Abstract
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 In0.27Ga0.73As/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.
Highlights
Owing to the rapid progress in the field of ultrafast lasers, the excitation and detection of short coherent acoustic phonons (CAPs) have become possible and have been intensively studied in various materials via ultrafast time-resolved spectroscopy [1,2,3,4,5,6,7,8]
We have described and discussed CAPs excited in the same multiple quantum well (MQW) structure by a single-pump-pulse asynchronous optical sampling (ASOPS) system based on two Yb:KYW lasers, where an acoustic wavepacket temporal sequence was produced, due to spatially periodically distributed triple-quantum wells (QWs) stacks [39,40]
The optical power of the second pump light is kept equal to that of the first one at 27 mW, the time delay ∆T between two pump pulses is adjustable in the range from 0 to 0.5T (5.6τ) which equals 15.12 ps in our case by moving a variable delay stage, where T and τ denote the period of the acoustic wave-packet sequence and the period of oscillations within the individual acoustic wave-packet, respectively
Summary
Owing to the rapid progress in the field of ultrafast lasers, the excitation and detection of short coherent acoustic phonons (CAPs) have become possible and have been intensively studied in various materials via ultrafast time-resolved spectroscopy [1,2,3,4,5,6,7,8]. The straightforward way to manipulating CAPs on a picosecond time scale is to do so in tailored nanostructures and materials, since the properties of laser-induced coherent picosecond acoustic phonons are strongly dependent on the nature of the constituent layers, their thicknesses, and periods of layered structures or thin-films [18]. Even though the above approaches for CAP manipulation have displayed their feasibilities and versatilities, many questions remain open While these questions could be answered by investigating a large number of different sample structures and using a large set of parameters for the variation of extrinsic factors, we decide to invest the control over CAP dynamics optically. To the best of our knowledge, such a system has been applied to investigate the multi-pulse optical coherent control of CAPs by ultrafast time-resolved spectroscopy for the first time, which potentially stirs new applications and continuing progress in laser ultrasonics
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