Interaction of guided light in rib polymer waveguides with dielectrophoretically controlled nanoparticles

Abstract

This work demonstrates an optofluidic system, where dielectrophoretically controlled suspended nanoparticles are used to manipulate the properties of an optical waveguide. This optofluidic device is composed of a multimode polymeric rib waveguide and a microfluidic channel as its upper cladding. This channel integrates dielectrophoretic (DEP) microelectrodes and is infiltrated with suspended silica and tungsten trioxide nanoparticles. By applying electrical signals with various intensities and frequencies to the DEP microelectrodes, the nanoparticles can be concentrated close to the waveguide surface significantly altering the optical properties in this region. Depending on the particle refractive indices, concentrations, positions and dimensions, the light remains confined or is scattered into the surrounding media in the microfluidic channel.

Appendices

Appendix 1

Samples of the particle suspensions were placed on glass slides and dehydrated. The samples were then viewed using an FEI Nova SEM (Fig. 8a, b). The SEM images help to provide confirmation of the particles shape and dimensions.

Fig. 8

SEM image of a 450 nm diameter SiO₂ nanoparticles, b 80 nm diameter WO₃ nanoparticles

Appendix 2

SU-8 epoxy was chosen as the waveguide core material due to its relatively high refractive index and low loss over a wide visible wavelength range (Jiang et al. 2008). The waveguide was fabricated using SU-8 2002 (MicroChem) as the core and KMPR 1035 (MicroChem) as the lower cladding using nanoimprint lithography techniques (Kostovski et al. 2009).

First a layer of KMPR 1035 was spin coated at 3000 r/min for 30 s to form a film of thickness of 35 μm (Fig. 9a). The layer was then soft baked at 100°C for 14 min on a horizontally levelled hotplate. The KMPR layer was then exposed to UV light (Karl Suss MA6) with an exposure dose of 700 mJ/cm² and cross-linked during the successive post-exposure bake (PEB) for 3 min at 100°C (Fig. 9b). Following this, a 50-nm adhesion layer of chromium (Cr) followed by a 150-nm conductive film of gold (Au) was deposited by electron beam evaporation.

Fig. 9

System fabrication process

The DEP microelectrodes were then patterned from the gold film (Fig. 9c). This was achieved by spin coating AZ1512 (Microchemicals) with a rotational speed of 3000 r/min followed by soft baking at 90°C for 20 min (Fig. 9d). The photoresist was then exposed with a chrome mask containing patterns of the curved DEP electrodes (Fig. 9e). The photoresist was developed in AZ 1:4 developer for 20 s. The metal layers of Au and Cr were etched using wet chemical etching processes. Au was etched using Aqua Regia (HNO₃ + 3HCl) and Cr with Cr etchant (CH₃COOH + Ce(NH₄) 2(NO₃)₆ + H₂O) for 15 and 110 s, respectively. The photoresist was then removed using acetone, IPA, and DI water (Fig. 9f).

The core waveguide layer was fabricated by spin coating the SU-8 2002 photoresist at 3000 r/min to form a 2-μm film. This was deposited over the patterned microelectrodes (Fig. 9g). The waveguide cast master was fabricated using a diluted version of SU-8 photoresist spun to a thickness of 350 nm and patterned using a 5-μm width waveguide chrome mask. This master was then coated with a thick layer of polydimethylsiloxane (PDMS) enclosed in a shim and was cured to form a nano-imprint lithography stamp (Kostovski et al. 2009). This PDMS stamp was used to imprint the raised rib waveguides onto the sample forming a raised rib replica of the waveguide master. The imprinted sample was then soft baked whilst in contact with the PDMS stamp for 20 min at 65°C, avoiding higher temperatures to prevent thermal distortion of the PDMS stamp. The imprinted SU-8 was then exposed using ultra-violet (UV) light with a dose of 350 mJ/cm² (Fig. 9h). Post-exposure, the PDMS stamp was removed and the imprinted waveguide was cured by baking at 65°C for 1 min followed by 90°C for 10 min. The sample was then diced using a dicing machine (DAD420) to obtain optically smooth edges (Fig. 9i). The platform including optical waveguides and DEP electrodes was then integrated a 500 μm × 12 mm × 100 μm (w × l × h) microfluidic channel patterned in polydimethylsiloxane (PDMS) which was fabricated using soft lithography techniques (McDonald et al. 2000).

A cast master for the microfluidic channel was then made using SU-8 2050 (Microchem) and spun at 1500 r/min for 30 s to obtain a 100-μm feature height. The master was then soft baked on a hotplate at 65°C for 3 min followed by 95°C for 10 min. Next the wafer master was exposed to UV with a dose of 200 mJ/cm² using a film mask and post-exposure baked at 65°C for 2 min followed by 95°C for 7 min. A 1:10 mixture of PDMS curing agent:elastomer (Sylgard 184) was mixed, degassed, and poured into microfluidic shims and cured at 75°C for 2 h in an oven. Prior to attachment to the microscope slide, the microfluidic reservoirs were punched with 0.75-mm holes and the channel surface was plasma treated for 2 min to enhance adhesion.