Trapping absorbing and non-absorbing aqueous aerosol particles using a universal 4-arm Laguerre-Gaussian mode light trap

Absorbing aerosols, such as brown carbon (BrC) and absorbing secondary organic aerosols (SOA), have attracted broad interest due to their importance for climate and human health. The pronounced time-dependence of light absorption during aging renders the precise estimation of their impact on global warming difficult. Single particle studies of such aerosols would be very useful to better understand their aging in the atmosphere through processes such as photochemistry. Previously proposed optical traps cannot continuously trap particles whose absorption state changes from strongly absorbing to non-absorbing or vice versa. Some of the traps presented can isolate absorbing and non-absorbing particles, but require mechanical alignment of the trap depending on the strength of particle absorption. However, mechanical realignment is not compatible with continuous trapping and observation. Here, we introduce a universal optical trap which does not require mechanical realignment. The versatility of the trap relies on four trapping beams - either vortex Laguerre-Gaussian (LG) or fundamental Gaussian beams - which are modulated with a spatial light modulator (SLM). The performance of the trap is demonstrated by trapping different types of absorbing and non-absorbing particles. We also show that the trap can be used to observe the photochemical reaction of aqueous droplets containing fulvic acid, a common component of BrC. Digital holography measurements demonstrate that the confinement of the particles in the trap can be controlled by changing the orbital angular momentum (OAM) of the LG beams. Spectroscopic measurements, such as fluorescence and Raman scattering, are shown to be possible in all configurations of the proposed trap.