Small Mode Research Articles

Strongly localized nanolaser based on the vertical dipole governed by bound states in the continuum

Ultrasmall mode volumes and strongly localized fields are crucial for the miniaturization and performance enhancement of nanolasers. Here, we demonstrate a nanolaser based on a vertical dipole resonance coupled to its mirror in a periodic array of nanopillars on an Ag mirror, governed by symmetry-protected bound states in the continuum (BICs), which possess extremely small mode volumes and high field enhancement. A nanolaser with a strongly localized field size of only ∼λ/300 (where λ is the resonant wavelength) can be explored by using this vertical dipole. Compared to a vertical dipole nanolaser without a silver mirror, the effective mode volume can be reduced by an order of magnitude, and the threshold can decrease from 5.85 to 0.337 μJ/mm2. Additionally, by controlling further investigated the angle of the incident light, we can also adjust the threshold of the nanolaser. When the incidence angle is adjusted from 9° to 1°, the threshold can be reduced from 1.43 to 0.299 μJ/mm2.

Investigation of pedestal parameters and divertor heat fluxes in small ELM regimes in DIII-D

Abstract Divertor heat flux and its correlation with pedestal parameters within various small edge localized mode (ELM) regimes, including high beta poloidal, type-II and ELMs with negative triangularity H-modes were investigated in DIII-D. The parallel energy fluences of type-II and high beta poloidal small ELM regimes fall below the linear scaling with pedestal electron pressure for type-I ELMs put forward in Eich et al 2017 (Nucl. Mater. Energy 12 84–90). The negative triangularity of H-mode ELMs follow the Eich scaling for type-I ELMs. The parallel heat flux and total heat loads to the divertor were determined using high-time resolution infrared thermography, while pedestal parameters were obtained through self-consistent kinetic equilibrium reconstructions. Linear regressions for the type-II and high beta poloidal regimes demonstrate that an equivalent 7.5 MA small ELM scenario in ITER would fall below the ~5 MJ m − 2 leading edge melting limit for tungsten (Gunn et al 2017 Nucl. Fusion 57 046025). Utilizing fast thermography, the scrape-off layer power fall-off length for both inter-ELM and intra-ELM was determined and compared to the Eich scaling with poloidal magnetic field in Eich et al (ASDEX Upgrade Team and JET EFDA Contributors 2013 Nucl. Fusion 53 093031). Except for the high beta poloidal scenario, all the small ELM regimes during both inter- and intra-ELM periods had power fall-off lengths ( λ q ) larger then would be expected from the B pol , MP − 1 scaling associated with type-I ELMs, signifying their potential in managing heat loads and offering a solution for core–edge integration.

Dependence of ELM instability on separatrix density in EAST long-pulse H-mode plasmas

Abstract The transition from small edge-localized modes (ELMs) to large ELMs has been repetitively observed in minute-scale long-pulse high-confinement mode (H-mode) discharges in 2017 EAST campaign. The appearance of large ELMs is found to be strongly correlated with the decrease in separatrix density due to the gradual decrease in fuel recycling during the long-pulse H-mode operations (LPHOs). By numerical scan of separatrix density with a fixed temperature profile, it has been found that the dependence of ELM instability on separatrix density is related to the competition between the ion diamagnetic stabilizing effect and destabilizing effect of pressure gradient and current density in the pedestal region, shedding light on a comprehensive understanding of different roles of separatrix density in ELM instability observed in EAST experiments. With a high separatrix density, the ideal ballooning mode could be destabilized near the separatrix, which is thought to help achieve small ELMs in EAST LPHOs. In 2021 EAST campaign, the experiment of large ELMs control has been performed through actively changing fuel recycling by moving strike point location (SPL) on the lower tungsten divertor target plate. It has been demonstrated that the mitigation of large ELMs is strongly correlated with the significant increase in separatrix density, which is thought to be attributed to a higher ionization source in the scrape-off layer (SOL) region by SOLPS-ITER simulation. The high ionization source in the SOL region is believed to provide a strong fueling effect near the separatrix and thus raise the local density, which is considered as an important reason for triggering ballooning instabilities near the separatrix and achieving small ELMs.

A long-pulse small edge-localized-mode high-confinement plasma with detachment feedback control by floating potential in an experimental advanced superconducting tokamak in a metal wall environment

Abstract One of the key challenges facing the magnetic fusion research is to demonstrate the compatibility between high confinement and radiative divertor in long-pulse discharges with a metal wall environment. A small edge localized mode (ELM) high confinement plasma is obtained in the upgraded lower divertor of Experimental Advanced Superconducting Tokamak (EAST) with an energy confinement factor H98~1.1 and a Greenwald density fraction fGW ~ 0.65 maintained for 26 s, and periodical detachment is achieved through active control of neon impurity seeding in this long-pulse discharge. For the divertor region, partial detachment is achieved periodically on the outer divertor target plates with the plasma temperature near the outer strike point decreased to below 5 eV and peak surface temperature on the outer divertor target plates maintained below 350C. The peak heat flux of the lower outer divertor decreases significantly and its profile along the target becomes very flat in the detached state. Two low-frequency (<10 kHz) fluctuations that are related to rippling mode caused by a resistive instability appear in the detached state. For the pedestal region, the electron pressure profile is flatter and ELM amplitude is smaller in the detached state than that in the attached state. Edge coherent mode (ECM) appears in the attached state and disappears in the detached state. To achieve this experimentally, a new impurity seeding feedback control scheme is applied, where the floating potential measured by divertor Langmuir probes is used as the feedback sensor, which is more reliable in the long-pulse discharges with high heat fluxes, thus more suitable for application in future devices. This work provides a new approach for the actively controlled radiative divertor as a solution to the divertor heat loads of the future fusion reactors.

Nonlinear simulations of GAEs in NSTX-U

A set of nonlinear simulations has been performed in order to study the nonlinear evolution of unstable global Alfvén eigenmodes in the National Spherical Torus Experiment-Upgrade (NSTX-U). Results of the single toroidal mode number, n, simulations are compared with a full nonlinear simulation (all toroidal harmonics included). In single-n simulations, the conservation of two integrals of motion of a particle in a cyclotron resonance with a monochromatic wave is demonstrated, resulting in a one-dimensional evolution of the particle distribution in (E,μ,pϕ) phase-space. Nonlinear simulations (both single-n and full nonlinear) show a significant redistribution of the resonant fast ions, especially in the pitch parameter. Thus, the changes in the resonant particle's parallel and perpendicular energies can be several times larger than the total particle energy change, with only a small fraction transferring into the excitation of the mode itself. This implies that even a relatively small amplitude mode can significantly modify the beam distribution in the resonant region. For the NSTX-U case considered, the single-n simulation results are close to full nonlinear simulation only for the most unstable mode, in which case the saturation amplitudes and changes in the fast ion distribution are comparable. In contrast, peak amplitudes of subdominant modes in all-n simulations are smaller by a factor of 3–10 compared to single-n runs due to the flattening of the beam ion distribution by the fastest growing mode.

Stabilized conformal planar cavity with continuous adjustable resonant frequency

A conformal planar cavity with filling dielectrics is conformally transformed from a concave cavity by transformation optics. The proposed conformal planar cavity offers remarkable advantages, including high stability, high Q factor, tunable resonant frequency, and small mode volume. In contrast to concave cavities, it offers a significant advantage by being integrable onto a chip. Additionally, when compared to planar cavities based on metasurfaces, which lack the capability for continuous resonant frequency adjustment, the proposed conformal cavity offers a tunable resonant frequency that can be continuously adjusted by simply changing the cavity length without the need for redesigning the filling dielectrics. Only dielectrics with permittivity ranging from 1 to 1.7 are required inside the conformal cavity, which can be easily obtained at various frequency bands. The tunability and stability of the conformal planar cavity is numerically verified, and experimental demonstrations at microwave band are performed.

On the Optimum Distribution of Multiple Helmholtz Resonators for Annular Combustors

Abstract The combustors of many modern land-based gas turbines and aero-engines are annular. This kind of combustors often suffer from thermoacoustic oscillations, with the occurrence of mostly the circumferential first-order, second-order and third-order oscillation modes in the annular combustion chamber. To solve this problem, passive control methods are widely used adding passive acoustic dampers such as Helmholtz resonators (HRs). Depending on the type and number of HRs, and the possible positions over the circumference to install them, there could easily have millions, or even billions, of possible arrangment patterns. Finding a good design is the key to solve this problem. In this paper, we perform a theoretical and numerical study of an annular combustor installed with multiple types of HRs over the circumference. Firstly, a simple annular duct with arbitrary distributions of these HRs is studied analytically by solving the nonlinear eigenvalue problem of a 1D network model. Based on the results of the analytical method, the impact of the HRs on the acoustic modes of the combustion chamber is studied and the optimum arrangement of multiple resonators is obtained. This arrangement usually gives a null (or small) mode splitting strength and a good damping effect. Finally, we apply the optimum arrangement to damp the thermoacoustic modes that have been captured by numerical simulation for a real annular combustor. We use numerical simulations based on solving the 3D Helmholtz equation in COMSOL to verify the feasibility of the optimum arrangement.

Thermo-Optic Switch with High Tuning Efficiency Based on Nanobeam Cavity and Hydrogen-Doped Indium Oxide Microheater

We propose and experimentally demonstrate an efficient on-chip thermo-optic (TO) switch based on a photonic crystal nanobeam cavity (PCNC) and a hydrogen-doped indium oxide (IHO) microheater. The small mode volume of the PCNC and the close-range heating through the transparent conductive oxide IHO greatly enhance the coupling between the thermal field and the optical field, increasing the TO tuning efficiency. The experimental results show that the TO tuning efficiency can reach 1.326 nm/mW. And the rise time and fall time are measured to be 3.90 and 2.65 μs, respectively. In addition, compared with the conventional metal microheater, the measured extinction ratios of the switches are close (25.8 dB and 27.6 dB, respectively), indicating that the IHO microheater does not introduce obvious insertion loss. Our demonstration showcases the immense potential of this TO switch as a unit device for on-chip large-scale integrated arrays.

Road user opinions and needs regarding small modular autonomous electric vehicles: Differences between elderly and non‐elderly in Norway

AbstractThis study examines road user opinions regarding small modular autonomous electric vehicles, focusing on the differences between the elderly and non‐elderly populations in Norway. The data allowed for a comparison between 193 respondents under 65 years old and 208 respondents over 65 years old. The results highlighted significant differences between the two groups about the vehicles, their usability, and the likeliness of using them as public transport if implemented in the future. Traffic safety and personal security were found to be decisive aspects, for respondents over 65 years old being more worried about safety and security than their counterparts. Trust that the authorities will ensure the safe implementation of such vehicles in the current transportation system was also significantly different between the two groups, with the younger generations having more trust in the authorities than the older group. The results shed light on road user opinions about a small modular transport mode, particularly on those over 65 years old, indicating a need for research efforts to better identify how this new form of public transport should be implemented in the future to improve the mobility of all travellers and meet the needs of the seniors.

Facilitated core-edge integration through divertor nitrogen seeding in the HL-2A tokamak

A divertor detachment with sustainable high-performance plasmas has been achieved through divertor nitrogen seeding in the HL-2A tokamak. The closed divertor structure facilitates achieving high divertor neutral pressure due to its efficient particle containment effect as demonstrated by both experimental and SOLPS simulation results, which aids in achieving the divertor detachment. The radiative divertor is conducive to high-frequency small edge localized modes (ELMs) operation, characterized by a reduced pedestal ion temperature gradient ∇Ti and the enhanced electron density gradient ∇ne . The presence of minority nitrogen at the edge plasma, which results from the divertor nitrogen seeding, contributes to the increase of ion temperature Ti in the confinement region by suppressing the turbulence through the impurity dilution effect and E×B shear. The increased ion temperature Ti and electron density ne compensate the energy loss resulting from the increased edge radiation during detachment, which contributes to the confinement sustainment combined with reduced pedestal energy loss caused by small ELMs. This work advances our understanding of the fundamental physics governing closed divertor detachment with sustainable high-performance plasmas through divertor nitrogen seeding.

Giant strong coupling in a Q-BICs' tetramer metasurface.

Due to their ultrahigh Q-factor and small mode volume, bound states in the continuum (BICs) are intriguing for the fundamental study of the strong coupling regime. However, the strong coupling generated by BICs in one metasurface is not always strong enough, which highly limits its efficiency in applications. In this work, we realize a giant strong coupling of at most 60 meV in a quasi-BICs' (Q-BICs) tetramer metasurface composed of four Si cylinders with two different sets of diagonal lengths. The Q-BICs are induced from two types of electric quadrupole (EQ), for which detuning can be flexibly controlled by manipulating the C4v symmetry breaking Δd. The giant Rabi splitting in our proposed metasurface performs more than 15 times of the previous works, which provides more possibilities for important nonlinear and quantum applications, such as nanolaser and quantum optics.

A probabilistic framework for external pitting corrosion growth modelling for buried steel pipelines considering soil properties

This paper introduces a novel framework for developing reliable probabilistic predictive corrosion growth models for buried steel pipelines using pipeline inspection data. The framework adopts a power-law function of time model formulation, accounting for nonconstant damage growth rates, and considers the correlation between defect depth and length growth models. The proposed framework explicitly incorporates local influential soil properties in the model formulation; thus, it requires no segmentation and homogenous defect growth assumption and provides defect-specific growth models. The framework is applicable regardless of the availability of matched or non-matched defect data. For corrosion initiation time estimation, two different approaches are proposed: one is to use a Poisson process to account for defect occurrence, which can also predict newly generated defects since the last inspection, and the other is to use multivariate linear regression of soil and pipe properties. The statistics of unknown model parameters are assessed using a Bayesian updating framework in which the model error can be incorporated. The proposed framework is applied using two different sets of data: one set of inline inspection (ILI) data and one set of field excavation data. A case study is conducted, where time-dependent system reliability of an in-service pipeline is assessed considering small leak and burst failure modes using the developed defect growth models. The impact of the growth model accuracy on the probability of failure is investigated, and the importance analysis is performed to identify the most influential random variables to the probability of failure.

High-performance CO detection based on a PhC cavity in terahertz band

Combined with the light absorption from molecular vibration, photonic crystal (PhC) cavity structures have gradually shown great potential in gas detection, particularly for toxic gases. We proposed a PhC cavity with a high-quality factor of 1.24 × 106 and a small mode volume of 2.3 × 10−4 (λ/n)3, which was used for carbon monoxide detection. To reduce the interference of other gases, we set the resonance frequency in the terahertz band. The numerical analysis shows that the structure has good selectivity and high sensitivity, and the linear fitting of the results provides the possibility to realize the application, which has great competitiveness in the same type of sensor structure. Additionally, we also proved that the interference of H2O and CO2 on the CO sensing can be ignored, and it supports the detection of CO without pre-drying.

High-Q Fabry-Pérot Cavity Based on Micro-Lens Array for Refractive Index Sensing

Fabry-Pérot (FP) microcavities have attracted tremendous attention in recent years due to their favorable optical characteristics of the high quality (Q) factor and small mode volume. In this work, we presented a novel approach that utilized the soft lithography and imprinting technology to incorporate the convex micro-lens array structure into the FP (FP-lens) cavity. A strong mode-profile restriction of the micro-lens simultaneously reduced the mode volume and enhanced the Q factor, exhibiting high tolerance to non-parallelism of mirrors compared with that of the plane-plane FP (PP-FP) microcavities. In the experiment, the Q factor of the FP-lens cavity was measured to be 8.145×104, which exhibited a 5.6-fold increase than that of the PP-FP cavity. Furthermore, we experimentally measured the refractive index sensing performance of the FP-lens cavity with the sensitivity of 594.7 nm/RIU and a detection limit of 4.26×10−7 RIU. On the basis of this superior sensing performance, the FP-lens cavity has the great potential for applications in biosensors.

Ultra-low threshold deep ultraviolet generation in a hollow-core fiber.

Tunable ultrashort pulses in the ultraviolet spectral region are in great demand for a wide range of applications, including spectroscopy and pump-probe experiments. While laser sources capable of producing such pulses exist, they are typically very complex. Notably, resonant dispersive-wave (RDW) emission has emerged as a simple technique for generating such pulses. However, the required pulse energy used to drive the RDW emission, so far, is mostly at the microjoule level, requiring complicated and expensive pump sources. Here, we present our work on lowering the pump energy threshold for generating tuneable deep ultraviolet pulses to the level of tens of nanojoules. We fabricated a record small-core antiresonant fiber with a hollow-core diameter of just 6 μm. When filled with argon, the small mode area enables higher-order soliton propagation and deep ultraviolet (220 to 270 nm) RDW emission from 36 fs pump pulses at 515 nm with the lowest pump energy reported to date (tens of nanojoules). This approach will allow the use of low-cost and compact laser oscillators to drive nonlinear optics in gas-filled fibers for the first time to our knowledge.

Effect of exceptional point on the performance of a bistable all-optical switch.

Microring cavities based on whispering-gallery modes (WGMs) have a very high-quality factor (Q) and a small mode volume, greatly improving the interaction between light and matter, which has attracted great attention in microlaser, nonlinear, and sensing fields. Plasmonics in the microcavity further enhance compression of the optical field. Recently, research on enhanced optical sensing sensitivity and low threshold laser based on exceptional points (EPs) is quite impressive. In this work, we propose a new, to our knowledge, all-optical switch by using the bistable effect under the EP of an ultra-compact plasmonic racetrack resonator and perform numerical simulations using the finite-difference time-domain (FDTD) method. The introduction of EPs further enhances the localization of the light field and thus improves the Kerr nonlinear effect of the microcavity; low threshold optical bistability is achieved. The results show that the device under an EP has a relatively lower threshold (input optical power threshold of 2.2 MW/cm2), shorter switching time (1.725 ps), and significantly improved switching contrast (17.16 dB) compared with those without EP. Our research lays the groundwork for optical switches that are chip-integrated, have low power consumption, and exhibit short switching times.

Coherent optical coupling to surface acoustic wave devices

Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>105 measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems.

Numerical Analysis of Optical Properties Using Octagonal Shaped Ge15As15Se17Te53 Chalcogenide Photonic Crystal Fiber

A novel Ge15As15Se17Te53 chalcogenide-based photonic crystal fiber with an octagonal cladding was designed which exhibits low confinement loss and high nonlinearity. The dispersion properties were investigated over a wide wavelength range of up to 11 µm. Based on preliminary simulation results, three fibers with suitable characteristic quantities are proposed for supercontinuum generation. Among these fibers, fiber #F1 (Ʌ = 3.1 µm, d/Ʌ = 0.3) exhibits all-normal dispersion with a small value of −0.374 ps/[nm.km]. At the same time, the highest nonlinear coefficient of 2876.861 W−1.km−1 at the pump wavelength of 5.45 µm is also found. The #F2 fiber (Ʌ= 4.5 µm, d/Ʌ = 0.3) has anomalous flat dispersion with one zero dispersion wavelength. This fiber has the smallest dispersion and lowest confinement loss values of 0.281 ps/[nm.km] and 1.360×10−9 at 5.7 µm pump wavelength, respectively. With the anomalous dispersion, #F3 fiber (Ʌ = 3.5 µm, d/Ʌ = 0.3) offers a small effective mode area of 20.892 µm2 and a low confinement loss of 4.974×10−8 dB/m at 5.25 µm wavelength. The proposed fibers can be a low peak power, broadband supercontinuum source, suitable for application fields such as optical communication and biomedical sensors.

Time varying reliability analysis of corroded gas pipelines using copula and importance sampling

Ensuring the safety of gas pipelines is crucial for the reliable and efficient transportation of natural gas. This study introduces a methodology for evaluating the time-varying reliability of corroded gas pipelines. The proposed approach employs both importance sampling and copula theory to effectively address the uncertainties associated with random variables involved and provide efficient estimates of failure probability. Small leak and burst failure modes are specifically investigated within this study. The methodology first establishes a step-by-step procedure for analysis. Subsequently, the study investigates the failure probability of corroded gas pipelines, taking into account both small leak and burst failures, under scenarios involving single and multiple defects. Furthermore, the study examines the influence of correlation strength and dependence structure among the involved random variables using the copula theory. Additionally, the generation of newly formed defects is thoroughly investigated, along with its impact on failure probability. Numerical examples are provided to evaluate the performance of the proposed methodology, comparing it with the benchmark failure probability estimated through Monte Carlo simulation. The results validate the precision and efficiency of the proposed methodology, while offering practical and insightful suggestions for reliability analysis of real-world gas pipelines.

Graphene-integrated microring cavity for electronically controlled molecular fingerprinting

Microring cavities supporting whispering-gallery modes (WGMs) have an exceptionally high quality factor (Q) and a small mode volume, greatly improving the interaction between light and matter, which has attracted great attention in various microscale/nanoscale photonic devices and potential applications. Recently, two-dimensional van der Waals (vdW) materials such as graphene have emerged as a potential platform for next-generation biosensing by enabling the confinement of light fields at the nanoscale. Here, we propose what we believe to be a novel approach to achieve molecular fingerprint retrieval by integrating graphene into a microring cavity and conducting numerical simulations using the finite-difference time-domain (FDTD) method. The hybrid cavity exhibits high-quality WGMs with a high Q factor of up to 800. Moreover, the resonant wavelength can be electronically controlled through modulation of graphene’s Fermi level, enabling coverage of the entire free spectral range at infrared frequencies. By depositing a thin layer of biomolecular material (e.g., CBP) onto the surface of our hybrid cavity, we are able to accurately read out the absorption spectrum at multiple spectral points, thereby achieving broadband fingerprint retrieval for the targeted biomolecule. Our results pave the way for highly sensitive, chip-integrated, miniaturized, and electrically modulated infrared spectroscopy biosensing.