Diffraction-based nonlinear model for the design of broadband adiabatic up-conversion imaging.

In recent years, mid-infrared parametric upconversion imaging, a nonlinear optical method that involves converting mid-infrared light into visible images, has significantly advanced and has shown considerable potential for various applications, including biomedical imaging and remote sensing. While diffraction-based parametric upconversion imaging modeling in standard thin birefringence crystals have been addressed, the numerical framework developed so far fails to address long aperiodic poled crystals. Specifically, diffraction-based analysis of the recent broadband adiabatic frequency upconversion imaging, which allows simultaneous image upconversion of extremely broadband signals is still lacking. Here, we introduce a diffraction-based numerical simulation framework for predicting the evolution of the nonlinear image/signal generation in upconversion imaging systems. This generalized framework can handle both periodically and aperiodically poled crystal designs. Specifically, the model captures faithfully and addresses the varying image magnification arising from upconversion at a Fourier plane of a multiwavelength object. The numerical simulations are validated by experimental measurements of broadband upconversion 3-5 µm mid-IR images to the visible-NIR, showing a good agreement. Moreover, the model allows the exploration of the trade-offs in the spectral span when moving to the full visible range. Our numerical framework will be useful for the interpretation of experimental results obtained in an imaging setting with nonlinear optical elements.

Machine learning for comprehensive prediction of high risk for Alzheimer's disease based on chromatic pupilloperimetry.

Currently there are no reliable biomarkers for early detection of Alzheimer's disease (AD) at the preclinical stage. This study assessed the pupil light reflex (PLR) for focal red and blue light stimuli in central and peripheral retina in 125 cognitively normal middle age subjects (45-71 years old) at high risk for AD due to a family history of the disease (FH+), and 61 age-similar subjects with no family history of AD (FH-) using Chromatic Pupilloperimetry coupled with Machine Learning (ML). All subjects had normal ophthalmic assessment, and normal retinal and optic nerve thickness by optical coherence tomography. No significant differences were observed between groups in cognitive function and volumetric brain MRI. Chromatic pupilloperimetry-based ML models were highly discriminative in differentiating subjects with and without AD family history, using transient PLR for focal red (primarily cone-mediated), and dim blue (primarily rod-mediated) light stimuli. Features associated with transient pupil response latency (PRL) achieved Area Under the Curve Receiver Operating Characteristic (AUC-ROC) of 0.90 ± 0.051 (left-eye) and 0.87 ± 0.048 (right-eye). Parameters associated with the contraction arm of the rod and cone-mediated PLR were more discriminative compared to parameters associated with the relaxation arm and melanopsin-mediated PLR. Significantly shorter PRL for dim blue light was measured in the FH+ group in two test targets in the temporal visual field in right eye that had highest relative weight in the ML algorithm (mean ± standard error, SE 0.449 s ± 0.007 s vs. 0.478 s ± 0.010 s, p = 0.038). Taken together our study suggests that subtle focal changes in pupil contraction latency may be detected in subjects at high risk to develop AD, decades before the onset of AD clinical symptoms. The dendrites of melanopsin containing retinal ganglion cells may be affected very early at the preclinical stages of AD.

Inverse design of unparametrized nanostructures by generating images from spectra.

Recently, there has been an increasing number of studies applying machine learning techniques for the design of nanostructures. Most of these studies train a deep neural network (DNN) to approximate the highly nonlinear function of the underlying physical mapping between spectra and nanostructures. At the end of training, the DNN allows an on-demand design of nanostructures, i.e., the model can infer nanostructure geometries for desired spectra. While these approaches have presented a new paradigm, they are limited in the complexity of the structures proposed, often bound to parametric geometries. Here we introduce spectra2pix, which is a DNN trained to generate 2D images of the target nanostructures. By predicting an image, our model architecture is not limited to a closed set of nanostructure shapes, and can be trained for the design of a much wider space of geometries. We show, for the first time, to the best of our knowledge, a successful generalization ability, by designing completely unseen shapes of geometries. We attribute the successful generalization to the ability of a pixel-wise architecture to learn local properties of the meta-material, therefore mimicking faithfully the underlying physical process. Importantly, beyond synthetical data, we show our model generalization capability on real experimental data.

Angular super-resolution retrieval in small-angle X-ray scattering.

Small-angle X-ray scattering (SAXS) techniques enable convenient nanoscopic characterization for various systems and conditions. Unlike synchrotron-based setups, lab-based SAXS systems intrinsically suffer from lower X-ray flux and limited angular resolution. Here, we develop a two-step retrieval methodology to enhance the angular resolution for given experimental conditions. Using minute hardware additions, we show that translating the X-ray detector in subpixel steps and modifying the incoming beam shape results in a set of 2D scattering images, which is sufficient for super-resolution SAXS retrieval. The technique is verified experimentally to show superior resolution. Such advantages have a direct impact on the ability to resolve finer nanoscopic structures and can be implemented in most existing SAXS apparatuses both using synchrotron- and laboratory-based sources.

High-resolution multi-scan compact Fourier transform-infrared spectrometer.

The Fourier transform-infrared (FT-IR) spectrometer is a widely used high-resolution spectral characterization method in materials, chemicals, and more. However, the inverse relation between the spectral resolution and the interferometer's arm length yields a tradeoff between spectral resolution and spectrometer footprint. Here, we introduce a novel method to overcome this traditional FT-IR resolution limit. The enhanced high-resolution multi-scan compact FT-IR spectrometer we present achieves an effectively long interferogram by combining multiple short FT-IR scans. Simulation and experimental results demonstrate a significant increase in the spectral resolution of a FT-IR spectrometer by employing our interferogram stitching algorithm.

Plasmonic nanostructure design and characterization via Deep Learning.

Nanophotonics, the field that merges photonics and nanotechnology, has in recent years revolutionized the field of optics by enabling the manipulation of light-matter interactions with subwavelength structures. However, despite the many advances in this field, the design, fabrication and characterization has remained widely an iterative process in which the designer guesses a structure and solves the Maxwell's equations for it. In contrast, the inverse problem, i.e., obtaining a geometry for a desired electromagnetic response, remains a challenging and time-consuming task within the boundaries of very specific assumptions. Here, we experimentally demonstrate that a novel Deep Neural Network trained with thousands of synthetic experiments is not only able to retrieve subwavelength dimensions from solely far-field measurements but is also capable of directly addressing the inverse problem. Our approach allows the rapid design and characterization of metasurface-based optical elements as well as optimal nanostructures for targeted chemicals and biomolecules, which are critical for sensing, imaging and integrated spectroscopy applications.

Pulse shaping of broadband adiabatic SHG from a Ti-sapphire oscillator.

We experimentally demonstrate an efficient broadband second-harmonic generation (SHG) process with a tunable mode-locked Ti:sapphire oscillator. We have achieved a robust broadband and efficient flat conversion of more than 35 nm wavelength by designing an adiabatic aperiodically poled potassium titanyl phosphate crystal. Moreover, we have shown that with such efficient flat conversion, we can shape and control broadband second-harmonic pulses. More specifically, we assign a spectral phase of absolute value and π-step, which allows wavelength tunable intense pump-probe and amplitude modulation of the broadband second-harmonic output. Such spectral phases serve as a proof of concept for other pulse-shaping applications for nonlinear spectroscopy and imaging.

Publisher's Note: Coherence-Driven Topological Transition in Quantum Metamaterials [Phys. Rev. Lett. 116, 165502 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.165502.

Coherence-Driven Topological Transition in Quantum Metamaterials.

We introduce and theoretically demonstrate a quantum metamaterial made of dense ultracold neutral atoms loaded into an inherently defect-free artificial crystal of light, immune to well-known critical challenges inevitable in conventional solid-state platforms. We demonstrate an all-optical control, on ultrafast time scales, over the photonic topological transition of the isofrequency contour from an open to closed topology at the same frequency. This atomic lattice quantum metamaterial enables a dynamic manipulation of the decay rate branching ratio of a probe quantum emitter by more than an order of magnitude. Our proposal may lead to practically lossless, tunable, and topologically reconfigurable quantum metamaterials, for single or few-photon-level applications as varied as quantum sensing, quantum information processing, and quantum simulations using metamaterials.

Experimental Realization of Two Decoupled Directional Couplers in a Subwavelength Packing by Adiabatic Elimination.

On-chip optical data processing and photonic quantum integrated circuits require the integration of densely packed directional couplers at the nanoscale. However, the inherent evanescent coupling at this length scale severely limits the compactness of such on-chip photonic circuits. Here, inspired by the adiabatic elimination in a N-level atomic system, we report an experimental realization of a pair of directional couplers that are effectively isolated from each other despite their subwavelength packing. This approach opens the way to ultradense arrays of waveguide couplers for integrated optical and quantum logic gates.

An ultrathin invisibility skin cloak for visible light.

Metamaterial-based optical cloaks have thus far used volumetric distribution of the material properties to gradually bend light and thereby obscure the cloaked region. Hence, they are bulky and hard to scale up and, more critically, typical carpet cloaks introduce unnecessary phase shifts in the reflected light, making the cloaks detectable. Here, we demonstrate experimentally an ultrathin invisibility skin cloak wrapped over an object. This skin cloak conceals a three-dimensional arbitrarily shaped object by complete restoration of the phase of the reflected light at 730-nanometer wavelength. The skin cloak comprises a metasurface with distributed phase shifts rerouting light and rendering the object invisible. In contrast to bulky cloaks with volumetric index variation, our device is only 80 nanometer (about one-ninth of the wavelength) thick and potentially scalable for hiding macroscopic objects.

Macroscale Transformation Optics Enabled by Photoelectrochemical Etching.

Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients for transformation optical devices. This technique allows the fabrication of macroscale devices with large refractive index gradients. Patterned porous layers can also be lifted from the substrate and transferred to other materials, creating more possibilities for novel devices.

Adiabatic elimination-based coupling control in densely packed subwavelength waveguides.

The ability to control light propagation in photonic integrated circuits is at the foundation of modern light-based communication. However, the inherent crosstalk in densely packed waveguides and the lack of robust control of the coupling are a major roadblock toward ultra-high density photonic integrated circuits. As a result, the diffraction limit is often considered as the lower bound for ultra-dense silicon photonics circuits. Here we experimentally demonstrate an active control of the coupling between two closely packed waveguides via the interaction with a decoupled waveguide. This control scheme is analogous to the adiabatic elimination, a well-known procedure in atomic physics. This approach offers an attractive solution for ultra-dense integrated nanophotonics for light-based communications and integrated quantum computing.

Anti-Hermitian plasmon coupling of an array of gold thin-film antennas for controlling light at the nanoscale.

Open quantum systems consisting of coupled bound and continuum states have been studied in a variety of physical systems, particularly within the scope of nuclear, atomic, and molecular physics. In the open systems, the effects of the continuum decay channels are accounted for by indirect non-Hermitian couplings among the quasibound states. Here we explore anti-Hermitian coupling in a plasmonic system for spatially manipulating light on the nanoscale. We show that by utilizing the anti-Hermitian coupling, plasmonic antennas closely packed within only λ/15 separations can be individually excited from the far field, which are otherwise indistinguishable from each other. This opens a new venue for the nanoscale lightwave control, wavelength multiplexing, and spectrum splitting.

Near-field characterization of extraordinary optical transmission in sub-wavelength aperture arrays.

Extra ordinary transmission through arrays of subwavelength apertures has been investigated using near-field scanning optical microscopy. For such studies arrays were fabricated to give maximum resonance enhancement of light transmission at the wavelength of illumination that was used (532 nm). To define this enhancement a design was employed that allowed in one field of view of a near-field image the investigation of single apertures of dimension that was similar to what was incorporated into the sub-wavelength hole array. Significant asymmetry in the transmission and the propagation of the light along the aperture array was detected. This non-uniformity could be explained by polarization of the incident light, edge effects and the geometry of the array. The results support a hypothesis of both enhanced transmission due to surface plasmons and a non-diffracting beaming as a function of distance effect in the propagation of the light from the array.