Flexible Photonics Research Articles

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

Transparent and Flexible Photon Sieve Made with Cellulose Nanofiber by Micro-Nano Structure Molding

Transparent and Flexible Photon Sieve Made with Cellulose Nanofiber by Micro-Nano Structure Molding

The Green Synthesis of 2D Copper Nanosheets and Their Light Absorption.

In this study, a new green synthesis method for two-dimensional (2D) copper nanosheets is developed using methylsulfonylmethane (DMSO2). The chemical composition and light absorption of 2D copper nanosheets are also studied. A new green method is mainly to utilize DMSO2, which is environmentally friendly enough to be considered a food-grade chemical, unlike the conventional method using toxic chemicals, such as ammonia and hydrazine (N2H4). With a reducing agent, the aggregation of uncertain copper products was produced in the absence of DMSO2, while 2D copper nanosheets were formed in the presence of DMSO2. The optimum concentration of DMSO2 as a surfactant was determined to be 2 M, resulting in large surface areas with regular edges. FTIR spectrum confirmed C–H bonding from DMSO2 used to synthesize 2D copper nanosheets. The light absorption peak was revealed at 800 nm in the UV–vis spectrum. This proposed new green method not only has a simpler process than the conventional methods, such as hydrothermal method and chemical bath deposition, but also substitutes toxic chemicals with DMSO2. 2D copper nanosheets can be used for various applications, including conductive filler or ink in the flexible electronics and laser photonics fields.

Enhanced Metal-Insulator Transition in Freestanding VO2 Down to 5 nm Thickness.

Ultrathin freestanding membranes with a pronounced metal-insulator transition (MIT) have huge potential for future flexible electronic applications as well as provide a unique aspect for the study of lattice-electron interplay. However, the reduction of the thickness to an ultrathin region (a few nm) is typically detrimental to the MIT in epitaxial films, and even catastrophic for their freestanding form. Here, we report an enhanced MIT in VO2-based freestanding membranes, with a lateral size up to millimeters and the VO2 thickness down to 5 nm. The VO2 membranes were detached by dissolving a Sr3Al2O6 sacrificial layer between the VO2 thin film and the c-Al2O3(0001) substrate, allowing the transfer onto arbitrary surfaces. Furthermore, the MIT in the VO2 membrane was greatly enhanced by inserting an intermediate Al2O3 buffer layer. In comparison with the best available ultrathin VO2 membranes, the enhancement of MIT is over 400% at a 5 nm VO2 thickness and more than 1 order of magnitude for VO2 above 10 nm. Our study widens the spectrum of functionality in ultrathin and large-scale membranes and enables the potential integration of MIT into flexible electronics and photonics.

From flexible electronics to flexible photonics: A brief overview

The development of the information technology and, very recently, of new application scenarios like Internet of Things (IoT) and Industry 4.0 has pushed the research towards new technological platforms. In this frame, the spatial freedom permitted by flexible short-range connections and flexible devices has become as important as “classical” parameters such as low weight, low power consumption, and electromagnetic immunity. Accordingly, first the flexible electronics and later the flexible photonics have produced innovative components and devices. The present paper aims at presenting a brief overview of this broad area, underlining the achievements and the remaining challenges in the different routes to the manufacturing of flexible photonic devices. Material platforms remain at the core of such developments, and it is interesting to note that glassy materials still constitute a fundamental piece in the present and future scenario. • A short summary of the early steps of flexible electronics is presented. • Early developments of flexible photonics are discussed. • Various material platforms of flexible photonics are reviewed: ∙ Semiconductor on polymer flexible structures ∙All-polymeric structures ∙Glass on polymer structures ∙Flexible glass substrates and components • Further potential developments are outlined

Flexible single-crystalline GaN substrate by direct deposition of III-N thin films on polycrystalline metal tape

Demonstration of a mechanically flexible single-crystalline GaN substrate by direct epitaxial growth on metal foil with significantly reduced processing costs and versatile functionality on flexible electronics and photonics.

Flexible fiber-laser ultrasound sensor for multiscale photoacoustic imaging

Photoacoustic imaging (PAI) is a noninvasive biomedical imaging technology capable of multiscale imaging of biological samples from organs down to cells. Multiscale PAI requires different ultrasound transducers that are flat or focused because the current widely-used piezoelectric transducers are rigid and lack the flexibility to tune their spatial ultrasound responses. Inspired by the rapidly-developing flexible photonics, we exploited the inherent flexibility and low-loss features of optical fibers to develop a flexible fiber-laser ultrasound sensor (FUS) for multiscale PAI. By simply bending the fiber laser from straight to curved geometry, the spatial ultrasound response of the FUS can be tuned for both wide-view optical-resolution photoacoustic microscopy at optical diffraction-limited depth (~1 mm) and photoacoustic computed tomography at optical dissipation-limited depth of several centimeters. A radio-frequency demodulation was employed to get the readout of the beat frequency variation of two orthogonal polarization modes in the FUS output, which ensures low-noise and stable ultrasound detection. Compared to traditional piezoelectrical transducers with fixed ultrasound responses once manufactured, the flexible FUS provides the freedom to design multiscale PAI modalities including wearable microscope, intravascular endoscopy, and portable tomography system, which is attractive to fundamental biological/medical studies and clinical applications.

Programmable High-Resolution Spectral Processor in C-band Enabled by Low-Cost Compact Light Paths

The flexible photonics spectral processor (PSP) is an indispensable element for elastic optical transmission networks that adopt wavelength division multiplexing (WDM) technology. The resolution and system cost are two vital metrics when developing a PSP. In this paper, a high-resolution 1 × 6 programmable PSP is investigated and experimentally demonstrated by using low-cost compact spatial light paths, which is enabled by a 2 K (1080p) liquid crystal on silicon (LCoS) and two cascaded transmission gratings with a 1000 line/mm resolution. For each wavelength channel, the filtering bandwidth and power attenuation can be manipulated independently. The total insertion loss (IL) for six ports is in the range of 5.9~9.4 dB over the full C-band. The achieved 3-dB bandwidths are able to adjust from 6.2 GHz to 5 THz. Furthermore, multiple system experiments utilizing the proposed PSP, such as flexible spectral shaping and optical frequency comb generation, are carried out to validate the feasibility for the WDM systems.

Solvent Responsive Self‐Folding of 3D Photosensitive Graphene Architectures

Stimuli responsive self‐folding structures with 2D layered materials (2DLMs) are important for flexible electronics, wearables, biosensors, bioelectronics, and photonics. Previously, strategies have been developed to self‐fold 2D materials to form various robots, sensors, and actuators. Still, there are limitations with scalability and a lack of design tools to obtain complex structures for reversible actuation, high integration, and reliable function. Herein, a mass‐producible strategy for creating monolayer graphene‐based reversible self‐folding structures using either gradient or differentially cross‐linked films of a negative epoxy photoresist widely used in microfluidics and micromechanical systems, namely, SU8 is demonstrated. Wafer‐scale patterning and integration of complex and functional devices in the form of rings, polyhedra, flowers, and bidirectionally folded origami birds are achieved. Also, gold (Au) electrodes to realize functional graphene–Au Schottky interfaces with enhanced photoresponse and 3D angle sensitive detection are integrated. The experiments are guided and rationalized by theoretical methods including coarse‐grained models, specifically developed for the tunable mechanics of this photoresist that simulate the folding dynamics, and finite element method (FEM) electromagnetic simulations. This work suggests a comprehensive framework for the rational design and scalable fabrication of complex 3D self‐actuating optical and electronic devices through the folding of 2D monolayer graphene.

Transparent and Flexible Inorganic Perovskite Photonic Artificial Synapses with Dual‐Mode Operation

AbstractWith the rapid development of artificial intelligence, the simulation of the human brain for neuromorphic computing has demonstrated unprecedented progress. Photonic artificial synapses are strongly desirable owing to their higher neuron selectivity, lower crosstalk, wavelength multiplexing capabilities, and low operating power compared to their electric counterparts. This study demonstrates a highly transparent and flexible artificial synapse with a two‐terminal architecture that emulates photonic synaptic functionalities. This optically triggered artificial synapse exhibits clear synaptic characteristics such as paired‐pulse facilitation, short/long‐term memory, and synaptic behavior analogous to that of the iris in the human eye. Ultraviolet light illumination‐induced neuromorphic characteristics exhibited by the synapse are attributed to carrier trapping and detrapping in the SnO2 nanoparticles and CsPbCl3 perovskite interface. Moreover, the ability to detect deep red light without changes in synaptic behavior indicates the potential for dual‐mode operation. This study establishes a novel two‐terminal architecture for highly transparent and flexible photonic artificial synapse that can help facilitate higher integration density of transparent 3D stacking memristors, and make it possible to approach optical learning, memory, computing, and visual recognition.

Flexible and Stretchable Photonics: The Next Stretch of Opportunities

Conventional electronic and photonic devices are inherently 2D and rigid because of the substrates on which they are fabricated. However, the world is not flat and stiff: There are many application...

Ultra-thin curved visible microdisk lasers with three-dimensional whispering gallery modes

Abstract Microdisk lasers are important components in photonic integrated circuits (PICs), of which the whispering gallery modes (WGMs) are usually confined within a two-dimensional (2D) planar slab. Here, owing to the strain relaxation of quantum wells by wet-etching method, we present ultra-thin curved visible microdisk lasers with single-mode lasing emission and a high quality factor of ∼17,000, which enable a 3D spatial intensity distribution of WGMs and provide an extra degree of freedom for the confined photons compared with the conventional 2D in-plane WGMs. The curved microdisk lasers with a 3D spatial profile of WGMs may provide attractive applications in flexible and multilevel photon sources for the PICs.

Homogeneous Model for the Nanoparticle Growth in Polymer Matrices

A promising method of the fabrication of polymer nanocomposite materials relies on the in situ generation of the nanoparticles (NPs) in the matrix due to the thermal or photochemical destruction of some sort of precursor. By means of selective transformation of the clean polymer to the nanocomposite using the direct laser writing or by applying a photomask, one can construct the multiscale structured composite materials for numerous applications, including flexible electronics, photonics, and plasmonics. In this work, we propose a model for the nanoparticle generation and growth in polymers based on Smoluchowski’s equations. Unlike the typical liquids, the diffusion of the particles in polymer matrices does not obey the Stokes–Einstein law. The steep degradation of the NP’s diffusivity with its size results in the self-organization of the particle mass distribution into two fractions. One includes the small clusters that are constantly generated due to the precursor destruction. The other consists of the nearly constant number of relatively large particles that gather the small clusters, acting as heterogeneous growth centers. Such quasi-heterogeneous behavior is possible without any initially existing growth centers, such as impurities and interfaces. The difference of this quasi-heterogeneous behavior from the one suggested by true heterogeneous models is discussed. The possible effects of the nonspherical particle shape and stable configurations of small clusters containing the “magic number” of atoms are studied.

Ultra-thin glass as a substrate for flexible photonics

Flexible photonics is an emerging field in optical materials for several frontier applications. New ultra-thin glasses with thicknesses ranging from tens to hundreds of microns are potential candidates as substrates for flexible photonics applications. The improvement in mechanical performance, specifically retained strength, for such glasses is achieved by composition design and ion exchange post-processing. The main physical effects to be considered are the introduction of residual stress profiles and refractive index modifications. Both aspects may interfere with flexible photonics application of these glass substrates. The physics of stress buildup and relaxation are discussed, as well as the impact on refractive index. The discussion also considers promising glass compositions widely used in consumer electronics applications.

Flexible photonic devices based on dielectric antennas

Flexible and stretchable photonics are emerging fields aiming to develop novel applications where the devices need to conform to uneven surfaces or whenever lightness and reduced thickness are major requirements. However, owing to the relatively small refractive index of transparent soft matter including most polymers, these materials are not well adapted for light management at visible and near-infrared frequencies. Here we demonstrate simple, low cost and efficient protocols for fabricating Si1−xGex-based, sub-micrometric dielectric antennas over record scales (50 mm wafers) with ensuing hybrid integration into different plastic supports. The transfer process has a near-unity yield: up to 99.94% for disordered structures and 99.5% for the ordered counterpart. Finally, we benchmark the optical quality of the dielectric antennas with light scattering measurements, demonstrating the control of the islands structural color and the onset of sharp Mie modes after encapsulation in plastic. Thanks to the ease of implementation of our fabrication methods, these results are relevant for the integration of SiGe-based dielectric Mie resonators in flexible substrates over large surfaces.

Hyperuniform disordered waveguides and devices for near infrared silicon photonics

We introduce a hyperuniform-disordered platform for the realization of near-infrared photonic devices on a silicon-on-insulator platform, demonstrating the functionality of these structures in a flexible silicon photonics integrated circuit platform unconstrained by crystalline symmetries. The designs proposed advantageously leverage the large, complete, and isotropic photonic band gaps provided by hyperuniform disordered structures. An integrated design for a compact, sub-volt, sub-fJ/bit, hyperuniform-clad, electrically controlled resonant optical modulator suitable for fabrication in the silicon photonics ecosystem is presented along with simulation results. We also report results for passive device elements, including waveguides and resonators, which are seamlessly integrated with conventional silicon-on-insulator strip waveguides and vertical couplers. We show that the hyperuniform-disordered platform enables improved compactness, enhanced energy efficiency, and better temperature stability compared to the silicon photonics devices based on rib and strip waveguides.

Solution Processed Polymer-ABX4 Perovskite-Like Microcavities

Thanks to solution processability and broad emission in the visible spectral range, 2D hybrid perovskite-like materials are interesting for the realization of large area and flexible lighting devices. However, the deposition of these materials requires broad-spectrum solvents that can easily dissolve most of the commercial polymers and make perovskites incompatible with flexible photonics. Here, we demonstrated the integration of broadband-emitting (EDBE)PbCl4 (where EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)) thin films with a solution-processed polymer planar microcavities, employing a sacrificial polymer multilayer. This approach allowed for spectral and angular redistribution of the perovskite-like material, photoluminescence, that can pave the way to all-solution-processed and flexible lightning devices that do not require complex and costly fabrication techniques.

Ultrathin Silicon Nanomembrane in a Tubular Geometry for Enhanced Photodetection

AbstractAdvanced 3D mesostructures have prominent applications in flexible electronics, photonics, mechanics, and biomedicine. Here, 3D tubular silicon photodetectors are demonstrated by self‐rolling ultrathin silicon nanomembranes with enhanced broadband photodetection and responsivity. Strain distributions within the unique circular structures are investigated with micro‐Raman spectroscopy, while the tensile strain of the rolled‐up microtubes can be controlled with the tube diameter by modifying the membrane thickness and etching parameters during the rolled‐up process. The tubular photodetector exhibits increased photocurrent and suppressed dark current compared with planar devices over a wide incident angle. This study provides a novel approach to construct high‐performance omnidirectional Si‐based photodetectors and offers promising prospects for design and manufacture of complex 3D optoelectronic devices.

Flexible and Stretchable Photonic Sensors Based on Modulation of Light Transmission

AbstractEmerging technologies, such as soft robotics, electronic skin, wearable devices, and flexible display, demand the practical application of photonic sensors that are bendable and stretchable, as conventional photonic sensors are fabricated with rigid materials and substrates, rendering their applications in deformable state or soft systems infeasible. Thus, the development of flexible and stretchable photonic sensors, which can be wrapped onto curved surfaces and easily deformed into different shapes and sizes, has inspired great research interest. This article presents recent progress in the development of flexible and stretchable photonic sensors, particularly photonic sensors based on the modulation of light transmission, from the aspects of materials synthesis and selection, structure design, fabrication methods, sensing characteristics, and performances. The challenges and future research opportunities related to flexible and stretchable photonic sensors and their potential application prospects are also discussed.

Ultrafast Energy Transfer of Both Bright and Dark Excitons in 2D van der Waals Heterostructures Beyond Dipolar Coupling.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have shown great potential in ultrathin and flexible optoelectronic and photonics devices. Besides emissive bright excitons, they also possess rich non-emissive dark excitons including momentum-forbidden indirect excitons and spin-forbidden triplet-like excitons, which could be dominant species under optical or electrical excitation in 2D optoelectronic and photonic devices. Efficient harvesting of both bright and dark excitons from TMDs and understanding the exciton-transfer mechanism consequently are not only of fundamental interest but also a technological challenge. Here, by combining steady-state photoluminescence spectroscopy and ultrafast transient reflectance spectroscopy, we show efficient exciton harvesting by ultrafast energy transfer in WSe2/MoTe2 van der Waals heterostructures, leading to the photoluminescence enhancement of MoTe2. The energy transfer occurs with near-unity yield and in an ultrafast (∼200 fs) manner for both bright and dark excitons, suggesting a dominant Dexter-type energy-transfer process consisting of simultaneous transfer of both electron and hole in van der Waals coupled 2D layers at ultimate proximity. This result is beyond the conventional dipole-dipole coupling mechanism typically assumed at 2D interfaces and offers a path to high speed and enhanced light harvesting and emission applications based on 2D heterostructures.