Vertical optical ring resonators fully integrated with nanophotonic waveguides on silicon-on-insulator substrates

Nanogap Enabled Trajectory Splitting and 3D Optical Coupling in Self-Assembled Microtubular Cavities

We report the generation of multiple sets of 3D confined resonant modes in a single microtube cavity owing to nanogap induced resonant trajectory splits. The optical field largely overlaps in the split resonant trajectories, enabling strong optical coupling of 3D confined resonant light. The anticrossing feature and modes changing-over were demonstrated as direct evidence of strong coupling. In such an optical coupling system, the spatial optical field distribution of 3D coupling modes was experimentally mapped under the strong coupling regime, which allows direct observation of the energy transfer process between two hybrid states. Numerical calculations based on a quasi-potential model and the mode detuning process are in excellent agreement with the experimental results. The generation of multiple sets of 3D confined resonant modes and their efficient coupling in a single microcavity are of high interest for directional coupling with a higher degree of freedom to realize on-chip integration with elevated functionalities such as multiplexing, 3D lasing, and signal processing.

Investigation of Side Wall Roughness Effect on Optical Losses in a Multimode Si3N4 Waveguide Formed on a Quartz Substrate

This article presents the results of the study of the influence of the most significant parameters of the side wall roughness of an ultra-thin silicon nitride lightguide layer of multimode integrated optical waveguides with widths of 3 and 8 microns. The choice of the waveguide width was made due to the need to provide multimode operation for telecommunication wavelengths, which is necessary to ensure high integration density. Scattering in waveguide structures was measured by optical frequency domain reflectometry (OFDR) of a backscattering reflectometer. The finite difference time domain method (FDTD) was used to study the effect of roughness parameters on optical losses in fabricated waveguides, the roughness parameters that most strongly affect optical scattering were determined, and methods of its significant reduction were specified. The prospects for implementing such structures on a quartz substrate are justified.

Progress of Waveguide Ring Resonators Used in Micro-Optical Gyroscopes

Micro-optical gyroscopes (MOGs) are a type of high-accuracy gyroscope, which have the advantages of miniaturization, low cost, and satisfactory operating power. The quality factor (Q) of the waveguide ring resonators (WRRs) is very important to the performance of MOGs. This paper reviews various MOGs using WRRs made from different materials, including silica, indium phosphide, calcium fluoride, and polymer WRRs. The different architectures of the MOGs are reviewed, such as double-ring resonator MOGs and multiple-ring resonator MOGs. Candidate high-Q WRRs for MOGs, including silicon nitride, lithium niobite, calcium fluoride, and magnesium fluoride WRRs, are also reviewed. The manufacturing process, Q, and integration density values are compared. Summarizing the advanced WRRs and calculating the shot-noise-limited sensitivity are helpful processes in selecting suitable materials to fabricate MOGs.

Broadband High-Efficiency Grating Couplers for Perfectly Vertical Fiber-to-Chip Coupling Enhanced by Fabry-Perot-like Cavity

We propose a broadband high-efficiency grating coupler for perfectly vertical fiber-to-chip coupling. The up-reflection is reduced, hence enhanced coupling efficiency is achieved with the help of a Fabry-Perot-like cavity composed of a silicon nitride reflector and the grating itself. With the theory of the Fabry-Perot cavity, the dimensional parameters of the coupler are investigated. With the optimized parameters, up-reflection in the C-band is reduced from 10.6% to 5%, resulting in an enhanced coupling efficiency of 80.3%, with a 1-dB bandwidth of 58 nm, which covers the entire C-band. The minimum feature size of the proposed structure is over 219 nm, which makes our design easy to fabricate through 248 nm deep-UV lithography, and lowers the fabrication cost. The proposed design has potential in efficient and fabrication-tolerant interfacing applications, between off-chip light sources and integrated chips that can be mass-produced.

Self-Assembled 3D Nanosplit Rings for Plasmon-Enhanced Optofluidic Sensing

Plasmonic sensors are commonly defined on two-dimensional (2D) surfaces with an enhanced electromagnetic field only near the surface, which requires precise positioning of the targeted molecules within hotspots. To address this challenge, we realize segmented nanocylinders that incorporate plasmonic (1-50 nm) gaps within three-dimensional (3D) nanostructures (nanocylinders) using electron irradiation triggered self-assembly. The 3D structures allow desired plasmonic patterns on their inner cylindrical walls forming the nanofluidic channels. The nanocylinders bridge nanoplasmonics and nanofluidics by achieving electromagnetic field enhancement and fluid confinement simultaneously. This hybrid system enables rapid diffusion of targeted species to the larger spatial hotspots in the 3D plasmonic structures, leading to enhanced interactions that contribute to a higher sensitivity. This concept has been demonstrated by characterizing an optical response of the 3D plasmonic nanostructures using surface-enhanced Raman spectroscopy (SERS), which shows enhancement over a 22 times higher intensity for hemoglobin fingerprints with nanocylinders compared to 2D nanostructures.

Efficiency Enhanced Grating Coupler for Perfectly Vertical Fiber-to-Chip Coupling

In this work, a bidirectional grating coupler for perfectly vertical coupling is proposed. The coupling efficiency is enhanced using a silicon nitride (Si₃N₄) layer above a uniform grating. In the presence of Si₃N₄ layer, the back-reflected optical power into the fiber is diminished and coupling into the waveguide is increased. Genetic algorithm (GA) is used to optimize the grating and Si₃N₄ layer simultaneously. The optimal design obtained from GA shows that the average in-plane coupling efficiency is enhanced from about 57.5% (−2.5 dB) to 68.5% (−1.65 dB), meanwhile the average back-reflection in the C band is reduced from 17.6% (−7.5 dB) to 7.4% (−11.3 dB). With the help of a backside metal mirror, the average coupling efficiency and peak coupling efficiency are further increased to 87% (−0.6 dB) and 89.4% (−0.49 dB). The minimum feature size of the designed device is 266 nm, which makes our design easy to fabricate through 193 nm deep-UV lithography and lowers the fabrication cost. In addition, the coupler proposed here shows a wide-band character with a 1-dB bandwidth of 64 nm and 3-dB bandwidth of 96 nm. Such a grating coupler design can provide an efficient and cost-effective solution for vertical fiber-to-chip optical coupling of a Wavelength Division Multiplexing (WDM) application.

Three-Dimensional Microtubular Devices for Lab-on-a-Chip Sensing Applications

The rapid advance of micro-/nanofabrication technologies opens up new opportunities for miniaturized sensing devices based on novel three-dimensional (3D) architectures. Notably, microtubular geometry exhibits natural advantages for sensing applications due to its unique properties including the hollow sensing channel, high surface-volume ratio, well-controlled shape parameters and compatibility to on-chip integration. Here the state-of-the-art sensing techniques based on microtubular devices are reviewed. The developed microtubular sensors cover microcapillaries, rolled-up nanomembranes, chemically synthesized tubular arrays, and photoresist-based tubular structures via 3D printing. Various types of microtubular sensors working in optical, electrical, and magnetic principles exhibit an extremely broad scope of sensing targets including liquids, biomolecules, micrometer-sized/nanosized objects, and gases. Moreover, they have also been applied for the detection of mechanical, acoustic, and magnetic fields as well as fluorescence signals in labeling-based analyses. At last, a comprehensive outlook of future research on microtubular sensors is discussed on pushing the detection limit, extending the functionality, and taking a step forward to a compact and integrable core module in a lab-on-a-chip analytical system for understanding fundamental biological events or performing accurate point-of-care diagnostics.

Multiplexing and tuning of a double set of resonant modes in optical microtube cavities monolithically integrated on a photonic chip

In this Letter, we experimentally demonstrate a monolithic integration of two vertically rolled-up microtube resonators (VRUMs) on polymer-based 1×5 multimode interference waveguides to achieve 3D multi-channel coupling. In this configuration, different sets of resonant modes are simultaneously excited at S-, C-, and L- telecom bands, demonstrating an on-chip multiplexing, based on a vertical-coupling configuration. Moreover, the resonant wavelength tuning and consequently the overlapping of resonant modes are accomplished via covering the integrated VRUMs by liquid. A maximum sensitivity of 330 nm/refractive index unit is achieved. The present work would be a critical step for the realization of massively parallel optofluidic sensors with higher sensitivity and flexibility for signal processing, particularly in a 3D-integrated photonic chip.

High-Q Fabry⁻Pérot Micro-Cavities for High-Sensitivity Volume Refractometry

This work reports a novel structure for a Fabry⁻Pérot micro cavity that combines the highest reported quality factor for an on-chip Fabry⁻Pérot resonator that exceeds 9800, and a very high sensitivity for an on-chip volume refractometer based on a Fabry⁻Pérot cavity that is about 1000 nm/refractive index unit (RIU). The structure consists of two cylindrical Bragg micromirrors that achieve confinement of the Gaussian beam in the plan parallel to the chip substrate, while for the perpendicular plan, external fiber rod lenses (FRLs) are placed in the optical path of the input and the output of the cavity. This novel structure overcomes number of the drawbacks presented in previous designs. The analyte is passed between the mirrors, enabling its detection from the resonance peak wavelengths of the transmission spectra. Mixtures of ethanol and deionized (DI)-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer. The design criteria are detailed and the modeling is based on Gaussian-optics equations, which depicts a scenario closer to reality than the usually used ray-optics modeling.

Label-Free Biological and Chemical Sensing Using Whispering Gallery Mode Optical Resonators: Past, Present, and Future

Sensitive and rapid label-free biological and chemical sensors are needed for a wide variety of applications including early disease diagnosis and prognosis, the monitoring of food and water quality, as well as the detection of bacteria and viruses for public health concerns and chemical threat sensing. Whispering gallery mode optical resonator based sensing is a rapidly developing field due to the high sensitivity and speed of these devices as well as their label-free nature. Here, we describe the history of whispering gallery mode optical resonator sensors, the principles behind detection, the latest developments in the fields of biological and chemical sensing, current challenges toward widespread adoption of these devices, and an outlook for the future. In addition, we evaluate the performance capabilities of these sensors across three key parameters: sensitivity, selectivity, and speed.

Optical microtube cavities monolithically integrated on photonic chips for optofluidic sensing

Microtubular optical resonators are monolithically integrated on photonic chips to demonstrate optofluidic functionality. Due to the compact subwavelength-thin tube wall and a well-defined nanogap between polymer photonic waveguides and the microtube, excellent optical coupling with extinction ratios up to 32 dB are observed in the telecommunication relevant wavelength range. For the first time, optofluidic applications of fully on-chip integrated microtubular systems are investigated both by filling the core of the microtube and by the microtube being covered by a liquid droplet. Total shifts over the full free spectral range are observed in response to the presence of the liquid medium in the vicinity of the microtube resonators. This work provides a vertical coupling scheme for optofluidic applications in monolithically integrated so-called "lab-in-a-tube" systems.

Integrative optofluidic microcavity with tubular channels and coupled waveguides via two-photon polymerization

Miniaturization of functional devices and systems demands new design and fabrication approaches for lab-on-a-chip application and optical integrative systems. By using a direct laser writing (DLW) technique based on two-photon polymerization (TPP), a highly integrative optofluidic refractometer is fabricated and demonstrated based on tubular optical microcavities coupled with waveguides. Such tubular devices can support high quality factor (Q-factor) up to 3600 via fiber taper coupling. Microtubes with various diameters and wall thicknesses are constructed with optimized writing direction and conditions. Under a liquid-in-tube sensing configuration, a maximal sensitivity of 390 nm per refractive index unit (RIU) is achieved with subwavelength wall thickness (0.5 μm), which offers a detection limit of the devices in the order of 10^-5 RIU. Such tubular microcavities with coupled waveguides underneath present excellent optofluidic sensing performance, which proves that TPP technology can integrate more functions or devices on a chip in one-step formation.

Localized Surface Plasmons Selectively Coupled to Resonant Light in Tubular Microcavities

Vertical gold nanogaps are created on microtubular cavities to explore the coupling between resonant light supported by the microcavities and surface plasmons localized at the nanogaps. Selective coupling of optical axial modes and localized surface plasmons critically depends on the exact location of the gold nanogap on the microcavities, which is conveniently achieved by rolling up specially designed thin dielectric films into three-dimensional microtube cavities. The coupling phenomenon is explained by a modified quasipotential model based on perturbation theory. Our work reveals the coupling of surface plasmon resonances localized at the nanoscale to optical resonances confined in microtubular cavities at the microscale, implying a promising strategy for the investigation of light-matter interactions.

Optofluidic Fabry-Pérot Micro-Cavities Comprising Curved Surfaces for Homogeneous Liquid Refractometry-Design, Simulation, and Experimental Performance Assessment

In the scope of miniaturized optical sensors for liquid refractometry, this work details the design, numerical simulation, and experimental characterization of a Fabry-Pérot resonator consisting of two deeply-etched silicon cylindrical mirrors with a micro-tube in between holding the liquid analyte under study. The curved surfaces of the tube and the cylindrical mirrors provide three-dimensional light confinement and enable achieving stability for the cavity illuminated by a Gaussian beam input. The resonant optofluidic cavity attains a high-quality factor (Q)-over 2800-which is necessary for a sensitive refractometer, not only by providing a sharp interference spectrum peak that enables accurate tracing of the peak wavelengths shifts, but also by providing steep side peaks, which enables detection of refractive index changes by power level variations when operating at a fixed wavelength. The latter method can achieve refractometry without the need for spectroscopy tools, provided certain criteria explained in the details are met. By experimentally measuring mixtures of acetone-toluene with different ratios, refractive index variations of 0.0005 < Δn < 0.0022 could be detected, with sensitivity as high as 5500 μW/RIU.