AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Optical Trap for Both Transparent and Absorbing Particles in Air Using a Single Shaped Laser Beam for Measuring Raman Spectra
YONG-LE PAN, Brandon Redding, Chuji Wang, Steven Hill, Joshua Santarpia, US Army Research Lab
Abstract Number: 420 Working Group: Instrumentation and Methods
Abstract There is a need for improved real-time on-line instruments for studying and monitoring aerosols, especially bio- and chemical threats. Although spectra-based technology (mass, fluorescence, laser-induced-breakdown) have been developed, Raman spectra can provide more information for better discrimination but its emission is very weak. Collecting an adequate Raman spectrum from a single micron-sized particle requires measurement times of at least seconds. Trapping the particle in a very small region then is highly desired.
Existing optical traps are designed for either absorbing or transparent particles, primarily relying on either photophoretic or radiative pressure force, respectively. Absorbing particles are trapped in the low intensity region, while transparent particles are trapped near the highest intensity spot. To our knowledge, a single optical trap capable of capturing particles of either types has not been demonstrated. However, many applications require the ability to trap particles regardless of their morphology and absorptivity. In addition, high numerical aperture (NA) optics (typically >0.9) are required to produce a strong enough gradient force to trap transparent airborne particles (e.g. laser tweezers), which complicates system integration.
We present a technique that enables trapping both absorbing and transparent particles in air using a single shaped laser beam. The optical geometry produces a low-light-intensity region for photophoretic trapping of absorbing particles while simultaneously reducing the scattering force near the focal spot to enabling radiative pressure trapping of transparent particles using relatively low NA optics (~0.55 for a particle with a refractive index of 1.5). Experiments demonstrated trapping of both absorbing and transparent particles with either spherical or spatially irregular morphologies. Raman spectra from a variety of trapped bioaerosol particles were also measured.
This general purpose optical trapping scheme could improve the versatility of laser trapping systems designed for airborne particles, and enable extensive on-line characterization of aerosols when coupled with particle interrogation techniques. This research was supported by the Defense Threat Reduction Agency (DTRA) and ARL mission funds.