Wearable Photonics Research Articles

Bio-inspired stretchable and self-healable nanocomposite gelatin hydrogel with low silica nanoparticle content and brilliant angle/strain-independent structural colors

Stretchable and self-healable structural-colored nanocomposite gelatin hydrogel with photonic structure of random dense/loose regions of non-close-packed silica nanoparticles (SiO2 NPs) is developed via convenient and scalable polymer-mediated assembly. The formation of the special photonic structure with overall low SiO2 NP content is characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The small-angle X-ray scattering test indicates the existence of short-range order, which is consistent with the TEM and SEM results. The dense photonic structure consisting of SiO2 NP chains and domains, and the loose structure consisting of abundant solitary SiO2 NPs, accounts for pronounced and uniform structural colors via combined effects of coherent scattering of light by dense structure, and Mie scattering of solitary NPs. These structural colors are independent of viewing angles and large mechanical strains. The dynamic physical and chemical bonding of the gelatin hydrogel network matrix enables self-healing functions and good deformability. Moreover, salt treatment of the structural-colored gelatin hydrogel networks enhances physical crosslinking and the mechanical strength of the hydrogel. The resilient, flexible and self-healable structural-colored nanocomposite hydrogel with low SiO2 NP content has great potential in wearable photonics, cosmetics, and various other color-related fields. This work also sheds light on enhancing non-iridescent structural colors through advanced photonic design.

Wrinkled Graphene Structure and Localized Surface Plasmon Resonance Induced Stretchable White Random Lasers Based on GLN‐functionalized 2D WS2 Quantum Dots

AbstractThis study presents investigations into the fabrication, characterization, and performance analysis of stretchable white random lasers based on 2D glutamine(GLN)‐functionalized WS2 quantum dots (QDs) enhanced by the integration of Au nanoparticles (NPs) and a wrinkled graphene structure. Wrinkled graphene holds the potential for achieving transient population inversion through electron collisions. Incorporating Au NPs introduces localized surface plasmon resonance (LSPR), which enhances the light‐matter interaction, resulting in reduced lasing thresholds. The GLN‐functionalized WS2 QDs exhibit strong photoluminescence emission, and their integration with a wrinkled graphene structure and Au NPs creates a synergistic effect that enhances the emission efficiency and enables the realization of white random lasing. The extensive characterization and analysis of the emission spectra under different deformation ratios provide valuable insights into the tunability and reliability of these devices, as well as the importance of light trapping due to the wrinkled graphene structure. The findings of this study underscore the significant potential of LSPR and wrinkled graphene structure induced stretchable and white random lasers based on 2D GLN‐functionalized WS2 QDs. These lasers may have a promising application in the field of flexible and wearable photonics, which is a critical step towards the development of next‐generation optoelectronic devices with improved performance.

Wide gamut dynamic color manipulation based on dielectric metasurface on a stretchable substrate

Dynamic color adjustment technology has broad prospects in invisibility, anti-counterfeiting, and display, and is an important research direction in structural color. This study investigates the excitation of magnetic dipole resonances in the visible region by titanium dioxide (TiO2) metasurfaces using elastic dielectric polydimethylsiloxane (PDMS) and applied to dynamic color modulation. The near-field interaction between the resonant dielectric elements can be modulated using the mechanical strain of the PDMS substrate, enabling the advection of the resonance peak of the structure throughout the visible range, thereby achieving a wide color gamut. The color structure of TiO2 metasurface is sensitive to the refractive index of the surrounding medium. Moreover, active switching between the “ON” state (highly reflective at a selected wavelength) and the “OFF” state (transparent) can be achieved through the deformation of the flexible PDMS substrate. The proposed method can be applied to fields such as sensor displays and spectroscopy and for studying stretchable and wearable photonics.

Mid-Infrared Intersubband Cavity Polaritons in Flexible Single Quantum Well

Strong and ultrastrong coupling between intersubband transitions in quantum wells and cavity photons have been realized in mid-infrared and terahertz spectral regions. However, most previous works employed a large number of quantum wells on rigid substrates to achieve coupling strengths reaching the strong or ultrastrong coupling regime. In this work, we experimentally demonstrate ultrastrong coupling between the intersubband transition in a single quantum well and the resonant mode of photonic nanocavity at room temperature. We also observe strong coupling between the nanocavity resonance and the second-order intersubband transition in a single quantum well. Furthermore, we implement for the first time such intersubband cavity polariton systems on soft and flexible substrates and demonstrate that bending of the single quantum well does not significantly affect the characteristics of the cavity polaritons. This work paves the way to broaden the range of potential applications of intersubband cavity polaritons including soft and wearable photonics.

Highly Sensitive Strain Sensor Based on Microfiber Coupler for Wearable Photonics Healthcare

Flexible strain sensors are essential components of wearable smart devices that perceive and respond to strain stimulations. However, the sensitivity and response time of most sensors require further improvement to detect subtle strains related to human bodies. Herein, an ultrasensitive flexible optical sensor with fast response time has been built based on a microfiber coupler encapsulated in polydimethylsiloxane. Benefiting from dramatic change of coupling ratio of the microfiber coupler under weak strain, this flexible strain sensor exhibits ultrahigh strain sensitivity (gauge factor, GF = 900), low detection limit (0.001%), ultrafast response time (<0.167 ms), wide sensing range (0.45%), and superior durability and stability (>10 000 cycles). Real‐time capturing and recognizing of respiration, broadband sound signals, and pulse waves at different sites of human body have been demonstrated based on this highly sensitive microfiber coupler sensor. Moreover, simultaneous detection of the wrist pulse and human voice has been achieved based on the frequency division multiplexing technology. This flexible photonics strain sensor could serve as the prototype of ultrasensitive flexible optical sensors with fast response time for the development of high performance and wearable healthcare devices.

In Situ Super-Hindrance-Triggered Multilayer Cracks for Random Lasing in π-Functional Nanopolymer Films.

In situ self-assembly of semiconducting emitters into multilayer cracks is a significant solution-processing method to fabricate organic high-Q lasers. However, it is still difficult to realize from conventional conjugated polymers. Herein, we create the molecular super-hindrance-etching technology, based on the π-functional nanopolymer PG-Cz, to modulate multilayer cracks applied in organic single-component random lasers. Massive interface cracks are formed by promoting interchain disentanglement with the super-steric hindrance effect of π-interrupted main chains, and multilayer morphologies with photonic-crystal-like ordering are also generated simultaneously during the drop-casting method. Meanwhile, the enhancement of quantum yields on micrometer-thick films (Φ = 40% to 50%) ensures high-efficient and ultrastable deep-blue emission. Furthermore, a deep-blue random lasing is achieved with narrow linewidths ~0.08 nm and high-quality factors Q ≈ 5,500 to 6,200. These findings will offer promising pathways of organic π-nanopolymers for the simplification of solution processes applied in lasing devices and wearable photonics.

A Review on Metasurface: From Principle to Smart Metadevices

Metamaterials are composed of periodic subwavelength metallic/dielectric structures that resonantly couple to the electric and magnetic fields of the incident electromagnetic waves, exhibiting unprecedented properties which are most typical within the context of the electromagnetic domain. However, the practical application of metamaterials is found challenging due to the high losses, strong dispersion associated with the resonant responses, and the difficulty in the fabrication of nanoscale 3D structures. The optical metasurface is termed as 2D metamaterials that inherent all of the properties of metamaterials and also provide a solution to the limitation of the conventional metamaterials. Over the past few years, metasurfaces; have been employed for the design and fabrication of optical elements and systems with abilities that surpass the performance of conventional diffractive optical elements. Metasurfaces can be fabricated using standard lithography and nanoimprinting methods, which is easier campared to the fabrication of the counterpart 3 days metamaterials. In this review article, the progress of the research on metasurfaces is illustrated. Concepts of anomalous reflection and refraction, applications of metasurfaces with the Pancharatanm-Berry Phase, and Huygens metasurface are discussed. The development of soft metasurface opens up a new dimension of application zone in conformal or wearable photonics. The progress of soft metasurface has also been discussed in this review. Meta-devices that are being developed with the principle of the shaping of wavefronts are elucidated in this review. Furthermore, it has been established that properties of novel optical metasurface can be modulated by the change in mechanical, electrical, or optical stimuli which leads to the development of dynamic metasurface. Research thrusts over the area of tunable metasurface has been reviewed in this article. Over the recent year, it has been found that optical fibers and metasurface are coagulated for the development of optical devices with the advantages of both domains. The metasurface with lab-on fiber-based devices is being discussed in this review paper. Finally, research trends, challenges, and future scope of the work are summarized in the conclusion part of the article.‬‬‬‬‬‬‬

Flexible GaAs photodetector arrays hetero-epitaxially grown on GaP/Si for a low-cost III-V wearable photonics platform.

We demonstrate flexible GaAs photodetector arrays that were hetero-epitaxially grown on a Si wafer for a new cost-effective and reliable wearable optoelectronics platform. A high crystalline quality GaAs layer was transferred onto a flexible foreign substrate and excellent retention of device performance was demonstrated by measuring the optical responsivities and dark currents. Optical simulation proves that the metal stacks used for wafer bonding serve as a back-reflector and enhance GaAs photodetector responsivity via a resonant-cavity effect. Device durability was also tested by bending 1000 times and no performance degradation was observed. This work paves a way for a cost-effective and flexible III-V optoelectronics technology with high durability.

P‐149: Enhanced Electro‐Optic Performances of the Optically Isotropic Liquid Crystal Displays Utilizing PEDOT: PSS

Optically isotropic liquid crystal (OILC) composite film in which nano‐sized LC droplets are embedded in a polymer matrix is highly useful for bendable displays and wearable photonics but a high operating voltage due to field shielding effect of the insulting matrix and low Kerr constant needs to be overcome. To address this challenge, a small amount of conductive materials PEDOT: PSS is doped into the OILCs. The LC cells with 2 and 3.5 wt% PEDOT: PSS doped OILCs exhibit improved driving voltages by 6.5% and 11.7% and on‐state transmittance by 10.3% and 30.1%, respectively, compared to conventional one. In addition, faster switching response times are achieved while the dark level remains to be unaltered.

Wearable Triboelectric/Aluminum Nitride Nano-Energy-Nano-System with Self-Sustainable Photonic Modulation and Continuous Force Sensing.

Wearable photonics offer a promising platform to complement the thriving complex wearable electronics system by providing high‐speed data transmission channels and robust optical sensing paths. Regarding the realization of photonic computation and tunable (de)multiplexing functions based on system‐level integration of abundant photonic modulators, it is challenging to reduce the overwhelming power consumption in traditional current‐based silicon photonic modulators. This issue is addressed by integrating voltage‐based aluminum nitride (AlN) modulator and textile triboelectric nanogenerator (T‐TENG) on a wearable platform to form a nano‐energy‐nano‐system (NENS). The T‐TENG transduces the mechanical stimulations into electrical signals based on the coupling of triboelectrification and electrostatic induction. The self‐generated high‐voltage from the T‐TENG is applied to the AlN modulator and boosts its modulation efficiency regardless of AlN's moderate Pockels effect. Complementarily, the AlN modulator's capacitive nature enables the open‐circuit operation mode of T‐TENG, providing the integrated NENS with continuous force sensing capability which is notably uninfluenced by operation speeds. Furthermore, a physical model is proposed to describe the coupled AlN modulator/T‐TENG system. With the enhanced photonic modulation and the open‐circuit operation mode enabled by synergies between the AlN modulator and the T‐TENG, optical Morse code transmission and continuous human motion monitoring are demonstrated for practical wearable applications.

Guided-mode resonances in flexible 2D terahertz photonic crystals

In terahertz (THz) photonics, there is an ongoing effort to develop thin, compact devices such as dielectric photonic crystal (PhC) slabs with desirable light–matter interactions. However, previous works in THz PhC slabs have been limited to rigid substrates with thicknesses ∼ 100 s of micrometers. Dielectric PhC slabs have been shown to possess in-plane modes that are excited by external radiation to produce sharp guided-mode resonances with minimal absorption for applications in sensors, optics, and lasers. Here we confirm the existence of guided resonances in a membrane-type THz PhC slab with subwavelength ( λ 0 / 6 − λ 0 / 12 ) thicknesses of flexible dielectric polyimide films. The transmittance of the guided resonances was measured for different structural parameters of the unit cell. Furthermore, we exploited the flexibility of the samples to modulate the guided modes for a bend angle of θ ≥ 5 ∘ , confirmed experimentally by the suppression of these modes. The mechanical flexibility of the device allows for an additional degree of freedom in system design for high-speed communications, soft wearable photonics, and implantable medical devices.

Liquid‐Crystal‐Mediated Active Waveguides toward Programmable Integrated Optics

AbstractOptics, playing an ever‐important role in life, is developing toward miniaturization and integration. Particularly, the field programming of integrated optics will benefit its functionality and durability, thus is highly pursued. Here, through precisely controlling the space‐variant orientations of anisotropic liquid crystals (LCs), various graded‐index waveguides are demonstrated in a homogeneous medium. Straight/bending waveguides and ring resonators are fabricated via LC photopatterning. They exhibit pronounced polychromatic light‐guiding performance. Thanks to their intrinsic external‐stimuli responsivity, the thermal switchability and polarization tunability are verified. By selectively polymerizing, robust functional components and active connecting waveguides can be realized on the same chip. It provides a practical and universal solution to programmable integrated optics, and extends the applications of microstructured soft matter even in wearable photonics.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response.

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO2 metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

Paper-like flexible optically isotropic liquid crystal film for tunable diffractive devices.

We have demonstrated a paper-like diffractive film in which nano-structured liquid crystal droplets are embedded in elastomeric monomer incorporated polymer matrix by polymerization induced phase-separation. The film with voltage-tunable phase grating exhibits an optically isotropic phase with high transparency and an effective chromatic diffraction for an incident white light with sub-millisecond switching time. In addition, the proposed diffractive film is exhibiting excellent chemical stability against organic and inorganic solvents. In this paper, the diffraction properties of test films depending on incident polarization direction, wavelength, and spatial dispersion are characterized. Easy processing and optically isotropic nature of the film imparts potential applications to flexible electro-optic devices that can be widely implemented in wearable photonics.

Cascade‐Microphase‐Separation‐Induced Hierarchical Photonic Structures in Supramolecular Organogel for Deformation‐Insensitive Structural Colors

AbstractPhotonic materials with tunable structural colors in response to various stimuli have enabled diverse sensing and displaying applications. However, the changeable colors caused by mechanical deformations may severely confuse the optical readouts for the upcoming wearable photonics. Herein, an extremely stretchable supramolecular organogel is demonstrated with deformation‐insensitive structural colors through an unconventional cascade‐microphase‐separation process among the silica nanoparticles (SiO2 NPs), silicone oils, and hydrogen‐bonded polymer gel. The resultant sparse and abundant microscale SiO2 NP aggregates with proper sizes, shapes, and random distributions in the gel matrix serve as rigid structural color domains and secure invariant colors to the whole soft sample under extreme mechanical deformations. Moreover, the dynamic hydrogen bonding of the organogel matrix offers the nanocomposites shear‐thinning and easy‐molding property, making them ideal for color paints in versatile coating applications. The deformable organogel nanocomposites have great potentials in various fields requiring stable colors and open up a new avenue for the development of wearable photonic devices.