Optical sensor based on two-dimensional photonic crystals for measuring glucose in urine

Abstract

The design and simulation of two bio-optical sensors for the noninvasive measurement of glucose levels in urine of diabetic patients are presented. While the sensors are of the same shape and structure, their quality factors (QFs), sensitivities, and sizes are different. The structure developed is based on two-dimensional (2D)-hexagonal-latticed photonic crystals containing silicon rods immersed in urine. By creating line and point defects in the structure, two resonant modes are localized in the photonic bandgap (the photonic bandgap is a frequency range corresponding to mirror action for incident light of frequency within that range). Sensor 1 design was based on localized resonant mode 1, while sensor 2 uses localized resonant mode 2. The mechanism of these sensors relies on the change of the refractive index of urine as a result of the change in glucose concentration, which in turn causes shifts in resonant modes of the sensors. For tuning in the wavelength of 1.55 μm, the scaling method is utilized. The major difference between the proposed sensors and previously reported bio-optical sensors is the determination of glucose level based on the shift in resonant wavelengths. Previous works relied on the change of the amplitude of emerging waves for the same purpose. Consequently, the sensors presented here exhibit a higher QF and operate within the wavelength window of 1.55 μm. QF calculations and sensitivity simulations for sensor 1 yielded approximate values of 3800 and 500 nm/refractive index unit (RIU), respectively. For sensor 2, QF and sensitivity were 5540 and 500 nm / RIU, respectively. The sizes of sensor 1 and sensor 2 are 14.93 × 8.8 μm2 and 15.7 × 9.47 μm2, respectively. The 2D finite difference time domain and plane wave expansion methods were relied upon in the simulations.