Rapid and selective actuation of 3D-printed shape-memory composites via microwave heating.

Three-dimensional (3D) printing allows the fabrication of complex shapes with high resolutions. However, the printed structures typically have fixed shapes and functions. Four-dimensional printing allows the shapes of 3D-printed structures to be transformed in response to external stimuli. Among the external stimuli, light has unique advantages for remote thermal actuation. However, light absorption in opaque structures occurs only near the sample surface; thus, actuation can be slow. Here, we propose and experimentally demonstrate the rapid and selective actuation of 3D-printed shape-memory polymer (SMP) composites using microwave heating. The SMP composite filaments are prepared using different amounts of graphite flakes. Microwave radiation can penetrate the entire printed structures and induce rapid heating. With sufficient graphite contents, the printed SMP composites are heated above their glass transition temperature within a few seconds. This leads to rapid thermal actuation of the 3D-printed SMP structures. Finally, dual-material 3D printing is demonstrated to induce selective microwave heating and control actuation motion. Our experiments and simulations indicate that microwave heating of SMP composites can be an effective method for the rapid and selective actuation of complex structures.

Chiral electroluminescence from thin-film perovskite metacavities.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

Topological Control of 2D Perovskite Emission in the Strong Coupling Regime.

Momentum space topology can be exploited to manipulate radiation in real space. Here we demonstrate topological control of 2D perovskite emission in the strong coupling regime via polaritonic bound states in the continuum (BICs). Topological polarization singularities (polarization vortices and circularly polarized eigenstates) are observed at room temperature by measuring the Stokes parameters of photoluminescence in momentum space. Particularly, in symmetry-broken structures, a very large degree of circular polarization (DCP) of ∼0.835 is achieved in the perovskite emission, which is the largest in perovskite materials to our knowledge. In the strong coupling regime, lower polariton modes shift to the low-loss spectral region, resulting in strong emission enhancement and large DCP. Our reciprocity analysis reveals that DCP is limited by material absorption at the emission wavelength. Polaritonic BICs based on 2D perovskite materials combine unique topological features with exceptional material properties and may become a promising platform for active nanophotonic devices.

Multipole resonance and Vernier effect in compact and flexible plasmonic structures.

Spoof surface plasmons in corrugated metal surfaces allow tight field confinement and guiding even at low frequencies and are promising for compact microwave photonic devices. Here, we use metal-ink printing on flexible substrates to construct compact spoof plasmon resonators. We clearly observe multipole resonances in the microwave frequencies and demonstrate that they are still maintained even under significant bending. Moreover, by combining two resonators of slightly different sizes, we demonstrate spectral filtering via the Vernier effect. We selectively address a target higher-order resonance while suppressing the other modes. Finally, we investigate the index-sensing capability of printed plasmonic resonators. In the Vernier structure, we can control the resonance amplitude and frequency by adjusting a resonance overlap between two coupled resonators. The transmission amplitude can be maximized at a target refractive index, and this can provide more functionalities and increased design flexibility. The metal-ink printing of microwave photonic structures can be applied to various flexible devices. Therefore, we expect that the compact, flexible plasmonic structures demonstrated in this study may be useful for highly functional elements that can enable tight field confinement and manipulation.

High sensitivity bolometers based on metal nanoantenna dimers with a nanogap filled with vanadium dioxide.

One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO2) was studied for long-wavelength infrared detection. Because VO2 is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer's temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

Circularly Polarized Emission from Organic-Inorganic Hybrid Perovskites via Chiral Fano Resonances.

Organic-inorganic hybrid perovskites hold great potential for various optoelectronic devices with exceptional properties. Although the direct generation of circularly polarized emission from perovskites would enable various compact devices, achieving a large degree of circular polarization (DCP) at room temperature still remains challenging. Herein, we demonstrate that DCP can be strongly enhanced at the narrow mode position of chiral Fano resonances. In our design, a perovskite film is spin-coated on a symmetry-broken structure with a relatively large feature size. A large DCP of more than 0.5 is achieved at room temperature without the direct patterning of the perovskite layer. Reciprocity calculation reveals that chiral field enhancement enables the emission of opposite helicity to couple into counter-propagating slab modes and leads to a large DCP. Our design is very general and scalable. Our work may lead to circularly polarized light sources based on various perovskite materials.

Multicolor 4D printing of shape-memory polymers for light-induced selective heating and remote actuation.

Four-dimensional (4D) printing can add active and responsive functions to three-dimensional (3D) printed objects in response to various external stimuli. Light, among others, has a unique advantage of remotely controlling structural changes to obtain predesigned shapes. In this study, we demonstrate multicolor 4D printing of shape-memory polymers (SMPs). Using color-dependent selective light absorption and heating in multicolor SMP composites, we realize remote actuation with light illumination. We experimentally investigate the temperature changes in colored SMPs and observe a clear difference between the different colors. We also present simulations and analytical calculations to theoretically model the structural variations in multicolor composites. Finally, we consider a multicolor hinged structure and demonstrate the multistep actuation by changing the color of light and duration of illumination. 4D printing can allow complex, multicolor geometries with predesigned responses. Moreover, SMPs can be reused multiple times for thermal actuation by simply conducting thermomechanical programming again. Therefore, 4D printing of multicolor SMP composites have unique merits for light-induced structural changes. Our study indicates that multicolor 4D printing of SMPs are promising for various structural changes and remote actuation.

Enhancement of sub-bandgap light absorption in perovskite semiconductor films via critical coupling.

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

Femtosecond laser irradiation of molecular excitonic films for nanophotonic response control and large-area patterning.

Molecular excitonic films such as J-aggregate thin films can show an optically metallic response in the visible region and can be considered as alternative materials for plasmonics. However, there was no direct, top-down method to modify the optical response over a large area. Here, we demonstrate the femtosecond (fs) laser processing of J-aggregate films on the centimeter scale. With proper laser conditions, optically metallic films (Re[ε] < 0) were modified to dielectric ones (Re[ε] > 0) with large changes in optical responses. We performed various optical spectrum measurements to investigate the effect of fs-laser irradiation. Our results demonstrate that the strong modification of the optical response can be induced over a large area by fs-laser processing and this can be useful for novel nanophotonic studies.

3D printing of twisting and rotational bistable structures with tuning elements.

Three-dimensional (3D) printing is ideal for the fabrication of various customized 3D components with fine details and material-design complexities. However, most components fabricated so far have been static structures with fixed shapes and functions. Here we introduce bistability to 3D printing to realize highly-controlled, reconfigurable structures. Particularly, we demonstrate 3D printing of twisting and rotational bistable structures. To this end, we have introduced special joints to construct twisting and rotational structures without post-assembly. Bistability produces a well-defined energy diagram, which is important for precise motion control and reconfigurable structures. Therefore, these bistable structures can be useful for simplified motion control in actuators or for mechanical switches. Moreover, we demonstrate tunable bistable components exploiting shape memory polymers. We can readjust the bistability-energy diagram (barrier height, slope, displacement, symmetry) after printing and achieve tunable bistability. This tunability can significantly increase the use of bistable structures in various 3D-printed components.

Theoretical investigations on microwave Fano resonances in 3D-printable hollow dielectric resonators.

High-index dielectric structures have recently been studied intensively for Mie resonances at optical frequencies. These dielectric structures can enable extreme light manipulation, similar to that which has been achieved with plasmonic nanostructures. In the microwave region, dielectric resonators and metamaterials can be fabricated directly using 3D printing, which is advantageous for fabricating structurally complicated 3D geometries. It is therefore especially suitable for the fabrication of subwavelength structures. Here we report theoretical investigations on microwave Fano resonances in 3D-printable dielectric materials and structures. In particular, we propose and analyse 3D-printable, hollow, dielectric resonators with relatively low refractive indices, which exhibit sharp Fano resonances. We can control the interaction between bright and dark modes in a coupled dielectric particle pair by adjusting the inner-hole size, and thus we can increase the radiative Q factors further. We also find that Fano resonances in these hollow dielectric resonators are very sensitive to an index change in the surrounding medium, which could be useful for long-distance environmental sensing. New possibilities and opportunities are opening up with the rapid development of 3D-printing technologies. Our findings and the detailed investigations reported here can provide useful guidelines for future photonic devices based on 3D-printable materials and structures.

Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene.

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.

Active switching and tuning of sharp Fano resonances in the mid-infrared spectral region.

We propose and analyze a scheme for active switching and spectral tuning of mid-infrared Fano resonances. We consider dielectric resonators made of semiconductor cylinder arrays and block pairs, and theoretically investigate their optical response change due to carrier generation. Owing to sharp optical resonances in these structures and large dielectric constant variations with carrier densities, the significant spectral tuning of Fano resonances is achievable. Furthermore, selective optical pumping in coupled semiconductor structures can even enable dynamic switching of Fano resonances. This leads to a drastic change in the scattering spectra as well as in the near-field intensity. We also observe a stark difference between Fano resonances in cylinder arrays and block pairs. To understand this unusual behavior, we adopt the two coupled oscillator model, and extract the relevant Fano resonance parameters that explain this difference. Our findings and in-depth analyses can be useful for molecular sensors and switching devices in the technologically important mid-infrared spectral region.

General Strategy for Broadband Coherent Perfect Absorption and Multi-wavelength All-optical Switching Based on Epsilon-Near-Zero Multilayer Films.

We propose a general, easy-to-implement scheme for broadband coherent perfect absorption (CPA) using epsilon-near-zero (ENZ) multilayer films. Specifically, we employ indium tin oxide (ITO) as a tunable ENZ material, and theoretically investigate CPA in the near-infrared region. We first derive general CPA conditions using the scattering matrix and the admittance matching methods. Then, by combining these two methods, we extract analytic expressions for all relevant parameters for CPA. Based on this theoretical framework, we proceed to study ENZ CPA in a single layer ITO film and apply it to all-optical switching. Finally, using an ITO multilayer of different ENZ wavelengths, we implement broadband ENZ CPA structures and investigate multi-wavelength all-optical switching in the technologically important telecommunication window. In our design, the admittance matching diagram was employed to graphically extract not only the structural parameters (the film thicknesses and incident angles), but also the input beam parameters (the irradiance ratio and phase difference between two input beams). We find that the multi-wavelength all-optical switching in our broadband ENZ CPA system can be fully controlled by the phase difference between two input beams. The simple but general design principles and analyses in this work can be widely used in various thin-film devices.

Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared.

Perfect absorption (PA) of incident light is important for both fundamental light-matter interaction studies and practical device applications. PA studies so far have mainly used resonant nanostructures that require delicate structural patterning. Here, we realize tunable and broadband PA in the near-infrared region using relatively simple thin film coatings. We adjust the growth condition of an ITO film and control its epsilon-near-zero (ENZ) wavelength. We show that this results in highly tunable PA in the telecommunication window. Then, using an ITO multilayer of different ENZ wavelengths, we demonstrate broadband PA that covers a wide range of near-infrared wavelengths. The use of ENZ coatings makes PA adjustable during the film growth and does not require any structural patterning afterward. It also facilitates the chip-scale integration of perfect absorbers with other device components. Broadband PA relaxes the single wavelength condition in previous PA studies, and thus it is suitable for many practical device applications, including sensors, photodetectors, and energy harvesting devices.

Epsilon-near-zero strong coupling in metamaterial-semiconductor hybrid structures.

We present a new type of electrically tunable strong coupling between planar metamaterials and epsilon-near-zero modes that exist in a doped semiconductor nanolayer. The use of doped semiconductors makes this strong coupling tunable over a wide range of wavelengths through the use of different doping densities. We also modulate this coupling by depleting the doped semiconductor layer electrically. Our hybrid approach incorporates strong optical interactions into a highly tunable, integrated device platform.

Nanofocusing of light using three-dimensional plasmonic mode conversion.

Efficient nanofocusing of light into a gap plasmon waveguide using three-dimensional mode conversion in a strip plasmonic directional coupler is proposed. Unlike conventional nanofocusing using tapering structures, a plasmonic directional coupler converts E(z)-type odd mode energy into E(y)-type gap plasmon mode by controlling phase mismatch and gap spacing. The simulation result shows the maximum electric field intensity increases up to 58.1 times the input intensity, and 17.3% of the light is focused on the nano gap region.

Active tuning of mid-infrared metamaterials by electrical control of carrier densities.

We demonstrate electrically-controlled active tuning of mid-infrared metamaterial resonances using depletion-type devices. The depletion width in an n-doped GaAs epilayer changes with an electric bias, inducing a change of the permittivity of the substrate and leading to frequency tuning of the resonance. We first present our detailed theoretical analysis and then explain experimental data of bias-dependent metamaterial transmission spectra. This electrical tuning is generally applicable to a variety of infrared metamaterials and plasmonic structures, which can find novel applications in chip-scale active infrared devices.

Power flow from a dipole emitter near an optical antenna.

Current methods to calculate the emission enhancement of a quantum emitter coupled to an optical antenna of arbitrary geometry rely on analyzing the total Poynting vector power flow out of the emitter or the dyadic Green functions from full-field numerical simulations. Unfortunately, these methods do not provide information regarding the nature of the dominant energy decay pathways. We present a new approach that allows for a rigorous separation, quantification, and visualization of the emitter output power flow captured by an antenna and the subsequent reradiation power flow to the far field. Such analysis reveals unprecedented details of the emitter/antenna coupling mechanisms and thus opens up new design strategies for strongly interacting emitter/antenna systems used in sensing, active plasmonics and metamaterials, and quantum optics.

Plasmonic beaming and active control over fluorescent emission.

Nanometallic optical antennas are rapidly gaining popularity in applications that require exquisite control over light concentration and emission processes. The search is on for high-performance antennas that offer facile integration on chips. Here we demonstrate a new, easily fabricated optical antenna design that achieves an unprecedented level of control over fluorescent emission by combining concepts from plasmonics, radiative decay engineering and optical beaming. The antenna consists of a nanoscale plasmonic cavity filled with quantum dots coupled to a miniature grating structure that can be engineered to produce one or more highly collimated beams. Electromagnetic simulations and confocal microscopy were used to visualize the beaming process. The metals defining the plasmonic cavity can be utilized to electrically control the emission intensity and wavelength. These findings facilitate the realization of a new class of active optical antennas for use in new optical sources and a wide range of nanoscale optical spectroscopy applications.