Hybrid achromatic microlenses with high numerical apertures and focusing efficiencies across the visible

Abstract

Compact visible wavelength achromats are essential for miniaturized and lightweight optics. However, fabrication of such achromats has proved to be exceptionally challenging. Here, using subsurface 3D printing inside mesoporous hosts we densely integrate aligned refractive and diffractive elements, forming thin high performance hybrid achromatic imaging micro-optics. Focusing efficiencies of 51-70% are achieved for 15μm thick, 90μm diameter, 0.3 numerical aperture microlenses. Chromatic focal length errors of less than 3% allow these microlenses to form high-quality images under broadband illumination (400-700 nm). Numerical apertures upwards of 0.47 are also achieved at the cost of some focusing efficiency, demonstrating the flexibility of this approach. Furthermore, larger area images are reconstructed from an array of hybrid achromatic microlenses, laying the groundwork for achromatic light-field imagers and displays. The presented approach precisely combines optical components within 3D space to achieve thin lens systems with high focusing efficiencies, high numerical apertures, and low chromatic focusing errors, providing a pathway towards achromatic micro-optical systems.

Fig. 1. Volumetric integration of diffractive and refractive components for compact hybrid achromatic microlenses.

a Artistic rendering of a subsurface hybrid lens focusing white light. The low refractive index PSiO₂ host medium is represented by a blue cube surrounding the printed diffractive and refractive optical components. In the real system, the cube extends in X and Y, but was truncated in the illustration. White light enters the hybrid lens’ aperture from the bottom and is focused to a single point. b, c Geometric and GRIN hybrid doublet achromats. Experimentally measured confocal fluorescence images of achromatic hybrid doublets fabricated within the volume of the host medium. Both geometric (b) and GRIN (c) refractive components are integrated above flat diffractive lenses. d, e Lower and higher NA diffractive lenses. Experimentally measured confocal fluorescence images of diffractive lenses fabricated within the volume of the PSiO₂ host medium. The diffractive lenses are used as control samples. The diffractive lens shown in d is identical to the diffractive component used in the hybrid lenses. The diffractive lens in e has a higher NA and was designed to have the same focal length as the hybrid lenses at a wavelength of 633 nm, allowing for a more direct comparison. The black regions surrounding the experimentally imaged microlenses in a–d are occupied by PSiO₂. Scale bars are 15 μm.

Fig. 2. Focal profiles of diffractive and hybrid lenses at visible wavelengths.

a, b X-Z confocal scans of a lower NA diffractive lens (a) and a higher NA diffractive lens (b) focusing 633, 612, 542, and 488 nm light. These lenses experience strong negative dispersion. c, d X-Z confocal scans of a GRIN hybrid lens (c) and a geometric hybrid lens (d) focusing 633, 612, 542, and 488 nm light. These lenses experience reduced negative dispersion. For all lenses, the horizontal white dotted lines are the approximate reference focal planes for 633 nm light. The horizontal white solid lines represent the approximate interface between the PSiO₂ host medium and air. The lens cartoons represent the approximate locations of the printed lenses. All scans are 90 μm in X and are to scale with each other in Z.

Fig. 3. Characterization of focal length and focusing efficiency.

a Graph of focal length plotted against wavelength for lower and higher numerical aperture (NA) diffractive lenses, geometric hybrid lenses, and GRIN hybrid lenses. Each data point was averaged across ten printed lenses. The diffractive lenses experienced stronger negative dispersion than the hybrid lenses. b Graph of measured focusing efficiency plotted against wavelength for each type of lens. Each data point was averaged across the ten printed lenses. Error bars represent the maximum and minimum values across the ten samples. c Schematic depicting the experimental setup for measuring microlens focusing efficiency. Microlenses (left of the figure) are placed on a motorized stage and illuminated by collimated lasers. A 50:50 beam splitter transmits half of the light to a camera and half to a power meter, allowing the simultaneous alignment of the pinhole and the power measurement.

Fig. 4. White light imaging with subsurface microlenses.

a Illustration of chromatic imaging by a high NA diffractive lens under broadband white light illumination. The lens focuses light from an object and forms a point spread function (PSF) and an image some distance away. Due to dispersion, the white light is separated into individual color components, resulting in a rainbow color-blurred image. The insets show experimentally collected images measured at different planes 30 μm apart. The chromatic blurring causes a different colored image (red, orange, green, cyan, or blue) to be formed at each plane (no single white light image is formed). The scale bar is 45 μm for the insets. b Illustration of achromatic imaging by a hybrid doublet under broadband white light illumination. The PSF and a white image of the object is formed at single planes. Insets show experimentally measured white light images formed by the geometric hybrid achromat and the GRIN hybrid achromat at their image planes. The images have a yellowish tint because a warm white LED was used as the illumination. The scale bar is 45 μm for the insets.

Fig. 5. Hybrid doublets with increased numerical apertures (NAs).

Confocal fluorescence intensity images of hybrid doublets with increasingly high NAs from 0.3 to 0.471. Diffractive components were designed with 3 (a, b), 5 (c, d), 6 (e, f), or 9 (g, h) zones. Refractive components were defined to be either plano-convex (a, c, e, g) or biconvex (b, d, f, h). Broadband visible wavelength images formed are shown to the right of each respective doublet. Scale bars are 15 μm for the fluorescence images and 45 μm for the images of the resolution target formed by the hybrid microlenses.

Fig. 6. Imaging with an achromatic microlens array.

a Optical microscope image of the achromatic microlens array illuminated by a white LED. Each microlens in the array is identical to the lens in Fig. 1b. b Array of images formed by the microlens array in a under broadband white light illumination. Light-field information of groups 6 and 7 on the USAF resolution target is collected. The inset shows a rendered image that was reconstructed from the collected array of images. Scale bars are 45 μm.