Ultrafast laser pulses, with pulse widths as short as femtoseconds and even attoseconds, enable precise measurements with extremely high temporal resolutions. These capabilities have fostered numerous ultrafast spectroscopic detection and imaging techniques, establishing themselves as crucial tools in frontier photophysical research and applications. Several Nobel Prizes over the past 40 years underscore the promising future of ultrafast optics. However, the wave nature of ultrafast laser pulses imposes limitations on spatial resolution and penetration depth in certain scenarios. The optical diffraction limit of ultrafast laser pulses, for instance, sets an intrinsic physical constraint on spatial resolution. Additionally, strong optical absorption and scattering in opaque media restrict penetration depths to the nanometer scale. Therefore, it is imperative for the ultrafast optics community to develop complementary spatiotemporal imaging with ultrahigh resolution. Ultrafast acoustic pulses emitted during the interaction between ultrafast laser pulses and thin-film photoacoustic transducers hold promise for overcoming current limitations, which will open a new pathway for achieving spatiotemporal imaging with ultrahigh resolution. Firstly, the ultrahigh frequencies of these acoustic pulses, approaching THz, correspond to wavelengths below 10 nm, allowing for spatial resolutions in the order of a few nanometers. The mechanical wave nature of ultrafast acoustic pulses significantly strengthens their ability to penetrate opaque media. Secondly, these pulses can have extremely short durations, measured in picoseconds or even sub-picoseconds, enabling the functional imaging of buried interfaces and defects with unprecedented spatiotemporal precision. Thirdly, ultrafast acoustic pulses can be readily detected by ultrafast laser pulses, which provides a versatile approach for developing spatiotemporal imaging applicable across various materials and systems. Emerging sources of ultrafast acoustic pulses, such as 2D semiconductor heterostructures, show great potential for generating tunable ultrafast acoustic emissions. These sources offer a novel platform for studying interfacial photophysics and advancing the functional imaging capabilities of critical micro-devices and chips. Recent advancements in harnessing ultrafast acoustic pulses from 2D semiconductor heterostructures underscore their pivotal role in future studies of spatiotemporal imaging with ultrahigh resolutions. In this review, we will explore the history, recent progress, and future research opportunities in ultrafast acoustic pulses.

We review ultrfast acoustic pulses and discuss the challenge of controllable emission for ultrafast acoustic pulses. Firstly, we introduce optical emission schemes of ultrafast acoustic pulses (Fig. 1), where photoacoustic transducers of metal and semiconductor thin-film heterostructures are simultaneously excited by ultrafast laser pulses. We discuss the optical detection schemes of ultrafast acoustic pulses (Fig. 2), including the surface optical detection scheme of acoustic echoes in nontransparent media and the optical tracking detection scheme of acoustic traveling waves in transparent media. Secondly, we comprehensively present the history of ultrafast acoustic pulses, tracing it back to Bell’s pioneering exploration of the photoacoustic effect in 1880. Milestones of modern ultrafast acoustic pulse research are provided in detail (Fig. 3). Then, we point out the limitations of conventional photoacoustic energy materials, which pose a challenge for further advancements in this research field. Subsequently, we discuss the extraordinary photoacoustic energy conversion potential of 2D semiconducting materials. Recent progress and examples of coherent acoustic phonon oscillations in 2D semiconducting materials and heterostructures are presented (Figs. 4-6). These studies demonstrate that 2D semiconducting materials and heterostructures can serve as new ultrafast photoacoustic transducers capable of efficiently emitting ultrafast acoustic pulses with controllable wave parameters such as tunable wavelength, frequency, and pulse width. Next, recent experimental progress of ultrafast acoustic pulses employing 2D semiconducting heterostructures is further introduced (Figs. 7-8). The controllable emission properties of ultrafast acoustic pulses in 2D semiconducting heterostructures are of broad significance for realizing new ultrafast acoustic pulse emission sources, enabling functional imaging with ultrahigh spatiotemporal resolutions. Finally, we look ahead to the future development and applications of ultrafast acoustic pulses.

As a unique carrier of ultrafast energy and information, ultrafast acoustic pulses offer physical advantages such as ultrashort pulse width, nanometer wavelength, and high penetration depth compared to ultrafast laser pulses. When combined with ultrafast optical spectroscopic techniques, fundamental studies and applications of ultrafast acoustic pulses can boost the interdisciplinary development of ultrafast optics. Although 2D layered semiconducting materials have presented great potential in constructing efficient and controllable sources of ultrafast acoustic pulses, the physical mechanism of photoacoustic conversion in van der Waals heterointerfaces and ultrafast measurement techniques needs comprehensive demonstration. Achieving ultrafast acoustic pulses with wavelengths in the nanometer range and pulse widths in the sub-picosecond domain presents challenges in developing corresponding spatiotemporal imaging techniques and instruments. We believe that research into controllable emission of ultrafast acoustic pulses will be applied to the noninvasive evaluation of micro-devices and chips, which will eventually pave the way for functional imaging with ultrahigh spatiotemporal resolutions.