Ge2Sb2Te5-based efficient switching between a cross-polarization conversion and a circular-to-linear polarization conversion.

The terahertz (THz) band has a great potential for the development of communication technology, but it has not been fully utilized due to the lack of practical devices, especially actively controllable multifunctional devices. Here, we propose and demonstrate a Ge2Sb2Te5 (GST)-based metamaterial device, where an actively controllable function is experimentally verified by inducing the crystallization process with thermal activation. Cross-polarization conversion in the reflection mode and circular-to-linear polarization conversion in the transmission mode are obtained under crystalline and amorphous GST conditions, respectively. The combination of GST and THz waves has a wide range of applications and will further advance the THz field.

Tunable parity-time symmetry vortex laser from a phase change material-based microcavity.

Traditional light sources cannot emit an electromagnetic (EM) field with an orbital angular momentum (OAM), limiting their applications in modern optics. The recent development of the OAM laser, mainly based on micro- and nanostructures, can satisfy the increasing requirements for on-chip photonics and information capacities. Nevertheless, the photonic structures have fixed parameters that prevent these OAM lasers from being dynamically tuned. Here, we propose tunable vortex lasing from a microring cavity integrated by a phase change material, Ge2Sb2Te5 (GST225). By modulating the complex refractive index to create an exceptional point (EP) to break the degeneracy of whispering gallery modes with opposite orientations, the microlaser working at the EP can impart an artificial angular momentum, thus emitting vortex beams with well-defined OAM. The grating scatter on the edge of the microring can provide efficient vertical radiation. The vortex laser wavelength from the GST225/InGaAsP dual-layered microring cavity can be dynamically tuned by switching the state of GST225 between amorphous and crystalline without changing the microring geometry. We construct an electric-thermal model to show the tuning range of operating wavelengths (EPs) from 1544.5 to 1565.9 nm in ~25 ns. Our study on high-speed tunable PT-symmetry vortex lasers facilitates the next generation of integrated optoelectronic devices for optical computing and communications in both classical and quantum regions.

Erratum: Roadmap for phase change materials in photonics and beyond.

[This corrects the article DOI: 10.1016/j.isci.2023.107946.].

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.

Plasmonic Chiral Metasurface-Induced Upconverted Circularly Polarized Luminescence from Achiral Upconversion Nanoparticles.

Chirality induction, transfer, and manipulation have aroused great interest in achiral nanomaterials. Here, we demonstrate strong upconverted circularly polarized luminescence from achiral core-shell upconversion nanoparticles (UCNPs) via a plasmonic chiral metasurface-induced optical chirality transfer. The Yb3+-sensitized core-shell UCNPs with good dispersity exhibit intense upconversion luminescence of Tm3+ and Nd3+ through the energy transfer process. By spin-coating the core-shell UCNPs on this chiral metasurface, strong enhancement and circular polarization modulation of upconversion luminescence can be achieved due to resonant coupling between surface plasmons and upconversion nanoparticles. In the UCNPs-on-metasurface composite, a significant upconversion luminescence enhancement can be achieved with a maximum enhancement factor of 32.63 at 878 nm and an overall enhancement factor of 11.61. The luminescence dissymmetry factor of the induced upconverted circularly polarized luminescence can reach 0.95 at the emission wavelength of 895 nm. The UCNPs-on-metasurface composite yields efficient modulation for the emission intensity and polarization of UCNPs, paving new pathways to many potential applications in imaging, sensing, and anticounterfeiting fields.

Nonvolatile chirality switching in terahertz chalcogenide metasurfaces.

Actively controlling the polarization states of terahertz (THz) waves is essential for polarization-sensitive spectroscopy, which has various applications in anisotropy imaging, noncontact Hall measurement, and vibrational circular dichroism. In the THz regime, the lack of a polarization modulator hinders the development of this spectroscopy. We theoretically and experimentally demonstrate that conjugated bilayer chiral metamaterials (CMMs) integrated with Ge2Sb2Te5 (GST225) active components can achieve nonvolatile and continuously tunable optical activity in the THz region. A THz time-domain spectroscopic system was used to characterize the device, showing a tunable ellipticity (from ‒36° to 0°) and rotation of the plane polarization (from 32° to 0°) at approximately 0.73 THz by varying the GST225 state from amorphous (AM) to crystalline (CR). Moreover, a continuously tunable chiroptical response was experimentally observed by partially crystallizing the GST225, which can create intermediate states, having regions of both AM and CR states. Note that the GST225 has an advantage of nonvolatility over the other active elements and does not require any energy to retain its structural state. Our work allows the development of THz metadevices capable of actively manipulating the polarization of THz waves and may find applications for dynamically tunable THz circular polarizers and polarization modulators for THz emissions.

Acoustic transmissive cloaking with adjustable capacity to the incident direction.

Zero-refractive-index (ZRI) phononic crystals (PhCs), in which acoustic waves can be transmitted without phase variations, have considerable potential for engineering wavefronts and thus are applicable to invisibility cloaking. However, the creation of the transmissive cloaking achieved by ZRI-PhCs is challenging under an oblique incidence, which substantially hinders their practical applications. Here, we experimentally demonstrate acoustic transmissive cloaking with the adjustable capacity to the incident direction. Acoustic transmissive cloaking of arbitrarily shaped obstacles can be obtained through a hybrid acoustic structure consisting of one outer layer of a programmable phase-engineered metasurface (PPEM) and one inner layer of a double zero-refractive-index (DZRI)-PhC. The DZRI-PhC is functionally the same as an equiphase area and can guide acoustic waves around the obstacle, a process known as acoustic tunneling. The PPEM perpendicularly transfers the incident acoustic waves to the DZRI-PhC and allows the emergent waves from the DZRI-PhC to transmit along the original incident direction. The DZRI-PhC is made of an array of iron squares in the air. The reciprocal of the effective bulk modulus and the effective mass density is approximately zero at a frequency of 3015 Hz (0.5187 v 0 /a) originating from the zeroth-order Fabry-Pérot (FP) resonance that possesses infinite phase velocities. Each meta-atom of the outer metasurface consists of a line channel and four shunted Helmholtz resonators, which have effective masses that are engineered by a mechanics system. The amplitude and phase of the sound waves propagating through each meta-atom can be controlled continuously and dynamically, enabling the metasurface to obtain versatile wavefront manipulation functions. Acoustic cloaking is visually demonstrated by experimentally scanning the acoustic field over the hybrid structure at a frequency of 3000 Hz (0.5160 v 0 /a). Our work may provide applications with great potential, including underwater ultrasound, airborne sound, acoustic communication, imaging, etc.

Sieving nanometer enantiomers using bound states in the continuum from the metasurface.

Enantioseparation of chiral molecules is an important aspect of life sciences, chemical syntheses, and physics. Yet, the prevailing chemical techniques are not effective. Recently, a few types of plasmonic apertures have been theoretically proposed to distinguish between chiral molecules that vary based on their handedness under circularly polarized illumination. Both analytic calculations and numerical simulation demonstrated that enantioselective optical sieving could be obtained at the nanoscale using a large chiral optical force based on plasmonic resonance enhanced near-field chiral gradients in the aperture. Nevertheless, scaling this scheme to chiral entities of a few nanometer size (i.e., proteins and DNA) faces formidable challenges owing to the fabrication limit of a deeply sub-nanometer aperture and the intense power levels needed for nanoscale trapping. In contrast, by extending the Friedrich-Wintgen theory of the bound states in the continuum (BIC) to photonics, one may explore another mechanism to obtain enantioselective separation of chiral nanoparticles using all-dielectric nanostructures. Here, we present a metasurface composed of an array of silicon (Si) nanodisks embedded with off-set holes, which supports a sharp high-quality (Q) magnetic dipolar (MD) resonance originating from a distortion of symmetry-protected BIC, so called quasi-BIC. We, for the very first time, show that such a quasi-BIC MD resonance can markedly improve the chiral lateral force on the paired enantiomers with linearly polarized illumination. This quasi-BIC MD resonance can enhance the chirality density gradient with alternating sign at each octant around the Si nanodisk, while exhibiting a small gradient for the electromagnetic (EM) density. This offers a chiral lateral force that is 1 order larger in magnitude compared to the non-chiral lateral forces on sub-2 nm chiral objects with a chirality parameter of ±0.01. Moreover, the quasi-BIC MD resonance can excite four pairs of diverse optical potential wells (-13k B T) that are distributed uniformly along the outer edge of the resonator, enabling a simultaneous separation of four paired enantiomers. Our proposed dielectric metasurface may move forward the techniques of enantioseparation and enantiopurification, taking a novel perspective to advanced all-optical enantiopure synthesis.

Terahertz multi-level nonvolatile optically rewritable encryption memory based on chalcogenide phase-change materials.

Fast and efficient information processing and encryption, including writing, reading, and encryption memory, is essential for upcoming terahertz (THz) communications and information encryption. Here, we demonstrate a THz multi-level, nonvolatile, optically rewritable memory and encryption memory based on chalcogenide phase-change materials, Ge2Sb2Te5 (GST). By tuning the laser fluence irradiated on GST, we experimentally achieve multiple intermediate states and large-area amorphization with a diameter of centimeter-level in the THz regime. Our memory unit features a high operating speed of up to 4 ns, excellent reproducibility, and long-term stability. Utilizing this approach, hexadecimal coding information memories are implemented, and multiple writing-erasing tests are successfully carried out in the same active area. Finally, terahertz photoprint memory is demonstrated, verifying the feasibility of lithography-free devices. The demonstration suggests a practical way to protect and store information and paves a new avenue toward nonvolatile active THz devices.

Ultrathin Suspended Chiral Metasurfaces for Enantiodiscrimination.

Chiral metasurfaces can exhibit a strong circular dichroism, but it is limited by the complicated fabrication procedure and alignment errors. Here, a new type of self-aligned suspended chiral bilayer metasurface with only one-step electron beam lithography exposure is demonstrated. A significant optical chirality of 221° µm-1 can be realized using suspended metasurfaces with a thickness of 100 nm. Furthermore, this study experimentally demonstrates that such a structure is capable of label-free discrimination of the chiral molecules at zeptomole level, exhibiting a much higher sensitivity (orders of magnitude) compared to the conventional circular dichroism spectroscopy. The fundamental principles for chiral sensing using molecules-metasurfaces interactions are explored. Benefiting from the giant chiroptical response, the proposed metadevice may offer promising applications for ultrathin circular polarizers, chiral molecular detectors, and asymmetry information processing.

Reconfigurable label-free shape-sieving of submicron particles in paired chalcogenide waveguides.

Up-to-date particle sieving schemes face formidable challenges for sieving label-free submicron molecules with similar sizes and dielectric constants but diverse shapes. Herein, optical sorting of polystyrene particles with various shapes is illustrated in optofluidic nanophotonic paired waveguide (ONPW) composed of chalcogenide semiconductor Sb2Se3. The Sb2Se3-ONPW creates the coupling length (CL) between the neighboring hot spots that can be actively modulated via the transition of Sb2Se3 between amorphous (AM) and crystalline (CR) phases. Submicron particles interfere with the coupled hotspots, which can exert various optical torques on the particles according to their profiles. In the model system, spherical (diameter of 0.5 µm) and rod-shaped (diameter of 0.5 µm, length of 1.5 µm) polystyrene particles were employed to mimic two types of bacteria, namely, Staphylococcus aureus and rod-shaped Escherichia coli, respectively. For the AM state, the CL value is ∼7.0 µm, enabling the structure to trap the sphere stably in the hot spots. For the CR state, the CL value becomes ∼25 µm, leading to stable trapping of the rod-shaped particle. In this work, the working wavelength was fixed at 1.55 µm at which both AM- and CR-Sb2Se3 are transparent. Our scheme may offer a paradigm shift in shape-selective sieving of biomolecules and fulfill the requirements of the new-generation lab-on-chip techniques, where the integrated manipulation system must be much more multifunctional and flexible.

Enhanced dielectric and conductivity properties of carbon-coated SiC nanocomposites in the terahertz frequency range.

C-coated SiC nanocomposites (SiC@C NCs) were one-step synthesized under a mixture atmosphere of Ar and CH4 using a DC arc-discharge plasma method. The microstructure of the composites could be controlled by varying the volume ratio of Ar and CH4. A strong response to the terahertz (THz) field was observed due to the existence of a graphite shell. The dielectric properties of SiC@C NCs can be enhanced by altering the thickness of the graphite shell. The thicker graphite shell results in a stronger absorption of THz waves and an enhanced real part of conductivity. Fitting the measured conductivity data using the Drude-Smith model reveals that the carrier transport in the SiC@C NCs and its counterpart, the SiC nanoparticles, is dominated by backscattering. The SiC@C NCs with enhanced conductivity are believed to be fundamental materials for various functionalized optoelectronic devices.

Nonintrusively Adjusting Structural Colors of Sealed Two-Dimensional Photonic Crystals: Immediate Transformation between Transparency and Intense Iridescence and Their Applications.

Responsive photonic crystals (PCs), which can adjust structural colors in response to external stimuli, show great potential applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting. In contrast, conventional structure-intrusive adjustment manners that external stimuli directly interact with the ordered arrays may lead to structural damage or longer response time. Here, a noninvasive adjustment of the structural colors of two-dimensional (2D) PCs (2D-PCs) is explored based upon diffraction theory. Sealed 2D-PCs and 2D inverse opal photonic crystal (IOPC) flexible devices are prepared. They are highly transparent in air but immediately exhibit intense viewing angle-dependent structural colors after being dipped in water. The mechanism of transparent-iridescent immediate transformation is explained by Bragg's law. The design mechanism is examined by numerical simulation and spectral shifts in different external media. We demonstrate its applications in the fields of information encryption and anticounterfeiting by using the transparent-iridescent immediate transformation of sealed 2D-PC patterns and 2D IOPC free-standing films sealed on the product surface. Because of the strong contrast between transparency and intense iridescence, reversible and immediate transformation, and durability, sealed 2D-PCs and 2D IOPC flexible devices designed by the noninvasive adjustment strategy will lead to a variety of new applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting.

A reconfigurable hyperbolic metamaterial perfect absorber.

Metamaterial (MM) perfect absorbers are realised over various spectra from visible to microwave. Recently, different approaches have been explored to integrate tunability into MM absorbers. Particularly, tuning has been illustrated through electrical-, thermal-, and photo-induced changes to the permittivity of the active medium within MM absorbers. However, the intricate design, expensive nanofabrication process, and the volatile nature of the active medium limit the widespread applications of MM absorbers. Metal-dielectric stack layered hyperbolic metamaterials (HMMs) have recently attracted much attention due to their extraordinary optical properties and rather simple design. Herein, we experimentally realised a reconfigurable HMM perfect absorber based on alternating gold (Au) and Ge2Sb2Te5 (GST225) layers for the near-infrared (N-IR) region. It shows that a red-shift of 500 nm of the absorptance peak can be obtained by changing the GST225 state from amorphous to crystalline. The nearly perfect absorptance is omnidirectional and polarisation-independent. Additionally, the absorptance peak can be reversibly switched in just five nanoseconds by re-amorphising the GST225, enabling a dynamically reconfigurable HMM absorber. Experimental data are validated numerically using the finite-difference time-domain method. The absorber fabricated using our strategy has advantages of being reconfigurable, uncomplicated, and lithography-free over conventional MM absorbers, which may open up a new path for applications in energy harvesting, photodetectors, biochemical sensing, and thermal camouflage techniques.

Vectorial holography-mediated growth of plasmonic metasurfaces.

Nowadays, the electromagnetic properties of artificial photonic materials can be well-tuned via designs over their composition and geometries. However, engineering the properties of artificial materials at the nanoscale is challenging and costly. Here we demonstrate a facile and low-cost method for fabricating large-area silver nanoparticle metasurfaces (AgNPMSs) by using the vectorial holography-mediated growth technique. The AgNPMS, which can be regarded as a hologram device, possesses excellent chiroptical properties. The vectorial holographic technique may open avenues for fabricating novel chiroptical metamaterials with large degrees of freedom, which can be further used for beam steering, photocatalysis, biosensing, etc.

Nonlinear Diatomic Metasurface for Real and Fourier Space Image Encoding.

In linear optics, the metasurface represents an ideal platform for encoding optical information because of its unprecedented abilities of manipulating the intensity, polarization, and phase of light wave with subwavelength meta-atoms. However, controlling various degrees of freedom of light in nonlinear optics remains elusive. Here, we propose a nonlinear plasmonic metasurface working in the near-infrared regime that can simultaneously encode optical images in the real and Fourier spaces. This is achieved by designing a diatomic meta-molecule, which enables the independent control of the nonlinear geometric phase, polarization, and intensity of second harmonic waves. The proposed nonlinear diatomic metasurface provides an ultracompact platform for implementing multidimensional optical information encoding and may hold great potential in optical information security and optical anticounterfeiting.

Tuning of Classical Electromagnetically Induced Reflectance in Babinet Chalcogenide Metamaterials.

Metamaterials analog of electromagnetically induced reflectance (EIR) has attracted intense attentions since they can provide various applications for novel photonic devices such as optical detectors with a high sensitivity and slow-light devices with a low loss. The development of dynamic photonic devices desires a tunable EIR feature in metamaterials. However, most metamaterials-induced EIR is not spectrally controllable particularly for the near-infrared (NIR) region. Herein, a tuning of EIR is illustrated in Babinet chalcogenide metamaterials in the NIR region. The EIR response is created by weak hybridization of two dipolar (bright) modes of the paired Au slots. Such a mode interference can be engineered through non-volatile phase transition to the refractive index of the Ge2Sb2Te5 (GST), resulting in an active controlling of the reflection window. A 15% spectral tuning of the reflectance peak is observed experimentally in the NIR region as switching the GST state between amorphous and crystalline.

Extraordinary Multipole Modes and Ultra-Enhanced Optical Lateral Force by Chirality.

Strong mode coupling and Fano resonances arisen from exceptional interaction between resonant modes in single nanostructures have raised much attention for their advantages in nonlinear optics, sensing, etc. Individual electromagnetic multipole modes such as quadrupoles, octupoles, and their counterparts from mode coupling (toroidal dipole and nonradiating anapole mode) have been well investigated in isolated or coupled nanostructures with access to high Q factors in bound states in the continuum. Albeit the extensive study on ordinary dielectric particles, intriguing aspects of light-matter interactions in single chiral nanostructures is lacking. Here, we unveil that extraordinary multipoles can be simultaneously superpositioned in a chiral nanocylinder, such as two toroidal dipoles with opposite moments, and electric and magnetic sextupoles. The induced optical lateral forces and their scattering cross sections can thus be either significantly enhanced in the presence of those multipoles with high-Q factors, or suppressed by the bound states in the continuum. This work for the first time reveals the complex correlation between multipolar effects, chiral coupling, and optical lateral force, providing a distinct way for advanced optical manipulation.

Tunable Grain Orientation of Chalcogenide Film and Its Application for Second Harmonic Generation.

To date, the second harmonic generation (SHG) has a great effect on photonic devices. However, it is a formidable challenge to achieve reconfigurable SHG. Hereby, we experimentally demonstrate the SHG response from the oriented Ge2Sb2Te5 (GST) grains induced by polarized laser pulses for the first time. The orientation of GST grains is found to be perpendicular to the polarization direction of the pump laser. Such unique ordered structures result in a periodic change of SHG intensity with the input polarization angle of the pump laser rotating every 180°. These findings may pave avenues for generating nonlinear optical sources with a simple process, scalability, and switchable functionality.

Breakup Dynamics of Semi-dilute Polymer Solutions in a Microfluidic Flow-focusing Device.

Droplet microfluidics involving non-Newtonian fluids is of great importance in both fundamental mechanisms and practical applications. In the present study, breakup dynamics in droplet generation of semi-dilute polymer solutions in a microfluidic flow-focusing device were experimentally investigated. We found that the filament thinning experiences a transition from a flow-driven to a capillary-driven regime, analogous to that of purely elastic fluids, while the highly elevated viscosity and complex network structures in the semi-dilute polymer solutions induce the breakup stages with a smaller power-law exponent and extensional relaxation time. It is elucidated that the elevated viscosity of the semi-dilute solution decelerates filament thinning in the flow-driven regime and the incomplete stretch of polymer molecules results in the smaller extensional relaxation time in the capillary-driven regime. These results extend the understanding of breakup dynamics in droplet generation of non-Newtonian fluids and provide guidance for microfluidic synthesis applications involving dense polymeric fluids.