Multi-level micro-optics enables broadband, multi-plane. computer-generated holography

Transcription

1 Multi-level micro-optics enables broadband, multi-plane computer-generated holography Monjurul Meem, and Rajesh Menon 1,2,a) 1 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA. 2 Oblate Optics, Inc Brixton Place, San Diego CA 92130, USA. a) rmenon@eng.utah.edu ABSTRACT We demonstrate full-color (broadband) image projection via non-absorbing, multi-level microoptical devices that we call polygrams. By appropriate design of the polygram topography, we experimentally demonstrate the projection of: (1) a full-color image by illumination with the flashlight of any mobile phone, (2) 2 distinct images in the visible and in the invisible (nearinfrared) bands, (3) multiple distinct images in multiple planes, and (4) magnified images by illumination with a point source. Such polygrams could have important applications in anticounterfeiting.

2 Broadband image projection can be achieved with absorbing color filters as in conventional image projectors. However, these are very inefficient. The same may be achieved with non-absorbing patterned dielectrics, which could be programmable (eg. spatial-light modulators) [1] or fixed (generally referred to as holograms or computer-generated holograms). Traditionally, these devices are sensitive to the wavelength of illumination, achieving high efficiencies only in narrow bandwidths. Recently, there have been several attempts to improve the broadband performance of such devices either by using metamaterials [2,3] or via multi-level diffractive structures [4]. Metamaterials-based holograms require deep sub-wavelength minimum features and relatively large aspect ratios, which render them very challenging for practical applications and also are very inefficient, if they exploit plasmonics. We previously demonstrated the design, fabrication and characterization of multi-level diffractive holograms that can project full-color images when illuminated by collimated white light [4,5]. We refer to such devices as polygrams. The constituent element of the polygram is a square pixel, whose minimum width is determined by the fabrication technology, and whose height is determined via a nonlinear optimization procedure. The goal of optimization is to maximize the diffraction efficiency averaged over all the wavelengths of interest. In order to ensure manufacturability, we enforce additional constraints on the number of height levels. By exploiting the intrinsic chromaticity of diffraction as well as the material dispersion, we show that it is possible to achieve efficient image projection not only for broadband illumination, but also for illumination with multiple distinct spectra as well as onto multiple image planes. All the devices reported here have minimum feature width of 20mm, maximum feature height of 2.6mm (100 levels) and total size of 10mm X 10mm. Figure 1 shows a polygram that is designed to project a full-color image, when illuminated by the flashlight of any mobile phone. The image was projected onto a white screen and photographed, and

3 compared to the simulated image. Rich saturated colors spanning the entire visible range (blue, green, red) are clearly visible. Unlike previous work, this result suggests that spatial collimation is not critical [4]. light from cell phone flash (a) experiment polygram image z=50mm (b) simulation cell phone flash polygram 20µm image projected on screen Design 10mm 2.6µm 0 (f) 10mm (e) (d) 400µm (c) Fig. 1. Polygram illuminated by flashlight of a mobile phone. (a) Schematic and (b) photograph of the experiment. (c) Designed height map of the polygram. Bottom Inset: Magnified view of a small portion. (d) Optical micrograph of a small portion of the fabricated polygram. (e) Simulated and (e) experimental full-color images when the polygram is illuminated by the flash of a mobile phone. The polygram was fabricated using grayscale lithography in a transparent polymer [4]. We can extend the technology to multiple spectral bands as illustrated in Fig. 2, where one polygram produces either a full-color image or an invisible image, when illuminated by a white light source or a near-infrared source, respectively. This was predicted via simulations, [5] but this is the first experimental demonstration. Such a device could readily enable both covert and overt security for anti-counterfeiting.

4 polygram (c) λ=850nm image (d) Visible collimated input (a) 10mm 20µm z=375mm (b) target 10mm Fig. 2. One polygram produces different images under different wavelengths. (a) Schematic of the experiment. (b) 3D optical micrograph of a portion of the fabricated polygram. Experimental images of (c) a kangaroo at l=850nm (d) a parrot under visible (white light) illumination (target image is shown on right inset). In the Fresnel-diffraction regime, wavelength and propagation distance are inter-changeable. Therefore, one would expect the polygram to be able to project different images in different image planes as illustrated in Fig. 3. In other words, the projected image changes in a controlled manner as the light propagates. When illuminated by collimated white light, the polygram shown in Figs. 3(b-c), projects the image 1 ( + ) at a distance of 40mm and the image 2 ( x ) at a distance of 60mm (see supplementary video 1 showing the transition). Another pair of achromatic images (digits 7 and 9 ) projected by a different polygram are shown in Fig. 3(e). Previously, we demonstrated this with laser illumination [6], but this is the first demonstration with a broadband illumination. (a) polygram image1 image2 image1 image2 image1 image2 collimated input Sim. z1=40mm z2=60mm (b) (c) 10mm Expt. m 10m (d) m 10m 10mm (e)

5 Fig. 3. One polygram produces 2 different images at 2 different image planes. (a) Schematic of the experiment. (b) Designed height map of the polygram. (c) Optical micrograph of a small portion of the fabricated polygram. Simulated (top row) and Experimental (bottom row) images of image 1 and image 2 for the polygram in (c) are shown in (d), and for a different polygram are shown in (e). The illumination was a collimated white light. The experimental images were obtained by photographing the image projected onto a white screen. The technology can be extended to 3 or more planes as well. In Fig. 4, we illustrate the example of 3 different images in 3 different planes (see supplementary video 2). (a) polygram ge1 ge2 age3 ima ima im image1 image2 image3 Sim. collimated input z1=40m m z2=80mm z3=120mm Expt. m 10m (b) Fig. 4. One polygram produces 3 different images at 3 different image planes. (a) Schematic of the experiment. (b) Simulated (top row) and Experimental (bottom row) images of images 1, 2 and 3. The illumination was a collimated white light. The experimental images were obtained by photographing the image projected onto a white screen. For the first time, we demonstrate that the polygram belongs to a class of broadband diffractive optics that act as lenses [7,8]. One can consider the image projected by the polygram as an engineered, broadband point-spread function (PSF). This implies that if the polygram is illuminated by a point source,

6 the image formed would be the convolution of the point (with appropriate magnification) and the PSF. This offers an approach to create magnified image projection, an example of which is illustrated in Fig. 5 (see supplementary video 3). (a) pointsource magnified image polygram (b) collimated (c) point-source z z 7.5mm 16 mm Fig. 5. Image magnification with a point source. (a) Schematic of experiment. Images produced by a polygram (b) under white collimated light and (c) under white point-source showing magnification of the image (note the change in scale). The point-source was obtained by placing an opaque screen with a ~1mm aperture over a white-led flashlight. In summary, here we demonstrate that appropriate design of multi-level diffractive structures can enable efficient image projection over broadband and multiple spectral bands, and onto multiple image planes. Since such structures may be fabricated at low cost over large areas and at high speed via various imprinting technologies, these can be readily used in anti-counterfeiting applications. In addition, we point out that metamaterials and metasurfaces for image projection does not offer any advantage over polygrams. In fact, polygrams are far easier to manufacture. Funding. Office of Naval Research (ONR) (N ); The University of Utah USTAR program.

7 Acknowledgment. We thank Brian Baker, Steve Pritchett and Christian Bach for fabrication advice, and Christian Skipper (Keyence) for providing the 3D optical micrograph in Fig. 2. The University of Utah has applied for a patent covering the subject technology. One of us (RM) is co-founder of Oblate Optics, which is commercializing this technology. REFERENCES 1. J. Cho, S. Kim, S. Park, B. Lee and H. Kim, DC-free on-axis holographic display using a phaseonly spatial-light modulator, Opt. Lett. 43 (14) (2018). 2. M. Khorasaninejad, A. Ambrosio, P. Kanhaiya and F. Capasso, Broadband and chiral binary dielectric meta-holograms, Science Adv. 2(5) e (2016). 3. L. Wang, S. Kruk, H. Tang, T. Li, I. Kravchenko, D. N. Neshev and Y. S. Kivshar, Grayscale transparent metasurface holograms, Optica 3 (12) (2016) 4. N. Mohammad, M. Meem, X. Wan and R. Menon, Full-color, large-area, transmissive holograms enabled by multi-level diffractive optics, Sci. Rep. 7, 5789 (2017). 5. G. Kim, J-A. Dominguez-Caballero and R. Menon, Design and analysis of multi-wavelength diffractive optics, Opt. Exp. 20(2), (2012). 6. P. Wang and R. Menon, Optical lithography on oblique and multi-plane surfaces using diffractive phase masks, Journal of Micro/Nanolithography, MEMS and MOEMS (JM3) 14(2), (2015). 7. M. Meem, A. Majumder and R. Menon, Full-color video and still imaging using two flat lenses, Opt. Exp. 26(21) (2018) 8. N. Mohammad, M. Meem, B. Shen, P. Wang and R. Menon, Broadband imaging with one planar diffractive lens, Sci. Rep (2018). Author Contributions

8 MM performed fabrication and analyzed the results. RM provided design guidance and analyzed the results. All authors prepared and edited the manuscript. Competing Interests Statement RM is co-founder of Oblate Optics, Inc., which is commercializing technology discussed in this manuscript. The University of Utah has filed for patent protection for technology discussed in this manuscript. Materials and Correspondence Correspondence and materials requests should be addressed to RM at

9 Multi-level micro-optics enables broadband, multi-plane computer-generated holography Supplementary Information Monjurul Meem, and Rajesh Menon 1,2,a) 1 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA. 2 Oblate Optics, Inc Brixton Place, San Diego CA 92130, USA. a) rmenon@eng.utah.edu Figure S1 shows a photograph of the experimental setup. opaque screen polygram Fig. S1. Photograph of experimental setup. Figure S2 shows a photograph of the setup with point source used for the results in Fig. 5.

10 point source hologram opaque screen front view of point source Fig. S2. Photograph of experimental setup with point source. Media. Supplementary Video 1 shows the projected image changing as the screen is moved away from the polygram described in Fig. 3. Supplementary Video 2 shows the projected image changing as the screen is moved away from the polygram described in Fig. 4. Supplementary Video 3 shows the projected image changing magnification as the point source is moved away from the polygram described in Fig. 5. In this case, the polygram-screen distance is not changed.

11 Brief Literature review of Meta-holograms Metaholograms that multiplex different images in different polarization states have been demonstrated in transmission [1], in reflection [2], with circular polarized light [3,4], but these suffer from poor efficiency and require sub-100nm structures in silicon. Relative poor quality images in 3 planes was demonstrated with a meta-hologram at a single illumination wavelength, but these again require sub-100nm features, are polarization sensitive (only operate for circular polarized light), exhibit poor efficiency [5]. This has been extended using plasmonic metaholograms to project portions of images at multiple discrete laser illuminations (again with polarization sensitivity) in transmission [6,7] and in reflection [8]. Furthermore, we emphasize that these references completely ignore previous work, were multiplane images were demonstrated at a single wavelength with multi-level diffractive holograms [10] and full-color (broadband) images were demonstrated with LED (incoherent and unpolarized) illumination [11]. To summarize, polygrams are the only technology that has currently demonstrated: Broadband (white) LED illumination Multi-plane, broadband images Different images in very distinct spectra regimes (visible and near-ir) Image magnification enabled by point source Simple fabrication due to the huge minimum features involved (20mm in our examples) References for this section: 1. R. Zhao, B. Sain, Q. Wei, C. Tang, X. Li, T. Weiss, L. Huang, Y. Wang, and T. Zentgraf, Multichannel vectorial holographic display and encryption, Nature News, 28-Nov-

12 2018. [Online]. Available: [Accessed: 17-Jan-2019]. 2. "High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images," 3. D. Wen, F. Yue, G. Li, G. Zheng, K. Chan, S. Chen, M. Chen, K. F. Li, P. W. H. Wong, K. W. Cheah, E. Y. B. Pun, S. Zhang, and X. Chen, "Helicity multiplexed broadband metasurface holograms," 4. J. P. B. Mueller, N. A. Rubin, R. C. Devlin, B. Groever, and F. Capasso, "Metasurface Polarization Optics: Independent Phase Control of Arbitrary Orthogonal States of Polarization," 5. "Strain Multiplexed Metasurface Holograms on a Stretchable Substrate," 6. X. Li, L. Chen, Y. Li, X. Zhang, M. Pu, Z. Zhao, X. Ma, Y. Wang, M. Hong, and X. Luo, "Multicolor 3D meta-holography by broadband plasmonic modulation," 7. "Visible-Frequency Dielectric Metasurfaces for Multiwavelength Achromatic and Highly Dispersive Holograms," 8. "Aluminum Plasmonic Multicolor Meta-Hologram," 9. P. Wang and R. Menon, Optical lithography on oblique and multi-plane surfaces using diffractive phase masks, Journal of Micro/Nanolithography, MEMS and MOEMS (JM3) 14(2), (2015).

13 N. Mohammad, M. Meem, X. Wan and R. Menon, Full-color, large-area, transmissive holograms enabled by multi-level diffractive optics, Sci. Rep. 7, 5789 (2017).