Central Dip Research Articles

Determining the absolute number density of a thermal vapor via photon correlations

We propose a technique to determine the absolute number density N of an alkali-metal vapor confined within a nanocell. This method is based on near-resonant driving of the vapor's constituent atoms and determining the mean interparticle distance (or equivalently the temperature) from the power spectrum associated with the offset photon intensity-intensity correlation function g(2)(τ)−1. In our investigation, we treat the atoms as an average of interacting and radiating dipole pairs randomly positioned within the nanocell. We observe that the power spectrum of the emitted light has a central dip for interparticle distances corresponding to ∼λ/5 and that this result is robust to variations in the driving Rabi frequency and the average detuning. With our proposed method, we can overcome the limitations in defining the absolute number density N, which is currently typically deduced from the temperature of heating elements applied externally to the cell. Published by the American Physical Society 2024

An advanced fabrication method for Yb3+-Doped optical fibers featuring AlPO4 core glass

Ytterbium (Yb)-doped fibers using aluminophosphosilicate (Yb-APS) host glass offer significant advantages over conventional aluminosilicate (Yb-AS) fibers, such as strong photodarkening resistance, higher RE-ion solubility and lower Numerical Aperture (NA), making them ideal for high-power laser applications. However, achieving optimal performance in Yb-APS fiber fabrication poses various challenges, including maintaining a precise Al3+/P5+ ratio (1:1), minimizing the central dip in the refractive index profile (RIP), ensuring proper dopant distribution, and reducing background loss. We introduce a new fabrication method for Yb-APS fiber to address these challenges. This approach entails first synthesizing amorphous Yb³⁺: AlPO4 particles, then incorporating them into the fiber core using a hybrid technique that merges Vapor Phase Delivery (VPD) with solution doping (SD). We extensively utilized various materials and optical characterizations, including XRD, FTIR, Raman spectroscopy, FESEM, and XPS, to fine-tune the composition of Yb³⁺: AlPO4. The optical characterization of the developed preform sample, including absorption, emission, and luminescent lifetime measurements, was conducted to assess the fabrication technique's suitability. The results confirmed the successful incorporation of Yb-ions into the preform core, with peaks consistent with reported values. The fiber's attenuation loss was measured, showing approximately 10 dB/km at 1200 nm. This indicates that the particles dissolved smoothly during preform and fiber processing, resulting in minimal scattering loss in the fiber core. The developed fiber demonstrated NA of 0.1, uniform RIP along the preform length with significant reduction of the central dip formation and improved spectral performance (∼1.7X broader bandwidth) compared with an equivalent Yb-AS fiber. These findings suggest the Yb-APS fiber's overall effectiveness and structural robustness, highlighting the potential of the proposed fabrication method for high-power fiber laser applications.

INTERACTION BETWEEN A BEYOND-BAND DISCRETE SOLITON AND A DIRAC SOLITON IN BINARY WAVEGUIDE ARRAYS

We investigate the intricate interactions between a beyond-band discrete soliton (BBDS) and Dirac soliton in a binary waveguide array (BWA). Through comprehensive analysis, we demonstrate that the behavior of these soliton interactions remains largely unaffected by the presence of central peaks or dips at the center of each soliton, or the initial phase differences between them. Our research highlights the critical role of the BBDS intensity in determining the nature of its interaction with Dirac solitons. We reveal that low-intensity BBDS exhibit consistent attraction towards Dirac solitons across multiple encounters, while high-intensity BBDSs show an initial phase of attraction followed by a repulsion phase, causing a subsequent divergence. These findings not only enhance our understanding of soliton interactions on a fundamental level but also set the stage for the development of innovative optical communication and signal processing technologies leveraging the distinctive properties of BBDSs and Dirac solitons.

Specular transformation of electromagnetic partially coherent beams with a wave-folding interferometer

We consider a class of specular or anti-specular vector beams, by illuminating stochastic vector beams into a prism-based wavefront-folding interferometer. Such transform is applied to various genuine model input beams, and then the properties of the resulting fields are discussed. Numerical results show that the specular nature of these vector fields not only creates sharp internal spectral density distribution, but also produces novel polarization patterns with oscillations or a central dip on the degree of polarization-profile. Such optical characteristics can be flexibly modulated by the correlation structure of the source. We also suggest that the specular transform could be efficiently employed in developing novel partially coherent vector beams.

Shoulder and Knee Arthroscopy Access Point: Prospective Comparison of Sonographic and Palpatory Detection - Which Method is Better for Novices?

Purpose Arthroscopy is one of the most common interventions in orthopedics. Hence it is important to train users early in order to ensure the safest possible identification of access portals (AP). This prospective study aimed to compare a palpatory (PalpMethod) with a sonographic (SonoMethod) method for AP location in the shoulder and knee joints. Materials and Methods The study included trainee doctors (n=68) attending workshops (lasting approx. 90 minutes). In these workshops a teaching video initially demonstrated the PalpMethod and SonoMethod of AP identification. An experienced operator first marked the access portals on the test subject with a UV pen (determined ideal point [DIP]). Adhesive film was then affixed to the puncture regions. Subsequently participants marked on shoulders and knees first the point determined by palpation, then the point determined by sonography. Analysis involved DIP visualization with a UV lamp and employed a coordinate system around the central DIP. In addition, participants completed an evaluation before and after the workshop. Results The analysis included 324 measurements (n=163 shoulders and n= 161 knees). The majority of participants had not previously attended any courses on manual examination (87.9%) or musculoskeletal ultrasound (93.9%). Overall, the markings participants made on the shoulder using the SonoMethod were significantly closer to the DIP than those made by the PalpMethod (Palp 18.8mm ± 14.5mm vs. Sono 11.2mm ± 7.2mm; p<0.001). On the knee, however, the markings made by the PalpMethod were significantly closer to the DIP overall (Palp 8.0mm ± 3.2mm vs. Sono 12.8mm ± 5.2mm; p<0.001). Conclusion The results show that the SonoMethod produces more accurate markings on the shoulder, while the PalpMethod is superior for the knee.

Propagation properties of specular and antispecular twisted Gaussian Schell-model beams

We introduce a class of specular and antispecular twisted Gaussian Schell-model beams, which are generated by inserting a twisted Gaussian Schell-model beam into a wavefront folding interferometer (WFI). The analytical expression for the cross-spectral density function of the beam propagating in free space is derived, and the statistical properties of the optical field are investigated in detail. The results show that the twisted effect is still maintained after the transformation, and the spectral density of the light field always rotates to 90 degrees around the axis during propagation. Furthermore, with appropriate optical field adjustment, the twist effect of the spectral degree of coherence (DOC) can be observed, but in opposite directions to the irradiance profile. We also find that the twisted phase not only controls the rotation of the field, but also effectively modulates the overall spot contour. For the far-field spectral density distribution, a larger twist effect will induce a smaller ellipticity of the beam spot. However, the intensity pattern in the central area is mainly determined by the phase difference of WFI. To be specific, the specular twisted field always has a sharp central peak during propagation, and in the antispecular case it has a central dip. Besides, the DOC distribution can be flexibly adjusted by the source coherence, the twisted phase and the phase difference of the WFI. The results of our work have important applications in the fields of free-space beam communication and particle trapping.

Stationary states and switching dynamics of the self-defocusing nonlinear coupled system with PT-symmetric [formula omitted]-wavenumber Scarf II potential

The stationary states and switching dynamics of the parity-time (PT) symmetric coupled system with k-wavenumber Scarf II potential in the self-defocusing nonlinear regime have been analysed. The eigenmodes with central peaks are evolved in the linear regime, whereas those with central dips are evolved in the defocusing nonlinear regime, for ground, 2nd, and 4th excited states. The threshold condition of the PT-symmetry breaking and the effect of coupling, self-defocusing nonlinearity, and width of the potential on the high and low-frequency modes of the ground and excited states have been analysed. The stability of the eigenmodes is verified using the linear stability analysis. The power-switching dynamics between the gain and lossy channels of the system has been analysed. The input power-dependent switching and the effects of gain/loss and potential width on the switching are discussed.

Slip distribution inversion of seismic sub-fault dip iteration using gradient based optimizer algorithm

This paper considers setting different dips for different sub-faults to fit the actual rupture situation based on the fault rupture of the 2013 Lushan MS7.0 earthquake. Meanwhile, combined with the coseismic GNSS data of the Lushan earthquake, the source parameters and sliding distribution of the Lushan earthquake fault are inversed. Firstly, we use the gradient based optimizer (GBO) in nonlinear inversion to obtain the source parameters of this seismic fault. The inversion results indicate that the strike of the fault is 206.52°, the dip is 44.10°, the length is 21.92 km, and the depth is 12.79 km. To refine the sliding distribution of the seismic fault, the seismic fault is divided into 3 × 3 sub-faults. Then, we fix the central sub-fault dip of 44.10°; the dip of other sub-faults is obtained by iteration. After that, the model is further divided into a fault layer model composed of 23 × 19 sub fault slices, and using the Matlab fitting function is used to fit the dip of the 23 × 19 sub faults. Finally, the Lushan seismic fault plane is established as a shovel structure with steep upper and gentle lower, steep south and gentle north. The slip distribution inversion results indicate that the depth of the slip peak is 13 km, the corresponding maximum slip momentum is 0.67 m, the seismic moment is 1.10 × 1019 N·m and the corresponding moment magnitude is MW6.66. The results above are consistent with the research results of seismology.

Impact of the central refractive index dip of fibers on high-power applications

Central refractive index dip is a common phenomenon in the fibers fabricated by the modified chemical vapor deposition (MCVD) technology, which is the main fabrication technique for high-power laser fibers. In this paper, we present a numerical analysis of the dip effect on high-power-related parameters for the first time, to the best of our knowledge. Three aspects including mode field parameter, beam quality, and bending performance are studied under different dip parameters and bending radii. It is found that the dip is possible to increase the effective mode area and the bending loss, which offers a flexible way to suppress the non-linear effects and filter the higher-order modes by optimizing the dip parameters. Besides, different from the mode area and bending loss, beam quality exhibits an interesting trend when the dip radius increases. The results could facilitate a comprehensive understanding of the dip fiber properties, which also offer guidance to evaluate and design the fiber with central refractive index dip for high-power applications.

Strain and curvature optical fiber laser sensor based on an accurate RI-regulated FMF with high modal power balance

In this paper, a ring fiber laser sensor based on a three layered-core few modes fiber (TL-FMF) is proposed. The home-made TL-FMF is spliced with two pieces of single mode fiber (SMF) at both sides as the sensing and filtering component. In the TL-FMF, an intermediate with higher refractive index (RI) is sandwiched between a central dip and a sub-high RI layer. Due to the special RI, the TL-FMF has an equidistant modal effective RI difference and a balanced modal power of different modes, enabling a highly uniform interference spectrum. Then applying the TL-FMF in proposed ring fiber laser sensor, the sensing experimental results show that its sensitivities in strain and curvature are −2.78 pm/με and −6.17 nm/m−1, respectively, and shows an excellent characteristic of temperature insensitivity. This fiber laser sensor implements the dual-parameter measurement with anti-cross sensitivity, which offers a promising candidate in fiber fabrication monitoring, deformation measurement and bending detection.

Gold-coated split laser-induced periodic surface structures as refractometric sensors

The generation of surface plasmon resonances (SPR) in laser-induced periodic surface structures (LIPSS) allows their application in the field of optical sensing, such as the detection of refractive index variations in gases and liquids. We have fabricated gold-coated LIPSS nanostructures on stainless steel substrates by using femtosecond laser nano-ablation. This technique is a low-cost and high-throughput fabrication method applicable to fast and large-scale manufacturing. The depth profile of the fabricated LIPSS shows a central dip at the top of each ripple that split the geometry. The actual topography is modeled and included in a computational electromagnetism package to obtain the expected optical response under the experimental conditions. The measured and simulated spectral reflectances are compared, and the differences are explained by the departure of the fabricated LIPSS from the ideal topography. The experiments and simulations showed excellent agreement for the main spectral characteristics, like the Fano-like lineshapes of the spectral reflectance. This fitting provides the values used to determine the refractometric performance of the fabricated device, that shows a sensitivity of 518 nm/RIU and a figure of merit of 32 RIU−1 for an aqueous analyte. Our experimental results show that the fabricated devices are competitive in terms of cost and simplicity when compared to existing devices with similar performance.

The super-soft source phase of the recurrent nova V3890 Sgr

Context.The 30-yr recurrent symbiotic nova V3890 Sgr exploded on 2019 August 28 and was observed with multiple X-ray telescopes.Swiftand AstroSat monitoring revealed slowly declining hard X-ray emission from shocks between the nova ejecta and the stellar wind of the companion. Later, highly variable super-soft-source (SSS) emission was seen. AnXMM-Newtonobservation during the SSS phase captured the high degree of X-ray variability in terms of a deep dip in the middle of the observation.Aims.This observation adds to the growing sample of diverse SSS spectra and allows spectral comparison of low- and high-state emission to identify the origin of variations and subsequent effects of such dips, all leading to new insights into how the nova ejecta evolve.Methods.Based on an initial visual inspection, quantitative modelling approaches were conceptualised to test hypotheses of interpretation. The light curve was analysed with a power spectrum analysis before and after the dip and with an eclipse model to test the hypothesis of occulting clumps as in U Sco. A phenomenological spectral model (SPEX) was used to fit the complex Reflection Grating Spectrometer (RGS) spectrum accounting for all known atomic physics. A blackbody source function was assumed, as in all atmosphere radiation transport models, while the complex radiation transport processes were not modelled. Instead, one or multiple absorbing layers were used to model the absorption lines and edges, taking into account all state-of-the-art knowledge of atomic physics.Results.In addition to the central deep dip, there is an initial rise of similar depth and shape, and, after the deep dip, there are smaller dips of ~10% amplitude, which might be periodic over 18.1-min. Our eclipse model of the dips yields clump sizes and orbital radii of 0.5–8 and 5–150 white dwarf radii, respectively. The simultaneousXMM-NewtonUV light curve shows no significant variations beyond slow fading. The RGS spectrum contains both residual shock emission at short wavelengths and the SSS emission at longer wavelengths. The shock temperature has clearly decreased compared to an earlierChandraobservation (day 6). The dip spectrum is dominated by emission lines as in U Sco. The intensity of underlying blackbody-like emission is much lower with the blackbody normalisation yielding a similar radius to that of the brighter phases, while the lower bolometric luminosity is ascribed to lowerTeff. This would be inconsistent with clump occultations unless Compton scattering of the continuum emission reduces the photon energies to mimic a lower effective temperature. However, systematic uncertainties are high. The absorption lines in the bright SSS spectrum are blueshifted by 870 ± 10 km s−1before the dip and are slightly faster, 900 ± 10 km s−1, after the dip. The reproduction of the observed spectrum is astonishing, especially that only a single absorbing layer is necessary while three such layers are needed to reproduce the RGS spectrum of V2491 Cyg. The ejecta of V3890 Sgr are thus more homogeneous than many other SSS spectra indicate. Abundance determination is in principle possible but highly uncertain. Generally, solar abundances are found, except for N and possibly O, which are higher by an order of magnitude.Conclusions.High-amplitude variability of SSS emission can be explained in several ways without having to give up the concept of constant bolometric luminosity. Variations in the photospheric radius can expose deeper lying plasma that could pulse with 18.1 min and that would yield a higher outflow velocity. Also, clump occultations are consistent with the observations.

Investigating the origin of observed central dips in radial metallicity profiles

ABSTRACT Radial metallicity trends provide a key indicator of physical processes such as star formation and radial gas migration within a galaxy. Large integral field unit surveys allow for detailed studies of these radial variations, with recent observations detecting central dips in the metallicity, which may trace the impact of various evolutionary processes. However, the origin of these dips has not been conclusively determined, with suggestions that they may be diagnostic dependent. In this paper, we use the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey to investigate whether the observed dips represent genuine decreases in the central metallicity, or whether they could be an artefact of the diagnostic used. Using a sub-sample of 758 local star-forming galaxies at low inclinations, we investigate in detail the impact of using different strong line diagnostics on the shapes of the returned profiles, and the prevalence of dips. We find no clear evidence of the dips being caused by changing values of the ionization parameter within galaxies. To investigate physical causes, we explore both global and spatially resolved parameters, finding that galaxies exhibiting central dips in the O3N2 metallicity profile have on average lower H α equivalent width values out to $R/R_{\rm {e}} \sim 1.5$, and higher values of DN(4000) in the central regions. We additionally find a higher prevalence of dips in galaxies with high stellar mass, and lower values of global specific star formation rate, suggesting a possible link to central quenching. Nevertheless, these results are dependent on the diagnostic used, suggesting caution should be taken when interpreting observed features in galaxy metallicity gradients.

Fiber laser with built-in FBG and LPFG for simultaneous two parameters sensing

This work aims to measure the liquid level and temperature at the same time. We propose a fiber laser based on Bragg grating (FBG) and a long-period fiber grating (LPFG) for simultaneously two-parameter sensing. It has one wavelength peak induced by the FBG and one central dip induced by the LPFG. The resulting temperature varies from 25 °C to 50 °C in 7.5 nm wavelength drifts of LPFG with linearity R 2 of 0.9994. The sensitivity of the FBG liquid level sensing will be affected by the temperature change, ranging from—0.12 nm cm−1 at low temperature to—0.004 nm cm−1 at high temperature, and the linearity R 2 is about 0.9967. Thanks to the laser cavity, the proposed fiber laser with an extensive dynamic range could be used for remote monitoring.

Fabrication of nanoporous silica rods from curable nanocomposites and their application in Yb-doped fiber lasers

Rare-earth (RE) doped silica glass is essential for constructing high-power fiber lasers. However, the achievement of high doping concentrations with a uniform RE ion dispersion in the SiO2 glass matrix is hindered by clustering and inhomogeneous distribution. Herein, a novel technique—UV-curable nanocomposite technology (UV-CNCT)—is demonstrated for fabricating a nanoporous silica rod with a high doping concentration and uniform dispersion of RE ions in a SiO2 glass matrix. A preform with an Yb-doped P2O5-Al2O3-SiO2 core and pure SiO2 cladding, and a single cladding fiber with a doping concentration of 2.1 wt% (for Yb2O3) are fabricated. A linear all-fiber resonant cavity is constructed to analyze the laser performance. Experimental results reveal the highly uniform dispersion of dopants at a relatively high concentration for both the preform and fiber without aggregation, center dip. The proposed technology is expected to be an effective approach for the fabrication of high-quality RE-doped silica glass for advanced fiber lasers.

Molecules with ALMA at Planet-forming Scales (MAPS). XIII. HCO+ and Disk Ionization Structure

Abstract We observed HCO+ J = 1 − 0 and H13CO+ J = 1 − 0 emission toward the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 as part of the MAPS project. HCO+ is detected and mapped at 0.″3 resolution in all five disks, while H13CO+ is detected (S/N &gt; 6σ) toward GM Aur and HD 163296 and tentatively detected (S/N &gt; 3σ) toward the other disks by a matched filter analysis. Inside a radius of R ∼ 100 au, the HCO+ column density is flat or shows a central dip. At outer radii (≳100 au), the HCO+ column density decreases outward, while the column density ratio of HCO+/CO is mostly in the range of ∼10−5–10−4. We derived the HCO+ abundance in the warm CO-rich layer, where HCO+ is expected to be the dominant molecular ion. At R ≳ 100 au, the HCO+ abundance is ∼3 × 10−11 − 3 × 10−10, which is consistent with a template disk model with X-ray ionization. At the smaller radii, the abundance decreases inward, which indicates that the ionization degree is lower in denser gas, especially inside the CO snow line, where the CO-rich layer is in the midplane. Comparison of template disk models with the column densities of HCO+, N2H+, and N2D+ indicates that the midplane ionization rate is ≳10−18 s−1 for the disks around IM Lup, AS 209, and HD 163296. We also find hints of an increased HCO+ abundance around the location of dust continuum gaps in AS 209, HD 163296, and MWC 480. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Stable filamentary structures in atmospheric pressure microwave plasma torch

This paper experimentally investigates the processes governing the single- and multi-filament regimes in an atmospheric pressure microwave (MW) torch operated in argon. Optical emission spectroscopy and spectral imaging are the principal diagnostics techniques which are employed. MW power is found to be the main parameter controlling the number of filaments. The single-filament regime exhibits many properties typical for surface wave discharges, e.g. a linear decrease in electron density along the axis or the existence of a central dip in the radial/lateral emission profiles. Simple geometric quantities, such as the length or thickness of the filament(s), vary almost linearly with the input MW power, and exhibit discontinuities at successive filament splitting events. These take place at similar values of filament maximum thickness, and may be due to skin-depth limited power transfer. The presence and chemistry of a low-emission intensity plasma shell surrounding the filament(s) is also investigated. The gas temperature is estimated from the OH band and complemented by Schlieren imaging, which revealed that a much larger cone of gas is being heated by filaments than is their diameter.

MCVD-based GRIN-axicon for the generation of scalable Bessel-Gauss beams.

In this Letter, we introduce a graded-index (GRIN)-lens combination named GRIN-axicon, which is a versatile component capable of generating high-quality scalable Bessel-Gauss beams. To the best of our knowledge, the GRIN-axicon is the only optical component that can be introduced in both larger-scale laboratory setups and miniaturized all-fiber optical setups, while having an easy control of the dimensioning of the generated focal line. We show that a GRIN lens with a hyperbolic secant refractive index profile with a sharp central dip and no ripples generates a Bessel-Gauss beam with a high-intensity central lobe when coupled to a simple lens. Such fabrication characteristics are very suitable for the modified chemical vapor deposition (MCVD) process and enable easy manufacturing of an adaptable component that can fit in any optical setup.

Phase control of the transmission in cavity magnomechanical system with magnon driving

We investigate the coherent control of the transmission spectrum in a cavity magnetomechanical system consisting of microwave photon, magnon, and phonon modes, where the microwave cavity is driven by a strong pump field and a weak probe field, and the magnon is driven by a weak microwave source. Different from a single transparency window in the absence of the phonon–magnon interaction, two transparency windows and three absorption dips can be observed in the presence of the phonon–magnon interaction, which originates from the joint interaction of phonon–magnon and photon–magnon. In addition, two absorption dips located at both sides of the central absorption dip can be modulated asymmetrically into amplification and absorption by varying the magnetic field amplitude of the magnon driving field. Interestingly enough, the relative phase of applied fields could have profound effects on both the transmission spectrum and the group delay of the output field by choosing the appropriate magnetic field amplitude of the magnon driving field. The transmission group delay can be switched between positive to negative and vice versa by adjusting the relative phase between the applied fields. The present results illustrate the potential to utilize the relative phase for controlling the microwave signal in the cavity magnomechanical system, as well as guidance in the design of information transduction and quantum sensing.

Partially Coherent Dual Nonparaxial Accelerating Beams

AbstractPartially coherent dual nonparaxial accelerating beams are constructed from the perspective of a superposition of fully coherent modes and incoherent interference effects among these modes. It is found that divergence angle and partial coherence of such beams can be altered by a curvature parameter. At the beginning of the propagation, such beams focus in the case of constructive interference, but they leave a central dip in the destructive case. Additionally, the extension of a crossed‐shaped intensity distribution caused by interference is slower than that of the transverse beam frame evidently. In particular, moving along the nonparaxial accelerating trajectory on the micrometer scale, the main lobes of these beams can diverge to a nearly flat angle. As such, these beams are likely to have applications in microscopic particle manipulation and coherence image processing.