Ultraviolet Circularly Polarized Luminescence in Chiral Perovskite Nanoplatelet-Molecular Hybrids: Direct Binding Versus Efficient Triplet Energy Transfer.

The development of ultraviolet circularly polarized light (UVCPL) sources has the potential to benefit plenty of practical applications but remains a challenge due to limitations in available material systems and a limited understanding of the excited state chirality transfer. Herein, by constructing hybrid structures of the chiral perovskite CsPbBr3 nanoplatelets and organic molecules, excited state chirality transfer is achieved, either via direct binding or triplet energy transfer, leading to efficient UVCPL emission. The underlying photophysical mechanisms of these two scenarios are clarified by comprehensive optical studies. Intriguingly, UVCPL realized via the triple energy transfer, followed by the triplet-triplet annihilation upconversion processes, demonstrates a 50-fold enhanced dissymmetry factor glum. Furthermore, stereoselective photopolymerization of diacetylene monomer is demonstrated by using such efficient UVCPL. This study provides both novel insights and a practical approach for realizing UVCPL, which can also be extended to other material systems and spectral regions, such as visible and near-infrared.

Stable and Ultrafast Blue Cavity-Enhanced Superfluorescence in Mixed Halide Perovskites.

Cavity-enhanced superfluorescence (CESF) in quantum dot (QD) system is an ultrafast and intense lasing generated by combination of quantum coupling effect and optically stimulated amplification effect, which can provide a new idea for realizing high quality blue light sources and address the limitation of conventional inefficient blue light sources. Modifying halide composition is a straightforward method to achieve blue emission in perovskite QD system. However, the spectral instability introduced by photoinduced halide phase segregation and low coupling efficiency between QDs and optical cavities make it challenging to achieve stable blue CESF in such halide-doped QD system. Herein, long-range-ordered, densely packed CsPbBr2 Cl QD-assembled superlattice microcavities in which the two core issues can be appropriately addressed are developed. The QD superlattice structure facilitates excitonic delocalization to decrease exciton-phonon coupling, thus alleviating photoinduced phase segregation. By combination of theoretical analysis and temperature-dependent photoluminescence (PL) measurements, the underlying photoinduced phase segregation mitigation mechanism in mixed halide superlattices is clarified. Based on the CsPbBr2 Cl QD superlattices with regularly geometrical structures, in which the gain medium can be strongly coupled to the naturally formed microcavity, stable and ultrafast (3 ps) blue CESF with excellent optical performance (threshold ≈33 µJ cm-2 , quality factor ≈1900) is realized.

Switchable ultra-broadband terahertz wave absorption with VO2-based metasurface.

Metamaterial absorbers (MMAs) offer a novel and flexible method to realize perfect absorption in specific frequencies, especially in the THz range. Despite the exotic abilities to manipulate light, most previously reported MMAs still suffer from limited bandwidth and tunability. Here we present a thermally switchable terahertz (THz) metasurface that exhibits ultra-broadband absorption and high-transmission characteristics at different ambient temperatures. Our simulations demonstrate that at room temperature the structure is highly transparent. When the ambient temperature reaches 358 K, the proposed design exhibits an ultra-broadband absorption from 0.398 to 1.356 THz with the absorptivity maintaining above 90% and the relative absorption bandwidth reaches up to 109.2%. The structure is demonstrated to be insensitive to the incident angle. Moreover, the bandwidth of such a structure can easily be expanded or reduced by cascading or removing the rings, providing high scalability in practical applications. Such a thermally switchable THz metasurface may have potential applications in various fields, such as optical switching, THz imaging, modulating and filtering.

Demonstration of Thermally Tunable Multi-Band and Ultra-Broadband Metamaterial Absorbers Maintaining High Efficiency during Tuning Process.

Metamaterial absorbers (MMAs) with dynamic tuning features have attracted great attention recently, but most realizations to date have suffered from a decay in absorptivity as the working frequency shifts. Here, thermally tunable multi-band and ultra-broadband MMAs based on vanadium dioxide (VO2) are proposed, with nearly no reduction in absorption during the tuning process. Simulations demonstrated that the proposed design can be switched between two independently designable multi-band frequency ranges, with the absorptivity being maintained above 99.8%. Moreover, via designing multiple adjacent absorption spectra, an ultra-broadband switchable MMA that maintains high absorptivity during the tuning process is also demonstrated. Raising the ambient temperature from 298 K to 358 K, the broadband absorptive range shifts from 1.194-2.325 THz to 0.398-1.356 THz, while the absorptivity remains above 90%. This method has potential for THz communication, smart filtering, detecting, imaging, and so forth.

Transparent ultra-wideband double-resonance-layer metamaterial absorber designed by a semiempirical optimization method.

In this study, a transparent ultra-wideband double-resonance-layer absorber was designed using a semiempirical optimization method. In this method, an equivalent circuit model, genetic algorithm, and parameter fitting are employed to reduce the computation time and improve the design flexibility. Simulations and measurements show that the as-designed absorber can achieve ultrawide microwave absorption in the range of 2.00 to 11.37 GHz with a fractional bandwidth of 140.2%. Furthermore, electric field and surface current distributions show that the broad bandwidth was derived from the good matching of the absorption peaks in the two resonance layers. In addition, the target waveband of the as-designed absorber covered the wavebands of WiFi and radio-frequency identification, as well as part of the 5G waveband. This makes the proposed absorber a good candidate for daily electromagnetic pollution reduction.

Tunable and transparent broadband metamaterial absorber with water-based substrate for optical window applications.

A tunable and transparent metamaterial absorber (MMA) with a water-based substrate is presented, with high optical transparency and broadband microwave absorptivity. In the material structure, indium-tin-oxide (ITO) films are included as the resonant pattern and reflective layers, and distilled water is combined with polymethyl methacrylate (PMMA) to produce the dielectric substrate. By effectively designing its structural parameters, the proposed absorber achieves >90% absorptivity, covering an ultrawide frequency range of 5.8-16.2 GHz, while the average optical transmittance is ∼70.18% over a wavelength range of 400-800 nm. Moreover, owing to a specific design feature, the absorber has high polarization insensitivity and wide-incident-angle stability for transverse electric (TE) and transverse magnetic (TM) polarization waves. Furthermore, the absorption properties of the absorber can be further tuned by controlling the thickness of the water substrate. Both numerical simulations and experimental measurements demonstrate the excellent performance of the device, showing its strong potential for use in optical windows within military and medical equipment.

High-performance broadband electromagnetic interference shielding optical window based on a metamaterial absorber.

An excellently transparent metamaterial-based electromagnetic interference (EMI) shielding window with broadband absorption is presented theoretically and demonstrated experimentally. The window is composed of double split circular ring (SCR) elements whose absorption spectra feature two mild resonant peaks. Indium-tin-oxide (ITO) with resonant patterns is utilized as the material to induce high ohmic loss and broaden the absorption bandwidth. The window achieves strong absorptivity, > 90%, covering an ultrawide frequency range of 7.8-18.0 GHz. Moreover, the measured shielding effectiveness (SE) of the window is > 18.25 dB, at 7.0-18.0 GHz, while the average optical transmittance is fixed at ∼73.10% in the visible-near-infrared (Vis-NIR) region of 400-1,500 nm. Further, the absorption mechanism is revealed by designing an equivalent circuit model and studying the distributions of the electric field and surface currents of the structure. Furthermore, a specific design feature also makes our device insensitive to the incident angle and the polarization state of the impinging microwave. The 90% absorption and shielding performance of the proposed optical window avail it for a wide range of great potential applications, such as the displays of military and medical precision devices.

Large-scale, low-cost, broadband and tunable perfect optical absorber based on phase-change material.

Metamaterial-based electromagnetic absorbers have attracted much attention recently, but most previous realizations suffer from issues of narrow bandwidth, time-consuming and high-cost fabrication methods, and/or fixed functionalities, and so are unfavorable for practical applications. Here, we demonstrate experimentally a large-scale, broadband, polarization-independent, and tunable metamaterial absorber, which works for both visible and near-infrared light. A lithography-free and low-cost method was utilized to fabricate a centimeter-sized metamaterial sample in a metal-insulator-metal (MIM) configuration with nano-scale precision, in which a phase-change material, Ge2Sb2Te5 (GST), was adopted as the insulating spacer of the MIM structure. With two different resonance mechanisms working together, the proposed device was shown to exhibit high absorptivity (>80%) within a broad wavelength band (480-1020 nm). By thermally tuning the phase state of the GST layer, we can dramatically enlarge the working bandwidth of the metamaterial absorber by shifting one absorption peak by about 470 nm. These findings may stimulate many potential applications in, for example, solar cells, energy harvesting, smart sensing/imaging, and color printing.

Near-field imaging of the multi-resonant mode induced broadband tunable metamaterial absorber.

Metamaterial absorbers with tunability have broad prospects for mid-infrared absorption applications. While various methods have been proposed to control absorption, how to analyse and present the physical image of absorption mechanism in depth is still expected and meaningful. Here, we present experimental spatial near-field distributions of a multi-resonant mode induced broadband tunable metamaterial absorber by using near-field optical microscopy. The absorber is constructed by a metal double-sized unit cell and a metallic mirror separated by a thin Ge2Sb2Te5 (GST) spacer. To clearly obtain the physical images, we used a hybrid unit cell consisting of four square resonators to produce two absorption peaks at 7.8 µm and 8.3 µm. The resonance central-wavelength exhibits a redshift while switching the GST thin film from amorphous to crystalline phase. The near-field amplitude and phase optical responses of the absorber are directly observed at absorption frequencies when GST is in both phases, respectively. This work will pave the way for the fundamental science field and inspire potential applications in optical tunable absorption control.

Double-layer metal mesh etched by femtosecond laser for high-performance electromagnetic interference shielding window.

An excellent transparent electromagnetic interference (EMI) shielding window is proposed and demonstrated theoretically and experimentally. The window is composed of double layers of Au-Ni composite mesh, separated by the quartz-glass substrate. The simulation exhibits that the shielding effectiveness (SE) of the double-layer mesh can be improved by increasing the thickness of the substrate in the low frequency range far below the first interfere valley. The measured SE of the proposed structure reaches over 37.61 dB covering an ultra-wide frequency ranging from 150 MHz to 5 GHz, with a maximal SE of 75.84 dB at 3.58 GHz, while the average optical transmittance of the double-layer mesh maintains ∼76.35% at 400-900 nm. Moreover, femtosecond laser direct writing processing technology is used to manufacture the double-layer metal grids, the fabricated grids are not easy to be scuffed off and has a longer operating life. Such a high-performance EMI shielding window has great potential applications in precision optical monitoring instrument and military devices.

Hybridization-induced broadband terahertz wave absorption with graphene metasurfaces.

Electromagnetic (EM) wave absorption plays a vital role in photonics. While metasurfaces are proposed to absorb EM waves efficiently, most of them exhibit limited bandwidth and fixed functionalities. Here, we propose a broadband and tunable terahertz (THz) absorber based on a graphene-based metasurface, which is constructed by a single layer of closely patterned graphene concentric double rings and a metallic mirror separated by an ultrathin SiO2 layer. Plasmonic hybridization between two graphene rings significantly enlarges the absorption bandwidth, which can be further tuned by gating the graphene. Moreover, the specific design also makes our device insensitive to the incident angle and polarization state of impinging EM waves. Our results may inspire certain wave-modulation-related applications, such as THz imaging, smart absorber, tunable sensor, etc.