New hand-held spectrometers matching the same performance capacity of large, benchtop instruments have been designed by physicists from Harvard University. The details of the innovation are published in APL Photonics, from AIP Publishing. The team derived inspiration from their cutting-edge work in meta-lenses. The hand-held spectrometers have the potential for several applications from health care diagnostics to food and environmental monitoring.
Spectrometers are extensively used to quantify the presence of a number of chemical or biological compounds according to their interaction with light. However, to be a useful instrument for users, such as food-safety inspectors working in the field or physicians at the bedside, spectrometers must be portable, cost-efficient, and user-friendly without requiring specialized training or equipment. However, there is an innate trade-off between the performance and size of the spectrometer. To maintain performance while minimizing spectrometer size, the team has created a spectrometer with meta-lenses that integrate the functionalities of a conventional grating and focusing mirror into one component, as well as having much higher ability to spatially divide wavelengths (the so-called dispersion). Altogether, the spectrometer’s overall size is considerably minimized without compromising performance.
This research has its roots all the way back to 2011, when we were investigating the fundamental properties of light as it interacts with two dimensional metamaterials (metasurfaces) and discovered generalized laws for the refraction and reflection of light for metasurface, which are powerful generalizations of the textbook laws valid for ordinary surfaces.
Federico Capasso of Harvard.
In contrast to conventional refractory lenses that are millimeters in thickness and have a typical curved surface, a meta-lens is a totally planar or flat lens comprising of millions of nanostructures.
Using lithographic methods, correct fabrication and placement of these nanostructures enables similar or improved functionalities compared to conventional lenses. These meta-lenses can be tailor-made according to a user's specifications, and produced in large scale using the same foundries that manufacture computer chips.
For these reasons, we believe meta-lenses to be game-changers. 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.
"The potential applications of these new smaller spectrometers are significant for portable monitoring of biological and chemical compounds" said Alex Zhu, lead author of the paper. "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 timescale of minutes to hours, as opposed to days or weeks from usual chemistry-based methods." This is applicable for environmental monitoring as well: Data about pollutants, or harmful chemicals could be gathered and processed from the field in real time at a number of locations using ultra-compact, high performance spectrometers.
The subsequent step toward achieving the complete potential of these meta-spectrometers is to enhance the prototype’s performance for both the spectral resolution and the working wavelength range. Then it can be used for a broad range of analyses, including highly specialized ones to detect gene markers or proteins (Raman spectroscopy), which usually involve difficult processes with advanced equipment in a full-scale laboratory.
The goal is to be able to achieve comparable levels of performance with a simple 'plug-and-play' two-component device, i.e., a meta-lens and a detector, which together function as a meta-spectrometer. The potential for this already exists in the meta-lens technology; it is simply a question of finding the right configurations and making it work.