With the growing interest in the optical imaging of ultrafast phenomena in transparent objects, from shock wave to neuronal action potentials, high contrast imaging at high frame rates has become desirable. While phase sensitivity provides the contrast, the frame rates and sequence depths are highly limited by the detectors. Here, we present phase-sensitive compressed ultrafast photography (pCUP) for single-shot real-time ultrafast imaging of transparent objects by combining the contrast of dark-field imaging with the speed and the sequence depth of CUP. By imaging the optical Kerr effect and shock wave propagation, we demonstrate that pCUP can image light-speed phase signals in a single shot with up to 350 frames captured at up to 1 trillion frames per second. We expect pCUP to be broadly used for a vast range of fundamental and applied sciences.
Since its first appearance, phase-sensitive imaging methods, such as phase contrast, differential interference contrast, and dark-field imaging, have completely changed the way we study transparent objects by rendering the phase delay caused by the object without using any exogenous contrast agents, such as fluorescence tags (1–3). The application of phase imaging covers a vast range of fields, including biological microscopy, optical metrology, and astronomy (4–10). Recent advances in phase imaging have also reached a breakthrough where these imaging techniques can now break the diffraction limit and achieve high-resolution unlabeled imaging of transparent objects in three dimensions (3D) (11–18). By challenging the limits of imaging, phase imaging has now become essential for new scientific discoveries, especially in biological sciences, by allowing label-free optical detection of nanoscale subcellular activities (19–24).
Following the previous advances in contrast, resolution, and 3D imaging capability, attempts have been made to improve the speed of phase imaging for the potential applications in studying a variety of ultrafast events, such as ultrashort laser pulse’s propagation, laser-induced damages, and shock wave (25–32). Moreover, with the growing interest in optical detection of neuronal action potentials, the field of phase imaging has started to seek a marked improvement in speed to match the propagation speed of neuronal action potentials (33–35). Recently, several techniques have succeeded in detecting ultrafast phase signals, including the light-in-flight recording by digital holography (LIF-DH), the time-resolved holographic polarization microscopy (THPM), and the ultrafast framing camera (UFC) (36–40). Although these techniques achieve high–frame rate imaging, their sequence depths (i.e., the number of frames per movie) are limited by several factors, such as the number of imaging pulses (THPM), the trade-off between the sequence depth and the field of view (LIF-DH), and the number of array detectors (UFC). The typical sequence depths reported for these techniques are 16 frames per movie at the maximum.
To overcome these limitations and to achieve ultrafast phase imaging that is capable of real-time imaging of ultrafast events, we present the phase-sensitive compressed ultrafast photography (pCUP) system, which combines the phase-sensitive dark-field imaging technique with CUP (41). CUP is based on the compressed sensing theory and the streak camera technology to achieve receive-only single-shot ultrafast imaging of up to 350 frames per event at 100 billion frames/s (Gfps). Since CUP operates as a passive detector, it can be coupled to many optical imaging systems (42–45). By combining CUP with dark-field microscopy, we show that pCUP can image ultrafast phase signals with a noise-equivalent sensitivity of 3 mrad and at an improved speed of 1 trillion frames/s (Tfps). We also demonstrate the ultrafast real-time phase imaging capability of pCUP by imaging three different events: phase signals from transparent 50-nm-diameter SiO2 beads in immersion oil, traveling phase signals induced by the optical Kerr effect in a crystal, and propagating phase signals caused by laser-induced shock wave in water.
pCUP consists of two parts, a dark-field microscope system and an upgraded lossless-encoding CUP (LLE-CUP) detection system (Fig. 1) (43, 44). The dark-field imaging has been achieved by blocking the unscattered light at the Fourier plane using a beam block built from an anodized aluminum disc attached to a glass coverslip. The size of the beam block depends on the magnification of the system used for different experiments, and for each experiment, the block size is selected to provide the maximum signal-to-background ratio (SBR) and signal-to-noise ratio (SNR) (detailed in the Supplementary Materials). In particular, a pump pulse (represented as the red and magenta beam paths in Fig. 1) generates a transient phase dynamics ϕ(x, y; t). Here, x and y denote the transverse Cartesian coordinates, and t denotes time. The transient event is probed by an imaging pulse (represented as the green beam path) with an incident intensity I0(x, y; t). The details of the pump and probe pulses will be specified in the following sections in Results. The intensity distribution of a dark-field image is represented by I(x, y, t) = I0(x, y; t)[1 − cos ϕ(x, y; t)] (detailed in the Supplementary Materials). The dark-field image suppresses the background and the associated fluctuation, and the enhanced spatial sparsity is ideal for CUP. The signal from the dark-field microscope is separated through a 10:90 (transmission/reflection) beam splitter to form images at the external complementary metal-oxide semiconductor (CMOS) camera and at the entrance plane of the LLE-CUP system. The external CMOS camera captures a time-unsheared (i.e., time-integrated) view. The image formed at the entrance plane of the LLE-CUP system is then relayed to a digital micromirror device (DMD; DLP LightCrafter, Texas Instruments), with a ×1/3 magnification, and split into two complementary views, generated by the pseudorandom binary pattern loaded onto the DMD. The two complementary views are collected by the stereoscope objective and then are passed through two dove prisms with a 90° rotation from each other. The dove prisms flip one of the views in the x direction and the other in the y direction (Fig. 1), and thus, the two views are 180° rotated from each other. Therefore, in the streak camera, the two views experience shearing in opposite directions relative to the image coordinates to provide an improved lossless encoding.