reduce the size of these sensors, [4][5][6][7][8][9][10][11][12] since it exhibits a unique surface condition with tailored spectral properties and strong electric field enhancement, which results in a high sensitivity to the surroundings. [13][14][15][16][17][18][19][20][21] To date, most approaches toward metamaterial inspired sensors are based on perfect light absorption. [17,19,22,23] However, the measurement of their reflectance makes both optical alignment and chip integration challenging. [24] Furthermore, metamaterial sensors that are designed for the M-IR are rarely discussed because robust sources, detectors, and efficient components are limited. [25] In addition, despite metamaterial sensors exhibiting excellent sensitivity at their designed frequency, demonstrations of wideband or frequency-swept detection are uncommon. This is because the metamaterial optical response is usually fixed by its dimensions and dielectric properties. [26] Consequently, measurements at different wavelengths using a single metamaterial device are impossible. Recently, metamaterials that are tuned by integrating graphene, [27] vanadium dioxide, [28] liquid crystals, [29] and metal hydrides [30] were investigated. Particularly, the graphene [27] and vanadium dioxide [28] based metasurfaces are promising for dynamically tuning plasmonic induced transparency.Note, these tuning mechanisms tend to be impractical in the M-IR region because the materials involved have a strong Drude contribution to the dielectric function. The M-IR region is, however, an important spectral band where there exists an atmospheric transparency window and molecular vibrational fingerprints. [31][32][33] Chalcogenide phase change materials (PCMs) have a remarkable portfolio of properties. [34][35][36] In particular, unlike silica glasses, the low phonon energies of chalcogenides allow them to be transparent in the M-IR. [37] The fast switching between two structural states of the phase change data storage material, Ge 2 Sb 2 Te 5 (GST), make it ideal for active photonic devices. [34,[38][39][40][41][42][43][44][45][46][47][48] Notably, in the M-IR region GST exhibits a pronounced contrast in the real part of the permittivity (ε r ), and a negligibly small ratio of the imaginary (ε i ) to the ε r , indicating low absorptive losses. [49,50] In addition, it is now possible to design the nanostructure of GST to achieve specific switching characteristics. [51,52] Thus, there is evidence that chalcogenide PCMs can enable practical spectroscopically programmable M-IR metamaterials.In this work, we demonstrate a phase change material tuned transmissive M-IR metasurface. The metasurface consists of an array of Au square pillars stacked above a GST switchable layer. Contrary to other metal-dielectric-metal trilayer reflective metamaterials, our design works in transmission mode by avoidingThe intense light-matter interaction of plasmonic metasurfaces provides an appealing platform for optical sensing. To date, most metasurface sensors are not spectrally tuned. Mo...

The narrowband surface plasmon resonance of metallic nanostructures was once thought to limit the bandwidth of absorptance, yet recent demonstrations show that it can be harnessed using mechanisms such as multiple resonances, impedance matching, and slow-light modes to create broadband absorptance. However, in the visible spectrum, realization of absorbers based on patterned plasmonic nanostructures is challenging due to strict fabrication tolerances. Here we experimentally compare two different candidates for visible light broadband high absorptance. The first candidate is planar thin film dual layers of Ge2Sb2Te5 and aluminum (Al), while the second structure employs ultrathin Al grating/Ge2Sb2Te5 dual layers. In both cases, the absorbers yield a measured absorptance greater than 78% in the visible. A remarkably high-absorptance bandwidth of 120 nm was measured and associated with the large imaginary part of Ge2Sb2Te5 dielectric function. We find that the simple dual-layer planar structure is an effective absorber in the near-infrared, but its absorptance is less effective in the visible. However, for visible wavelengths the grating structure can blue-shift the absorptance peak to 422 nm. The simple geometries of the plasmonic absorbers facilitate fabrication over large areas. It has practical applications in light harvesting, sensing, and high-resolution color printing.

Separating enantiomers is vital in chemical syntheses, life sciences, and physics. However, the usual chemical processes are inefficient. Recently, plasmonic nanostructures have drawn considerable attention for manipulating nanoparticles; however, only a few approaches are proposed to discriminate between entities that differ in terms of their handedness. This is because the chiral polarizability is much smaller than the electric polarizability, and therefore the non‐chiral gradient force dominates over the chiral gradient force. This limit means that the enantioselective sorting of chiral nanoparticles is a formidable challenge. A plasmonic nanostructure consisting of a disc‐double split ring resonator exhibiting a dipole–octupole (DO) Fano resonance (FR) is designed and fabricated. It is theoretically demonstrated that such a DO‐FR can markedly enhance the chiral gradient force on the paired enantiomers. The coaxial channel of the resonator possessing high chirality density gradients around the DO‐FR is derived. This provides an enhanced chiral gradient force that dominates over the non‐chiral gradient forces on sub‐10 nm chiral nanoparticles. Enantiomeric pairs can thus experience distinct potential wells in terms of signs. This proposed structure may advance the techniques of enantiopurification and enantioseparation, bringing a new perspective to state‐of‐the‐art all‐optical enantiopure synthesis.