Researchers at the Massachusetts Institute of Technology’s Lincoln Laboratory, Cambridge, report that they have developed a microscope in which optical microscopy is combined with laser spectroscopy to enable identification of individual micron-sized particles. They demonstrated the technology by measuring infrared spectra of individual three-micron spheres made of silica or acrylic.
The technique works by illuminating particles with both an infrared laser and a green laser. The infrared laser energizes the particles, causing them to heat up and expand. The green laser light is then scattered by these heated particles. A visible-wavelength camera monitors this scattering, tracking physical changes of the individual particles through the microscope’s lens.
The instrument can be used to identify the material composition of individual particles by tuning the infrared laser to different wavelengths and collecting the visible scattered light at each wavelength. The slight heating of the particles does not impart any permanent changes to the material, making the technique ideal for nondestructive analysis.
The ability to excite particles with infrared light and then look at their scattering with visible wavelengths – a process called photothermal modulation of Mie scattering – has been used since the 1980s. This new work uses more advanced optical components to create and detect the Mie scattering, and is the first to use an imaging configuration to detect multiple species of particles.
The microscope’s use of visible wavelengths for imaging gives it a spatial resolution of around one micron, compared to the roughly ten-micron resolution of traditional infrared spectroscopy methods. This increased resolution allows the technique to distinguish and identify individual particles that are extremely small and close together.
The microscope is a simple optical setup consisting of compact components that will allow the instrument to be miniaturized into a portable device about the size of a shoebox.
Paper: R.M. Sullenberger, S.M. Redmond, D. Crompton, A.M. Stolyarov, W.D. Herzog, “Spatially-Resolved Individual Particle Spectroscopy using Photothermal Modulation of Mie Scattering,” Opt. Lett., Vol. 42, Issue 2, 203-206, (2017).
DOI: 10.1364/OL.42.000203.
