The manipulation of light propagation is a key area in optics and optoelectronics. Photonic crystals, periodic arrays of dielectric structures, are crucial components in integrated photonics, enabling control over light transmission and regulation. Their unique optical properties allow for the construction of various photonic devices, including light emitting devices, optical receivers, switches, modulators, and resonators. While 1D and 2D photonic crystals are relatively easier to fabricate, creating 3D structures presents significant challenges due to the need for refractive index variation in three spatial directions. Laser lithography, particularly ultrafast laser processing, offers a potential solution for 3D fabrication. Ultrafast lasers, with their high intensity and ultrashort pulse duration, can overcome crystal bandgaps and enable direct micromachining without significant thermal effects. Femtosecond lasers have been employed to create photonic crystals, especially 3D structures in transparent materials. However, achieving nanoscale structures is limited by the optical diffraction limit. Recent advancements have demonstrated nanostructure fabrication at the tens of nanometer scale using laser direct writing lithography, but these methods are often limited to planar or non-transparent materials. To overcome these limitations, parallel laser processing techniques, such as microlens array lithography (MLA) and laser interference lithography (LIL), have been developed. While MLA enables rapid fabrication of large-scale patterns, it requires customized micro-lens arrays for different structures. LIL, though advantageous for pattern switching, is generally limited to planar structures. This work introduces a multi-beam laser lithography strategy utilizing beam shaping to achieve customizable 3D nanostructure fabrication in crystals, overcoming the limitations of previous techniques.

The literature review extensively covers existing methods for fabricating photonic crystals, highlighting the challenges associated with creating 3D structures and nanoscale features. It discusses the use of ultrafast lasers in micromachining and their advantages in creating 3D photonic crystals in transparent materials. The review also examines the limitations of single-beam laser direct writing, emphasizing the challenges of achieving nanoscale precision and the need for multiple scans. Existing parallel processing techniques like MLA and LIL are analyzed, pointing out their individual strengths and limitations in terms of flexibility, scalability, and the ability to create 3D structures. This thorough review sets the stage for the proposed multi-beam approach.

The proposed multi-beam laser lithography method employs spatial light shaping using a spatial light modulator (SLM) to generate a customizable multi-beam light field. A Dammann grating phase is used to generate a simple multi-beam field, while a Fresnel phase is used to further control the spatial distribution of the foci. Superimposing these phases creates a complex multi-beam light field. The multi-beam laser direct writing is performed by scanning the shaped light field across the YAG crystal. The laser energy is controlled to induce structural changes without material removal or ripple formation. After laser inscription, the modified regions are etched using acid to create hollow channel structures. The gap width between parallel channels is controlled by adjusting the SLM phase holograms. Debye diffraction integration is used to theoretically analyze the multi-beam light field and predict the intensity distribution in a tightly focused condition. The experimental setup utilizes a femtosecond laser system with an oil-immersed objective lens (NA=1.42) for tight focusing. The fabricated structures are characterized using optical microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).

The study successfully demonstrates the fabrication of parallel channels with controllable nanoscale gaps in YAG crystals using the multi-beam laser lithography technique. The gap width can be precisely controlled from hundreds of nanometers to several micrometers by adjusting the SLM phase holograms. The number of stripes in the binary phase mask hologram directly affects the gap width, showing a positive correlation when the gap size is in the nanoscale and a linear relationship when the size is in the micrometer scale. Pulse energy also influences the gap and channel width; the gap width shows a negative correlation with pulse energy, while the channel width shows a positive correlation. The method efficiently produces multiple channels simultaneously, significantly improving the fabrication speed compared to single-beam methods. The fabrication of complex channel arrays with arbitrary size and gap width is achieved through the use of superimposed phase holograms. Raman and XPS analysis confirms the structural changes induced by the laser processing, revealing lattice defects and Al ion transformations. These findings support the effectiveness and precision of the multi-beam approach.

The results demonstrate the successful implementation of a novel multi-beam laser lithography technique for the fabrication of high-precision nanoscale photonic crystals. The ability to control the gap width at the nanoscale, combined with the parallel processing capability, significantly improves the efficiency and flexibility of photonic crystal fabrication. The method's ability to produce complex channel arrays with arbitrary geometries opens up possibilities for creating advanced photonic devices. The structural analysis using Raman and XPS confirms the uniformity and stability of the multi-beam process, supporting its potential for large-scale fabrication. The findings have important implications for the development of integrated photonics, offering a powerful tool for creating sophisticated optical devices with improved performance and miniaturization.

This study presents a novel multi-beam laser lithography method for efficient and precise fabrication of nanoscale photonic crystals. The method allows for precise control over the gap width between parallel channels, enabling the creation of complex structures. The use of superimposed SLM phase holograms enhances the flexibility and efficiency of the process. Future research could explore the application of this technique to other materials and the integration of these nanostructures into functional photonic devices.

The current study focuses on the fabrication of photonic crystals in YAG crystals. The applicability of this method to other materials needs further investigation. The precise control of laser parameters is crucial for achieving consistent results. Further optimization of the laser parameters and etching process could potentially lead to even smaller feature sizes and improved control over the nanostructure geometry.