Optical Peak Research Articles

Study on optical absorption of silicene nanodisks under strain modulation

Using the tight-binding Hamiltonian approach, we theoretically study the position of energy level structures and optical absorption peaks by applying stress forces laterally or longitudinally of triangular zigzag silicene nanodisks (TZSN). We consider the effect of strain on the structure. We found that by applying a lateral force similar to that applied to the stretched structure (the force applied to the compressed structure), all absorption peaks can be observed to move to the left or to the right as a whole in the optical absorption spectrum, and this shift is more pronounced at the main absorption peaks, indicating the possibility of TZSN as a strain sensor, and we also considered which size of TZSN would be more suitable as a sensor. On the other hand, it is found that the compressive strain can effectively reduce the light absorption intensity when the incident energy is less than 1.4 eV (0.89 µm). The work described in the system can be used as new material is applied to the sensor and imaging equipment, etc.

GRB-SN Association within the Binary-driven Hypernova Model

Observations of supernovae (SNe) Ic occurring after the prompt emission of long gamma-ray bursts (GRBs) are addressed within the binary-driven hypernova (BdHN) model where GRBs originate from a binary composed of a ∼10M ⊙ carbon–oxygen (CO) star and a neutron star (NS). The CO core collapse gives the trigger, leading to a hypernova with a fast-spinning newborn NS (νNS) at its center. The evolution depends strongly on the binary period, P bin. For P bin ∼ 5 min, BdHNe I occur with energies 1052–1054 erg. The accretion of SN ejecta onto the NS leads to its collapse, forming a black hole (BH) originating the MeV/GeV radiation. For P bin ∼ 10 min, BdHNe II occur with energies 1050–1052 erg and for P bin ∼ hours, BdHNe III occur with energies below 1050 erg. In BdHNe II and III, no BH is formed. The 1–1000 ms νNS originates, in all BdHNe, the X-ray-optical-radio afterglows by synchrotron emission. The hypernova follows an independent evolution, becoming an SN Ic, powered by nickel decay, observable after the GRB prompt emission. We report 24 SNe Ic associated with BdHNe. Their optical peak luminosity and time of occurrence are similar and independent of the associated GRBs. From previously identified 380 BdHN I comprising redshifts up to z = 8.2, we analyze four examples with their associated hypernovae. By multiwavelength extragalactic observations, we identify seven new episodes, theoretically explained, fortunately not yet detected in Galactic sources, opening new research areas. Refinement of population synthesis simulations is needed to map the progenitors of such short-lived binary systems inside our galaxy.

Exploring the electronic transport, magnetic, and optical properties of strongly correlated systems: A numerical analytical continuation approach

The electronic, magnetic and optical properties of the double perovskites Ca2NiOsO6 and Fe-doped derivative were calculated using the full potential linearized augmented plane wave technique through the GGA + U with PBE exchange correlation functionals. The calculations show that both of the systems are half-metallic with Fe-doped system as a ferromagnet, whereas the undoped system shows the ferrimagnetic ordering. Additionally, the study is extended for calculating the Mott parameters through dynamical mean field theory (DMFT) with the maximum entropy model (MEM). It is found that the MIT model parameters (U, [Formula: see text]) for Ca2NiOsO6 and Ca2Fe[Formula: see text]Ni[Formula: see text]OsO6 systems are (5.7[Formula: see text]eV, 6.0[Formula: see text](eV)[Formula: see text]) and (6.0[Formula: see text]eV, 6.0[Formula: see text](eV)[Formula: see text]), respectively. Furthermore, the calculations agree with optical Drude peak analysis. The optical conductivity, reflectivity, absorptivity, ELOSS function, dielectric function refractive index and sum-rule are also explored in relation to the photoinduced behaviors of the materials.

SN 2021gno: a calcium-rich transient with double-peaked light curves

ABSTRACT We present extensive ultraviolet (UV) and optical photometric and optical spectroscopic follow-up of supernova (SN) 2021gno by the ‘Precision Observations of Infant Supernova Explosions’ (POISE) project, starting less than 2 d after the explosion. Given its intermediate luminosity, fast photometric evolution, and quick transition to the nebular phase with spectra dominated by [Ca ii] lines, SN 2021gno belongs to the small family of Calcium-rich transients. Moreover, it shows double-peaked light curves, a phenomenon shared with only four other Calcium-rich events. The projected distance from the centre of the host galaxy is not as large as other objects in this family. The initial optical light-curve peaks coincide with a very quick decline of the UV flux, indicating a fast initial cooling phase. Through hydrodynamical modelling of the bolometric light curve and line velocity evolution, we found that the observations are compatible with the explosion of a highly stripped massive star with an ejecta mass of $0.8\, M_\odot$ and a 56Ni mass of 0.024 M⊙. The initial cooling phase (first light-curve peak) is explained by the presence of an extended circumstellar material comprising ∼$10^{-2}\, {\rm M}_{\odot }$ with an extension of $1100\, R_{\odot }$. We discuss if hydrogen features are present in both maximum-light and nebular spectra, and their implications in terms of the proposed progenitor scenarios for Calcium-rich transients.

AT2018dyk revisited: a tidal disruption event candidate with prominent infrared echo and delayed X-ray emission in a LINER galaxy

ABSTRACT The multiwavelength data of nuclear transient AT2018dyk, initially discovered as a changing-look low-ionization nuclear emission-line region (LINER) galaxy, has been revisited by us and found being in agreement with a tidal disruption event (TDE) scenario. The optical light curve of AT2018dyk declines as a power-law form approximately with index –5/3 yet its X-ray emission lags behind the optical peak by ∼140 d, both of which are typical characteristics for TDEs. The X-ray spectra are softer than normal active galactic nuclei (AGNs) although they show a slight trend of hardening. Interestingly, its rising time-scale belongs to the longest among TDEs while it is nicely consistent with the theoretical prediction from its relatively large supermassive black hole (SMBH) mass (∼107.38M⊙). Moreover, a prominent infrared echo with peak luminosity ∼7.4 × 1042 erg s−1 has been also detected in AT2018dyk, implying an unusually dusty subparsec nuclear environment in contrast with other TDEs. In our sample, LINERs share similar covering factors with AGNs, which indicates the existence of the dusty torus in these objects. Our work suggests that the nature of nuclear transients in LINERs needs to be carefully identified and their infrared echoes offer us a unique opportunity for exploring the environment of SMBHs at low accretion rate, which has been so far poorly explored but is crucial for understanding the SMBH activity.

Computational search for efficient single-photon emitters among the substitutional doping defects in two-dimensional GaSe

Single photon emitters (SPE) in the near-infrared (NIR) spectrum range are critical elements in quantum communication technology. We apply here computational methods to reveal local defects in the GaSe monolayer with properties favorable for such SPEs. Based on an educated guess, we selected twelve defects YX in GaSe, where the Y dopant substitutes the X host atom, X = Ga or Se, and Y = C, Si, Ge, N, P, or As. Our calculations show that two defects, NGa, and PGa, are dynamically unstable. For the remaining defects, we apply the linear response GW method and Bethe-Salpeter equation (BSE) to calculate the electronic structure and optical excitation spectra. Some defects are found to have two sharp peaks (occupied and unoccupied) of the density of the electronic states within the band gap. The NSe-, PSe-, and AsSe-defects have intense sharp an optical excitation peak at energies around 0.8 eV (λ = 1550 nm). Analysis of the formation of the BSE eigenstates associated with these peaks suggests that these excitations can cause a narrow zero-phonon line emission. We thus propose the NSe-, PSe-, and AsSe-defects in the GaSe monolayer as promising SPE in NIR region.

Optical absorption and luminescent behaviour of Ruddlesden-Popper type Sr2(Ce1-xFex)O4 (x = 0–1.0) oxides

An attempt has been made to synthesise iron-doped Sr2(Ce1-xFex)O4 (x = 0–1.0) oxides and characterized with regards to optical absorption and photoluminescence. It exhibits single (orthorhombic), dual (orthorhombic + tetragonal), and single (tetragonal) phase formation with x = 0–0.03, 0.05–0.80, and 0.90–1.0, respectively. The peaks observed at ∼2.71 and ∼2.44 eV (say; x = 0) in photoluminescent spectra are assigned to the charge transfer transition between Ce–O2 and Ce–O1, respectively. These states are arising due to the length difference between terminal and equatorial Ce–O bonds in the CeO6 octahedron. The composition Sr2FeO4 (x = 1.0) depicts peaks at 2.66 and 2.56 eV related to the charge transfer transition (CTT) between Fe–O of FeO6 octahedra. The excitation spectrum Sr2(Ce0.80Fe0.20)O4 exhibits two peaks around 5.84 and 4.90 eV related to the different Ce4+ - O2− distances in the lattice. Moreover, shifting of peaks refers to red shift in spectra and arises due to decrease in Ce–O bond length. The optical absorption peak at 261 nm (say; x = 0) is attributed to transitions from O(2p) to Ce(4f) in the O2− and Ce4+ states. The band gap decreases (3.73–3.59 eV) with iron, and supporting our photoluminescent spectra.

Optical properties of methyl-substituted germanane monolayer in the presence of the external magnetic field, strain and spin-orbit coupling.

In this research, we use the tight-binding model, which includes spin-orbit coupling and an external magnetic field, to describe the optical properties of the methyl-substituted germanane (GeCH3) monolayer. We have applied the Kubo formula, linear response theory, and Green's function approach to calculate the optical absorption coefficient of the GeCH3monolayer. Here, the effects of an external magnetic field, strain, spin-orbit coupling, temperature, and electron/hole doping on the frequency dependence behavior of the optical conductivity have been investigated in detail. Our numerical results show that with increasing the external magnetic field, strain, and electron doping, the weight of the Drude increases. The optical absorption peak decreases and shifts to higher frequencies by applying an external magnetic field, strain, and electron doping. Controlling the optical and electronic properties of GeCH3is leading to use this structure it in the electronic and optoelectronic industries.

Two Candidate Obscured Tidal Disruption Events Coincident with High-energy Neutrinos

Recently, three optical tidal disruption event (TDE) candidates discovered by the Zwicky Transient Facility (ZTF) have been suggested to be coincident with high-energy neutrinos. They all exhibit unusually strong dust infrared echoes, with their peak times matching the neutrino arrival time even better than the optical peaks. We hereby report on two new TDE candidates that are spatially and temporally coincident with neutrinos by matching our sample of mid-infrared outbursts in nearby galaxies (MIRONG) with Gold alerts of IceCube high-energy neutrino events up to 2022 June. The two candidates show negligible optical variability according to their ZTF light curves and can therefore be classified as part of the growing population of obscured TDE candidates. The chance probability of finding two such candidates is about ∼3% by redistributing the MIRONG sources randomly in the Sloan Digital Sky Survey footprint, which will be as low as ∼0.1% (or ∼0.2%) if we limit to sources with increased fluxes (or variability amplitudes) comparable with the two matched sources. Our findings further support the potential connection between high-energy neutrinos and TDEs in dusty environments by increasing the total number of neutrino-associated TDE and TDE candidates to five, although the underlying physics remains poorly understood.

AT 2021loi: A Bowen Fluorescence Flare with a Rebrightening Episode Occurring in a Previously Known AGN

The optical-ultraviolet transient AT 2021loi is located at the center of its host galaxy. Its spectral features identify it as a member of the Bowen fluorescence flare (BFF) class. The first member of this class was considered to be related to a tidal disruption event, but enhanced accretion onto an already active supermassive black hole was suggested as an alternative explanation. Having occurred in a previously known unobscured active galactic nucleus, AT 2021loi strengthens the latter interpretation. Its light curve is similar to those of previous BFFs, showing a rebrightening approximately 1 yr after the main peak (which was not explicitly identified but might be the case in all previous BFFs). An emission feature around 4680 Å, seen in the preflare spectrum, strengthens by a factor of ∼2 around the optical peak of the flare and is clearly seen as a double-peaked feature then, suggesting a blend of N iii λ4640 with He ii λ4686 as its origin. The appearance of O iii λ3133 and possible N iii λλ4097, 4103 (blended with Hδ) during the flare further support a Bowen fluorescence classification. Here we present ZTF, ATLAS, Keck, Las Cumbres Observatory, NEOWISE-R, Swift AMI, and Very Large Array observations of AT 2021loi, making it one of the best-observed BFFs to date. It thus provides some clarity on the nature of BFFs but also further demonstrates the diversity of nuclear transients.

Delayed appearance and evolution of coronal lines in the TDE AT2019qiz

ABSTRACT Tidal disruption events (TDEs) occur when a star gets torn apart by a supermassive black hole as it crosses its tidal radius. We present late-time optical and X-ray observations of the nuclear transient AT2019qiz, which showed the typical signs of an optical-UV transient class commonly believed to be TDEs. Optical spectra were obtained 428, 481, and 828 rest-frame days after optical light-curve peak, and a UV/X-ray observation coincided with the later spectrum. The optical spectra show strong coronal emission lines, including [Fe vii], [Fe x], [Fe xi], and [Fe xiv]. The Fe lines rise and then fall, except [Fe xiv] that appears late and rises. We observe increasing flux of narrow H α and H β and a decrease in broad H α flux. The coronal lines have full width at half-maximum ranging from ∼150−300 km s−1, suggesting they originate from a region between the broad- and narrow-line emitting gas. Between the optical flare and late-time observation, the X-ray spectrum softens dramatically. The 0.3–1 keV X-ray flux increases by a factor of ∼50, while the hard X-ray flux decreases by a factor of ∼6. Wide-field Infrared Survey Explorer fluxes also rose over the same period, indicating the presence of an infrared echo. With AT2017gge, AT2019qiz is one of two examples of a spectroscopically confirmed optical-UV TDE showing delayed coronal line emission, supporting speculations that Extreme Coronal Line Emitters in quiescent galaxies can be echos of unobserved past TDEs. We argue that the coronal lines, narrow lines, and infrared emission arise from the illumination of pre-existing material likely related to either a previous TDE or active galactic nucleus activity.

Theoretical Light-curve Models of the Symbiotic Nova CN Cha—Optical Flat Peak for 3 Yr

CN Cha is a slow symbiotic nova characterized by a 3 yr long optical flat peak followed by a rapid decline. We present theoretical light curves for CN Cha, based on hydrostatic approximation, and estimate the white-dwarf (WD) mass to be ∼0.6 M ☉ for a low metal abundance of Z = 0.004. These kinds of flat-peak novae are border objects between classical novae having a sharp optical peak and extremely slow novae, the evolutions of which are too slow to be recognized as nova outbursts on a human timescale. Theoretically, there are two types of nova envelope solutions—static and optically thick wind—in low-mass WDs (≲0.7 M ☉). Such a nova outburst begins first in a hydrostatic manner, and later it could change to an optically thick wind evolution, due to perturbation by the companion star in the nova envelope. Multiple peaks are a reflection of the relaxation process of the transition. CN Cha supports our explanation of the difference between long-lasting flat-peak novae like CN Cha and multiple-peak novae like V723 Cas, because the companion star is located far outside, and does not perturb, the nova envelope in CN Cha.

First-principles calculations to investigate structural, elastic, electronic, and optical properties of XSrCl3 (X = Li, Na)

The computational analysis of the compounds XSrCl3X=Li and Na is the subject of this research work. We utilized Density Functional Theory (DFT) to analyze the structural, optical, elastic, and electronic characteristics of XSrCl3(X=Li and Na) using the WIEN2K software. Birch Murnaghan curve optimization is performed to ascertain the structural details of both compounds. Using structural analysis, we learned that every compound displays a stable structural state at its optimum or ground state. The IRelast Package, which calculates elastic constants, is used to determine the elastic characteristics. Calculating these elastic characteristics reveals the compound's ductile or brittle nature, isotropy or anisotropy, and mechanical stability. Both materials are anisotropic and stable, as shown by an evaluation of elastic constants. Examining the Cauchy pressure, Poisson's ratio and Pugh ratio reveals that LiSrCl3 is a ductile compound while NaSrCl3 is brittle. We used generalized gradient approximation (GGA) to examine the electrical characteristics (Band Structure and Density of States). According to their computed band structures, these substances have wide indirect energy band gaps (R-Γ), or 3.57 eV for LiSrCl3 and 3.62 eV for NaSrCl3, which are indicative of semiconducting properties. To quantify the involvement of electronic states in the band structure, total and partial densities of states (TDOS and PDOS) are utilized. The optical characteristics of radiation up to 15 eV are estimated. Following an investigation of their optical properties, we measured these compounds' refractive indexes, absorption coefficients, and reflectivity. According to the anticipated electronic structure, the major optical spectrum peaks are listed.

Tradeoff Between Diversity and Multiplexing Gains in Block Fading Optical Wireless Channels

The diversity-multiplexing tradeoff (DMT) provides a fundamental performance metric for different multiple-input multiple-output (MIMO) schemes in wireless communications. In this paper, we explore the block fading optical wireless communication (OWC) channels and characterize the DMT in the presence of both optical peak- and average-power constraints. Three different fading distributions are considered, which reflect different channel conditions. In each channel condition, we obtain the optimal DMT when the block length is sufficiently large, and we also derive the lower and upper bounds of the DMT curve when the block length is small. These results are dramatically different from the existing DMT results in radio-frequency (RF) channels. These differences may be due to the fact that the optical input signal is real and bounded, while its RF counterpart is usually complex and unbounded.

Optical Color of Type Ib and Ic Supernovae and Implications for Their Progenitors

Type Ib and Ic supernovae (SNe Ib/Ic) originate from hydrogen-deficient massive star progenitors, of which the exact properties are still much debated. Using SN data in the literature, we investigate the optical B − V color of SNe Ib/Ic at the V-band peak and show that SNe Ib are systematically bluer than SNe Ic. We construct SN models from helium-rich and helium-poor progenitors of various masses using the radiation hydrodynamics code STELLA and discuss how the B − V color at the V-band peak is affected by the 56Ni to ejecta mass ratio, 56Ni mixing, and the presence/absence of a helium envelope. We argue that the dichotomy in the amount of helium in the progenitors plays the primary role in making the observed systematic color difference at the optical peak, in favor of the most commonly invoked SN scenario that SNe Ib and SNe Ic progenitors are helium rich and helium poor, respectively.

A DFT study of structural, electronic and optical properties of Lead-free and Ge based cubic perovskite RbGeX‌3 (X= I, Br and Cl)

The current research uses density functional theory (DFT) approximations in conjunction with the plane wave-pseudopotential method to investigate structural, electronic, and optical properties of Pb-free cubic perovskite RbGeX3 (X= I, Br and Cl) materials. More specifically, Norm-conserving pseudopotential has been employed to describe the ion and valence electrons interaction, and Perdew-Burke-Ernzerhof (PBE) flavor is used to represent the exchange-correlation part of the energy of the GGA approximation. Our calculated lattice constants are 5.95, 5.55, and 5.29 Å for RbGeX3 (where X=I, Br, and Cl), respectively, and they are of are in good agreement with available empirical and other values. The band structure shows the direct band gap nature of the three compounds under research here and our values of the band gap energy E_g are in good agreement with the other available results. Materials under research show response to the electromagnetic radiation starting from the infrared region to the very high energies (~33 eV). The RbGeI3 has the lowest E_g value at the low region energies and the highest optical response peaks but RbGeCl3 has the highest optical response peaks at energies located near ~20 eV. Our results show that these materials are good candidates for photo electronic applications including solar cells.

Molarity Influenced Interesting Electrochromic Optical Modulation Peak Shift in Nebulized Spray Deposited Mn3O4 Films

Herein, we report the investigations on the electrochromic properties of nebulized-spray deposited Mn3O4 thin films in Na2SO4 aqueous solution as a function of molar concentration, for the first time. Phase analysis reveals that the films possess a tetragonal structure. From the Raman study, strong Mn2+ breathing vibration (in Mn–O) occurred in tetrahedral sites (of spinel Mn3O4). At 0.02 M, the film surface is covered with very-tiny particles with 84% highest optical transparency average. Both transmittance and absorbance related properties of electrochromic states are consider here, while introducing the notions of transmittance modulation (ΔT), absorbance modulation (ΔA), transmittance modulation efficiency (TME) and absorbance modulation efficiency (AME (or) coloration efficiency). When the concentration increases, the red shift was occurred at highest peaks of ΔT, ΔA, TME and AME with respect to the decrease in optical band gap. The maximum AME and TME of 25.064 cm2 C−1 (at 369.1 nm) and 17.542 cm2 C−1 (at 438.6 nm) were obtained for prepared samples. After the 100th cycle, the average AME (and TME) values in the UV and visible regions are decreases from 18.910 to 2.783 cm2 C−1 (2.884 to 1.060 cm2 C−1) and from 11.089 to 4.772 cm2 C−1 (11.346 to 4.684 cm2 C−1), respectively, indicating that the film is electrochromically active in the visible region even after the 100th cycle.

The optical gain of dilute bismuth GaAs nanowires under the joint uniaxial stresses

Combining the effective-mass theory with valence-band anticrossing model, the electronic structures and optical gain of dilute bismuth (Bi) GaAs nanowires under the uniaxial stresses are studied. The calculations manifest the band gap of GaAs1−xBix nanowires will be lowered with increasing bismide composition and [100] direction uniaxial stress, which can cause the red shift of optical gain peaks of nanowires. Moreover, it is found that, although almost pure optical gain along z direction can be obtained via applying single [100] direction stress, the first gain peaks are hard to be tuned to the optimal optical communication band, which makes it possible to impose another small uniaxial stress along [001] direction simultaneously to further redshift the gain peaks, and the ultimate results prove that the effect under the joint uniaxial stresses is remarkable. Our investigations mean that GaAs1−xBix nanowires is suitable for optoelectronic devices in optical communication band through appropriate modulated methods.

Optical Gain of a Spherical InAs Quantum Dot under the Effects of the Intense Laser and Magnetic Fields

In quantum dots, where confinement is strong, interactions between charge carriers play an essential role in the performance of semiconductor materials for optical gain. Therefore, understanding this phenomenon is critical for achieving new devices with enhanced features. In this context, the current study examines the optical properties of an exciton confined in a spherical InAs quantum dot under the influence of magnetic and intense laser fields. We investigate the oscillator strength, exciton lifetime, and optical gain, considering the effects of both external fields. We also pay particular attention to the influence of quantum dot size on the results. Our calculations show that the two external fields have opposite effects on our findings. Specifically, the applied magnetic field increases the oscillator strength while the intense laser reduces it. In addition, the optical gain peaks are redshifted under the application of the intense laser, whereas the magnetic field causes a blueshift of the peak threshold. We also find that both external perturbations significantly influence the exciton lifetime. Our study considers the outcomes of both the exciton’s ground (1s) and first excited (1p) states. The theoretical results obtained in this study have promising implications for optoelectronic devices in the ∼3–4 μm wavelength range only through the control of quantum dot sizes and external perturbations.

Optical polarization from colliding stellar stream shocks in a tidal disruption event.

A tidal disruption event (TDE) occurs when a supermassive black hole rips apart a passing star. Part of the stellar material falls toward the black hole, forming an accretion disk that in some cases launches a relativistic jet. We performed optical polarimetry observations of a TDE, AT 2020mot. We find a peak linear polarization degree of 25 ± 4%, consistent with highly polarized synchrotron radiation, as is typically observed from relativistic jets. However, our radio observations, taken up to 8 months after the optical peak, do not detect the corresponding radio emission expected from a relativistic jet. We suggest that the linearly polarized optical emission instead arises from shocks that occur during accretion disk formation, as the stream of stellar material collides with itself.