Tailoring the transmission of liquid-core waveguides for wavelength filtering on a chip

The combination of integrated optics and microfluidics in planar optofluidic devices carries the potential for novel compact and ultra-sensitive detection in liquid and gaseous media. Single molecule fluorescence detection sensitivity in planar beam geometry was recently demonstrated in liquid-core antiresonant reflecting optical waveguides (ARROWs) fabricated on a silicon chip. A key component of a fully integrated single-molecule sensor is the addition of an optical filtering capability to separate excitation beams from much weaker generated fluorescence or scattering signals. This capability will eventually allow for integration of the photodetector on the same chip as the optofluidic sensing part. It has been theoretically shown that the wavelength-dependent transmission of liquid-core ARROWs can be tailored to efficiently separate excitation and fluorescence. Here, we present the wavelength dependent transmission of air-core ARROW waveguides, using a highly nonlinear photonic crystal fiber to generate a broadband excitation spectrum, and the design of liquid-core ARROW waveguides with integrated filter function. The air-core waveguide loss shows pronounced wavelength dependence in good agreement with the design, demonstrating the potential of tailoring the optical properties of liquid-core waveguides to accommodate single-molecule sensing on a chip. We also present an ARROW design to produce wavelength-dependent transmission that is optimized for fluorescence resonance energy transfer (FRET) studies with high transmission at 573 nm and 668 nm, and low transmission at 546 nm.