Femtosecond laser fabrication of optical sensors integrated in a lab-on-a-chip

Lab-on-a-chips (LOCs) are becoming one of the most powerful tools of analytical chemistry, with a broad application in life sciences, biotechnology and drug development. They incorporate in a single substrate the functions of a biological laboratory (i.e. microfluidic channels, reservoirs, valves, pumps and sensors). Their main advantages include the possibility of working with small sample quantities (from nano- to picoliters), high sensitivity, speed of analysis and the possibility of measurement automation and standardization. Direct on-chip integration of photonic devices for detection of biomolecules flowing in the microchannels is one of the main objectives of current research in this field, which becomes particularly challenging in case of analytes that cannot be chemically labeled. In fact in this case an interferometric detection is often needed and optical devices, such as interferometers, have to be integrated. Ultrafast laser writing of waveguides in glasses proves to be a very flexible, simple and well suited method also for this kind of applications. As a post-processing technique it doesn't affect the fabrication of the fluidic part and its unique three-dimensional capabilities allow realization of devices with complex design. In this work we report on the use of femtosecond laser pulses to fabricate a Mach-Zehnder interferometer, integrated with a microfluidic channel; this provides label-free sensing, by means of refractive index measurements, of samples flowing in the microchannel.