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

(1)本研究项目的目的是研制一种能够以T比特/S的速度工作的超快全光调制器件，其发展的重点是(A)利用有代表性的研究者提出和研究的全光调制方法，以及(B)利用有望具有100fs超快布居驰豫时间的GaN/AlGaN量子阱。(2)首先，研究了GaN/AlGaN量子阱的生长条件，以最大限度地增加量子阱的数量。研究发现，通过在AlGaN势垒层之间引入GaN缓冲层，可以避免由于晶格失配而在生长层中产生裂纹。在此基础上，成功地生长了一种具有200个势垒层的多量子阱，在4.5μm波长下出现了30-40%的子带间吸收峰。(3)子带间布居弛豫时间决定了t-…的调制速度采用单色泵浦-探针法对器件进行了测量。实验首次表明，弛豫时间最短可达100-200飞秒。研制了全光调制实验的测量系统，利用差频混频晶体产生中红外脉冲，利用水池产生白光。为了进行比较和试验，测量了由常规的GaAsAlGaAs量子阱构成的器件的调制特性。精确地研究了GaN/AlGaN和GaN/AlGaN量子阱中载流子弛豫动力学的差异。(5)从实际应用的角度出发，研究了GaN/AlGaN量子阱中子带间跃迁波长的缩短。通过改变生长方法从MOVPE到MBE，获得了小于1.55fj.m的跃迁波长。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(泵浦)光源。较少

(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