Optical Analog Research Articles

Optical analog to adiabatic passages in two coupled nonlinear waveguides by resonance-locked inverse engineering

Abstract We discuss in this paper the optical analog to rapid adiabatic passage (RAP) and two-state stimulated Raman adiabatic passage (STIRAP) in coupled waveguides with nonlinearity. With the increase of the power of light propagating in waveguides, the nonlinearity can significantly prevent the two adiabatic passages from being successfully simulated. Nevertheless, we find that the negative role of the nonlinearity can be avoided under the assistance of the method of resonance-locked inverse engineering. The method implies redesigning the longitudinally varying detuning of the propagation constants. We further point out that the variation of the detuning does not have to be exactly the same as what the method suggests. As long as the detuning has similar trend to what the method shows, the RAP and two-state STIRAP can still be realized in nonlinear waveguides. This makes the waveguide structure more simpler. Based on our study, the applications of two-state adiabatic passages in designing directional couplers and beam splitters are extended from linear to nonlinear waveguides.

Miniaturized on-chip optical differentiator based on 2F-structured metasurfaces.

Analog optical computing based on Fourier optics has attracted ever-growing attention, offering unprecedented low power consumption and high parallelism computation at the speed of light. Typically, classical optical 4F systems have been widely employed as one of the most common approaches for analog optical computing. However, most existing schemes replicate the original architecture relying on two Fourier transforms and one spatial-frequency filtering, resulting in bulky size and complex structure. Here, we propose a novel, to the best of our knowledge, on-chip 2F structure that achieves ultra-miniaturized optical analog computing. Taking advantage of the exceptional design flexibility of metasurfaces, we reduce the optical path length through a combination of phase compensation and complex amplitude modulation, thereby significantly simplifying the system structure without sacrificing accuracy compared to the traditional 4F system. As a proof-of-concept demonstration, we design and fabricate an on-chip optical differentiator on a silicon-on-insulator platform, achieving 84.01% and 79.81% differentiation accuracy in simulation and experiment, respectively.

A simple model of a gravitational lens from geometric optics

We propose a simple geometric optics analog of a gravitational lens with a refractive index equal to one at large distances and scaling like n(r)2=1+C2/r2, where C is a constant. We obtain the equation for ray trajectories from Fermat's principle of least time and the Euler equation. Our model yields a very simple ray trajectory equation. The optical rays bending, reflecting, and looping around the lens are all described by a single trigonometric function in polar coordinates. Optical rays experiencing fatal attraction are described by a hyperbolic function. We use our model to illustrate the formation of Einstein rings and multiple images.

Performance Evaluation of Optical Reflection Tolerance and Its Improvement Techniques in Various Optical Analog Transmitters

Performance Evaluation of Optical Reflection Tolerance and Its Improvement Techniques in Various Optical Analog Transmitters

Holographic multiplexing metasurface with twisted diffractive neural network

As the cornerstone of AI generated content, data drives human-machine interaction and is essential for developing sophisticated deep learning agents. Nevertheless, the associated data storage poses a formidable challenge from conventional energy-intensive planar storage, high maintenance cost, and the susceptibility to electromagnetic interference. In this work, we introduce the concept of metasurface disk, meta-disk, to expand the capacity limits of optical holographic storage by leveraging uncorrelated structural twist. We develop a physical twisted neural network to describe the optical behavior of the meta-disk and conduct a comprehensive lateral error analysis, where the meta-disk stores large volumes of information through internal structural multiplexing. Two-layer 640 µm x 640 µm meta-disk is sufficient to store over hundreds of high-fidelity images with SSIM of 0.8. By harnessing advanced three-dimensional (3D) printing technology, optical holographic storage is experimentally demonstrated with Pancharatnam-Berry metasurfaces. Our technology provides essential backing for the next generation of optical storage, display, encryption, and multifunctional optical analog computing.

Broadband and parallel multiple-order optical spatial differentiation enabled by Bessel vortex modulated metalens

Optical analog image processing technology is expected to provide an effective solution for high-throughput and real-time data processing with low power consumption. In various operations, optical spatial differential operations are essential in edge extraction, data compression, and feature classification. Unfortunately, existing methods can only perform low-order or selectively perform a particular high-order differential operation. Here, we propose and experimentally demonstrate a Bessel vortex modulated metalens composed of a single complex amplitude metasurface, which can perform multiple-order radial differential operations over a wide band by presetting the order of the corresponding Bessel vortex. This architecture further enables angle multiplexing to create multiple information channels that synchronously perform multi-order spatial differential operations, indicating the superiority of the proposed devices in parallel processing. Our approach may find various applications in artificial intelligence, machine vision, autonomous driving, and advanced biomedical imaging.

Purely spatial frame-dragging and oblate spherodial refractive index as an optical analog to Kerr spacetime

Abstract This paper presents an optical analogy to the lightlike geodesics in spacetime with frame-dragging effects, particularly in Kerr spacetime. This was done by correspondingly equating parts of the deflection angle expression of the lightlike geodesics to that of the optical analog with gradient refractive index (GRIN). The resulting conditions of analogy separates the GRIN from the frame-dragging effects, yielding an oblate spheroidally symmetric GRIN expression and a new optical Lagrangian which situates such medium in a non-Euclidean background space. Such space is demostrated to possess a purely spatial frame-dragging effect. This analogy might suggest a generalized notion of analog magnetism or provide additional insights to studies on gravitational lensing and construction of optical analogs in the laboratory setting.

Optical orbital angular momentum analogy to the Stern-Gerlach experiment.

Symmetry breaking has been shown to reveal interesting phenomena in physical systems. A notable example is the fundamental work of Otto Stern and Walther Gerlach [Stern and Zerlach, Z. Physik9, 349 (1922)10.1007/BF01326983] nearly 100 years ago demonstrating a spin angular momentum (SAM) deflection that differed from classical theory. Here we use non-separable states of SAM and orbital angular momentum (OAM), known as vector vortex modes, to demonstrate how a classical optics analogy can be used to reveal this non-separability, reminiscent of the work carried out by Stern and Gerlach. We show that by implementing a polarization insensitive device to measure the OAM, the SAM states can be deflected to spatially resolved positions.

Optical analog computing for salient object detection in complex scenes via dielectric metasurface

Optical analog computing overcomes the throughput and computational speed limitations of traditional digital computing, facilitating salient object detection on platforms that rapidly process massive image data, such as optical remote sensing. Further device miniaturization and integration can be achieved by replacing filters in optical analog computing with metasurface. Based on the combination of optical analog computing and dielectric metasurface, we proposed an optical analog cross-correlation operation operating in the mid-infrared wavelength band to achieve the real-time and compact salient object detection. Even in extremely complex scenes, this cross-correlation operation can be used to identify and segment target patterns based on several target features accurately. The proposed optical analog cross-correlation operation may find applications in object tracking, medical image segmentation, and person re-identification.

High-performance optical dialogue for power electronic system

High-accuracy signal and high-quality power transmissions play an essential role in power electronics systems. Nowadays, the severe electromagnetic interference (EMI) by the switching converter threatens the stable operation of conventional signal and power transmissions. To mitigate the effects of EMI, optical fiber is a better solution due to its unmatched isolation properties. Here, we perform a transmission configuration, named optical dialogue, based on a single fiber for simultaneous bidirectional analog optical signal (AOS) and analog optical power (AOP) transmission, to ensure EMI immunity. This optical dialogue transmission not only utilizes AOS to achieve high-accuracy signal transmission, but also utilizes AOP to achieve high-quality power transmission. We validate the proposed optical dialogue transmission using discrete off-the-shelf components, demonstrating the high frequency signal transmission and high efficiency power transmission. Our proposed optical dialogue transmission offers a high-performance optical solution for signal and power transmission, which is significant step forward in the development of more accurate, reliable, and safer power electronic systems.

Dual-polarized transmissive metasurfaces for near-unitary-NA analog spatial computing empowered by impedance matching and mismatching.

In recent years, due to the increasing requirement for real-time and massive data processing, optical analog computation has arisen as a promising alternative to digital computation. Optical spatial differentiation plays a fundamentally important role in various emerging technologies, including augmented reality, autonomous driving, and object recognition. However, previous demonstrations encountered several limitations, such as the dependence on polarization states and a typically limited numerical aperture (NA) of about 0.5, especially in the transmission mode. Here, a new, to our knowledge, design strategy based on the evolution between impedance matching and mismatching in a metasurface is proposed to fill this gap, which can perform dual-polarized second-order derivative for image processing. Our scheme enables high transmission under dual polarization over an 85° incident angle range (NA = 0.996), resulting in more than twofold spatial resolution. Our work paves the way for polarization-insensitive high-resolution signal and image processing in the terahertz region.

Nanosecond Laser Debonding Using an Ultrathin Absorber Layer in a Transparent Composite: Making a Light‐Powered Exploding Bolt

AbstractLocalized heating of an adhesive interface can separate glued substrates for recycling or reconfiguring. Light provides a way to deposit energy at this type of buried interface, but the photon energy must still be transformed into thermal energy. The deposition of an ultrathin (<1 µm) absorbing layer before gluing provides a way to build this photothermal capability into the bonded structure. To demonstrate this concept, a submicron absorbing layer composed of the dye Fluorescein 27 (F27) is applied to a poly(methyl methacrylate) (PMMA) substrate that is glued to a second PMMA piece using a commercial cyanoacrylate adhesive. The laser debonding is characterized as function of dye layer thickness, applied pressure, and laser pulse fluence. A 450 nm thick dye layer retained a high adhesive strength of 3.5 MPa but can be debonded by a single 532 nm nanosecond laser pulse with a fluence ≤ 0.7 J cm−2. Decomposition of the absorbing layer resulted in rapid gas evolution that left the micron‐scale surface morphology of the PMMA substrate intact. This system enabled the fabrication of an optical analog to the exploding bolt, where a single laser pulse can impulsively separate a structural fastener.

Optical continuous pulse position modulation-based analog RoF via frequency-domain position estimation.

Analog radio-over-fiber (A-RoF) solutions for mobile fronthaul are regaining wide attention due to their high spectral efficiency and low complexity. However, the performance of A-RoF is usually limited by the fiber link fidelity. In this Letter, we propose and experimentally demonstrate an optical continuous pulse position modulation-based analog radio-over-fiber (OCPPM-RoF) scheme, in which the amplitudes of wireless waveforms are mapped to the time-domain positions of optical pulses to decouple the additive noise. The de-modulation of OCPPM-RoF signals is performed by a frequency-domain continuous position estimation (FD-CPE) algorithm including cross-power spectrum calculation and weighted least square for accurate delay estimation. In the experiment, by adopting appropriate pulse width factors, the recovered signal-to-noise ratio (SNR) can be flexibly adjusted within a wide range from 31.8 to 54.0 dB, supporting the transmission of various formats from 256-QAM to 65536-QAM. The results indicate that OCPPM-RoF can be a promising candidate for future fronthaul.

Non-linear media in weakly curved spacetime: optical solitons and probe pulses for gravimetry

That light propagating in a gravitational field gets frequency-shifted is one of the basic consequences of any metric theory of gravity rooted in the equivalence principle. At the same time, also a time dependent material’s refractive index can frequency-shift light propagating in it. The mathematical analogy between the two effects is such that the latter has been used to study the optical analogue of a black-hole spacetime. Here, we combine these two effects by showing that light propagation in non-linear media in the presence of a moving refractive index perturbation can lead to a gravity-dependent blueshift. We find that the predicted blueshift surpasses the gravitational redshift even if the medium is considered to be perfectly stiff. In realistic scenarios, by far the strongest frequency shift arises due to the deformation of the dielectric medium and the corresponding photoelastic change of refractive index. This has the potential to facilitate optical sensing of small gravity gradients.

High-order Autler–Townes splitting in electrically tunable photonic molecules

Whispering gallery mode optical microresonators represent a promising avenue for realizing optical analogs of coherent light–atom interactions, circumventing experimental complexities. All-optical analogs of Autler–Townes splitting have been widely demonstrated, harnessing coupled optical microresonators, also known as photonic molecules, wherein the strong coupling between resonant fields enables energy level splitting. Here, we report the characterizations of Autler–Townes splitting in waveguide-coupled microring dimers featuring mismatched sizes. By exploiting backscattering-induced coupling via Rayleigh and Mie scatterers in individual rings, high-order Autler–Townes splitting has been realized, yielding supermode hybridization in a multi-level system. Upon resonance detuning using an integrated phase shifter, intra-cavity coupling-induced splitting becomes almost indistinguishable at the zero-detuning point where the strong inter-cavity coupling counteracts the imbalance of backscattering strengths in individual rings. Through demonstrations on the maturing silicon photonics platform, our findings establish a framework of electrically tunable photonic molecules for coupling-mediated Autler–Townes splitting, offering promising prospects for on-chip signal generation and processing across classical and quantum regimes.

Generalized Huygens' Condition as the Fulcrum of Planar Nonlocal Omnidirectional Transparency: From Meta‐Atoms to Metasurfaces

AbstractFresnel reflection has been known for centuries to fundamentally impede efficient transmittance across planar interfaces, especially at grazing incidence. In this work, the generalized Huygens' condition (GHC) is introduced to resolve such intricacies in metasurface (MS) designs and allow omnidirectional transparency. Compared to common numerical metamaterial approaches, the analytical framework herein yields surprisingly simple closed‐form conditions, carefully leveraging the natural nonlocal mechanisms endowed in planar electromagnetic structures. At the meta‐atom level, the GHC is met by balancing traditional tangential susceptibilities of Huygens' MSs with their unconventional normal counterparts; the latter facilitates the key requisite of vanishing backscattering at the challenging grazing incidence scenario. At the MS level, this central insight is sheerly utilized to engineer realistic all‐angle transparent printed‐circuit‐board (PCB) cascaded admittance sheets. Thoroughly validated in simulation and experiment, this universal GHC demonstrates a resourceful venue for practical implementation of advanced nonlocal devices, e.g., flat optical components, optical analog computers, and spaceplates.

All-optically controlled mode-coupling induced transparency with tunable efficiency in a microsphere resonator.

The optical analogs of electromagnetically induced transparency (EIT) have attracted vast attention recently. The generation and manipulation of EIT in microcavities have sparked research in both fundamental physics and photonic applications, including light storage, slow light propagation, and optical communication. In this Letter, the generation and tuning of an all-optically controlled mode-coupling induced transparency (MCIT) are proposed, experimentally demonstrated, and theoretically analyzed. The MCIT effect originated from the intermodal coupling between the plethora of modes generated in our fabricated optical microcavity, and the tuning of the transparency mode utilized the cavity's thermal bistability nature. Furthermore, based on our method, a novel, to the best of our knowledge, controlling of the mode shifting efficiency is also achieved with an increase up to two times and more. The proposed scheme paves a unique, simple, and efficient way to manipulate the induced transparency mode, which can be useful for applications like cavity lasing and thermal sensing.

Nonlinear encoding in diffractive information processing using linear optical materials

Nonlinear encoding of optical information can be achieved using various forms of data representation. Here, we analyze the performances of different nonlinear information encoding strategies that can be employed in diffractive optical processors based on linear materials and shed light on their utility and performance gaps compared to the state-of-the-art digital deep neural networks. For a comprehensive evaluation, we used different datasets to compare the statistical inference performance of simpler-to-implement nonlinear encoding strategies that involve, e.g., phase encoding, against data repetition-based nonlinear encoding strategies. We show that data repetition within a diffractive volume (e.g., through an optical cavity or cascaded introduction of the input data) causes the loss of the universal linear transformation capability of a diffractive optical processor. Therefore, data repetition-based diffractive blocks cannot provide optical analogs to fully connected or convolutional layers commonly employed in digital neural networks. However, they can still be effectively trained for specific inference tasks and achieve enhanced accuracy, benefiting from the nonlinear encoding of the input information. Our results also reveal that phase encoding of input information without data repetition provides a simpler nonlinear encoding strategy with comparable statistical inference accuracy to data repetition-based diffractive processors. Our analyses and conclusions would be of broad interest to explore the push-pull relationship between linear material-based diffractive optical systems and nonlinear encoding strategies in visual information processors.

Spin–orbit optical broadband achromatic spatial differentiation imaging

Spatial optical analog differentiation allows ultrahigh-speed and low-power-consumption of image processing, as well as label-free imaging of transparent biological objects. Optical analog differentiation with broadband and incoherent sources is appealing for its multi-channels and multi-task information processing, as well as the high-quality differentiation imaging. Currently, broadband and incoherent optical differentiation is still challenging. Here, a compact and broadband achromatic optical spatial differentiator is demonstrated based on the intrinsic spin–orbit coupling in a natural thin crystal. By inserting a uniaxial crystal just before the camera of a conventional microscope, the spin to orbit conversion will embed an optical vortex to the image field and make a second-order topological spatial differentiation to the field, thus an isotropic differential image will be captured by the camera. The wavelength-independent property of the intrinsic spin–orbit coupling effect allows us to achieve broadband analog computing and achromatic spatial differentiation imaging. With this differentiation imaging method, both amplitude and pure phase objects are detected with high contrast. Transparent living cells and biological tissues are imaged with their edge contours and intracellular details protruded in the edge detection mode and edge enhancement mode, respectively. These findings pave the way for optical analog computing with broadband incoherent light sources and concurrently drive the advancement of high-performance and cost-effective phase contrast imaging.

Optical Spin Waves.

Chirality is inherent to a broad range of systems, including solid-state and wave physics. The precession (chiral motion) of the magnetic moments in magnetic materials, forming spin waves, has various properties and many applications in magnetism and spintronics. We show that an optical analogue of spin waves can be generated in arrays of plasmonic nanohelices. Such optical waves arise from the interaction between twisted helix eigenmodes carrying spin and orbital angular momenta. We demonstrate that these optical spin waves are reflected at the interface between successive domains of enantiomeric nanohelices, forming a heterochiral lattice regardless of the wave propagation direction within the lattice. Optical spin waves may be applied in techniques involving photon spin, ranging from data processing and storage to quantum optics.