and large bandwidth in sum frequency conversion using the adiabatic inversion scheme,and the quasi-phased matched technique in analogy with adiabatic population transfer mechanism of two-level systems.The geometrical representation of the process of sum frequency generation with different parameters are showed intuitively in the bloch sphere. Finally,we study the adiabatic condition for achieving both high efficiency and large bandwidth in sum frequency conversion, obtaining its analytical expression as a function of crystal length and input frequency bandwidth. The the key position of high efficiency energy conversion and theoretical bandwidth response range in sum frequency conversion are determined by the position of energy conversion of two superposition fields or diathermanous condition.