Optical guiding of absorbing nanoclusters in air

Abstract

Two effects of the light-matter interaction are widely used for micromanipulation of particles with laser beams [1]: radiation pressure [2], originating from a direct transfer of momentum from photons, and electric dipole (gradient) force acting on polarized particles [3]. In gaseous media, however, the heating of absorbing particles by light leads to much stronger radiometric forces [4] considered until now only as an obstacle which prevents a stable trapping [5]. In contrast, here we show that photophoretic force [6,7] can be specifically tailored to trap and manipulate absorbing particles in open air. We demonstrate experimentally the optical guiding of clusters of carbon nanoparticles [8,9] in the form of nanofoam fragments of arbitrary shape, with the size in the range 0.1-10 micrometers, and for laser powers lower than one milli Watt. The optical trap is created by two counter-propagating "doughnut" vortex beams [10,11], and it allows simultaneous trapping of several particles as well as their stable positioning and controlled guiding along the optical axis. Only a small fraction of operating power is actually absorbed because the particles are trapped in the region of vanishing intensity at the vortex core. Thus the alteration of physical and chemical properties of airborne particles is minimal in the trap, this feature is important for experiments with aerosols [12]. Furthermore, the non-contact and remote optical trapping of absorbing aerosol particles can be applied to detect, monitor, and possibly reduce the exposure to engineered nanomaterials in air [13], a necessary tool [14] for the studies of impact of nanotechnology on health [15] and environment [16]. The photophoretic trapping can be also employed to simulate, on laboratory scales, the processes studied in atmospheric [17,18] and planetary [19,20] sciences.When a photon is absorbed by a small particle its momentum contributes towards radiation pressure while its energy dissipates in heat. The latter leads to thermal forces [4] deliberately avoided in optical tweezers [1]: an optical trap utilizing a gradient force produced by a single strongly focused laser beam [3]. The applications of optical tweezers range from trapping of colloidal particles [21,22] and living cells [23] to manipulation of single molecules [24] and atoms [25]. Yet these applications are limited by the condition of vanishing radiometric forces and exclude, for example, trapping of strongly absorbing particles in air [21,26]. Indeed, if the surface of an aerosol particle is nonuniformly heated by an incident light, the gas molecules rebound off the surface with different velocities creating an integrated force on the particle, this effect was discovered by Ehrenhaft [6] and termed photophoresis (PP) [4,7]. A rough comparison [27] of the radiation pressure force, F a = P/c, exerted by a beam with power P , and the PP force, F pp = P/3v, for particles with zero thermal conductivity [17], shows that for air at room temperature the later dominates by several orders of magnitude,...

Photophoretic trapping-Raman spectroscopy for single pollens and fungal spores trapped in air

“…Raman spectroscopy combined with rapid methods for trapping particles [63] is one of the promising techniques to explore. In this work, we have combined photophoretic [44,48,51,52,63] trapping and Raman spectroscopy to characterize single bioaerosol particles in air. In particular, a single light absorbing pollen or fungal spore is trapped in air by photophoretic forces, and Raman spectroscopy is used as an interrogator, to form photophoretic trapping Raman spectroscopy (PTRS).…”

“…The relative strength of these forces depends on the absorptivity of the particle, the thermal conductivity of the particle, and the particle morphology, on the pressure and thermal conductivity of the surrounding gas, and on the strength and geometry of the optical field. Previous studies found that for strongly absorbing particles such as carbon nanofoam, the photophoretic force can be orders of magnitude stronger than the radiative force [48]. We performed an initial estimation assuming homogenous spherical particles illuminated by a plane wave using the approximations and derivations in Ref.…”

“…We performed an initial estimation assuming homogenous spherical particles illuminated by a plane wave using the approximations and derivations in Ref. [45][46][47][48][49][50][51][52]. We used Eq.…”

Advanced optical characterization of micro solid immersion lens

“…Absorbing (i.e. non-transparent) particles in liquids and gaseous medium are also repelled and pushed away from intensity maxima [37][38][39] . In addition, a dark trap is more beneficial for some applications than a bright trap for a simple reason: the light may not interact with the trapped object.…”

Helicity-dependent three-dimensional optical trapping of chiral microparticles

“…This technique has been introduced to capture transparent solid glass microspheres by a pair of counterpropagating moderately focused Gaussian beams 19,20 . Counterpropagating schemes with high-order optical modes such as Laguerre-Gaussian beams have also been proposed for trapping and manipulation of absorbing particles 21,22 . Here we consider both options by using circularly polarized light beams with identical helicity, power and bell-or doughnut-shaped transverse intensity profiles.…”