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

半导体量子阱(QWS)中的子带间跃迁(ISB-T)引起了人们的极大兴趣。ISB-T具有很大的跃迁几率和很快的能量驰豫时间(约Lps)。到目前为止，量子阱结构中的光学现象主要集中在这种ISB-T或传统的带间跃迁(IB-T)上。通过同时利用ISB-T和IB-T来研究这一现象在物理上是非常有趣的，并可能导致新的器件应用领域，如超快全光器件。基于这一概念，最近提出并证明了利用ISB共振光对IB共振光进行调制，其中调制方案有两种：(I)利用n掺杂的量子阱，(Ii)利用非掺杂的量子阱。前者利用导带中第一个子带到第二个子带的实际载流子跃迁，而后者利用它们之间的准虚拟跃迁。从皮秒到飞秒…的超快调制在这项工作中，我们已经成功地在以前的n掺杂量子阱的情况下，利用ISB共振光调制了超快的IB共振光。实验所用的样品具有多量子阱结构，30个周期的GaAs势垒(7.6 nm)和Al_&lt；0.3&gt；Ga_&&lt；0.7&gt；作为势垒层(14.1 nm)，其中势垒层是选择性地n掺杂Si(n-3.0x10^&lt；18&gt；Cm^&lt；-3&gt；)。用密度矩阵理论进行的理论计算表明，当ISB共振光强为1 mW/cm^2时，IB吸收变化约3000 cm^~(-1)~gt；这一值足以满足实际器件的性能要求。量子阱被AlGaAs包层沙切，加工成脊波导结构，使IB共振光通过脊波导光传输，IB共振光在传输过程中受到ISB共振光的调制。利用自由电子激光器(FEL)作为ISB共振光，以10赫兹的重复频率输出宽度约为15微米的宏脉冲。每个宏脉冲包含一串持续时间为45 ns的300-400个超短微脉冲。对用于本研究的晶片的ISB-T吸收饱和的泵浦和探头测量表明，微脉冲宽度短至-5ps。自由电子激光脉冲从衬底一侧以布鲁斯特角入射到样品上，偏振态为TM。另一方面，采用波长为830 nm的连续半导体激光器作为IB谐振光，通过物镜入射到样品的脊形波导中，利用上升和下降时间分别为400ps的高速Si-Pin光电探测器探测到IB谐振光，并在自由电子激光微脉冲信号触发的数字化示波器中进行分析。结果发现，调制信号被清楚地观察为对应于每个自由电子激光微脉冲的负尖峰。这些负峰表明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，这受到如上所述的检测系统的限制。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共振光和有ISB共振光的IB共振光的强度。需要注意的是，由于检测系统的响应时间约为800ps，而实际调制时间将快至-5ps(FEL微脉宽)，因此实际调制深度应比观察到的调制深度大100倍。观察到的上升时间远快于带间弛豫时间(通常为-几纳秒)，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，我们找不到限制速度的因素。为了证实这一点，我们还测量了调制深度与自由电子激光脉冲能量的关系。当有效脉冲能量超过-5pJ(有效峰值功率约为300kW/cm~2)时，调制深度饱和为0.8~0.9%。实验结果也支持了本实验实现了近乎全开关。我们还用InGaAs./AlAs量子阱系统成功地将子带间位移缩短到1.9 mm。较少

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