3D-printed gradient-index Lüneburg lens is fabricated at optical wavelengths
One of the holy grails of gradient-index (GRIN) optics fabrication is to make a 3D Lüneburg lens, which is a sphere-shaped lens with a refractive index that has only a dependence on the radius of the sphere; invented by by Rudolf Lüneburg in 1944, it is a prime example of spherically symmetric gradient-index lenses because the particular index gradient that it has causes the lens to focus an incoming collimated beam from any direction to a point on the sphere's surface opposite from the incoming beam.
It has previously has been impossible to fabricate a Lüneburg lens for use at optical wavelengths. Now, Researchers from the University of Illinois at Urbana-Champaign and Stanford University (Palo Alto, CA) have developed a 3D printing process that can make just this type of lens. Poised to benefit are various areas of imaging, computing, and communications.1
The study was led by University of Illinois Urbana-Champaign researchers Paul Braun and Lynford Goddard and is the first to demonstrate the ability to adjust how light refracts through a lens with submicrometer precision.
In the lab, the team uses direct-laser writing to create the lenses by solidifying liquid polymers; the resulting prototype was microsopic in size at only 15 μm in diameter.
“We addressed the refractive index limitations by printing inside of a nanoporous scaffolding support material,” Braun says. “The scaffold locks the printed micro-optics into place, allowing for the fabrication of a 3D system with suspended components.”
The researchers theorize that this refractive index control is a result of the polymer-setting process. “The amount of polymer that gets entrapped within the pores is controlled by the laser intensity and exposure conditions,” Braun says. “While the optical properties of the polymer itself do not change, the overall refractive index of the material is controlled as a function of laser exposure.”
The team fabricated several devices: a flat lens, the world’s first visible-light Lüneburg lens, achromatic doublets in a single printing step, and an all-pass ring resonator that was coupled to a subsurface 3D waveguide.
Team members said they expect that their method will significantly impact the manufacturing of complex optical components and imaging systems and will be useful in advancing personal computing.
“A great example of the application of this development will be its impact on data transfer within a personal computer,” Goddard says. “Current computers use electrical connections to transmit data. However, data can be sent at a significantly higher rate using an optical waveguide because different colors of light can be used to send data in parallel. A major challenge is that conventional waveguides can only be made in a single plane and so a limited number of points on the chip can be connected. By creating three-dimensional waveguides, we can dramatically improve data routing, transfer speed, and energy efficiency.”
Source: https://news.illinois.edu/view/6367/1565551394
REFERENCE:
1. Christian R. Ocier et al., Light: Science & Applications (2020); https://doi.org/10.1038/s41377-020-00431-3.
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John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.