Single-shot compressed ultrafast photography: a review

Introduction

Recently, a research group led by Prof. Shian Zhang and Prof. Zhenrong Sun from State Key Laboratory of Precision Spectroscopy, East China Normal University, published an invited review entitled "single-shot compressed ultrafast photography: a review" in Advanced Photonics (Dalong Qi, Shian Zhang, Chengshuai Yang, Yilin He, Fengyan Cao, Jiali Yao, Pengpeng Ding, Liang Gao, Tianqing Jia, Jinyang Liang, Zhenrong Sun, Lihong V. Wang. Single-shot compressed ultrafast photography: a review[J]. Advanced Photonics, 2020, 2(1): 014003). In this mini-review, the authors gave a comprehensive introduction on compressed ultrafast photography (CUP), including the background, work principles, technical improvements, technical extensions, related applications, and future prospects.

Background

The birth of transient optical imaging can be traced back to the 1870s, pioneered by a legendary American photographer E. Muybridge, who captured a galloping horse, as shown in Fig. 1(a). Nowadays, scientists have visualized the ultrafast lattice vibrational wave in a single exposure, which is shown in Fig. 1(b). This made the single-shot ultrafast imaging technology show irreplaceable advantages in the face of unrepeatable and irreversible dynamics that traditional pump-detection technology can hardly deal with, such as optical rogue waves, irreversible structural dynamics in chemical reactions, and shockwave generation in inertial confinement nuclear fusion.

Figure 1 (a) Capture of a galloping horse, and (b) ultrafast lattice vibration visualized by a single-shot ultrafast optical imaging technique.

Principle

Among the various single-shot ultrafast imaging methods, the compressed ultrafast photography (CUP) equipped an ultrafast temporal resolution and a high data compression ratio by combining the compressed sensing (CS) with a streak camera. The CUP has achieved the world record of an imaging speed of 10 trillion frames per second (Tfps) and hundreds of frames in a single exposure. The experimental setup is schematically shown in Fig. 2(a), where the dynamic scene was transferred to the digital micromirror device (DMD) for spatial random encoding by an imaging system. Then the encoded scene was reflected and entered into a wide-open streak camera for temporal-spatial conversion and integration to form a two-dimensional image E(m, n). In the image reconstruction, a variety of optimization algorithms were employed to estimate the original I(x, y, t) from the measured E(m, n), such as two-step iterative shrinkage/thresholding (TwIST) and augmented Lagrangian (AL) algorithms. Figure 2(b) shows the optical Mach cone captured by the CUP system with an imaging speed of 10 Tfps.

Figure 2 (a) The schematic diagram of CUP, and (b) the optical Mach cone captured by CUP.

Extensions

Based on CUP, the CS has been further applied to many different fields to compensate for their respective limitations. Here were a few examples. First, the compressed ultrafast spectrum-temporal (CUST) photography utilized a chirped femtosecond laser pulse as the active illumination, and employed the CS to achieve an imaging speed of 3.85 Tfps and a spectral resolution of 0.25 nm (Fig. 3(a)). Furthermore, the theoretical verifications of compressed ultrafast electron diffraction imaging and microscopy extended this modality from the photons to the electrons (Fig. 3(b)). In addition, the compressed optical-streaking ultrahigh-speed photography (COSUP) realized the all-optical design by replacing the streak camera with a galvanometer scanner (Fig. 3(c)). These extensions have greatly advanced the applications of CUP like techniques.

Figure 3 The schematic diagrams of (a) CUST photography, (b) compressed ultrafast electron diffraction and microscopy, and (c) COSUP.

Applications

The original and updated CUP systems have been widely used to observe complex ultrafast phenomena and transient scenes. The applications included but not limited in the capture of flying photons in reflection and refraction (Figs. 4(a) and (b)), the continuous 3D surface detection (Fig. 4(c)), the spatiotemporal measurement of ultrashort pulses, and the secure transmission of image information.


Figure 4 Flying photons in (a) reflection and (b) refraction, and (c) continuous imaging of 3D objects.

Prospects

In recent years, CUP has made great progress. However, it still lags in spatial resolution or pixel counts per frame. In terms of hardware, electro-optical deflectors and spectral components have been introduced to enrich the capabilities. In terms of software, more intelligent algorithms are in urgent demands to further improve the stability and spatial resolution of reconstructed images. Due to its characteristics of passive imaging, CUP is easy to combine with microscopic or astronomical platforms to explore ultrafast dynamic processes in a wider range of spatial scales, which needs to be further studied by researchers.