Significance Asynchronous optical sampling (ASOPS) based on pump- probe spectroscopy is a powerful technique for ultrafast optical measurements. It leverages linear scanning between different pulse sequences of dual- comb lasers to enable equivalent sampling of pump pulse signals by delayed probe pulses. This allows for the observation of transient responses in samples on a femtosecond or even sub-femtosecond scale. Traditional electronic systems are limited by the bandwidth of detector circuits, rendering them unable to directly detect instantaneous processes on picosecond or femtosecond time scale. Optical sampling technology overcomes this limitation by converting temporal resolution to spatial resolution, significantly expanding the effective bandwidth for sampling ultrafast processes. By utilizing equivalent sampling on ultrafast time scales, it reconstructs signals bypassing the limitations of electronic detection bandwidth, providing a simple and effective method for studying photoinduced transient properties of materials, such as carrier dynamics, surface acoustics, and ultrafast magnetodynamics. In addition, with scanning rates up to the kHz range, ASOPS pump- probe spectroscopy translates ultrafast behavior into a series of frames observable by humans, facilitating detailed analysis. Compared to other microscale spectroscopy techniques, such as electron spectroscopy, high-energy X-ray technology, and particle scattering spectroscopy, pump- probe spectroscopy requires less stringent experimental environments and sample preparation, making it a widely used in fields like femtosecond chemistry, physics, and biology. Progress Conventional ASOPS requires sampling across the entire pump pulse cycle, typically resulting in window lengths of nanoseconds. Although different transient processes require different time windows, most only need one or two hundred picoseconds to meet observation requirements, leading to a low duty cycle and a significant portion of irrelevant acquired data. Furthermore, the timing jitter caused by the laser drift of repetition rate on the femtosecond time scale is significant and complex active frequency stabilization devices are required, therefore increasing system cost and complexity. To improve sampling efficiency and address these limitations, researchers have developed various techniques based on traditional ASOPS in recent years, including electronically controlled optical sampling (ECOPS), optical sampling by cavity tuning (OSCAT), arbitrary detuning asynchronous optical sampling (AD-ASOPS), heterodyne interferometry via rep- rate exchange (PHIRE), variable repetition frequency ASOPS (VRF-ASOPS), and hybrid ASOPS. ECOPS enhances the effective duty cycle by actively manipulating the repetition rate. It uses two synchronized femtosecond lasers with identical repetition rates. A tunable phase signal modulates the probe laser while the pump laser maintains a constant repetition rate. This allows detection pulses to effectively " walk " across the pump pulse in the time domain through alternating modulation, enabling rapid and adaptable optical sampling. Since ECOPS involves undersampling at nonlinear intervals, interpolation for unequal time steps is necessary when frequency domain data is needed. Initial calibration is also required to adjust the scanning zero behavior. OSCAT offers a more cost-effective and compact solution compared to ASOPS and ECOPS. It uses a single femtosecond laser, splitting the same pulse into pump and probe beams with one beam experiencing a fixed delay introduced by a long fiber. The scanning range is determined by the repetition frequency tuning range and this fixed delay. While OSCAT has advantages in cost and compactness, its flexibility for different applications is limited by predefined scanning windows, and the asymmetry introduced by long fibers often requires additional dispersion compensation. AD-ASOPS employs two independent, free- running lasers for pump and probe, operating at different and potentially unstable repetition rates. It continuously monitors and evaluates the instantaneous frequency of the two lasers, identifying coincidence events where the pulses overlap. The result is reconstructed from these events through post- calibration, avoiding the complexity of traditional asynchronous sampling devices but introducing statistical data and longer sampling times. PHIRE is similar to OSCAT in that both input the laser output of a single comb into a Michelson interferometer with asymmetric fiber and use delay accumulation from long fibers to generate scanning pulse pairs. PHIRE inserts a modulator in the other arm to quickly switch repetition rates. VRF- ASOPS detects the instantaneous hot- electron excitation generated from a trigger sample, such as 100 nm platinum film, converting its corresponding repetition- frequency difference to response pulse pairs. This enables ASOPS measurement without stabilizers or feedback loops. The temporal resolution changes by adjusting the intracavity PZT. Hybrid ASOPS utilizes the high repetition frequency of the soliton microcomb to achieve parallel multi- point sampling of the frequency waveform generated by the measured radiation signal. This improves the acquisition rate without sacrificing resolution. The time- resolved spectra of hybrid ASOPS combine high resolution, high speed, and broad bandwidth, making it a powerful tool for exploring the complexities of materials at ultrafast timescales. Conclusions and Prospects Recent years have seen increased attention to combining ultrafast spectroscopy and ultrafast microscopy. Ultrafast microscopy technology is based on the development of spectroscopy technology to study temporal and spatial dynamics simultaneously. Ultrafast microscopy, using ultra- short pulse sequences, provides high time sensitivity and high spatial resolution, key for studying zero- dimensional and one-dimensional materials, such as single molecules, metal nanoparticles, semiconductor quantum dots, or carbon nanotube, as well as local regions of two- dimensional heterogeneous chemical systems, such as organic semiconductor films, polycrystalline perovskite films, and biological media like cells or chromophores. With the mature development of ultrafast pump- probe spectroscopy, new ultrafast spectroscopy or microscopy technologies have emerged. These include the new time- domain stretch spectrometer combining dual- frequency- comb pump- probe technology and time- domain stretch spectrometer, attosecond-pump attosecond-probe based on full X-ray with sub-femtosecond resolution for understanding electron dynamics in quantum systems, and attosecond transient absorption caused by infrared pump and X-ray detection, paving the way for new imaging techniques in attosecond condensed matter system. The development of electrical phase- locking technology and optical frequency comb technology is pushing ASOPS pump- probe system towards miniaturization and lightweight, making them more reliable and cost-effective, and laying a solid foundation for further system improvements and developments.
