Integrated photonics multi-waveguide devices for optical trapping and Raman spectroscopy: design, fabrication and performance demonstration

Abstract

We realized integrated photonics multi-waveguide devices for optical trapping and Raman spectroscopy of particles in a fluid. In these devices, multiple beams directed towards the device center lead to a local field enhancement around this center and thus counteract the effect of light concentration near the facets, which is a disadvantage of dual-waveguide traps. Thus, a trapping region is created around the center, where a single particle of a size in a wide range can be trapped and studied spectroscopically, free from the influence of surfaces. We report the design (including simulations), fabrication and performance demonstration for multi-waveguide devices, using our Si3N4 waveguiding platform as the basis. The designed ridge waveguides, optimized for trapping and Raman spectroscopy, emit narrow beams. Multiple waveguides arranged around the central microbath result from fanning out of a single input waveguide using Y-splitters. A second waveguiding layer is implemented for detection of light scattered by the trapped particle. For reliable filling of the device with sample fluid, microfluidic considerations lead to side channels of the microbath, to exploit capillary forces. The interference of the multiple beams produces an array of hot spots around the bath center, each forming a local trap. This property is clearly confirmed in the experiments and is registered in videos. We demonstrate the performance of a 2-waveguide and a 16-waveguide device, using 1 and 3 μm polystyrene beads. Study of the confined Brownian motion of the trapped beads yields experimental values of the normalized trap stiffness for the in-plane directions. The stiffness values for the 16-waveguide device are comparable to those of tightly focused Gaussian beam traps and are confirmed by our own simulations. The Raman spectra of the beads (in this work measured via an objective) show clear peaks that are characteristic of polystyrene. In the low-wavenumber range, the spectra have a background that most likely originates from the Si3N4 waveguides.

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“…1, FAU; 2, excitation-waveguide circuitry; 3, microfluidic bath with the central trapping region; 4, detection-waveguide circuitry; 5, light from the trap that is coupled out by the detection waveguides. Here, the detection waveguides collect light as a result of direct illumination and scattering (from Loozen et al(53)). (B) Device type 2: Spiral waveguide with the Raman pump light traveling inside the waveguide.…”

Optimisation and scaling effect of dual-waveguide optical trapping in the SOI platform

“…Later on, they experimentally demonstrated dual-waveguide optical traps based on a composite SiO 2 -Si 3 N 4 waveguide structure called the TripleX platform [18][19][20], on which they successfully trapped polystyrene beads and realised on-chip single-beam Raman spectroscopy. Recently, They also proposed and demonstrated a multi-waveguides device for trapping and Raman spectroscopy [21]. Olav Gaute Hellesø demonstrated similar dualwaveguide optical traps using waveguide loops made of 150-180nm tantalum pentoxide (Ta 2 O 5 ) on oxidised silicon substrates, where polystyrene microspheres and red blood cells were propelled along the waveguides and stably trapped in the gap area [22,23].…”