Liquid core waveguides by UV modification of nanoporous polymer

Abstract

Liquid core waveguides are fabricated from a self-assembled nanoporous polymer, with a porosity of 40%. The high porosity results in an effective refractive index of 1.26 for visible light, i.e. below the refractive index of aqueous solutions. However, since the polymer is hydrophobic, fluids to not initially penetrate into the pores. We show that the inner surface of the pores can be rendered selectively hydrophilic by exposing them to ultraviolet light thorough a photo mask 1 . As liquids infiltrate the exposed regions and replace air, the refractive index is raised to 1.42, and thus these areas can function as liquid core waveguides. Fig. 1 illustrates this principle.The nanoporous polymer is fabricated from a diblock copolymer where 1,2-polybutadiene (PB) is covalently bound to Polydimethylsiloxane (PDMS). When cross linked at the correct temperature the polymer self-assembles into the desired gyroid morphology. The pores are subsequently formed by etching away the PDMS block 2,3 .A photo of a water infiltrated waveguide is shown in Fig. 2. Light from a He-Ne laser (632.8 nm) is launched in a fiber coupled to the chip. The transmitted light is collected with another fiber and analyzed in an optical parametric oscillator. Waveguides with similar bends but different lengths, as shown in the inset of Fig. 3, were defined to measure the propagation loss. As the main panel of Fig. 3 is showing the loss was measured to 0.62 dB/mm. With an improved exposure and casting procedure this loss is expected to be minimized further.The versatility, low loss, and simple fabrication, where waveguides are written with UV light, is very promising for application in cheap integrated optofluidic analysis systems, where small samples and large interaction lengths are required.