Ultrafast optical imaging is typically measured in picoseconds, whereas high-speed electronic cameras image at the millisecond and microsecond timescale. To bridge the gap between these technologies, a research team at the University of Tokyo developed a technology they’re calling “spectrum circuit,” a precision optical circuit that allows superfine images to be taken over multiple timescales at high speed. Using the new technology, the researchers demonstrated nanosecond photography as well as ultrawide-time-range, high-speed photography. Spectrum circuit produces nondamaging laser pulses that are set to emit at different time intervals. In addition to tunable time intervals, the pulse trains produced by spectrum circuit have short pulse durations and high temporal accuracy. Pulse manipulation is performed in free space while retaining the optical power and short pulse duration. An underwater shock wave moving through a HeLa cell. Using their spectrum circuit technology, the researchers could see the difference between how the shock wave moved inside and outside of a cell submerged in water. They noted that the results suggested that the cell structure shifts with the visualized wavefront position (shown in the red/orange line in the image). Courtesy of Saiki et al., 2023. The researchers combined spectrum circuit with a single-shot optical imaging technique called sequentially timed all-optical mapping photography (STAMP). Using spectrum circuit to provide a time-stretch approach at the nanosecond timescale, the researchers achieved superfine-exposure, nanosecond-scale photography based on STAMP. The single-shot, nanosecond photography demonstrated high spatial resolution, high signal-to-noise ratio, and a superfine-exposure time about 103 times shorter than the frame interval. The researchers realized single-shot, multi-timescale, ultrafast photography by combining spectrum circuit and STAMP with other photonic time-stretch techniques to capture femtosecond-nanosecond dynamics, and with high-speed cameras as detectors to capture microsecond-millisecond dynamics. Although the researchers used spectrum circuit with STAMP, use of the technology could be extended to applications like burst pulse generation and spectroscopy by optimizing each parameter design or optics. A diverse range of ultrafast light sources with broad bandwidths could be used, including supercontinuum white light lasers, mid-infrared lasers, and ultraviolet lasers. The researchers attribute the versatility of spectrum circuit to its free-space operation based on a combination of dispersive optics and mirrors. The researchers demonstrated the new imaging technique by photographing a microscopic shock wave passing through a single cell. They captured the shock wave propagating through a biological cell with a 1.5-ns frame interval and 44-ps exposure time, while suppressing image blur and avoiding damage to the cell. “For the first time in history, as far as we know, we have directly observed the interaction between a biological cell and a shock wave, and experimentally demonstrated that the velocity of the shock wave propagating inside the cell is faster than the outside of the cell,” researcher Takao Saiki said. “Furthermore, our approach has enabled us to demonstrate high-speed photography across a wide time range, which includes picosecond (one-trillionth of a second), nanosecond (one-billionth of a second) and millisecond (one-thousandth of a second) timescales.” Images of laser ablation taken using the ultrawide-time-range, high-speed camera. Applying this new imaging technology, the researchers could see the propagating shock wave and plasma and the progress of laser processing over multi-timescales (~10 to 100 ps, ~1 to 10 ns, and ~1 to 100 ms). Courtesy of Saiki et al., 2023. The researchers used the same approach to visualize the effects of laser ablation on glass. They focused an ultrashort laser pulse — just 35 fs long — onto a glass plate. Using spectrum circuit, they observed the resulting shock waves and the laser’s effect on the glass. They visualized femtosecond laser processing over multiple timescales (25-ps, 2.0-ns, and 1-ms frame intervals), and showed that the plasma generated at the picosecond timescale affected subsequent shock wave formation at the nanosecond timescale. “We could see the interplay between different physical processes taking place over time, and how they took shape,” said Keiichi Nakagawa, professor at the University of Tokyo. “Our technology provides opportunities to reveal useful, but unknown, high-speed phenomena by enabling us to observe and analyze such ultrafast processes.” Nakagawa said that the team plans to use the new imaging technique to visualize how cells interact with acoustic waves like those used in ultrasound and shock wave therapy. “By doing this, we aim to understand the primary physical processes that activate subsequent therapeutic effects in the human body,” he said. The team also wants to use spectrum circuit to improve laser processing techniques, by identifying the physical parameters that could enable faster, more precise, more consistent, and more cost-effective manufacturing. "We expect to make broad contributions in various fields, from biomedicine to manufacturing, materials, the environment, and energy,” Nakagawa said. The research was published in Science Advances (www.doi.org/10.1126/sciadv.adj8608).
