Fast Switching “On/Off” Chiral Surface Plasmon Polaritons in Graphene-Coated Ge₂Sb₂Te₅ Nanowire

Plasmonic nanowire was found to generate chiral surface plasmon polaritons (SPPs), with the perspective of enhancing molecular spectroscopy for biosensing applications. However, the lack of chiroptical switches exhibits significant limitations in detecting a multitude of various analytes with a high sensitivity and in asymmetric catalysis to provide a switchable stereoselectivity. Here, we numerically and analytically propose a graphene-coated Ge₂Sb₂Te₅ (GST225) nanowire to solve this problem. We highlight that the chiral SPPs propagating along the nanowire can be reversibly switched between “on” (transparent) and “off” (opaque) as transiting the state of GST225 core between amorphous and crystalline. Moreover by changing the Fermi energy of the graphene coating layer, the hybrid nanowire can produce either achiral or chiral SPPs at the output of nanowire. A thermal-electric model is put forward to study the temporal variation of the temperature of the GST225 core. Transiting the structural phase of GST225 in nanosecond was theoretically demonstrated. Our proof of concept permits the preparation of circularly polarized light source with a fast switching “on/off” function. We foresee its potential applications for tunable nanophotonic devices, plasmonic nanowire networks, on-chip biosensing, etc., where the active controlling of SPPs is necessary

Fast Switching “On/Off” Chiral Surface Plasmon Polaritons in Graphene-Coated Ge₂Sb₂Te₅ Nanowire

Plasmonic nanowire was found to generate chiral surface plasmon polaritons (SPPs), with the perspective of enhancing molecular spectroscopy for biosensing applications. However, the lack of chiroptical switches exhibits significant limitations in detecting a multitude of various analytes with a high sensitivity and in asymmetric catalysis to provide a switchable stereoselectivity. Here, we numerically and analytically propose a graphene-coated Ge₂Sb₂Te₅ (GST225) nanowire to solve this problem. We highlight that the chiral SPPs propagating along the nanowire can be reversibly switched between “on” (transparent) and “off” (opaque) as transiting the state of GST225 core between amorphous and crystalline. Moreover by changing the Fermi energy of the graphene coating layer, the hybrid nanowire can produce either achiral or chiral SPPs at the output of nanowire. A thermal-electric model is put forward to study the temporal variation of the temperature of the GST225 core. Transiting the structural phase of GST225 in nanosecond was theoretically demonstrated. Our proof of concept permits the preparation of circularly polarized light source with a fast switching “on/off” function. We foresee its potential applications for tunable nanophotonic devices, plasmonic nanowire networks, on-chip biosensing, etc., where the active controlling of SPPs is necessary

Wideband Absorbers in the Visible with Ultrathin Plasmonic-Phase Change Material Nanogratings

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

Gate-Programmable Electro-Optical Addressing Array of Graphene-Coated Nanowires with Sub-10 nm Resolution

The rapid development of highly integrated photonic circuits has been driving electro-optic (EO) devices to increasingly compact sizes, with the perspective of being able to control light at the nanoscale. However, tunability with spatial resolution below 10 nm scale with conventional approaches, such as metallic nanowires, remains a challenge. Here, we show a graphene-coated nanowire system aiming at beam spatial modulation at a deeply subwavelength scale. By analytically and numerically investigating the eigenmodal properties of this system, we found that beam power can propagate along either a swinging or a helical path in the hybrid nanowire. In particular, the period of the swing beam and the chirality and period of the helix beam can be flexibly controlled by tuning the chemical potential of graphene via the gate voltage. Significantly, due to its good modal confinement, such a beam can be independently manipulated even in the presence of another nanowire at a separation of 40 nm, which opens a realistic path toward gate-programmable EO addressing or data storage with ultrahigh density (64 terabyte/μm). At the same time, by fulfilling the phase matching condition between the two supported guided modes operating at different wavelengths, either a full band or band-tunable terahertz wave at the nanoscale may be achieved by nonlinear difference frequency generation. Our proposed hybrid nanowire system opens interesting potentials to accomplish gate-programmable EO devices at sub-10 nm scale

Controlling Lateral Fano Interference Optical Force with Au–Ge₂Sb₂Te₅ Hybrid Nanostructure

We numerically demonstrate that a pronounced dipole–quadrupole (DQ) Fano resonance (FR) induced lateral force can be exerted on a dielectric particle 80 nm in radius (<i>R</i>_sphere = 80 nm) that is placed 5 nm above an asymmetric bow-tie nanoantenna array based on Au/Ge₂Sb₂Te₅ dual layers. The DQ-FR-induced lateral force achieves a broad tuning range in the mid-infrared region by changing the states of the Ge₂Sb₂Te₅ dielectric layer between amorphous and crystalline and in turn pushes the nanoparticle sideways in the opposite direction for a given wavelength. The mechanism of lateral force reversal is revealed through optical singularity in the Poynting vector. A thermal–electric simulation is adopted to investigate the temporal change of the Ge₂Sb₂Te₅ film’s temperature, which demonstrates the possibility of transiting the Ge₂Sb₂Te₅ state by electrical heating. Our mechanism by tailoring the DQ-FR-induced lateral force presents clear advantages over the conventional nanoparticle manipulation techniques: it possesses a pronounced sideways force under a low incident light intensity of 10 mW/μm², a fast switching time of 2.6 μs, and a large tunable wavelength range. It results in a better freedom in flexible nanomechanical control and may provide a new means of biomedical sensing and nano-optical conveyor belts

Beam Steering of Nonlinear Optical Vortices with Phase Gradient Plasmonic Metasurfaces

The generation of photons with spin and orbital angular momentum is of great importance in the fields of classical and quantum optical communications. Recent studies show that optical vortices with on-demand angular momentum can be realized with geometric phase-controlled metasurfaces. However, such optical vortices have two spin-locked orbital angular momentum states, which are difficult to distinguish in the same propagating direction. While the beam steering of the optical vortices can be easily realized in the linear optical regime, it remains elusive in the nonlinear optical counterpart. Here, we propose to generate and spatially separate the spin-locked second-harmonic vortex beams through phase gradient plasmonic metasurfaces. Based on the concept of the nonlinear geometric phase, the fork-type phase distributions are encoded onto the metasurfaces by using gold meta-atoms with a threefold rotational symmetry. Under the pumping of fundamental waves in the near-infrared regime, the spin-locked optical vortices at second-harmonic frequency are generated and then projected to different diffraction orders. The proposed strategy may have important applications in high-dimensional optical information processing

Observation and Manipulation of Visible Edge Plasmons in Bi₂Te₃ Nanoplates

Noble metals, like Ag and Au, are the most intensively studied plasmonic materials in the visible range. Plasmons in semiconductors, however, are usually believed to be in the infrared wavelength region due to the intrinsic low carrier concentrations. Herein, we observe the edge plasmon modes of Bi₂Te₃, a narrow-band gap semiconductor, in the visible spectral range using photoemission electron microscopy (PEEM). The Bi₂Te₃ nanoplates excited by 400 nm femtosecond laser pulses exhibit strong photoemission intensities along the edges, which follow a cos⁴ dependence on the polarization state of incident beam. Because of the phase retardation effect, plasmonic response along different edges can be selectively exited. The thickness-dependent photoemission intensities exclude the spin–orbit induced surface states as the origin of these plasmonic modes. Instead, we propose that the interband transition-induced nonequilibrium carriers might play a key role. Our results not only experimentally demonstrate the possibility of visible plasmons in semiconducting materials but also open up a new avenue for exploring the optical properties of topological insulator materials using PEEM