Femtosecond laser microfabrication of optical waveguides in commercial microfluidic lab-on-a-chip

One of the main challenges of lab-on-a-chip technology is the on-chip integration of photonic functionalities by manufacturing optical waveguides for sensing biomolecules flowing in the microchannels. Such integrated approach has many advantages over traditional free-space optical sensing, such as compactness and portability, enhanced sensitivity and the possibility of parallel excitation at multiple points in the channel. Femtosecond laser pulses allow the microfabrication of optical waveguides into glass substrates, by exploiting nonlinear absorption in the focal volume, to induce a local refractive index modification. It is a direct, maskless, three-dimensional fabrication technique, which is particularly suited for waveguide inscription in already formed LOCs. In this work we report on the use of femtosecond laser pulses to inscribe optical waveguides in commercial fused silica LOCs in different geometries with respect to the microfluidic channels. We use the astigmatic beam shaping technique, previously developed by our group, to obtain waveguides with circular cross-section employing a 1 kHz repetition rate amplified Ti:sapphire laser. We demonstrate single mode waveguides in the visible with propagation losses lower than 1 dB/cm. Such waveguides provide highly selective excitation of small volumes of fluorescent solutions filling the channels and integrated collection of the fluorescent signal. Application to capillary electrophoresis will be discussed.