Microring Cavity Research Articles

Loss reduction in electromechanically tunable microring cavities

Nanophotonic structures coupled with mechanics enable large effective index perturbation. To date, however, the relation between index tuning and induced optical loss has not been considered in detail. In this work we present an in-depth study of optical loss mechanisms in an electromechanically-tunable waveguide filter. Gradient electric forces modify the coupling between a microring optical cavity and a suspended micromechanical (MEMS) perturber resulting in a measured tuning greater than one free-spectral range (FSR) and an effective index tuning of 3×10-2. We examine various loss contributions and find, for certain conditions, a surprising reduction in loss with greater MEMS-induced mode perturbation. Modeling confirms the device behavior and loss mitigation is discussed.

Development of silicon microneedle arrays with spontaneously generated micro-cavity ring for transdermal drug delivery

Transdermal drug delivery techniques based on microneedle arrays can painlessly administer controlled amount of drugs through penetration of the epidermis. Solid microneedles arrays can be employed in this process after coating the tip with the liquid agent before insertion. In this work, we report a low cost technique for the formation of pyramidal microneedle arrays based on crystalline silicon. A combination of crystallographic etching, thermal oxidation and subsequent HNA treatment was employed. Optimization of the process parameters led to the generation of pyramidal microneedle arrays where the tip diameter can be reduced without loss of overall structural robustness. Micro-cavities with size 4–10 μm in diameter can be reproducibly generated in a ring surrounding the tip, while maintaining an otherwise smooth microneedle facet wall. Optical fluorescence imaging indicates that fluids can be exclusively trapped in these microcavities, with implications in transdermal drug delivery.

A semi-empirical integrated microring cavity approach for 2D material optical index identification at 1.55 μm

Abstract Atomically thin 2D materials such as transition metal dichalcogenides (TMDs) provide a wide range of basic building blocks with unique properties, making them ideal for heterogeneous integration with a mature chip platform for advances in optical communication technology. The control and understanding of the precise value of the optical index of these materials, however, is challenging, as the standard metrology techniques such as the millimeter-large ellipsometry is often not usable due the small lateral 2D material flake dimension. Here, we demonstrate an approach of passive tunable coupling by integrating few layers of MoTe2 onto a microring resonator connected to a waveguide bus. We find the TMD-to-ring circumference coverage length ratio required to precisely place the ring into a critical coupling condition to be about 10% as determined from the variation of spectral resonance visibility and loss as a function of TMD coverage. Using this TMD-ring heterostructure, we further demonstrate a semiempirical method to determine the index of a 2D material (n MoTe2 of 4.36+0.011i) near telecommunication-relevant wavelength. The placement, control, and optical property understanding of 2D materials with integrated photonics pave the way for further studies of active 2D material-based optoelectronics and circuits.

Cantilever-based freestanding InGaP/InGaAlP quantum wells microring lasers

Cantilever-based freestanding microring lasers were demonstrated based on InGaP/InGaAlP quantum wells emitting in the red region. The rotational symmetry of the microring cavity is broken by introducing the connected cantilevers, which induce degenerated mode splitting and single mode lasing emission. The connected cantilevers can not only provide mechanical support for freestanding microring lasers but also function as waveguides for efficient light extraction. In addition, the application of cantilever-based microring lasers had been studied for a refractive index sensor with a measured sensitivity of ∼7.3 nm per refractive index unit and flexible local strain gauges with the measured gauge factor of ∼6.33 nm per stretching unit.

All-Optical Wavelength Multicasting in Quadruple Resonance-Split Coupled Silicon Microring Cavity

We demonstrate an all-optical four-channel wavelength multicasting in a coupled silicon microring resonator system. The scheme is based on two-photon-absorption-induced free carrier dispersion in silicon. The coupled cavity facilitates resonance splitting that is utilized as individual channels for multicasting. Using the split resonances, we achieve an aggregate multicasted data rate of 48 Gbps (4 × 12 Gbps). Moreover, we also present a detailed analysis and performance of the multicasting architecture using bit error rate and eye diagram measurements.

Spectral Behaviors of Unidirectional Lasing from Various Semiconductor Square Ring Microcavities

Spectral behaviors of lasing from semiconductor square ring microcavities with structures for unidirectional laser oscillation were investigated. When a tapered structure was introduced, the lasing envelope shifted to a shorter wavelength region. Statistical estimate of the additional loss caused by the tapered structure was carried out by analyzing spectral data from many sets of cavities with various sizes. When a saw-edged structure was introduced, the unidirectional lasing functioned well but no apparent spectral shift was observed due to negligible additional loss.

Curved Nanomembrane-Based Concentric Ring Cavities for Supermode Hybridization

We report the mode interactions and resonant hybridization in nanomembrane-formed concentric dual ring cavities supporting whispering gallery mode resonances. Utilizing a rolled-up nanomembrane with subwavelength thickness as an interlayer, dual concentric microring cavities are formed by coating high-index nanomembranes on the inner and outer surfaces of the rolled-up dielectric nanomembrane. In such a hybrid cavity system, the conventional fundamental mode resonating along a single ring orbit splits into symmetric and antisymmetric modes confined by concentric dual ring orbits. Detuning of the coupled supermodes is realized by spatially resolved measurements along the cavity axial direction. A spectral anticrossing feature is observed as a clear evidence of strong coupling. Upon strong coupling, the resonant orbits of symmetric and antisymmetric modes cross over each other in the form of superwaves oscillating between the concentric rings with opposite phase. Notably, the present system provides high flexibilities in controlling the coupling strength by varying the thickness of the spacer layer and thus enables switching between strong and weak coupling regimes. Our work offers a compact and robust scheme using curved nanomembranes to realize novel cavity mode interactions for both fundamental and applied studies.

Quantum frequency conversion for multiplexed entangled states generated from micro-ring silicon chip.

Silicon-on-chip photonic circuits are among some very promising platforms for generating nonclassical photonic quantum state, because of its low loss, small footprint, and compatibility with complementary metal-oxide-semiconductor (CMOS) and telecommunications techniques. Dense wavelength division multiplexing (DWDM) is a leading technique for enhancing the transmission capacity of both classical and quantum communications. To bridge the frequency gap between silicon-chip and other quantum systems, such as quantum memories, a quantum interface is indispensable. Here, we demonstrate a quantum interface for multiplexed energy-time entanglement states, which are generated on a silicon micro-ring cavity that is based on frequency up-conversion. By switching the pump wavelength, energy-time entanglement from any channel can be selected at will after being up-converted. The high visibilities of two-photon interference over three channels after frequency up-conversion clearly prove that the entanglement is fully preserved during the quantum frequency conversion (QFC) process. Our work provides new perspectives regarding channel capacity enhancement in quantum communications and for quantum resources being transferred between two different quantum systems.

Tunable Orbital Angular Momentum Radiation from Angular-Momentum-Biased Microcavities.

Lasers and light emitters do not typically radiate fields with orbital angular momentum (OAM). Here we show that a suitable scheme of spatiotemporal modulation of a microring cavity laser can impart a synthetic angular momentum, resulting in beams with well-defined OAM. The phenomenon relies on a traveling wave modulation of the refractive index of the microring, which breaks the degeneracy of oppositely oriented whispering gallery modes. In parallel, a static structural grating on the periphery of the microring enables efficient vertical radiation. The proposed structure is inherently tunable and can also emit fields with zero net OAM while retaining toroidal energy distributions similar to the effect of an axicon lens.

Characteristics of a Vertical Metal-Insulator-Metal Microring Cavity

We propose a vertical metal-insulator-metal (MIM) plasmonic cavity that comprises top Ag and bottom InGaAsP microring structures on an Ag substrate. A full vectorial numerical simulation was used to characterize the fundamental optical properties of the transverse-magnetic-like plasmonic whispering-gallery-modes (WGMs) excited in a variety of MIM plasmonic cavities. In addition, we exploit the controllability of far-field radiations using the single MIM microring cavity and successfully demonstrate strong directional and localized vertical radiations from the plasmonic WGMs by introducing a number of nanoscale disks along the circumference of the cavity. Our plasmonic cavity can be applied as a component of nanoscale photonic integrated circuits as well as spatially localized out-of-plane optical sensors.

Front Cover: Self‐Assembled Organic Crystalline Microring Cavities with High Q‐Factors (ChemNanoMat 9/2018)

Front Cover: Self‐Assembled Organic Crystalline Microring Cavities with High Q‐Factors (ChemNanoMat 9/2018)

Optomechanical Cavities for All-Optical Photothermal Sensing

Cavity optomechanics enables strong coupling of optics and mechanics. Although remarkable progress has been made, practical applications of cavity optomechanics are only recently being realized. In this work we propose an all-optical sensing technique enabling the measurement of photothermally induced strains with ultrahigh-resolution. We demonstrate an optomechanical sensor consisting of a silicon nitride (Si3N4) microring cavity that is evanescently coupled to a suspended SiNx micromechanical (MEMS) oscillator. Experiments show that MEMS resonances are excited purely via cavity-enhanced gradient optical forces. However, small levels of absorption in the oscillator result in photothermally induced strains that shift the mechanical resonance frequencies. By measuring absorption-induced frequency shifts our technique enables high-resolution with nanostrain sensitivity corresponding to fJ-levels of absorption. As a demonstration, we perform absorption spectroscopy on the MEMS device and measure the known Si–H absorption feature of deposited silicon nitride. The unprecedented sensitivity enabled by absorption-induced frequency shifts enables entirely new sensors in fields ranging from materials and chemical sensing to bolometers and imaging arrays.

Selective Pump Focusing on Individual Laser Modes in Microcavities

We demonstrate selective pump focusing for highly isolated single-mode lasers in microdisk and microring cavities, and achieve lasing action from a microdisk cavity underneath a scattering medium. The spatial profile of the pumping light evolves by an iterative feedback process and is optimized to maximize the field overlap with a selected cavity mode. The high order of mode selectivity and high resolving power are obtained in a multimode cavity in the presence of significant modal overlaps. As a result of the adaptive optical pumping, we successfully achieve the efficient energy transfer to a microdisk underneath a random scattering medium and observe lasing action through the scattering medium. We believe that our selective pumping procedure will pave the way for the development of low-threshold, single-mode nanolasers embedded in various materials.

Self‐Assembled Organic Crystalline Microring Cavities with High Q‐Factors

AbstractOrganic crystalline microstructures are recently a target of intense research from the point of view of photonic devices such as microcavities and optical waveguides. A bottom‐up method based upon self‐assembly produces organic microstructures including oriented needles. A top‐down approach, in turn, enabled the fabrication of microstructures of different morphologies and configurations such as microrings and microdisks. In the present study, by using a bottom‐up technique we have developed organic microrings of high crystallinity with minimized crystal imperfections. We have found out that the said microrings function as resonators with high quality (Q) factors. We observe a series of sharply resolved peaks in their photoluminescence spectra and assign those peaks to whispering gallery modes formed in the microrings. Their mode numbers are precisely determined from their mode spacings with the maximum of Q‐factor being ∼2000. The number is extremely high compared with those of other organic microrings of the same size.

Enhanced response of non-Hermitian photonic systems near exceptional points

This paper theoretically and numerically studies the response characteristics of non-Hermitian resonant photonic systems operating near an exceptional point (EP), where two resonant eigenmodes coalesce. It is shown that a system near an EP can exhibit a non-Lorentzian frequency response, whose line shape and intensity strongly depend on the modal decay rate and coupling parameters for the input waves, unlike a normal Lorentzian response around a single resonance. In particular, it is shown that the peak intensity of the frequency response is inversely proportional to the fourth power of the modal decay rate and can be significantly enhanced with the aid of optical gain. The theoretical results are numerically verified by a full wave simulation of a microring cavity with gain. In addition, the effects of the nonlinear gain saturation and spontaneous emission are discussed. The response enhancement and its parametric dependence may be useful for designing and controlling the excitation of eigenmodes by external fields.

Ultra-low-threshold InGaN/GaN quantum dot micro-ring lasers

In this work, we demonstrate ultra-low-threshold, optically pumped, room-temperature lasing in GaN microdisk and micro-ring cavities containing InGaN quantum dots and fragmented quantum wells, with the lowest measured threshold at a record low of 6.2 μJ/cm2. When pump volume decreases, we observe a systematic decrease in the lasing threshold of micro-rings. The photon loss rate, γ, increases with increasing inner ring diameter, leading to a systematic decrease in the post-threshold slope efficiency, while the quality factor of the lasing mode remains largely unchanged. A careful analysis using finite-difference time-domain simulations attributes the increased γ to the loss of photons from lower-quality higher-order modes during amplified spontaneous emission.

Optimal design of a microring cavity optical modulator for efficient RF-to-optical conversion.

The efficiency of optical sideband generation with a microring resonator modulator as a function of modulator parameters is studied taking into account the photon dynamics inside the resonator. The best achievable modulation efficiency is determined for any choice of the resonator intrinsic quality factor, and analytic solutions for the optimum modulator parameters, namely the coupling coefficient and the detuning between the frequencies of the input laser light and the microring resonance, are provided. This analysis is carried out both for a narrowband RF signal, in which case the modulator is optimized for the center frequency of this signal, and for wideband signals, when high conversion efficiency over a wide range of RF frequencies is desired. The obtained results are expected to be useful coherent optical links, direct detection RF receivers, and optical wavelength converters.

Efficient wavelength conversion with low operation power in a Ta2O5-based micro-ring resonator.

The Ta2O5-based micro-ring resonator with an unloaded quality factor of 182,000 has been demonstrated to realize efficient nonlinear wavelength generation. The propagation loss of the resonator is 0.5 cm-1, and the buildup factor of the ring resonator is estimated to be ∼50. With a high buildup factor of the ring structure, the four-wave-mixing (FWM) conversion efficiency of -30 dB is achieved in the resonator with a pump power of 6mW. Based on power-dependent FWM results, the nonlinear refractive index of Ta2O5 is estimated to be 1.4×10-14 cm2/W at a wavelength of ∼1550 nm. The demonstration of an enhanced FWM process in the Ta2O5-based micro-ring cavity implies the possibility of realizing FWM-based optical parametric oscillation in a Ta2O5-based micro-ring resonator.

On Sagnac frequency splitting in a solid-state ring Raman laser.

We report on an accurate measurement of the frequency splitting of an optical rotating ring microcavity made out of calcium fluoride. By measuring the frequencies of the clockwise and counter-clockwise coherent Raman emissions confined in the cavity modes, we show that the frequency splitting is inversely proportional to the refractive index of the cavity host material. The measurement has an accuracy of 1% and unambiguously confirms the classical theoretical prediction based on special theory of relativity. This Letter also demonstrates the usefulness of the ring Raman microlaser for rotation measurements.

Optimisation of spontaneous four-wave mixing in a ring microcavity

A theory of spontaneous four-wave mixing in a ring microcavity is developed. The rate of emission of biphotons for pulsed and monochromatic pumping with allowance for the dispersion of group velocities is analytically calculated. In the first case, pulses in the form of an increasing exponential are considered, which are optimal for excitation of an individual resonator mode. The behaviour of the group velocity dispersion as a function of the width and height of the waveguide is studied for a specific case of a ring microcavity made of silicon nitride. The results of the numerical calculation are in good agreement with the experimental data. The ring microcavity is made of two types of waveguides: completely etched and half etched. It is found that the latter allow for better control over the parameters in the manufacturing process, making them more predictable.