Rajesh Menon, University of Utah & Oblate Optics; Monjurul Meem, University of Utah; Apratim Majumder, University of Utah
Keywords: flat optics, diffractive lenses, achromatic lenses, depth of focus, imaging.
Abstract:
The weight of many imaging systems are dominated by optics, especially at large apertures. Flat lenses can mitigate this problem by: (1) The weight of one flat lens can be many orders of magnitude lower than that of a corresponding refractive lens; (2) A flat lens can perform the function of multiple conventional lenses, thereby reducing the number of components; and (3) The flat lens can enable new capabilities that are not accessible with conventional lenses as summarized below.
Although diffractive flat lenses have been used in imaging before, they have poor performance due to aberrations (chromatic and off-axis). We have combined new theoretical insights, nonlinear optimization based design, and high-resolution grayscale nanofabrication to demonstrate flat Multi-level Diffractive Lenses (MDLs) with high numerical aperture (NA > 1.3 under water immersion) (low f/#) [1,2] and covering the visible,[3-5] near infra-red,[6] short wave-infrared,[7] longwave infrared [8], ultra-violet [9] and the Terahertz bands.[10] We have also combined two MDLs to create telescopes with varying magnification. [11]
The weight of an MDL is dominated by its support substrate. By keeping this substrate very thin, one can achieve orders of magnitude reduction over the weight of the refractive counterpart. The MDLs can be fabricated directly into dielectric materials like glass, Silicon, but also could be patterned in transparent polymers. Since the device thickness of the MDL can be small (~ 2l), then even absorptive polymer films could be used with good performance. Scalability to large apertures is feasible by either segmented MDLs or by patterning large areas via imprint lithography or combinations of the two. In this presentation, we will describe our advancements to achieve large aperture imaging.
As mentioned earlier, the MDL can perform the function of multiple conventional lenses. This concept can be illustrated by a single MDL that is achromatic over a large bandwidth, such as the visible band [3-5]. But we can a step further, and show that MDLs can achieve achromaticity over bandwidths that are several orders of magnitude larger than what is possible with conventional lens systems. For example, we showed that a single MDL can be achieve achromatic imaging (and focusing) from the visible (450nm) to the LWIR (15mm) without changing its focal length.[12] A second example can be seen in an MDL, whose depth of focus was increased by several orders of magnitude over that of a conventional lens. [13] Compared to metalenses, MDLs have the advantages of easier manufacturing and thereby, easier scaling to larger areas. [14]
Flat lenses provide an important toolbox for optical design with degrees of design freedom that are not accessible with conventional lenses. This technology has the potential to enable imaging systems that are not only lightweight, but also have functions that are not previously possible, such as ultra-broadband imaging through a single optical aperture.
References:
[1] D. Chao, et al, Immersion Zone-Plate-Array Lithography, J. Vac. Sci. Technol. B, 23(6), 2657-2661 (2005).
[2] M. Meem, et al, Large-area, high-NA multi-level diffractive lens via inverse design, Optica 7(3) 252-253 (2020).
[3] M. Meem, et al, Inverse-designed flat lens for imaging in the visible & near-infrared with diameter > 3mm and NA=0.3, Appl. Phys. Lett. 117(4) 041101 (2020).
[4] N. Mohammad, M. Meem, B. Shen, P. Wang and R. Menon, Broadband imaging with one planar diffractive lens, Sci. Rep. 8 2799 (2018).
[5] P. Wang, N. Mohammad and R. Menon, Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing, Sci. Rep. 6, 21545 (2016).
[6] S. Banerji, et al, Ultra-thin near infrared camera enabled by a flat multi-level diffractive lens, Opt. Lett. 44(22) 5450-5452 (2019).
[7] M. Meem, et al, Imaging across the Short-Wave Infra-Red (SWIR) Band via a Flat Multilevel Diffractive Lens, OSA Continuum 2 (10) 2968-2974 (2019).
[8] M. Meem, et al, Broadband lightweight flat lenses for longwave-infrared imaging, PNAS Oct 2019, 201908447.
[9] S. Banerji & B. Sensale-Rodriguez, Inverse designed achromatic flat lens operating in the ultraviolet, OSA Continuum 3(7) 1917-1929 (2020).
[10] S. Banerji & B. Sensale-Rodriguez, A computational design framework for efficient, fabrication error-tolerant, planar THz diffractive optical elements. Scientific reports, 9(1), 5801 (2019).
[11] M. Meem, A. Majumder and R. Menon, Full-color video and still imaging using two flat lenses, Opt. Exp. 26(21) 26866-26871 (2018).
[12] M. Meem, et al, Imaging from the visible to the longwave infrared via an inverse-designed flat lens, arXiv:2001.03684 (2020).
[13] S. Banerji, et al, Diffractive flat lens enables Extreme Depth-of-focus Imaging, Optica 7(3) 214-2017 (2020).
[14] S. Banerji, M. Meem, B. Sensale-Rodriguez and R. Menon, Imaging with flat optics: metalenses or diffractive lenses?, Optica 6(6) 805-810 (2019).
Date of Conference: September 14-17, 2021
Track: Optical Systems & Instrumentation