Optical trapping and coherent anti-Stokes Raman scattering (CARS) spectroscopy of submicron-size particles

Optical trapping combined with coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time as a new technique for the chemical analysis of individual particles over an extended period of time with high temporal resolution. Single submicron-size particles suspended in aqueous media are optically trapped and immobilized using two tightly focused collinear laser beams from two pulsed Ti:Sapphire laser sources. The particles can remain stably trapped at the focus for many tens of minutes. The same lasers generate a CARS vibrational signal from the molecular bonds in the trapped particle when the laser frequencies are tuned to a vibrational mode of interest, providing chemical information about the sample. The technique is characterized using single polystyrene beads and unilamellar phospholipid vesicles as test samples and can be extended to the study of living biological samples. This novel method could potentially be used to monitor rapid dynamics of biological processes in single particles on short time scales that cannot be achieved by using other vibrational spectroscopy techniques.