LAUSR

From the early optical-tweezers approach, which uses a single tightly focused laser beam to levitate dielectric or absorbing micron-sized particles, to the recently developed optical traps such as the universal optical trap (UOT), which can trap particles of arbitrary chemical and physical properties in different media, optical trapping (OT) has evolved significantly over the last decades. Research in OT has been extended from single-particle control to single-particle measurements. One of the most rapid developments in OT is the combination of OT with advanced laser spectroscopic techniques to achieve on-trap single-particle studies. To date, a wide variety of single particles including carbons, dusts, metal oxides, pollens, spores, organic/inorganic droplets, etc have been stably trapped in air and characterized using Raman spectroscopy, cavity ringdown spectroscopy, light scattering, or laser-induced breakdown spectroscopy, etc. As single particles can be trapped stably in the UOT for long periods of time, temporal evolution of the chemical and physical properties of trapped particles can also be studied. Very recently, even chemical reactions of a single particle under controlled atmospheric environments have been investigated. This review updates the most recent developments in OT, with a particular emphasis on laser-spectroscopy measurements of single particles trapped in air.