Investigation of an ultrafast all optical modulation device using three levels in a GaN/AlGaN quantum well

(1)本课题的研究目的是研制一种工作速度为t比特/秒的超快全光调制器件。发展的重点是：(a)利用代表研究者提出和研究的全光调制方法，(b)利用GaN / AlGaN量子阱，预计具有100 fs的超快种群弛豫时间。(2)首先研究了GaN / AlGaN量子阱的生长条件，使量子阱数量最大化。研究发现，通过在氮化镓阻挡层之间引入氮化镓缓冲层，可以避免由于晶格失配而在生长层中产生的裂纹。因此，成功地生长了一个200层的多量子阱，在4.5 μm波长处显示出30-40%的强亚带间吸收峰。(3)其次，采用单色泵浦探针法测量子带间人口弛豫时间，该时间决定了器件的调制速度。实验首次证明了弛豫时间短至100-200 fs。结果表明，该调制装置的响应时间可短至100 fs。(4)研制了全光调制实验测量系统，该系统利用差频混频晶体产生中红外脉冲，利用水池在水池中产生白光。为了比较和试验，测量了由传统GaAs/AlGaAs量子阱组成的器件的调制特性。精确地研究了GaAs/AlGaAs量子阱与GaN/AlGaN量子阱之间载流子弛豫动力学的差异。(5)为了实际应用，研究了GaN/AlGaN量子阱中子带间跃迁波长的缩短。将生长方式由MOVPE改为MBE，过渡波长短于1.55fj。已获取M。这是一个非常令人鼓舞的结果，因为我们可以利用一个小型半导体激光器作为控制光源（泵浦）。少

(1) The purpose of this research project is to develop an ultrafast all-optical modulation device which can operate at a speed of T-bits/s. The points of the developments are (a) utilization of the all-optical modulation method which has been proposed and investigated by the representative researcher, and (b) utilization of GaN / AlGaN quantum wells which are expected to have an ultra fast population relaxation time of 100 fs.(2) At first, the growth condition of GaN / AlGaN quantum well was investigated to maximize the numbers of quantum wells. It was found that the generation of the crack in the growth layer due to the lattice mismatch can be avoided by introducing a GaN buffer layer in between the AlGaN barrier layers. Therefore a multiple-quantum-well with 200 well layers has been successfully grown, which showed a strong intersubband absorption peak of 30-40% at a wavelength of 4.5 μm.(3) Next, the intersubband population relaxation time, which determines the modulation speed of t … More he device, was measured by one-color pump-probe method. It was experimentally shown for the first time that the relaxation time is as short as 100-200 fs. The result indicated that the response time of the modulation device can be as short as 100 fs.(4) The measurement system for the all-optical modulation experiment has been developed, which utilizes a differential frequency mixing crystal to generate a mid-infrared pulse and a water cell to generate a white-light generation in the water cell. For the comparison and trial, the modulation characteristics of the device consist of conventional GaAs/AlGaAs quantum wells was measured. The deference of the carrier relaxation dynamics between the GaAs/AlGaAs quantum wells and GaN/AlGaN quantum wells was precisely investigated.(5) The shortening of the intersubband transition wavelength in a GaN/AlGaN quantum well was investigated for the purpose of practical applications. By changing the growth method from MOVPE to MBE, a transition wavelength shorter than 1.55fj.m has been obtained. This is a very encouraging result since we can utilize a small semiconductor laser for the source of the control (pump) light. Less