Laser-based particle analysis technique looks promising for future air-pollution monitoring

A technique developed by researchers at Max Planck Institute for the Science of Light and Friedrich-Alexander University (both in Erlangen, Germany) for continuously monitoring both the size and optical properties of individual airborne particles could offer a better way to monitor air pollution.¹ It is especially promising for analyzing fine particulate matter measuring less than 2.5 μm in size (PM2.5), which can reach deep into the lungs and cause health problems.

In the new analysis approach, airborne particles are trapped inside a laser beam (3 W CW, 1064 nm wavelength) by optical forces and propelled forwards by radiation pressure. The trapping force is strong enough to overcome the gravitational force acting on very small particles such as PM2.5 and automatically aligns the particles with a hollow-core photonic crystal fiber (HC-PCF) with a central core that allows the LP₀₁ mode to propagate. The core is hollow and surrounded by a glass microstructure that confines light inside the fiber.

Once aligned, the laser light propels the particle into the fiber, causing the laser light inside the fiber to scatter and create a detectable reduction in the fiber transmission. The researchers developed a new signal processing algorithm to retrieve useful information from the particle-scattering data in real-time. After detection, the particle simply ejects from the fiber without degrading the device.

“Air pollution has become an essential problem in many countries,” says research team leader Shangran Xie from Philip St. J. Russell's group at Max Planck Institute for the Science of Light. “Since our setup is very simple and compact, it should be possible to turn it into a table-top device for continuously monitoring airborne PM2.5 in urban areas and industrial sites. The most unique feature of our technique is that it can count the number of particles — which is related to the level of pollution — while simultaneously providing detailed real-time information on particle size distribution and chemical dispersion. This additional information could be useful for fast and continuous pollution monitoring in sensitive areas, for example.”

“The transmission signal from the fiber also lets us measure time-of-flight, which is the time the particle takes to travel through the fiber,” explains Abhinav Sharma, the doctoral student working on this project. “The drop in fiber transmission together with the time-of-flight information allow us to unambiguously calculate the particle size and refractive index. The refractive index can assist in identifying the particle material because this optical property is already known for most common pollutants.”

Nanometer-level resolution
The researchers tested their technique using polystyrene and silica particles of several different sizes. They found that the system could precisely separate particle types and could measure the 0.99-μm silica particle with a resolution as small as 18 nm.

The researchers plan to test the system’s ability to analyze particles which are more commonly found in the atmosphere. They also want to demonstrate the technique’s ability to perform measurements in liquid, which would be useful for water-pollution monitoring. They have filed a patent on this technique and plan to continue to develop prototype devices, such as ones that could be used to monitor air pollution outside the lab.

Source: https://www.osa.org/en-us/about_osa/newsroom/news_releases/2019/new_particle_analysis_for_air_monitoring/

REFERENCE:

1. A. Sharma et al., Optics Express, 27, 24, 34496-34504 (2019); doi: https://doi.org/10.1364/OE.27.034496