INFORMATION TECHNOLOGY

On-demand manufacturing of optical shading structures


Introduction

Two-photon lithography (TPL) is an advanced manufacturing technology that enables the fabrication of complex three-dimensional (3D) structures with printing resolutions of hundreds of nanometers, widely used in the manufacture of advanced optical devices, such as apochromatic X-ray lenses, fiber optic end-face imaging devices, etc. The photosensitive resin commonly used printed structure is transparent and suitable as an optical component. However, mixing opaque and transparent structures simultaneously in a single device is challenging, often requiring multiple material systems or manually adding ink after manufacturing, with drawbacks such as complex processes and cumbersome handling.

Recently, a research team from the Agency for Science, Technology and Research of Singapore and the Singapore University of Technology and Design published a report entitled “Fabrication of opaque and transparent 3D structures using a single material via two-photon polymerisation lithography” at Light: Advanced Manufacturing”.

In this paper, the researchers propose an on-demand printing strategy to achieve transparent and opaque 3D structures based on a single photosensitive resin. Controlled printing of opaque structures with a minimum feature size of about 10 μm is realized, and the structure exhibits less than 15% transmittance at a thickness of about 30 μm. Further, this process is applied to print an opaque aperture integrated with a transparent lens, resulting in an improved imaging contrast.

How it works

The fabrication of opaque structures based on TPL technology is a continuous process that defines each region by controlling the amount of laser exposure, as shown in the 2×2 checkerboard pattern illustration in Figure 1. First, print transparent areas using regular exposure conditions. Second, a filled structure (vertical plane in the x and y directions) is fabricated in an opaque area to confine the liquid resin to a smaller profile. Next, the smaller resin pool is overexposed at high doses layer by layer, exhibiting opacity. The filled structure minimizes the movement of microscopic bubbles and prevents them from coalescing into larger bubbles that interfere with the printing process. At the same time, the filled structure also acts as a support, preventing it from collapsing during temperature rises. As a result, transparent and opaque structures can be selectively manufactured in a single resin material. The structures in this study were fabricated using Nanoscribe GmbH’s commercial TPL system (Photonic Professional GT), IP-S photosensitive resin, and a 25x objective.

Figure 1: Schematic diagram of selective manufacturing of transparent and opaque structures

Source: Light: Advanced Manufacturing

Process development

In general, when the exposure power is higher than the threshold power of the photosensitive resin, lower scanning speed and longer exposure time will lead to bubble formation and material decomposition. The uniformity of material decomposition and the opacity of the laser irradiation area depend on the exposure dose, which is directly proportional to the laser power and inversely proportional to the scanning speed. To determine the optimal laser dose for fabricating opaque structures, a point array exposure was performed based on the pulsed exposure mode, and Figure 2 shows the laser exposure parameter sweep results. At lower exposure doses, the dot structure is clearly contoured without any distortion, and the size of the dot varies according to the laser dose. When the exposure dose is increased, damaged structures gradually appear. Also, the exposure time required to cause damage gradually decreases with increasing power.

Figure 2: Laser exposure parameter sweep results for a point array structure

Source: Light: Advanced Manufacturing

Selective manufacturing of opaque structures

When the opaque area is subdivided into smaller resin pools with filled brackets, the damage is limited to a small volume, and the bubbles formed are small with minimal impact on the overall structure. The researchers speculate that this is due to the large number of liquid-solid interfaces that reduce heat dissipation and contribute to local heat accumulation. Figure 3a shows the checkerboard pattern fabrication results without/with filled brackets, demonstrating the effectiveness of this overexposure method based on filled brackets. Figure 3b shows the visible band transmission spectrum of opaque structures and transparent structures of different thicknesses. In addition, multiple samples with the same thickness were tested for transmittance to ensure repeatability. And when the thickness exceeds 30 μm, the transmittance is always less than 15%, as shown in Figure 3c. Figure 4d(i) shows an abstract black-and-white pattern with a thickness of 50 μm, demonstrating that the developed method has the manufacturing capability to realize geometries such as curved shapes and sharp corners. Moreover, the average roughness of the interface between the dark and light areas is 4.3 μm. Further, the coffee cup shown in Figure 3d(ii) demonstrates the ability of the process to fabricate true 3D opaque structures. The first row shows a schematic of the model of an empty, half-full, and full cup, and a transmission microscope image of the fabricated structure corresponding to the design is shown in the second row.

Figure 3: Selective manufacturing of opaque structures

Source: Light: Advanced Manufacturing

Integrated Fresnel lens with opaque aperture

An opaque structure in an optical system that blocks light in a specific area and reduces the effects of stray light. Figure 4 shows the results of a Fresnel lens with an opaque aperture used to block any stray light during imaging. Figure 4b shows the imaging results of Fresnel lenses with and without aperture on the USAF 1951 negative resolution target, respectively. Figure 4c shows the line-pair intensity quantification of element 5 in group 5 in the imaging plot, with an increase in image contrast from 9% (no aperture) to 24% (with aperture).

Figure 4: Imaging results of a Fresnel lens with an integrated opaque aperture

Source: Light: Advanced Manufacturing

summary

In summary, the laser decomposition conditions of TPL were studied, and a process was developed to create opaque region structures in transparent optical elements. Opaque structures are printed using high laser power to induce laser decomposition of liquid resin within microscopic compartments, enabling local heat accumulation. The filled structure within the contour helps to reduce the coalescence of microbubbles formed during overexposure. In the future, a focus will be placed on multiple 3D patterns and pattern densities to determine the finest feature sizes that can present opaque structures, further reducing the transmittance of opaque structures. At the same time, explore the effectiveness of this technology in fabricating black complex 3D structures. Further, high-power laser, resin chemistry, and exposure techniques are used to improve the resolution and printing speed of opaque structures. (Source: Advanced Manufacturing WeChat public account)

Related paper information:https://doi.org/10.37188/lam.2023.025

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