Optical configurations for photophoretic trap of single particles in air

Since Ashkin's pioneering work in the 1970's, optical trapping (OT) and manipulation have become an indispensable tool in diverse research fields. Today, there are multiple optical trapping schemes in use. In this article, we explore six different optical trapping schemes based on the photophoretic force (PPF). Within these schemes we explore 21 variants differing in such details as laser source, power, beam shape, and focusing optics. We evaluate and rate the trapping quality and performance of the six trapping schemes in terms of four key aspects: simplicity, robustness, flexibility, and efficiency. One of the schemes is novel: we introduce a simple, high quality scheme using a confocal design in which one trapping beam is effectively converted to two counter-propagating beams. The versatility of this new trapping scheme is demonstrated via application of the scheme to cavity ringdown spectroscopy. We hope this exploration of the diversity of PPF trapping schemes will extend applications of OT by providing researchers with information to assist in the selection of specific optical trapping schemes from the first-of-its-kind list of 21 configurations presented herein.