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At least 397 records · Page 22

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DOE OSTI2019

Design of a Selective Filter Based on One-dimensional Superconductor Photonic Crystal

In this paper, we present a new design and analysis of one-dimensional superconductor photonic crystal (1D-SPC).The transmission spectrum and dispersion relation of this structure are obtained by the use of the transfer matrix method and Bloch theorem. The defective layers existence allows obtaining a new composite structure with tunable optical properties. Considering the transmission matrix of each layer, we can obtain the transmission matrix of the entire structure. The obtained results are presented in terms of the transmission spectra for both TE and TM modes. Numerical results of the effects of temperature, superconductor layer’s thickness, and incident angle on the transmission spectra are presented. We discuss the effect of defect layers inserted between 1D-SPC structure on the transmission spectra. The number of defect modes can be controlled by adjusting the number of defect layers in the structure. The proposed approach is approved by comparison of computed results with previous published data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Landau quantization in tilted Weyl semimetals with broken symmetry

Broken symmetry and tilting effects are ubiquitous in Weyl semimetals (WSMs). Therefore, it is crucial to understand their impacts on the materials’ electronic and optical properties. Here, using a realistic four-band model for WSMs, which incorporates both the symmetry breaking and tilting effects, we study its Landau quantization and the associated magneto-absorption spectrum. We show that the Landau bands in tilted WSMs can be obtained by considering a non-tilt Hamiltonian through Lorentz boost. Furthermore, broken symmetry effects can generate an additional term in the Hamiltonian, which equivalently lead to band reconstruction. Our work provides a more realistic view of the magnetic field response of WSMs that shall be taken into account in relevant future device applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2021

Superradiance and Subradiance due to Quantum Interference of Entangled Free Electrons

When multiple quantum emitters radiate, their emission rate may be enhanced or suppressed due to collective interference in a process known as super- or subradiance. Such processes are well known to occur also in light emission from free electrons, known as coherent cathodoluminescence. Unlike atomic systems, free electrons have an unbounded energy spectrum, and, thus, all their emission mechanisms rely on electron recoil, in addition to the classical properties of the dielectric medium. To date, all experimental and theoretical studies of super- and subradiance from free electrons assumed only classical correlations between particles. However, dependence on quantum correlations, such as entanglement between free electrons, has not been studied. Recent advances in coherent shaping of free-electron wave functions motivate the investigation of such quantum regimes of super- and subradiance. In this Letter, we show how a pair of coincident path-entangled electrons can demonstrate either super- or subradiant light emission, depending on the two-particle wave function. By choosing different free-electron Bell states, the spectrum and emission pattern of the light can be reshaped, in a manner that cannot be accounted for by a classical mixed state. We show these results for light emission in any optical medium and discuss their generalization to many-body quantum states. Furthermore, our findings suggest that light emission can be sensitive to the explicit quantum state of the emitting matter wave and possibly serve as a nondestructive measurement scheme for measuring the quantum state of many-body systems.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

Mid-infrared frequency doubling using strip-loaded silicon nitride on epitaxial barium titanate thin film waveguides

Broadband mid-infrared (mid-IR) frequency doubling was demonstrated using nonlinear barium titanate (BTO) thin films. The device has a strip-loaded waveguide structure consisting of top silicon nitride (SiN) strips and an underneath BTO guiding layer. The epitaxial BTO was deposited on a strontium titanate (STO) substrate by pulsed-laser deposition. Through a SiN grating coupler, the pumping mid-IR light at wavelength λ = 3.30 − 3.45 µ m was coupled into the nonlinear BTO layer, where the spectrum of the near-infrared (NIR) second-harmonic generation was characterized. The developed BTO waveguides provide a platform for mid-IR nonlinear integrated photonics and on-chip quantum optics.

42 ENGINEERING↗

DOE OSTI2022

Metaplectic geometrical optics for ray-based modeling of caustics: Theory and algorithms

The optimization of radio frequency-wave (RF) systems for fusion experiments is often performed using ray-tracing codes, which rely on the geometrical-optics (GO) approximation. However, GO fails at caustics such as cutoffs and focal points, erroneously predicting the wave intensity to be infinite. This is a critical shortcoming of GO, since the caustic wave intensity is often the quantity of interest, e.g., RF heating. Full-wave modeling can be used instead, but the computational cost limits the speed at which such optimizations can be performed. Here, we have developed a less expensive alternative called metaplectic geometrical optics (MGO). Instead of evolving waves in the usual x (coordinate) or k (spectral) representation, MGO uses a mixed X$\equiv$Ax+Bk representation. By continuously adjusting the matrix coefficients A and B along the rays, one can ensure that GO remains valid in the X coordinates without caustic singularities. The caustic-free result is then mapped back onto the original x space using metaplectic transforms. Here, we overview the MGO theory and review algorithms that will aid the development of an MGO-based ray-tracing code. We show how using orthosymplectic transformations leads to considerable simplifications compared to previously published MGO formulas. We also prove explicitly that MGO exactly reproduces standard GO when evaluated far from caustics (an important property that until now has only been inferred from numerical simulations), and we relate MGO to other semiclassical caustic-removal schemes published in the literature. Finally this discussion is then augmented by an explicit comparison of the computed spectrum for a wave bounded between two cutoffs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Considerations in upconversion: A practical guide to sum-frequency generation spectrometer design and implementation

Here, in this tutorial review, we discuss how the choice of upconversion pulse shape in broadband vibrational sum-frequency generation (SFG) spectrometer design impacts the chemical or physical insights one can obtain from a set of measurements. A time-domain picture of a vibrational coherence being mapped by a second optical field is described and the implications of how this mapping, or upconversion process, takes place are given in the context of several popular and emerging approaches found in the literature. Emphasis is placed on broadband frequency-domain measurements, where the choice of upconversion pulse enhances or limits the information contained in the SFG spectrum. We conclude with an outline for a flexible approach to SFG upconversion using pulse-shaping methods and a simple guide to design and optimize the associated instrumentation.

47 OTHER INSTRUMENTATION↗

DOE OSTI2020

Self-trapped hole and impurity-related broad luminescence in $β$-Ga 2 O 3

Here we explore the luminescence properties of self-trapped holes and impurity-related acceptors using one-dimensional configuration coordinate diagrams derived from hybrid functional calculations. The photoluminescence spectrum of as-grown $β$-Ga 2 O 3 typically consists of a broad band in the wavelength region from ultraviolet to green and is often dominated by an impurity independent ultraviolet band that is commonly attributed to self-trapped holes. Here, we use the self-trapped hole as a benchmark to evaluate the accuracy of the theoretical defect luminescence spectra and estimate the optical properties of Mg Ga , Be Ga , Ca Ga , Cd Ga , Zn Ga , Li Ga , and N O acceptor impurities, as well as their complexes with hydrogen donors. We also explore $V$ Ga acceptors complexed with hydrogen and Si Ga donor impurities. The results show that these defects can give rise to broad luminescence bands peaking in the infrared to visible part of the spectrum, making them potential candidates for the defect origin of broad luminescence bands in $β$-Ga 2 O 3 .

36 MATERIALS SCIENCE↗

DOE OSTI2024

Fundamental optical transitions in hexagonal boron nitride epilayers

Fundamental optical transitions in hexagonal boron nitride (h-BN) epilayers grown on sapphire by metal–organic chemical vapor deposition (MOCVD) using triethylboron as the boron precursor have been probed by photoluminescence (PL) emission spectroscopy. The low temperature (10 K) PL spectrum exhibits two groups of emission lines. The first group includes the direct observation of the free exciton and impurity bound exciton (BX) transitions and phonon replicas of the BX transition, whereas the second group is attributed to the direct observation of the band-to-band transition and its associated phonon replicas. The observations of zero-phonon lines of the band-to-band and exciton transitions, which are supposedly forbidden or “dark” in perfect h-BN crystals, result from a relaxed requirement of momentum conservation due to symmetry-breaking in the presence of high concentrations of impurities/defects and strain, which in turn provided more deterministic values of the energy bandgap (E g ), exciton binding energy (E x ), and binding energy of impurity bound excitons (E BX ) in h-BN epilayers. Excitonic parameters of h-BN epilayers grown by MOCVD, carbon-free chemical vapor deposition, and high purity h-BN bulk materials are compared and discussed. The present results, together with available information in the literature, represent a significant improvement in the understanding of the fundamental optical properties and excitonic parameters of h-BN ultrawide bandgap semiconductors.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Influence of local symmetry on lattice dynamics coupled to topological surface states

Herein we investigate coupled electron-lattice dynamics in the topological insulator Bi 2 Te 3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wave vectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman and infrared branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wave functions via ultrafast optical excitation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Benchmark of few-level quantum theory vs ab initio numerical solutions for the strong-field Autler–Townes effect in photoionization of hydrogen

Abstract The temporal and spectral consequences of an intermediate resonance en route to photoionization are investigated theoretically in two ways: by solving few-level model equations and by ab initio numerical solution of the time-dependent Schrödinger equation, in both cases for hydrogen in three dimensions. The model consists of atomic states resonantly field-dressed in a three-level reduction of the hydrogen atom that consists of the 2 p –3 d (Balmer) transition and one energetically-distant continuum state. The model’s level occupation probabilities are derived from three Schrödinger amplitude equations and are benchmarked against an ab initio numerical solution for the hydrogen electron’s wavefunction under the same field. We examine contrasts between the results of the two approaches with a particular focus on Autler–Townes doublets that appear in the photoelectron spectrum.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

A new redback pulsar candidate 4FGL J2054.2+6904

ABSTRACT The Fermi catalogue contains about 2000 unassociated γ-ray sources. Some of them were recently identified as pulsars, including so-called redbacks and black widows, which are millisecond pulsars in tight binary systems with non- and partially-degenerate low-mass stellar companions irradiated by the pulsar wind. We study a likely optical and X-ray counterpart of the Fermi source 4FGL J2054.2+6904 proposed earlier as a pulsar candidate. We use archival optical data as well as Swift/XRT and SRG/eROSITA X-ray data to clarify its nature. Using Zwicky Transient Facility data in g and r bands spanning over 4.7 yr, we find a period of ≈7.5 h. The folded light curve has a smooth sinusoidal shape with the peak-to-peak amplitude of ≈0.4 mag. The spectral fit to the optical spectral energy distribution of the counterpart candidate gives the star radius of 0.5 ± 0.1 R⊙ and temperature of 5500 ± 300 K implying a G2–G9-type star. Its X-ray spectrum is well fitted by an absorbed power law with the photon index of 1.0 ± 0.3 and unabsorbed flux of ≈2 × 10−13 erg s−1 cm−2. All the properties of 4FGL J2054.2+6904 and its presumed counterpart suggest that it is a member of the redback family.

Karpova, A. V. (ORCID:0000000242115856)↗

DOE OSTI2021

Intrinsic spatial chirp of subcycle terahertz pulsed beams

Here, we present an analysis of spatial chirp in subcycle pulsed beams corroborated by spatiotemporal electro-optic sampling measurements of terahertz radiation. Modeling the subcycle pulsed beam as a superposition of monochromatic Gaussian beams, free-space propagation is shown to directly lead to lateral spatial chirp with the spectrum on-axis substantially bluer than the overall energy spectrum. The two-transverse + one-temporal dimensional profile of terahertz subcycle pulsed beams from organic crystals DSTMS and OH1 are measured and compared with observed spatiospectral correlations consistent with the model.

43 PARTICLE ACCELERATORS↗

DOE OSTI2019

On the Amplification of Terahertz Radiation by High-Q Resonant Plasmons in a Periodic Graphene Bilayer under Plasmon-Mode Anticrossing

The spectrum of amplification of terahertz radiation in a structured active graphene bilayer consisting of two identical periodic graphene microribbon arrays separated by a thin dielectric barrier layer is theoretically investigated. The system supports optical and acoustic plasmon modes. The resonant frequencies of the optical and acoustic modes change oppositely with the dielectric-layer thickness, which allows plasmon-mode anticrossing. It is shown that the investigated graphene structure is characterized by a strong plasmon response and giant terahertz-radiation amplification at plasma resonance frequencies in the vicinity of the anticrossing between the optical and acoustic plasmon modes at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

SN 2019yvq Does Not Conform to SN Ia Explosion Models

We present new photometric and spectroscopic observations of SN 2019yvq, a Type Ia supernova (SN Ia) exhibiting several peculiar properties including an excess of UV/optical flux within days of explosion, a high Si ii velocity, and a low peak luminosity. Photometry near the time of first light places new constraints on the rapid rise of the UV/optical flux excess. Furthermore, a near-infrared spectrum at +173 days after maximum light places strict limits on the presence of H or He emission, effectively excluding the presence of a nearby nondegenerate star at the time of explosion. New optical spectra, acquired at +128 and +150 days after maximum light, confirm the presence of [Ca ii] λ7300 and persistent Ca ii NIR triplet emission as SN 2019yvq transitions into the nebular phase. The lack of [O i]$\lambda 6300$ emission disfavors the violent merger of two C/O white dwarfs (WDs) but the merger of a C/O WD with a He WD cannot be excluded. We compare our findings with several models in the literature postulated to explain the early flux excess including double-detonation explosions, 56 Ni mixing into the outer ejecta during ignition, and interaction with H- and He-deficient circumstellar material. Each model may be able to explain both the early flux excess and the nebular [Ca ii] emission, but none of the models can reconcile the high photospheric velocities with the low peak luminosity without introducing new discrepancies.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Toward a More Complete Optical Census of Active Galactic Nuclei via Spatially Resolved Spectroscopy

While emission line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single-fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin–Philips–Terlevich line flux ratio diagnostics classify each galaxy’s central 3" spectrum as LINER or star-forming, while >10% of the spaxels in the galaxy’s MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7–10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low-luminosity AGNs, dust-obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single-fiber spectra.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

A multiwavelength study of the flat-spectrum radio quasar NVSS J141922−083830 covering four flaring episodes

We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J141922−083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog. Relatively bright flares (>3 magnitudes) were observed on 2015 February 21 (MJD 57074) and 2018 September 8 (MJD 58369) in the optical band with the MASTER Global Robotic Net telescopes. Optical spectra obtained with the Southern African Large Telescope on 2015 March 1 (MJD 57082), during outburst, and on 2017 May 30 (MJD 57903), during quiescence, showed emission lines at 5325 Å and at ≈3630 Å that we identified as the Mg II III] 1909 Å lines, respectively, and hence derived a redshift z = 0.903. Analysis of Fermi-Large Area Telescope (LAT) data was performed in the quiescent regime (5 yr of data) and during four prominent flaring states in 2014 February–April, 2014 October–November, 2015 February–March, and 2018 September. We present spectral and timing analysis with Fermi-LAT . We report a hardening of the gamma-ray spectrum during the last three flaring periods, with a power-law spectral index Γ = 2.0–2.1. The maximum gamma-ray flux level was observed on 2014 October 24 (MJD 56954) at (7.57 ± 1.83) × 10 −7 ph cm −2 s −1 . The multiwavelength spectral energy distribution (SED) during the 2015 February–March flare supports the earlier evidence of this blazar to belong to the FSRQ class. The SED can be well represented with a single-zone leptonic model with parameters typical of FSRQs, but also a hadronic origin of the high-energy emission cannot be ruled out.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

The interplay of excitonic delocalization and vibrational localization in optical lineshapes: A variational polaron approach

The dynamics of molecular excitonic systems are complicated by a competition between electronic coupling (which drives delocalization) and vibrational-electronic (vibronic) interactions (which tend to encourage electronic localization). A particular challenge of molecular systems is that they typically possess a large number of independent vibrations, with frequencies often spanning the entire spectrum of relevant electronic energy gaps. Recent spectroscopic observations and numerical simulations on a water-soluble chlorophyll-binding protein (WSCP) reveal a transition between two regimes of vibronic behavior, a Redfield-like regime in which low-frequency vibrations respond to a delocalized excitonic state, and a Förster-like regime where high-frequency vibrations act as incoherent excitations on individual pigments. Although numerical simulations can reproduce these effects, there is a need for a simple, systematic theory that accurately describes the smooth transition between these two regimes in experimental spectra. Here we address this challenge by generalizing the variational polaron transform approach of [Bloemsma et al., Chem. Phys. 481, 250 (2016)] to include arbitrary bath densities for systems with or without symmetry. We benchmark this theory against both numerical matrix-diagonalization methods and experimental 77 K fluorescence spectra for two WSCP variants, obtaining quite satisfactory agreement in both cases. Here, we apply this theory to offer an explanation for the large loss in apparent electronic coupling in the WSCP Q57K mutant and to examine the likely impact of the interplay between excitonic delocalization and vibrational localization on vibrational sideband shapes and apparent coupling strengths in high-resolution optical spectra for chlorophyll-protein complexes such as WSCP.

14 SOLAR ENERGY↗

DOE OSTI2022

Radio and X-Ray Observations of the Luminous Fast Blue Optical Transient AT 2020xnd

We present deep X-ray and radio observations of the fast blue optical transient (FBOT) AT 2020xnd/ZTF 20acigmel at z = 0.2433 from 13 days to 269 days after explosion. AT 2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT 2018cow. AT 2020xnd shows luminous radio emission reaching L ν ≈ 8 × 10 29 erg s -1 Hz -1 at 20 GHz and 75 days post-explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at L X ≈ 6 × 10 42 erg s -1 . Interpreting the radio emission in the context of synchrotron radiation from the explosion's shock interaction with the environment, we find that AT 2020xnd launched a high-velocity outflow (v ~ 0.1c–0.2c) propagating into a dense circumstellar medium (effective $\dot{M}\approx {10}^{-3}\,{M}_{\odot }$ yr -1 for an assumed wind velocity of v w = 1000 km s -1 ). Similar to AT 2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT 2020xnd a high-redshift analog to AT 2018cow, and establish AT 2020xnd as the fourth member of the class of optically luminous FBOTs with luminous multiwavelength counterparts.

79 ASTRONOMY AND ASTROPHYSICS↗