arXiv subjects

Hongbo Hu

Publications and source records attributed to Hongbo Hu.

At least 37 records · Page 2Linked to original sources

arXiv2017

Tungsten disulfide-gold nanohole hybrid metasurfaces for nonlinear metalens in the visible region

Recently, nonlinear hybrid metasurface comes into an attractive new concept in the research of nanophotonics and nanotechnology. It is composed of semiconductors with an intrinsically large nonlinear susceptibility and traditional plasmonic metasurfaces, offering opportunities for efficiently generating and manipulating nonlinear optical responses. A high second-harmonic generation (SHG) conversion efficiency has been demonstrated in the mid-infrared region by using multi-quantum-well (MQW) based plasmonic metasurfaces. However, it has yet to be demonstrated in the visible region. Here we present a new type of nonlinear hybrid metasurfaces for the visible region, which consists of a single layer of tungsten disulfide (WS2) and a phased gold nanohole array. The results indicate that a large SHG susceptibility of ~0.1 nm/V at 810 nm is achieved, which is 2~3 orders of magnitude larger than that of typical plasmonic metasurfaces. Nonlinear metalenses with the focal lengths of 30 μm, 50 μm and 100 μm are demonstrated experimentally, providing a direct evidence for both generating and manipulating SH signals based on the nonlinear hybrid metasurfaces. It shows great potential applications in designing of integrated, ultra-thin, compacted and efficient nonlinear optical devices, such as frequency converters, nonlinear holography and generation of nonlinear optical vortex beam.

physics.optics↗

arXiv2017

The Giant Radio Array for Neutrino Detection (GRAND): Present and Perspectives

The Giant Radio Array for Neutrino Detection (GRAND) aims at detecting ultra-high energy extraterrestrial neutrinos via the extensive air showers induced by the decay of tau leptons created in the interaction of neutrinos under the Earth's surface. Consisting of an array of $\sim10^5$ radio antennas deployed over $\sim 2\times10^5\,\rm {km}^2$, GRAND plans to reach, for the first time, an all-flavor sensitivity of $\sim1.5\times10^{-10} \,\rm GeV\, cm^{-2} \,s^{-1}\, sr^{-1}$ above $5\times10^{17}$ eV and a sub-degree angular resolution, beyond the reach of other planned detectors. We describe here preliminary designs and simulation results, plans for the ongoing, staged approach to the construction of GRAND, and the rich research program made possible by GRAND's design sensitivity and angular resolution.

astro-ph.IM↗

arXiv2017

EAS age determination from the study of the lateral distribution of charged particles near the shower axis with the ARGO-YBJ experiment

The ARGO-YBJ experiment, a full coverage extensive air shower (EAS) detector located at high altitude (4300 m a.s.l.) in Tibet, China, has smoothly taken data, with very high stability, since November 2007 to the beginning of 2013. The array consisted of a carpet of about 7000 m$^2$ Resistive Plate Chambers (RPCs) operated in streamer mode and equipped with both digital and analog readout, providing the measurement of particle densities up to few particles per cm$^2$. The unique detector features (full coverage, readout granularity, wide dynamic range, etc) and location (very high altitude) allowed a detailed study of the lateral density profile of charged particles at ground very close to the shower axis and its description by a proper lateral distribution function (LDF). In particular, the information collected in the first 10 m from the shower axis have been shown to provide a very effective tool for the determination of the shower development stage ("age") in the energy range 50 TeV - 10 PeV. The sensitivity of the age parameter to the mass composition of primary Cosmic Rays is also discussed.

astro-ph.HE↗

arXiv2017

Search for Gamma Ray Bursts with the ARGO-YBJ Detector in Shower Mode

The ARGO-YBJ detector, located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China), was a full coverage air shower array dedicated to gamma ray astronomy and cosmic ray studies. The wide field of view (~ 2 sr) and high duty cycle (> 86%), made ARGO-YBJ suitable to search for short and unexpected gamma ray emissions like gamma ray bursts (GRBs). Between 2007 November 6 and 2013 February 7, 156 satellite-triggered GRBs (24 of them with known redshift) occurred within the ARGO-YBJ field of view. A search for possible emission associated to these GRBs has been made in the two energy ranges 10-100 GeV and 10-1000 GeV. No significant excess has been found in time coincidence with the satellite detections nor in a time window of one hour after the bursts. Taking into account the EBL absorption, upper limits to the energy fluence at 99% of confidence level have been evaluated,with values ranging from ~ 10-5 erg cm-2 to ~10-1 erg cm-2.

astro-ph.HE↗

arXiv2017

The Giant Radio Array for Neutrino Detection

The Giant Radio Array for Neutrino Detection (GRAND) is a planned array of ~200 000 radio antennas deployed over ~200 000 km2 in a mountainous site. It aims primarly at detecting high-energy neutrinos via the observation of extensive air showers induced by the decay in the atmosphere of taus produced by the interaction of cosmic neutrinos under the Earth surface. GRAND aims at reaching a neutrino sensitivity of 5.10$^{11}$ E$^{-2}$ GeV$^{-1}$cm$^{-2}$s$^{-1}$sr$^{-1}$ above 3.10$^{16}$ eV. This ensures the detection of cosmogenic neutrinos in the most pessimistic source models, and ~50 events per year are expected for the standard models. The instrument will also detect UHECRs and possibly FRBs. Here we show how our preliminary design should enable us to reach our sensitivity goals, and discuss the steps to be taken to achieve GRAND.

astro-ph.IM↗

arXiv2016

Detection of thermal neutrons with the PRISMA-YBJ array in Extensive Air Showers selected by the ARGO-YBJ experiment

We report on a measurement of thermal neutrons, generated by the hadronic component of extensive air showers (EAS), by means of a small array of EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement Array), novel devices based on a compound alloy of ZnS(Ag) and $^{6}$LiF. This array has been operated within the ARGO-YBJ experiment at the high altitude Cosmic Ray Observatory in Yangbajing (Tibet, 4300 m a.s.l.). Due to the tight correlation between the air shower hadrons and thermal neutrons, this technique can be envisaged as a simple way to estimate the number of high energy hadrons in EAS. Coincident events generated by primary cosmic rays of energies greater than 100 TeV have been selected and analyzed. The EN-detectors have been used to record simultaneously thermal neutrons and the air shower electromagnetic component. The density distributions of both components and the total number of thermal neutrons have been measured. The correlation of these data with the measurements carried out by ARGO-YBJ confirms the excellent performance of the EN-detector.

astro-ph.IM↗

arXiv2015

Highly-sensitive detection of the lattice distortion in single bent ZnO nanowires by second-harmonic generation microscopy

Nanogenerators based on ZnO nanowires (NWs) realize the energy conversion at nanoscale, which are ascribed to the piezoelectric property caused by the lattice distortion of the ZnO NWs. The lattice distortion can significantly tune the electronic and optical properties, and requires a sensitive and convenient measurement. However, high-resolution transmission electron microscopy (HRTEM) technique provides a limited sensitivity of 0.01 nm on the variation of the lattice spacing and requires vacuum conditions. Here we demonstrate a highly-sensitive detection of the lattice distortion in single bent ZnO NWs by second-harmonic generation (SHG) microscopy. As the curvature of the single bent ZnO NW increases to 21 mm-1 (<4% bending distortion), it shows a significant decrease (~70%) in the SHG intensity ratio between perpendicular and parallel excitation polarization with respect to c-axis of ZnO NWs. Importantly, the extraordinary non-axisymmetrical SHG polarimetric patterns are also observed, indicating the twisting distortion around c-axis of ZnO NWs. Thus, SHG microscopy provides a sensitive all-optical and non-invasive method for in situ detecting the lattice distortion under various circumstances.

physics.optics↗

arXiv2015

4.5 years multi-wavelength observations of Mrk 421 during the ARGO-YBJ and Fermi common operation time

We report on the extensive multi-wavelength observations of the blazar Markarian 421 (Mrk 421) covering radio to gamma-rays, during the 4.5 year period of ARGO-YBJ and Fermi common operation time, from August 2008 to February 2013. In particular, thanks to the ARGO-YBJ and Fermi data, the whole energy range from 100 MeV to 10 TeV is covered without any gap. In the observation period, Mrk 421 showed both low and high activity states at all wavebands. The correlations among flux variations in different wavebands were analyzed. Seven large flares, including five X-ray flares and two GeV gamma-ray flares with variable durations (3-58 days), and one X-ray outburst phase were identified and used to investigate the variation of the spectral energy distribution with respect to a relative quiescent phase. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to few keV. The TeV gamma-ray flux increases up to 0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV gamma-rays is found to vary depending on the flare, a feature that leads us to classify flares into three groups according to the GeV flux variation. Finally, the one-zone synchrotron self-Compton model was adopted to describe the emission spectra. Two out of three groups can be satisfactorily described using injected electrons with a power-law spectral index around 2.2, as expected from relativistic diffuse shock acceleration, whereas the remaining group requires a harder injected spectrum. The underlying physical mechanisms responsible for different groups may be related to the acceleration process or to the environment properties.

astro-ph.HE↗

arXiv2015

The Giant Radio Array for Neutrino Detection

High-energy neutrino astronomy will probe the working of the most violent phenomena in the Universe. The Giant Radio Array for Neutrino Detection (GRAND) project consists of an array of $\sim10^5$ radio antennas deployed over $\sim$200000km$^2$ in a mountainous site. It aims at detecting high-energy neutrinos via the measurement of air showers induced by the decay in the atmosphere of $τ$ leptons produced by the interaction of the cosmic neutrinos under the Earth surface. Our objective with GRAND is to reach a neutrino sensitivity of $3\times10^{-11}E^{-2}$GeV$^{-1}$cm$^{-2}$s$^{-1}$sr$^{-1}$ above $3 \times10^{16}$eV. This sensitivity ensures the detection of cosmogenic neutrinos in the most pessimistic source models, and about 100 events per year are expected for the standard models. GRAND would also probe the neutrino signals produced at the potential sources of UHECRs. We show how our preliminary design should enable us to reach our sensitivity goals, and present the experimental characteristics. We assess the possibility to adapt GRAND to other astrophysical radio measurements. We discuss in this token the technological options for the detector and the steps to be taken to achieve the GRAND project.

astro-ph.HE↗

arXiv2015

Study of the diffuse gamma-ray emission from the Galactic plane with ARGO-YBJ

The events recorded by ARGO-YBJ in more than five years of data collection have been analyzed to determine the diffuse gamma-ray emission in the Galactic plane at Galactic longitudes 25° < l < 100° and Galactic latitudes . The energy range covered by this analysis, from ~350 GeV to ~2 TeV, allows the connection of the region explored by Fermi with the multi-TeV measurements carried out by Milagro. Our analysis has been focused on two selected regions of the Galactic plane, i.e., 40° < l < 100° and 65° < l < 85° (the Cygnus region), where Milagro observed an excess with respect to the predictions of current models. Great care has been taken in order to mask the most intense gamma-ray sources, including the TeV counterpart of the Cygnus cocoon recently identified by ARGO-YBJ, and to remove residual contributions. The ARGO-YBJ results do not show any excess at sub-TeV energies corresponding to the excess found by Milagro, and are consistent with the predictions of the Fermi model for the diffuse Galactic emission. From the measured energy distribution we derive spectral indices and the differential flux at 1 TeV of the diffuse gamma-ray emission in the sky regions investigated.

astro-ph.IM↗

arXiv2015

The analog Resistive Plate Chamber detector of the ARGO-YBJ experiment

The ARGO-YBJ experiment has been in stable data taking from November 2007 till February 2013 at the YangBaJing Cosmic Ray Observatory (4300 m a.s.l.). The detector consists of a single layer of Resistive Plate Chambers (RPCs) ( about 6700 m^2}) operated in streamer mode. The signal pick-up is obtained by means of strips facing one side of the gas volume. The digital readout of the signals, while allows a high space-time resolution in the shower front reconstruction, limits the measurable energy to a few hundred TeV. In order to fully investigate the 1-10 PeV region, an analog readout has been implemented by instrumenting each RPC with two large size electrodes facing the other side of the gas volume. Since December 2009 the RPC charge readout has been in operation on the entire central carpet (about 5800 m^2). In this configuration the detector is able to measure the particle density at the core position where it ranges from tens to many thousands of particles per m^2. Thus ARGO-YBJ provides a highly detailed image of the charge component at the core of air showers. In this paper we describe the analog readout of RPCs in ARGO-YBJ and discuss both the performance of the system and the physical impact on the EAS measurements.

astro-ph.IM↗

arXiv2015

The cosmic ray proton plus helium energy spectrum measured by the ARGO-YBJ experiment in the energy range 3-300 TeV

The ARGO-YBJ experiment is a full-coverage air shower detector located at the Yangbajing Cosmic Ray Observatory (Tibet, People's Republic of China, 4300 m a.s.l.). The high altitude, combined with the full-coverage technique, allows the detection of extensive air showers in a wide energy range and offer the possibility of measuring the cosmic ray proton plus helium spectrum down to the TeV region, where direct balloon/space-borne measurements are available. The detector has been in stable data taking in its full configuration from November 2007 to February 2013. In this paper the measurement of the cosmic ray proton plus helium energy spectrum is presented in the region 3-300 TeV by analyzing the full collected data sample. The resulting spectral index is $γ= -2.64 \pm 0.01$. These results demonstrate the possibility of performing an accurate measurement of the spectrum of light elements with a ground based air shower detector.

hep-ex↗

arXiv2015

Search for GeV Gamma Ray Bursts with the ARGO-YBJ Detector: Summary of Eight Years of Observations

The search for Gamma Ray Burst (GRB) emission in the energy range 1-100 GeV in coincidence with the satellite detection has been carried out using the Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ) experiment. The high altitude location (4300 m a.s.l.), the large active surface ($\sim$ 6700 m$^2$ of Resistive Plate Chambers), the wide field of view ($\sim 2~$sr, limited only by the atmospheric absorption) and the high duty cycle ($>$ 86 %) make the ARGO-YBJ experiment particularly suitable to detect short and unexpected events like GRBs. With the scaler mode technique, i.e., counting all the particles hitting the detector with no measurement of the primary energy and arrival direction, the minimum threshold of $\sim$ 1 GeV can be reached, overlapping the direct measurements carried out by satellites. During the experiment lifetime, from December 17, 2004 to February 7, 2013, a total of 206 GRBs occurring within the ARGO-YBJ field of view (zenith angle $θ$ $\le$ 45$^{\circ}$) have been analyzed. This is the largest sample of GRBs investigated with a ground-based detector. Two lightcurve models have been assumed and since in both cases no significant excess has been found, the corresponding fluence upper limits in the 1-100 GeV energy region have been derived, with values as low as 10$^{-5}~$erg cm$^{-2}$. The analysis of a subset of 24 GRBs with known redshift has been used to constrain the fluence extrapolation to the GeV region together with possible cutoffs under different assumptions on the spectrum.

astro-ph.HE↗

arXiv2015

The Knee of the Cosmic Hydrogen and Helium Spectrum below 1 PeV Measured by ARGO-YBJ and a Cherenkov Telescope of LHAASO

The measurement of cosmic ray energy spectra, in particular for individual species, is an essential approach in finding their origin. Locating the "knees" of the spectra is an important part of the approach and has yet to be achieved. Here we report a measurement of the mixed Hydrogen and Helium spectrum using the combination of the ARGO-YBJ experiment and of a prototype Cherenkov telescope for the LHAASO experiment. A knee feature at 640+/-87 TeV, with a clear steepening of the spectrum, is observed. This gives fundamental inputs to galactic cosmic ray acceleration models.

astro-ph.HE↗

arXiv2014

Identification of the TeV Gamma-ray Source ARGO J2031+4157 with the Cygnus Cocoon

The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by $Fermi$-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from November 2007 to January 2013, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of $10^{\circ}\times 10^{\circ}$, finding a source extension $σ_{ext}$= 1$^{\circ}$.8$\pm$0$^{\circ}$.5. The observed differential energy spectrum is $dN/dE =(2.5\pm0.4) \times 10^{-11}(E/1 TeV)^{-2.6\pm0.3}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by $Fermi$-LAT, and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with Supernova Remnants indicates that the particle acceleration inside a superbubble is similar to that in a SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.

astro-ph.HE↗

arXiv2014

Energy Spectrum of Cosmic Protons and Helium Nuclei by a Hybrid Measurement at 4300 m a.s.l

The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured, below the so-called "knee", by using a hybrid experiment with a wide field-of-view Cherenkov telescope and the Resistive Plate Chamber (RPC) array of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium nuclei have been well separated from other cosmic ray components by using a multi-parameter technique. A highly uniform energy resolution of about 25% is achieved throughout the whole energy range (100 TeV - 700 TeV). The observed energy spectrum is compatible with a single power law with index gamma=-2.63+/-0.06.

astro-ph.HE↗

arXiv2013

TeV gamma-ray survey of the Northern sky using the ARGO-YBJ detector

The ARGO-YBJ detector is an extensive air shower array that has been used to monitor the northern $γ$-ray sky at energies above 0.3 TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination band from $-10^{\circ}$ to $70^{\circ}$, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to $\sim$1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than 5 standard deviations. Several excesses are also reported as potential $γ$-ray emitters. The features of each source are presented and discussed. Additionally, $95\%$ confidence level upper limits of the flux from the investigated sky region are shown. Specific upper limits for 663 GeV $γ$-ray AGNs inside the ARGO-YBJ field of view are reported. The effect of the absorption of $γ$-rays due to the interaction with extragalactic background light is estimated.

astro-ph.HE↗

arXiv2013

Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment

We report the observation of a very high energy γ-ray source, whose position is coincident with HESS J1841-055. This source has been observed for 4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its emission is detected with a statistical significance of 5.3 standard deviations. Parameterizing the source shape with a two-dimensional Gaussian function we estimate an extension σ=(0.40(+0.32,-0.22}) degree, consistent with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x 10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy range 0.9-50 TeV. The integral γ-ray flux above 1 TeV is 1.3-+0.4 Crab units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the flux determination between HESS and ARGO-YBJ, and possible counterparts at other wavelengths are discussed.

astro-ph.HE↗