To obtain an arbitrary polarization in three dimensions is of significant interest for many applications. Here, we demonstrate a configuration to achieve it. After phase coding by binary phase plates, two cylindrical polarized beams are superposed and focused by a high-numerical aperture lens to obtain transversally and radially polarized components in the focus vicinity. By adjusting their intensity and phase difference, the desired three-dimensional polarization can be achieved. The proportion of the desired polarization is extremely high and the focus spot is smaller than the diffraction-limited spot size.
We introduce three types of analog optical accelerators for enhancing the performance of electronic computing. These include (1) physical computing using complex optical dynamics in silicon for acceleration of scientific computing, (2) spectrotemporal processing and sparse coding using dispersion basis functions and (3) photonic computing primitives for performing nonlinear mathematical operations.
Autonomous vehicles and robots, key components of the 4th industrial revolution, must be able to observe their 3D environment in realtime. Currently, the 3D video camera using time-of-flight imaging is the most viable solution however raster speeds are currently limited by the speed of mechanical scanners or by the wavelength tuning speed of pulsed lasers. By adapting techniques from ultrafast time-stretch imaging, a new Lidar platform scans orders of magnitude faster than today's commercial line-scanning pulsed-Lidar systems.
Photonic time-stretch has established world's fastest real-time spectrometers and cameras with applications in biological cell screening, tomography, microfluidics, velocimetry and vibrometry. In time-stretch imaging, the target's spatial information is encoded in the spectrum of the broadband laser pulses, which is stretched in time and then detected by a single-pixel detector and digitized by a real-time ADC, and processed by a CPU or a dedicated FPGA or GPU. In time-of-flight LiDAR measurement, the maximum detectable distance scales with the temporal duration of the chirped illumination source. The bearing angle is proportional to the bandwidth of the source. In order to have a large detection angle and depth, a large chirp-bandwidth product is required. Various methods have been proposed to generate a chirped output to realize time-stretch, including single mode fibers, dispersion compensating fibers, chirped Bragg grating, and chromo-modal dispersion (CMD). But none of those methods provide the chirped source with a large time-bandwidth product. Moreover, the chirp profile and the operating wavelength can be changed with minimum freedom in those methods. In this study, we demonstrate the discrete time-stretch method that can generate the giant time-bandwidth product with arbitrary nonlinear chirp at operating wavelength from the visible to the infrared. A chirped pulse train with chirp time-bandwidth product at the order of 106 is easily feasible, rendering time-of-flight imaging of long-ranging distance and large bearing angle possible. We show its application in spectral-temporal LIDAR with the foveated vision at MHz refresh rate.
Constructing stable electrode/electrolyte interphase with fast interfacial kinetics is vital for fast-charging batteries. Herein, we investigate the interphase that forms between a high-voltage Na3 V2 (PO4 )2 F3 cathode and the electrolytes consisting of 3.0, 1.0, or 0.3 M NaClO4 in an organic carbonate solvent (47.5 : 47.5 : 5 mixture of EC: PC: FEC) during charging up to 4.5 V at 55 °C. It is found that a higher anion/solvent ratio in electrolyte solvation structure induces anion-dominated interphase containing more inorganic species and more anion derivatives (Cx ClOy ), which leads to a larger interfacial Na+ transport resistance and more unfavorable gas evolution. In comparison, a low anion/solvent ratio derives stable anion-tuned interphase that enables better interfacial kinetics and cycle ability. Importantly, the performance of a failed cathode is restored by triggering the decomposition of Cx ClOy species. This work elucidates the role of tuning interphase in fast-charging batteries.
The two-dimensional free boundary problem for a surface dissolving in potential flow owing to concentrated sources of dissolving agent c is formulated and solved. The surface boundary evolves quasi-steadily, and the resulting steady advection–diffusion equation for c, and Laplace's equation for the velocity potential form a coupled pair of conformally invariant PDEs. The dynamics of the coupled fluid flow and evolving surface depends on the Péclet number: Pe=UL/D, where U and L are typical velocity and lengthscales respectively, and D is the diffusivity of c. The conformally invariant property is exploited in finding an equation of the Polubarinova–Galin class for the conformal map from the unit ζ-disk to the evolving domain in physical space. The equation is solved numerically and the time-evolution of the dissolving surface determined. For a single concentrated source with flow initially parallel to a flat surface, a Pe-dependent scallop-like surface shape typically develops. The problem involving a periodic array of concentrated sources aligned parallel to an initially flat surface is also solved.
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