Linewidth Variation Research Articles

Long-term monitoring of large-scale magnetic fields across optical and near-infrared domains with ESPaDOnS, Narval, and SPIRou

Context. Dynamo models describing the generation of stellar magnetic fields for partly and fully convective stars are guided by observational constraints. Zeeman-Doppler imaging has revealed a variety of magnetic field geometries and, for fully convective stars in particular, a dichotomy: either strong, mostly axisymmetric, and dipole-dominated or weak, non-axisymmetric, and multipole-dominated. This dichotomy is explained either by dynamo bistability (i.e., two coexisting and stable dynamo branches) or by long-term magnetic cycles with polarity reversals, but there is no definite conclusion on the matter. Aims. Our aim is to monitor the evolution of the large-scale field for a sample of nearby M dwarfs with masses between 0.1 and 0.6 M⊙, which is of prime interest to inform distinct dynamo theories and explain the variety of magnetic field geometries studied in previous works. This also has the potential to put long-term cyclic variations of the Sun’s magnetic field into a broader context. Methods. We analysed optical spectropolarimetric data sets collected with ESPaDOnS and Narval between 2005 and 2016, and near-infrared SPIRou data obtained between 2019 and 2022 for three well-studied, active M dwarfs: EV Lac, DS Leo, and CN Leo. We looked for secular changes in time series of longitudinal magnetic field, width of unpolarised mean-line profiles, and large-scale field topology as retrieved with principal component analysis and Zeeman-Doppler imaging. Results. We retrieved pulsating (EV Lac), stable (DS Leo), and sine-like (CN Leo) long-term trends in longitudinal field. The width of near-infrared mean-line profiles exhibits rotational modulation only for DS Leo, whereas in the optical it is evident for both EV Lac and DS Leo. The line width variations are not necessarily correlated to those of the longitudinal field, suggesting complex relations between small- and large-scale field. We also recorded topological changes in the form of a reduced axisymmetry for EV Lac and transition from a toroidal-dominated to poloidal-dominated regime for DS Leo. For CN Leo, the topology remained predominantly poloidal, dipolar, and axisymmetric, with only an oscillation in field strength. Conclusions. Our results show a peculiar evolution of the magnetic field for each M dwarf individually, with DS Leo and EV Lac manifesting more evident variations than CN Leo. These findings confirm that M dwarfs with distinct masses and rotation periods can undergo magnetic long-term variations and suggest an underlying variety of cyclic behaviours of their magnetic fields.

Directional Filter Design and Simulation for Superconducting On-Chip Filter-Banks

Many superconducting on-chip filter-banks suffer from poor coupling to the detectors behind each filter. This is a problem intrinsic to the commonly used half-wavelength filter, which has a maximum theoretical coupling of 50 %. In this paper, we introduce a phase-coherent filter, called a directional filter, which has a theoretical coupling of 100 %. In order to study and compare different types of filter-banks, we first analyze the measured filter frequency scatter, losses, and spectral resolution of a DESHIMA 2.0 filter-bank chip. Based on measured fabrication tolerances and losses, we adapt the input parameters for our circuit simulations, quantitatively reproducing the measurements. We find that the frequency scatter is caused by nanometer-scale line width variations and that variances in the spectral resolution is caused by losses in the dielectric only. Finally, we include these realistic parameters in a full filter-bank model and simulate a wide range of spectral resolutions and oversampling values. For all cases, the directional filter-bank has significantly higher coupling to the detectors than the half-wave resonator filter-bank. The directional filter eliminates the need to use oversampling as a method to improve the total efficiency, instead capturing nearly all the power remaining after dielectric losses.

Analysis of Optical and Microwave Transmission by Linewidth Variation of Metal Mesh for FSS in GPTS

Analysis of Optical and Microwave Transmission by Linewidth Variation of Metal Mesh for FSS in GPTS

Spin exchange dynamics in 4H SiC monocrystals with different nitrogen donor concentrations

4H silicon carbide (SiC) polytype is preferred over other SiC polytypes for high-power, high-voltage, and high-frequency applications due to its superior electrical, thermal, and structural characteristics. In this manuscript, we provide a comprehensive study of the spin coupling dynamics between conduction electrons and nitrogen (N) donors in monocrystalline 4H SiC with various concentrations of uncompensated N donors from 1017 to 5 × 1019 cm−3 by continuous wave, pulsed electron paramagnetic resonance (EPR), and microwave perturbation techniques at T = 4.2–300 K. At low temperatures, two triplets due to N donors in cubic (Nk) hexagonal (Nh) positions and triplet arisen from spin-interaction between Nh and Nk were observed in 4H SiC having Nd − Na ≈ 1017 cm−3. A single S-line (S = 1/2) dominates the EPR spectra in all investigated 4H SiC monocrystals at high temperatures. It was established that this line occurs due to the exchange coupling of localized electrons (dominate at low temperatures) and non-localized electrons (dominate at high temperatures). The localized electrons were attributed to Nh for Nd − Na ≈ 1017 cm−3 and Nk donors for Nd − Na ≥ 5 × 1018 cm−3. We have concluded that the conduction electrons in 4H SiC monocrystals are characterized by g|| = 2.0053(3) and g⊥ = 2.0011(3) for Nd − Na ≤ 5 × 1018 cm−3 and g|| = 2.0057(3) and g⊥ = 2.0019(3) for Nd – Na ≈ 5 × 1019 cm−3. Using the theoretical fitting of the temperature variation of S-line EPR linewidth in 4H SiC having Nd – Na ≤ 5 × 1018 cm−3, the energy levels of 57–65 meV that correlate with the valley-orbit splitting values for Nk donors in 4H SiC monocrystals were obtained.

Control of the Bright-Dark Exciton Splitting Using the Lamb Shift in a Two-Dimensional Semiconductor.

We investigate the exciton fine structure in atomically thin WSe_{2}-based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting Δ between the bright and dark exciton is measured by photoluminescence spectroscopy. We demonstrate that Δ can be tuned by a few meV as a result of a significant Lamb shift of the optically active exciton that arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measure strong variations of the bright exciton radiative linewidth as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field fluctuations. We find a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control we demonstrate here in the weak light-matter coupling regime should apply to any semiconductor structures.

Polarimetric parity-time symmetry fiber laser with switchable and tunable orthogonal single-polarization narrow-linewidth output

Parity-time (PT) symmetry has garnered significant attention in the field of photonics owing to its remarkable properties. High-performance lasers, which require simultaneous gain and loss, serve as an ideal platform for harnessing the exceptional properties of PT symmetry. In this study, we present and experimentally verify a polarimetric PT-symmetry fiber laser that uses a polarization-maintaining fiber Bragg grating (PM-FBG). This design allows for tunable, switchable dual-wavelength operation with narrow linewidth and orthogonal single-polarization characteristics. By converting two equal-length yet distinct polarization states into two spatial subspaces sharing a common wavelength space, we can achieve PT symmetry. Regulating the polarization controllers (PCs) enables interconversion between PT-symmetry phases. During the PT-symmetric phase (or PT exact phase), a dual-wavelength laser is realized with wavelength fluctuations below 0.098 nm and power fluctuations below 2.175 dB. At this stage, light propagates through both ordinary and extraordinary wavelength-spaces. During the PT-broken phase, one waveguide has a net gain and the other incurs a net loss, leading to a single longitudinal mode (SLM) laser output. The wavelength and power fluctuations of left-wavelength λ1 and right-wavelength λ2 remain below 0.002 nm and 0.770 dB over a 20-minute period, during single-wavelength operation. By adjusting the axial strain on the PM-FBG, the laser output can be tuned within the wavelength range of 1549.754 nm to 1551.690 nm, while maintaining a linewidth variation of less than 0.130 kHz. As the wavelength is adjusted, the azimuth sum of λ1 and λ2 consistently remains close to 90 degrees, accompanied by degrees of polarizations exceeding 99%.

Vliyanie tolshchiny inkapsuliruyushchikh sloev na kachestvo geterostruktur na osnove MoSe2

The dependence of the width of exciton and trion photoluminescence lines in MoSe2 monolayers on the thickness of hexagonal boron nitride encapsulating layers has been investigated. The possibility of variation of the exciton photoluminescence linewidth due to the interaction of excitons with the modes of resonator cavities made up of a silicon substrate and a boron nitride top layer has been checked. This interaction may significantly change the photoluminescence linewidth owing to the Parcell effect. Measurements taken of samples with different thicknesses of the bottom and top boron nitride layers have not revealed any influence of the Parcell effect on the linewidth. It has turned out however that the linewidth narrows by several times with increasing boron nitride bottom layer thickness from 10 to 100 nm and reaches 2 meV at a thickness of 100 nm. Supposedly, such narrowing of the photoluminescence line is associated with a decrease in the density of submicron bubbles, which takes place at longitudinal stress relaxation in the thicker layer of boron nitride.

A thermally controlled high-Q metasurface lens

Dynamic metasurface control is a promising yet challenging prospect for next generation optical components. Here, we design and characterize a thermally controllable metasurface lens, with a high-quality-factor (high-Q) resonance working as both the basis of the lensing behavior and method for efficient modulation. Our high-Q lens is constructed via a zone plate architecture comprised of alternating regions with and without resonant character. Non-resonant regions block transmission, while resonant regions—with measured Qs up to ∼1350—transmit only on resonance. By leveraging the thermo-optic effect, we dynamically control the spectral position of the high-Q resonance to achieve wavelength selectivity of the focusing behavior. Due to the sharp spectral linewidth and amplitude variation of the high-Q resonance, thermal tuning can further result in metasurface switching, where the lensing behavior is changed between on and off states. For a device utilizing only moderate Q-factors of ∼350, the resonance's FWHM can be shifted with temperature changes of only 50 °C, and the device can be fully switched off when operating at 100 °C. Our work provides an initial experimental demonstration of dynamic control of a local high-Q wavefront shaping metasurface.

Problems in Observation and Identification of Torsional Waves in the Lower Solar Atmosphere

Registering periodic variations of spectral line widths serves as the main method for observing torsional Alfv\'{e}n waves. Theoretically, the method seems valid, yet it entails a number of caveats when applied to data. For instance, the amplitudes of these observations should vary with changes of the location on the disk, and they should be associated with no intensity oscillations. We analyze extensive observational material of periodic non-thermal variations of line widths in coronal holes and facular regions in a number of spectral lines: H{\alpha}, He I 10830 {\AA}, Ca II 8542 {\AA}, Ba II 4554 {\AA}. In most cases, we detected associated intensity oscillations at similar frequencies. Besides, we observed no centre-to-limb dependency. This calls for a discussion on the practical validity of the method and on the alternative explanations for the nature of non-thermal variations of spectral line widths. Based on our observations, we consider registering line profile broadening a necessary but not sufficient means for unambiguous identification of torsional Alfv\'{e}n waves in the lower solar atmosphere.

Damping in yttrium iron garnet films with interface

We report strong damping enhancement in yttrium iron garnet (YIG) film due to spin inhomogeneity at the interface. The growth-induced thin interfacial gadolinium iron garnet (GdIG) layer antiferromagnetically (AFM) exchange couples with the top YIG layer. The out-of-plane angular variation of ferromagnetic resonance linewidth $\mathrm{\ensuremath{\Delta}}H$ reflects a large inhomogeneous distribution of the effective magnetization $\mathrm{\ensuremath{\Delta}}4\ensuremath{\pi}{M}_{\mathrm{eff}}$ due to the presence of the aforementioned AFM exchange coupling between the interfacial-GdIG and top-YIG layer. We probe the spin inhomogeneity at the YIG-GdIG interface by performing an in-plane angular variation of resonance field ${H}_{r}$, leading to a unidirectional feature. The large extrinsic $\mathrm{\ensuremath{\Delta}}4\ensuremath{\pi}{M}_{\mathrm{eff}}$ contribution, apart from the inherent intrinsic Gilbert contribution, manifests enhanced precessional damping in the YIG film.

Spectroscopic valuation of phonon lifetime and surface characteristics of Zn0.96Mg0.04S thin film.

The anharmonic properties of the longitudinal optical (LO) phonon mode of Mg-doped ZnS (Zn0.96 Mg0.04S) are investigated using the Balkanski and Klemens models on the temperature-dependent Raman spectra. The variation in the position of the Raman line, peak width, and phonon lifetime with temperature was fitted using three and four phonon decay mechanisms. The values of the anharmonic fitting parameters indicated low anharmonicity. A lifetime of ∼0.17ps at 90K indicated a fast phonon decay. In addition, the thin film is analyzed to evaluate its surface characteristics using Raman mapping that showed chemical homogeneity over a large area of the film. Furthermore, we analyzed spatial variations of Raman line intensity, peak area, linewidth, and line position of the LO phonon mode. Raman analysis helped in understanding the phonon-phonon interaction mechanism in Zn0.96 Mg0.04S thin films.

Single polarization, narrow linewidth hybrid laser based on selective polarization mode feedback

We demonstrate a single polarization, narrow linewidth hybrid laser by using a semiconductor gain chip and a birefringence silica-on-silicon waveguide Bragg grating. The hybrid laser shows good linewidth and single polarization characteristics with the selective polarization mode feedback of high birefringence grating. The temperature phase modulation on the characteristics of the potentially lasing modes is displayed. In addition, the performances of different mode power, side mode suppress ratio, spectrum and linewidth during temperature tuning are characterized. We based on the rate equation method and the chirp factor reduction theory analysis the variation of laser linewidth with power and lasing wavelength. The best linewidth achieves 4.36 kHz and the best polarization extinction ratio is 38.6 dB. The output power in fiber is 6.58 mW.

A CMOS-Compatible Carrier-Injection Plasmonic Micro-Ring Modulator with Resonance Tuning by Carrier Concentration

A complementary metal-oxide-semiconductor (CMOS)-compatible carrier-injection plasmonic micro-ring modulator (CIPMRM) is designed and analyzed theoretically. The CIPMRM has a compacted footprint of 49.3 μm2 (R = 2 μm), a bit rate of 36.5 Gbps, insertion loss of −9.8 dB, a static extinction ratio of 21.7 dB, and energy consumption of 4.40 pJ/bit as 2.2 V peak-to-peak voltage is applied at 1550 nm. Besides, the method of resonance tuning by carrier concentration is proposed to compensate for the wavelength mismatch between the CIPMRM resonance and the laser, resulting from temperature and line width variation of the CIPMRM. This method has a faster response time and a greater ability to shift the resonant wavelength compared with the method of thermo-optic resonance tuning. The proposed scheme provides a route for realizing the compacted size modulator for optoelectronic integration.

Thermal conductivity estimation of silicon bulk material based on modified Raman thermal technique

The thermal conductivity of single-crystal silicon was investigated by using Raman spectroscopy. The laser simultaneously acted as an excitation source and a heating source. The correlations between Raman spectra with both temperature and laser power for single-crystal silicon, which has a potential relationship with the thermal conductivity, were estimated. The results showed the localized compressive stress caused by laser heating would underestimate Raman peak shift. So, the temperature was determined based on the variation of linewidth. The predicted thermal conductivity of single-crystal silicon is 125 W/m ⋅ K, which is comparable to the theoretical value.

Rotational Modulation of Spectroscopic Zeeman Signatures in Low-mass Stars

Abstract Accurate tracers of the stellar magnetic field and rotation are cornerstones for the study of M dwarfs and for reliable detection and characterization of their exoplanetary companions. Such measurements are particularly challenging for old, slowly rotating, fully convective M dwarfs. To explore the use of new activity and rotation tracers, we examined multiyear near-infrared (NIR) spectroscopic monitoring of two such stars—GJ 699 (Barnard’s Star) and Teegarden’s Star—carried out with the Habitable-zone Planet Finder spectrograph. We detected periodic variations in absorption line widths across the stellar spectrum, with higher amplitudes toward longer wavelengths. We also detected similar variations in the strength and width of the 12435.67 Å neutral potassium (K i) line, a known tracer of the photospheric magnetic field. Attributing these variations to rotational modulation, we confirm the known 145 ± 15 day rotation period of GJ 699, and measure the rotation period of Teegarden’s Star to be 99.6 ± 1.4 days. Based on simulations of the K i line and the wavelength dependence of the line-width signal, we argue that the observed signals are consistent with varying photospheric magnetic fields and the associated Zeeman effect. These results highlight the value of detailed line profile measurements in the NIR for diagnosing stellar magnetic field variability. Such measurements may be pivotal for disentangling activity and exoplanet-related signals in spectroscopic monitoring of old, low-mass stars.

Electron-phonon coupling, thermal expansion coefficient, resonance effect, and phonon dynamics in high-quality CVD-grown monolayer and bilayer MoSe2

Probing phonons, quasiparticle excitations, and their coupling has enriched our understanding of two-dimensional (2D) materials and proved to be crucial for developing their potential applications. Here, we report comprehensive temperature 4--330 K and polarization-dependent Raman measurements on monolayer (1L) and bilayer (2L) ${\mathrm{MoSe}}_{2}$. Phonon modes up to the fourth order are observed, including forbidden Raman and infrared modes, and understood considering the Fr\"ohlich mechanism of exciton-phonon coupling. Most notably, anomalous variations in the phonon linewidths with temperature point at the significant role of electron-phonon coupling in these systems, especially for the out-of-plane (${A}_{1g}$) and shear modes (${E}_{2g}^{2}$), which is found to be more prominent in the narrow-gapped 2L than the large gapped 1L. We deciphered the ambiguity in symmetry assignments, especially to the peaks \ensuremath{\sim}170 and $\ensuremath{\sim}350\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ via polarization-dependent measurements. Temperature-dependent thermal expansion coefficient, an important parameter for device performance, is carefully extracted for both 1L and 2L by monitoring the temperature dependence of the real part of the phonon self-energy parameter. Our temperature-dependent in-depth Raman studies pave a way for uncovering the deeper role of phonons in these 2D layered materials from a fundamental as well as application point of view.

Subsiding strain-induced In-Ga intermixing in InAs/InxGa1−xAs sub-monolayer quantum dots for room temperature photodetectors

Strain – induced intermixing in sub – monolayer (SML) quantum dots (QDs) affects primarily the dot size distribution in an erratic way and also the inter – dot coupling efficiency for carrier transport. In this study, we have explored the role of strain and carrier confinement effects in order to control the dot size distribution by varying InAs QDs coverage (X: 0.3 and 0.5 ML) and stacks (Y: 4, 6, and 8) simultaneously. The size variation affects the position of localized levels inside QDs primarily as it is the inter – dot coupling that decides the luminescence efficiency. Next, through multiple stacking of QD layers, strain variation becomes inhomogenous that facilitate a higher degree of In – Ga intermixing which necessitates to estimate the amount of Indium (%) present inside such In–rich islands to progress for a superior optical performance. Hence, we combine and address these above concerns in great detail. The 20 K ground-state (GS) photoluminescence (PL) energy for 0.3 and 0.5 ML samples were centered at 1.24–1.33 eV, and 1.11–1.19 eV respectively. The PL linewidth and activation energy variation with temperature validated that there exists two possible thermal escape pathways for carrier recombination. Besides, PL – excitation (PLE) studies comprehended the influence of phonon and excited states (ES) in the samples. Raman analysis helped us calculate the In% inside QDs by figuring out the shift in InAs QD phonon modes (amount of compressive strain). Furthermore, the structural analysis by high-resolution X-ray diffraction (HRXRD) was done to calculate the strain moderated defect density and also the effect of in – plane compressive strain to the reduction in quantum confinement. The increase in compressive strain inside QDs caused more In–Ga intermixing in 0.3 ML samples and higher defect densities. Altogether we see that sample with 0.5 ML, 4 stacks showed vertically-correlated dot growth (from HR-XTEM) with minimum strain (−0.03383 a.u.), dislocation density (4.887 x 1011 cm−2) and higher In content (62.41%). This high degree of excitonic carrier confinement and strain tuning makes SML QDs an attractive candidate for room-temperature (RT) operable photodetector (PD) devices.

Sunlight Control of Ferromagnetic Damping in Photovoltaic/Ferromagnetic Heterostructures

AbstractThe inexorable trend of spintronics is to develop efficient spin‐control methods. For example, photovoltaic spintronics driven by sunlight has great potential in energy‐saving applications. Here, a ferromagnetic/PV heterojunction is prepared on Si‐substrate as Ta (4 nm)/Co (1 nm)/ZnO (from 10 to 58 nm)/PTB7‐Th: PC71BM/Pt (3 nm) to study how visible light affected magnetic dynamics. A linewidth variation as high as 812.55 Oe is achieved by optimizing the thickness of ZnO film used to enhance the electron transfer. Due to the photovoltaic effect, reversible ferromagnetic resonance (FMR) linewidth switches are displayed, which is attributed to the 3d orbitals occupancy of Co induced the TMS contribution and inhomogeneous broadening. This work proposes a sunlight controllable magnetic damping heterostructure for developing fast, small, energy‐efficient spintronics applications.

Enhancing precision in bioprinting utilizing fuzzy systems

Bioprinting facilitates the generation of complex, three-dimensional (3D), cell-based constructs for a variety of applications. Although multiple bioprinting technologies have been developed, extrusion-based systems have become the dominant technology due to the diversity of substrate materials (i.e. bioinks) that can be accommodated, either individually or in combination. Each bioink has unique material properties and extrusion characteristics that limit bioprinting precision, particularly when generating identically shaped constructs from different bioinks. Here, we aimed to achieve high precision (i.e. repeatability) across samples by generating bioink-specific printing parameters using a systematic approach. We hypothesized that a Fuzzy system could be used as a soft computing method to tackle the inherent vagueness and imprecision in 3D bioprinting data and uncover the optimal printing parameters for a specific bioink that would result in high precision. Our Fuzzy model was used to approximate and quantify the precision and ease of printability for two common bioinks - type I collagen and Pluronic F127, with or without dilution in αMEM culture media. The model consisted of three inputs (pressure, speed, and bioink dilution percentage) and a single output (line width). Using this system, we introduce the Bioink Precision Index (BPI), a metric that can be used to quantify and compare the precision of any bioink regardless of bioprinting technique and environmental parameters. To validate BPI, we demonstrate a significant increase (+54%) in line width variation between parameter sets with high (16.6) and low (7.5) BPI. Finally, we estimate that printing with parameters optimized using BPI would increase the line width precision for collagen (+15%) and Pluronic F127 (+29%) as compared to the manufacturer's recommended printing parameters.

Comparative Study of Computer Vision Based Line Followers Using Raspberry Pi and Jetson Nano

The line follower robot is a mobile robot which can navigate and traverse to another place by following a trajectory which is generally in the form of black or white lines. This robot can also assist human in carrying out transportation and industrial automation. However, this robot also has several challenges with regard to the calibration issue, incompatibility on wavy surfaces, and also the light sensor placement due to the line width variation. Robot vision utilizes image processing and computer vision technology for recognizing objects and controlling the robot motion. This study discusses the implementation of vision based line follower robot using a camera as the only sensor used to capture objects. A comparison of robot performance employing different CPU controllers, namely Raspberry Pi and Jetson Nano, is made. The image processing uses an edge detection method which detect the border to discriminate two image areas and mark different parts. This method aims to enable the robot to control its motion based on the object captured by the webcam. The results show that the accuracies of the robot employing the Raspberry Pi and Jetson Nano are 96% and 98%, respectively.