Multiple trap Laguerre-Gaussian holographic optical tweezers using a multiplexed ferroelectric SLM

Abstract

A holographic optical tweezers system is constructed using time-shared multiplexing to generate multiple optical traps. Computer generated Fresnel zone plates are used to produce the required laser focuses for standard optical traps while helical zone plates are used to generate Laguerre-Gaussian (LG) laser modes that produce doughnut shaped focuses. Standard Fresnel zone plates are used for trapping non-biological matter whereas LG modes are used in trapping delicate biological matter that is susceptible to damage from excessive laser radiation. A reflective high speed ferroelectric spatial light modulator (SLM), which is used to display the zone plate images, is capable of multiplexing a maximum of 24 binary images at a refresh rate of up to 1440Hz. By programming the SLM to display one binary hologram per bit plane, a commercial 24 bit colour SLM is used to provide 24 multiplexed traps. The ferroelectric holographic system enables multiple independently movable traps using time-shared multiplexing without the need for mechanical movement within the system. The software developed to display the zone plates uses Open GL acceleration to allow fast smooth movement in real time. Open GL utilises the graphics processing unit (GPU) common on many computers today which greatly increases the frame rate of the images displayed on the SLM. Use of the apparatus is demonstrated by the trapping and manipulation of colloidal particles and yeast cells. Preliminary results indicate that the use of LG zone plates for trapping biological cells results in significantly less damage than standard Fresnel zone plates.