AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Photophoretic Trapping of Absorbing Particles in Air and Measurement of Their Single-Particle Raman Spectra
YONG-LE PAN, Steve Hill, Mark Coleman, US Army Research Laboratory
Abstract Number: 470 Working Group: Instrumentation and Methods
Abstract A new method is demonstrated for optically trapping micron-sized absorbing particles in air and obtaining their single-particle Raman spectra. A 488-nm Gaussian beam from an Argon ion laser is transformed by conical lenses (axicons) and other optics into two counter-propagating hollow beams, which are then focused tightly to form hollow conical beams near the trapping region. The combination of the two coaxial conical beams, with focal points shifted relative to each other along the axis of the beams, generates a low-light-intensity biconical region totally enclosed by the high-intensity light at the surface of the bicone. Particles within this region are trapped by the photophoretic forces that push particles toward the low-intensity center of this region.
Using this system, individual micron-sized particles or aggregates of Johnson grass smut spores (an example fungal spore), riboflavin, carbon black, nigrosin, and carbon nanotubes were trapped in open air for times up to hours. Raman spectra from individual trapped particles made from carbon nanotubes were obtained in less than 1 second. The strong photophoretic force, which has been calculated to be orders of magnitude larger than the radiation pressure force, could supply a stronger optical trap than laser tweezers. This photophoretic trapping technique could lead to the development of an on-line real-time single-particle Raman spectrometer for characterization of absorbing aerosol particles.