All-optical tuning of EIT-like dielectric metasurfaces by means of chalcogenide phase change materials

Active control of an electromagnetically induced transparency analogue in a coupled dual bound states in the continuum system integrated with graphene

Electronically induced transparency (EIT) is a coherent optical phenomenon that induces interference within atoms, allowing certain specific frequencies of light to pass through atomic media without being absorbed. However, EIT systems face challenges related to narrow transparency windows and precise control of slow light. We propose an interference structure based on a coupled dual bound states in the continuum (BIC) system to emulate the EIT-like effect. By integrating quasi-BIC (bright mode) with BIC (dark mode), our design successfully achieves an EIT-like effect in a narrow bright mode with a full width at half maximum (FWHM) of less than 1 nm. Its notable features are the bright mode's wide tunability achieved through structural parameter adjustment and a significant group delay of up to 14.43 ps. Additionally, integrating graphene into the BIC structure introduced a form of active tunability akin to the EIT-like effect. We numerically calculate the coupling structure, and its intrinsic mechanism is analyzed. Analysis based on coupled-mode theory confirms that this active modulation primarily stems from changes in the BIC structure's loss. Due to its special frequency selectivity and insensitivity to the polarization of the light source, this narrow-band EIT-like structure is particularly suitable for high-precision optical sensing and spectroscopy. The significant group delay of this structure enhances the interaction between light and matter, improving the accuracy and efficiency of optical signal control and data transmission, opening up new avenues for slow light applications and making significant progress in the development of active tunable optical switches and modulators.

Roadmap for phase change materials in photonics and beyond

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their diverse applications in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum. It discusses various device configurations and optimization techniques, including deep learning-based metasurface design. The integration of PCMs with Photonic Integrated Circuits and advanced electric-driven PCMs are explored. PCMs hold great promise for multifunctional device development, including applications in non-volatile memory, optical data storage, photonics, energy harvesting, biomedical technology, neuromorphic computing, thermal management, and flexible electronics.

Polarization-insensitive electromagnetically induced transparency and its sensing performance based on spoof localized surface plasmons in vanadium dioxide-based terahertz metasurfaces

The multi-layer terahertz metasurfaces are designed to achieve polarization-insensitive electromagnetically induced transparency (EIT) effect and its sensing performance based on spoof localized surface plasmons (S-LSPs). The unit cell of the proposed metasurfaces is comprised of a metallic spiral (MS) structure, square metal frame (SMF) structure, and vanadium dioxide (VO2) layer. The EIT effect is realized by the bright-bright coupling between spoof electric localized surface plasmons (S-ELSPs) and electric dipole, which can be proved by the multipole scattering theory. The maximum value of transmission amplitude at the transparent window is 0.91, and the modulation depth can reach 51% by adjusting the conductivity of VO2. The theoretical results based on the two-particle model show excellent agreement with the simulated results. Moreover, the change of polarization angle has little effect on the EIT effect and the proposed metasurfaces show polarization-insensitive characteristics. The slow light effect of the proposed metasurfaces can also be dynamically controlled by tuning the conductivity of VO2. Due to the high Q value of the transparent window, the proposed metasurfaces exhibit excellent sensing performance, and the sensitivity is 0.172 THz RIU-1. Our study provides a method for the fabrication of EIT metasurfaces and has a broad application prospect in slow light devices, sensors, and modulators.

Terahertz biosensor integrated with Au nanoparticles to improve the sensing performance

A terahertz (THz) sensor is presented based on a metasurface integrated with Au nanoparticles (AuNPs). It was used to improve the sensitivity of the detection of whey protein and to enhance the sensing index of group delay. This demonstrates that AuNPs can improve the sensing performance of the biosensor. The internal mechanism can be explained by the modified perturbation theory and the coupled harmonic oscillator model. This study provides a means of enhancing the sensitivity of the THz biosensor.

Reconfigurable Radiation Angle Continuous Deflection of All-Dielectric Phase-Change V-Shaped Antenna

All-dielectric optical antenna with multiple Mie modes and lower inherent ohmic loss can achieve high efficiency of light manipulation. However, the silicon-based optical antenna is not reconfigurable for specific scenarios. The refractive index of optical phase-change materials can be reconfigured under stimulus, and this singular behavior makes it a good candidate for making reconfigurable passive optical devices. Here, the optical radiation characteristics of the V-shaped phase-change antenna are investigated theoretically. The results show that with increasing crystallinity, the maximum radiation direction of the V-shaped phase-change antenna can be continuously deflected by 90°. The exact multipole decomposition analysis reveals that the modulus and interference phase difference of the main multipole moments change with the crystallinity, resulting in a continuous deflection of the maximum radiation direction. Thus, the power ratio in the two vertical radiation directions can be monotonically reversed from -12 to 7 dB between 20% and 80% crystallinity. The V-shaped phase-change antenna exhibits the potential to act as the basic structural unit to construct a reconfigurable passive spatial angular power splitter or wavelength multiplexer. The mechanism analysis of radiation directivity involving the modulus and interference phase difference of the multipole moments will provide a reference for the design and optimization of the phase-change antenna.

Low-loss ultrafast and non-volatile all-optical switch enabled by all-dielectric phase change materials

All-optical switches show great potential to overcome the speed and power consumption limitations of electrical switching. Owing to its nonvolatile and superb cycle abilities, phase-change materials enabled all-optical switch (PC-AOS) is attracting much attention. However, realizing low-loss and ultrafast switching remains a challenge, because previous PC-AOS are mostly based on plasmonic metamaterials. The high thermal conductance of metallic materials disturbs the thermal accumulation for phase transition, and eventually decreases the switching speed to tens of nanoseconds. Here, we demonstrate an ultrafast switching (4.5 ps) and low-loss (2.8 dB) all-optical switch based on all-dielectric structure consisting of Ge₂Sb₂Te₅ and photonic crystals. Its switching speed is approximately ten thousand times faster than the plasmonic one. A 5.4 dB on-off ratio at 1550 nm has been experimentally achieved. We believe that the proposed all-dielectric optical switch will accelerate the progress of ultrafast and energy-efficient photonic devices and systems.

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All-dielectric phase change materials are used to achieve low loss all optical switch

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Only 15 nm phase change film is used for laser induced ultrafast switching

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Up to 7.4 dB switching contrast can be realized in the Near Infrared Spectrum

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Nano-hole array metasurface enables polarization insensitive optical filtering

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Plasmon-Induced Transparency for Tunable Atom Trapping in a Chiral Metamaterial Structure

Plasmon-induced transparency (PIT), usually observed in plasmonic metamaterial structure, remains an attractive topic for research due to its unique optical properties. However, there is almost no research on using the interaction of plasmonic metamaterial and high refractive index dielectric to realize PIT. Here, we report a novel nanophotonics system that makes it possible to realize PIT based on guided-mode resonance and numerically demonstrate its transmission and reflection characteristics by finite element method simulations. The system is composed of a high refractive-index dielectric material and a two-dimensional metallic photonic crystal with 4-fold asymmetric holes. The interaction mechanism of the proposed structure is analyzed by the coupled-mode theory, and the effects of the parameters on PIT are investigated in detail. In addition, we first consider this PIT phenomenon of such fields on atom trapping (⁸⁷Rb), and the results show that a stable 3D atom trapping with a tunable range of position of about ~17 nm is achieved. Our work provides a novel, efficient way to realize PIT, and it further broadens the application of plasmonic metamaterial systems.

Plasmonic Elliptical Nanohole Arrays for Chiral Absorption and Emission in the Near-Infrared and Visible Range

Chiral plasmonic nanostructures with tunable handedness-dependent absorption in the visible and infrared offer chiro-optical control at the nanoscale. Moreover, coupling them with emitting layers could lead to chiral nanosources, important for nanophotonic circuits. Here, we propose plasmonic elliptical nanohole arrays (ENHA) for circularly dependent near-infrared and visible emission. We first investigate broadband chiral behavior in an Au-ENHA embedded in glass by exciting it with plane waves. We then study the coupling of ENHA with a thin emitting layer embedded in glass; we focus on the emission wavelengths which provided high chirality in plane-wave simulations. Our novel simulation set-up monitors the chirality of the far-field emission by properly averaging a large set of homogeneously distributed, randomly oriented quantum sources. The intrinsic chirality of ENHA influences the circular polarization degree of the emitting layer. Finally, we study the emission dependence on the field distribution at the excitation wavelength. We demonstrate the chiral absorption and emission properties for Au-ENHA emitting in the near-infrared range, and for Ag-ENHA which is excited in green range and emits in the Lumogen Red range. The simple geometry of ENHA can be fabricated with low-cost nanosphere lithography and be covered with emission gel. We thus believe that this design can be of great importance for tunable chiral nanosources.

Bidirectional Electromagnetically Induced Transparency Based on Coupling of Magnetic Dipole Modes in Amorphous Silicon Metasurface

A bidirectional electromagnetically induced transparency (EIT) arising from coupling of magnetic dipole modes is demonstrated numerically and experimentally based on nanoscale a-Si cuboid-bar metasurface. Analyzed by the finite-difference time-domain (FDTD) Solutions, both the bright and dark magnetic dipole mode is excited in the cuboid, while only the dark magnetic dipole mode is excited in the bar. By breaking the symmetry of the cuboid-bar structure, the destructive interference between bright and dark magnetic dipole modes is induced, resulting in the bidirectional EIT phenomenon. The position and amplitude of simulated EIT peak is adjusted by the vertical spacing and horizontal spacing. The EIT metasurface was fabricated by Electron-Beam Lithography and deep silicon etching technique on the a-Si film deposited by Plasma-Enhanced Chemical Vapor Deposition. Measured by a convergent spectrometer, the fabricated sample achieved a bidirectional EIT peak with transmission up to 65% and 63% under forward and backward incidence, respectively. Due to the enhanced magnetic field induced by the magnetic dipole resonance, the fabricated bidirectional EIT metasurface provides a potential way for magnetic sensing and magnetic nonlinearity.

Active Modulation of an All-Dielectric Metasurface Analogue of Electromagnetically Induced Transparency in Terahertz

In this work, an analogue of electromagnetically induced transparency (EIT) is excited by a periodic unit consisting of a silicon rectangular bar resonator and a silicon ring resonator in terahertz (THz). The analogue of the EIT effect can be well excited by coupling of the "bright mode" and the "dark mode" supported by the bar and the ring, respectively. Using the semimetallic properties of graphene, active control of the EIT-like effect can be realized by integrating a monolayer graphene into THz metamaterials. By adjusting the Fermi energy of graphene, the resonating electron distribution changes in the dielectric structures, resulting in the varying of the EIT-like effect. The transmission can be modulated from 0.9 to 0.3 with the Fermi energy of graphene placed under the ring resonator mold varying from 0 to 0.6 eV, while a modulation range of 0.9-0.3 corresponds to Fermi energy from 0 to 0.3 eV when graphene is placed under the rectangular bar resonator. Our results may provide potential applications in slow light devices and an ultrafast optical signal.

Independent tuning of bright and dark meta-atoms with phase change materials on EIT metasurfaces

The realization of tunable metasurfaces is of fundamental importance for boosting the electromagnetic field control ability. Especially, it is important to put forward new modulation methods to further understand their underlying modulation mechanism and expand their application range. In this paper, tunable electromagnetically induced transparency (EIT) metasurfaces based on the phase change material Ge2Sb2Te5 (GST) are proposed and experimentally demonstrated. Different from previous modulation methods of directly introducing the GST film below the metasurfaces, here a two-step lithography method is introduced to combine independent GST strips with bright and dark meta-atoms in the EIT structures, respectively, achieving the independent modulation of the EIT-like spectra. In addition, by applying temporal coupled-mode theory (TCMT), the EIT-like spectra with different GST crystallization levels were analysed and the corresponding characteristic parameters were determined simultaneously. These fitting results reveal that GST strips can modulate the resonances of the bright and dark meta-atoms independently by shifting the resonant frequency and increasing the decay rate, which in turn result in the different modulation features of the EIT-like spectra. This method improves the degree of freedom of active modulation and provides a new route for tunable slow light devices.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge₂Sb₂Te₅ (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Optical radiation manipulation of Si-Ge2Sb2Te5 hybrid metasurfaces

Active optical metadevices have attracted growing interest for the use in nanophotonics owing to their flexible control of optics. In this work, by introducing the phase-changing material Ge₂Sb₂Te₅ (GST), which exhibits remarkably different optical properties in different crystalline states, we investigate the active optical radiation manipulation of a resonant silicon metasurface. A designed double-nanodisk array supports a strong toroidal dipole excitation and an obvious electric dipole response. When GST is added, the toroidal response is suppressed, and the toroidal and electric dipoles exhibit pronounced destructive interference owing to the similarity of their far-field radiation patterns. When the crystallization ratio of GST is varied, the optical radiation strength and spectral position of the scattering minimum can be dynamically controlled. Our work provides a route to flexible optical radiation modulation using metasurfaces.

Experimental Demonstration of Electromagnetically Induced Transparency in a Conductively Coupled Flexible Metamaterial with Cheap Aluminum Foil

We propose a conductively coupled terahertz metallic metamaterial exhibiting analog of electromagnetically induced transparency (EIT), in which the bright and dark mode antennae interact via surface currents rather than near-field coupling. Aluminum foil, which is very cheap and often used in food package, is used to fabricate our metamaterials. Thus, our metamaterials are also flexible metamaterials. In our design, aluminum bar resonators and aluminum split ring resonators (SRRs) are connected (rather than separated) in the form of a fork-shaped structure. We conduct a numerical simulation and an experiment to analyze the mechanism of the proposed metamaterial. The surface current due to LSP resonance (bright mode) flows along different paths, and a potential difference is generated at the split gaps of the SRRs. Thus, an LC resonance (dark mode) is induced, and the bright mode is suppressed, resulting in EIT. The EIT-like phenomenon exhibited by the metamaterial is induced by surface conducting currents, which may provide new ideas for the design of EIT metamaterials. Moreover, the process of fabricating microstructures on flexible substrates can provide a reference for producing flexible microstructures in the future.

Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO2 phase change layers

Active, ultra-fast external control of the emission properties at the nanoscale is of great interest for chip-scale, tunable and efficient nanophotonics. Here we investigated the emission control of dipolar emitters coupled to a nanostructure made of an Au nanoantenna, and a thin vanadium dioxide (VO₂) layer that changes from semiconductor to metallic state. If the emitters are sandwiched between the nanoantenna and the VO₂ layer, the enhancement and/or suppression of the nanostructure's magnetic dipole resonance enabled by the phase change behavior of the VO₂ layer can provide a high contrast ratio of the emission efficiency. We show that a single nanoantenna can provide high magnetic field in the emission layer when VO₂ is metallic, leading to high emission of the magnetic dipoles; this emission is then lowered when VO₂ switches back to semiconductor. We finally optimized the contrast ratio by considering different orientation, distribution and nature of the dipoles, as well as the influence of a periodic Au nanoantenna pattern. As an example of a possible application, the design is optimized for the active control of an Er³⁺ doped SiO₂ emission layer. The combination of the emission efficiency increase due to the plasmonic nanoantenna resonances and the ultra-fast contrast control due to the phase-changing medium can have important applications in tunable efficient light sources and their nanoscale integration.

Near-infrared modulation by means of GeTe/SOI-based metamaterial

Today, nanophotonics still lacks components for modulation that can be easily implementable in existing silicon-on-insulator (SOI) technology. Chalcogenide phase change materials (PCMs) are promising candidates for tuning in the near infrared: at the nanoscale, thin layers can provide enough contrast to control the optical response of a nanostructure. Moreover, all-dielectric metamaterials allow for resonant behavior without having ohmic losses in the telecom range. Here, a novel hybridization of a SOI-based metamaterial with PCM GeTe is experimentally investigated. A metamaterial based on Si nanorods, covered by a thin layer of GeTe, is designed and fabricated. Switching GeTe from amorphous to crystalline leads to a rather high resonance-governed reflection contrast at 1.55 μm. Additional confocal Raman imaging is done to differentiate the crystallized zones of the metamaterials' unit cell. The findings are in good agreement with numerical analysis and show good perspectives of all-dielectric tunable near-infrared nanophotonics.

Broadband efficient modulation of light transmission with high contrast using reconfigurable VO2 diffraction grating

Ultra-compact dynamically reconfigurable modulation of optical transmission has been widely studied by using subwavelength-spaced resonant metasurface structures containing reconfigurable optical materials. However, it has been difficult to achieve high transmissivity, large modulation depth, and broad bandwidth simultaneously with the conventional resonance-based metasurface schemes. Here, we propose a reconfigurable phase-transition diffractive grating, made of thick VO₂ ridge waveguides, for achieving the above-mentioned three goals simultaneously in the near-infrared range. Based on the large dielectric-to-plasmonic transition characteristic of VO₂ in the near-infrared range, diffraction directivity of dual-VO₂ ridge waveguide is designed to be tuned by thermally driven phase transition of VO₂ for transverse electrically polarized illumination. Then, the diffractive VO₂ ridge waveguide grating composed of the periodically arranged dual VO₂ ridge waveguides is designed with on-state efficiency around 0.3 and minimum modulation depth about 0.35 over a broad bandwidth of 550 nm (1100-1650 nm). The working principle and excellent modulation performance are thoroughly verified through numerical and experimental studies.