A microscope that can chemically identify μm-sized particles could one day be used in airports and other high-security venues to rapidly screen people for microscopic amounts of potentially dangerous materials. The technique, which was developed by researchers at the Massachusetts Institute of Technology’s Lincoln Laboratory, uses photothermal modulation of Mie scattering (PMMS) to enable concurrent spatial and spectral discrimination of individual μm-sized particles, and uses an imaging configuration to detect multiple species of particles. “We’re actually imaging the area that we’re interrogating,” said researcher Alexander Stolyarov. “This means we can simultaneously probe multiple particles on the surface at the same time.” Multiple species of micron-sized particles are simultaneously illuminated by an infrared laser and a green laser beam. Absorption of the infrared laser light by the particles increases their temperatures, causing them to expand and slightly altering their optical properties. These changes are unique to the material composition of each particle and can be measured by examining the modulation of scattered green light from each particle. Courtesy of Ryan Sullenberger, MIT Lincoln Laboratory. The researchers measured the IR absorption spectra of individual 3-µm acrylic and silica spheres. Trace quantities of material were deposited onto an IR-transparent substrate and simultaneously illuminated by a wavelength-tunable intensity-modulated quantum cascade pump laser and a continuous-wave 532-nm probe laser. Absorption of the pump laser by the particles resulted in direct modulation of the scatter field of the probe laser. The probe light scattered from the interrogated region was imaged onto a visible-wavelength camera, enabling simultaneous probing of spatially separated individual particles. Physical changes of the individual particles could be tracked using the microscope’s lens. By tuning the wavelength of the pump laser, the researchers were able to obtain the IR absorption spectrum and use the instrument to identify the material composition of individual particles. The slight heating of the particles did not cause any permanent changes, making the technique suitable for nondestructive analysis. The microscope’s use of visible wavelengths for imaging gives it a spatial resolution of around 1 μm, compared to the roughly 10-μm resolution of traditional IR spectroscopy methods. The increased resolution makes it possible to distinguish and identify individual particles that are extremely small and close together. “If there are two very different particles in the field of view, we're able to identify each of them,” said Stolyarov. “This would never be possible with a conventional infrared technique because the image would be indistinguishable.” The microscope combines a quantum cascade laser with a very stable visible laser source and a commercially available scientific-grade camera. “We are hoping to see an improvement in high-power wavelength-tunable quantum cascade lasers," said Sullenberger. "A more powerful infrared laser enables us to interrogate larger areas in the same amount of time, allowing more particles to be probed simultaneously.” The instrument employs a simple optical setup consisting of compact components that would allow it to be miniaturized into a portable device about the size of a shoebox. It uses IR spectroscopy to detect the IR fingerprint of unknown materials without the use of bulky IR detectors. “The most important advantage of our new technique is its highly sensitive, yet remarkably simple design,” said researcher Ryan Sullenberger. “It provides new opportunities for nondestructive chemical analysis while paving the way toward ultra-sensitive and more compact instrumentation.” The researchers plan to test their microscope on additional materials, including particles that are not spherical in shape. They also want to test their setup in more realistic environments that might contain interferents in the form of particles that aren't from the chemical of interest. “The presence of interferents is perhaps the biggest challenge I anticipate we will need to overcome," said Stolyarov. "Although contamination is a problem for any technique measuring absorption from small amounts of materials, I think our technique can solve that problem because of its ability to probe one particle at a time.” The microscope's ability to identify individual particles could make it useful for fast detection of chemical threats or controlled substances. Its high sensitivity is also ideal for scientific analysis of very small samples or for measuring the optical properties of materials. The research was published in Optics Letters, a journal of the Optical Society of America (doi: 10.1364/OL.42.000203).