Regular laser-induced periodic surface structures (LIPSS) were efficiently fabricated on indium tin oxide (ITO) films by femtosecond laser direct writing with a cylindrical lens. It was found that randomly distributed nanoparticles and high spatial frequency LIPSSs (HSFL) formed on the surface after a small number of cumulative incident laser pulses per spot, and regular low spatial frequency LIPSSs (LSFL) appeared when more laser pulses accumulated. The mechanism of the transition was studied by real-time absorptance measurement and theoretical simulation. Results show that the interference between incident laser and surface plasmon polaritons (SPPs) excited by random surface scatterers facilitates the formation of prototype LSFLs, which in turn enhances light absorption and SPP excitation following laser pulses. The effects of scanning velocity and laser fluence on LSFL quality were discussed in detail. Moreover, large-area extremely regular LSFL with a diameter of 30 mm were efficiently fabricated on an ITO film by femtosecond laser direct writing with the cylindrical lens. The fabricated LSFLs on the ITO film demonstrate vivid structural color. During LSFL processing, the decrease of ITO film thickness leads to the increase of near-infrared optical transmittance.

Dynamic tunable metasurfaces are of great interest for their optical modulation properties. This study proposes a metasurface with a rectangular hole etched from a silicon square. By converting this rectangular hole into a trapezoid, we disrupt the symmetry, transforming the symmetry-protected bound states in the continuum (BICs) into a quasi-BIC state, achieving triple Fano resonances with a maximum Q factor of 1074. The results of the multipole analysis suggest resonance modes are toroidal dipole, electric quadrupole and magnetic dipole, respectively. A typical dielectric/dielectric/metal structure is then formed by adding an Au layer below the original structure. The polarized-light absorption of the metasurface is found to be unaffected by the angle of incident light. An analysis of the thickness of the Si is studied on the effect of absorption. Eventually, a single layer of graphene is incorporated at the bottom of the Si. The dynamic modulation of the three absorption peaks of the composite metasurface is achieved by controlling the bias voltage to alter the Fermi level E _f of graphene. The Si–graphene–Au structure has a sensitivity of 252.5 nm RIU⁻¹ and the maximum performance value of 126.25 RIU⁻¹ at E _f = 1 eV. These results indicate that this composite metasurface has potential applications in the research of sensor direction.

The enhancement and selective excitation of coherent anti-Stokes Raman scattering (CARS) and the suppression of background noise are very important problems for real-time detection at the single-molecule level. Optimizing the plasmonic substrate to ensure that all the hot spots of the pump, probe, Stokes, and anti-Stokes light are at the same position is the key to increasing the CARS signal to reach the level of single-molecule detection. The selective excitation of the target CARS peak and the suppression of the other peaks are the key to improving the signal-to-noise ratio. In this paper, we present a theoretical study to control the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure. By optimizing the modulation of the pump, Stokes, and probe pulse, one CARS peak is maximized, while the other two are suppressed to zero. Fano resonance is applied to simultaneously enhance the four surface plasmon modes of the pump, probe, Stokes, and anti-Stokes light and to ensure that all the hot spots are located at the same position by adjusting the size of the disk-ring structure. The hot spots of the four pulses are concentrated in the disk-ring gap with a deviation distance of less than 2 nm, and the intensity of the CARS is enhanced by 1.43 × 10¹² times, which is much higher than the requirement of single-molecule detection. The time, frequency, and phase distribution of the input and the response of the four pulses are studied in detail. It was found that the selective excitation and the spectra of CARS are both well preserved.