June 2015
Spotlight Summary by Richard Bowman
Optical trap for both transparent and absorbing particles in air using a single shaped laser beam
When light hits an airborne particle, it can impart both energy and momentum. Each of these physical mechanisms can cause the particle to move; energy absorbed by a particle can heat up one side of it, which in turn causes air molecules to bounce off it with higher velocity. The extra recoil from molecular collisions on the hot side of the particle gives rise to a force that pushes the particle away from regions of high light intensity - the "photophoretic" force. Momentum, on the other hand, is usually important when trapping transparent particles. When the particle is at the focal point of a beam, the light passes through unchanged. If it is displaced, however, the light is deflected, and the recoil from this change in momentum pushes the particle back towards the focus.
These two regimes of optical trapping have generally been quite separate areas of experimentation. Photophoretic forces are several orders of magnitude greater than those arising from light's momentum, and so traps for absorbing particles have often focused on particles many microns across, manipulating them over large distances - even up to metres. Conventional optical trapping uses light's momentum, and the requirements this places on the optics generally means that high-powered microscope objectives are required, with short working distances. Particles trapped generally range from a few microns to a few hundred nanometres; larger particles than this are usually too heavy when working in air.
Redding and Pan demonstrate a simple approach here, capable of forming a trap using either photophoretic or refractive forces. Their innovation is to use a ring-shaped beam; prepared using two conical lenses, the ring beam sends most of the light through the edges of the focussing lens to the particle, rather than putting most of the power in the centre, as usual, meaning most of the light propagates at a relatively large angle from the optical axis. Concentrating power at high angles has previously been shown to increase the axial stiffness and decrease the scattering force in a conventional trap. Here, the authors show a ring beam allows stable traps to be formed using much lower numerical aperture, meaning that an aspheric lens with a relatively long focal length and working distance can be used in place of an objective.
While transparent particles will be trapped at the focal point of the beam, the cone of light leading to the focus is hollow - this means there is a cone-shaped dark region just above the focus, which acts as a cup-shaped trap for absorbing particles via the photophoretic force. Consequently, both transparent and absorbing particles are trapped by the beam, always along the optical axis but in one case at the focal point and in the other just above it.
Trapping both kinds of particle with a single beam has potential uses in aerosol analysis, where it is desirable to capture aerosols of any nature, without having to switch the beam. Integrating optical analysis such as Raman spectroscopy would allow aerosol particles to be usefully trapped and characterised. Using the same lens for trapping and analysis would enable particles held in the trap to be analysed whether they were in the focus or just behind it, either by scanning the focus or simply by allowing slightly out of focus light to be detected. The increase in working distance and decrease of the numerical aperture required should also find myriad uses: objective lenses place serious limits on the sample geometries that can be used in optical tweezers, so being able to form stable traps with just an aspheric lens is a valuable tool.
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These two regimes of optical trapping have generally been quite separate areas of experimentation. Photophoretic forces are several orders of magnitude greater than those arising from light's momentum, and so traps for absorbing particles have often focused on particles many microns across, manipulating them over large distances - even up to metres. Conventional optical trapping uses light's momentum, and the requirements this places on the optics generally means that high-powered microscope objectives are required, with short working distances. Particles trapped generally range from a few microns to a few hundred nanometres; larger particles than this are usually too heavy when working in air.
Redding and Pan demonstrate a simple approach here, capable of forming a trap using either photophoretic or refractive forces. Their innovation is to use a ring-shaped beam; prepared using two conical lenses, the ring beam sends most of the light through the edges of the focussing lens to the particle, rather than putting most of the power in the centre, as usual, meaning most of the light propagates at a relatively large angle from the optical axis. Concentrating power at high angles has previously been shown to increase the axial stiffness and decrease the scattering force in a conventional trap. Here, the authors show a ring beam allows stable traps to be formed using much lower numerical aperture, meaning that an aspheric lens with a relatively long focal length and working distance can be used in place of an objective.
While transparent particles will be trapped at the focal point of the beam, the cone of light leading to the focus is hollow - this means there is a cone-shaped dark region just above the focus, which acts as a cup-shaped trap for absorbing particles via the photophoretic force. Consequently, both transparent and absorbing particles are trapped by the beam, always along the optical axis but in one case at the focal point and in the other just above it.
Trapping both kinds of particle with a single beam has potential uses in aerosol analysis, where it is desirable to capture aerosols of any nature, without having to switch the beam. Integrating optical analysis such as Raman spectroscopy would allow aerosol particles to be usefully trapped and characterised. Using the same lens for trapping and analysis would enable particles held in the trap to be analysed whether they were in the focus or just behind it, either by scanning the focus or simply by allowing slightly out of focus light to be detected. The increase in working distance and decrease of the numerical aperture required should also find myriad uses: objective lenses place serious limits on the sample geometries that can be used in optical tweezers, so being able to form stable traps with just an aspheric lens is a valuable tool.
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Article Information
Optical trap for both transparent and absorbing particles in air using a single shaped laser beam
Brandon Redding and Yong-Le Pan
Opt. Lett. 40(12) 2798-2801 (2015) View: Abstract | HTML | PDF