Femtosecond laser technology has developed considerably in the past decades, promoting the progress of scientific areas such as ultrafast optics, strong field physics, super-resolution imaging, and precision measurement. In a typical femtosecond pump-probe experiment, the evolution dynamics of the microscopic structures of matter is excited and observed with micrometer to nanometer spatial resolutions and femtosecond to attosecond temporal resolutions. For a pump laser pulse, complete characterization of its spatiotemporal field distribution information such as pulse duration, dispersion, and wavefront distortion allows researchers to accurately control the laser-matter interaction process. For a probe pulse, the evolution history of the excited matter’s optical properties is encoded in the spatiotemporal amplitude and phase modulations of the probe pulse. Therefore, it is necessary to develop techniques for the spatiotemporal characterization of femtosecond laser fields in three dimensions, along two transverse spatial directions and one longitudinal temporal direction.
Conventionally, a series of femtosecond laser-pulse characterization techniques have been developed and most of them focus on measuring the "longitudinal" temporal profile of the laser field. For example, autocorrelation and frequency-resolved optical gating have matured and are widely applied worldwide in ultrafast optics laboratories. Moreover, commercial devices based on these techniques are available in the market. For obtaining the transverse spatial distribution information of a laser pulse, multiple techniques and devices are developed. Laser beam profilers are widely applied by academic and industrial users for transverse spatial intensity profiles of lasers. Phase information or wavefront distribution is obtained using wavefront sensors such as Shack-Hartmann devices. To obtain a three-dimensional spatiotemporal profile of a femtosecond laser field, the results of the longitudinal temporal profile characterization and the beam profile measurements are usually combined.
However, the conventional approach of combining the temporal characterization and spatial measurement results is not appropriate for complicated optical fields. First, with an increase in the peak power of femtosecond laser pulses to the petawatt level, the large-aperture optical elements in the petawatt laser facilities introduce severe spatially and temporally dependent optical field distortions. Such distortions are called the spatiotemporal coupling effect, and they have to be characterized simultaneously in the spatial and temporal domains. Second, complicated optical fields with optical vortices or structured polarization have attracted increasing attention of researchers in recent years and require advanced spatiotemporal characterization techniques. Third, some laser-driven short-wavelength light sources such as high harmonic radiations have spatiotemporal coupling nature. A complete characterization of their spatiotemporal profile enables studies on strong field physics in attosecond time scales.
Therefore, this paper aims at reviewing the recent progress in the spatiotemporal femtosecond laser-field characterization techniques, especially emphasizing their capability of simultaneously revolving two-dimensional spatial information and one-dimensional temporal information.
This paper is organized as follows. After a brief introduction in the first section, the second section reviews the techniques used to measure femtosecond laser pulses in the longitudinal time domain, as the traditional time-domain measurement techniques form the fundamentals of spatiotemporally resolved measurement techniques developed in recent years. According to the mechanism of the interaction between the pulse to be measured and the reference pulse in the time domain, the time-domain femtosecond laser-pulse measurement techniques are divided into three categories: intensity autocorrelation-based measurement (Fig. 1), frequency-domain interferometry-based measurement (Fig. 2), and phase modulation-based measurement (Fig. 3).
The third section is the most important part of this paper, which reviews the development of the spatiotemporally resolved femtosecond laser-field characterization techniques. We first discuss the applications of imaging spectrometers in different femtosecond laser spatiotemporal characterization techniques (Fig. 4). As the time-domain information is obtained from the spectral measurement, imaging spectrometers provide additional spatially resolved information simultaneously along the entrance slit direction. However, the entrance slit also blocks the light distributed along the direction perpendicular to it; therefore, almost all pulse spatiotemporal measurement techniques based on imaging spectrometers provide only one-dimensional lateral spatial information or two-dimensional spatial information by scanning the entrance position of the laser field on the slit in an unstable time-consuming way. Specific technologies include SEA-SPIDER, SRSI-ETE, and CROAK.
Alternative methods other than imaging spectrometers have been explored to measure the spectrum of the incident laser field. The related techniques are divided into two categories: multispectral and hyperspectral imaging methods. In multispectral imaging methods such as STRIPED-FISH and HAMSTER, a few spectral components of the incident femtosecond laser pulse are imaged and measured and the full spectrum is obtained through an interpolation scheme if the laser-pulse spectrum is simple. Figure 5 shows the principles and the experimental setup of the STRIPED-FISH technique as a typical multispectral imaging technique, and previous experimental results are shown. When the incident laser field has a complex spectral structure, hyperspectral imaging methods should be applied to resolve fine structures of the spectrum. Figure 6 shows the details of the typical hyperspectral imaging techniques, TERMITES and its derivative INSIGHTS, which adopt the principles of spatially resolved Fourier transform spectroscopy and measure the three-dimensional spatiotemporal amplitude and phase distributions of a femtosecond laser field in multiple shots. However, single-shot three-dimensional characterization of an optical field is still challenging.
Finally, we have briefly reviewed some techniques to determine the spatiotemporal coupling effect of a femtosecond laser pulse without obtaining the three-dimensional optical field distribution. These techniques are effective and simple in optical configurations, which are especially useful for pulse characterization of petawatt laser pulses in large laser facilities.
We expect to solve three problems in the near future. First, although the TERMITES and INSIGHT techniques can resolve the three-dimensional optical field information of a femtosecond laser pulse, single-shot measurement is yet to be achieved. Because of the development of the compressive sensing principle, an optical compressed imaging technique called CASSI (Fig. 7) provides a new idea of hyperspectrally measuring the spatial distribution of each spectral component in a single shot. However, CASSI can only obtain the intensity information of a laser pulse, leaving the phase distribution unknown. Based on CASSI, we have also proposed two global three-dimensional phase retrieval schemes, and we expect that the three-dimensional optical field distribution of an arbitrary femtosecond laser field should be determined in a single shot. Preliminary experimental results have justified our proposals.
Second, the application of spatiotemporal pulse characterization in ultrafast pump-probe experiments has been proposed. The evolution information of the optical properties of the pump laser-excited matter is encoded in the amplitude and phase modulations of the probe laser pulse; thus, a thorough characterization of the probe pulse reveals the dynamics of the femtosecond laser-matter interaction. So far, most of the probe characterization techniques are the multispectral imaging techniques, such as the STAMP and FINOPA techniques (Fig. 8), realizing the temporal resolving capability by linearly mapping the chirped probe spectral components to different time delays.
Finally, we have extended the femtosecond laser characterization techniques to the measurement of high harmonic generation. High harmonic generation is a type of laser-like coherent light source in the extreme ultraviolet or soft X-ray spectral range with small divergence, excellent spatial and temporal coherence, high brightness, and femtosecond or attosecond pulse duration. After reviewing the spatially integrated time-domain measurement techniques such as autocorrelation, RABBITT, and CRAB (or attosecond streaking camera), we have discussed the in situ measurement techniques for obtaining the spatiotemporal information of high harmonic radiation or isolated attosecond pulses (Fig. 9).
