Ultra-wide free spectral range, enhanced sensitivity, and removed mode splitting SOI optical ring resonator with dispersive metal nanodisks

Abstract

A refractive index sensor with a free spectral range that is unlimited by neighboring mode spacing (10 fold increase with respect to 20 nm of an unmodified ring), based on an optical silicon-on-insulator microring resonator patterned with periodically arranged set of gold nanodisks, is presented and numerically verified. It is shown that the particular periodic arrangement of nanodisks selects a single resonance from a wide set of ring resonator modes and removes mode splitting. Extraction of the waveguided electromagnetic energy into evanescent plasmonic modes enhances light-analyte interaction and increases device sensitivity to variation of refractive index up to 176 nm/RIU (about 2-fold increase compared to the unmodified ring), which is useful for sensor applications. Proof of the concept is presented by finite-difference time-domain simulations of a design readily practicable by means of modern nanotechnology.

A simple and novel method to obtain an FSR free silicon ring resonator

“…Similar with former literature, 10,11 we also widen the FSR by means of internal reflection inside the ring. However, the significant improvement lies in the choice of reflector: an all-silicon loop-MZI based reflector.…”

“…10,11,14 Both approaches have some drawbacks. For the first one, the design accuracy and fabrication tolerance needs careful attention as the resonance wavelength as well as the bandwidth of each ring need to be perfectly matched, otherwise strong resonance splitting might be induced.…”

Design of a single all-silicon ring resonator with a 150 nm free spectral range and a 100 nm tuning range around 1550 nm

“…Moreover, Bragg gratings and metal particles are accompanied with extra loss, namely radiation loss and absorption loss. As clearly shown in [13], the Q factor as well as the extinction ratio of the resonance is quite poor. Again, these methods, even though they make use of deliberate backreflection, cannot compensate for unintentional, stochastic backscattering along the circumference of the ring.…”

“…The modified power transmission P r at the resonance frequencies ω 1 or ω 2 are given in Eq. (13). Note that in Eq.…”

“…Some work from other researchers has been done to implement ring resonators with a very wide FSR [11,[13][14][15][16]. Apart from drastically reducing the bend radius, these methods can be classified in two categories: the use of the Vernier effect in multiple rings [15,16] and the use of intraring reflectors, which require complicated processing or CMOS-incompatible materials [13,14,17].…”