The advancement of nonvolatile reconfigurable photonic elements has led to the development of Sb2S3 as a desirable phase change material for optical phase modulation in visible and near-infrared wavebands. However, achieving long-lasting cycling durability of the phase change remains a formidable challenge. Traditional characterization methods like transmittance or reflectance measurement fail to provide a quantitative assessment of the degree of phase change. To address this, the study focuses on enhancing the cycling durability of complete phase change in Sb2S3 films by manipulating the wavelength of a femtosecond laser for amorphization. To accurately measure optical transmission phase shifts during phase change experiments, a novel liquid-crystal retardance matching technique based on the Mach-Zehnder interferometer is developed. This approach allows for a direct and quantitative assessment of the degree of phase change without relying on an optical model. The findings reveal that longer wavelengths and multi-pulse irradiation with increasing pulse energy significantly enhance the cycling durability of the phase change. Additionally, the cost-effective customization of nonvolatile photonic elements is demonstrated using laser direct writing, eliminating the need for lithography. It is anticipated that this work will drive advancements in Sb2S3-based reconfigurable devices and contribute to the broader field of nonvolatile reconfigurable photonic elements.

Low-cost large-area chirality meta-devices (CMDs) with adjustable optical chirality are of great interest for polarization-sensitive imaging, stereoscopic display, enantioselectivity analysis, and catalysis. Currently, CMDs with adjusted chiroptical responses in the mid-infrared to terahertz band have been demonstrated by exploiting photocarriers of silicon, pressure, and phase-change of GSTs but are still absent in the visible band, which in turn limits the development of chiral nanophotonic devices. Herein, by employing a phase-change material (Sb2S3), we present a protocol for the fabrication of wafer-scale visible-band enantiomeric CMDs with handedness, spectral, and polarization adjustability. As measured by circular dichroism, the chirality signs of CMDs enantiomers can be adjusted with Sb2S3 from amorphous to crystalline, and the chirality resonance wavelength can also be adjusted. Our results suggest a new type of meta-devices with adjustable chiroptical responses that may potentially enable a wide range of chirality nanophotonic applications including highly sensitive sensing and surface-enhanced nanospectroscopy.

Optical characteristics of square-latticed metasurfaces are analyzed by equations derived from a dipole-quadrupole system. For the metasurface composed of Si nanospheres, meta-atoms maintain the anapole characteristics of the single scatterer, which are not affected by the lattice period. The far-field response of the metasurface thus can be engineered without significantly disturbing near-fields in meta-atoms. In addition, as exhibited with Ag/Si core-shell nanospheres, the transmittance of metasurface at anapole can also be modulated and even nulled by shifting the lattice magnetic dipole and quadrupole resonances. In a small-period metasurface, the near-field interaction of adjacent meta-atoms manipulates the lattice resonance behavior, leading to significant modulation in transmittance at anapole.

Optical chirality enhancement is highly demanded for enantioselective interaction of circularly polarized light with chiral molecules. The chirality enhancement in the coaxial air hole of a hollow silicon disk depends on three aspects, namely, the enhancements of electric and magnetic fields and a factor determined by the phases of their field components. In the spectral regime of dipole resonances, maximum chirality enhancement with sign consistency and uniform spatial distribution in the air hole can be obtained in association with both magnetic dipole resonance and anapole. Due to dipolar interference, the chirality is nulled at their coincidence, around which the sign of chirality is reversed. Maximum chirality with both positive and negative signs can be found between magnetic dipole resonance and anapole in the vicinity of their coincidence. This situation is maintained under size scaling so that the operation wavelength can be broadly tuned. The optical chirality can be further improved by merely adjusting the hole radius, by which the optimal spatially averaged optical chirality enhancement factor can reach 39 and −23. The simple strategy for optimizing Mie resonators presented in this work may benefit the design of Mie resonator‐based achiral metasurfaces for chirality detection application.

We demonstrate a nanometric displacement sensor with a switchable measuring range by using a single silicon nanoantenna. It is revealed that the interference between the longitudinal and transverse dipolar scattering can be well tuned by moving the nanoantenna in the focal field of the cylindrical vector beam. As a result, a position related scattering directivity is found and is used as a displacement sensor with a 4.5 nm lateral resolution. Interestingly, the measuring range of this displacement sensor can be extended by twice through simply changing the excitation from the azimuthally polarized beam to the radially polarized beam. Our results provide a facile way to tune the measuring range of the nanometric displacement sensor and may open up an avenue to super-resolution microscopy and optical nanometrology.

Sb2S3 has great potential to become a good phase-change material for visible and near-infrared wavebands due to its low loss and high refractive index contrast, so great interest lies in the technology development. This work investigates the intermediate phase-change states and their cycling durabilities by employing a continuous-wave laser for crystallization and a femtosecond laser for amorphization. By considering stratified partial amorphization due to non-uniform intensity distribution along propagation in a film, a double-layer model is proposed to describe intermediate states, which fits experimental data better than the mixture models in effective medium approximation. The phase-change degree is then defined as the ratio of the amorphization depth to the total film thickness, which can be controlled by combination of the pulse energy and the number of pulses in multi-pulse femtosecond laser irradiation for amorphization. The cycling durability is improved by reducing pulse energy and increasing the number of pulses. The experimentally achieved maximum cycling durabilities are 30, 1000, and 7000 cycles for 90%, 60%, and 20% phase-change degrees. The implementation of intermediate states with improved cycling durability may promote the development of Sb2S3-based reconfigurable photonic devices.

Indium tin oxide (ITO) has a wide range of applications at its epsilon-near-zero (ENZ) wavelength due to its unique optical properties. Post-annealing is a simple way to tune the ENZ wavelength. We show that the ENZ wavelength of ITO films could be red-shifted over a wide range from 1200 nm to 1550 nm by thermal annealing in air for durations up to 130 min at 330 degrees. Optical transmittance and reflectance spectra were measured for these ITO samples, along with electron densities, to extract the Drude model parameters of plasma frequency, damping factor, electron mobility and effective mass. The results show that the changes in electron density and effective mass collectively cause the red-shift in the plasma frequency and ENZ wavelength. The oxygen uptake and crystallite size increase during the annealing in the air are the main reasons for the change in electronic properties. This versatile method of tuning the ITO's ENZ wavelength can expand the wavelength range for applications and adapt it to working at wavelengths of plasmonic devices in the telecommunication wavebands.

We present the plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles (Ag NPs) fabricated using a femtosecond pulse. Theoretical analysis indicates Ag NPs with a diameter of ∼100 nm have excellent linear response within the visible band, and the electric field intensity enhancement factor reaches ∼105 under excitation of continuous light of 632.8 nm. Meanwhile, the simulation result of second-order surface nonlinear optical response shows that the second harmonic conversion efficiency of the Ag NPs dimer is two orders of magnitude higher than that of a single Ag NP, under excitation of a femtosecond pulse. In experiment, the linear response of Ag NPs is examined using surface-enhanced Raman spectroscopy (SERS) with a Raman enhancement factor of ∼1.7 × 1010, revealing the excellent linear optical response of Ag NPs. Moreover, the spectra of the second harmonic can be measured clearly under conditions of an average pump power of 40 μW, revealing the excellent second-order surface nonlinear optical response of Ag NPs.

Plasmonic nanocavities enable extreme light-matter interaction by pushing light down to the nanoscale. The dipolar feature of bright modes allows coupling with the external excitation from free space but results in a radiating background, whereas nonradiating dark plasmon modes can hardly be excited. Here, we report for the first time on strong coupling between dark plasmon and anapole modes in a hybrid metal-dielectric nanostructure. With the aid of vanishing dipole characteristics of the anapole and dark plasmons, the hybrid modes exhibit minimum far-field scattering and maximum near-field enhancement. The dark mode coupling in the metal-dielectric nanostructure offers a nonradiating air cavity with greatly improved field enhancement in a broadened band, thus providing a background-free experimental platform for spectroscopic applications. The proposed approach to dark plasmon excitation, i.e., via anapole, may boost practical exploitation of dark plasmons by allowing linearly polarized light illumination and scalable arrays of individual nanostructure units.

Au-nanoparticle (Au-NP) substrates for surface-enhanced Raman spectroscopy (SERS) were fabricated by grid-like scanning a Au-film using a femtosecond pulse. The Au-NPs were directly deposited on the Au-film surface due to the scanning process. The experimentally obtained Au-NPs presented local surface plasmon resonance effect in the visible spectral range, as verified by finite difference time domain simulations and measured reflection spectrum. The SERS experiment using the Au-NP substrates exhibited high activity and excellent substrate reproducibility and stability, and a clearly present Raman spectra of target analytes, e.g. Rhodamine-6G, Rhodamine-B and Malachite green, with concentrations down to 10−9 M. This work presents an effective approach to producing Au-NP SERS substrates with advantages in activity, reproducibility and stability, which could be used in a wide variety of practical applications for trace amount detection.

Au-nanoparticles (Au-NPs) substrates for surface-enhanced Raman spectroscopy (SERS) are fabricated by scanning an Au-film using femtosecond pulse. The Au-NPs SERS substrates exhibit high activity and excellent reproducibility.