Ultrafast Interband-Resonant Light Modulation by Intersubband-Resonant Light in Quantum Wells

半导体量子阱中的子带间跃迁（ISB-T）引起了广泛的关注。ISB-T具有较大的跃迁概率和非常快的能量弛豫时间（约lps）。迄今为止，量子阱结构中的光学现象主要集中在ISB-T或传统的带间跃迁（IB-T）上。通过同时利用ISB-T和IB-T来研究这些现象在物理上是非常有趣的，并且可能导致超快全光器件等新的器件应用领域。基于这一概念，最近提出并证明了isb谐振光对ib谐振光的调制，其中调制方案有两种：(i)利用n掺杂的qw和（ii）利用未掺杂的qw。前者利用导带中第一子带到第二子带的实际载流子跃迁，后者利用它们之间的准虚拟跃迁。从皮秒到飞米特的超快调制，在这些方案中可以预期更大的秒范围。在本工作中，我们成功地在原n掺杂QW的情况下，用isb谐振光进行了超快的ib谐振光调制。实验样品具有多量子阱结构，具有30个周期的GaAs阱（7.6nm）和MBE在GaAs衬底上生长的Al_<0.3>Ga_<0.7>As势垒层（14.1nm），其中势垒层选择性地掺杂Si （n-3.0 × 10^<18>cm^<-3>）。利用密度矩阵理论进行理论计算，预测在使用1MW/cm^2的isb共振光强时，ib吸收变化约为3000cm^<-1>。这个吸收变化值对于器件的实际性能来说足够大。量子阱被AlGaAs包层夹在中间，加工成脊状波导结构，使ib谐振光通过它传播。ib谐振光在通过波导传播时被isb谐振光调制。利用自由电子激光器（FEL）以10Hz的重复频率输出宽约15mum的大脉冲。每个微脉冲包含一列300-400个持续时间为45ns的超短微脉冲。利用泵浦和探针测量硅片ISB-T的吸收饱和度，发现微脉冲宽度短至-5ps。在TM极化下，FEL脉冲以布鲁斯特角从衬底侧入射到样品上。另一方面，使用波长为830nm的连续波半导体激光器作为ib谐振光，通过物镜入射到样品的脊波导上。调制后的ib谐振光由上升和下降时间分别为400ps的高速硅脚光电探测器检测，并由FEL微脉冲信号触发的数字化示波器进行分析。结果发现，调制信号明显为负尖峰，对应于每个FEL微脉冲。这些负尖峰表明，ISB激发引起的ib共振光的吸收系数增加与理论预测一致。观测到的IB信号的下降和上升时间分别约为400ps，受上述检测系统的限制。观测到的调制深度m为0.8-0.9%，其中m定义为m= (I_1-I_2) /I_1: I_1和I_2分别为无isb谐振光和有isb谐振光的ib谐振光强度。需要注意的是，实际的调制深度应该比观测到的深度大100倍以上，因为探测系统的响应时间约为800ps，而实际的调制时间将快至-5ps （FEL微脉冲宽度）。观测到的上升时间比带间弛豫时间（通常为-几个纳秒）快得多，这表明调制速度不受价带空穴积累的限制，除了我们提出的调制原理外，我们无法找到限速因素。为了证实这一点，我们还测量了调制深度对FEL脉冲能量的依赖性。当有效脉冲能量超过-5pJ （qw的有效峰值功率约为300kW/cm^2）时，调制深度达到0.8 ~ 0.9%。实验结果也证明了本实验实现了几乎完全的开关。通过使用InGaAs，我们还成功地将子带间传输缩短到1.9mm。/唉，QW系统。少

There is much interest in intersubband-transition (ISB-T) in semiconductor quantum wells (QWs). The ISB-T has a large transition probability and a very fast energy relaxation time (about lps). The optical phenomena in the QW structure thus far have been mainly concentrated on either this ISB-T or a conventional interband-transition (IB-T). To investigate the phenomena by the simultaneous utilizetion of the ISB-T and IB-T is physically very interesting and may lead to new device application fields such as ultrafast all-optical devices.Based on this concept, the IB-resonant light modulation by the ISB-resonant light has been recently proposed and demonstrated, where modulation schemes are two-fold : (i) utilization of n-doped QWs and (ii) utilization of undoped QWs. The former makes use of the actual carrier transition from the first to second subbands in the conduction band, while the latter utilizes the quasi-virtual transition between them. Ultrafast modulation from picosecond to femt … More second range can be expected in these schemes.In this work, we have succeeded in ultrafast IB-resonant light modulation by the ISB-resonant light in the case of former n-doped QW.The sample utilized for the experiment has a multiple quantum well structure with 30 periods of GaAs wells (7.6nm) and Al_<0.3>Ga_<0.7>As barrier layrs (14.1nm) grown by MBE on a GaAs substrate, where the barrier layrs were selectively n-doped with Si (n-3.0x10^<18>cm^<-3>). The theoretical calculation using a density matrix theory predicted that the IB-absorption change of about 3000cm^<-1> can be expected by using an ISB-resonant light intensity of 1MW/cm^2. This value of absorption change is large enough for actual device performance. The QWs were sandwitched by AlGaAs cladding layrs and processed to the ridge waveguide structure to let the IB-resonant light propagate through it.The IB-resonant light is modulated by ISB-resonant light when it propagates through the waveguide. A free-electron laser (FEL) was utilized for the ISB-resonant light which delivered macropulses with a width of about 15mum at a 10Hz repetition rate. Each macropulse contained a train of 300-400 ultrashort micropulses with a 45ns duration. A pump and probe measurement of the absorption saturation of the ISB-T of the wafer utilized for this study revealed that the micropulse width was as short as -5ps. The FEL pulse was incident to the sample from the substrate side with a Brewster's angle with TM polarization. On the other hand, a cw semiconductor laser with a wavelength of 830nm was used for the IB-resonant light, which was incident to the ridge waveguide of the sample through an objective lens.The modulated IB-resonant light was detected by a high-speed Si-pin photodetector whose rise and fall times were 400ps, respectively, and was analyzed by a digitizing oscilloscope triggered by the FEL micropulse signal. It was found that the modulation signals were clearly observed as negative spikes, corresponding to each FEL micropulses. These negative spikes indicate the increase of absorption coefficient for the IB-resonant light induced by the ISB excitation in accordance with the theoretical prediction. The fall and rise times of the observed IB signal were about 400ps, respectively, which were limited by the detection system as described above. The observed modulation depth m was found to be 0.8-0.9%, where m was defined as m= (I_1-I_2) /I_1 : I_1 and I_2 are the intensities of the IB-resonant light without and with ISB-resonant light, respectively.It should be noted that the actual modulation depth should be more than 100 times larger than the observed one, because the response time of the detection system was about 800ps, while the actual time of the modulation would be as fast as -5ps (FEL micropulse width). The rise time observed is much faster than the band-to-band relaxation time (typically, -a few nanosecond), which indicates that the modulation speed is not limited by the hole accumulation in the valence band, and we could not find the speed limitation factors except for our proposed modulation principle. To confirm this, we have also measured the dependence of the modulation depth on the FEL pulse energy. The modoulation depth saturated to be 0.8-0.9% when the effective pulse energy exceeds -5pJ (effective peak power supplied to the QWs was about 300kW/cm^2). The result also supports that the almost full switching was achieved by this experiment. We have also succeeded in shortening of the intersubband trasnsition down to 1.9mm by using InGaAs./AlAs QW system. Less