Lithium niobate crystals with deep microhole structures are excellent photonic-crystal devices with modulation properties of wavelength selection. However, current fabrication methods, such as focused ion beam etching, chemical etching, or conventional laser drilling, remain a considerable challenge for obtaining microholes with high-aspect-ratios in lithium niobate crystals. This paper presents a strategy for the one-step fabrication of uniform deep microhole arrays with a 700∶1 aspect ratio within lithium niobate crystals using the ultrafast laser temporal Bessel shaping technique. This efficient and high-quality strategy for fabricating deep microhole arrays has excellent process stability. The prepared lithium niobate microhole array has remarkable selective beam transmittance, and we hope that this strategy can be used as a promising method for fabricating lithium niobate photonic-crystals.
In this study, the original femtosecond Gaussian beam was transformed into a zero-order Bessel beam using a series of beam shaping units and the energy distribution of the femtosecond Bessel beam was calculated via COMSOL simulations. The one-step fabrication of deep microholes was realized using the high peak power of the femtosecond laser and by adjusting the spatial energy distribution of the Bessel beam. By matching the pulse frequency and the speed of the moving stage, stable and uniform fabrication of large-area deep microhole arrays could be achieved by varying beam energy and the relative focal position. The resulting microhole morphology and aspect ratio were evaluated using scanning electron microscope, confocal laser scanning microscope, and optical microscope. Additionally, the beam transmission test was performed on the microhole arrays, verifying the structure’s excellent selective beam transmission ability.
The femtosecond Bessel beam obtained after beam shaping successfully realized the fabrication of microhole arrays with a 700∶1 aspect ratio. Varying the laser power can effectively adjust the morphology and aspect ratio of the fabricated microhole. With an increase in laser power, the diameter and depth of the microhole become larger but the aspect ratio gradually decreases. At the same time, an increase in laser power can lead to a side lobe etching effect on the sample surface, resulting in degradation of the device performance or even its damage. Variation in the relative focal position slightly changes the microhole diameter but considerably affects the depth of the microhole. Furthermore, maximum utilization of the Bessel beam energy can be achieved when the Bessel beam is focused at the center of the sample, and a complete through-hole of a 500 μm thick lithium niobate crystal is realized. This high-aspect-ratio microhole array demonstrates excellent selective transmission of light beams in the 450510 nm range.
In this study, a femtosecond Bessel beam is successfully used to rapidly produce a uniform array of microholes with an aspect ratio of 700∶1 inside a lithium niobate crystal. The effects of laser output power and relative focal position on the microhole’s morphology, depth, and aspect ratio are systematically studied and summarized. The laser power range for inhibiting the side lobe etching effect and the design principles of the microhole array are presented. The high-aspect-ratio lithium niobate photonic-crystal filter is fabricated based on the optimization of the processing parameters, and the wavelength-selective transmission of the structure for beams in the range of 450510 nm is demonstrated through the transmission spectrum measurements. The efficient and reliable processing of high-aspect-ratio microhole structures provides a new pathway that is worth exploring for the fabrication of lithium-niobate-based photonic-crystal devices.
