A flexible ultrasonic array transducer able to be attached to the body has the potential to achieve long-term continuous unconstrained ultrasound (US) imaging. However, the quality of reconstructed US images is affected by the accuracy of the estimated array shape because the array shape is primitive for time-delay calculation in delay-and-sum beamforming. In this study, we propose an algorithm for estimating the array shape only from the backscattered US signal without using any external device. The proposed algorithm is based on the assumption that beam-summed images reconstructed using an array shape estimated at higher accuracy would have smaller entropy. The array shape is estimated by searching for the shape with minimal entropy, which was used as the index of the beam-summed image quality. Simulation experiments and phantom experiments were used to evaluate the proposed algorithm. In the simulation experiments, three different array shapes with 2.0 MHz, 20 elements, and 0.8 mm pitch transducers were estimated. In the phantom experiments, the array shapes of commercially available linear, convex, and concave transducers were estimated. The results showed that the proposed algorithm can estimate the correct array shape with an average element position error of less than one-eighth of the wavelength of the transmitted signals. These results indicate that the proposed algorithm can achieve sufficiently accurate shape estimation and has the potential to enable clear US imaging with flexible array transducers.

Single-shot optical imaging based on ultrashort lasers has revealed nonrepetitive processes in subnanosecond timescales beyond the recording range of conventional high-speed cameras. However, nanosecond photography without sacrificing short exposure time and image quality is still missing because of the gap in recordable timescales between ultrafast optical imaging and high-speed electronic cameras. Here, we demonstrate nanosecond photography and ultrawide time-range high-speed photography using a spectrum circuit that produces interval-tunable pulse trains while keeping short pulse durations. We capture a shock wave propagating through a biological cell with a 1.5-ns frame interval and 44-ps exposure time while suppressing image blur. Furthermore, we observe femtosecond laser processing over multiple timescales (25-ps, 2.0-ns, and 1-ms frame intervals), showing that the plasma generated at the picosecond timescale affects subsequent shock wave formation at the nanosecond timescale. Our technique contributes to accumulating data of various fast processes for analysis and to analyzing multi-timescale phenomena as a series of physical processes.

We present sequentially timed all-optical mapping photography (STAMP) with a slicing mirror in a branched 4f system for an increased number of frames without sacrificing pixel resolution. The branched 4f system spectrally separates the laser light path into multiple paths by the slicing mirror placed in the Fourier plane. Fabricated by an ultra-precision end milling process, the slicing mirror has 18 mirror facets of differing mirror angles. We used the boosted STAMP to observe dynamics of laser ablation with two image sensors which captured 18 subsequent frames at a frame rate of 126 billion frames per second, demonstrating this technique’s potential for imaging unexplored ultrafast non-repetitive phenomena.

The production of ultrashort pulse trains is important for various applications, such as laser ablation,^{1 – 5} ultrafast imaging,^{6 – 10} and acoustic wave generation.¹¹ Laser-processing methods using pulse trains, such as double-pulse processing^1,4 and burst processing,^2,3,5 have been widely applied to improve processing accuracy and efficiency. In ultrafast imaging, pulse trains are used to capture the spatiotemporal dynamics of the optical properties of target objects.^{6 – 10} In acoustic wave generation, multicycle acoustic waves can be tuned in the GHz frequency range by changing the time interval of the pulse train absorbed by the transducer.¹¹ One of the typical pulse trains contains pulses of different wavelengths. In this spectrally separated pulse train, the spatiotemporal profiles, such as pulse duration, and wavefront of each pulse can be individually modulated by pulse-shaping techniques using a diffraction grating, lens, and spatial light modulator (SLM).^12,13 Such spatiotemporal pulse shaping is crucial for the electron dynamics control in laser ablation,^14,15 fluorescence imaging,¹⁶ and manipulation of terahertz signals,¹⁷ among others. In addition, a spectrally separated pulse train is essential as a probe for sequentially timed all-optical mapping photography (STAMP),⁶ which is a single-shot imaging technique that can capture two-dimensional burst images of ultrafast events. Therefore, producing spectrally separated ultrashort pulse trains and controlling their individual properties have promising implications in different applications.