Deep learning convolutional neural networks generally involve multiple-layer, forward-backward propagation machine-learning algorithms that are computationally costly. In this work, we demonstrate an alternative scheme to convolutional neural nets that reconstructs an original image from its optically preprocessed, Fourier-encoded pattern. The scheme is much less computationally demanding and more noise robust, and thus suited for high-speed and low-light imaging. We introduce a vortex phase transform with a lenslet-array to accompany shallow, dense, “small-brain” neural networks. Our single-shot coded-aperture approach exploits the coherent diffraction, compact representation, and edge enhancement of Fourier-transformed spiral phase gradients. With vortex encoding, a small brain is trained to deconvolve images at rates 5–20 times faster than those achieved with random encoding schemes, where greater advantages are gained in the presence of noise. Once trained, the small brain reconstructs an object from intensity-only data, solving an inverse mapping without performing iterations on each image and without deep learning schemes. With vortex Fourier encoding, we reconstruct MNIST Fashion objects illuminated with low-light flux ( 5 nJ / cm 2 ) at a rate of several thousand frames per second on a 15 W central processing unit. We demonstrate that Fourier optical preprocessing with vortex encoders achieves similar accuracies and speeds 2 orders of magnitude faster than convolutional neural networks.

Nonmagnetic media can be magnetized by light via processes referred to as an inverse Faraday effect (IFE). With nonmagnetic metal nanostructures, the IFE is dominated by the presence of light‐induced solenoidal surface currents or plasmons with orbital angular momenta, whose properties depend on both the light and nanostructure geometry. Here, through a systematic study of gold nanodisks with different sizes, we demonstrate order‐of‐magnitude enhancement of the IFE compared to a bare gold film. Large IFE signals occur when light excites the dipolar plasmonic resonance of the gold nanodisk. We observe that the spectral response of the IFE signal mirrors the spectral response of time‐dependent thermo‐transmission signals. Our careful quantitative experimental measurements and analysis offer insight into the magnitude of IFE in plasmonic structures for compact, low‐power, magneto‐optic applications.

There is wide interest in understanding and leveraging the nonlinear plasmon-induced potentials of nanostructured materials. We investigate the electrical response produced by spin-polarized light across a large-area bottom-up assembled 2D plasmonic crystal. Numerical approximations of the Lorentz forces provide quantitative agreement with our experimentally-measured DC voltages. We show that the underlying mechanism of the spin-polarized voltages is a gradient force that arises from asymmetric, time-averaged hotspots, whose locations shift with the chirality of light. Finally, we formalize the role of spin-orbit interactions in the shifted intensity patterns and significantly advance our understanding of the physical phenomena, often related to the spin Hall effect of light.

We experimentally, analytically, and numerically demonstrate the nonlinear photo-induced plasmon-assisted magnetic response that occurs with metallic nanoparticles in aqueous solution. We measure the scattered spectra from solutions of gold nanospheres (10⁻⁷ fill factor) and observe appreciable changes when simultaneously applying DC magnetic fields and illuminating samples with light. The magnetic response is achieved using light from a solar simulator at unprecedented low illumination intensities (< 1W/cm²) and is sustained when the magnetic field is removed. Distinctly different behavior is observed depending on the circular-polarization handedness given a fixed magnetic field. Nanoparticle aggregation is more likely to occur when the circular-polarization trajectory opposes the solenoid current that produces the magnetic field. Using Mie’s theoretical solution, we show how vortex orbital surface currents lead to an increased and anisotropic electrical conductivity, which shifts the scattered spectra in agreement with experimental results. The single-nanoparticle plasmon-induced magnetization, which couples the scattered and incident electric fields, changes sign with orthogonal circular-polarization handedness.

The inverse Faraday effect is an optomagnetic phenomenon that describes the ability of circularly polarized light to induce magnetism in solids. The capability of light to control magnetic order in solid state materials and devices is of interest for a variety of applications, such as magnetic recording, quantum computation, and spintronic technologies. However, significant gaps in understanding about the effect persist, such as what material properties govern the magnitude of the effect in metals. In this work, we report time-resolved measurements of the specular inverse Faraday effect in nonmagnetic metals, i.e., the magneto-optic Kerr effect induced by circularly polarized light. We measure this specular inverse Faraday effect in Cu, Pd, Pt, W, Ta, and Au at a laser wavelength of 783 nm. For Ta and W, we investigate both α and β phases. We observe that excitation of these metals with circularly polarized light induces significant circular dichroism. This nonlinear magneto-optical response to circularly polarized light is an order of magnitude larger in α-W than other metals, e.g., Pt or Au, and is greater than nearly all other reported values for the inverse Faraday effect in other materials. Our results benchmark the range of the inverse Faraday effect that can be observed in nonmagnetic metals and provide insight into what material properties govern the inverse Faraday effect in metals.