Photophoretic trapping of absorbing particles in air and measurement of their single-particle Raman spectra

U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, USA.
Optics Express (Impact Factor: 3.49). 02/2012; 20(5):5325-34. DOI: 10.1364/OE.20.005325
Source: PubMed


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, which is a type of bottle beam. Particles within this region are trapped by the photophoretic forces that push particles toward the low-intensity center of this region. Raman spectra from individual trapped particles made from carbon nanotubes are measured. This trapping technique could lead to the development of an on-line real-time single-particle Raman spectrometer for characterization of absorbing aerosol particles.

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Available from: Coralyn Hill, Oct 13, 2014
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    ABSTRACT: We demonstrate optical trapping and manipulation of micron-sized absorbing air-borne particles with a single focused Gaussian beam. Transportation of trapped nonspherical particles from one beam to another is realized, and the underlying mechanism for the trapping is discussed by considering the combined action of several forces. By employing a specially-designed optical bottle beam, we observe stable trapping and optical transportation of light-absorbing particles from one container to another that is less susceptible to ambient perturbation.
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    ABSTRACT: We demonstrate a method for optical trapping and Raman spectroscopy of micron-sized, airborne absorbing particles using a single focused laser beam. A single Gaussian beam at 532 nm is used to trap and precisely manipulate absorbing airborne particles. The fluctuation of the position of the trapped particles is substantially reduced by controlling the power of the laser beam with a position-sensitive detector and a locking circuit. Raman spectra of the position-stabilized particles or clusters are then measured with an objective and CCD spectrograph.
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    ABSTRACT: We measure the periodic circular motion of single absorbing aerosol particles that are optically trapped with a single focused Gaussian beam and rotate around the laser propagation direction. The scattered light from the trapped particle is observed to be directional and change periodically at 0.4–20 kHz. The instantaneous positions of the moving particle within a rotation period are measured by a high-speed imaging technique using a charge coupled device camera and a repetitively pulsed light-emitting diode illumination. The centripetal acceleration of the trapped particle as high as ∼20 times the gravitational acceleration is observed and is attributed to the photophoretic forces.
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