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NASA NTRS2014

Frequency-Modulated, Continuous-Wave Laser Ranging Using Photon-Counting Detectors

Optical ranging is a problem of estimating the round-trip flight time of a phase- or amplitude-modulated optical beam that reflects off of a target. Frequency- modulated, continuous-wave (FMCW) ranging systems obtain this estimate by performing an interferometric measurement between a local frequency- modulated laser beam and a delayed copy returning from the target. The range estimate is formed by mixing the target-return field with the local reference field on a beamsplitter and detecting the resultant beat modulation. In conventional FMCW ranging, the source modulation is linear in instantaneous frequency, the reference-arm field has many more photons than the target-return field, and the time-of-flight estimate is generated by balanced difference- detection of the beamsplitter output, followed by a frequency-domain peak search. This work focused on determining the maximum-likelihood (ML) estimation algorithm when continuous-time photoncounting detectors are used. It is founded on a rigorous statistical characterization of the (random) photoelectron emission times as a function of the incident optical field, including the deleterious effects caused by dark current and dead time. These statistics enable derivation of the Cramér-Rao lower bound (CRB) on the accuracy of FMCW ranging, and derivation of the ML estimator, whose performance approaches this bound at high photon flux. The estimation algorithm was developed, and its optimality properties were shown in simulation. Experimental data show that it performs better than the conventional estimation algorithms used. The demonstrated improvement is a factor of 1.414 over frequency-domainbased estimation. If the target interrogating photons and the local reference field photons are costed equally, the optimal allocation of photons between these two arms is to have them equally distributed. This is different than the state of the art, in which the local field is stronger than the target return. The optimal processing of the photocurrent processes at the outputs of the two detectors is to perform log-matched filtering followed by a summation and peak detection. This implies that neither difference detection, nor Fourier-domain peak detection, which are the staples of the state-of-the-art systems, is optimal when a weak local oscillator is employed.

Erkmen, Baris I.↗

DOE OSTI2025

Distinct Optical Excitation Mechanisms of a Coherent Magnon in a van der Waals Antiferromagnet

The control of antiferromagnets with ultrashort optical pulses has emerged as a prominent field of research. Tailored laser excitation can launch coherent spin waves at terahertz frequencies, yet a comprehensive description of their generation mechanisms is still lacking despite extensive efforts. Using terahertz emission spectroscopy, we investigate the generation of a coherent magnon mode in the van der Waals antiferromagnet NiPS 3 under a range of photoexcitation conditions. By tuning the pump photon energy from transparency to resonant with a 𝑑−𝑑 transition, we reveal a striking change in the coherent magnon’s dependence on the pump polarization, indicating two distinct excitation mechanisms. Furthermore, our findings provide a strategy for the manipulation of magnetic modes via photoexcitation around subgap electronic states.

Light-induced magnetic effects↗

DOE OSTI2022

Black Hole Flares: Ejection of Accreted Magnetic Flux through 3D Plasmoid-mediated Reconnection

Magnetic reconnection can power bright, rapid flares originating from the inner magnetosphere of accreting black holes. We conduct extremely high-resolution (5376 × 2304 × 2304 cells) general-relativistic magnetohydrodynamics simulations, capturing plasmoid-mediated reconnection in a 3D magnetically arrested disk for the first time. We show that an equatorial, plasmoid-unstable current sheet forms in a transient, nonaxisymmetric, low-density magnetosphere within the inner few Schwarzschild radii. Magnetic flux bundles escape from the event horizon through reconnection at the universal plasmoid-mediated rate in this current sheet. The reconnection feeds on the highly magnetized plasma in the jets and heats the plasma that ends up trapped in flux bundles to temperatures proportional to the jet's magnetization. The escaped flux bundles can complete a full orbit as low-density hot spots, consistent with Sgr A* observations by the GRAVITY interferometer. Reconnection near the horizon produces sufficiently energetic plasma to explain flares from accreting black holes, such as the TeV emission observed from M87. The drop in the mass accretion rate during the flare and the resulting low-density magnetosphere make it easier for very-high-energy photons produced by reconnection-accelerated particles to escape. The extreme-resolution results in a converged plasmoid-mediated reconnection rate that directly determines the timescales and properties of the flare.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1978

Astronomical monochromatic imaging as applied to the Io sodium cloud

Four types of instruments for recording monochromatic astronomical images are compared: filters; slitless spectrographs; multislit spectrographs; and interferometers. Each of these instruments has been used on the sodium emission cloud of Io, with varying degrees of success. Multislit spectrographs and interferometers encode the signal, and it can be extracted with a noise level close to that for photon statistics. This is not normally the case for filters and slitless spectrographs. On balance, it is found that a Mach-Zehnder interferometer provides the best system for imaging faint monochromatic signals on a brighter continuum background.

Murcray, F. J.↗

NASA NTRS1990

Starburst outflows from nearby galaxies

Starburst outflows from NGC 5461, 1569 and M82 are discussed. The Sc I galaxy, M101, is reknowned for the kpc-size superassociations of star clusters and HII regions that dominate its spiral arms. NGC 5461 is one of the brightest of these superassociations, rivaling the Large Magellanic Cloud in H alpha luminosity. The NGC 5461 superassociation is dominated by a single unresolved HII region of outstanding luminosity (approx. 1000 Orion nebulae). Detailed examination of corresponding continuum images indicates that only the southern plume has any sort of stellar counterpart. The other plumes are clearly diffuse with no underlying hot stars. An image of NGC 1569 is discussed. Besides showing the peculiar arm noted by Zwicky (1971) and the filamentary extensions to the North and South (as noted by Hodge 1974), this image also reveals two arc-like features of diffuse ionized gas to the South. Both arcs are concentric with the bright center of the galaxy - where the super star clusters, A and B are located. The inner arc (Arc 1) appears to follow the same curve as the SW arm thus suggesting that the two features represent limb-brightened fragments of vast superbubble that was blown out by a central starburst sometime in the past. As the classic starburst galaxy, M82 displays all the luminous hallmarks of intense high-mass star formation and outflow activity. The diffuse H alpha and x ray emitting gas along the minor axis provides especially good evidence for a bipolar outflow of hot gas which is shock heating the swept-up interstellar medium (ISM) to temperatures of approx. 10(exp 4) K. An image shows the H alpha emission within the disk and along the minor axis. Another image shows the same field in the light of near-infrared. Both figures are based on charge coupled device images taken with the McGraw-Hill 1.3 m telescope (Waller 1989). The longer wavelength emission clearly shows a more extended morphology along the major axis. The morphological discrepancy is most likely due to the greater obscuration by dust suffered by the H alpha photons. A third image shows the map of visual extinction that researchers derive from a pixel-by-pixel comparison of the (SIII) and H alpha fluxes. The greatest extinctions are evident along an arc that includes two especially obscured regions on opposite sides of the bursting nucleus.

Waller, William H.↗

NASA NTRS1991

The featureless continua and hydrogen lines of Seyfert 2 galaxies

Optical and ultraviolet spectra taken in similar sized large apertures for 15 Seyfert 2 galaxies are presented. Measurements of emission-line strengths of Ly-alpha, H-alpha, and H-beta are used together with measurements of the ultraviolet slopes to estimate the ratio of recombination photons to ionizing photons. The photon ratios indicate that the ionizing continuum is being emitted anisotropically for at least six of the eight objects with UV slope measurements. The median value of the UV slope is indistinguishable from the UV slope of Seyfert 1 galaxies. This result suggests that in the occultation/reflection picture of Seyfert nuclei, the reflecting particles are free electrons and not dust. Previous observations of variability in Mrk 477 are verified, and it is shown that this galaxy exhibits characteristics of both Seyfert 1 and Seyfert 2 galaxies.

Kinney, A. L.↗

NASA NTRS1994

X-ray spectrum and variability of the quasar PG 1211+143

We present preliminary results of an ASCA observation of the classic soft-excess quasar PG 1211+143. The overall ASCA spectrum can be characterized by a blackbody with a temperature of approximately 125 eV (quasar frame) and a power law with photon index of approximately 2. Simultaneous ROSAT data are suggestive of further steepening of the spectrum just below the ASCA band. Comparison with previous observations shows that the soft flux in the 0.1-2 keV band varies by at least a factor of approximately 16, scaling roughly as the square of the hard flux in the 2-10 keV band over a timescale of approximately 13.5 yr. We also find evidence of short-term amplitude variability of up to a factor of approximately 2 on a timescale of approximately 2 x 10(exp 4) sec, in both the soft and hard flux so that the soft and hard photons are likely to originate from the same, compact, region. The data rule out variable absorption (cold or ionized) as the origin of the soft excess, favoring an intrinsic emission component. However, we argue against optically thin emission for the 'blue bump' in PG 1211+143. The large amplitude soft X-ray variability may be indicative of variations in the effective temperature, or peak, of the soft component. There is only marginal evidence for Fe K line emission between 6-7 keV in the quasar frame.

Yaqoob, Tahir↗

NASA NTRS1994

COMPTEL upper limits on gamma-ray line emission from Supernova 1991T

The imaging Compton telescope COMPTEL on board the Compton Gamma-Ray Observatory (CGRO) measures gamma-rays in the energy range 0.75-30 MeV with an energy resolution of 9.7% full width at half maximum (FWHM) at 1 MeV. From June 15 to 28, 1991 and again from October 3 to 17, 1991 the region containing the supernova SN 1991T was observed. A search for gamma-ray line emission from the supernova yields no detection of line emission from the supernova. 2 sigma upper limits for the two predicted lines at 847 keV and at 1.238 MeV of approximately equal to 3 x 10(exp -5) photons/(sq cm)(s) were derived. These limits are compared with the predictions of some theoretical models and constraints imposed by these limits on these models are discussed.

Lichti, G. G.↗

NASA NTRS2010

Highlights of GeV Gamma-Ray Astronomy

Because high-energy gamma rays are primarily produced by high-energy particle interactions, the gamma-ray survey of the sky by the Fermi Gamma-ray Space Telescope offers a view of sites of cosmic ray production and interactions. Gamma-ray bursts, pulsars, pulsar wind nebulae, binary sources, and Active Galactic Nuclei are all phenomena that reveal particle acceleration through their gamma-ray emission. Diffuse Galactic gamma radiation, Solar System gamma-ray sources, and energetic radiation from supernova remnants are likely tracers of high-energy particle interactions with matter and photon fields. This paper will present a broad overview of the constantly changing sky seen with the Large Area Telescope (LAT) on the Fermi spacecraft.

Thompson, David J.↗

NASA NTRS2014

Inferred Cosmic-Ray Spectrum from Fermi-LAT Gamma-Ray Observations of the Earths Limb

Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly-discovered features may offer a clue to the origin of high-energy CRs. We use the Fermi Large Area Telescope observations of the -ray emission from the Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range approx. 90 GeV-6 TeV (derived from a photon energy range 15 GeV-1 TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index 2.68 +/- 0.04 and 2.61 +/- 0.08 above approx. 200 GeV, respectively.

fermi↗

NASA NTRS1989

Nonthermal synchrotron radiation and pair annihilation in gamma-ray bursts

A self-consistent Monte Carlo simulation of quantized synchrotron emission, pair production, and pair annihilation in strong (2 x 10 to the 12th G or greater) magnetic fields is presented as a model of the production of annihilation lines in gamma-ray burst sources. The calculation determines the self-consistent photon spectrum and pair distribution functions resulting from a nonthermal injection of electrons into a region of homogeneous, strong magnetic field. The results indicate that the appearance of observable annihilation features in gamma-ray burst spectra requires isotropic or fan-beamed injection of electrons with energies of at least 5 MeV. The lines are narrow because higher energy electrons escape annihilation and do not contribute to Doppler-broadening. Injection beamed along the field did not produce observable annihilation lines in any of the cases studied. As a result, emission at energies much greater than 1 MeV produced through beaming seems to be incompatible with observable annihilation in this model. These results are discussed in relation to observed features in gamma-ray burst spectra.

Preece, Robert↗

DOE OSTI2025

Observation and origin of multi-THz quantum beats in superfluorescent emission from argon

We report observation of multi-THz quantum beats in the amplitude of superfluorescence in pure argon gas, using a time delayed 780-nm probe beam that interrogates the relevant population densities. The superfluorescence is induced by three-photon absorption of an ultrashort 260.5-nm laser pulse that creates coherent and coupled populations in multiple closely spaced 3⁢d and 5⁢s Ar levels. Argon pressure and pump laser focusing parameters are seen to modify the frequency of the quantum beats, which is attributed to the absorption of the self-generated, nonresonant third harmonic of the pump from Ar near the zero of the real part of the overlapping linear susceptibilities of the closely spaced 3⁢d and 5⁢s excited states. The self-absorption of the phase-matched THG photons creates a coherent population of atoms which acts like an atomic excited state (3⁢ω-induced state). Furthermore, we have also observed a new two-step superfluorescent cascade that results from the population redistribution enabled by the 780-nm probe pulse between the 3⁢d'[5/2] 3 and the 3⁢ω-induced state and gives rise to a previously unknown transition that emits on 1587 nm.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2021

Single-Particle Spectroscopy as a Versatile Tool to Explore Lower-Dimensional Structures of Inorganic Perovskites

The remarkable defect tolerant nature of inorganic cesium halide perovskites leading to near unity photoluminescence (PL) quantum yield (QY) and narrow emission linewidth across the entire visible spectrum, have provided a tantalizing platform for a development of a plethora of light-emitting applications. Recently, lower dimensional (2D, 1D, and 0D) perovskites have attracted further attention due to the enhanced thermal, photo and chemical stability as compared to their three-dimensional (3D) analogues. Combination of the external size quantization and internal octahedral organization provides a unique opportunity to study and harness “multidimensional” electronic properties engineered both on atomic scale and the nanoscale. However, crucial research to understand the elementary charge carrier dynamics in lower dimensional perovskites lags far behind the enormous effort to incorporate them into optoelectronic devices. In this Perspective, we provide a review of recent developments that focus on studies of the dynamics of excitonic complexes in Cs-based perovskite nanocrystals using single-particle time-resolved PL spectroscopy and photon correlation measurements. Single photon statistical studies not only offer the unprecedented level of detail to directly assess various recombination pathways but also provide insights into specifics of carrier’s localization. Further, we discuss the underlying physicochemical processes that govern PL emission and draw attention to a number of attributes within this class of the materials, especially lower-dimensional perovskites that may indicate the common origin of the PL emission, as well as provide a route map for the vast unexplored territories where single particle spectroscopy can be a powerful tool to unravel crucial information.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS2007

The Use of the BAT Instrument on SWIFT for the Detection of Prompt Gamma-Ray Emission from Novae

Gamma-rays are expected to be emitted during and immediately following a nova explosion due to the annihilation of positrons emitted by freshly produced short-lived radioactive isotopes. The expected gammaray emission is relatively short-lived and as nova explosions are unpredictable, the best chance of detecting the gamma-rays is with n wide field instrument. At the time when the flux is expected to rcach its peak, most of the gamma-ray production is at depths such that the photons suffer several Compton scatterings before escaping, degrading their energy down to the hard X-ray band (10s of keV). SWIFT/BAT is a very wide field coded mask instrument working in the energy band 14-190 keV and so is very well suited to the search for such gamma-rays. A retrospective search is being made in the BAT data for evidence for gamma-ray emission from the direction of novae at around the time of their explosion. So far the only positive detection is of RS Ophiuchi and in this case the emission is probably due to shock heating.

Skinner, Gerry↗

DOE OSTI2023

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. We aim to acquire a set of very low-redshift SFGs that exhibit signs of a hard radiation field being present. A subsequent analysis of their emission line properties is intended to shed light on how the conditions prevalent in these objects compare to those predicted to be present in early SFGs that are thought to be LyC emitters (LCEs). We used archival spectroscopic SDSS DR12 data to select a sample of low-redshift He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an active galactic nucleus, which serves as our only selection criterion. We performed a population spectral synthesis with FADO to reconstruct these galaxies’ star-formation histories (SFHs). Utilising the spectroscopic information at hand, we constrained the predominant ionisation mechanisms in these galaxies and inferred information on interstellar medium (ISM) conditions relevant for the escape of LyC radiation. Our final sample consists of eighteen ionised, metal-poor galaxies (IMPs). These low-mass (6.2 ≤ log(M * /M ⊙ ) ≤ 8.8), low-metallicity (7.54 ≤log(O/H) + 12 ≤ 8.13) dwarf galaxies appear to be predominantly ionised by stellar sources. We find large [OIII] 5007/[OII] 3727 ratios and [SII] 6717,6731/Hα deficiencies, which provide strong indications for these galaxies to be LCEs. At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions ≳10%. The IMPs’ SFHs exhibit strong similarities and almost all galaxies appear to contain an old (> 1 Gyr) stellar component, while also harbouring a young, two-stage (~10 Myr and < 1 Myr) starburst, which we speculate might be related to LyC escape. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme ISM conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z < 0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Electrically controllable chirality in a nanophotonic interface with a two-dimensional semiconductor

Chiral nanophotonic interfaces enable propagation direction-dependent interactions between guided optical modes and circularly dichroic materials. Electrical tuning of interface chirality would aid active, switchable non-reciprocity in on-chip optoelectronic and photonic circuitry, but remains an outstanding challenge. Here, in this paper, we report electrically controllable chirality in a nanophotonic interface with atomically thin monolayer tungsten diselenide (WSe 2 ). Titanium dioxide waveguides are directly fabricated on the surface of low-disorder, boron nitride-encapsulated WSe 2 . Following integration, photoluminescence from excitonic states into the waveguide can be electrically switched between balanced and directionally biased emission. The operational principle leverages the doping-dependent valley polarization of excitonic states in WSe 2 . Furthermore, the nanophotonic waveguide can function as a near-field source for diffusive exciton fluxes, which display valley and spin polarizations that are inherited from the interface chirality. Our versatile fabrication approach enables the deterministic integration of photonics with van der Waals heterostructures and could provide optical control over their excitonic and charge-carrier behaviour.

42 ENGINEERING↗

NASA NTRS1997

Spectral and Temporal Properties of Galactic Black Hole Systems

Kusunose, Mineshige & Yamada (1996; hereafter KMY) extended the model of Kusunose & Mineshige (1995) to the Galactic black hole candidates by considering nonthermal electron injection with gamma(EQ\0(,\s\up2(less than),\s\do-l(_))) 10. The effects of pair escape and advection on the disk structure and general relativistic effects on the emission spectrum were also examined. They found that the energy spectral index (alpha)(sub x) of the power law X-rays is about-0.8 and-2.0 when 1(sub soft)/1 = 0.2 and 2, respectively, where 1(sub soft)/1 is the ratio of the compactness of the injected soft photons to that of the gravitational energy. The power law index was found to be nearly independent of the mass accretion which is consistent with the observed luminosity independence. The model with small 1(sub soft)/1 (less than 1) shows promise for explaining the low state observed in Galactic black hole candidates. Model fits were provided for GX339-4 and Cyg X- 1 data from COMPTEL and OSSE on the Compton Gamma Ray Observatory. The difference in emission spectra between thermal disks and the model of KMY appears only in the energy range greater than 100 keV. Li, Kusunose and Liang (1996) studied stochastic particle acceleration to produce nonthermal particle distributions which then were used in the model of Kusunose & Mineshige (1995) to model the spectrum above 1 Mev from GBHC's. Under certain conditions, stochastic electron acceleration overcame Coulomb and Compton losses resulting in a suprathermal electron population. Good fits were obtained by COMPTEL and OSSE observations of Cyg X-1 and GRO J0422+23. Kusunose & Mineshige (1996a) examined the role of electron-positron pairs in advection-dominated disks. They found that the results for advection-dominated disks without pairs are not qualitatively changed by including pairs. Summaries of work sponsored by this grant are given in Wheeler, Kim, Moscoso, Kusunose & Mineshige (1996) and Kusunose (1996) Work was also done on developing a model for an e(+-) pair wind from the inner disk region of a black hole. The model consists of three zones: a pair production/annihilation zone at the base of the wind, a pair annihilation zone slightly further out from the disk and a wind zone extending to infinity where no annihilation occurs. The model assumes an input X-ray / gamma-ray power-law spectrum as a function of photon energy and angular distribution. Pairs are created in the pair production/annihilation zone via photon-photon, photonparticle and particle-particle collisions. The bulk velocity of the pairs is obtained from the momentum component perpendicular to the disk taking into account the radiation pressure acceleration of the pairs. Energy balance in the pair production/annihilation zone is calculated by assuming that the momentum component parallel to the disk is thermalized and by taking into account heating/cooling via anisotropic thermal Compton scattering along with the cooling processes of bremsstrahlung and synchrotron radiation. The pair density is also calculated. The outer boundary of the pair production/annihilation zone is where the optical depth for photon-photon pair produciton is unity. Beyond this point only pair annihilation will occur in the pair annihilation zone. When the pair density becomes small, pair annihilation will be negligible and the pairs will flow freely to infinity. In this model we have found that the X-ray power-law spectral index, alpha(sub x) is the primary parameter which determines the density, temperature and velocity of the pair production/annihilation zone (Moscoso, Kusunose & Wheeler 1996).

Wheeler, J. Craig↗

DOE OSTI2021

Harnessing the photonic local density of states in graphene moiré superlattices

In this work we investigate the electromagnetic local density of states (LDOS) near a twisted bilayer graphene (TBG) deposited over a general isotropic substrate. The band structure of the TBG is calculated within a tight-binding framework, and then used to determine the TBG's conductivity. The latter presents a nontrivial dependence upon the angle of twist, which shows up in the LDOS, allowing for a moiré pattern-dependent quantum emission. For some specific twist angles we show that it is possible to either enhance or decrease the LDOS by an order of magnitude at selected frequencies when compared to the monolayer. Furthermore, this impressive variation is explained in terms of the presence/absence of well defined surface plasmon polaritons (SPPs). Altogether our findings demonstrate that TBG is an alternative, versatile material platform for controlling spontaneous emission and enhancing light-matter interactions, and pave the way for further studies and applications of quantum emission in the emerging class of two-dimensional moiré materials.

36 MATERIALS SCIENCE↗