Developed at Harvard SEAS, metalenses bring benchtop performance to small handheld spectrometer
Compact metasurface lenses serve as both high-dispersion grating and focusing elements for spectroscopy.
|A drawing of a section of an off-axis metalens developed at Harvard SEAS shows a spatially varying pattern of rectangles, which in the fabricated device are titanium dioxide (TiO2) nanofins on a glass substrate. The section shown is about 3 µm across. (Image: John Wallace, drawn from the Fig. 3a photo in Ref. 1)|
Handheld spectrometers that use metalenses (lenses made of a metamaterial surface rather than ground and polished glass) have been created by Federico Capasso's group at Harvard University's John A. Paulson School of Engineering and Applied Sciences (SEAS; Cambridge, MA).1 The handheld spectrometers are capable of the same performance as large benchtop instruments, and are potentially useful for health-care diagnostics to environmental and food monitoring.
The metalenses replace a spectrometer's traditional grating and focusing mirrors; in addition, the metasurface grating's dispersion is greater than that of traditional gratings. As a result, the overall size of the spectrometer is significantly reduced without sacrificing performance.
Nothing but metalenses
Using only metalenses and a CMOS camera and having a total beam-propagation length of only a few centimeters, the spectrometer achieves a spectral resolution down to 0.3 nm and a wavelength range of more than 170 nm.
A metalens is a planar lens made up of a grid of nanostructures. Using lithographic techniques, proper placement and fabrication of these nanostructures can enable similar or better functionalities that traditional lenses. These metalenses can be customized to a user's specifications and mass-produced using the same foundries that produce computer chips.
"For these reasons, we believe metalenses to be game-changers," says Capasso. "In fact, our work on metalenses in the visible, published last year, was hailed by Science magazine as one of the top breakthroughs of the year in 2016." (Laser Focus World wrote about these achievements too, here and here and here.)
"The potential applications of these new smaller spectrometers are significant for portable monitoring of biological and chemical compounds," says Alex Zhu, one of the researchers. "For example, physicians could bring hospital-level diagnostic capabilities to patients in the field where sophisticated equipment and highly trained personnel are not available, providing data on a time scale of minutes to hours, as opposed to days or weeks from usual chemistry-based methods."
The same is true for environmental monitoring: Data about pollutants, or toxic chemicals could be collected and processed in real time on site at various locations with ultra-compact, high performance spectrometers.
Potential for Raman spectroscopy
The next step toward realizing the full potential of these "meta-spectrometers" is to improve both the working wavelength range and spectral resolution. This would allow it to be used for a wide variety of analyses, including Raman spectroscopy (used, for example, to identify proteins or gene markers), which typically involve onerous processes with sophisticated equipment in a full-size laboratory.
The research was partially funded by the Air Force Office of Scientific Research (AFOSR).
1. A. Y. Zhu et al., APL Photonics, Feb. 7, 2017; doi: 10.1063/1.494259; http://aip.scitation.org/doi/full/10.1063/1.4974259