Slot Waveguide Ring Resonators Research Articles

On-Chip Chemical Sensing Using Slot-Waveguide-Based Ring Resonator

We demonstrate static and dynamic bulk refractive index measurement using slot-waveguide based ring resonator. A detailed simulation is performed to optimize the slot-waveguide geometry for maximum sensitivity. The on-chip measurements are performed using a slot waveguide ring resonator by applying aqueous solutions of acids and bases at different concentrations. Based on these measurements, we derive a relation of refractive index change per unit change in concentration of the liquids in water. We experimentally measured a maximum sensitivity of 476 nm/RIU, enabling a limit of detection ( LOD ) of ${1.05}\times {10}^{-{5}}$ RIU. Finally, we demonstrate dynamic refractive index on-chip measurement by diluting potassium bicarbonate with hydrogen peroxide.

Electric Field-Induced Linear Electro-Optic Effect Observed in Silicon-Organic Hybrid Ring Resonator

In this work, we report on the observation of the electric field-induced linear electro-optical effect in a silicon-based slot waveguide ring resonator covered by the nonlinear optical polymer Poly[(methyl methacrylate)-co-(Disperse Red 1 methacrylate)]. The device is fabricated in a photonic integrated circuit technology and intensity modulation is demonstrated to work in a direct-detection setup. As unique feature, the electric field-induced linear electro-optical effect allows to modulate the optical carrier wave with a time-varying electric field, while a static electric field can control its amplitude with no need for additional photonic components. Here, a linear tuning of the modulation-amplitude as function of the applied static electric field is demonstrated. This work paves a way toward novel integrated photonic device concepts.

Direct observation and simultaneous use of linear and quadratic electro-optical effects

We report on the direct observation and simultaneous use of the linear and quadratic electro-optical effect and propose a method by which higher-order susceptibilities of electro-optical materials can be determined. The evaluation is based on the separation of the second- and third-order susceptibilities and the experimental technique uses a slot waveguide ring resonator fabricated in integrated photonic circuit technology, which is embedded by a guest-host polymer system consisting of the azobenzene dye Disperse Red 1 in a poly(methyl methacrylate) matrix as an active electro-optical material. The contribution of both effects on the electro-optical response under the influence of static and time-varying electrical fields is investigated. We show that the quadratic electro-optical effect has a significant influence on the overall electro-optical response even with acentric molecular orientated molecules. Our findings have important implications for developing electro-optical devices based on polymer-filled slot waveguides and give rise to advanced photonic circuits.

Silicon Slot Waveguide Electro-Optic Kerr Effect Modulator

The theoretical background to calculate the effective index tuning using the ultrafast electro-optic Kerr effect and a design of slot waveguide geometry are presented. Analysis of the slot waveguide ring resonator with silicon nanocrystals as the active medium to implement the CMOS compatible electro-optic Kerr effect modulator indicates a modulation bandwidth of 51 GHz, therefore, allowing for 90 Gb/s data rate transmission, and a low energy consumption of 22.99 fJ/bit.

Advanced Nanophotonics: Silicon-Organic Hybrid Technology

Integrated photonic devices have gained increasing research interests. Especially silicon photonics have become very attractive for various optical applications. Using silicon-on-insulator as a material platform provides the ability to fabricate photonic devices with electronic devices on a single chip. Driven by substantial research investments, the integration of photonic devices on silicon-on-insulator substrates has reached a degree of maturity that already permits industrial adoption. However, silicon has the disadvantage of linear electro-optical effects, and, therefore, advanced modulation formats are difficult to realize when using silicon-based high-speed modulators. Hence, a new approach was proposed: the silicon-organic hybrid technology. This technology is a viable extension of the silicon-on-insulator material system for efficient high-speed modulation. We herewith present our theoretical and experimental investigations of the silicon-organic hybrid slot-waveguide ring resonator. The advanced device design is described in detail, which allows using both, the efficient silicon-on-insulator strip-waveguides and the silicon-organic hybrid slot-waveguides in single ring resonator. For the first time, we report the transmission spectra of such a resonator covered with an electro-optical polymer.

Highly sensitive refractive index sensing by fast detuning the critical coupling condition of slot waveguide ring resonators.

We experimentally investigate refractive index sensing in silicon slot waveguide ring resonators by the detection of the giant shift of the ring transmission spectrum envelope enabled by the following specific conditions: the slot waveguide cross section as well as the ring couplers have been designed to lead to a V-shaped microring resonator spectrum modulated by the classical frequency comb and exhibiting quality factor peaks of 2000-6000 around λ=1.5 μm. By tracking the spectrum envelope wavelength shift, sensitivity up to S=1,300 nm per refraction index unit (RIU) is reported when the slots are filled by liquids with refraction index values close to 1.33.

Analysis of silicon-on-insulator slot waveguide ring resonators targeting high Q-factors.

Vertical slot waveguide micro-ring resonators in silicon photonics have already been demonstrated in previous works and applied to several schemes, including sensing and hybrid nonlinear optics. Their performances, first quantified by the reachable Q-factors, are still perceived to be restrained by larger intrinsic propagation losses than those suffered by simple Si wire waveguides. In this Letter, the optical loss mechanisms of slot waveguide micro-ring resonators are thoroughly investigated with a special focus on the coupler loss contribution that turns out to be the key obstacle to achieving high Q-factors. By engineering the coupler design, slotted ring resonators with a 50 μm radius are experienced with a loaded Q-factor up to 10 times improvement from Q=3,000 to Q=30,600. The intrinsic losses due to the light propagation in the bent slot ring itself are proved to be as low as 1.32±0.87 dB/cm at λ=1,550 nm. These investigations of slot ring resonators open high performance potentials for on-chip nonlinear optical processing or sensing in hybrid silicon photonics.

Novel Ring Resonator Combining Strong Field Confinement With High Optical Quality Factor

Slot waveguide ring resonators appear promising candidates for several applications in silicon photonics. Strong field confinement, high device tunability, and low power consumption are beneficial properties compared with strip waveguides. Slot waveguide ring resonators suffer, however, from rather low optical quality factors due to optical losses. This letter proposes and experimentally demonstrates a novel concept based on a partially slotted ring and a strip-to-slot mode converter. An exceptional high quality factor of ${\sim }10^{5}$ has been measured.

Slot waveguide ring resonators coated by an atomic layer deposited organic/inorganic nanolaminate.

In this study, slot waveguide ring resonators patterned on a silicon-on-insulator (SOI) wafer and coated with an atomic layer deposited nanolaminate consisting of alternating layers of tantalum pentoxide and polyimide were fabricated and characterized. To the best of our knowledge, this is the first demonstration of atomic layer deposition (ALD) of organic materials in waveguiding applications. In our nanolaminate ring resonators, the optical power is not only confined in the narrow central air slot but also in several parallel sub-10 nm wide vertical polyimide slots. This indicates that the mode profiles in the silicon slot waveguide can be accurately tuned by the ALD method. Our results show that ALD of organic and inorganic materials can be combined with conventional silicon waveguide fabrication techniques to create slot waveguide ring resonators with varying mode profiles. This can potentially open new possibilities for various photonic applications, such as optical sensing and all-optical signal processing.

Nanophotonic Biosensor Technologies for Lab on Chip Applications—a Focus Article on Optical Biosensors from Three EC Lab on Chip Projects with a Comparison to the State of Art

This paper compares the performances of various nanophotonic biosensors developed in three recent European Lab-on-Chip collaborations: SABIO, INTOPSENS and POSITIVE. These are attractive for biosensing due to their small footprint, high Q-factors and compatibility with on-chip optics and microfluidics enabling integrated sensor arrays for compact lab-on-chip (LOC) applications. Many applications typically require the addressing of a number of issues including: improving limit of detection, managing the influence of temperature, parallelization of the measurement for higher throughput and on-chip referencing, efficient light-coupling strategies to simplify alignment, and packaging of the nanophotonics chip and integration with microfluidics. For ring resonator-based sensors, volumetric sensitivities of 246 nm/RIU and 2169 nm/RIU and limits of detection of 5 × 10−6 RIU and 8.3 × 10−6 RIU were reported from SABIO (at 1.3 μm) and INTOPSENS (at 1.5 μm), respectively. For SABIO, this was for an eight-channel Si3N4 slot-waveguide ring resonator sensor array whilst for INTOPSENS this was for an individual Si Vernier cascade sensor. In POSITIVE for porous alumina-based membrane sensors, a volumetric limit of detection (LOD) was reported at 5 × 10−6 RIU but more importantly, in contrast to the sensors from the other two projects, the standard deviation of the measured values was below 5 %, sensing response times were fast and small sample volumes could be used (<100 μl). For biosensing within SABIO, a surface limit of detection of 0.9 pg/mm2 for anti-BSA on a gluteraldehyde-covered surface was recorded corresponding to a 125 ng/ml anti-BSA solution, whilst Si slot-waveguide ring resonators have reported 2 pg/mm2 and 10 ng/ml for biotin on a streptavidin-coated surface. In contrast, in POSITIVE, for an assay of β-lactoglobulin-anti-β-lactoglobulin-anti-rabbit-IgG-streptavidin-conjugated CdSe quantum dots, a noise floor for individual measurements of 3.7 ng/ml (25 pM) was obtained, with an overall statistical, or formal assay LOD of 33.7 ng/ml (225 pM), for total assay times of under 1 h. With similar volumetric limits of detection, the sensors are still poorer than that of the state of art in nanophotonic sensors; however, the POSITIVE device compared favourably to it at least for total assay times, response times and minimum volumes of analyte necessary.

Optimized Coupler Design for Slot Waveguide Ring Resonators

The highly sensitive nature of slot waveguide micro-resonator power couplers is discussed with respect to the resonator geometry and evanescent coupler design. A modified, tri-bar, evanescent field coupler is demonstrated that shows reduced sensitivity to ring radius as compared with conventional couplers, while maintaining a compact beat-length of only a few micrometers. With the measurement of bending losses of curved slot waveguides, critically coupled ring resonators were designed and fabricated, exhibiting a resonance extinction exceeding 22 dB.

Efficient Poling of Electro‐Optic Polymers in Thin Films and Silicon Slot Waveguides by Detachable Pyroelectric Crystals

Pyroelectric crystals are used as a conformal and detachable electric field source to efficiently pole electro-optic (E-O) polymers in both parallel-plate (transverse) and in-plane (quasi-longitudinal) configurations. Large Pockels coefficients in poled thin films and high tunability of resonance wavelength shift in hybrid polymer silicon slot waveguide ring-resonator modulators have been achieved using this method.

Advances in Nanophotonic Sensing Technologies During Three International Label-Free Lab-On-Chip Projects

We review the results from the use of various integrated nanophotonic sensors for label-free biosensing developed in three recent European biosensor collaborations: SABIO, INTOPSENS, and POSITIVE. Nanophotonic transducers are attractive for label-free biosensing due to their small footprint, high Q-factors, and compatibility with on-chip optics and microfluidics. This enables integrated sensor arrays for compact labs-on-chip. One application of label-free sensor arrays is for point-of-care medical diagnostics. Bringing such powerful tools to the single medical practitioner is an important step towards personalized medicine, but requires addressing a number of issues: improving limit of detection, managing the influence of temperature, parallelization of the measurement for higher throughput and on-chip referencing, efficient light-coupling strategies to simplify alignment, and packaging of the nanophotonics chip and integration with microfluidics. From SABIO, we report a volume sensing sensitivity of 240 nm/RIU and detection limit of 5 × 10−6 RIU, and a surface sensing limit of detection (LOD) of 0.9 pg/mm2 for at 1.3 μm for an eight-channel slot-waveguide ring resonator sensor array, within a microfluidics integrated compact cartridge. In INTOPSENS, ongoing efforts have so far resulted in various nanophotonic transducer designs with volume sensing sensitivities as great as 2,169 nm/RIU and LODs down to 8.3 × 10−6 RIU at 1.5 μm. Early experiments from the POSITIVE project have demonstrated volumetric sensitivities as high as 1,247 nm/RIU at 1.5 μm.

On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index sensor array

We present an experimental study of an integrated slot-waveguide refractive index sensor array fabricated in silicon nitride on silica. We study the temperature dependence of the slot-waveguide ring resonator sensors and find that they show a low temperature dependence of -16.6 pm/K, while at the same time a large refractive index sensitivity of 240 nm per refractive index unit. Furthermore, by using on-chip temperature referencing, a differential temperature sensitivity of only 0.3 pm/K is obtained, without individual sensor calibration. This low value indicates good sensor-to-sensor repeatability, thus enabling use in highly parallel chemical assays. We demonstrate refractive index measurements during temperature drift and show a detection limit of 8.8 x 10-6 refractive index units in a 7 K temperature operating window, without external temperature control. Finally, we suggest the possibility of athermal slot-waveguide sensor design.

A packaged optical slot-waveguide ring resonator sensor array for multiplex label-free assays in labs-on-chips

We present the design, fabrication, and characterisation of an array of optical slot-waveguide ring resonator sensors, integrated with microfluidic sample handling in a compact cartridge, for multiplexed real-time label-free biosensing. Multiplexing not only enables high throughput, but also provides reference channels for drift compensation and control experiments. Our use of alignment tolerant surface gratings to couple light into the optical chip enables quick replacement of cartridges in the read-out instrument. Furthermore, our novel use of a dual surface-energy adhesive film to bond a hard plastic shell directly to the PDMS microfluidic network allows for fast and leak-tight assembly of compact cartridges with tightly spaced fluidic interconnects. The high sensitivity of the slot-waveguide resonators, combined with on-chip referencing and physical modelling, yields a volume refractive index detection limit of 5 x 10(-6) refractive index units (RIUs) and a surface mass density detection limit of 0.9 pg mm(-2), to our knowledge the best reported values for integrated planar ring resonators.

Optical Slot-Waveguide Based Biochemical Sensors

Slot-waveguides allow light to be guided and strongly confined inside a nanometer-scale region of low refractive index. Thus stronger light-analyte interaction can be obtained as compared to that achievable by a conventional waveguide, in which the propagating beam is confined to the high-refractive-index core of the waveguide. In addition, slot-waveguides can be fabricated by employing CMOS compatible materials and technology, enabling miniaturization, integration with electronic, photonic and fluidic components in a chip, and mass production. These advantages have made the use of slot-waveguides for highly sensitive biochemical optical integrated sensors an emerging field. In this paper, recent achievements in slot-waveguide based biochemical sensing will be reviewed. These include slot-waveguide ring resonator based refractometric label-free biosensors, label-based optical sensing, and nano-opto-mechanical sensors.

Reconfiguration of microring resonators by liquid adhesion

We present a procedure to reconfigure microring resonators based on liquid surface adhesion. Droplets of organic solvents were deposited over Si3N4/SiO2 strip- and slot-waveguide ring resonators, and the transmission spectra were measured several hours after the evaporation of the droplets at room temperature. Our measurements show that the optical properties of the microrings are significantly modified by liquid adhered to the waveguides, persisting even 96 h after bulk evaporation. Liquid-solid interfacial forces slow down liquid evaporation at the nanoscale, enabling permanent photonic configurations. Rewriteability is achieved by removing the adhered liquid with heat.