利用摻雜之硒化鎵晶體產生兆赫波之研究

Abstract

本計畫將尋找適當的元素摻雜於非線性半導體硒化鎵晶體中，以提高 其產生兆赫波之效率。近來已有研究群利用硒化鎵晶體產生一可調波長之 兆赫波幅射。也因為此種晶體在兆赫波波段吸收率較低，所以在眾多非線 性光學晶體中最被看好的材料之一。不過其本身的均勻度及光學透明度， 卻成為產生高功率兆赫波幅射之限制。一般來說，利用差頻產生兆赫波的 方法中，增加晶體厚度應該可增加兆赫波的輻射功率。但到目前為止，實 驗上並未觀察到此現象。因此，透過離子半徑較大元素的摻雜來增加其極 化率或均勻度以提高兆赫波的輻射功率。另外，利用偶極天線的結構來產 生兆赫波幅射，同樣透過不同元素的摻雜，也可增加其電阻率及載子之遷 移率以提高兆赫波的輻射效率，同時更可研究其載子之動力學性質。所以， 不管利用何種方式產生兆赫波輻射，本計劃的目標在透過不同元素之摻 雜，同時配合尋找穩定的長晶條件，以得到一具有高均勻度、高機械強度、 高透光率、高電阻率等特性之高品質非線性半導體硒化鎵晶體，進而實現 高功率及波長可調之兆赫波幅射光源。最後，將利用此兆赫波幅射進行高 溫超導體、多鐵錳氧化物、磁性半導體或半導體量子點之超快動力學研究。

This proposal is intended to find a new doping element in nonlinear optical semiconducting crystals, GaSe, for high efficiency terahertz (THz) generation. GaSe crystals which will be the main subject of the research within this proposal are among the most prospective IR-range crystals. In last years the possibility to create tunable terahertz generation sources on its base was demonstrated by experiments. GaSe possesses the lowest absorption in the terahertz range among known nonlinear crystals. Therefore the crystals are prospective for THz generation by nonlinear-optical conversions (difference frequency generation, DFG). On the other hand, the efficiency of conversion and the power of output radiation are limited by crystal properties such as optical transparency and homogeneity. In particular at DFG the increasing crystal length must give higher conversion efficiency that is not observed in real experiments. The answer should be given what limits the possibilities of the crystals and whether it is possible to improve them, for example by introducing chemical elements with bigger atomic radius and higher polarizability. The second method to generate THz is employing of dipole antenna scheme. For enhancement of efficiency of such devices it is necessary to use materials with high resistance and carrier mobility. Electrical transport properties of GaSe can be considerably modified by doping and it looks reasonable to investigate dipole antennas based on doped crystals, which also should give information on their electronic properties. Within this proposal it is planed to find conditions of growth of large homogeneous GaSe crystals with lower absorption in THz region and investigation of possibilities to enhance their characteristics by the way of their doping for creation on their base of optical elements for THz range generators and frequency converters. In particular the problem is to reveal the mechanisms determining the optical transparency of nonlinear optical crystals in the specified range. The task is to develop scientifically based and stable schemes to produce nonlinear optical single crystals with improved parameters for applications in the THz range. Finally, after getting the suitable nonlinear optical crystals for high power and widely tunable wavelength THz generation, we could study the ultrafast dynamics in some interesting materials such as high Tc superconductor, multiferroics manganites, magnetic semiconductors or semiconductor quantum dots et. al..