On-chip optical trapping and Raman spectroscopy using a TripleX dual-waveguide trap.

Martijn Boerkamp,Thijs Van Leest,Rene Heideman,Jacob Caro,Marcel Hoekman,Jeroen Heldens,Arne Leinse

doi:10.1364/oe.22.030528

Abstract

We present a new approach to the dual-beam geometry for on-chip optical trapping and Raman spectroscopy, using waveguides microfabricated in TripleX technology. Such waveguides are box shaped and consist of SiO2 and Si3N4, so as to provide a low index contrast with respect to the SiO2 claddings and low loss, while retaining the advantages of Si3N4. The waveguides enable both the trapping and Raman functionality with the same dual beams. Polystyrene beads of 1 μm diameter can be easily trapped with the device. In the axial direction discrete trapping positions occur, owing to the intensity pattern of the interfering beams. Trapping events are interpreted on the basis of simulated optical fields and calculated optical forces. The average transverse trap stiffness is 0.8 pN/nm/W, indicating that a strong trap is formed by the beams emitted by the waveguides. Finally, we measure Raman spectra of trapped beads for short integration times (down to 0.25 s), which is very promising for Raman spectroscopy of microbiological cells.

Highlights

Today’s strong interest in characterization of biological samples with optical methods has initiated several research directions, including classification and identification of biological cells with Raman spectroscopy [1,2]
With a dual-waveguide trap based on waveguides fabricated in TripleX technology, a novel design of the dual-beam trap, we have optically trapped polystyrene beads from an aqueous suspension flowing through an integrated fluidic channel
A transverse trapping force as high as 1.3 pN can be exerted on a bead with a power of 5.1 mW supplied to the trap

Summary

Introduction

Today’s strong interest in characterization of biological samples with optical methods has initiated several research directions, including classification and identification of biological cells with Raman spectroscopy [1,2]. A salient difference with conventional optical tweezers [11] is that for a dual-waveguide trap with coherently excited modes in either waveguide, the force in the direction parallel to the beams is oscillatory as a result of beam interference [10]. This gives rise to multiple stable trapping positions, a situation similar to that of optical lattices used to study trapped atoms [12]. With our device we successfully trap polystyrene beads and induce a clear Raman effect in the trapped beads

Design of the dual-waveguide trap and experimental setup

Optical field of the dual-waveguide trap

Trapping experiments

Force calculations and optical power considerations

Raman spectroscopy

Conclusions