Compact Optical Devices Research Articles

Metasurface-enabled 3D imaging via local bright spot gray scale matching using the structured light dot array.

Three-dimensional (3D) imaging is widely utilized in various applications, such as light detection, autonomous vehicles, and machine vision. However, conventional 3D imaging systems often rely on bulky optical components. Metasurfaces, as next-generation optical devices, possess flexible wavefront modulation capabilities and excellent combination with computer vision algorithms. Here, we propose a large field-of-view (FOV) structured light dot array projection device based on a metasurface, covering a 2π-FOV, for projecting coded point clouds in Fourier space. We explore a local bright spot gray scale matching algorithm for depth extraction, enabling 3D imaging. This algorithm simplifies the data processing flow and optimizes depth extraction and feature matching processes through a customized region gray scale comparison. As a result, it effectively reduces computational complexity and enhances tolerance to image quality fluctuations. The proposed approach provides new possibilities for developing compact and high-performance planar 3D optical imaging devices, which will drive the advancement of fields such as computer vision and artificial intelligence.

Rapid-response, low-detection-limit, positive-negative air pressure sensing: GaN chips integrated with hydrophobic PDMS films

Despite the importance of positive and negative pressure sensing in numerous domains, the availability of a single sensing unit adept at handling this dual task remains highly limited. This study introduces a compact optical device capable of swiftly and precisely detecting positive and negative pressures ranging from −35 kPa to 35 kPa. The GaN chip, which serves as a core component of the device, is monolithically integrated with light-emitting and light-detecting elements. By combining a deformable PDMS film coated with a hydrophobic layer, the chip can respond to changes in optical reflectance induced by pressure fluctuations. The integrated sensing device has low detection limits of 4.3 Pa and −7.8 Pa and fast response times of 0.14 s and 0.22 s for positive and negative pressure variations, respectively. The device also demonstrates adaptability in capturing distinct human breathing patterns. The proposed device, characterized by its compactness, responsiveness, and ease of operation, holds promise for a variety of pressure-sensing applications.

A tunable narrow single-mode bandpass filter using graphene nanoribbons for THz applications

This paper presents a tunable, single-mode narrowband optical filter designed for terahertz applications utilizing graphene nanoribbons. To attain optimal conditions, the filter was devised in three steps. It is composed of two input and output waveguides and a T-shaped resonator with nanoscale dimensions. The transmission spectrum analysis employs the three-dimensional finite difference time domain and coupled mode theory methods. Tunability is achieved through the adjustment of the nanoribbon size and the chemical potential of graphene. The filter demonstrates remarkable performance metrics, including a maximum transmission spectrum efficiency of 99%, a full width at half maximum (FWHM) of 0.115 THz, a quality factor (Q-factor) of 100, and a free spectral range (FSR) of 45 THz. The presented structure holds significant promise for integrated optical components and compact optical devices, showcasing its applicability in the terahertz frequency range. Furthermore, the inherent sensitivity of this structure to changes in the refractive index of the substrate positions it as a potential sensor.

Non-interleaved quad-channel metasurfaces for simultaneous nanoprinting and holography enabled by phase and bidirectional intensity modulation

Metasurfaces, with their unparalleled ability to manipulate light-field characteristics, have brought forth numerous advantages in ultra-compact and high-resolution image displays, particularly in multifunctional metadevices for simultaneous nanoprinting and holography. However, current efforts are hindered by challenges such as increased fabrication complexity, low efficiencies, or being confined to classic two/three-channel configurations, limiting their further applications. In this study, we propose a single-cell non-interleaved metasurface platform capable of simultaneously and independently generating two holographic images in the far field and two nanoprinting images in the near field. Through a spin-decoupled phase modulation, the metasurface can produce two far-field holographic images under orthogonal circularly polarized incidences. By fully unlocking the Malus's law-assisted bidirectional intensity modulation, the metasurface enables the reconstruction of two near-field nanoprinting images with two distinct orthogonal polarization optical setups as decoding keys. Moreover, the quad-channel metasurface we proposed exhibits broadband response and on-off binary switching performance enabled by the phase transition of Ge2Sb2Se4Te1. We envision that the proposed design paradigm opens up new possibilities for expanding the functionality and enhancing the capacity of metasurfaces without burdening their design, thereby paving the way for compact multifunctional optical devices in image display, information encoding, anti-counterfeiting, and other related fields.

Rapid, non-contact identification of organic solvents: Monolithic GaN chips incorporating PDMS/PS photonic crystals

The prompt identification of organic solvents is essential in many practical scenarios. This work introduces a compact optical device for rapid, non-contact detection of organic solvents. The fabrication of the GaN chip employs a monolithic integration approach, enabling the simultaneous formation of on-chip components responsible for light emission and light detection. The self-assembled polystyrene (PS) spheres embedded in a polydimethylsiloxane (PDMS) layer as photonic crystals can effectively induce distinct optical changes when exposed to organic solvents. By analyzing the magnitude and recovery time of the photocurrent signals, up to seven types of organic samples can be distinguished. Different from traditional optical sensing systems, the proposed chip-level integration strategy eliminates the need for external assembly of optical components, resulting in an ultracompact millimeter-sized sensing unit. The developed device also offers features of simplicity of use and a short measurement time of less than 10 s, making it a feasible solution for quick detection in emergency scenarios.

In vivo optimization of the experimental conditions for the non-invasive optical assessment of breast density

In this study, time domain diffuse optical spectroscopy is performed in the range 600–1100 nm on 11 healthy volunteers with a portable system for the quantitative characterization of breast tissue in terms of optical properties and optically-derived blood parameters, tissue constituent concentrations, and scattering parameters. A measurement protocol involving different geometries (reflectance and transmittance), subject’s positions (sitting and lying down), probing locations (outer, lower, and inner breast quadrants), and source-detector distances (2 and 3 cm) allowed us to investigate the effect of tissue heterogeneity and different measurement configurations on the results with the aim of identifying the best experimental conditions for the estimate of breast density (i.e., amount of fibro-glandular tissue in the breast) as a strong independent risk factor for breast cancer. Transmittance results, that in previous studies correlated strongly with mammographic density, are used as a reference for the initial test of the simpler and more comfortable reflectance measurement configuration. The higher source-detector distance, which probes deeper tissue, retrieves optical outcomes in agreement with higher average density tissue. Similarly, results on the outer quadrants indicate higher density than internal quadrants. These findings are coherent with breast anatomy since the concentration of dense fibro-glandular stroma is higher in deep tissue and towards the external portion of the breast, where the mammary gland is located. The dataset generated with this laboratory campaign is used to device an optimal measurement protocol for a future clinical trial, where optical results will be correlated with conventional mammographic density, allowing us to identify a subset of wavelengths and measurement configurations for an effective estimate of breast density. The final objective is the design of a simplified, compact and cost-effective optical device for a non-invasive, routine assessment of density-associated breast cancer risk.

Broadband Complex Amplitude‐Modulated Metasurfaces for Nanoprinting and Vectorial Hologram with Continuously Varying Linear Polarization Distributions

AbstractThe multi‐dimensional light‐field manipulation capability of metasurfaces positions them as promising candidates for displaying nanoprinting and holographic images at ultra‐short distances with subwavelength resolution. In recent years, merging nanoprinting and holographic images into a single‐layer metasurface has emerged as a research focus to enhance information storage capacity. However, existing multi‐channel metasurface designs often limit the number of polarization states available for the holographic image. Here, a scheme is proposed and demonstrated to encode both a continuous grayscale image and a vectorial hologram with a continuously varying linear polarization distribution onto a complex amplitude‐modulated metasurface. The nanoprinting and holographic images generated from dielectric metasurface exhibit broadband response for the visible light. This method paves the way for compact optical devices tailored for applications in information encoding, high‐density optical storage, and information anti‐counterfeiting.

Exploring the efficacy of subwavelength gratings as short-wavelength infrared filters

Advancements in nanofabrication technology have greatly facilitated research on nanostructures and their associated properties. Among these structures, subwavelength components have emerged as promising candidates for ultra-compact optical elements, can potentially supplant conventional optical components and enable the realization of compact and efficient optical devices. Spectral analysis within the infrared spectrum offers a wealth of information for monitoring crop health, industrial processes, and target identification. However, conventional spectrometers are typically bulky and expensive, driving an increasing demand for cost-effective spectral sensors. Here we investigate three distinct subwavelength grating structures designed to function as narrowband filters within the short-wavelength infrared (SWIR) range. Through simple adjustments to the period of grating strips, these filters selectively transmit light across a wide wavelength range from 1100 to 1700 nm with transmission exceeding 70% and full width at half maximum (FWHM) down to 6 nm. Based on a simple design, the results present great potential of subwavelength grating filters for multiband integration and developing ultra-compact spectral sensors.

Compact and cost-effective laser-powered speckle contrast optical spectroscopy fiber-free device for measuring cerebral blood flow.

In the realm of cerebrovascular monitoring, primary metrics typically include blood pressure, which influences cerebral blood flow (CBF) and is contingent upon vessel radius. Measuring CBF noninvasively poses a persistent challenge, primarily attributed to the difficulty of accessing and obtaining signal from the brain. Our study aims to introduce a compact speckle contrast optical spectroscopy device for noninvasive CBF measurements at long source-to-detector distances, offering cost-effectiveness, and scalability while tracking blood flow (BF) with remarkable sensitivity and temporal resolution. The wearable sensor module consists solely of a laser diode and a board camera. It can be easily placed on a subject's head to measure BF at a sampling rate of 80Hz. Compared to the single-fiber-based version, the proposed device achieved a signal gain of about 70 times, showed superior stability, reproducibility, and signal-to-noise ratio for measuring BF at long source-to-detector distances. The device can be distributed in multiple configurations around the head. Given its cost-effectiveness, scalability, and simplicity, this laser-centric tool offers significant potential in advancing noninvasive cerebral monitoring technologies.

Multispectral Polarization States Generation with a Single Metasurface

AbstractPolarization state generation plays an important role in optics and are applied in many fields. Multispectral polarization manipulation can increase information capacity. To meet the requirement of miniaturization and integration, it is desirable to generate multispectral polarization states with a compact optical device. a metasurface device is proposed and experimentally demonstrated, which can simultaneously realize wavelength separation and polarization generation. A geometric metasurface is used to realize the phase and wavelength multiplexing. Six polarization states with different wavelengths are generated. Customized polarization states with predesigned wavelength information can add extra degrees of freedom for carrying information, opening a new window for portable polarization optics where low‐cost and miniaturized systems are in high demand.

End-to-end optimization of single-shot monocular metasurface camera for RGBD imaging

RGBD cameras that capture color and depth information have a wide range of applications such as robotics, autonomous driving, and cons umer electronics. Traditional RGBD imaging usually requires multi-cameras or extra active illumination which inevitably leads to bulky or complex imaging systems, hindering the growing demand for compact integrated optical devices. Optical metasurface have emerged as a powerful substitute to the traditional diffractive optical element due to the superior dispersion manipulation and extremely compact size. However, it is still a great challenge to realize a single-shot monocular metasurface camera, due to the strong and unignorable wavelength-dependent aberrations. In this work, we demonstrate an end-to-end compact single-shot monocular metasurface camera for RGBD imaging. By utilizing end-to-end joint optimization framework to learn metasurface physical structure in conjunction with deep neural networks reconstruction algorithm, the multidimensional light field information RGBD of a scene in a single shot can be reconstructed within a depth range of 0.5 m. Compared with traditional lens-based RGBD imaging, our proposed metasurface-based imaging achieves an improvement of about 2 dB in chromatic imaging and a 4 times increase in depth estimation accuracy. Our proposed scheme could facilitate the further development of the intelligent computational meta-optics in diverse fields ranging from machine vision to biomedical imaging.

Highly selective single-mode graphene bandpass filter based on Wilkinson power divider structure

In this paper, we introduce a novel graphene nanoribbon bandpass filter designed using the Wilkinson power divider structure for single-mode operation. Through the three-dimensional finite-difference time-domain (3D-FDTD) technique, we analyze transmission spectra and electromagnetic field distributions. Our primary goals are to achieve high transmission efficiency, improved adjustability, and an excellent quality factor (Q-factor). The proposed single-mode graphene nanoribbon bandpass filter exhibits outstanding performance characteristics, including a 45 THz free spectral range (FSR), 95 % transmission efficiency, and a full width at half maximum (FWHM) of 0.14 THz at the resonant frequency of 15 THz. By changing the chemical potential (or bias voltage) to manipulate the conductivity and dielectric constant of the graphene surface, we successfully tune the center frequency of the filter within the 10–20 THz range. This flexibility, combined with exceptional transmission efficiency and Q-factor, positions our filter as a prime candidate for seamless integration into compact optical devices. Additionally, by adjusting the refractive index of the substrate, we achieve a sensitivity exceeding 7 THz/RIU or 7500 nm/RIU, making our filter a promising sensor for applications requiring precise detection and measurement capabilities.

Room-Temperature Strong Coupling of Few-Exciton in a Monolayer WS2 with Plasmon and Dispersion Deviation.

Engineering room-temperature strong coupling of few-exciton in transition-metal dichalcogenides (TMDCs) with plasmons promises to construct compact and high-performance quantum optical devices. But it remains unimplemented due to their in-plane excitons. Here, we demonstrate the strong coupling of few-exciton within 10 in monolayer WS2 with the plasmonic mode with a large tangential component of the electric field tightly trapped around the sharp corners of an Au@Ag nanocuboid, the fewest number of excitons observed in the TMDC family so far. Furthermore, we for the first time report a significant deviation with a relative difference of up to 100.6% between the spectrum and eigenlevel splitting dispersions, which increases with decreasing coupling strength. It is also shown that the coupling strength obtained by the conventional concept of both being equal to the measured spectrum splitting is markedly overestimated. Our work enriches the understanding of strong light-matter interactions at room temperature.

Stacking-Controlled Growth of rBN Crystalline Films with High Nonlinear Optical Conversion Efficiency up to 1.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2Dmaterials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

High quality factor metasurfaces for two-dimensional wavefront manipulation

The strong interaction of light with micro- and nanostructures plays a critical role in optical sensing, nonlinear optics, active optical devices, and quantum optics. However, for wavefront shaping, the required local control over light at a subwavelength scale limits this interaction, typically leading to low-quality-factor optical devices. Here, we demonstrate an avenue towards high-quality-factor wavefront shaping in two spatial dimensions based on all-dielectric higher-order Mie-resonant metasurfaces. We design and experimentally realize transmissive band stop filters, beam deflectors and high numerical aperture radial lenses with measured quality factors in the range of 202–1475 at near-infrared wavelengths. The excited optical mode and resulting wavefront control are both local, allowing versatile operation with finite apertures and oblique illumination. Our results represent an improvement in quality factor by nearly two orders of magnitude over previous localized mode designs, and provide a design approach for a new class of compact optical devices.

Tungsten disulfide based anisotropic metalens for refractive index sensing applications in visible region

Studies on metalenses are rapidly developing, aiming to introduce compact optical devices with superior properties. In this study, which can contribute to the existing literature under the heading of anisotropic metamaterials, the focusing performance of the metalens using the aperture-based Pancharatnam-Berry phase method is numerically analyzed. The proposed structure is formed by coating tungsten disulfide layer on silicon nitride membrane and then etching the whole structure according to the aperture Pancharatnam-Berry phase method. The designed metalens is capable of focusing with efficiencies of 9.7 %, 6.7 %, and 8.4 % at red, green, and blue wavelengths, respectively. At all three wavelengths, the resonative behavior is provided by the anisotropic behavior. Additionally, high numerical aperture values as 0.81, 0.81, and 0.76 are obtained for all three wavelengths. Moreover, refractive index sensing performance is analyzed by immersing the structure into different types of alcohol media, and in this respect, another unique aspect of the study is revealed.

Nonlocal resonances in pedestal high-index-contrast metasurfaces based on a silicon-on-insulator platform

Subwavelength control of the electromagnetic field distribution represents one of the current challenges in photonics research. In this field, diffractive metasurfaces with spatially extended (i.e., nonlocal) resonant modes have recently gained great interest for their versatility in molding the electromagnetic field beyond the approximation of independent resonators. In this framework, we design a high-contrast nonlocal metasurface featuring modes situated in the third operating spectral window of optical communications. The optical properties of the fabricated metasurface are investigated by linear spectroscopy and numerical simulations. Our analysis allows to discern the magnetic and electric nature of the bounded modes by means of polarization-resolved reflectance measurements. Our results represent a step toward integrated nano-photonics in the telecom regime and open promising opportunities for the development of compact nonlinear optical devices.

Reversibly reconfigurable GSST metasurface for broadband beam steering and achromatic focusing in the long-wave infrared.

Active optical metasurfaces promise compact, lightweight, and energy-efficient optical systems with unprecedented performance. Chalcogenide phase-change material Ge2Sb2Se4Te1 (GSST) has shown tremendous advantages in the design of mid-infrared active metasurfaces. However, most of the GSST-based active metasurfaces can only work efficiently within a narrow frequency range. Furthermore, their design flexibility and reversible switching capability are severely restricted by the melting of GSST during re-amorphization. Here, we propose broadband, reversibly tunable, GSST-based transmissive metasurfaces operating in the long-wave infrared spectrum, where the GSST micro-rods are cladded by refractory materials. To accurately evaluate the performance of the proposed metasurfaces, two figures of merits are defined: FOMΦ for the evaluation of wavefront matching, and FOMop for the assessment of the overall performance incorporating both wavefront modulation efficiency and switching contrast ratio. For the proof of concept, two meta-devices are numerically presented: a multifunctional deflector that offers continuous beam steering and long-wave pass filtering simultaneously, and a large-area (1 cm × 1 cm) broadband (11-14 µm) varifocal metalens with the ability of achromatic imaging (12.5-13.5 µm). In particular, the metalens features high FOMop values over 16 dB in the achromatic band, with the average focusing efficiency approximating 70% (60%) in amorphous (crystalline) state and a spectral switching contrast ratio surpassing 25 dB. Our design scheme provides an additional degree of freedom for dynamic modulation and offers a novel approach for achieving high-efficiency mid-infrared compact optical devices.

Self-design of arbitrary polarization-control waveplates via deep neural networks

The manipulation of polarization states beyond the optical limit presents advantages in various applications. Considerable progress has been made in the design of meta-waveplates for on-demand polarization transformation, realized by numerical simulations and parameter sweep methodologies. However, due to the limited freedom in these classical strategies, particular challenges arise from the emerging requirement for multiplex optical devices and multidimensional manipulation of light, which urge for a large number of different nanostructures with great polarization control capability. Here, we demonstrate a set of self-designed arbitrary wave plates with a high polarization conversion efficiency. We combine Bayesian optimization and deep neural networks to design perfect half- and quarter-waveplates based on metallic nanostructures, which experimentally demonstrate excellent polarization control functionalities with the conversion ratios of 85% and 90%. More broadly, we develop a comprehensive wave plate database consisting of various metallic nanostructures with high polarization conversion efficiency, accompanying a flexible tuning of phase shifts (0–2π) and group delays (0–10 fs), and construct an achromatic metalens based on this database. Owing to the versatility and excellent performance, our self-designed wave plates can promote the performance of multiplexed broadband metasurfaces and find potential applications in compact optical devices and polarization division multiplexing optical communications.

A versatile geometric metasurface for generating both focused vortex and vector beams

Singular beam has attracted lots of research interests because of its great potential for applications in various fields such as optical communications and super-resolution imaging. To meet the increasing demands of integrated photonic systems, miniaturized devices for generating singular beams are required. Here, a coherent approach is proposed to design metasurfaces for polarization-controllable generation of focused singular beams with phase or polarization singularity. Metasurfaces with different topological charges configuration are designed, and the capability of generating focused optical vortices with arbitrary topological charges pair and required focusing performance is demonstrated. Furthermore, it is demonstrated that both focused vortices and cylindrical vector beams can be generated and controlled easily by incident polarization. The flexibility and versatility of the proposed approach may provide new opportunities to realize compact and multifunctional singular optical devices for applications including optical tweezers, light-matter interaction, optical communication, and other related fields.