Optical pulling is the attraction of objects back to the light source by the use of optically induced “negative forces”. The light-induced photophoretic force is generated by the momentum transfer between the heating particles and surrounding gas molecules and can be several orders of magnitude larger than the radiation force and gravitation force. Here, we demonstrate that micron-sized absorbing particles can be optically pulled and manipulated towards the light source over
a long distance in air with a collimated Gaussian laser beam based on a negative photophoretic force. A variety of airborne absorbing particles can be pulled by this optical pipeline to the region where they are optically trapped with
another focused laser beam and their chemical compositions are characterized with Raman spectroscopy. We found that
micron-sized particles are pulled over a meter-scale distance in air with a pulling speed of 1-10 cm/s in the optical pulling pipeline and its speed can be controlled by changing the laser intensity. When an aerosol particle is optically
trapped with a focused Gaussian beam, we measured its rotation motion around the laser propagation direction and
measured its Raman spectroscopy for chemical identification by molecular fingerprints. The centripetal acceleration of
the trapped particle as high as ~20 times the gravitational acceleration was observed. Optical pulling over large distances
with lasers in combination with Raman spectroscopy opens up potential applications for the collection and identification
of atmospheric particles.
Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped micro-particle, but is generally less effective for individual nano-sized objects in the 10-100 nm range. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap (SWOT) with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus is more stable and sensitive in measuring nanoparticles in liquid with 4-8 fold increase in the Raman signals. It can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, polystyrene beads (100 nm), SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles with a low laser power of a few milliwatts. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints.
A theory is presented to show the analytic expression of ultra-short pulses existing in a non-instantaneous-response optical fiber with complex parameters. The analytical investigations show that the explicit solitary solutions can be found in form of Jacobi elliptic functions when the imaginary parts of the parameters fulfill a linear relationship. It is found that the single Jacobi elliptic function solutions have two free parameters while hybrid Jacobi elliptic function solutions have only one free parameter.