Low-cost spectroscopy technique could allow detection of microscopic amounts of chemicals for applications in security, law enforcement and research
Researchers have developed a microscope that can chemically identify individual micron-sized particles. The new approach 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.
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.
Credit: Ryan Sullenberger, MIT Lincoln Laboratory
In the journal Optics Letters, from The Optical Society (OSA), researchers from the Massachusetts Institute of Technology's Lincoln Laboratory, USA, demonstrated their new microscope by measuring infrared spectra of individual 3-micron 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 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."
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.
Probing spectral fingerprints
Infrared spectroscopy is typically used to identify unknown materials because almost every material can be identified by its unique infrared absorption spectrum, or fingerprint. The new method detects this infrared fingerprint without using infrared detectors. These detectors add significant bulk to traditional instruments which is limiting for portable devices because of their requirement for cooling.
The new technique works by illuminating particles with both an infrared laser and a green laser. The 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 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."
The new microscope's use of visible wavelengths for imaging gives it a spatial resolution of around 1 micron, compared to the roughly 10-micron 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."
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 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."
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.
This material is based upon work supported by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract No. FA8721-05-C-0002 and/or FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering.
About Optics Letters
Optics Letters offers rapid dissemination of new results in all areas of optics with short, original, peer-reviewed communications. Optics Letters covers the latest research in optical science, including optical measurements, optical components and devices, atmospheric optics, biomedical optics, Fourier optics, integrated optics, optical processing, optoelectronics, lasers, nonlinear optics, optical storage and holography, optical coherence, polarization, quantum electronics, ultrafast optical phenomena, photonic crystals and fiber optics.
About The Optical Society
Founded in 1916, The Optical Society (OSA) is the leading professional organization for scientists, engineers, students and business leaders who fuel discoveries, shape real-life applications and accelerate achievements in the science of light. Through world-renowned publications, meetings and membership initiatives, OSA provides quality research, inspired interactions and dedicated resources for its extensive global network of optics and photonics experts. For more information, visit osa.org/100.
Rebecca B. Andersen
The Optical Society
The Optical Society
Joshua Miller | EurekAlert!
Researchers take next step toward fusion energy
16.11.2017 | Texas A&M University
Desert solar to fuel centuries of air travel
16.11.2017 | SolarPACES
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.
During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....
The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...
Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
17.11.2017 | Physics and Astronomy
17.11.2017 | Health and Medicine
17.11.2017 | Studies and Analyses