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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

NEAR-INFRARED IMAGING POLARIMETRY OF THE SERPENS CLOUD CORE: MAGNETIC FIELD STRUCTURE, OUTFLOWS, AND INFLOWS IN A CLUSTER FORMING CLUMP

We made deep near-infrared (JHKs) imaging polarimetry toward the Serpens cloud core, which is a nearby, active cluster forming region. The polarization vector maps show that the near-infrared reflection light in this region mainly originates from SVS 2 and SVS 20, and enable us to detect 24 small infrared reflection nebulae associated with young stellar objects. Polarization measurements of near-infrared point sources indicate an hourglass-shaped magnetic field, of which the symmetry axis is nearly perpendicular to the elongation of the C{sup 18}O (J = 1-0) or submillimeter continuum emission. The bright part of C{sup 18}O (J = 1-0), submillimeter continuum cores as well as many Class 0/I objects are located just toward the constriction region of the hourglass-shaped magnetic field. Applying the Chandrasekhar and Fermi method and taking into account the recent study on the signal integration effect for the dispersion component of the magnetic field, the magnetic field strength was estimated to be {approx}100 {mu}G, suggesting that the ambient region of the Serpens cloud core is moderately magnetically supercritical. This suggests that the Serpens cloud core first contracted along the magnetic field as an elongated cloud, which is perpendicular to the magnetic field, and that the central part then contracted across the magnetic field due to the high density in the central region of the cloud core, where star formation is actively continuing. Comparison of this magnetic field with previous observations of molecular gas and large-scale outflows suggests a possibility that the cloud dynamics are controlled by the magnetic field, protostellar outflows, and gravitational inflows. Furthermore, the outflow energy injection rate appears to be larger than the dissipation rate of the turbulent energy in this cloud, indicating that the outflows are the main source of turbulence and that the magnetic field plays an important role both in allowing the outflow energy to escape from the central region of the cloud core and enabling the gravitational inflows from the ambient region to the central region. These characteristics appear to be in good agreement with the outflow-driven turbulence model and imply the importance of the magnetic field to continuous star formation in the center region of the cluster forming region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

MODELING THE NEAR-ULTRAVIOLET BAND OF GK STARS. I. LOCAL THERMODYNAMIC EQUILIBRIUM MODELS

We present a grid of LTE atmospheric models and synthetic spectra that covers the spectral class range from mid-G to mid-K, and luminosity classes from V to III, that is dense in T{sub eff} sampling ({Delta}T {sub eff} = 62.5 K), for stars of solar metallicity and moderately metal-poor scaled solar abundance ([A/H]=0.0 and -0.5). All models have been computed with two choices of atomic line list: (1) the 'big' line lists of Kurucz that best reproduce the broadband solar blue and near-UV f{sub {lambda}} level, and (2) the 'small' lists of Kurucz and Peytremann that provide the best fit to the high-resolution solar blue and near-UV spectrum. We compare our model spectral energy distributions to a sample of stars carefully selected from the large catalog of uniformly re-calibrated spectrophotometry of Burnashev with the goal of determining how the quality of fit varies with stellar parameters, especially in the historically troublesome blue and near-UV bands. We confirm that our models computed with the 'big' line list recover the derived T {sub eff} values of the PHOENIX NextGen grid, but find that the models computed with the 'small' line list provide greater internal self-consistency among different spectral bands, and closer agreement with the empirical T{sub eff} scale of Ramirez and Melendez, but not to the interferometrically derived T{sub eff} values of Baines et al. We find no evidence that the near-UV band discrepancy between models and observations for Arcturus ({alpha} Boo) reported in two works by Short and Hauschildt is pervasive, and that Arcturus may be peculiar in this regard.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

NEAR-INFRARED THERMAL EMISSION FROM TrES-3b: A Ks-BAND DETECTION AND AN H-BAND UPPER LIMIT ON THE DEPTH OF THE SECONDARY ECLIPSE

We present H- and Ks-band photometry bracketing the secondary eclipse of the hot Jupiter TrES-3b using the Wide-field Infrared Camera on the Canada-France-Hawaii Telescope. We detect the secondary eclipse of TrES-3b with a depth of 0.133{sup +0.018}{sub -0.016}% in the Ks band (8{sigma})-a result that is in sharp contrast to the eclipse depth reported by de Mooij and Snellen. We do not detect its thermal emission in the H band, but place a 3{sigma} limit of 0.051% on the depth of the secondary eclipse in this band. A secondary eclipse of this depth in Ks requires very efficient day-to-nightside redistribution of heat and nearly isotropic reradiation, a conclusion that is in agreement with longer wavelength, mid-infrared Spitzer observations. Our 3{sigma} upper limit on the depth of our H-band secondary eclipse also argues for very efficient redistribution of heat and suggests that the atmospheric layer probed by these observations may be well homogenized. However, our H-band upper limit is so constraining that it suggests the possibility of a temperature inversion at depth, or an absorbing molecule, such as methane, that further depresses the emitted flux at this wavelength. The combination of our near-infrared measurements and those obtained with Spitzer suggests that TrES-3b displays a near-isothermal dayside atmospheric temperature structure, whose spectrum is well approximated by a blackbody. We emphasize that our strict H-band limit is in stark disagreement with the best-fit atmospheric model that results from longer wavelength observations only, thus highlighting the importance of near-infrared observations at multiple wavelengths, in addition to those returned by Spitzer in the mid-infrared, to facilitate a comprehensive understanding of the energy budgets of transiting exoplanets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

HYDROGEN PERMITTED LINES IN THE FIRST NEAR-IR SPECTRA OF Th 28 MICROJET: ACCRETION OR EJECTION TRACERS?

We report the first near-infrared detection of the bipolar microjet from T Tauri star ThA 15-28 (hereafter Th 28). Spectra were obtained with Very Large Telescope (VLT)/ISAAC for the slit both perpendicular and parallel to the flow to examine jet kinematics and gas physics within the first arcsecond from the star. The jet was successfully detected in both molecular and atomic lines. The H{sub 2} component was found to be entirely blueshifted around the base of the bipolar jet. It shows that only the blue lobe is emitting in H{sub 2} while light is scattered in the direction of the red lobe, highlighting an asymmetric extinction and/or excitation between the two lobes. Consistent with this view, the red lobe is brighter in all atomic lines. Interestingly, the jet was detected not only in [Fe II], but also in Br{gamma} and Pa{beta} lines. Though considered tracers mainly of accretion, we find that these high excitation hydrogen permitted lines trace the jet as far as 150 AU from the star. This is confirmed in a number of ways: the presence of the [Fe II] 2.13 {mu}m line which is of similarly high excitation; H I velocities which match the jet [Fe II] velocities in both the blue and red lobe; and high electron density close to the source of >6 x 10{sup 4} cm{sup -3} derived from the [Fe II] 1.64, 1.60 {mu}m ratio. These near-infrared data complement Hubble Space Telescope Imaging Spectrograph (HST/STIS) optical and near-ultraviolet data for the same target which were used in a jet rotation study, although no rotation signature could be identified here due to insufficient angular resolution. The unpublished HST/STIS H{alpha} emission is included here alongside the other H I lines. Identifying Br{gamma} and Pa{beta} as tracers of ejection is significant because of the importance of finding strong near-infrared probes close to the star, where forbidden lines are quenched, which will help understand accretion ejection when observed with high spatial resolution instruments such as VLTI/AMBER.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

STRONG NEW CONSTRAINTS ON THE EXTRAGALACTIC BACKGROUND LIGHT IN THE NEAR- TO MID-INFRARED

Direct measurements of the extragalactic background light (EBL) in the near-IR to mid-IR waveband are extremely difficult due to an overwhelming foreground from the zodiacal light that outshines the faint cosmological diffuse radiation field by more than an order of magnitude. Indirect constraints on the EBL are provided by {gamma}-ray observations of active galactic nuclei. Using the combination of the Fermi Gamma-Ray Space Telescope together with the current generation of ground-based air Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) provides unprecedented sensitivity and spectral coverage for constraining the EBL in the near- to mid-IR. In this paper, we present new limits on the EBL based on the analysis of the broadband spectra of a select set of {gamma}-ray blazars covering 200 MeV to several TeV. The EBL intensity at 15 {mu}m is constrained to be 1.36 {+-} 0.58 nW m{sup -2} sr{sup -1}. We find that the fast evolution and baseline EBL models of Stecker et al., as well as the model of Kneiske et al., predict significantly higher EBL intensities in the mid-IR (15 {mu}m) than is allowed by the constraints derived here. In addition, the model of Franceschini et al. and the fiducial model of DomInguez et al. predict near- to mid-IR ratios smaller than that derived from our analysis. Namely, their intensities in the near-IR are too low while their intensities in the mid-IR are marginally too high. All of the aforementioned models are inconsistent with our analysis at the >3{sigma} level.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The discovery of cometary activity in near-Earth asteroid (3552) Don Quixote

The near-Earth object (NEO) population, which mainly consists of fragments from collisions between asteroids in the main asteroid belt, is thought to include contributions from short-period comets as well. One of the most promising NEO candidates for a cometary origin is near-Earth asteroid (3552) Don Quixote, which has never been reported to show activity. Here we present the discovery of cometary activity in Don Quixote based on thermal-infrared observations made with the Spitzer Space Telescope in its 3.6 and 4.5 μm bands. Our observations clearly show the presence of a coma and a tail in the 4.5 μm but not in the 3.6 μm band, which is consistent with molecular band emission from CO{sub 2}. Thermal modeling of the combined photometric data on Don Quixote reveals a diameter of 18.4{sub −0.4}{sup +0.3} km and an albedo of 0.03{sub −0.01}{sup +0.02}, which confirms Don Quixote to be the third-largest known NEO. We derive an upper limit on the dust production rate of 1.9 kg s{sup –1} and derive a CO{sub 2} gas production rate of (1.1 ± 0.1) × 10{sup 26} molecules s{sup –1}. Spitzer Infrared Spectrograph spectroscopic observations indicate the presence of fine-grained silicates, perhaps pyroxene rich, on the surface of Don Quixote. Our discovery suggests that CO{sub 2} can be present in near-Earth space over a long time. The presence of CO{sub 2} might also explain that Don Quixote's cometary nature remained hidden for nearly three decades.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Main-belt asteroids with WISE/NEOWISE: Near-infrared albedos

We present revised near-infrared albedo fits of 2835 main-belt asteroids observed by WISE/NEOWISE over the course of its fully cryogenic survey in 2010. These fits are derived from reflected-light near-infrared images taken simultaneously with thermal emission measurements, allowing for more accurate measurements of the near-infrared albedos than is possible for visible albedo measurements. Because our sample requires reflected light measurements, it undersamples small, low-albedo asteroids, as well as those with blue spectral slopes across the wavelengths investigated. We find that the main belt separates into three distinct groups of 6%, 16%, and 40% reflectance at 3.4 μm. Conversely, the 4.6 μm albedo distribution spans the full range of possible values with no clear grouping. Asteroid families show a narrow distribution of 3.4 μm albedos within each family that map to one of the three observed groupings, with the (221) Eos family being the sole family associated with the 16% reflectance 3.4 μm albedo group. We show that near-infrared albedos derived from simultaneous thermal emission and reflected light measurements are important indicators of asteroid taxonomy and can identify interesting targets for spectroscopic follow-up.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A NUMERICAL SIMULATION OF COSMIC-RAY MODULATION NEAR THE HELIOPAUSE

Based on a hybrid galactic cosmic-ray transport model, which incorporated MHD global heliospheric data into Parker’s cosmic-ray transport equation, we studied the behavior of the transport of galactic cosmic rays and the corresponding gradients in their flux near the heliopause (HP). We found that, (1) by increasing the ratio of the parallel diffusion coefficient to the perpendicular diffusion coefficient in the interstellar magnetic field of the outer heliosheath, the simulated radial flux near the HP increases as well. As the ratio multiplying factor reached 10{sup 10}, the radial flux experienced a sudden jump near the HP, similar to what Voyager 1 observed in 2012. (2) The effect of changing the diffusion coefficients’ ratio on the radial flux variation depends on the energy of the cosmic rays, the lower the energy, the more pronounced the effect is. (3) The magnitude of the diffusion coefficients also affect the radial flux near the HP, the modulation beyond the HP varies by adjusting the magnitude multiplying factor.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Near-surface modification of defective KTaO 3 by ionizing ion irradiation

The synergistic effect of nuclear (S n ) and electronic (S e ) energy loss observed in some ABO 3 perovskites has attracted considerable attention due to the real possibility to modify various near-surface properties, such as the electronic and optical properties, by patterning ion tracks in the defective near-surface regions. In this study, we show that low-energy ion-induced disordering in conjunction with ionizing ion irradiation (18 MeV Si, 21 MeV Ni and 91.6 MeV Xe) is a promising approach for tailoring ion tracks in the near-surface of defective KTaO 3 . Experimental characterization and computer simulations reveal that the size of these latent ion tracks increases with S e and level of pre-existing damage. These results further reveal that the threshold S e value (S e th ) for track creation increases with decreasing pre-damage level. The values of S e th increase from 5.02 keV nm -1 , for a pre-existing fractional disorder of 0.53 in KTaO 3 , to 10.81 keV nm -1 for pristine KTaO 3 . Above these thresholds, amorphous latent tracks are produced due local melting and rapid quenching. Below a disorder fraction of 0.08 and S e ≤ 6.68 keV nm -1 , the synergistic effect is not active, and damage accumulation is suppressed due to a competing ionization-induced damage annealing process. These results indicate that, depending on S e and the amount of pre-existing damage, highly ionizing ions can either enhance or suppress damage accumulation in KTaO 3 , thus providing a pathway to tailoring defects states. Comprehending the conflicting roles of highly ionizing ions in defective ABO 3 oxides is vital for understanding and predictive modeling of ion-solid interactions in complex oxides, as well as for achieving control over ion track size in the near-surface of defective KTaO 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Helium Flux on Near-surface Helium Accumulation in Plasma-Exposed Tungsten

We report results of object kinetic Monte Carlo (OKMC) simulations aimed at understanding the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially defect-free and has a W(100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 10 22 to 4 × 10 25 He/m 2 s, with 100 eV helium atoms impinging on a W(100) surface up to a maximum fluence of 4×10 19 He/m 2 . In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface in initially pristine tungsten. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for 10 22 He/m 2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. Finally, we present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Carnegie Supernova Project-II: Extending the Near-infrared Hubble Diagram for Type Ia Supernovae to z ~0.1

The Carnegie Supernova Project-II (CSP-II) was an NSF-funded, four-year program to obtain optical and near-infrared observations of a "Cosmology" sample of ~100 Type Ia supernovae located in the smooth Hubble flow (0.03 $\lesssim$ z $\lesssim$ 0.10). Light curves were also obtained of a "Physics" sample composed of 90 nearby Type Ia supernovae at z ≤ 0.04 selected for near-infrared spectroscopic timeseries observations. The primary emphasis of the CSP-II is to use the combination of optical and near-infrared photometry to achieve a distance precision of better than 5%. Here in this paper, details of the supernova sample, the observational strategy, and the characteristics of the photometric data are provided. In a companion paper, the near-infrared spectroscopy component of the project is presented.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

HT Lyn and IR Lyn: Two Semi-detached-type Near-contact Binaries with Stable Orbital Period

Abstract We presented the first photometric and orbital period investigations for two near-contact binaries: HT Lyn and IR Lyn. The light-curves solutions derived from both our ground-based and various surveys’ observations suggested that HT Lyn and IR Lyn are two semi-detached-type near-contact binaries with the secondary and primary components filling the Roche lobe. Combining the eclipse timings derived from several surveys’ data and our observations with those reported in the literature, we modified the orbital periods and revealed that their orbital periods were stable for the last two decades. The absolute physical parameters were well determined by using the empirical relations and the Gaia-distance-based method. Similar to other near-contact binaries, the primary components of HT Lyn and IR Lyn are evolving at the main-sequence stage, while their less-massive secondary components show the over-sized and over-luminosity features and should be more evolved than their primary ones. Combining the stable orbital period with the semi-detached configurations, we infer that the two near-contact binaries may be just located in the critical phase and evolving from the current semi-detached phase to the detached or contact one.

Astronomy & Astrophysics↗
DOE OSTI2021

The statistical mechanics of near-BPS black holes

Due to the failure of thermodynamics for low temperature near-extremal black holes, it has long been conjectured that a 'thermodynamic mass gap' exists between an extremal black hole and the lightest near-extremal state. For non-supersymmetric near-extremal black holes in Einstein gravity with an AdS2 throat, no such gap was found. Rather, at that energy scale, the spectrum exhibits a continuum of states, up to non-perturbative corrections. In this paper, we compute the partition function of near-BPS black holes in supergravity where the emergent, broken, symmetry is PSU(1, 1|2). To reliably compute this partition function, we show that the gravitational path integral can be reduced to that of a $\mathcal{N}=4$ supersymmetric extension of the Schwarzian theory, which we define and exactly quantize. In contrast to the non-supersymmetric case, we find that black holes in supergravity have a mass gap and a large extremal black hole degeneracy consistent with the Bekenstein–Hawking area. Furthermore, our results verify a plethora of string theory conjectures, concerning the scale of the mass gap and the counting of extremal micro-states.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ALMA OBSERVATIONS OF THE GALACTIC CENTER: SiO OUTFLOWS AND HIGH-MASS STAR FORMATION NEAR Sgr A*

ALMA observations of the Galactic center with a spatial resolution of 2.''61 Multiplication-Sign 0.''97 resulted in the detection of 11 SiO (5-4) clumps of molecular gas within 0.6 pc (15'') of Sgr A*, interior to the 2 pc circumnuclear molecular ring. The three SiO (5-4) clumps closest to Sgr A* show the largest central velocities, {approx}150 km s{sup -1}, and the broadest asymmetric line widths with full width zero intensity (FWZI) {approx}110-147 km s{sup -1}. The remaining clumps, distributed mainly to the NE of the ionized mini-spiral, have narrow FWZI ({approx}18-56 km s{sup -1}). Using CARMA SiO (2-1) data, Large Velocity Gradient modeling of the SiO line ratios for the broad velocity clumps constrains the column density N(SiO) {approx}10{sup 14} cm{sup -2}, and the H{sub 2} gas density n{sub H{sub 2}} = (3-9) x 10{sup 5} cm{sup -3} for an assumed kinetic temperature 100-200 K. The SiO clumps are interpreted as highly embedded protostellar outflows, signifying an early stage of massive star formation near Sgr A* in the last 10{sup 4}-10{sup 5} yr. Support for this interpretation is provided by the SiO (5-4) line luminosities and velocity widths which lie in the range measured for protostellar outflows in star-forming regions in the Galaxy. Furthermore, spectral energy distribution modeling of stellar sources shows two young stellar object candidates near SiO clumps, supporting in situ star formation near Sgr A*. We discuss the nature of star formation where the gravitational potential of the black hole dominates. In particular, we suggest that external radiative pressure exerted on self-shielded molecular clouds enhances the gas density, before the gas cloud becomes gravitationally unstable near Sgr A*. Alternatively, collisions between clumps in the ring may trigger gravitational collapse.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Discovering faint and high apparent motion rate near-Earth asteroids using a deep learning program

ABSTRACT Although many near-Earth objects have been found by ground-based telescopes, some fast-moving ones, especially those near detection limits, have been missed by observatories. We developed a convolutional neural network for detecting faint fast-moving near-Earth objects. It was trained with artificial streaks generated from simulations and was able to find these asteroid streaks with an accuracy of 98.7 per cent and a false positive rate of 0.02 per cent on simulated data. This program was used to search image data from the Zwicky Transient Facility (ZTF) in four nights in 2019, and it identified six previously undiscovered asteroids. The visual magnitudes of our detections range from ∼19.0 to 20.3 and motion rates range from ∼6.8 to 24 deg d−1, which is very faint compared to other ZTF detections moving at similar motion rates. Our asteroids are also ∼1–51 m diameter in size and ∼5–60 lunar distances away at close approach, assuming their albedo values follow the albedo distribution function of known asteroids. The use of a purely simulated data set to train our model enables the program to gain sensitivity in detecting faint and fast-moving objects while still being able to recover nearly all discoveries made by previously designed neural networks which used real detections to train neural networks. Our approach can be adopted by any observatory for detecting fast-moving asteroid streaks.

Wang, Franklin (ORCID:0000000344887355)↗
DOE OSTI2026

Electronic coherence evolution at the nearly commensurate-incommensurate CDW boundary of 1⁢T−TaS 2

Transition-metal dichalcogenides host a variety of charge-density-wave phases that couple lattice, charge, and correlation effects. In 1⁢𝑇−TaS 2 , the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum-resolved insight. In this work, we use temperature-dependent angle-resolved photoemission spectroscopy to track the electronic structure across this transition. We observe a suppression of quasiparticle spectral weight at the Brillouin-zone center, coincident with the transport anomaly, but without clear evidence of a full band-gap opening. The transition appears to involve momentum-dependent redistribution of spectral weight, consistent with a loss of coherence that reshapes the Fermi surface while leaving conduction dispersions largely intact. These results suggest that the nearly commensurate–incommensurate transition may not align with a conventional metal-insulator transition picture, but rather as an electronic reconstruction driven by loss of coherence. Our work provides new microscopic insight into the resistivity anomaly near room temperature and may guide design principles for collective electronic switching in transition-metal dichalcogenides.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Near-field-induced optical force on a metal particle and C{sub 60}: Real-time and real-space electron dynamics simulation

Optical forces induced by a near field are calculated for a 1-mm-sized metal particle mimicked by a jellium model and for C{sub 60} in the framework of real-time and real-space time-dependent density-functional theory combined with a nonuniform light-matter interaction formalism, fully taking account of multipole interaction. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in C{sub 60}. We also found that the optical force depends linearly on the intensity of the near field.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Validity of Machine Learning in the Quantitative Analysis of Complex Scanning Near-Field Optical Microscopy Signals Using Simulated Data

Scattering-type scanning near-field optical microscope (s-SNOM) is a modern technique for subdiffractional optical imaging and spectroscopy. Over the past two decades, tremendous efforts have been devoted to modeling complex tip-sample interactions in s-SNOM, aimed at understanding the electrodynamics of materials at the nanoscale. However, due to complexities in analytical methods and the limited computation power for fully numerical simulations, compromises must be made to facilitate the modeling of tip-sample interaction, such as using quasistatic approximation or unrealistic tip geometries. Here, we apply a variety of widely utilized machine-learning methods, including k nearest neighbor and feedforward neural network etc. to study the phase-resolved spectroscopic near-field response. With only a small set of training data, which is simulated using the finite-dipole model, we demonstrate that the relation between the experimental near-field signal and sample optical constant can be one to one mapped without the need for tip modeling: for a given material with a moderate dielectric function, its complex near-field spectrum can be accurately determined within the mid-IR spectral range, and vice versa. Our preliminary study sets the stage for future exploration using real experimental data. Our method is beneficial for processing the increasing amount of data accumulated across many research groups and especially useful for user facilities such as synchrotron-based national laboratories where a large amount of data is generated on a daily basis.

36 MATERIALS SCIENCE↗