A high-resolution strain-gauge nanolaser

Actively tunable laser action in GeSn nanomechanical oscillators

Mechanical forces induced by high-speed oscillations provide an elegant way to dynamically alter the fundamental properties of materials such as refractive index, absorption coefficient and gain dynamics. Although the precise control of mechanical oscillation has been well developed in the past decades, the notion of dynamic mechanical forces has not been harnessed for developing tunable lasers. Here we demonstrate actively tunable mid-infrared laser action in group-IV nanomechanical oscillators with a compact form factor. A suspended GeSn cantilever nanobeam on a Si substrate is resonantly driven by radio-frequency waves. Electrically controlled mechanical oscillation induces elastic strain that periodically varies with time in the GeSn nanobeam, enabling actively tunable lasing emission at >2 μm wavelengths. By utilizing mechanical resonances in the radio frequency as a driving mechanism, this work presents wide-range mid-infrared tunable lasers with ultralow tuning power consumption.

A Wearable Thin-Film Hydrogel Laser for Functional Sensing on Skin

Flexible photonics offers the possibility of realizing wearable sensors by bridging the advantages of flexible materials and photonic sensing elements. Recently, optical resonators have emerged as a tool to improve their oversensitivity by integrating with flexible photonic sensors. However, direct monitoring of multiple psychological information on human skin remains challenging due to the subtle biological signals and complex tissue interface. To tackle the current challenges, here, we developed a functional thin film laser formed by encapsulating liquid crystal droplet lasers in a flexible hydrogel for monitoring metabolites in human sweat (lactate, glucose, and urea). The three-dimensional cross-linked hydrophilic polymer serves as the adhesive layer to allow small molecules to penetrate from human tissue to generate strong light--matter interactions on the interface of whispering gallery modes resonators. Both the hydrogel and cholesteric liquid crystal microdroplets were modified specifically to achieve high sensitivity and selectivity. As a proof of concept, wavelength-multiplexed sensing and a prototype were demonstrated on human skin to detect human metabolites from perspiration. These results present a significant advance in the fabrication and potential guidance for wearable and functional microlasers in healthcare.

Lasing Emission from Soft Photonic Crystals for Pressure and Position Sensing

In this report, we introduce a 1D photonic crystal (PhC) nanocavity with waveguide-like strain amplifiers within a soft polydimethylsiloxane substrate, presenting it as a potential candidate for highly sensitive pressure and position optical sensors. Due to its substantial optical wavelength response to uniform pressure, laser emission from this nanocavity enables the detection of a minimum applied uniform pressure of 1.6‱ in experiments. Based on this feature, we further studied and elucidated the distinct behaviors in wavelength shifts when applying localized pressure at various positions relative to the PhC nanocavity. In experiments, by mapping wavelength shifts of the PhC nanolaser under localized pressure applied using a micro-tip at different positions, we demonstrate the nanocavity's capability to detect minute position differences, with position-dependent minimum resolutions ranging from tens to hundreds of micrometers. Furthermore, we also propose and validate the feasibility of employing the strain amplifier as an effective waveguide for extracting the sensing signal from the nanocavity. This approach achieves a 64% unidirectional coupling efficiency for leading out the sensing signal to a specific strain amplifier. We believe these findings pave the way for creating a highly sensitive position-sensing module that can accurately identify localized pressure in a planar space.

Integrated Flexible Microscale Mechanical Sensors Based on Cascaded Free Spectral Range-Free Cavities

Photonic mechanical sensors offer several advantages over their electronic counterparts, including immunity to electromagnetic interference, increased sensitivity, and measurement accuracy. Exploring flexible mechanical sensors on deformable substrates provides new opportunities for strain-optical coupling operations. Nevertheless, existing flexible photonics strategies often require cumbersome signal collection and analysis with bulky setups, limiting their portability and affordability. To address these challenges, we propose a waveguide-integrated flexible mechanical sensor based on cascaded photonic crystal microcavities with inherent deformation and biaxial tensile state analysis. Leveraging the advanced multiplexing capability of the sensor, for the first time, we successfully demonstrate 2D shape reconstruction and quasi-distributed strain sensing with 110 μm spatial resolution. Our microscale mechanical sensor also exhibits exceptional sensitivity with a detected force level as low as 13.6 μN in real-time measurements. This sensing platform has potential applications in various fields, including biomedical sensing, surgical catheters, aircraft and spacecraft engineering, and robotic photonic skin development.

Speckle-based high-resolution multimodal soft sensing

Skin-like soft sensors are key components for human–machine interfaces; however, the simultaneous sensing of several types of stimuli remains challenging because large-scale sensor integration is required with numerous wire connections. We propose an optical high-resolution multimodal sensing approach, which does not require integrating multiple sensors. This approach is based on the combination of an optical scattering phenomenon, which can encode the information of various stimuli as a speckle pattern, and a decoding technique using deep learning. We demonstrate the simultaneous sensing of three different physical quantities—contact force, contact location, and temperature—with a single soft material. Another unique capability of the proposed approach is spatially continuous sensing with an ultrahigh resolution of few tens of micrometers, in contrast to previous multimodal sensing approaches. Furthermore, a haptic soft device is presented for a human–machine interface. Our approach encourages the development of high-performance smart skin-like sensors.

Strain-Enabled Phase Transition of Periodic Metasurfaces

Phase transitions are universal in solid-state matters, as well as in periodic electromagnetic metasurfaces-the photonic analogues of crystals. Although such transitions dictate the properties of active metasurfaces, universal ways to describe the structure transition of periodic metasurfaces have not yet been established. Here, the authors report the strain-enabled phase transition (or lattice deformation) of stretchable metasurfaces with the crystallographic description. They analytically and experimentally demonstrate the phase transition of plasmonic lattices between two arbitrary 2D Bravais lattices under certain strain configurations. The strain-induced symmetry lowering of the structures gives rise to optical anisotropy upon polarization, namely, linearly and circularly polarized dichroism. They further demonstrate the potential of phase transition in information decoding with applied strain. Interpreting the phase transition of metasurfaces from a standpoint of symmetry would accelerate the discovery of emergent properties, and provide a generalizable approach to designing active metasurfaces.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

High-Q 1D rod-based nanocavities

We report an analysis of one-dimensional rod-based photonic crystal nanocavities. These cavities offer opportunities for dielectric materials which lack a matching low-refractive index substrate or are limited in under-etching possibilities to create slab-based PhC cavities. They offer high theoretical Q-values exceeding 10⁶ for transverse magnetic polarized modes with modal volumes below 2.5(λ/n)³. For practical implementations, we propose embedding these structures in a low-refractive index polymer. An analysis of intentionally introduced variations in a rod diameter reveals which design directions should be followed in order to create cavities that are most robust for fabrication-induced variations.

Ultralow-threshold laser using super-bound states in the continuum

Wavelength-scale lasers provide promising applications through low power consumption requiring for optical cavities with increased quality factors. Cavity radiative losses can be suppressed strongly in the regime of optical bound states in the continuum; however, a finite size of the resonator limits the performance of bound states in the continuum as cavity modes for active nanophotonic devices. Here, we employ the concept of a supercavity mode created by merging symmetry-protected and accidental bound states in the continuum in the momentum space, and realize an efficient laser based on a finite-size cavity with a small footprint. We trace the evolution of lasing properties before and after the merging point by varying the lattice spacing, and we reveal this laser demonstrates the significantly reduced threshold, substantially increased quality factor, and shrunken far-field images. Our results provide a route for nanolasers with reduced out-of-plane losses in finite-size active nanodevices and improved lasing characteristics.

Photonic skins based on flexible organic microlaser arrays

Flexible photonics is rapidly emerging as a promising platform for artificial smart skins to imitate or extend the capabilities of human skins. Organic material systems provide a promising avenue to directly fabricate large-scale flexible device units; however, the versatile fabrication of all-organic integrated devices with desired photonic functionalities remains a great challenge. Here, we develop an effective technique for the mass processing of organic microlaser arrays, which act as sensing units, on the chip of photonic skins. With a bilayer electron-beam direct writing method, we fabricated flexible mechanical sensor networks composed of coupled-cavity single-mode laser sources on pliable polymer substrates. These microlaser-based mechanical sensor chips were subsequently used to recognize hand gestures, showing great potential for artificial skin applications. This work represents a substantial advance toward scalable construction of high-performance and low-cost flexible photonic chips, thus paving the way for the implementation of smart photonic skins into practical applications.

Recent advances in nanocavities and their applications

High quality factor and small mode volume in nanocavities enable the demonstration of efficient nanophotonic devices with low power consumption, strong nonlinearity, and high modulation speed, due to the strong light-matter interaction. In this review, we focus on recent state-of-the-art nanocavities and their applications. We introduce single nanocavities including semiconductor nanowires, plasmonic cavities, and nanostructures based on quasi-bound states in the continuum (quasi-BIC), for laser, photovoltaic, and nonlinear applications. In addition, nanocavity arrays with unique feedback mechanisms, including BIC cavities, parity-time symmetry coupled cavities, and photonic topological cavities, are introduced for laser applications. These various cavity designs and underlying physics in single and array nanocavities are useful for the practical implementation of promising nanophotonic devices.

Piezotronic effect in AlGaN/AlN/GaN heterojunction nanowires used as a flexible strain sensor

1D semiconductor nanowires (NWs) have been extensively studied in recent years due to the predominant mechanical flexibility caused by a large surface-to-volume ratio and unique electrical and optical properties induced by the 1D quantum confinement effect. Herein, we use a top-down two-step preparation method to synthesize AlGaN/AlN/GaN heterojunction NWs with controllable size. A single NW is transferred to a flexible poly(ethylene terephthalate) substrate and fixed by indium tin oxide electrodes to form an ohmic contact for the strain sensor. An external mechanical stress is introduced to study the performance of the fabricated piezotronic strain sensor. The gauge factor is as high as 30 under compressive or tensile stress, which indicates a high sensitivity of the strain sensor. Periodic strain tests show the high stability and repeatability of the sensor. The working mechanism of the strain sensor is investigated and systematically analyzed under compressive and tensile strain. Here, we describe a strain sensor that shows a great application potential in wearable integrated circuits, in health-monitoring devices, and in artificial intelligence.

Recent Progress in Nanolaser Technology

Nanolasers are key elements in the implementation of optical integrated circuits owing to their low lasing thresholds, high energy efficiencies, and high modulation speeds. With the development of semiconductor wafer growth and nanofabrication techniques, various types of wavelength-scale and subwavelength-scale nanolasers have been proposed. For example, photonic crystal lasers and plasmonic lasers based on the feedback mechanisms of the photonic bandgap and surface plasmon polaritons, respectively, have been successfully demonstrated. More recently, nanolasers employing new mechanisms of light confinement, including parity-time symmetry lasers, photonic topological insulator lasers, and bound states in the continuum lasers, have been developed. Here, the operational mechanisms, optical characterizations, and practical applications of these nanolasers based on recent research results are outlined. Their scientific and engineering challenges are also discussed.

Mechanochromism in Structurally Colored Polymeric Materials

Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

Wavelength-Tunable Single-Mode Microlasers Based on Photoresponsive Pitch Modulation of Liquid Crystals for Information Encryption

Information encryption and decryption have attracted particular attention; however, the applications are frequently restricted by limited coding capacity due to the indistinguishable broad photoluminescence band of conventional stimuli-responsive fluorescent materials. Here, we present a concept of confidential information encryption with photoresponsive liquid crystal (LC) lasing materials, which were used to fabricate ordered microlaser arrays through a microtemplate-assisted inkjet printing method. LC microlasers exhibit narrow-bandwidth single-mode emissions, and the wavelength of LC microlasers was reversibly modulated based on the optical isomerization of the chiral dopant in LCs. On this basis, we demonstrate phototunable information authentication on LC microlaser arrays using the wavelength of LC microlasers as primary codes. These results provide enlightenment for the implementation of microlaser-based cryptographic primitives for information encryption and anticounterfeiting applications.

Manipulating Light-Matter Interactions in Plasmonic Nanoparticle Lattices

Rationally assembled nanostructures exhibit distinct physical and chemical properties beyond their individual units. Developments in nanofabrication techniques have enabled the patterning of a wide range of nanomaterial designs over macroscale (>in.²) areas. Periodic metal nanostructures show long-range diffractive interactions when the lattice spacing is close to the wavelength of the incident light. The collective coupling between metal nanoparticles in a lattice introduces sharp and intense plasmonic surface lattice resonances, in contrast to the broad localized resonances from single nanoparticles. Plasmonic nanoparticle lattices exhibit strongly enhanced optical fields within the subwavelength vicinity of the nanoparticle unit cells that are 2 orders of magnitude higher than that of individual units. These intense electromagnetic fields can manipulate nanoscale processes such as photocatalysis, optical spectroscopy, nonlinear optics, and light harvesting. This Account focuses on advances in exciton-plasmon coupling and light-matter interactions with plasmonic nanoparticle lattices. First, we introduce the fundamentals of ultrasharp surface lattice resonances; these resonances arise from the coupling of the localized surface plasmons of a nanoparticle to the diffraction mode from the lattice. Second, we discuss how integrating dye molecules with plasmonic nanoparticle lattices can result in an architecture for nanoscale lasing at room temperature. The lasing emission wavelength can be tuned in real time by adjusting the refractive index environment or varying the lattice spacing. Third, we describe how manipulating either the shape of the unit cell or the lattice geometry can control the lasing emission properties. Low-symmetry plasmonic nanoparticle lattices can show polarization-dependent lasing responses, and multiscale plasmonic superlattices-finite patches of nanoparticles grouped into microscale arrays-can support multiple plasmon resonances for controlled multimodal nanolasing. Fourth, we discuss how the assembly of photoactive emitters on the nanocavity arrays behaves as a hybrid materials system with enhanced exciton-plasmon coupling. Positioning metal-organic framework materials around nanoparticles produces mixed photon modes with strongly enhanced photoluminescence at wavelengths determined by the lattice. Deterministic coupling of quantum emitters in two-dimensional materials to plasmonic lattices leads to preserved single-photon emission and reduced decay lifetimes. Finally, we highlight emerging applications of nanoparticle lattices from compact, fully reconfigurable imaging devices to solid-state emitter structures. Plasmonic nanoparticle lattices are a versatile, scalable platform for tunable flat optics, nontrivial topological photonics, and modified chemical reactivities.

Metal Slot Color Filter Based on Thin Air Slots on Silver Block Array

The human eye perceives the color of visible light depending on the spectrum of the incident light. Hence, the ability of color expression is very important in display devices. For practical applications, the transmitted color filter requires high transmittance and vivid colors, covering full standard default color spaces (sRGB). In this paper, we propose a color filter with a silver block array on a silica substrate structure with nanoscale air slots where strong transmission is observed through the slots between silver blocks. We investigated the transmitted color by simulating the transmission spectra as functions of various structure parameters. The proposed structure with an extremely small pixel size of less than 300 nm covers 90% of sRGB color depending on the structure and has a narrow angular distribution of transmitted light.

Lasing threshold of thresholdless and non-thresholdless metal-semiconductor nanolasers

The recently developed plasmonic and photonic metal-semiconductor nanolasers feature unique properties, such as ultra-small mode volume and footprint, high Purcell factor, and ultra-fast modulation. However, it is often difficult to recognize when the transition to lasing occurs, while the most important feature of laser radiation, i.e., coherence, is available only above the lasing threshold. Here we systematically study the second-order coherence properties of metal-semiconductor nanolasers at both low- and high-pump rates. We find the lasing threshold using a clear coherence definition and derive a simple expression for the threshold pump current (optical pump power), which can be applied to most thresholdless and non-thresholdless metal-semiconductor nanolasers.

Stretchable Nanolasing from Hybrid Quadrupole Plasmons

This paper reports a robust and stretchable nanolaser platform that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism. Increasing the size of metal nanoparticles in an array can introduce ultrasharp lattice plasmon resonances with out-of-plane charge oscillations that are tolerant to lateral strain. By patterning these nanoparticles onto an elastomeric slab surrounded by liquid gain, we realized reversible, tunable nanolasing with high strain sensitivity and no hysteresis. Our semiquantum modeling demonstrates that lasing build-up occurs at the hybrid quadrupole electromagnetic hot spots, which provides a route toward mechanical modulation of light-matter interactions on the nanoscale.

Microscale local strain gauges based on visible micro-disk lasers embedded in a flexible substrate

Microscale local strain gauges with low-power consumption and large strain range were demonstrated by integrating microdisk lasers in a deformable and flexible polymer substrate. The lasing spectra of microdisk lasers were sensitive to substrate deformation and can be modulated by strains. The measured relative wavelength tuning under strains of the novel strain sensors illustrated a linear behavior with the gauge factor being ~4.0 nm and ~6.7 nm per stretching unit for microdisk lasers with the diameter of 1.2 μm and 1.5 μm, which corresponding to a smooth wavelength tuning of 1.5 nm and 2.6 nm under 36% strain, respectively. In addition, to being used as microscale strain gauges, the visible lasers on the deformable substrate can also function as a tunable light source for the photonic integrated circuits and flexible laser projection displays.

Bioinspired Stimuli-Responsive Color-Changing Systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect.

Proposal and numerical study of a flexible visible photonic crystal defect cavity for nanoscale strain sensors

A flexible photonic crystal cavity, consisting of a III-V active layer embedded in a flexible medium, with a line-defect by removing three air holes for nanoscale structural deformation detection is proposed and optimized. The cavity can hold the photonic band-gap modes with the fundamental mode located at approximately 686 nm, overlapping with the photoluminescence spectrum of the InGaP/InGaAlP quantum wells. Results of finite-difference time-domain simulations indicate that the L3 cavity features an ultra-compact mode volume of 10^-3 µm³ and high quality factor of 10⁴ at a sub-micron footprint within the studied visible wavelength. Theoretical optical strain sensitivities of approximately 4.5 and 3 nm per ε (1% strain for both) for the x and y directions are predicted, respectively. When the cavity is under large bending curvatures, the Q factor rapidly decreases from 8000 to 2000.

Compressible 1D photonic crystal nanolasers with wide wavelength tuning

We propose and demonstrate a tunable photonic crystal nanolaser consisting of 1D periodic nanorods wrapped in deformable polydimethylsiloxane. In addition to low-threshold and long-term lasing stability, the nanolaser also displays reproducible and reliable wavelength tuning with a large tunability of 7.7 nm under 1% compression. By further associating with stretching, a very wide wavelength-tunable range of 155 nm that almost spans the entire S+C+L telecommunication bands is successfully demonstrated with a single nanolaser device.

Characteristics of strain-sensitive photonic crystal cavities in a flexible substrate

High-index semiconductor photonic crystal (PhC) cavities in a flexible substrate support strong and tunable optical resonances that can be used for highly sensitive and spatially localized detection of mechanical deformations in physical systems. Here, we report theoretical studies and fundamental understandings of resonant behavior of an optical mode excited in strain-sensitive rod-type PhC cavities consisting of high-index dielectric nanorods embedded in a low-index flexible polymer substrate. Using the three-dimensional finite-difference time-domain simulation method, we calculated two-dimensional transverse-electric-like photonic band diagrams and the three-dimensional dispersion surfaces near the first Γ-point band edge of unidirectionally strained PhCs. A broken rotational symmetry in the PhCs modifies the photonic band structures and results in the asymmetric distributions and different levels of changes in normalized frequencies near the first Γ-point band edge in the reciprocal space, which consequently reveals strain-dependent directional optical losses and selected emission patterns. The calculated electric fields, resonant wavelengths, and quality factors of the band-edge modes in the strained PhCs show an excellent agreement with the results of qualitative analysis of modified dispersion surfaces. Furthermore, polarization-resolved time-averaged Poynting vectors exhibit characteristic dipole-like emission patterns with preferentially selected linear polarizations, originating from the asymmetric band structures in the strained PhCs.

Tunable nanoblock lasers and stretching sensors

Reconfigurable, reliable, and robust nanolasers with wavelengths tunable in the telecommunication bands are currently being sought after for use as flexible light sources in photonic integrated circuits. Here, we propose and demonstrate tunable nanolasers based on 1D nanoblocks embedded within stretchable polydimethylsiloxane. Our lasers show a large wavelength tunability of 7.65 nm per 1% elongation. Moreover, this tunability is reconfigurable and reliable under repeated stretching/relaxation tests. By applying excessive stretching, wide wavelength tuning over a range of 80 nm (spanning the S, C, and L telecommunication bands) is successfully demonstrated. Furthermore, as a stretching sensor, an enhanced wavelength response to elongation of 9.9 nm per % is obtained via the signal differential from two nanoblock lasers positioned perpendicular to each other. The minimum detectable elongation is as small as 0.056%. Nanoblock lasers can function as reliable tunable light sources in telecommunications and highly sensitive on-chip structural deformation sensors.