Photonic Crystal Slab Sensors in Microfluidics

This Ph.D. thesis discusses the design and application of so-called photonic crystal slab (PCS) sensors, with an emphasis on microfluidic integration. PCS sensors can measure the refractive index of fluids, which makes them universal in the sense that almost any change to a fluid will alter its refractive index.

The underlying phenomenon is called guidedmode resonance (GMR), which responds to changes in the refractive index of fluids only within the first few hundred nanometers from the sensor surface. PCS sensors can be fabricated entirely out of polymers, and read out using instrumentation as simple an LED and a photo diode.

This work has thus far resulted in three manuscripts and one patent, which are attached. The first manuscript, which has been submitted to Computer Physics Communications, describes an open-source algorithm, which integrates with many electromagnetics simulation tools to provide adaptive resolution.

This algorithm can routinely make GMR simulations more than 100 times faster. The second manuscript, submitted to Optics Express, describes the practical application of polymeric PCS sensors. As with any refractive index sensor, the devices are highly sensitive to temperature drift and fluctuations.

We demonstrate a facile method for real-time compensation of these thermal disturbances. The third manuscript, which has been published in Micromachines, concerns the integration of PCS sensors into a microfluidic H-filter. We show that by monitoring the refractive index (and thus concentration) gradient, the diffusion length of molecules in flow can be determined.

PCS sensors have been in development for the last fewdecades, and both readout systems and sensor substrates are commercially available. However, products on the market predominantly target high-end applications such as pharmaceutical development, where their high pricetags are justifiable. In order to lower the entry barrier of the technology, we patented what is currently being marketed as The NanoCuvette, a spectrophotometer cuvette with an embedded PCS sensor.

Along with user-friendly software, this enables the use of an existing, unmodified spectrophotometer to make refractometric measurements, such as bulk concentration measurements on non-absorbing compounds. This effectively lowers the entry barrier for PCS sensors from tens of thousands of euros to tens of euros.