Nimi Gopalakrishnan

UV patterned nanoporous solid-liquid core waveguides

Nanoporous Solid-Liquid core waveguides were prepared by UV induced surface modification of hydrophobic nanoporous polymers. With this method, the index contrast (deltan = 0.20) is a result of selective water infiltration. The waveguide core is defined by UV light, rendering the exposed part of a nanoporous polymer block hydrophilic. A propagation loss of 0.62 dB/mm and a bend loss of 0.81 dB/90 degrees for bend radius as low as 1.75 mm was obtained in these multimode waveguides.

Photolithographic fabrication of solid–liquid core waveguides by thiol-ene chemistry

In this work we demonstrate an efficient and cleanroom compatible method for the fabrication of solid–liquid core waveguides based on nanoporous polymers. We have used thiol-ene photo-grafting to tune and pattern the hydrophilicity of an originally hydrophobic nanoporous 1, 2-polybutadiene. The generated refractive index contrast between the patterned water-filled volume and the surrounding empty hydrophobic porous polymer allows for light confinement within the water-filled volume—the solid–liquid core. The presented fabrication process is simple and fast. It allows a high degree of flexibility on the type and grade of surface chemistry imparted to the large nanoporous area depending upon the application. The fabrication does not need demanding chemical reaction conditions. Thus, it can be readily used on a standard silicon lithography bench. The propagation loss values reported in this work are comparable with literature values for state-of-the-art liquid-core waveguide devices. The demonstrated waveguide function added to the nanoporous polymer with a very high internal surface area makes the system interesting for many applications in different areas, such as diagnostics and bio-chemical sensing.

Nanofiltering via integrated liquid core waveguides

We demonstrate and describe how nanoporous liquid core waveguides can exclude scattering particles, making them an ideal integrated platform for analysis of turbid solutions. Milk with 0.5% fat showed an optical propagation loss of 0.05 dB/mm at 633 nm in nanoporous waveguides compared to the 10.6 dB/mm loss in standard cuvette measurements. To examine the nanofiltering effect, waveguides were infiltrated with solutions containing Rhodamine B molecules (1 nm) and 22 nm red fluorescing polystyrene beads. With fluorescence spectroscopy we show that 22 nm beads are excluded, while Rhodamine B molecules penetrate the waveguides. This is further confirmed by fluorescence microscopy, also revealing a homogenous distribution of Rhodamine in the waveguide volume.

Di-block co-polymer derived nanoporous polymer liquid core waveguides

Nanoporous liquid core waveguides are fabricated by selectively UV modifying a nanoporous polymer. The starting point is a diblock polymer where 1,2-polybutadiene (PB) molecules are bound to PDMS. When the PB is cross linked it self-assembles into PB with a network of 14 nm diameter PDMS filled pores. When the PDMS is etched, the hydrophobic PB is left with a porosity of 44%. The polymer is subsequently UV exposed through a shadow mask. This renders the exposed part hydrophilic, making it possible for water to infiltrate these areas. Water infiltration raises the refractive index, thus forming a liquid core waveguide. Here we present both the fabrication scheme and characterization results for the waveguides.

Liquid core waveguides by UV modification of nanoporous polymer

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 mask1. 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.

UV defined nanoporous liquid core waveguides

Nanoporous liquid core waveguides, where both core and cladding are made from the same material, are presented. The nanoporous polymer used is intrinsically hydrophobic, but selective UV exposure enables it to infiltrate with an aqueous solution, thus raising the refractive index from 1.26 to 1.42. The waveguides are promising for integrated optofluidic sensor systems, where a long optical interaction length can be achieved with a small fluid sample. The propagation loss of a 200×200 °m waveguide is measured to 0.62 dB/mm.