Li C, Gusev V, Dekorsy T, Hettich M
Opt Express. 2019 Jun 24;27(13):18706-18730. doi: 10.1364/OE.27.018706.
We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a InGaAs/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.
我们展示了超快光学在皮秒激光超声学应用方面的一项进展——基于两台2 GHz的Yb:KYW激光器的高速异步光学采样（ASOPS）系统，在InGaAs/GaAs多量子阱（MQW）结构中对激光诱导的梳状相干声子进行光学控制。来自同一泵浦激光器的两个连续脉冲用于激发MQW结构。第二个泵浦光脉冲相对于第一个泵浦光脉冲具有可调的时间延迟，并且其强度也可以调节，这使得能够对声子进行幅度和相位调制。这产生了丰富的时间声学模式，包括幅度被抑制或增强、各种波包形状、不同的波包宽度、减小的波包周期以及波包内单周期振荡的不同相移。在频域中，各个梳状分量的幅度和相移分别呈现出与第二个泵浦延迟相关的余弦波状和锯齿波状变化，调制频率等于梳状分量频率本身。通过调节第二个泵浦强度来改变各个分量的幅度和相移，在特定时间延迟下，当两个泵浦脉冲强度之比约为1:1时，会出现幅度谷和突然的相位跳跃。为了解释MQW样品中的声学操控，我们建立了一个简化模型，其中假设在无限周期MQW结构中，产生和检测功能均为高斯或矩形包络的余弦应力波。该模型在很宽的时间延迟和强度比范围内与实验结果吻合良好。此外，通过应用启发式分析方法和非线性校正，改进后的计算结果与实验结果达到了极佳的一致性，从而能够预测和合成MQW结构中的相干声波模式。
