IntroductionThe use of photonic crystals as biosensors has gained considerable attention in recent years due to their compact size and attractive sensing characteristics [1-3]. Photonic crystal slabs (PCS) incorporate these valuable properties with the ability to couple light from free-space radiation modes into guided resonances by vertical light coupling . PCS-based sensors operate by detecting minute changes in the index of refraction of a surrounding medium by observing shifts in guided resonance frequencies. In the context of biosensing, a PCS integrated in a microfluidic flow channel may be functionalized with biorecognition (capture) molecules to detect small changes in index of refraction incurred by bound analytes, eliminating the need for external labels or "tags". Label-free approaches allow for rapid real-time detection, while minimizing sample preparation and potential interference with analytes.
We present a biosensor composed of a 250 nm thick silicon nitride (SiNx) PCS above a 2.36 µm layer of silicon dioxide (SiO2) on a silicon substrate, as shown in Fig. 1. SiNx is suitable as a PCS because of its low optical losses and compatibility with conventional clean room fabrication techniques.
Fig. 1. a) An SEM image of the PCS biosensor design in a 200 µm diameter mesa. b) Our PCS design integrated with a polydimethylsiloxane (PDMS) microfluidic channel.
Fig. 2. Photonic crystal biosensor functionalized with enzyme. Color difference (red, green, blue) due variation in the lattice parameters of each mesa.
Table 1 presents the quality factor, Q = ?/??, and S values of the resonances in comparison to simulations conducted using the transfer matrix method. Due to reduced scattering from the hole array, TM-like resonances will have a higher Q than TE-like resonances. We limited our designs to have simulated Q < 104 and have demonstrated values above 600 in experiment. In general, fabricated devices have a lower Q compared with theory.
The index of refraction of de-ionized water varies in literature between 1.31 to 1.33, therefore we calculate our S values based on the well calibrated Cargille liquids. Importantly, we note the S values are largely unaffected by the fabrication errors that reduce the Q values. This attribute is very important for achieving high biosensing sensitivity. Taking a conservative estimate of detecting shifts of ??/2 for TM-like peaks, the minimum detectable index of refraction change is ?n ? 3.25×10-3.
Our results show that TE-like and TM-like resonances have independent values for Q and S. Additional simulations we have conducted establish that TM-like resonances can be tuned to have a much higher Q and S for designs involving a suspended PCS structure.
In conclusion, we have fabricated PCS optofluidic biosensors as large as 500 µm and evaluated guided resonance mode properties and sensitivity values for TE-like and TM-like cases. The measured sensitivity and peak locations are in good agreement with simulation predictions. Currently, we are working towards demonstrating sensing with biological agents.