Co-investigator Brian Anthony
Aerosol particles are distributed in the atmosphere and can constitute serious health threats depending on their chemistry, size, and concentration. For instance, particles of different sizes are deposited in different parts of a lung airway and can lead to specific respiratory complications; and aerosols with certain functional groups can be more harmful than others. So, the comprehensive sensing of aerosol particles is critical for human health, particularly with timely monitoring of environmental pollution, industrial pollution, and defense threats. Most existing aerosol sensors are based on free-space detection methods using optical scatter- ing, IR spectroscopy, and electrical property determination. These sensors can suffer from poor sensitivity and be expensive and bulky.
We have developed an on-chip photonic aerosol spectrometer that can perform in situ particle sizing, counting, shape, and chemical characterization. The device is based on an integrated array of waveguide and microresonator structures built on a silicon nitride-on-insulator platform using simple UV photolithography. We have demonstrated that the sensors can estimate the size of particles ranging from 100 nm to 5 microns with particle concentrations over ~500 to 105 particles/cm3. An aerosol particle falling on the microresonator sensor interacts with the evanescent field of the resonators and acts as a scatterer causing energy loss. The interaction of these particles with the evanescent mode of the microresonators depends on the particle size, shape and count. Coupled with theoretical scattering models of Mie and Rayleigh, we use the measured data to extract physical properties of the airborne particles. The Q-factor of these resonators is as high as 105 enabling sensing resolution to that of an individual aerosol particle. Similarly, by selecting a combination of the resonant wavelengths in microresonators to develop infra-red spectrum sensitive to the distinctive bands of organic and inorganic functional groups inherent in molecularly structures aerosol particles, the spectrometer can be used to do chemical characterization of aerosol particles. This multi resonator platform is tailorable to single or multi-species detection that can be deployed for a variety of aerosol chemistry sensing applications. The technology offers various advantages in particle sensing modalities by offering improved sensitivity, response time and reduced cost and size of the device.