Racetrack Microresonators Research Articles

Spontaneous parametric downconversion photon pair generation in small footprint X-cut periodically poled lithium niobate micro-resonator.

Spontaneous parametric downconversion (SPDC) has been crucial for producing non-classical light, such as correlated photon pairs and squeezed states, essential for optical quantum technologies. Recently, thin-film lithium niobate (TFLN)-based devices have enabled high-performance SPDC in on-chip integrated photonic platforms. Here, we developed an X-cut periodically poled lithium niobate (PPLN) racetrack micro-resonator with a compact footprint and demonstrated bright SPDC photon pair generation characterized by distinct resonances and high extraction efficiency. We separately assessed the internal and loaded photon pair generation rates, measuring them at 4.525 MHz/µW and 62.73 kHz/µW, respectively. Our platform is integrated with maturing electro-/thermo-optic TFLN circuit elements and has meaningful potential for advancing active quantum photonic applications.

Electro-optical intensity and phase modulations of racetrack micro-resonator base on LiNbO3-silicon hybrid waveguide covered by electro-optical polymers

Electro-optical intensity and phase modulations of racetrack micro-resonator base on LiNbO3-silicon hybrid waveguide covered by electro-optical polymers

Analysis on dual Fano resonance in a coupled-resonator waveguide

Dual Fano resonance was demonstrated in a compact coupling system without additional large footprint tunable devices consisting of a grating-coupled waveguide and a micro-racetrack resonator on thin film lithium niobate. A multimode interference model was proposed for the dual Fano resonance system. The inverse design method was used to realize model fitting, validate our model, and analyze our model. Based on the parameters obtained by the inverse design, we further analyzed the influence of different parameters. Our research also shows that through the interaction of two Fano resonance modes, tunable line shape and enhanced extinction ratio can be realized in the transmission spectrum, which has potential applications in optical sensing.

Low temperature PECVD processes for the fabrication of integrated symmetrical resonant UV210 organic/semiconductor structures

The resonant structures used as sensors for investigating specific dark and opaque substances may pose difficulties due to their potential aggressiveness towards photonic chips and the risk of destruction upon contact. To avoid it, this study presents a detailed description of a novel symmetrical waveguide structure with UV210 enclosed between Si/SiO2 bi-layers implemented for integrated photonics. The fabrication process is thoroughly explained, accompanied by various analyses conducted for characterizations purposes. The structure is composed of an organic UV210 material developed using deep UV lithography at 248 nm and fabricated onto an oxidized silicon layer, resulting in a Si/SiO2 bi-layer configuration. This process enables the fabrication of µm-scale access waveguides and racetrack Micro-Resonators (MRs) with a 400-nanometers gap. Several multilayer families have been produced using various low-temperature PECVD processes to obtain symmetrical Si/SiO2/UV210/Si/SiO2 structures and then properly characterized using non-destructive analyses and imaging techniques. The advantage of structural symmetry is primarily related to the electromagnetism and guidance equations, which eliminate the requirement for a cut-off thickness (or frequency), enabling significant miniaturization. Additionally, the upper cladding composed of a Si/SiO2 bi-layer offers protection against potentially aggressive substances directly in contact with the organic waveguide core and MRs. Different bi-layer upper cladding families, produced under various PECVD conditions, were characterized using XPS to understand the bonding mechanism between the silicon and UV210 organic. In addition, ellipsometric analyses were conducted to determine the real and imaginary components of the refractive index of the global structure and the upper cladding bi-layer. Structure imaging were obtained by Raman analyses and optical statistical measurements were conducted on four different types of heterostructures, with silica temperatures ranging from 120°C to 150°C during the second PECVD process. The resonance analysis, performed through Free Spectral Range (FSR) measurements, allowed us to draw conclusions regarding the stability and reproducibility of the fabrication process, as well as the impact of PECVD processes conditions on the optical measurements. Such optoelectronic resonant elements and hybrid heterostructures enable the investigation of aggressive substances in direct contact.

Octave soliton microcombs in lithium niobate microresonators.

Soliton microcombs are regarded as an ideal platform for applications such as optical communications, optical sensing, low-noise microwave sources, optical atomic clocks, and frequency synthesizers. Many of these applications require a broad comb spectrum that covers an octave, essential for implementing the f - 2f self-referencing techniques. In this work, we have successfully generated an octave-spanning soliton microcomb based on a z-cut thin-film lithium niobate (TFLN) microresonator. This achievement is realized under on-chip optical pumping at 340 mW and through extensive research into the broadening of dual dispersive waves (DWs). Furthermore, the repetition rate of the octave soliton microcomb is accurately measured using an electro-optic comb generated by an x-cut TFLN racetrack microresonator. Our results represent a crucial step toward the realization of practical, integrated, and fully stabilized soliton microcomb systems based on TFLN.

Semiclassical theory of frequency combs generated by parametric modulation of optical microresonators

An optical microresonator, which parameters are periodically modulated in time, can generate optical frequency comb (OFC) spectral resonances equally spaced by the modulation frequency. Significant recent progress in realization of OFC generators based on the modulation of microresonator parameters boosted interest to their further experimental development and theoretical understanding of underlying phenomena. However, most of theoretical approaches developed to date were based on the lumped parameter models which unable to evaluate, analyse, and optimize the effect of spatial distribution of modulation inside microresonators. Here we develop the multi-quantum semiclassical theory of parametrically excited OFCs which solves these problems. As an application, we compare OFCs which are resonantly or adiabatically excited in a racetrack microresonator (RTM) and a Surface Nanoscale Axial Photonics (SNAP) bottle microresonator (SBM). The principal difference between these two types of microresonators consists in much slower propagation speed of whispering gallery modes along the SBM axis compared to the speed of modes propagating along the RTM waveguide axis. We show that, due to this difference, similar OFCs can be generated by an SBM with a much smaller size compared to that of the RTM. Based on the developed theory, we analytically express the OFC spectrum of microresonators through the spatial distribution of modulated parameters and optimize this distribution to arrive at the strongest OFCs generated with minimum power consumption.

Chip-scale spontaneous quasi-phase matched second harmonic generation in a micro-racetrack resonator

Chip-scale spontaneous quasi-phase matched second harmonic generation in a micro-racetrack resonator

High-Q chalcogenide racetrack resonators based on the multimode waveguide.

We propose and demonstrate a highquality (Q) factor racetrack resonator based on uniform multimode waveguides in high-index contrast chalcogenide glass film. Our design features two carefully designed multimode waveguide bends based on modified Euler curves, which enable a compact 180° bend and reduce the chip footprint. A multimode straight waveguide directional coupler is utilized to couple the fundamental mode without exciting higher-order modes in the racetrack. The fabricated micro-racetrack resonator shows a record-high intrinsic Q of 1.31×106 for selenide-based devices, with a relatively low waveguide propagation loss of only 0.38dB/cm. Our proposed design has potential applications in power-efficient nonlinear photonics.

Electro-optic control of the external coupling strength of a high-quality-factor lithium niobate micro-resonator.

Controlling the optical coupling between a micro-resonator and waveguide plays a key role in on-chip photonic circuits. Here, we demonstrate a two-point coupled lithium niobate (LN) racetrack micro-resonator that enables us to electro-optically traverse a full set of the zero-, under-, critical-, and over-coupling regimes with minimized disturbance of the intrinsic properties of the resonant mode. The modulation between the zero- and critical-coupling conditions cost a resonant frequency shift of only ∼344.2 MHz and rarely changed the intrinsic quality (Q) factor of 4.6 × 105. Our device is a promising element in on-chip coherent photon storage/retrieval and its applications.

Compact, spatial-mode-interaction-free, ultralow-loss, nonlinear photonic integrated circuits

Multi-mode waveguides are ubiquitously used in integrated photonics. Although interaction among different spatial waveguide eigenmodes can induce novel nonlinear phenomena, spatial mode interaction is typically undesired. Adiabatic bends, such as Euler bends, have been favoured to suppress spatial mode interaction. Here, we adapt and optimize Euler bends to build compact racetrack microresonators based on ultralow-loss, multi-mode, silicon nitride photonic integrated circuits. The racetrack microresonators feature a footprint of only 0.21 mm2 for 19.8 GHz free spectral range, suitable for tight photonic integration. We quantitatively investigate the suppression of spatial mode interaction in the racetrack microresonators with Euler bends. We show that the low optical loss rate (15.5 MHz) is preserved, on par with the mode interaction strength (25 MHz). This results in an unperturbed microresonator dispersion profile. We further generate a single dissipative Kerr soliton of 19.8 GHz repetition rate without complex laser tuning schemes or auxiliary lasers. The optimized Euler bends and racetrack microresonators can be building blocks for integrated nonlinear photonic systems, as well as linear circuits for programmable processors or photonic quantum computing.

Influence of the bus waveguide on the linear and nonlinear response of a taiji microresonator.

We study the linear and nonlinear response of a unidirectional reflector where a nonlinear breaking of the Lorentz reciprocity is observed. The device under test consists of a racetrack microresonator, with an embedded S-shaped waveguide, coupled to an external bus waveguide (BW). This geometry of the microresonator is known as "taiji" microresonator (TJMR). Here, we show that a full description of the device needs to consider also the role of the BW, which introduces (i) Fabry-Perot oscillations (FPOs) due to reflections at its facets, and (ii) asymmetric losses, which depend on the actual position of the TJMR. At sufficiently low powers the asymmetric loss does not affect the unidirectional behavior, but the FP interference fringes can cancel the effect of the S-shaped waveguide. However, at high input power, both the asymmetric loss and the FPOs contribute to the redistribution of energy between counterpropagating modes within the TJMR. This strongly modifies the nonlinear response, giving rise to counter-intuitive features where, due to the FP effect and the asymmetric losses, the BW properties can determine the violation of the Lorentz reciprocity and, in particular, the difference between the transmittance in the two directions of excitation.

Sensitivity and intrinsic limit of detection improvement in a photonic refractive-index sensor

In this work, we study the bulk refractive index (RI) sensitivity (S) and intrinsic limit of detection (ILOD) of a micro racetrack resonator (MRR) based RI-sensor. Sensing measurements are performed with solutions of sodium chloride (NaCl)-water. The sensing device includes an array of multiple silicon-oxy-nitride (SiON) MRRs, fibre-coupled to a vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm. We particularly choose 850 nm for sensing measurement, since the water absorption coefficient at this wavelength is three orders of magnitude lower than at 1550 nm (another conventional wavelength in biosensing). Different concentrations of NaCl–water solutions (spanning from 0.0% to 1.8% weight/weight), are flowed on top of the MRR surface in the cladding layer. Measuring of whispering gallery mode (WGM) resonance wavelength shifts in NaCl–water solutions, confirming a S and ILOD of 31.66 nm/RIU and 5.98×10−4 RIU, respectively. Furthermore, with exact engineering of the physical parameters of the system, the best S and ILOD of 325.85 nm/RIU and 6.14×10−5 RIU are found, respectively. The ILOD parameter delineates the minimum amount of RI or correspondingly minimum concentration of a biological solution, which can be detected by a RI-sensor. In this regard, this device can find applications in the exact measurement of NaCl in real biological sample solutions.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform.

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.

Integrated silicon-based suspended racetrack micro-resonator for biological solution sensing with high-order mode* *Project supported by the National Natural Science Foundation of China (Grant No. 62075057) and Ph. D. Program of Henan Normal University, China (Grant Nos. 5101239170010 and gd17167).

A biological sensing structure with a high-order mode () is designed, which is composed of a suspended racetrack micro-resonator (SRTMR) and a microfluidic channel. The mode characteristics, coupling properties, and sensing performances are simulated by using the finite element method (FEM). To analyze the mode confinement property, the confinement factors in the core and cladding of the suspended waveguide for the , , and are calculated. The simulation results show that the refractive index (RI) sensitivity of the proposed sensing structure can be improved by using the high-order mode (). The RI sensitivity for the mode is ∼ 201 nm/RIU, which is twice to thrice higher than those for the mode and the mode. Considering a commercial spectrometer, the proposed sensing structure based on the SRTMR achieves a limit of detection (LOD) of ∼ 4.7 × 10−6 RIU. Combined with the microfluidic channel, the SRTMR can possess wide applications in the clinical diagnostic assays and biochemical detections.

Design and Characterization of Q-Enhanced Silicon Nitride Racetrack Micro-Resonators

Q-enhanced racetrack micro-resonators for the silicon nitride photonics integration platform have been designed, fabricated, and characterized. The proposed geometries permit to mitigate the impact of radiation loss at curved waveguides, one of the major limitations of silicon nitride circuits, therefore providing an increase of the intrinsic Q factor of micro-resonators when compared with the conventional structures with the same bent radii. The schemes put forward in this work permit a reduction of the size of the devices that has a direct impact on the integration scale in this platform. When used in the curved sections of waveguides routing optical signals within an integrated photonic circuit, the lowering of the radiation loss also permits the use of smaller bent radii likewise contributing to the circuit integration density.

High-quality-factor optical microresonators fabricated on lithium niobate thin film with an electro-optical tuning range spanning over one free spectral range [Invited

We demonstrate high-quality (intrinsic Q factor ∼2.8 × 106) racetrack microresonators fabricated on lithium niobate thin film with a free spectral range (FSR) of ∼86 pm. By integrating microelectrodes alongside the two straight arms of the racetrack resonator, the resonance wavelength around 1550 nm can be red shifted by 92 pm when the electric voltage is raised from −100 V to 100 V. The microresonators with the tuning range spanning over a full FSR are fabricated using photolithography assisted chemo-mechanical etching.

High spectro-temporal purity single-photons from silicon micro-racetrack resonators using a dual-pulse configuration.

Single-photons with high spectro-temporal purity are an essential resource for quantum photonic technologies. The highest reported purity up until now from a conventional silicon photonic device is 92% without any spectral filtering. We have experimentally generated and observed single-photons with 98.0±0.3% spectro-temporal purity, an upper bound of the stimulated emission tomography, using a conventional micro-racetrack resonator and an engineered dual pump pulse.

On-chip scalable mode-selective converter based on asymmetrical micro-racetrack resonators

Abstract Mode division multiplexing (MDM) technology has been well known to researchers for its ability to increase the link capacity of photonic network. While various mode processing devices were demonstrated in recent years, the reconfigurability of multi-mode processing devices, which is vital for large-scale multi-functional networks, is rarely developed. In this paper, we first propose and experimentally demonstrate a scalable mode-selective converter using asymmetrical micro-racetrack resonators (MRRs) for optical network-on-chip. The proposed device, composed of cascaded MRRs, is able to convert the input monochromatic light to an arbitrary supported mode in the output waveguide as required. Thermo-optical effect of silicon waveguides is adopted to tune the working states of the device. To test the utility, a device for proof-of-concept is fabricated and experimentally demonstrated based on silicon-on-insulator substrate. The measured spectra of the device show that the extinction ratios of MRRs are larger than 18 dB, and modal crosstalk for selected modes are all less than −16.5 dB. The switching time of the fabricated device is in the level of about 40 μs. The proposed device is believed to have potential applications in multi-functional and intelligent network-on-chip, especially in reconfigurable MDM networks.

High-resolution notch filters and diplexers based on SU-8 inverted rib waveguides.

We performed an end-to-end process, ranging from design, fabrication, and characterization of integrated polymeric optical devices under a mass production technology. Inverted rib waveguides formed by SU-8 photoresist deposited on top of full wafers, with trenches in silica, were used as a platform to implement such optical devices. Narrowband spectral filters based on microracetrack resonators and diplexers based on directional couplers, both with high extinction ratios, are demonstrated. Full wafers of those devices were processed in a complementary metal-oxide-semiconductor foundry's 150mm-facility. We believe the results are promising for applications ranging from telecommunication components to sensing devices.

Design of silicon-wire waveguide ultracompact racetrack resonators: geometrical parameters for optimal coupling.

In this paper, it is shown that a suitable choice of the geometrical parameters of a silicon-wire waveguide microracetrack resonator structure can lead to a substantial improvement in the control of coupling coefficients and, hence, the design of ultracompact devices for high-performance channel add-drop filters and all-optical switching applications. On the one hand, some simple theoretical arguments and simulation results indicate that the reduction of the silicon-wire rectangular waveguide cross-section area (width×height) is possible, from standard (450 nm×220 nm) to (380 nm×200 nm) on both the bus and the resonator waveguides; this action, apart from still guaranteeing a quasi-TE single-mode operation, would provide an effective improvement into scale-of-integration by a 1.30 factor per device volume. On the other hand, it will be shown by a semianalytical method (analytical calculation + numerical simulation) that achieving the waveguide-racetrack optimal coupling condition for a particular application can be reduced to a prime calculation of the main resonator geometrical parameters (bend radius, straight length, air gap and overall coupling length). In particular, the design of high-performance ultracompact waveguide-racetrack resonator structures, with pre-established Q factor (Q≥2000), free spectral range (FSR≥15 nm), full width at half-maximum (FWHM≤5 nm), finesse (F≥40) or extinction ratio signals (ER≥20 dB) can be systematically obtained with this procedure.