Periodic arrangements of subwavelength pores form the foundation of artificial all-dielectric nanophotonic structures, enabling exceptional control and manipulation of light. High-performance nanophotonic devices, such as photonic crystals and metasurfaces, achieve revolutionary light manipulation via precisely engineered subwavelength architectures, demonstrating vast potential in optical communications, sensing, and quantum technologies. The core performance of these devices critically relies on constituent units possessing extreme depth-to-diameter ratios.
However, realizing such structures presents significant challenges. Rapid fabrication of high-aspect-ratio, high-density periodic nanostructures remains a fundamental challenge for glass or crystal-based nanophotonics. Conventional methods often face nonlinear absorption saturation and plasma defocusing, which drastically limit the achievable depth of the nanochannels.
In a new paper provisionally accepted in Light: Advanced Manufacturing , a team of scientists, led by Professor Jianrong Qiu from Zhejiang University and Professor Lijing Zhong from Ningbo University, have introduced a novel approach called wet-etching-assisted single-pulse nanolithography (WESPN). They successfully fabricated nanohole-clad waveguides with ultra-high depth-to-diameter ratios using femtosecond laser writing combined with spherical-aberration-enhanced focal stretching and selective wet etching.
The researchers employed a spherical-aberration-enhanced focusing strategy by directing a high-numerical-aperture objective to focus femtosecond laser pulses deep inside a single-crystal sapphire substrate. By introducing a deliberate refractive-index mismatch, the focal volume is significantly elongated along the optical axis, avoiding the formation of a conventional diffraction-limited focal spot. When combined with dynamic axial focal stitching, this technique overcomes depth limitations to form continuous, extreme-aspect-ratio nanoholes.
This technique achieves record depth-to-diameter ratios (>50,000:1) with nanoholes featuring sub-500-nm diameters and lengths up to 1,500 μm. The densely packed nanohole lattice acts as a low-index cladding, while light is guided through an unmodified crystalline core, demonstrating high mode purity (10.9 dB). Furthermore, by embedding fluorescent probes within the nanoholes, the waveguide effectively guides light to excite the probes, showcasing remarkable optical sensing capabilities.
The scientists summarize the impact of their work: "We demonstrate a fabrication strategy that overcomes the long-standing depth limitation of single-pulse nanolithography by combining spherical-aberration-mediated focal stretching with dynamic axial stitching." They added, "This unified approach offers a scalable route to functional photonic integration in hard crystalline substrates, opening new opportunities for quantum emitters, dense photonic circuits, and high-sensitivity biochemical sensing."
Light: Advanced Manufacturing
Deep-Nanohole-Clad Waveguides with Depth-to-Diameter Ratio up to 50,000 in Single Crystals via Femtosecond Laser Writing