Results and Discussion The transmissivity curves obtained from COMSOL are consistent with experimental data, and both show a significant increase in the surface transmissivity of lithium niobate with increasing nanostructure height. Upon comparing the experimental data with the simulations, the results for the transmissivity of the lithium niobate primary wafer are found to be consistent (Fig. 11), and the height and RMS roughness of the conical subwavelength structure on the lithium niobate surface increase with an increase in the incident energy, etching time, and ion beam current. When the incident energy is 1000 eV, ion beam cuttent is 40 mA, incident angle is 70°, and etching time is 120 min, the prepared structure appears to have a tapered geometry with a longitudinal height of 143.5 nm (Fig. 7). The peak transmissivity of this lithium niobate crystal sample is 83.5% in the visible wavelength range, which is approximately 12.5 percentage points higher than that of unmodified lithium niobate (Fig. 9).
Subwavelength structures have been at the forefront of recent research as they can facilitate the miniaturization and reduce the cost of optical devices. Optical devices based on the special electromagnetic properties of subwavelength structures can performs functions that cannot be achieved using ordinary optical devices. Moreover, with high integration, they have remarkably broad application prospects in the field of integrated optics. Lithium niobate (LiNbO3) is an artificial ferroelectric crystal that exhibits remarkable photoelectricity, optical refraction, and nonlinear optical effects. It also has desirable material properties such as a high refractive index, good high-temperature resistance, good corrosion resistance, and a wide transparency window, making it a material of choice for integrated optical circuit applications. To realize the excellent functionality of lithium niobate optics and achieve efficient integration, preparing subwavelength structures on the surface of lithium niobate is an ideal way to showcase the potential of their optical properties. However, preparing subwavelength lithium niobate structures by using methods such as focused ion beam etching, laser etching, and wet etching is not simple, economical, or efficient. A micro- or nanoprocessing technique that does not require the use of templates offers a simple and economic method to prepare subwavelength structures on a large scale. The development of such a method can play an important role in research on the application of lithium niobate optical devices.
In this study, the subwavelength structure on a lithium niobate surface was designed and prepared using simulation and experiment. Finite element method-based simulations were performed using COMSOL software, while the subwavelength structure on the lithium niobate surface was prepared using a low-energy ion-beam etching method. The transmissivity, roughness, longitudinal height of the nanostructures, and surface morphology of the etched lithium niobate samples were analyzed using a Lambda950 spectrophotometer and atomic force microscope, respectively. By simulating and analyzing the optical properties of the fabricated lithium niobate surface, the influence of the subwavelength conical structure and structural parameters on the optical properties of lithium niobate was analyzed.
The transmissivity of nanostructures generated on the lithium niobate surface at an incident angle of 70°, incident energy of 1000 eV, ion beam current of 40 mA, and different etching time were simulated using COMSOL. The simulated transmissivity curves are consistent with the measured curves, and as the nanostructure height increases, the surface transmissivity of lithium niobate increases significantly. When the ion beam incident angle is 70°, the incident energy of the ion beam is greater than 600 eV, ion beam current is greater than 40 mA, and etching time is greater than 60 min, a large number of conical nanostructures can be formed on the surface of lithium niobate, and the height and RMS roughness of the conical nanostructure increase with the increase in incident energy, etching time, and ion beam current. The transmissivity measurements of lithium niobate crystals etched with different ion beam parameters were performed separately via spectrophotometry. The results show that the higher the nanostructure on the surface of lithium niobate, the more evident the effect of increasing transmittance in the visible band; when the incident energy is 1000 eV, ion beam current is 40 mA, incident angle is 70°, etching time is 120 min, longitudinal height of 143.5 nm appears on the surface of lithium niobate. At this time, the peak transmissivity of the lithium niobate crystal sample is 83.5% in the visible wavelength range, which is approximately 12.5 percentage points higher than that of the original lithium niobate.
