Single-SLM 3D interactive micromanipulation based on the generalized phase contrast (GPC) approach

Abstract

ABSTRACT Using a single phase-only spatial light modulator (SLM), we present a compact GPC-based optical trapping system for interactively manipulating microscopic particles in three dimens ions (3D) and in real-time. We employ only one GPC 4 f setup, which transforms 2D phase into intensity patterns, and utilize the SLM to form two phase-encoding regions defined by two equally sized apertures – one centered at x = x 0 and the other at x = – x 0 (with the optical axis centered at x = 0). Reconfigurable intensity patterns associated with the two independently addressable SLM-apertures are relayed to the sample volume to form a dynamic array of counterpropagating-beam traps. We discuss the experimental demonstrations showing 3D trapping of micropartic les using the presented optical setup. Keywords: Optical trapping and manipulation, counterpropagating beams, generalized phase contrast, spatial light modulator 1. INTRODUCTION In the mesoscopic regime (length scales of a few nanometers to a few micrometers), very small forces due to light-matter interaction are nevertheless strong enough to significantly in fluence the motion of tiny particles. Arthur Ashkin was the first to experimentally demonstrate the use of optical forces to accelerate and even trap microscopic objects [1]. Ashkin’s pioneering experiments in 1970 triggered a lot of interest and so the field of optical trapping was born. Now a great deal of progress has been achieved in optical trapping, both in its applications and technique development [2, 3]. For example, optical trapping and manipulation of a plurality of microparticles, which opens up promising themes of studies for many interdisciplinary fields including those of biological and medical relevance, are now viable using reconfigurable patterns of optical fields [4-7]. With the advent of computer-addressable spatial light modulators (SLM), the reconfigurability of the light patterns that act as confining optical potential landscapes is made even more feasible with a great degree of user-control. The generalized phase contrast (GPC) method provides for an optically efficient means of producing arbitrary intensity patterns [8, 9]. Recently, GPC has been utilized in developing a multiple-beam three-dimensional (3D) trapping system for user-interactive micromanipulation of a plurality of particles and microorganisms in real-time [4, 5]. In these previous GPC-trapping architectures, multiple counterpropagating-beam (CB) traps are created using two addressable devices: a spatial light modulator (SLM), which provides a reconfigurable input phase pattern for the GPC system (for xy position control of traps), and a spatial polarization modulat or (SPM), which controls the power ratio in each CB trap and thereby adjusts the axial position of each particle. In this paper we demonstrate a GPC-trapping system that utilizes just one SLM to generate CB traps with adjustable power ratios. We employ a single GPC imaging setup for transforming phase into intensity patterns while the SLM provides two off-axis phase-encoding regions illuminated by tw o equally sized circular beams. Reconfigurable intensity