arXiv subjects

E. Hu

Publications and source records attributed to E. Hu.

6 recordsLinked to original sources

arXiv2006

Wavelength- and material-dependent absorption in GaAs and AlGaAs microcavities

The quality factors of modes in nearly identical GaAs and Al_{0.18}Ga_{0.82}As microdisks are tracked over three wavelength ranges centered at 980 nm, 1460 nm, and 1600 nm, with quality factors measured as high as 6.62x10^5 in the 1600-nm band. After accounting for surface scattering, the remaining loss is due to sub-bandgap absorption in the bulk and on the surfaces. We observe the absorption is, on average, 80 percent greater in AlGaAs than in GaAs and in both materials is 540 percent higher at 980 nm than at 1600nm.

physics.optics↗

arXiv2001

Photon Correlation Spectroscopy of a Single Quantum Dot

We report photon correlation measurements that allow us to observe unique signatures of biexcitons in a single self-assembled InAs quantum dot. Photon correlation measurements of biexciton emission exhibit both bunching and antibunching under continuous-wave excitation while only antibunching is observed under pulsed excitation. Cross-correlation between biexciton and single-exciton peaks reveal highly asymmetric features, demonstrating that biexciton and exciton emissions have strong correlations due to cascaded emission. The anticipated correlation between the polarization of exciton and biexciton emissions however, is absent under our excitation conditions. Photon correlation measurements also provide evidence for the identification of the charged exciton emission.

cond-mat↗

arXiv2001

FUSE Observations of Cooling Flow Gas in the Galaxy Clusters A1795 and A2597

We present far-ultraviolet spectroscopy of the cores of the massive cooling flow clusters Abell 1795 and 2597 obtained with FUSE. As the intracluster gas cools through 3x10^5 K, it should emit strongly in the OVI 1032,1038 resonance lines. We report the detection of OVI 1032 emission in A2597, with a line flux of 1.35 +/- 0.35x10^-15 erg/cm2/s, as well as detection of emission from CIII 977. A marginal detection of CIII 977 emission is also reported for A1795. These observations provide evidence for a direct link between the hot (10^7 K) cooling flow gas and the cool (10^4 K) gas in the optical emission line fients. Assuming simple cooling flow models, the OVI line flux in A2597 corresponds to a mass deposition rate of ~40 Msun/yr within the central 36 kpc. Emission from OVI 1032 was not detected in A1795, with an upper limit of 1.5x10^-15 erg/cm2/s, corresponding to a limit on the mass cooling flow rate of Mdot(28 kpc) < 28 Msun/yr. We have considered several explanations for the lack of detection of OVI emission in A1795 and the weaker than expected flux in A2597, including extinction by dust in the outer cluster, and quenching of thermal conduction by magnetic fields. We conclude that a turbulent mixing model, with some dust extinction, could explain our OVI results while also accounting for the puzzling lack of emission by FeXVII in cluster cooling flows.

astro-ph↗

arXiv1998

Current-voltage characteristics of semiconductor-coupled superconducting weak links with large electrode separations

We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the 2-D electron gas in InAs-based semiconductor quantum wells, with relatively large (typically 0.5 micron) separations between niobium electrodes. The devices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for more conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width of the device, taken perpendicular to the current flow. This behavior falls outside the range of established theories; we propose that it is a fluctuation effect caused by giant shot noise associated with multiple Andreev reflections. The possibility of non-equilibrium effects is discussed.

cond-mat.supr-con↗

arXiv1998

The Photometric Redshift Distribution and Evolutionary Properties of Galaxies up to z= 4.5 in the Field of the Quasar BR1202-0725

We present a deep BVrIK multicolor catalog of galaxies in the field of the high redshift (z=4.7) quasar BR 1202-0725. Reliable colors have been measured for galaxies selected down to R=25. The choice of the optical filters has been optimized to define a robust multicolor selection of galaxies at 3.8 1.5. At variance with brighter surveys (I<22.5) there is a tail in the distribution towards high redshifts up to z~4. The luminosity function at z~0.6 shows a steepening for M_B>-19. The observed cosmological ultraviolet luminosity density is computed in the overall redshift interval z=0.3-4.5 reaching a value 2x10^{19} W/Hz/Mpc^3 at z~0.8. We have derived in a homogeneous way, using the GISSEL libraries, the physical parameters connected with the fitted spectral energy distributions. The bulk of the blue intermediate redshift population with z=0.4-1 mostly consists of very young star-forming galaxies with a median starburst age of the order of a few 10^8 yr and typical mass in luminous stars ~2x10^8 Mo.

astro-ph↗

arXiv1997

Faint M-dwarfs and the structure of the Galactic disk

We use broadband photometry and low-resolution spectra of a complete sample of late-K and M dwarfs brighter than I=22 in three fields at high galactic latitude to study issues relating to galactic structure and large scale abundance gradients in the Galaxy. The observed starcounts in each field are a good match to the predictions of models based on deep starcount data in other intermediate-latitude fields, and these models identify the late-type stars as members of the Galactic disk. Abundances for these late type stars are estimated via narrowband indices that measure the strength of the TiO and CaH bands in their spectra. Our results show that the average abundance in the Galactic disk remains close to solar even at heights of more than 2 kpc above the Plane.

astro-ph↗