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Ultrafast all-optical modulation using nearinfrared intersubband transition

使用近红外子带间跃迁的超快全光调制

基本信息

批准号：
09450030
负责人：
NODA Susumu
金额：
$ 6.72万
依托单位：
KYOTO UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 1998
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-09450030/
关键词：
Quantem well Intersubband transition All optical transition Relaxation time エネルギー緩和 InGaAs AlAs ポンプ-プローブ法光通信フェムト秒 AlAs量子井戸

项目摘要

Intersubband transition (ISB-T) in quantum wells (QWs) has been drawing much attention in terms of novel device applications due to their unique physical properties, such as an ultrafast relaxation time. We have proposed an ultrafast all-optical modulation scheme by using n-doped QWs, where an interband transition (IB-T) is modulated by using the ISB-T. In this scheme, the modulation speed is considered to be limited by the relaxation time of the electrons excited by ISB-T, which is expected to be as fast as 〜 ps. However, the available transition wavelength of the ISB-T was limited to the midfar-infrared region (4 〜 20 μm) until recently, and there has been a strong need to shorten the transition wavelength for some applications such as all-optical modulators for optical fiber communications. Recently, we have achieved an ISB-T wavelength of 1.90 μm in 7-ML-wide InGaAs/AlAs multiple QWs grown coherently on a GaAs substrate. However, in our previous studies, it was observed that the ab … More sorption magnitude decreases as the transition wavelength is shortened by narrowing the well width. It is required to enhance the absorption magnitude of the short-wavelength ISB-Ts to as much as that of the conventional wavelength ISB-Ts in order to enable application for our ultrafast all-optical modulators.In this study, we tried to optimize the QW structure to obtain large absorption while conserving the short transition wavelength. First, we reviewed the results presented in the previous reports and discussed the origins of the decrease of the absorption. Theoretical calculation of the carrier distribution shows that the reduction of the absorption is caused by carrier leakage to the X minimum of the AlAs barrier from the Γ minimum of the InGaAs well. In order to regain the absorption magnitude, we undertake to suppress the carrier leakage by reducing the density of states in the AlAs X minimum and by increasing the energy spacing between Γ and X minima by (a) increasing the In composition of the well from 0.2 to 0.4, (b) suppressing the In segregation during the growth by decreasing the substrate temperature from 480 〜 520 ℃ to 400 ℃ and c decreasing the barrier width form 36ML to 8ML. The optimizations lead to the enhancement of the absorption magnitude of more than twelve-fold, and a peak absorption coefficient of 14000cmィイD1-1ィエD1 is obtained with an optimized structure. The results obtained here are considered very useful for device applications of the short-wavelength intersubband transitions.Then, we have carried out a femtosecond time-resolved measurement on InGaAs/AlAs and AlGaAs/GaAs quantum wells, of the proposed modulation scheme. At first, electron relaxation dynamics in short wavelength (〜 2.5 μm) intersubband transition in InGaAs/AlAs quantum wells was investigated. Femtosecond pump and probe measurement yields a relaxation time of 〜 2.7 ps, which is as fast as that observed for 〜 7.2 μm ISB-T in GaAs/AlGaAs QWs (〜 0.6 ps). The ISB relaxation time increases more rapidly than that predicted from the usual intersubband optical-phonon scattering model as the ISB-T energy increases. Our theoretical calculation, which takes into account not only intersubband scattering but also intrasubband energy relaxation process, agrees well with the experimental results. The intrasubband energy relaxation process is found to be important in the short wavelength region. To the next, we carried out the all-optical modulation experiment with sub-picosecond time resolution. Ultrafast modulation of the signal light has been clearly observed with relatively low control light pulse energy of 〜 5 fJ/μmィイD12ィエD1 the control light wavelength is tuned to the ISB absorption peak (7.2 μm). The modulation depth was 〜 8.5% (which corresponds to an increase of 1000cmィイD1-1ィエD1 in the IB absorption coefficient) and the modulation speed (measured from FWHM of the modulation curve) was as fast as 〜 1.3 ps. The modulation depth was observed to decrease when the control light wavelength was detuned from the ISB absorption peak, which clearly indicates that the modulation is induced by the ISB-T. The results show that the proposed modulation scheme is promising for ultrafast all-optical modulation or switching devices. Less

量子威尔斯（QW）中的子带间跃迁（ISB-T）由于其独特的物理性质（如超快弛豫时间）而在新型器件应用方面引起了广泛的关注。我们提出了一种利用n掺杂量子阱实现超快全光调制的方案，其中带间跃迁（IB-T）是通过使用ISB-T来调制的。在该方案中，调制速度被认为是由ISB-T激发的电子的弛豫时间所限制的，该弛豫时间预计与100 ps一样快。然而，直到最近，ISB-T的可用过渡波长仅限于中远红外区域（4 μm至20 μm），并且对于某些应用（例如用于光纤通信的全光调制器），强烈需要缩短过渡波长。最近，我们在GaAs衬底上相干生长了7-ML宽的InGaAs/AlAs多量子阱，实现了1.90 μm的ISB-T波长。然而，在我们以前的研究中，观察到AB ...更多信息随着阱宽度变窄而使跃迁波长变短，吸附量减小。为了使我们的超快全光调制器能够应用，需要将短波长ISB-Ts的吸收幅度提高到与传统波长ISB-Ts一样多。首先，我们回顾了以前的报告中提出的结果，并讨论了吸收减少的原因。载流子分布的理论计算表明，吸收的减少是由于载流子从InGaAs阱的最小值到AlAs势垒的X最小值的泄漏造成的。为了恢复吸收幅度，我们通过降低AlAs X最小值中的态密度和通过增加Γ和X最小值之间的能量间隔来抑制载流子泄漏，所述增加是通过（a）将阱的In组分从0.2增加到0.4，（B）通过将衬底温度从480 ~ 520 ℃降低到400 ℃来抑制生长过程中In的偏析;优化后的结构使吸收强度提高了12倍以上，峰值吸收系数达到14000 cm-2·d-1·d-1。本文的研究结果对短波子带间跃迁的器件应用具有重要意义。然后，我们对InGaAs/AlAs和AlGaAs/GaAs量子阱威尔斯进行了飞秒时间分辨测量。首先研究了InGaAs/AlAs量子威尔斯阱中短波长（~ 2.5 μm）子带间跃迁的电子弛豫动力学。飞秒抽运和探测测量得到的弛豫时间为2.7ps，与GaAs/AlGaAs量子阱中的7.2 μ mISB-T的弛豫时间（0.6ps）一样快。随着ISB-T能量的增加，ISB弛豫时间的增加比通常的子带间光学-声子散射模型预测的要快。我们的理论计算不仅考虑了子带间散射，而且考虑了子带内能量弛豫过程，与实验结果符合得很好。子带内的能量弛豫过程被发现是重要的，在短波长区域。接下来，我们进行了亚皮秒时间分辨率的全光调制实验。在较低的控制光脉冲能量为1.5fJ/μm的情况下，观察到信号光的超快调制，控制光的波长被调谐到ISB吸收峰（7.2 μm）。调制深度为108.5%（这对应于IB吸收系数增加1000 cm λ D 1 -1 λ D 1）并且调制速度（从调制曲线的半高宽测量）快至101.3ps。当控制光波长与ISB吸收峰失谐时，观察到调制深度减小，这清楚地表明调制是由ISB-T引起的。结果表明，该调制方案在超快全光调制或开关器件中具有良好的应用前景。少