Enhancing the Strength of an Optical Trap by Truncation

Vanessa R M Rodrigues,Swapnesh Panigrahi,Deepak Mathur,Jayashree A Dharmadhikari,Argha Mondal,Aditya K Dharmadhikari,Etienne Dague

doi:10.1371/journal.pone.0061310

Vanessa R M Rodrigues, Swapnesh Panigrahi + Show 5 more

Open Access

https://doi.org/10.1371/journal.pone.0061310

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Abstract

Optical traps (tweezers) are beginning to be used with increasing efficacy in diverse studies in the biological and biomedical sciences. We report here results of a systematic study aimed at enhancing the efficiency with which dielectric (transparent) materials can be optically trapped. Specifically, we investigate how truncation of the incident laser beam affects the strength of an optical trap in the presence of a circular aperture. Apertures of various sizes have been used by us to alter the beam radius, thereby changing the effective numerical aperture and intensity profile. We observe significant enhancement of the radial and axial trap stiffness when an aperture is used to truncate the beam compared to when no aperture was used, keeping incident laser power constant. Enhancement in trap stiffness persists even when the beam intensity profile is modulated. The possibility of applying truncation to multiple traps is explored; to this end a wire mesh is utilized to produce multiple trapping that also alters the effective numerical aperture. The use of a mesh leads to reduction in trap stiffness compared to the case when no wire mesh is used. Our findings lead to a simple-to-implement and inexpensive method of significantly enhancing optical trapping efficiency under a wide range of circumstances.

Highlights

The ability to focus a laser beam tightly, to the diffraction limit, can be effectively utilized to trap microscopic dielectric particles; this was established some decades ago in pioneering experiments conducted by Ashkin and co-workers [1]
If a is multiplied by the objective’s numerical aperture (NA), an effective NA can be assigned to the laser beam expansion; the optimal value of such an effective NA, NAopt, is the dimensionless parameter that becomes useful in comparing the performance of different objectives
In order to determine the size of the beam after 1006magnification, we introduced an aperture and replaced the quadrant photodiode (QPD) and lens L5 by a beam profiler

Summary

Introduction

The ability to focus a laser beam tightly, to the diffraction limit, can be effectively utilized to trap microscopic dielectric particles; this was established some decades ago in pioneering experiments conducted by Ashkin and co-workers [1]. For a specified laser power, the parameters that affect the trap stiffness are incident beam size and the objective’s numerical aperture (NA). We investigate and quantify how truncation of the incident laser beam in our optical trap affects its strength (trap stiffness).

Results

Conclusion