Picosecond-resolution phase-sensitive imaging of transparent objects in a single shot

Abstract

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.

Fig. 1. pCUP system configuration.

BB, beam block; BS, 90:10 (reflection/transmission) beam splitter; CL₁, 1000-mm focal length cylindrical lens; CL₂, 500-mm focal length cylindrical lens; CMOS, external CMOS camera (Grasshopper3, FLIR); DM, 805-nm short-pass dichroic mirror; DP₁ and DP₂, dove prisms (PS995, Thorlabs); HWP₁, 1064-nm half-wave plate; HWP₂, 800-nm half-wave plate; L_C1 and L_C2, 150-mm focal length lens; L_F, 75-mm focal-length Fourier lens; L_T, 200-mm focal-length tube lens; M₁, right-angle prism mirror; M₂, knife-edge right-angle prism mirror; M_c, 12.7-mm-diameter mirror; Ob, objective lens [Olympus, Plan N 20×/0.4 NA (numerical aperture)]; P, linear polarizer; S, sample; SO, stereoscope objective (MV PLAPO 2XC, Olympus); IP, the image plane of the dark-field microscope or the entrance plane of the LLE-CUP system. Inset: Dove prism flips the entering image along the y′-axis, thus, y″ = − y′. In the main figure, DP₁ flips the image along the x axis, and DP₂ flips the image along the y axis.

Fig. 2. pCUP imaging of 50-nm SiO₂ beads in immersion oil.

(A) Five representative frames from a 270-frame sequence imaged with a 532-nm, 5-ns pulsed laser at a frame rate of 20 Gfps. Scale bar, 30 μm. (B) Average normalized intensity over the three beads plotted over the entire imaging duration. At five representative points, the ranges from the minimum to the maximum intensities are shown using error bars. The dashed line shows a Gaussian fit of the normalized intensity, yielding an FWHM of 5.1 ns.

Fig. 3. pCUP imaging of optical Kerr effect inside a BGO crystal at 1 Tfps.

(A) Four representative frames from the 50-frame reconstructed sequence captured over 50 ps at 1 Tfps. The traveling pump pulse induces local refractive index changes in the BGO crystal and, thus, creates a phase delay on the imaging pulse. Scale bar, 2 mm. (B) The x position of the centroid of the phase signal for each frame is plotted. The dashed line shows the x-t relation for the speed of light in the BGO crystal, which agrees well with the measurement plot for the frames in the middle of the sequence, where the entire pump laser pulse is captured in the field of view.

Fig. 4. pCUP imaging of laser-induced shock wave propagation in water.

(A) Nine representative frames captured from four 10-ns sequences. The four sequences span from 0 to 10 ns, 10 to 20 ns, 25 to 35 ns, and 37 to 47 ns. The images show both the expanding shock wave front and the cavitation bubble in the middle. The time on each image represents the time passed from the initial shock wave generation. Scale bar, 50 μm. (B) Plot of the radial distance of the shock wave front from the center of the shock wave. The distance was measured by averaging 16 radial profiles for each frame. The dashed line represents the radial distance of the shock wave predicted by a model that agrees well with the measurement (47, 48).