Compact Optical Components Research Articles

Tunable dielectric BIC metasurface for high resolution optical filters

The dielectric metasurface has become a powerful tool for compact optical components with various wavefront controlling functionalities accompanied by negligible losses at the corresponding working frequencies. In this work, we propose a tunable all-dielectric metasurface as an optical filter with high resolution covering different optical communication bands, where tunability is realized by a combination of changing the incident angle and modulating the refractive index of an optical phase changing material (OPCM). When the incident angle varies, our optical filter based on a two-dimensional bound state in continuums (BIC) metasurface can achieve sequential, extremely sharp resonances. In addition, the resonance peaks could be further shifted to a different frequency band by the refractive index change of OPCM via pulsed laser heating. The proposed scheme can offer optical filters with high spectral resolution and large tunable working wavelength range, which greatly benefits from the topological property of BIC and large modulation depth of OPCM.

Fabrication of ultrathin flexible diffractive optical elements (DOEs) and application for laser speckle suppression

Compact and flexible optical components are expected to be utilized in laser-imaging systems. We introduce a novel technique for producing self-made ultrathin flexible diffractive optical elements (DOEs) with reciprocating movement that are effective in suppression of laser speckle. We developed a preparation method for acetone-PMMA and benzene-PMMA coating liquids, and obtained all the process parameters for producing ultrathin flexible PMMA DOEs. Experimental results revealed that the fabricated DOEs reduced the speckle contrasts for red, green, and blue lasers substantially; in addition, their transmittance increased significantly. The proposed techniques can be applied extensively in the development of other kind of flexible microoptics.

Integrated Polarization-Splitting Grating Coupler for Chip-Scale Atomic Magnetometer.

Atomic magnetometers (AMs) are widely acknowledged as one of the most sensitive kind of instruments for bio-magnetic field measurement. Recently, there has been growing interest in developing chip-scale AMs through nanophotonics and current CMOS-compatible nanofabrication technology, in pursuit of substantial reduction in volume and cost. In this study, an integrated polarization-splitting grating coupler is demonstrated to achieve both efficient coupling and polarization splitting at the D1 transition wavelength of rubidium (795 nm). With this device, linearly polarized probe light that experienced optical rotation due to magnetically induced circular birefringence (of alkali medium) can be coupled and split into individual output ports. This is especially advantageous for emerging chip-scale AMs in that differential detection of ultra-weak magnetic field can be achieved through compact planar optical components. In addition, the device is designed with silicon nitride material on silicon dioxide that is deposited on a silicon substrate, being compatible with the current CMOS nanofabrication industry. Our study paves the way for the development of on-chip AMs that are the foundation for future multi-channel high-spatial resolution bio-magnetic imaging instruments.

Thermally reconfigurable metalens.

Reconfigurable metalenses are compact optical components composed by arrays of meta-atoms that offer unique opportunities for advanced optical systems, from microscopy to augmented reality platforms. Although poorly explored in the context of reconfigurable metalenses, thermo-optical effects in resonant silicon nanoresonators have recently emerged as a viable strategy to realize tunable meta-atoms. In this work, we report the proof-of-concept design of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon metalens operating at a fixed, visible wavelength (632 nm). Importantly, we demonstrate continuous, linear modulation of the focal-length up to 21% (from 165 μm at 20 °C to 135 μm at 260 °C). Operating under right-circularly polarized light, our metalens exhibits an average conversion efficiency of 26%, close to mechanically modulated devices, and has a diffraction-limited performance. Overall, we envision that, combined with machine-learning algorithms for further optimization of the meta-atoms, thermally reconfigurable metalenses with improved performance will be possible. Also, the generality of this approach could offer inspiration for the realization of active metasurfaces with other emerging materials within field of thermo-nanophotonics.

3D Switchable Diffractive Optical Elements Fabricated with Two‐Photon Polymerization

AbstractDirect laser writing is demonstrated by two‐photon polymerization of multi‐element diffractive optical components that can be switched on and off with an applied voltage. By exploiting the 3D capabilities of the laser microfabrication technique, multiple diffractive optical elements are written into a single liquid crystal (LC) layer. The switching behavior of the diffractive optical elements is controlled by simply changing the write‐voltage of the anisotropic polymer structures during fabrication. Initially, 2D diffraction gratings are written at different depths within the LC layer. Each element is then activated by applying a voltage of sufficient amplitude that causes the second diffractive optical element to become inactive. This is then followed by a demonstration of multi‐element computer generated holograms that are written at different depths within the LC layer. By altering the magnitude of the applied voltage, different images/patterns can be observed in the replay field using a simple electrode configuration. These compact and transmissive LC optical components could excel in applications where a degree of switchability is required but a highly pixelated fully programmable device is excessive.

All‐Dielectric Meta‐Optics for High‐Efficiency Independent Amplitude and Phase Manipulation

Optical metasurfaces, composed of subwavelength scattering elements, demonstrate remarkable control over the transmitted amplitude, phase, and polarization of light. However, manipulating the amplitude upon transmission, without the use of surface waves, requires loss if a single metasurface is used. Herein, high‐efficiency independent manipulation of the amplitude and phase of a beam is described using two lossless phase‐only metasurfaces separated by a distance. With this configuration, optical components such as combined beam‐forming and splitting devices are experimentally demonstrated, as well as those for forming complex‐valued, 3D holograms. The compound meta‐optic platform provides a promising approach for achieving high‐performance holographic displays and compact optical components, while exhibiting a high overall efficiency.

A wide-field microscope utilizing two cellphones for health-care applications.

In this report, we report a wide field-of-view (FOV) bright field (BF) microscope with compact and portable optical components, mechanically attached to the existing camera unit of the cellphone. A white screen displayed on a cellphone as the illumination source to pump the sample of interest uniformly for the purpose of the reduction in assembly complexity and alignment. It offers a large FOV of 4.36 × 3.27 mm without digital zoom and a spatial resolution of 1.5μm. Furthermore, we also have demonstrated the potential application for diseases diagnosis and screening by imaging malaria-infected blood sample and iron deficiency anemia blood sample in resource-constrained settings where mobile phone infrastructure is already ubiquitous but microscope is notoriously scarce.

Combining Switchable Phase‐Change Materials and Phase‐Transition Materials for Thermally Regulated Smart Mid‐Infrared Modulators

AbstractPhase‐change materials (PCMs) and phase‐transition materials (PTMs) both show a large contrast in their respective optical properties upon switching, enabling compact optical components with diverse functionalities like sensing, thermal imaging, and data recording. However, their switching properties differ significantly, that is, the switching is non‐volatile for PCMs while volatile for PTMs. Here, new‐generation smart mid‐infrared modulators with switchable transmission, reflection, and absorption are demonstrated conceptually and experimentally, which combine one PCM (Ge3Sb2Te6 or In3SbTe2) with one PTM (VO2) as two active layers. The bottom VO2 layer is employed as a thermally regulated (modulated) dynamic mirror, facilitating the switching of transmission between “on” state (using VO2 in its semiconducting state at temperatures below its phase transition temperature Tc) and “off” state (metallic VO2 at temperatures above Tc). The PCM layer on top of the metallic VO2 layer is used either for continuously adjusting the absorption peak spectrally (by up to 1.8 µm using different phases of Ge3Sb2Te6) or for switching between absorption mode (A = 0.99 with amorphous In3SbTe2) and reflection mode (R = 0.85 with crystalline In3SbTe2). The presented concept of merging static, non‐volatile thermal switching (via PCMs) with dynamic, volatile thermal modulation (via PTMs) empowers a new generation of optical devices for smart optical switching, for example in spectrally tunable safety optical switches.

Lossless Complex-Valued Optical-Field Control with Compound Metaoptics

Forming a desired optical field distribution from a given source requires precise spatial control of a field's amplitude and phase. Low-loss metasurfaces that allow extreme phase and polarization control of optical fields have been demonstrated over the past few years. However, metasurfaces that provide amplitude control have remained lossy, utilizing mechanisms such as reflection, absorption, or polarization loss to control amplitude. Here, we describe the amplitude and phase manipulation of optical fields without loss, by using two lossless phase-only metasurfaces separated by a distance. We first demonstrate a combined beam-former and splitter optical component using this approach. Next, we show a high-quality computer-generated three-dimensional hologram. The proposed metaoptic platform combines the advantage of lossless, complex-valued field control with a physically small thickness. This approach could lead to low-profile, three-dimensional holographic displays, compact optical components, and high precision optical tweezers for micro-particle manipulation.

Portable NIR Spectroscopy Measuring Device for Transformer Oil DBPC Inhibitor Analysis

Monitoring the insulation system of power transformers is crucial in ensuring power transformers are operating in optimal conditions. Given the nondirect and destructive nature of conventional techniques, optical techniques have prevailed to be an effective alternative to monitor the condition of the insulation system. The recent development of small and compact optical components has also contributed to the utilization of optical techniques in onsite testing and online monitoring application. Therefore, this work proposed the measurement of inhibitor concentration in transformer oil by using a portable optical measuring device that utilizes near-infrared spectroscopy. The fundamental studies of measuring inhibitor in transformer oil by using a newly discovered wavelength at 1403 nm were validated, and its adaptation to a portable optical measuring device was discussed. A new mathematical model was formulated to estimate the inhibitor concentration in transformer oil samples on the basis of data obtained from the photodetector. Measurements and the conventional technique were conducted for performance comparison and prototype verification. Results showed a root-mean-square-error of 0.01708 and an average error of 2.11%. This proposed prototype opens up opportunities in onsite testing and online monitoring of inhibitor content concentration in transformer oil.

Engineering of the Second‐Harmonic Emission Directionality with III–V Semiconductor Rod Nanoantennas

AbstractThe ability to engineer nonlinear optical emission from nanostructures is a key challenge to create efficient and compact components for integrated devices. This paper shows a method to control and manipulate the directionality of second‐harmonic generation emission by engineering geometry and position of rod nanoantennas. Single and dimer nanoantennas are fabricated by slicing III–V semiconductor nanowires with focused ion beam milling. The nonlinear optical response of nanoantennas is tailored by adjusting their length and position to achieve a targeted phase difference. The studied GaAs nanoantennas have a wurtzite structure that allows to achieve preferable directions for the second‐harmonic emission compared to a typical bulk zinc blende structure from top‐down fabricated nanostructures. Wurtzite nanoantennas provide a pure electric dipole response at the second‐harmonic wavelength, which together with pi‐phase control of emitted light is used for designing nonlinear emission patterns. The simulation results show how to redirect the second‐harmonic beam up to 30° and how to tailor the emission profile by adding elements. This method of second‐harmonic generation manipulation and phase array engineering can be applied to different types of nanowires and nanostructures. Nonlinear beam steering with structures from nanowires will foster the creation of compact optical components for integrated circuits.

Generalized Hartmann-Shack array of dielectric metalens sub-arrays for polarimetric beam profiling

To define and characterize optical systems, obtaining the amplitude, phase, and polarization profile of optical beams is of utmost importance. Traditional polarimetry is well established to characterize the polarization state. Recently, metasurfaces have successfully been introduced as compact optical components. Here, we take the metasurface concept to the system level by realizing arrays of metalenses, allowing the determination of the polarization profile of an optical beam. We use silicon-based metalenses with a numerical aperture of 0.32 and a mean measured focusing efficiency in transmission mode of 28% at a wavelength of 1550 nm. Our system is extremely compact and allows for real-time beam diagnostics by inspecting the foci amplitudes. By further analyzing the foci displacements in the spirit of a Hartmann-Shack wavefront sensor, we can simultaneously detect phase-gradient profiles. As application examples, we diagnose the profiles of a radially polarized beam, an azimuthally polarized beam, and of a vortex beam.

Machinable ceramic for high performance and compact THz optical components

The properties of Shapal Hi-M Soft, an aluminum nitride based ceramic, as a material for THz optical components are investigated and compared with other ceramic materials. Shapal is a low-cost and machinable ceramic with a high-refractive index and low losses at THz frequencies: n = 2.65, α = 0.4 cm−1. To demonstrate the shaping capabilities, a Fresnel lens is fabricated with a micro-milling system. Additionally, a prism as an example of a bulk component is demonstrated. Both a THz time domain spectrometer and a vector network analyzer (0.75–1.1 THz) are used for the optical characterization and the absorption coefficient is precisely obtained in the VNA frequency range.

High‐Efficiency Broadband Mid‐Infrared Flat Lens

AbstractIntegrated photonic circuits and infrared imaging systems demand compact optical components. Dielectric diffractive optics enables miniaturization of the curved refractive optics into planar structure by encoding phase over a 2D plane. However, in the mid‐infrared wavelength range, such engineered dielectric surfaces are not efficient, because optically transparent dielectric scatterers with high index contrast in the mid‐infrared spectral range suffer from low bandwidth and high thermal noise in long wavelengths. Here, a planar optical lens based on ultrathin gold plasmonic nanostructure operating in the mid‐infrared spectral range is demonstrated. The design enables subwavelength focusing beyond the Abbe‐Rayleigh diffraction limit while maintaining high transmission efficiency (≈60%) with excellent agreement between electromagnetic simulations and confocal measurements. Single and bilayer flat lenses designed for subwavelength polarization‐dependent line and polarization‐independent point focusing, respectively, are demonstrated. Such geometry‐defined tunable optical response overcomes the challenges associated with the unavailability of mid‐infrared transparent materials for low footprint planar integration with thermal imaging systems.

Optimization-based Dielectric Metasurfaces for Angle-Selective Multifunctional Beam Deflection

Synthesization of multiple functionalities over a flat metasurface platform offers a promising approach to achieving integrated photonic devices with minimized footprint. Metasurfaces capable of diverse wavefront shaping according to wavelengths and polarizations have been demonstrated. Here we propose a class of angle-selective metasurfaces, over which beams are reflected following different and independent phase gradients in the light of the beam direction. Such powerful feature is achieved by leveraging the local phase modulation and the non-local lattice diffraction via inverse scattered field and geometry optimization in a monolayer dielectric grating, whereas most of the previous designs utilize the local phase modulation only and operate optimally for a specific angle. Beam combiner/splitter and independent multibeam deflections with up to 4 incident angles are numerically demonstrated respectively at the wavelength of 700 nm. The deflection efficiency is around 45% due to the material loss and the compromise of multi-angle responses. Flexibility of the approach is further validated by additional designs of angle-switchable metagratings as splitter/reflector and transparent/opaque mirror. The proposed designs hold great potential for increasing information density of compact optical components from the degree of freedom of angle.

Sharp bend in two-dimensional optical waveguide based on gradient refractive index structure.

In this work, we propose the design of a sharp bend in a two-dimensional optical waveguide which has super-ellipse curve boundaries and a gradient refractive index structure in its core. Numerical simulations are presented to show the efficient light propagation in the waveguide bend, as well as the efficient light coupling between the proposed waveguide bend and a straight waveguide, for TE0 and TM0 modes. The proposed design strategy is also useful for designing other compact optical and photonic components.

A novel lab-on-chip platform with integrated solid phase PCR and Supercritical Angle Fluorescence (SAF) microlens array for highly sensitive and multiplexed pathogen detection

Solid-phase PCR (SP-PCR) has become increasingly popular for molecular diagnosis and there have been a few attempts to incorporate SP-PCR into lab-on-a-chip (LOC) devices. However, their applicability for on-line diagnosis is hindered by the lack of sensitive and portable on-chip optical detection technology. In this paper, we addressed this challenge by combining the SP-PCR with super critical angle fluorescence (SAF) microlens array embedded in a microchip. We fabricated miniaturized SAF microlens array as part of a microfluidic chamber in thermoplastic material and performed multiplexed SP-PCR directly on top of the SAF microlens array. Attribute to the high fluorescence collection efficiency of the SAF microlens array, the SP-PCR assay on the LOC platform demonstrated a high sensitivity of 1.6 copies/µL, comparable to off-chip detection using conventional laser scanner. The combination of SP-PCR and SAF microlens array allows for on-chip highly sensitive and multiplexed pathogen detection with low-cost and compact optical components. The LOC platform would be widely used as a high-throughput biosensor to analyze food, clinical and environmental samples.

Compact On-Chip Optical Components Based on Multimode Interference Design Using High-Contrast Grating Hollow-Core Waveguides

Low-loss high-contrast grating (HCG) hollow-core waveguide (HCW) is shown to be a promising candidate for building a variety of compact multimode interference (MMI) based optical components. First, we show that HCG-HCWs demonstrate self-imaging property essential for designing MMI components. Furthermore, strong confinement of light in the waveguide core and low-index guiding leads to compact optical components. Simulation results show extremely compact 2 × 2 coupler with a length of 26 μm (16 λ), footprint (length × width) of 85 μm2 (35 λ2) and a 3-dB splitter with a length of 3.6 μm (2.4 λ), and a footprint of 12 μm 2 (5 λ2). Simulation results agree well with the predictions of MMI theory and the design can be easily extended to realize 1 × N splitters and N × N couplers. As a proof of concept, a 1 × 4 splitter with equal power splitting ratio and low insertion loss is designed. In addition, an ultracompact 2 × 2 optical switch based on HCG-HCW is proposed and simulated. The two waveguide channels are isolated by a single layer of HCG. By a small change of the refractive index of this HCG, light can switch between the channels. By proper optimization of HCG parameters, only higher order modes are excited in the MMI region resulting in an ultracompact optical switch with a switching length of 60 μm, ∼25× smaller than conventional 2 × 2 MMI coupler.

Silicon Photonic Integrated Circuits for Wavelength-Division Multiplexing Applications

Silicon photonics will provide low-cost, high-bandwidth and compact optical components for a wide range of applications in optical communications and interconnects. One of the cited key advantages is the capability of wavelength-division multiplexing (WDM). However, the nature of high-index contrast of silicon photonic devices leads to significant challenges when implementing on-chip WDM filters, which is one of the key components in WDM circuits. In this paper, we review several demonstrated silicon photonic WDM circuits based on monolithically integrated silicon nitride (SiN) arrayed-waveguide gratings (AWGs) and thermally tunable silicon microring filters. The use of SiN waveguides with lower index contrast than silicon waveguides enables the realization of high-performance AWGs. Meanwhile, they can evanescently couple to silicon waveguides with high efficiency. The thermally tunable silicon microrings can be used as modulators and wavelength (de)multiplexing filters to implement versatile WDM circuits. Reconfigurability of channel spacing and central wavelengths is achieved by individual tuning of the rings. In this paper, we review silicon photonic circuits for multiple-channel modulators, polarization-insensitive WDM receiver, and variable optical attenuators with multiplexer.

Dual Fluorescence-Activated Study of Tumor Cell Apoptosis by an Optofluidic System

We have developed a planar optofluidic chip for the analysis of tumor cell apoptosis by exciting dual fluorescence through the integrated fibers. In contrast to conventional cell apop- tosis study that requires commercial flow cytometer with a very high price tag and bulky optics, our design was realized via a sim- ple single-layer soft lithography fabrication process. The device performance was tested by characterizing the HeLa cell apoptosis induced by hydrogen peroxide. The performance of the proposed optofluidic chip was concurrently confirmed by fluorescent mi- croscopy and benchmarked against a commercial flow cytometer. This study demonstrated the capability of the proposed optofluidic chip for cell apoptosis assay. The major advantages of this device include simple fabrication process, compact optical components, reliable performance, and low manufacturing cost, making it a promising platform for future mass-producible, inexpensive, and disposable on-chip investigation of biological samples.