Meta-lenses Bring Benchtop Performance to Hand-Held, Small Spectrometer

Spectrometers are devices that are extensively employed to quantify the presence of numerous chemical or biological compounds based on their interaction with the light. But, to be a practical tool for users, like physicians at the side or food-safety inspectors out in the domain, spectrometers have to be portable, easy to use and cost-effective without specialized equipment or training.

Usually, there is an inherent trade-off between the performances of the spectrometer. To maintain the performance while reducing spectrometer size, this group of scientists has introduced a spectrometer incorporating meta-lenses that link the functionalities of a conventional focusing and grating mirror into a singular component, as well as having much greater ability to spatially separate wavelengths. In all, the overall size of the spectrometer is drastically reduced without sacrificing performance.

“Such study has its roots all the way back till 2011, when we were investigating the basic properties of light as it interacts with two dimensional metamaterials and discovered generalized laws for the reflection and refraction of light for metasurface, which are robust generalizations of the textbook laws valid for ordinary surfaces,” explains Febderico Capasso of Harvard.

Figure: A drawing section of an off-axis metalens developed at Harvard SEAS

Unlike conventional refractory lenses that are millimetres thick and have a characteristic curved surface, a meta-lens is an absolutely planar or flat lens made up of millions of nanostructures. Employing lithographic methods, proper placement and fabrication of such nanostructures allows better or similar functionalities compared to traditional lenses.

Such meta-lenses can be personalized to a user’s specifications, and mass-produced employing the same foundries that manufacture computer chips. “For such reasons, we consider meta-lenses to be game-changers,” says Capasso. “In fact, our work on metalenses in the visible, introduced last year, was hailed by Science magazine as one of the top breakthroughs of the year in 2016.”

“The potential applications of such novel smaller spectrometers are significant for portable monitoring of chemical and biological compounds,” says Alex Zhu, head author of the paper. “For instance, physicians could bring hospital-level diagnostic potentialities to patients in the field where sophisticate equipment and exceedingly trained personnel are not available, offering data on a timescale of minutes to hours, as against days or weeks from usual chemistry-based methods.”

The further step towards realizing the complete potential of such meta-spectrometers is to enhance the performance of the prototype for both the working wavelength range and spectral resolution.

The aim is to be able to accomplish comparable levels of performance with a simple, ‘play n plug’ two component device that is, a meta-lens and a detector that together function as a meta-spectrometer,” Zhu says. “The feasible for such already exists in the meta-lens technology, it is simply a question of finding the accurate configurations and making it work.”