High-resolution light field prints by nanoscale 3D printing

Conventional prints only store intensity and color information, lacking the directional light ray control needed for 3D image display. Light field prints (LFPs), however, encode directional information, allowing for perspective changes from varying viewing angles. This concept, dating back to Gabriel Lippmann's 1908 proposal, uses an array of tiny lenses to record sub-images with slightly shifted perspectives. LFPs offer an autostereoscopic advantage over stereoscopic prints, requiring no special glasses or laser illumination. However, fabrication limitations result in pixelated LFPs due to low spatial and angular resolution. This study addresses this limitation by employing advanced nanofabrication techniques from structural color printing, aiming to produce high-resolution LFPs with significantly improved image quality and potential applications in security and print media.

High-resolution structural color prints, particularly those using plasmonic nanostructures, can achieve pixel resolutions far exceeding conventional inkjet prints. While inkjet printing can create LFPs, the resolution is inadequate for applications like optical security devices requiring spatial resolution below 50 µm. Misregistration between color layers is also a concern in inkjet printing, unlike nanofabrication methods like electron beam lithography. Previous work demonstrated high-resolution multi-color motion effects by bonding a microlens array onto a plasmonic color print; however, this method involves multiple steps and challenging nanoscale manual alignment. This research proposes a single-step approach using two-photon polymerization lithography (TPL) to overcome these limitations.

This research utilizes two-photon polymerization lithography (TPL) to fabricate high-resolution LFPs in a single patterning step, eliminating the need for manual alignment. The TPL system (Nanoscribe GmbH Photonic Professional GT) precisely positions each volumetric pixel with 10 nm accuracy. Microlenses and structural color pixels are fabricated in discrete slicing height steps of 20 nm and 300 nm, respectively, both using IP-Dip photoresist (n ~1.55). Unlike plasmonic color pixels, this approach doesn't require metal deposition, simplifying the fabrication process. The microlenses and color pixels are arranged pseudorandomly to minimize moiré patterns and potentially encode information for security applications. Raytracing simulations determined the microlens focal length and acceptable viewing angle range. Electromagnetic wave simulations determined the microlens focal spot size and color change with viewing angle. The simulations guided the design of spherical plano-convex microlenses (diameter L = 21 µm, radius of curvature R = 22 µm, focal length F = 37 µm, NA ~0.28), designed to accommodate the limited viewing angle range (θ = 0–16°) of the structural color pixels. The initial pixel design comprised 3 × 3 pixels per display unit, each containing 5 × 5 nanopillars (pitch P = 5 µm). Later designs used 5 × 5 pixels (P = 3 µm, 3 × 3 nanopillars per pixel) and 15 × 15 pixels (P = 1 µm, single nanopillar per pixel). An algorithm was used to create an interlaced digital image mapping pixel positions to display units, ensuring proper depth and motion parallax. Fabrication involved TPL of IP-Dip photoresist on a glass substrate, varying nanopillar height (0.6–2.7 µm) and laser exposure time (0.04–0.32 ms) to create a wide range of colors. The laser scanned the surface shell, with uncured photoresist solidified by UV curing. The display units were fabricated in a pseudorandom arrangement with center-to-center separation between 29 and 45 µm. Optical and scanning electron microscopy (SEM) were used to characterize the fabricated structures. A white light-emitting diode lamp illuminated the LFP, and digital camera images were captured from various viewpoints to assess the 3D image quality and motion parallax.

The fabricated LFPs demonstrated high spatial resolution (29–45 µm) and high angular resolution (~1.6°). Images appeared unpixellated to the naked eye, even at close range. The color of the displayed 3D object remained consistent across the acceptable viewing angle range (0–16°), aligning with simulation results. Reducing pixel pitch from 5 × 5 nanopillars to a single nanopillar (~300 nm diameter) maintained clear, colorful images with smooth motion parallax, achieving a maximum pixel resolution of 25,400 dots per inch, the highest reported for LFPs. The angular sampling interval (ωα ~1.6°) and angular difference in perspective between input images (ω ~2.1°) were smaller than the threshold angle (δ = 2.3°) for smooth motion parallax, minimizing accommodation-vergence conflict. The maximum image depth was calculated to be 1.3 mm, resulting in a total depth range of 2.6 mm. The LFP functioned effectively in transmission mode, although reflection mode resulted in reduced brightness and contrast. The maximum amount of information stored in the LFP was calculated to be 0.98 Megabits per mm².

This study successfully demonstrated a nanoscale 3D printing approach for fabricating high-resolution LFPs. The single-step TPL fabrication process, along with the pseudorandom arrangement of microlenses and pixels, significantly improved the resolution and reduced the complexity of LFP creation. The ability to achieve high-resolution images using only a single nanopillar per pixel highlights the potential for high information density and image quality. The smooth motion parallax and the minimal accommodation-vergence conflict enhance the realism and viewing experience. The results have implications for both print media, where high-quality 3D images can enhance visual appeal, and for security applications, where the high resolution and difficulty of replication provide significant anti-counterfeiting capabilities.

This research presents a novel method for fabricating high-resolution light field prints using two-photon polymerization lithography. The resulting prints exhibit superior image quality, with high spatial and angular resolutions, and smooth motion parallax. The use of a single nanopillar per pixel achieves unprecedented resolution. Future work could focus on improving fabrication speed using parallel processing TPL systems, exploring higher refractive index materials to expand color gamut, and designing for wider viewing angles using metalenses and angle-insensitive structural color pixels. This technology holds significant promise for applications in high-value print media and robust security features.

The current fabrication process, using a serial patterning TPL system, limits upscaling and mass production. The 24-hour printing time for a 2 mm × 2 mm area needs improvement. The fabricated LFPs are fragile and require protection from damage. The color gamut of the current LFPs is limited (~40% sRGB) due to the use of IP-Dip photoresist, a low-refractive index material. Reflection mode operation reduces image quality due to light scattering and losses.