AsianScientist (Mar. 9, 2015) – Researchers at the Harvard School of Engineering and Applied Sciences (SEAS), together with the Singapore Institute of Manufacturing Technology (SIMTech), have created a novel lens that is flat, thin, and capable of focusing light without separating its colors. This research was published in Science.
Components that bend light—such as prisms, lenses and diffractive gratings—lie at the heart of optical technology. These components typically bend different wavelengths of light through different angles, causing the separation of different colors of light, also known as chromatic dispersion. Dispersion gives rise to everyday phenomena like rainbows and colorful reflections off CDs, and it is used routinely in the scientific world for applications like spectroscopy and even high-speed photography.
While dispersion is useful in some settings, it is harmful in others. When an image is captured through a lens that has dispersion, different colors will be imaged at slightly different locations. The resulting chromatic aberration results in a blurred image, causing problems in photography, astronomy and microscopy. As such, telescopes and cameras have to rely on achromatic lenses, made by combining multiple pieces of glass with different dispersion such that their light-spreading effects cancel each other out. While effective, these lenses are expensive and heavy, adding cost and complexity to the design of an optical system.
“We set out to address the problem of chromatic aberration by designing a completely different kind of lens known as a metasurface, consisting of a glass sheet overlaid with hundreds of tiny silicon antennae,” explains Dr. Patrice Genevet, a researcher at SIMTech who co-authored the present study.
“These antennae are small enough to fit several into a single wavelength of light, allowing them to concentrate and re-emit an incoming light wave in a precisely controlled manner. Varying their width, height, and arrangement thus allows for the design of a surface that can bend light in very specific ways.”
The researchers were able to design two particular arrangements of these antennae that are able to redirect different wavelengths of light without separating them, thus creating examples of what they called an “achromatic metasurface”.
Their first metasurface was capable of bending incident light of different wavelengths through the same angle, unlike a diffraction grating which bends different wavelengths by different amounts; their second metasurface was capable of focusing incident light of different wavelengths at the same distance, unlike a conventional lens which would focus them at different distances.
“This somehow simple demonstration of dispersion-less refraction and achromatic focusing of broadband light will certainly open new and promising research opportunities in field of flat optics,” Genevet continues.
For example, an optical metasurface could conceivably focus red, blue and green onto the same spot, allowing for faithful transmission and manipulation of colored images without a bulky conventional achromatic lens. This opens important prospects in the miniaturization and optimization of optical devices, and possibly paves the way for 3D colored holograms instead of current holograms that are constrained to having a single color.
Dispersion management at a flat interface could also find widespread applications in ultrafast and nonlinear optics. Since the fabrication process involves standard techniques and uses equipment currently adopted by the integrated circuit industry, flat achromatic optical interfaces can be manufactured on large scale. Although this technology is still at the level of basic research, it has the potential to be deployed beyond the boundary of research laboratories, targeting large public applications for example in the area of imaging and vision.
The article can be found at: Aieta et al. (2015) Multiwavelength Achromatic Metasurfaces by Dispersive Phase Compensation.
—–
Copyright: Asian Scientist Magazine; Photo: Patrice Genevet/SIMTech.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.
