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Compact new microscope chemically identifies micrometer-sized particles

Multiple types of micrometer-sized particles are simultaneously illuminated by a far-infrared laser and a green laser beam. Absorption of the infrared laser energy by the particles increases their temperatures, causing them to expand and slightly altering their visible-light 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. (credit: Ryan Sullenberger, MIT Lincoln Laboratory)

MIT researchers have developed a radical design for a low-cost, miniaturized microscope that can chemically identify individual micrometer-sized particles. It could one day be used in airports or other high-security venues as a highly sensitive and low-cost way to rapidly screen people for microscopic amounts of potentially dangerous materials. It could also be used for scientific analysis of very small samples or for measuring the optical properties of materials.

Optical setup for PMMS measurement scheme. A tunable far-IR laser (QCL) or pump projects laser light (shown in this illustration as red, but actually invisible far-IR, or thermal energy) and a 532 nm laser (probe) projects green light onto the same location on a sample, which consists of microspheres deposited onto a ZnSe substrate in this experiment. A visible-light camera fitted with a 16× microscopic lens images the particles directly. The white LED is used to help locate the particles. (credit: R. M. Sullenberger et al./ Optics Letters)

In an open-access paper in the journal Optics Letters, from The Optical Society (OSA), the researchers demonstrated their new “photothermal modulation of Mie scattering” (PMMS) microscope by measuring infrared spectra of individual 3-micrometer spheres made of silica or acrylic. The new technique uses 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.

The new microscope’s use of visible wavelengths for imaging gives it a spatial resolution of around 1 micrometer, compared to the roughly 10-micrometer resolution of traditional infrared spectroscopy methods. This increased resolution allows the new technique 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 most important advantage of our new technique is its highly sensitive, yet remarkably simple design,” said Ryan Sullenberger, associate staff at MIT Lincoln Labs and first author of the paper. “It provides new opportunities for nondestructive chemical analysis while paving the way towards ultra-sensitive and more compact instrumentation.”

Probing spectral fingerprints

A typical far-IR spectrometer (credit: NYU)

Infrared spectroscopy is typically used to identify unknown materials because almost every material can be identified by its unique far-infrared absorption spectrum, or fingerprint. The new method detects this fingerprint without using actual far-infrared detectors, which add significant bulk to traditional instruments. That limits their use as portable devices — also because of their requirement for cooling.

The new technique works by illuminating particles with both an far-infrared laser and a green laser. The far-infrared laser deposits energy into the particles, causing them to heat up and expand. The green laser light is then scattered by these heated particles. A visible-wavelength camera is used to monitor 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 far-infrared laser to different wavelengths and collecting the visible scattered light at each wavelength. The slight heating of the particles doesn’t impart any permanent changes to the material, making the technique ideal for non-destructive 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.

“We’re actually imaging the area that we’re interrogating,” said Alexander Stolyarov, technical staff and a co-author of the paper. “This means we can simultaneously probe multiple particles on the surface at the same time.”

Compact, tunable infrared laser

The development of compact, tunable quantum-cascade infrared lasers was a key enabling technology for the new technique. The researchers combined 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 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 interfering particles.

The work was supported by the U.S. Assistant Secretary of Defense for Research and Engineering under an Air Force contract.

* “By using a visible probe beam and camera for registering the particle absorption, we are able to spectroscopically identify individual particles that are spaced closer than the IR diffraction limit, which represents a significant improvement over conventional IR spectroscopic imaging techniques,” the authors note.


Abstract of Spatially-resolved individual particle spectroscopy using photothermal modulation of Mie scattering

We report a photothermal modulation of Mie scattering (PMMS) method that enables concurrent spatial and spectral discrimination of individual micron-sized particles. This approach provides a direct measurement of the “fingerprint” infrared absorption spectrum with the spatial resolution of visible light. Trace quantities (tens of picograms) of material were deposited onto an infrared-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 results in direct modulation of the scatter field of the probe laser. The probe light scattered from the interrogated region is imaged onto a visible camera, enabling simultaneous probing of spatially-separated individual particles. By tuning the wavelength of the pump laser, the IR absorption spectrum is obtained. Using this approach, we measured the infrared absorption spectra of individual 3 μm PMMA and silica spheres. Experimental PMMS signal amplitudes agree with modeling using an extended version of the Mie scattering theory for particles on substrates, enabling the prediction of the PMMS signal magnitude based on the material and substrate properties.

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