Double Resonance Research Articles

High‐Frequency Electron Paramagnetic Resonance and Electron‐Nuclear Double Resonance Spectroscopy Study of the Ga Vacancy in β‐Ga2O3

The Ga vacancy (VGa) defect in β‐Ga2O3, generated by proton irradiation, is studied using high‐frequency electron paramagnetic resonance (EPR) and electron‐nuclear double resonance spectroscopy. The previous X‐band EPR studies of this defect, attributed to VGa2−, are extended to higher frequencies (240 GHz) and lower temperatures (T = 6 K). The spin Hamiltonian parameters of the VGa2− center are determined: electron spin S = 1/2, g‐tensor: gb = 2.0313, gc = 2.0079, and ga = 2.0025; the hyperfine interaction parameters with 2 equivalent Ga neighbors: Ab = 14.0 G, Ac = 14.6 G, and Aa* = 12.8 G for 69Ga; the superhyperfine interaction with distant Ga neighbors ASHF(69Ga) = 11 MHz; and the quadrupole interaction Qb(69Ga) = 0.32 MHz and Qb(71Ga) = 0.22 MHz. These results shall allow to refine the assignment of this center to a split vacancy or an unrelaxed VGa2− defect.

Research on high-quality factor sensing characteristics of all-dielectric nanopore array metasurface based on POA-DELM

Abstract Based on the optical properties of symmetric structures independent of each other in the orthogonal direction, an all-dielectric nano-square hole array metasurface which is symmetric along the diagonal is proposed. By changing the size of square nanopores, the symmetry of the periodic unit structure is broken and the double Fano resonance can be excited. The influence of each structural parameter on the sensing performance of the metasurface is analyzed respectively. As the main structural parameters and performance index, the metasurface height and the lengths of the main and sub-diagonal square nanoholes are selected as the input parameters, and the figure of merit (FOM) value is used as the output value. Then the nonlinear mapping relationship between the input and the output is established through deep extreme learning machine (DELM). Different optimization algorithms are used to optimize the weighted FOM values globally. The four evaluation indicators including root mean square error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE), and model fit (R-squared, R2) are used to assess the training effectiveness of the model. It is shown that the indexes are 0.9986, 0.9725, 3.1612 and 0.9733 respectively, and the FOM values of the dual Fano resonance after pelican optimization algorithm ( POA ) optimization are as high as 9.88 × 103 and 1.28 × 105, which demonstrate the effectiveness of POA-DELM proposed in this paper.

Pulsed EPR Methods in the Angstrom to Nanometre Scale Shed Light on the Conformational Flexibility of a Fluoride Riboswitch.

Riboswitches control gene regulation upon external stimuli such as environmental factors or ligand binding. The fluoride sensing riboswitch from Thermotoga petrophila is a complex regulatory RNA proposed to be involved in resistance to F- cytotoxicity. The details of structure and dynamics underpinning the regulatory mechanism are currently debated. Here we demonstrate that a combination of pulsed electron paramagnetic resonance (ESR/EPR) spectroscopies, detecting distances in the angstrom to nanometre range, can probe distinct regions of conformational flexibility in this riboswitch. PELDOR (pulsed electron-electron double resonance) revealed a similar preorganisation of the sensing domain in three forms, i.e. the free aptamer, the Mg2+-bound apo, and the F--bound holo form. 19F ENDOR (electron-nuclear double resonance) was used to investigate the active site structure of the F--bound holo form. Distance distributions without a priori structural information were compared with in silico modelling of spin label conformations based on the crystal structure. While PELDOR, probing the periphery of the RNA fold, revealed conformational flexibility of the RNA backbone, ENDOR indicated low structural heterogeneity at the ligand binding site. Overall, the combination of PELDOR and ENDOR with sub-angstrom precision gave insight into structural organisation and flexibility of a riboswitch, not easily attainable by other biophysical techniques.

Radiation-Induced Paramagnetic Centers in Meso- and Macroporous Synthetic Opals from EPR and ENDOR Data

The paramagnetic defects and radiation-induced paramagnetic centers (PCs) in silica opals can play a crucial role in determining the magnetic and electronic behavior of materials and serve as local probes of their electronic structure. Systematic investigations of paramagnetic defects are essential for advancing both theoretical and practical aspects of material science. A series of silica opal samples with different geometrical parameters were synthesized and radiation-induced PCs were investigated by means of the conventional and pulsed X- and W-band electron paramagnetic resonance, and 1H/2H Mims electron-nuclear double resonance. Two groups of PCs were distinguished based on their spectroscopic parameters, electron relaxation characteristics, temperature and time stability, localization relative to the surface of silica spheres, and their origin. The obtained data demonstrate that stable radiation-induced E’ PCs can be used as sensitive probes for the hydrogen-containing fillers of the opal pores, for the development of compact radiation monitoring equipment, and for quantum technologies.

Silicon double-disk optomechanical resonators from wafer-scale double-layered silicon-on-insulator

Whispering Gallery Mode (WGM) optomechanical resonators are a promising technology for the simultaneous control and measurement of optical and mechanical degrees of freedom at the nanoscale. They offer potential for use across a wide range of applications such as sensors and quantum transducers. Double-disk WGM resonators, which host strongly interacting mechanical and optical modes co-localized around their circumference, are particularly attractive due to their high optomechanical coupling. Large-scale integrated fabrication of silicon double-disk WGM resonators has not previously been demonstrated. In this work, we present a process for the fabrication of double-layer silicon-on-insulator wafers, which we then use to fabricate functional optomechanical double silicon disk resonators with on-chip optical coupling. The integrated devices present experimentally observed optical quality factors of the order of 105 and a single-photon optomechanical coupling of approximately 15 kHz.

Silicon-based double fano resonances photonic integrated gas sensor

The telecommunication wavelengths are crucial for developing a photonic integrated circuit (PIC). The absorption fingerprints of many gases lie within these spectral ranges, offering the potential to create a miniaturized gas sensor for PIC. This work presents novel double Fano resonances within the telecommunication band, based on silicon metasurfaces for selective gas sensing applications. Our proposed design comprises periodically coupled nanodisk and nanobar resonators mounted on a quartz substrate. Fano resonances can be engineered across the range from λ = 1.52 μm to λ = 1.7 μm by adjusting various geometrical parameters. A double detection sensor of carbon monoxide (CO) at λ = 1.566 μm and nitrous oxide (N2O) at λ = 1.674 μm is developed. The sensor exhibits exceptional refractometric sensitivity to CO of 1,735 nm/RIU with an outstanding FOM of 11,570 at the first Fano resonance (FR1). In addition, the sensor shows a sensitivity to N2O of 194 nm/RIU accompanied by an FOM of 510 at the second Fano resonance (FR2). The structure reveals absorption losses of 6.3% for CO at the FR1, indicating the sensor selectivity to CO. The sensor is less selective at FR2 and limited to spectral shifts induced by each gas type. Our proposed design holds significant promise for the development of a highly sensitive double-sensing refractometric photonic integrated gas sensor.

Tunable mid-infrared absorber based on Dirac metamaterials

Abstract In this paper, a tunable metamaterial absorber based on Dirac semimetal is proposed, which consists of three different structures, from top to bottom, namely a double semicircular Dirac semimetal resonator, a silicon dioxide (SiO2) substrate, and a continuous vanadium dioxide (VO2) reflector layer. When the Fermi energy level of the Dirac semimetal is 10\ meV, the absorber absorbs more than 90% in the 39.06-84.76\ THz range. Firstly, taking advantage of the tunability of the conductivity of the Dirac semimetal, the dynamic tuning of the absorption frequency can be achieved by changing the Fermi energy level of the Dirac semimetal without the need to optimise the geometry and remanufacture the structure. Secondly, the structure has been improved by the addition of the phase change material VO2, resulting in a much higher absorption performance of the absorber. Since vanadium dioxide is a temperature-sensitive metal oxide with an insulating phase below the phase transition temperature (about 68℃) and a metallic phase above the phase transition temperature, this paper also analyses the effect of vanadium dioxide on the absorptive performance at different temperatures, with the aim of further improving the absorber performance.

Double localized surface plasmon‒enhanced-second-harmonic generation behavior of equilateral triangular aluminum nanoprisms

Abstract This paper presents second-harmonic generation (SHG) behaviors for different-sized equilateral triangular aluminum nanoprisms. These behaviors were observed under the double localized surface plasmon (LSP) resonance condition. The SHG conversion efficiency was substantially enhanced when the excitation and SHG wavelengths were simultaneously tuned to the LSP-resonance wavelengths. The fundamental and higher-order LSP modes could be used to enhance the SHG conversion efficiency. The nonlinear light‒matter interaction associated with the double LSP resonance was numerically analyzed while focusing on the spatial overlap of the near-field distributions between the excitation and SHG wavelengths. The double LSP resonance enhanced SHG behaviors have been realized by using several elegantly designed plasmonic metal nanostructures consisting of the multi-elements. Our present study demonstrates this double LSP resonance enhanced SHG behavior from single, simple-shape triangular aluminum nanoprism.

Formamidinium Incorporates into Rb-based Non-perovskite Phases in Solar Cell Formulations.

Organic-inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI3), are among the most promising emerging photovoltaic materials. However, the spontaneous phase transition from the photoactive perovskite phase to an inactive non-perovskite phase complicates the application of FAPbI3in solar cells. To remedy this, alkali metal cations, most often Cs+, Rb+or K+, are included during perovskite synthesis to stabilize the photoactive phase. The atomic-level mechanisms of stabilization are complex. While Cs+dopes directly into the perovskite lattice, Rb+does not, but instead forms an additional non-perovskite phase, and the mechanism by which Rb confers increased stability remains unclear. Here, we use1H-87Rb double resonance NMR experiments to show that FA+incorporates into the Rb-based non-perovskite phases (FAyRb1-yPb2Br5andδ-FAyRb1-yPbI3) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the1H and87Rb chemical shifts,1H-87Rb heteronuclear correlation spectra, and87Rb{1H} REDOR spectra. Simulation of the REDOR dephasing curves suggests up to ~60% FA+incorporation into the inorganic Rb-based phase for the bromide system. In light of these results, we hypothesize that the substitution of FA+into the non-perovskite phase may contribute to the greater stability conferred by Rb salts in the synthesis of FA-based perovskites.

Formamidinium Incorporates into Rb‐based Non‐perovskite Phases in Solar Cell Formulations

Organic–inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI3), are among the most promising emerging photovoltaic materials. However, the spontaneous phase transition from the photoactive perovskite phase to an inactive non‐perovskite phase complicates the application of FAPbI3 in solar cells. To remedy this, alkali metal cations, most often Cs+, Rb+or K+, are included during perovskite synthesis to stabilize the photoactive phase. The atomic‐level mechanisms of stabilization are complex. While Cs+ dopes directly into the perovskite lattice, Rb+ does not, but instead forms an additional non‐perovskite phase, and the mechanism by which Rb confers increased stability remains unclear. Here, we use 1H–87Rb double resonance NMR experiments to show that FA+ incorporates into the Rb‐based non‐perovskite phases (FAyRb1‐yPb2Br5 and δ‐FAyRb1‐yPbI3) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the 1H and 87Rb chemical shifts, 1H–87Rb heteronuclear correlation spectra, and 87Rb{1H} REDOR spectra. Simulation of the REDOR dephasing curves suggests up to ~60% FA+ incorporation into the inorganic Rb‐based phase for the bromide system. In light of these results, we hypothesize that the substitution of FA+ into the non‐perovskite phase may contribute to the greater stability conferred by Rb salts in the synthesis of FA‐based perovskites.

Aspartic Acid Binding on Hydroxyapatite Nanoparticles with Varying Morphologies Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulation.

Hydroxyapatite (HAP) exhibits a highly oriented hierarchical structure in biological hard tissues. The formation and selective crystalline orientation of HAP is a process that involves functional biomineralization proteins abundant in acidic residues. To obtain insights into the process of HAP mineralization and acidic residue binding, synthesized HAP with specific lattice planes including (001), (100), and (011) are structurally characterized following the adsorption of aspartic acid (Asp). The adsorption affinity of Asp on HAP surfaces is evaluated quantitatively and demonstrates a high dependency on the HAP morphological form. Among the synthesized HAP nanoparticles (NPs), Asp exhibits the strongest adsorption affinity to short HAP nanorods, which are composed of (100) and (011) lattice planes, followed by nanosheets with a preferential expression of the (001) facet, to which Asp displays a similar but slightly lower binding affinity. HAP nanowires, with the (100) lattice plane preferentially developed, show significantly lower affinity to Asp and evidence of multilayer adsorption compared to the previous two types of HAP NPs. A combination of solid-state NMR (SSNMR) techniques including 13C and 15N CP-MAS, relaxation measurements and 13C-31P Rotational Echo DOuble Resonance (REDOR) is utilized to characterize the molecular structure and dynamics of Asp-HAP bionano interfaces with 13C- and 15N-enriched Asp. REDOR is used to determine 13C-31P internuclear distances, providing insight into the Asp binding geometry where stronger 13C-31P dipolar couplings correlate with binding affinity determined from Langmuir isotherms. The carboxyl sites are identified as the primary binding groups, facilitated by their interaction with surface calcium sites. The Asp chelation conformations revealed by SSNMR are further refined with molecular dynamics (MD) simulation where specific models strongly agree between the SSNMR and MD models for the various surfaces.

Analysis of Low-Frequency Sound Absorption Performance and Optimization of Structural Parameters for Acoustic Metamaterials for Spatial Double Helix Resonators

Low-frequency noise absorbers often require large structural dimensions, constraining their development in practical applications. In order to improve space utilization, an acoustic metamaterial with a spatial double helix, called a spatial double helix resonator (SDHR), is proposed in this paper. An analytical model of the spatial double-helix resonator is established and verified by numerical simulations and impedance tube experiments. By comparing the acoustic absorption coefficients of the spatial double-helix resonator, it is shown that the results of the analytical model, the numerical model, and the experiments are in good agreement, proving the accuracy of the theoretical model. The effects of different structural parameters on the peak sound absorption coefficient and resonance frequency are quantitatively revealed. The impedance variation law of the model is obtained, and the resistance and reactance distributions at the resonance frequency are analyzed. In the optimization model, the Back Propagation (BP) network is used to construct the mapping between the structural parameters and the resonance frequency and sound absorption coefficient, and this is used as the constraints of the equation, which is combined with Wild Horse Optimization (WHO) to establish the BP-WHO optimization model to minimize the volume of the spatial double helix resonator. The results show that, for a given noise frequency, the optimized structural parameters enhance the space utilization without affecting the performance of the space double helix resonator.

Enhancing Spectrometer Performance with Unsupervised Machine Learning.

Solid-state NMR spectroscopy (SSNMR) is a powerful technique to probe structural and dynamic properties of biomolecules at an atomic level. Modern SSNMR methods employ multidimensional pulse sequences requiring data collection over a period of days to weeks. Variations in signal intensity or frequency due to environmental fluctuation introduce artifacts into the spectra. Therefore, it is critical to actively monitor instrumentation subject to fluctuations. Here, we demonstrate a method rooted in the unsupervised machine learning algorithm principal component analysis (PCA) to evaluate the impact of environmental parameters that affect sensitivity, resolution and peak positions (chemical shifts) in multidimensional SSNMR protein spectra. PCA loading spectra illustrate the unique features associated with each drifting parameter, while the PCA scores quantify the magnitude of parameter drift. This is demonstrated both for double (HC) and triple resonance (HCN) experiments. Furthermore, we apply this methodology to identify magnetic field B0 drift, and leverage PCA to "denoise" multidimensional SSNMR spectra of the membrane protein, EmrE, using several spectra collected over several days. Finally, we utilize PCA to identify changes in B1 (CP and decoupling) and B0 fields in a manner that we envision could be automated in the future. Overall, these approaches enable improved objectivity in monitoring NMR spectrometers, and are also applicable to other forms of spectroscopy.

Spectral prediction of all dielectric nanopore metasurface based on DBO-DNN model

Based on the optical properties of symmetric structures independent of each other in the orthogonal direction, a class of all-dielectric nanohole array metasurfaces symmetrical along the diagonal is designed. By adding nanopores of different shapes to break the symmetry of the periodic unit structure, the double Fano resonance is excited. The spectral characteristics of metasurfaces with the same structure type are studied by finitedifference timedomain (FDTD) method. The deep neural network (DNN) is used to establish the nonlinear mapping relationship between the input structural parameters and the transmission spectrum. The number of hidden layers in the DNN and the number of neurons in each layer are optimized by the dung beetle optimization (DBO) algorithm. Therefore, the number of hidden layers of the model is determined to be 5, and the number of neurons in each layer is 120, 30, 150, 60, 90, respectively. The mean square error (MSE) is used to evaluate the training effect of DNN after optimization search. After 35,000 epochs of training, MSE is reduced to 0.0003926. The influence of different gradient descent optimization algorithms on the prediction results is explored respectively, and it is found that Adamax is the most effective. The results show that the prediction model can predict the spectrum within 1 s. Compared with the traditional simulation method, the simulation time is effectively saved. Meet the requirements of efficient and rapid design of ultra-thin lenses. For the same type of metasurface structure, the transmission spectrum can be accurately predicted without multiple data sets.

Bayesian Probabilistic Inference of Nonparametric Distance Distributions in DEER Spectroscopy.

Double electron-electron resonance (DEER) spectroscopy measures distance distributions between spin labels in proteins, yielding important structural and energetic information about conformational landscapes. Analysis of an experimental DEER signal in terms of a distance distribution is a nontrivial task due to the ill-posed nature of the underlying mathematical inversion problem. This work introduces a Bayesian probabilistic inference approach to analyze DEER data, assuming a nonparametric distance distribution with a Tikhonov smoothness prior. The method uses Markov Chain Monte Carlo sampling with a compositional Gibbs sampler to determine a posterior probability distribution over the entire parameter space, including the distance distribution, given an experimental data set. This posterior contains all of the information available from the data, including a full quantification of the uncertainty about the model parameters. The corresponding uncertainty about the distance distribution is visually captured via an ensemble of posterior predictive distributions. Several examples are presented to illustrate the method. Compared with bootstrapping, it performs faster and provides slightly larger uncertainty intervals.

Optimization of 15N–13C double-resonance NMR experiments under low temperature magic angle spinning dynamic nuclear polarization conditions

Dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) solid-state NMR carried out at 25 K enables rapid acquisition of multi-dimensional 13C–15N correlation spectra for protein structure studies and resonance assignment. Under commonly used DNP conditions, solvent deuteration reduces 1H–15N cross polarization (CP) efficiencies, necessitates more careful optimization, and requires longer high-power 15N radio-frequency pulses. The sensitivity of 2D heteronuclear correlation experiments is potentially impaired. Here we show that 2D 15N-13C experiments based on 13C-15N transferred echo double resonance (TEDOR) methods outperform 2D experiments based on CP transfers in a fully deuterated solvent, and are competitive with CP-based experiments when the solvent is only partially deuterated. Additionally, we show that optimization of TEDOR-based 2D experiments is simpler than optimization of CP-based experiments under 25 K MAS conditions.

Crisis-induced vibrational resonance in a phase-modulated periodic structure.

Double vibrational resonance is reported for a driven oscillator in a periodic structure of the Josephson junction type with high-frequency phase modulation. We identify two distinct phase modulation effects, namely, resonant induction and resonant amplification, leading to the appearance of a double resonance. We analyze these vibrational resonance phenomena theoretically and numerically, and we show that the origin of the induced resonance is traceable to a transition from periodicity to quasiperiodicity associated with an attractor-merging crisis.

Design of metamaterial for broadband sound absorption through double resonances

Abstract Broadband acoustic absorber via a structure with sub-wavelength thickness is of great and continuing interest in research and engineering communities. To broaden the absorption band of acoustic absorbers, common methods often involve using septum to create a double-layer structure or replacing cavity walls with flexible materials. However, such approaches require strict restrictions of thickness and structure design for the absorbers. This work presents a metamaterial design method that uses an external frame to construct double resonant metamaterials, effectively improving broadband sound absorption performance. Specifically, this method can be decoupled from the design of original cavity-type absorber structure and external frame. The absorption performance is significantly improved by adding an external frame to wrap around a cavity-type acoustic absorber structure with air. Furthermore, utilizing the coaction of cascade coupling and external frame, the average absorption ratio of the metamaterial in high frequency domain (above 2000 Hz) can be improved by 6 times. Since this structural design broadens the absorption band without changing structural parameters of the original absorber, it has potentials to be applied in various engineering fields.

In-Cell DEER Spectroscopy of Nanodisc-Delivered Membrane Proteins in Living Cell Membranes.

Membrane proteins are integral to numerous cellular processes, yet their conformational dynamics in native environments remains difficult to study. This study introduces a nanodelivery method using nanodiscs to transport spin-labeled membrane proteins into the membranes of living cells, enabling direct in-cell double electron-electron resonance (DEER) spectroscopy measurements. We investigated the membrane protein BsYetJ, incorporating spin labels at key positions to monitor conformational changes. Our findings demonstrate successful delivery and high-quality DEER data for BsYetJ in both Gram-negative E. coli and Gram-positive B. subtilis membranes. The delivered BsYetJ retains its ability to transport calcium ions. DEER analysis reveals distinct conformational states of BsYetJ in different membrane environments, highlighting the influence of lipid composition on the protein structure. This nanodelivery method overcomes traditional limitations, enabling the study of membrane proteins in more physiologically relevant conditions.

A comprehensive study of non-Lorentzian resonant lineshapes in nested ring resonators for quantum and photonic applications

Microring resonators are pivotal in photonic and quantum technologies. Enhancing their versatility for various applications often involves integrating additional elements to modify their resonant lineshapes. In this regard, Nested Ring Resonators (NRRs) offer a novel approach to generate a variety of lineshapes without the need for supplementary components. The coupled resonant architecture of NRRs can induce phenomena such as Electromagnetically Induced Absorption, Electromagnetically Induced Transparency, Fano resonance, and double Fano resonance. This study comprehensively analyzes the different resonant lineshapes achievable with NRRs, highlighting their broad applicability. We identify the specific conditions required for each resonant phenomenon and closely examine their characteristics. The theoretical insights from our research enable the autonomous design and realization of various resonance lineshapes within an NRR, eliminating the need for additional components while maintaining device compactness for a wide range of practical applications.