Transmission enhancement of subwavelength grating microlens by tapered nanostructure
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Research Letter
Transmission enhancement of subwavelength grating microlens by tapered nanostructure Mao Ye and Xiaopeng Guo, Integrated Nano Optoelectronics Laboratory, University of Michigan, 4901 Evergreen Road, Dearborn, Michigan 48128, USA Yasha Yi, Integrated Nano Optoelectronics Laboratory, University of Michigan, 4901 Evergreen Road, Dearborn, Michigan 48128, USA; Energy Institute, University of Michigan, 2301 Bonisteel Blvd., Ann Arbor, Michigan 48109, USA Address all correspondence to Yasha Yi at [email protected] (Received 28 January 2018; accepted 29 March 2018)
Abstract The emerging planar subwavelength microlens has attracted wide attention recently. There exists a trade-off in the selection of phase shifter materials for the lens designed with linearly polarized incidence. In this work, we have discovered that it is possible to utilize tapered nanostructure to increase the transmission of phase shifters built with high refractive index materials. A typical grating microlens is demonstrated to examine the effectiveness of taper-enhancement effect—the focus efficiency is increased from 9% to 28% with the properly designed tapered sidewall. Our work will provide a novel method to enhance performance using high refractive index materials in the emerging microlens field.
Introduction Subwavelength microlens is one of the emerging planar micro-optical devices with great potential in a variety of applications.[1–8] Among this field, high refractive index contrast nanostructures show great advantages in areas such as spectral modification of optical wave front.[2] These advantages have enabled the microlens to be designed in a scale ranging from micrometers to several millimeters with focus size less than a micrometer. Based on the subwavelength nature of phase shifters, the physical realization of the microlens relies on nanofabrication techniques. To be specific, it can be either fabricated through reactive ion etching (top-down process) or deposition then lift-off (bottom-up process). Despite the difference in the process details, both of the two techniques have one common challenge that it is extremely difficult to fabricate structures with a high aspect ratio (e.g., a ratio higher than 5:1 thickness versus width). For planar grating lens that is designed under linearly polarized incidence (e.g., utilization of propagation phase,[2,4] not Pancharatnam–Berry phase), the different phase shift is created by altering the effective refractive index. To achieve enough phase shift in visible wavelength with limited thickness (e.g., low aspect ratio), researchers have looked into high refractive index materials such as GaN[3] and TiO2.[9] While high refractive index materials are enabling phase shifters more phase shift with comparatively low aspect ratio, their transmission may experience a significant drop. To design an optimum planar microlens, one should either increase the phase coverage of low-index material
nanostructures or increase the transmission of high-index material nanostructures. While the most effec
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