Second Harmonic Generation-Enhanced Photonic Nanojet Machining in Water Medium with Position Control for Submicrometer Ablation

Abstract To achieve higher machining resolutions, photonic nanojet (PNJ) machining has emerged as an alternative technique, reducing the size of beams during laser ablation. Herein, we introduce a water medium for PNJ machining. While PNJ propagates over longer distances in water, the microsphere does not need to be positioned near the workpiece, reducing the risk of sample damage and alignment errors. This research aims to evaluate the impact of second harmonic generation (SHG) on enhancing PNJ machining precision and efficiency in a water medium. Specifically, it examines how SHG affects PNJ beam diameter, ablation depth, and machining quality to increase precision and resolution. We conducted an investigation using three-dimensional finite-difference time-domain method and performed PNJ machining experiments. The propagation profile in the SHG-generated PNJ yielded a longer depth of focus. Moreover, ablation was observed up to 8.0 µm from the rear microsphere surface with SHG, making it more effective for precision microfabrication over longer distances. The smallest hole diameter in the x - and y -directions was 0.55 µm using the SHG beam. The advantage of SHG-based PNJ machining for ultraprecise applications was demonstrated by the consistent achievement of submicrometer hole diameters and deeper holes when using SHG. This confirms that SHG can balance between high precision and effective material removal. These collectively demonstrate that SHG-enhanced PNJ machining in a water medium not only allows for deeper and more precise ablation but also minimizes thermal effects and machining debris, making it a promising method for future high-resolution laser microfabrication applications.