We numerically demonstrate a novel ultra-broadband polarization-independent metamaterial perfect absorber in the visible and near-infrared region involving the phase-change material Ge 2 Sb 2 Te 5 (GST). The novel perfect absorber scheme consists of an array of high-index strong-absorbance GST square resonators separated from a continuous Au substrate by a low-index lossless dielectric layer (silica) and a high-index GST planar cavity. Three absorption peaks with the maximal absorbance up to 99.94% are achieved, owing to the excitation of plasmon-like dipolar or quadrupole resonances from the high-index GST resonators and cavity resonances generated by the GST planar cavity. The intensities and positions of the absorption peaks show strong dependence on structural parameters. A heat transfer model is used to investigate the temporal variation of temperature within the GST region. The results show that the temperature of amorphous GST can reach up to 433 K of the phase transition temperature from room temperature in just 0.37 ns with a relatively low incident light intensity of 1.11 × 10 8 W∕m 2 , due to the enhanced ultra-broadband light absorbance through strong plasmon resonances and cavity resonance in the absorber. The study suggests a feasible means to lower the power requirements for photonic devices based on a thermal phase change via engineering ultra-broadband light absorbers.

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We propose an efficient multiband absorber comprised of a truncated, one-dimensional periodic metal-dielectric photonic crystal and a reflective substrate. The reflective substrate is essentially an optically thick metallic film. Such a planar device is easier to fabricate compared to absorbers with complicated shapes. For a four-unit cell device, all four of the absorption peaks can be optimized with efficiencies higher than 95 percent. Moreover, those absorption peaks are insensitive to the polarization and incident angle. The influences of the geometrical parameters and the refractive index of the dielectric on the device performance also are discussed. Furthermore, we found that the number of absorption peaks within each photonic band precisely corresponds to the number of unit cells because the truncated photonic crystal lattices select resonant modes. We also show that the total absorption efficiency gradually increases when there are more periods of the metal-dielectric composite layer placed on top of the metallic substrate. We expect this work to have potential applications in solar energy harvesting and thermal emission tailoring.

Nanoscale active devices, such as all-optical modulators and electro-optical transducers, can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials. Here, we demonstrate control over absorption properties in such a heterostructure by coupling the localized surface plasmon resonance (LSPR) of gold nanoantennas to a phase change material (PCM), Ge2Sb2Te5 (GST). The peak absorption of this hybrid absorber approaches near unity at resonance due to the simultaneous excitations of electric and magnetic resonant modes. Moreover, such a hybrid absorber can realize arbitrary wavelength-selective spectral absorption in the mid-infrared region simply by altering the square nanoantennas side length. By controlling the total power of the incident light, the intermediate phases composed of different proportions of the amorphous and crystalline molecules of the GST can be correspondingly tailored, and thus the absorption can be continuously tuned, which provides a flexible and encouraging way to achieve active features once fabricated. Importantly, by converting GST from the amorphous to crystalline state or vice versa, the hybrid absorber can realize bidirectional switching of "ON" and "OFF" states, with an outperformed modulation depth of 98% (or 95%) and extinction ratio of −17.15 dB (or −12.98 dB), respectively, indicating its excellent optical modulation performance. Notably, all the stable and intermediate phases of the GST are stable at room temperature, and therefore no sustained external thermal consumption is needed to maintain a desired absorption band for the hybrid scheme. Additionally, the structure can tolerate a wide range of incident angles as well as show polarization-independent features. With these extraordinary optical responses, the proposed scheme could find potential applications in active photonic devices such as optical modulation, thermal imaging and optical switching.

Chiral metamaterials with versatile designs can exhibit orders of magnitude enhancement in chiroptical responses compared with that of the natural chiral media. Here, we propose an ease-of-fabrication three-dimensional (3D) chiral metamaterial consisting of vertical asymmetric plate-shape resonators along a planar air hole array with extraordinary optical transmission. It is theoretically shown that such chiral metamaterials simultaneously support five-fold plasmonic Fano resonance states and exhibit significant bisignate circular dichroism (CD) with amplitude as large as 0.8 due to the distinctive local electric field distributions. More interestingly, a "bridge" in the proposed double-plate-based architectures can act as a flipped ruler that is able to continuously manipulate optical chirality including the handedness-selective enhancement and the switching of CD signals. Importantly, the proposed designs have been readily fabricated by using a focused-ion-beam irradiation-induced folding technique and they consistently exhibited five-fold Fano resonances with strong CD effects in experiments. The studies are helpful for the understanding, designing and improvement of chiral optical systems towards potential applications such as ultrasensitive biosensing, polarimetric imaging, quantum information processing, etc.

Metal nanogratings as one of the promising architectures for effective light trapping in organic photovoltaics (OPVs) have been actively studied over the past decade. Here we designed a novel metal nanowall grating with ultra-small period and ultra-high aspect-ratio as the back electrode of the OPV device. Such grating results in the strong hot spot effect in-between the neighboring nanowalls and the localized surface plasmon effect at the corners of nanowalls. These combined effects make the integrated absorption efficiency of light over the wavelength range from 400 to 650 nm in the active layer for the proposed structure, with respect to the equivalent planar structure, increases by 102% at TM polarization and by 36.5% at the TM/TE hybrid polarization, respectively. Moreover, it is noted that the hot spot effect in the proposed structure is more effective for ultra-thin active layers, which is very favorable for the exciton dissociation and charge collection. Therefore such a nanowall grating is expected to improve the overall performance of OPV devices.

Conventional metasurface carpet cloak generally has a narrow working bandwidth and a small incident angle range, which is far from meeting the requirements of modern military for broadband and wide-angle cloaking. Based on graphene apertures and graphene patches, we propose the design of metasurface carpet cloaks for terahertz wave and explore their application potential in realizing broadband and wide-angle cloaking. Simulation results demonstrate that the carpet cloaks based on graphene apertures and patches can achieve a large working bandwidth of 35.7% and 52.6%, and a wide angular span of  ±30° and  ±40°, respectively. The cloaking performance and the working bandwidths of the two graphene metasurface carpet cloaks can be tuned by changing the graphene Fermi energy uniformly. The carpet cloak constructed by graphene patches has a larger bandwidth and a wider angle domain than that by graphene apertures, which can be understood by comparing the phase dispersion of the two graphene metasurfaces.