利用雙色飛秒脈衝激發凝態物質之兆赫波輻射源探討

Abstract

目前常見的兆赫波激發源及機制，如：使用脈衝激發並施加電壓使自由載子加速的光導天線、以二階非線性當中的光整流效應產生兆赫波的ZnTe晶體、以及基頻與倍頻光同時激發空氣之三階非線性特性所產生之兆赫波。而在非線性光學中可以得知，只要光強足夠，任何材料皆可以被激發出三階非線性的特性，故此研究工作利用雙色脈衝激發材料三階非線性特，藉由四波整流效應產生兆赫波，其中針對各種材料之三階非線性特性進行模擬，探討其對兆赫波激發源的發展及兆赫波顯微術繞射極限的突破的展望性。本篇工作是以中心波段為800奈米的脈衝，經過倍頻晶體後產生400奈米的脈衝，紅光與藍光同時作用於材料上，激發材料三階非線性特性，以四波整流的機制產生兆赫波。我們模擬各個材料產生之兆赫波電極化分佈，並針對各個材料產生之兆赫波電極化，與基頻光在ZnTe中光整流效應所產生之兆赫波電極化做比較。接著計算各材料的三階非線性效率，模擬所選之材料產生的兆赫波偏極化，進而了解什麼樣的材料適合以雙色光產生兆赫波。There are some common terahertz sources and generation mechanics, like the photoconductive antenna, which is driven by femtosecond pulse and applied a bias to radiate the terahertz, and the nonlinear crystal, ZnTe, which can be induced the second order nonlinear effect and the terahertz will be generated from optical rectification (OR). Different from the single-color induced terahertz generation mentioned above, terahertz also can be generated by four-wave rectification (FWR) from the third order nonlinear effect, which is a function of fundamental and second harmonic generated (SHG) pulses to coherently excite at the same time. According to the nonlinear optics, all materials can be induced the high order nonlinear effect under the strong enough incident energy. Here, we try to induce terahertz frequency by four-wave rectification from kinds of materials, and investigate the promising of the terahertz source development and terahertz microscopy to conquer the diffraction limit by the numerical simulation.In this work, we applied an 800nm femtosecond pulse and a 400nm pulse from SHG to induce the FWR from the sample to generate the terahertz pulse. We use Mathematica to calculate the terahertz polarization and try to optimize the optical conditions interact with the sample. Then we also compared the terahertz polarization efficiency with the terahertz generated from OR by ZnTe.