Comparative Study of Laser Fabrication of Plano-Concave And Plano-Convex Microlenses on Fused Silica And Sapphire

Researchers from the University of Virginia published the article "Laser Fabrication and Comparative Study of Planoconcave and Planoconvex Microlenses on Fused Silica and Sapphire" in the international journal Micromachines.

01 Paper Introduction
With the miniaturization of imaging systems, the demand for efficient micro-optical components continues to grow. Microlens arrays (MLAs), composed of many microlenses, function similarly to the compound eyes of insects. Recently, high-power lasers have been used to fabricate fused silica microlens arrays. Lasers can provide high temperatures with short pulse durations in localized regions to achieve surface patterning. Since the laser beam energy is highly localized, the extremely low thermal expansion coefficient of fused silica prevents cracks or fractures caused by thermal gradients.

02 Full-Text Overview
The paper reports the fabrication of plano-concave lenses using a picosecond 355 nm wavelength laser and a CO2 laser, as well as the fabrication of plano-convex microlens arrays on fused silica by combining picosecond laser patterning with CO2 laser processing. The paper reports on surface morphology, profile, roughness, optical transmission efficiency, and laser beam profiles of the microlenses, and also demonstrates the fabrication of plano-concave lenses on sapphire, an infrared transmission material.

03 Figures and Analysis
MLAs were designed by modeling in non-sequential mode using Zemax software, which provides data on reflected and transmitted rays, as shown in Figure 1.

Figure 1. (A) Zemax Opticstudio simulation setup. (B) Simulated Gaussian beam profile. (C) Measured profile of a HeNe laser beam. Irradiance is in W/cm².

Concave MLAs were fabricated on fused silica using a CO2 laser. The lens with a diameter of 300 μm shown in Figure 2A was fabricated using a laser power density of 43.6 kW/cm² and an exposure time of 0.5 s. Due to the periodicity and small diameter of the microlenses, optical diffraction rings were observed in the transmitted light images. To eliminate diffraction effects, microlenses were randomly spaced, as shown in Figure 2B. CO2 laser-induced melting and resolidification reshaped the surface into a concave form. The depth and curvature radius of the formed concave lenses depended on the laser fluence and irradiation time. Increased time or fluence produced higher curvature.

Figure 2. Concave lens fabricated using a CO² laser. (A) Zero overlap. (B) Random overlap.

The optical transmission efficiency of the concave MLA fabricated by CO2 laser reached 94.5% with an anti-reflection coating. Beam profile simulations at 5 mm and 10 mm from the lens using Zemax software are shown in Figures 3A and 3B, while experimental results are shown in Figures 3C and 3D. When a Gaussian beam passes through the MLA, the beam expands and homogenizes. The measured divergence angles along the X-axis and Y-axis were 6.13° and 8.04°, respectively. The simulated divergence angles along the X-axis and Y-axis were 2.56° and 4.58°, respectively. The simulation did not consider lens overlap.

Figure 3. Simulated beam profile of a Gaussian beam passing through a concave MLA fabricated using a CO² laser at different distances from the lens: (A) 5 mm and (B) 10 mm. Experimentally measured beam profiles at (C) 5 mm and (D) 10 mm.

Concave MLAs were also fabricated using a picosecond laser. Lenses with a diameter of 25 μm were fabricated at a laser energy density of 4.66 J/cm², as shown in Figure 4A. The lenses were randomly spaced to eliminate diffraction effects. SEM images of these lenses are shown in Figure 4B.

Figure 4. SEM images of a concave MLA fabricated on fused silica using a picosecond laser. (A) Single concave lens. (B) Concave lenses with random spacing.

The optical transmission efficiency of the concave MLA fabricated by picosecond laser reached 92.1% with an anti-reflection coating. Simulated beam profiles at 5 mm and 10 mm from the lenses are shown in Figures 5A and 5B, while experimental results are shown in Figures 5C and 5D. The measured divergence angles along the X-axis and Y-axis were 11.99° and 15.09°, respectively. The simulated divergence angles along the X-axis and Y-axis were 8.56° and 11.4°, respectively.

Figure 5. Simulated beam profile of a Gaussian beam passing through a concave MLA fabricated using a picosecond laser at different distances from the lens: (A) 5 mm and (B) 10 mm. Experimentally measured beam profiles after passing through a picosecond laser-fabricated concave MLA at different distances from the lens: (C) 5 mm and (D) 10 mm.

Cylindrical lenses were fabricated by forming vertical structures through periodic picosecond laser scribing with 50 μm spacing, as shown in Figure 6A. The vertical structures formed by the picosecond laser were then remelted by a CO2 laser with a power density of 25.2 kW/cm² to form cylindrical lenses, as shown in Figure 6B.

Figure 6. SEM images of (A) picosecond laser micromachined patterns and (B) cylindrical lenses after CO2 laser melting.

The transmission efficiency of the cylindrical MLA was 86.7%. Simulated beam profiles at 5 mm and 10 mm from the lenses are shown in Figures 7A and 7B, while experimental results are shown in Figures 7C and 7D. The measured divergence angles along the X-axis and Y-axis were 5.55° and 11.33°, respectively. The simulated divergence angles along the X-axis and Y-axis were 2.86° and 11.4°, respectively.

Figure 7. Simulated beam profiles of a Gaussian beam after passing through a cylindrical MLA at different distances from the lens: (A) 5 mm and (B) 10 mm. Experimentally measured beam profiles after passing through a cylindrical MLA at different distances from the lens: (C) 5 mm and (D) 10 mm.

04 Conclusion
The paper reports the fabrication of concave lenses on fused silica using picosecond and CO2 lasers, as well as cylindrical and convex lenses on fused silica by combining picosecond micromachining with CO2 laser-induced melting, and concave lenses on sapphire using picosecond lasers. The fabricated fused silica microlens arrays achieved a high transmittance of over 94%. Zemax software was used to simulate transmitted beam profiles at different distances from the microlens arrays, and the simulation results matched well with experimental measurements.

**--Cite the article published by 高能束加工技术 on August 18, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."