A tunable multiband chirped metasurface

AbstractWe numerically present a multiband double negative chirped metasurface (MS) in the near-infrared (N-IR) region. The MS was composed of a round nanoholes array (RNA) penetrating through metal/dielectric material/metal (AuAl2O3Au) trilayers. The chirp was excited by varying the positions of the RNA along the direction of incident electric (E) field vector inside the meta-atom. It is found that besides a multiband double negative refractive index (NRI), a spectral tuning of NRI is also unveiled by moving the neighbouring round holes closer to each other. Importantly, we also show that the chirped MS with large round hole resonators possesses a high value of the Figure-of-Merit (FOM) in the optical region

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 (Rsphere = 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

suplementary video 1 - trapping and anti-trapping several times small.mov

The phase transition of VO2 from monoclinic (we also refer to it as “Cold VO2”) to rutile (“Hot VO2”) is controlled optically by changing the intensity of the trapping beam. Those phases has radically different refractive index values, which affects strongly the optical trapping. We observe that at lower laser powers, the particle can be trapped by an attractive optical force, and once the laser power exceeds its critical value, the nanoparticle escapes as the optical force switches from attractive to repulsive. The switching between attractive and repulsive forces is reversible and same particle can be attracted/repelled repeatedly. The supplementary video 1 shows that the particle was repelled away from the laser focus much faster than the Brownian motion, which suggests that the optical force in this situation is repulsive.</p

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 (R = 80 nm) that is placed 5 nm above an asymmetric bow-tie nanoantenna array based on Au/GeSbTe dual layers. The DQ-FR-induced lateral force achieves a broad tuning range in the mid-infrared region by changing the states of the GeSbTe 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 GeSbTe film's temperature, which demonstrates the possibility of transiting the GeSbTe 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/μm, 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

Switchable optical trapping based on Mie-resonant phase-change nanoparticles

Optical tweezers have revolutionized the manipulation of nanoscale objects, offering versatile control over trapped particles. Typically, the manipulation and trapping capabilities of optical tweezers rely on adjusting either the trapping laser beams or the optical environment surrounding the trapped nanoparticles. In our study, we present a novel approach to achieve tunable and switchable trapping using optical tweezers. We utilize nanoparticles made of a phase-change material (vanadium dioxide or VO$_2$), trapped by a focused Gaussian beam. By varying the intensity of the trapping beam, we can optically control the phase and optomechanical properties of the vanadium dioxide nanoparticles. At lower intensities, the nanoparticle remains in its monoclinic phase, and it is trapped by attractive optical forces. However, at higher intensities, the optical beam induces a phase transition in the nanoparticle to the rutile phase, which dramatically alters the optical forces, transforming them from attractive to repulsive, thereby pushing the nanoparticle away from the beam. Importantly, this effect is reversible, allowing the same particle to be attracted and repelled repeatedly. The observed phenomenon is governed by Mie resonances supported by the nanoparticle and their alterations during the phase transition of the VO$_2$ material. Our findings introduce a versatile new addition to the optical tweezers toolbox for optomechanical manipulation of nanoparticles

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 cos(4) 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

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