Conventional Optical Elements Research Articles

Extended-depth-of-field imaging with an ultra-thin folded lens

Optical systems with extended depth of field (EDOF) are crucial for observation and measurement applications, where achieving compactness and a substantial depth of field (DOF) presents a considerable challenge with conventional optical elements. In this paper, we propose an innovative solution for the miniaturization of EDOF imaging systems by introducing an ultra-thin annular folded lens (AFL). To validate the practical feasibility of the theory, we design an annular four-folded lens with an effective focal length of 80.91 mm and a total thickness of only 8.50 mm. Simulation results show that the proposed folded lens has a DOF of 380.55 m. We further developed an AFL-based test system exhibiting a resolution of 0.11 mrad across a wide wavelength range of 486–656 nm. Additionally, we present experimental results from a miniature compact prototype, which further highlights the promising potential of folded lenses for long-range EDOF imaging.

Compact freeform-surface-based Offner imaging spectrometer with both a long-slit and broadband.

Current imaging spectrometers with conventional optical elements face major challenges in achieving a large field of view (FOV), broadband and compact structure simultaneously. In this paper, a compact freeform-surface-based Offner imaging spectrometer with both a long-slit and a broadband (CISLS) is proposed. To keep a long slit and an anastigmatic imaging, the slit off-axis amount of the initial system is within a specific range theoretically. While to achieve a compact structure, the slit off-axis amount should be away from the specific range and as small as possible. Based on the vector aberration theory and the analytical study, Zernike polynomial terms Z5 and Z6 introduce the astigmatism independent of FOV. They are utilized to well balance the astigmatism when the slit off-axis amount is away from the specific range, helping a miniaturization of the system. Other Zernike polynomial terms below the eighth order introduce the astigmatism related to FOV. They contribute to balancing the astigmatism that produced with the increasing of the FOV, thus achieving a wide FOV. The design results show that the proposed CISLS with a high spectral resolution of 2.7 nm achieves a long slit of 30 mm in length but a small size of only 60 mm × 64 mm × 90 mm in volume under a broadband from 400 nm to 1000 nm.

Tantalum pentoxide: a new material platform for high-performance dielectric metasurface optics in the ultraviolet and visible region

Dielectric metasurfaces, composed of planar arrays of subwavelength dielectric structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems on chip. The performance of a dielectric metasurface is largely determined by its constituent material, which is highly desired to have a high refractive index, low optical loss and wide bandgap, and at the same time, be fabrication friendly. Here, we present a new material platform based on tantalum pentoxide (Ta2O5) for implementing high-performance dielectric metasurface optics over the ultraviolet and visible spectral region. This wide-bandgap dielectric, exhibiting a high refractive index exceeding 2.1 and negligible extinction coefficient across a broad spectrum, can be easily deposited over large areas with good quality using straightforward physical vapor deposition, and patterned into high-aspect-ratio subwavelength nanostructures through commonly-available fluorine-gas-based reactive ion etching. We implement a series of high-efficiency ultraviolet and visible metasurfaces with representative light-field modulation functionalities including polarization-independent high-numerical-aperture lensing, spin-selective hologram projection, and vivid structural color generation, and the devices exhibit operational efficiencies up to 80%. Our work overcomes limitations faced by scalability of commonly-employed metasurface dielectrics and their operation into the visible and ultraviolet spectral range, and provides a novel route towards realization of high-performance, robust and foundry-manufacturable metasurface optics.

Rapid scan white light two-dimensional electronic spectroscopy with 100kHz shot-to-shot detection.

We demonstrate an approach to two-dimensional electronic spectroscopy (2DES) that combines the benefits of shot-to-shot detection at high-repetition rates with the simplicity of a broadband white light continuum input and conventional optical elements to generate phase-locked pump pulse pairs. We demonstrate this through mutual synchronization between the laser repetition rate, the acousto-optical deflector, the pump delay stage, and the CCD line camera, which allows for rapid scanning of pump optical delay synchronously with the laser repetition rate, while the delay stage is moved at a constant velocity. The resulting shot-to-shot detection scheme is repetition rate scalable and only limited by the CCD line rate and the maximum stage velocity. Using this approach, we demonstrate the measurement of an averaged 2DES absorptive spectrum in as much as 1.2s of continuous sample exposure per 2D spectrum. We achieve a signal-to-noise ratio of 6.8 for optical densities down to 0.05 with 11.6s of averaging at 100kHz laser repetition rate. Combining rapid scanning of mechanical delay lines with shot-to-shot detection as demonstrated here provides a viable alternative to acousto-optic pulse shaping approaches that is repetition-rate scalable, has comparable throughput and sensitivity, and minimizes sample exposure per 2D spectrum with promising micro-spectroscopy applications.

Single-shot wide-field full-stokes polarization imaging

Polarization is an intrinsic property of light. With the advancements in nanophotonics, it is now possible to achieve arbitrary polarization transformations in space. Incorporating special functions in different spatial dimensions expands the design freedom of metasurfaces and other optical devices. To understand and utilize the metasurface, we design a metasurface optical element to achieve single-shot, wide-field polarization imaging. Our demonstration exhibits a compact, full Stokes wide-field polarization imaging device without requiring conventional optical elements, such as moving parts or wide-field lens sets. Wide-field polarization imaging devices are potential candidates for machine vision and biological tissue imaging applications.

Metasurface‐Based Orbital Angular Momentum Multi‐Dimensional Demultiplexer and Decoder

AbstractLight beams possess various dimensions, such as wavelength, spin angular momentum (SAM), and orbital angular momentum (OAM), which are indispensable in numerous optical applications. However, traditional techniques entail multiple attempts to disclose desired optical information, utilizing unwieldy multi‐pass systems or mechanically moving parts that are cumbersome to integrate into compact and integrated optical systems. Here, a single‐layer dielectric metasurface platform is proposed that enables multi‐dimensional demultiplexing and decoding, circumventing the need for conventional bulk optical elements. The platform can demultiplex 3D light parameters (wavelength, SAM, and OAM) and focus them at distinct spatial positions on a designated focal plane, providing a superior alternative to far‐field beams for integration with optical fiber and communication chips. The study verifies 132‐bit independent channels for a multi‐dimensional demultiplexer and demonstrates the metasurface‐based component's ability to function as an elliptic polarization decoder through a 4‐bit elliptic polarization coding transmission experiment. This work provides a compact and efficient platform for demultiplexing multi‐dimensional SAM and OAM states over a broad wavelength range, with potential implications for optical communications, optical data storage, optical information encryption, and quantum information sciences.

Bi-channel compressive hyperspectral imager based on polarization conversion metasurface

Metasurfaces have demonstrated remarkable capabilities in manipulating light fields and reducing the size of conventional optical elements to ultrathin devices, and compressive hyperspectral imaging systems can effectively utilize the spectral modulations of metasurfaces through spectral transmission tuning. In this paper, a bi-channel compressive hyperspectral imager based on polarization conversion metasurfaces is proposed. The metasurface layer of this device is able to modulate and convert the polarized incidence into two outputs of different transmission spectra with orthogonal polarizations, providing two channels of hyperspectral information that can be detected by a polarization image sensor in one snapshot. Additionally, this design offers a compact size, high speed, and accurate reconstruction through the utilization of a macro-pixel segmentation method for compressive sensing algorithms, and it is firmly demonstrated that the proposed hyperspectral imaging technology surpasses other traditional compressive hyperspectral image recovery methods in terms of improved reconstruction quality. Therefore, it is envisioned that this technology will find significant applications in various fields, including food safety, biomedical imaging, precision agriculture, object detection and so on.

Smart Manipulation of Complex Optical Elements via Contact-adaptive Dry Adhesives.

The implementation of complex, high-precision optical devices or systems, which have vital applications in the aerospace, medical, and military fields, requires the ability to reliably manipulate and assemble optical elements. However, this is a challenging task as these optical elements require contamination-free and damage-free manipulation and come in a variety of sizes and shapes. Here, a smart, contact-adaptive adhesive based on magnetic actuation is developed to address this challenge. Specifically, the surface bio-inspired adhesives made of fluororubber facilitate contamination-free and damage-free adhesion. The stiffness modulation of packaged magnetorheological grease based on the magnetorheological effect endows the smart adhesive with a high conformability to the optical elements in the soft state, a high grip force in the stiff state, and the ability to quickly release the optical elements in the recovered soft state. The smart adhesive provides a versatile solution for reliably and quickly manipulating and assembling multiscale optical elements with planar or complex 3D shapes without causing surface contamination or damage. These extraordinary capabilities are demonstrated by the manipulation and assembly of various optical elements, such as convex/concave/ball lenses and extremely complex-shaped light guide plates. The proposed smart adhesive is a promising candidate for conventional optical element manipulation technologies.

High-performance polarization-controlled perfect optical vortex generation via all-dielectric metasurfaces in the visible region

Metasurfaces, with superiority in integration and flexibility compared to conventional bulky optical elements, open a perspective in generating perfect optical vortex (POV). However, the generation of POV beams with simultaneous high-efficiency, multi-functionality, and broadband is still challenging. Here, we design POV generators based on highly efficient all-dielectric metasurfaces and produce POV beams in simulation in the visible region with maximum generation efficiency of 74%. Such high efficiency originates from the stimulation of the resonance modes of polarization-dependent parity in the well-designed unit cell. By superposing two sets of phase profiles, polarization-controlled POV generators are obtained. Furthermore, the POV generator exhibits broadband property due to the geometric phase independent of the incident wavelengths. Our work provides an effective approach for generating controllable POV beams, promoting the construction of integrated and multi-functional optical systems.

A simple Bessel module with tunable focal depth and constant resolution for commercial two-photon microscope

The volumetric imaging of two-photon microscopy expands the focal depth and improves the throughput, which has unparalleled superiority for three-dimension samples, especially in neuroscience. However, emerging in volumetric imaging is still largely customized, which limits the integration with commercial two-photon systems. Here, we analyzed the key parameters that modulate the focal depth and lateral resolution of polarized annular imaging and proposed a volumetric imaging module that can be directly integrated into commercial two-photon systems using conventional optical elements. This design incorporates the beam diameter adjustment settings of commercial two-photon systems, allowing flexibility to adjust the depth of focus while maintaining the same lateral resolution. Further, the depth range and lateral resolution of the design were verified, and the imaging throughput was demonstrated by an increase in the number of imaging neurons in the awake mouse cerebral cortex.

Off-axis spectral confocal metasurfaces with linear dispersion in a broad bandwidth

Optical metasurfaces offer significant possibilities for miniaturization and expanding the capabilities of conventional optical elements due to their superb wavefront regulation abilities. In this paper, we propose a design approach that precisely controls the chromatic dispersion of anisotropy silicon nanobricks to create off-axis spectral confocal metasurfaces with linear dispersion in short-wave infrared range (1600 nm–2400 nm). Firstly, a single-layer off-axis spectral confocal metasurfaces with an aperture diameter of 60μm and a fixed x-direction focal length component of 50μm is presented to demonstrate the effectiveness of approach. At the maximum and minimum frequencies, the total focal lengths are 125μm and 83.33μm, respectively. Subsequently, we analyze the possible limitations of designing single-layer metasurfaces and overcome them by adopting doublet metasurfaces. An example of double-layer off-axis spectral confocal metasurfaces with a radius of 50μm and an off-axis distance of 80μm is showcased to introduce the approach, and the focal lengths range between the maximum and minimum frequencies is 66.67μm, larger than that of single-layer metasurfaces. The numerical simulation results validate the practicality of our design approach, which can be generalized for other customized functionalities metasurfaces with a broad bandwidth.

Single-shot quantitative amplitude and phase imaging based on a pair of all-dielectric metasurfaces

Quantitative amplitude and phase imaging (QAPI) has been an effective technique to examine label-free biomedical samples. Simple and reliable QAPI techniques realized by replacing conventional bulky optical elements with planar structures will improve the system portability and facilitate in vivo imaging applications. Here, we propose a single-shot QAPI method realized by simply inserting a pair of all-dielectric geometric phase metasurfaces into a traditional microscope. The first metasurface splits a linearly polarized incident beam into two circularly polarized components and the following metasurface deflects the two beams back toward their initial directions. The metasurface pair generates two laterally displaced replicas of the input object, of which the interference forms a retardance image with a bias retardation controlled by an analyzer. The amplitude and phase information of the object is reconstructed from four retardance images simultaneously recorded by a polarized camera. The metasurface pair can be placed near any conjugate plane of the object, which provides a flexible and robust configuration for QAPI, demonstrating its wide usage in live imaging.

Metasurface with Directional‐Controlled Asymmetric Transmissions

The diverse design freedom and exotic electromagnetic properties of metasurfaces enable them to achieve unprecedented capability and superiority with regard to conventional optical elements. Herein, a bifunctional microwave metasurface with an asymmetric bidirection transmission phase that can be controlled is reported. The proposed metasurface is composed of bilayered double‐arrow‐shaped structures sandwiching a subwavelength metal grating. Such design can achieve not only linear polarization (LP) conversion for two oppositely directed wave incidences but also phase control independently by introducing a rotation and adjusting the geometrical parameters of the double‐arrow‐shaped structures. The metasurface breaks the symmetry and realizes different functions for opposite propagation directions, such as deflection and focusing, orbital angular momentum (OAM) mode generation, and holographic imaging. The experimental results suggest that the proposed Janus metasurface can be further exploited as an attractive platform for various applications ranging from microwave reconfigurable beam antenna and multifunctional polarization control to optical imaging, sensing, and information processing.

Metasurface-Based Solid Poincaré Sphere Polarizer.

The combination of conventional polarization optical elements, such as linear polarizers and waveplates, is widely adopted to tailor light's state of polarization (SOP). Meanwhile, less attention has been given to the manipulation of light's degree of polarization (DOP). Here, we propose metasurface-based polarizers that can filter unpolarized incident light to light with any prescribed SOP and DOP, corresponding to arbitrary points located both at the surface and within the solid Poincaré sphere. The Jones matrix elements of the metasurface are inverse-designed via the adjoint method. As prototypes, we experimentally demonstrated metasurface-based polarizers in near-infrared frequencies that can convert unpolarized light into linear, elliptical, or circular polarizations with varying DOPs of 1, 0.7, and 0.4, respectively. Our Letter unlocks a new degree of freedom for metasurface polarization optics and may break new ground for a variety of DOP-related applications, such as polarization calibration and quantum state tomography.

Characterization and design of a freeform holographic optical element

ObjectiveThe aberrations of a conventional lens holographic optical element (HOE) are considerable because of the off-axis recording and diffraction nature. A freeform HOE was introduced to alleviate these aberrations. To design and optimize the recording wavefront of a freeform HOE, it is important to figure out the imaging properties of an HOE. MethodsWe systematically derived a general characterization of the conjugate imaging equation of an elementary HOE recorded by two spherical wavefronts using the scalar diffraction theory. In our design, each patch of a freeform HOE is considered as an elementary HOE. The recording wavefronts of a freeform HOE is designed by determining each elementary HOE through the conjugate imaging equation. ResultsThe image position predicted by the imaging equation is consistent with that of the finite ray-tracing method. The error when the imaging equation is applied to off-axis object points and different reconstruction wavelengths is eliminated effectively through coordinate transformation. Furthermore, based on this equation, a freeform HOE with 3 × 3 tiling elementary HOEs is designed to eliminate distortion and astigmatism of the HOE imaging system. A holographic printer to fabricate this freeform HOE is recommended.

Design, Fabrication and Analysis of a Hybrid-Order Monolithic Imaging Diffractive Lens on a Germanium Substrate

Diffractive optical elements are gradually replacing some conventional optical elements and becoming a key component of optical systems due to their unique phase modulation function. However, the imaging performance will be reduced due to the fact that this single-sided microstructured lens still produces chromatic aberration. Therefore, the key issue for the application of diffractive optical elements in optical systems is the reduction of chromatic aberration, and diffractive lenses with double-sided microstructures are proposed as a solution. This research describes the design and analysis of a 70-mm-diameter, 296-mm-focal-length double-sided microstructured hybrid-order monolithic imaging diffractive lens operating in the mid-wave infrared region (3.7-4.7 μm). The design minimizes chromatic aberration by up to 30 times compared to a standard harmonic diffractive lens and improves the image performance of a single-lens optical system operating in the infrared range. Experiments indicate that this design is capable of achieving single-lens imaging with high sensitivity for optical systems with a measured NETD ≤ 50 mK. The analysis of the experiments yielded suggestions for future research.

All Fiber Vector Magnetometer Based on Nitrogen-Vacancy Center.

Magnetometers based on nitrogen-vacancy (NV) centers in diamonds have promising applications in fields of living systems biology, condensed matter physics, and industry. This paper proposes a portable and flexible all-fiber NV center vector magnetometer by using fibers to substitute all conventional spatial optical elements, realizing laser excitation and fluorescence collection of micro-diamond with multi-mode fibers simultaneously and efficiently. An optical model is established to investigate multi-mode fiber interrogation of micro-diamond to estimate the optical performance of NV center system. A new analysis method is proposed to extract the magnitude and direction of the magnetic field, combining the morphology of the micro-diamond, thus realizing μm-scale vector magnetic field detection at the tip of the fiber probe. Experimental testing shows our fabricated magnetometer has a sensitivity of 0.73 nT/Hz1/2, demonstrating its feasibility and performance in comparison with conventional confocal NV center magnetometers. This research presents a robust and compact magnetic endoscopy and remote-magnetic measurement approach, which will substantially promote the practical application of magnetometers based on NV centers.

Thickness optimization algorithm to improve multilayer diffractive optical elements performance.

The diffractive zone thicknesses of conventional diffractive optical elements (DOEs) are generally obtained using the thin element approximation (TEA). However, the TEA yields inaccurate results in the case of thick multilayer DOEs (MLDOEs). The extended scalar theory (EST) is an alternative thickness optimization method that depends on the diffractive order and the optimization wavelength. We developed an algorithm to research suitable EST input parameters. It combines ray-tracing and Fourier optics to provide a performance estimate for each EST parameter pair. The resulting "best" MLDOE designs for three different material combinations are analyzed using rigorous finite-difference time-domain. Compared to the TEA, the proposed algorithm can provide performing zone thicknesses.

Three-dimensionally reconfigurable focusing of laser by mechanically tunable metalens doublet with built-in holograms for alignment

Abstract Metalenses have potential to replace various bulky conventional optical elements with ultrathin nanostructure arrays. In particular, active metalenses with reconfigurable focusing capability have attracted considerable interest from the academic and industrial communities. However, their tuning range is currently restricted by limited material properties and fabrication difficulties. Here, a hybrid optical system capable of three-dimensional relocation of a focal spot is proposed and experimentally demonstrated. The system comprises a mechanically actuated passive metalens doublet that can be easily fabricated with commonly available materials and processes. An incident laser can be focused to a desired point in three-dimensional space simply by rotating two metalenses or changing their separation. In addition, exploiting the polarization-multiplexing capability of metasurfaces, a hologram is incorporated to the metalenses to guide rotational and positional alignment of two metasurfaces. The ease of fabrication and alignment provided by this approach could widen its application to many practical fields.

A universal metasurface transfer technique for heterogeneous integration.

Metasurfaces offer a versatile platform for engineering the wavefront of light using nanostructures with subwavelength dimensions and hold great promise for dramatically miniaturizing conventional optical elements due to their small footprint and broad functionality. However, metasurfaces so far have been mainly demonstrated on bulky and planar substrates that are often orders of magnitude thicker than the metasurface itself. Conventional substrates not only nullify the reduced footprint advantage of metasurfaces, but also limit their application scenarios. The bulk substrate also determines the metasurface dielectric environment, with potentially undesired optical effects that undermine the optical performance. Here we develop a universal polymer-assisted transfer technique to tackle this challenge by decoupling the substrate employed on the fabrication of metasurfaces from that used for the target application. As an example, Huygens' metasurfaces with 120 nm thickness in the visible range (532 nm) are demonstrated to be transferred onto a 100 nm thick freestanding SiNx membrane while maintaining excellent structural integrity and optical performance of diffraction-limited focusing. This transfer method not only enables the thinnest dielectric metalens to the best of our knowledge, but also opens up new opportunities in integrating cascaded and multilayer metasurfaces, as well as the heterogeneous integration with nonconventional substrates and various electronic/photonic devices.