Optical sensor systems for biological and medical applications have been widely developed in order to satisfy the current
requirements such as a miniaturization, cost reduction, label-free detection and fast response. Here, we demonstrate a
highly sensitive optical sensor based on two cascaded microring resonators (MRRs) exploiting the Vernier effect. The
architecture consists of a filter MRR connected to a sensor MRR via a common waveguide. The external medium of the
filter MRR is isolated with a top cladding layer, while the sensor MRR interacts with the analyte sample via an opening.
The sensor chip, that includes an array of five cascaded MRRs, was designed and fabricated on a silicon nitride platform.
A first test has been performed with sodium chloride (NaCl) concentrations in deionized (DI) water providing a
sensitivity of 1.03 nm/% (6317 nm/RIU). A limit of detection of 3.16 x 10<sup>-6</sup> RIU was demonstrated for the current
sensor, respectively. Several concentrations of isopropanol in ethanol ranging from 0% to 10% were also investigated.
These preliminary measurements show a sensitivity as high as 0.95 nm/% at ~1535 nm compared to 0.02 nm/% from a
single sensor MRR. For a moderated alignment between the chip and cleaved optical fibers, tapered grating couplers are
included at the ends of waveguides. Hence, by combining the Vernier effect and the silicon nitride material, cascaded
MRRs will be a powerful optical configuration for biosensing applications in a wide operating wavelength range.
The progress in bioanalytics caused a growing demand of innovation in reliable, miniaturized and low cost optical sensor systems based on integrated optical devices. We present a detailed analysis of sensor elements for applications in aqueous solution based on two cascaded microring resonators (MRRs) by using the Vernier effect (VE). This approach is beneficial for ultra-high sensitivity at large fabrication tolerances, aspects of crucial importance for the practical detection of biomolecules such as peptides. The architecture consists of two silicon nitride microrings connected via a bus waveguide. The free spectral range (FSR) of individual rings is slightly different in order to achieve VE. Thereby the external refractive index of the reference ring is fixed; the second one varies due to the presence of the analyte. The precise operation is controlled by using spectral tuning via integrated micro-heaters. Theoretical analysis has been performed for different structural parameters. A sensitivity several orders of magnitude higher than in the case of a single ring can be predicted for TE and TM polarization, respectively. The first design of Vernier devices and its experimental characterization will be presented. The devices include tapered grating couplers in order to couple light between fibers and chip at moderate alignment tolerances in a reliable manner. Therefore, by combining the VE and the spectral tuning, cascaded MRRs are an optical configuration very promising for sensing applications.
We have recently demonstrated a particularly economic approach to analyze large arrays of microring resonator
(MRR) sensor elements coupled to a single bus waveguide. The sensor elements can be individually functionalized
to specifically promote the accumulation of target molecules. The binding of target molecules to the surface of a
particular MRR results in an increase of its resonance wavelengths which can be measured with high accuracy.
In order to measure the response of the individual MRR from an array to external stimuli, we employ a special
frequency modulation scheme in which each MRR is independently modulated and phase sensitive lock-in detection
is used to filter the respective frequency component from the superimposed complex transmission spectrum
of the bus waveguide. We fabricated test arrays comprising up to 12 MRR coupled to a single bus waveguide.
A silicon nitride based material system was chosen to realize the devices. Each element of an array is equipped
with a platinum heater electrode for thermo-optical modulation. A tunable laser system was used for optical
characterization and a clear readout of the individual MRR resonance frequencies was possible by employing the
modulation scheme above. Furthermore, we demonstrated a bulk refractive index sensitivity of 190 nm/RIU for
a frequency modulated MRR.
With our first results, we point out the large potential for multiplexed label-free detection of diverse bio molecular
compounds. Due to the miniaturization of the multisensor arrays the realization of portable sensor systems will
In this paper we demonstrate the ability to analyze a multiple of miniaturized bus-integrated sensor elements at
high sensitivity from the superimposed complex overall spectrum by individual frequency modulation of optical
microring resonators (MRR) fed by a single bus waveguide. The diverse sensor elements can be coated with
biochemically selective adlayers (e.g., antibody molecules) to specifically promote the accumulation of target
molecules on the MRR surface. Adhesion of target molecules results in an increase of the MRR resonance
frequencies which can be measured at high sensitivity with a picometer accuracy. Readout of each MRR from
the complex overall spectrum is performed by using phase sensitive lock-in detection to filter out the individual
and selective response to external stimuli. We fabricated test arrays with 12 MRR elements based on silicon
nitride material, each element integrated with a heater electrode for thermo-optical modulation of the MRR. A
clear readout of the individual MRR by using a tuneable laser source is accomplished in a simple and reliable
manner via lock-in detection despite strong overlap of the individual resonances. With our first results, we point
out the large potential for multiplexed label-free detection of diverse biomolecular compounds.
A microring resonator is used as a photonic sensor device for the detection of the explosive trinitrotoluene (TNT).
Selectivity is achieved by coating the sensor chip with specially designed receptor molecules. The measurand is the shift
in resonance frequency of the microring resonator induced by the change in effective index of refraction of the
waveguide materials due to adsorption/intercalation of the analyte. The response is linear with concentration and
reversible, i.e. the TNT molecules desorb from the sensor surface when it is flushed with carrier gas. This enables online
measurements since the sensor can be used again after flushing and no sampling is needed. Insensitivity to other
substances is demonstrated. Some chemically similar molecules induce a shift also, but the sensitivity is much lower.
The sensing limit for TNT is determined to be 0.5ppb. Simultaneous operation of two ring resonators is demonstrated,
proving the capability of a multi species monitoring when the rings are coated with different receptor molecules.