Browsing by Author "Zhang, X."
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Item Autoionization of very-high-n strontium Rydberg atoms(IOP Publishing, 2017) Zhang, X.; Fields, G.; Dunning, F.B.; Yoshida, S.; Burgdörfer, J.The autoionization of high n, n~280-430, strontium Rydberg states through excitation of the 5s 2S1/2→5p 2P1/2 transition in the core ion is investigated. The autoionization rates decrease rapidly as L is increased paving the way for production of long-lived two-electron-excited planetary atoms.Item Autoionization of very-high-n strontium Rydberg states(American Physical Society, 2018) Fields, G.; Zhang, X.; Dunning, F.B.; Yoshida, S.; Burgdörfer, J.We study, using a combination of experiment and theory, the excitation and decay of very-high-n(n∼280–430) strontium autoionizing Rydberg states formed by near-resonant driving of the 5s2S1/2→5p2P1/2 core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the nand ℓ quantum numbers of the Rydberg electron. The nonstationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.Item Characterizing high-n quasi-one-dimensional strontium Rydberg atoms(American Physical Society, 2014) Hiller, M.; Yoshida, S.; Burgdörfer, J.; Ye, S.; Zhang, X.; Dunning, F.B.; Rice Quantum InstituteThe production of high-n, n ∼ 300, quasi-one-dimensional (quasi-1D) strontium Rydberg atoms through two-photon excitation of selected extreme Stark states in the presence of a weak dc field is examined using a crossed laser-atom beam geometry. The dipolar polarization of the electron wave function in the product states is probed using two independent techniques. The experimental data are analyzed with a classical trajectory Monte Carlo simulation employing initial ensembles that are obtained with the aid of quantum calculations based on a two-active-electron model. Comparisons between theory and experiment highlight different characteristics of the product quasi-1D states, in particular, their large permanent dipole moments, ∼1.0 to 1.2n2ea0, where e is the electronic charge and a0 is the Bohr radius. Such states can be engineered using pulsed electric fields to create a wide variety of target statesItem Efficient three-photon excitation of quasi-one-dimensional strontium Rydberg atoms with n ~ 300(American Physical Society, 2014) Ye, S.; Zhang, X.; Dunning, F.B.; Yoshida, S.; Hiller, M.; Burgdörfer, J.; Rice Quantum InstituteThe efficient production of very-high-n, n ~ 300, quasi-one-dimensional (quasi-1D) strontium Rydberg atoms through three-photon excitation of extreme Stark states in the presence of a weak dc field is demonstrated using a crossed laser-atom beam geometry. Strongly polarized quasi-1D states with large permanent dipole moments ∼1.2n2 a.u. can be created in the beam at densities (∼106 cm−3) where dipole blockade effects should become important. A further advantage of three-photon excitation is that the product F states are sensitive to the presence of external fields, allowing stray fields to be reduced to very small values. The experimental data are analyzed using quantum calculations based on a two-active-electron model together with classical trajectory Monte Carlo simulations. These allow determination of the atomic dipole moments and confirm that stray fields can be reduced to _25 μV cm−1.Item Imaging the evolution of an ultracold strontium Rydberg gas(American Physical Society, 2013) McQuillen, P.; Zhang, X.; Strickler, T.; Dunning, F.B.; Killian, T.C.; Rice Quantum InstituteClouds of ultracold strontium 5s48s1S0 or 5s47d1D2 Rydberg atoms are created by two-photon excitation of laser-cooled 5s21S0 atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-ℓ) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the S states, which display isotropic attractive interactions, than for the D states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.Item Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs(Springer Nature, 2017) He, G.; Nathawat, J.; Kwan, C.-P.; Ramamoorthy, H.; Somphonsane, R.; Zhao, M.; Ghosh, K.; Singisetti, U.; Perea-López, N.; Zhou, C.; Elías, A.L.; Terrones, M.; Gong, Y.; Zhang, X.; Vajtai, R.; Ajayan, P.M.; Ferry, D.K.; Bird, J.P.The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS2 due to the narrow width of its energy bands. The various results presented here suggest that WS2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.Item Optical study of local strain related disordering in CVD-grown MoSe2ﾠmonolayers(AIP Publishing, 2016) Krustok, J.; Raadik, T.; Jaaniso, R.; Kiisk, V.; Sildos, I.; Marandi, M.; Komsa, H.-P.; Li, B.; Zhang, X.; Gong, Y.; Ajayan, P.M.We present temperature dependent micro-photoluminescence and room temperature photoreflectance spectroscopy studies on aged MoSe2 monolayers with high surface roughness. A0 and B0 exciton bands were detected at 1.512 eV and 1.72 eV, respectively, which are 50–70 meV lower than those commonly reported for high-quality samples. It is shown that the difference can be accounted for using a model of localized excitons for disordered MoSe2monolayers where the optical band gap energy fluctuations could be caused by random distribution of local tensile strain due to surface roughness. The density of localized exciton states is found to follow the Lorentzian shape, where the peak of this distribution is about 70 meV from the energy of delocalized states.Item Production of high-n strontium Rydberg atoms(IOP Publishing, 2014) Ye, S.; Zhang, X.; Killian, T.C.; Dunning, F.B.; Hiller, M.; Yoshida, S.; Burgdörfer, J.The photoexcitation of strontium Rydberg atoms with n~300 is being examined using a crossed laser-atom beam approach to enable study of quasi-stable two-electron excited states and of strongly-coupled Rydberg systems.Item Rydberg blockade effects at n∼300 in strontium(American Physical Society, 2015) Zhang, X.; Dunning, F.B.; Yoshida, S.; Burgdörfer, J.Rydberg blockade at n∼300, is examined using strontium nF31 Rydberg atoms excited in an atomic beam in a small volume defined by two tightly focused crossed laser beams. The observation of blockade for such states is challenging due to their extreme sensitivity to stray fields and the many magnetic sublevels associated with F states which results in a high local density of states. Nonetheless, with a careful choice of laser polarization to selectively excite only a limited number of these sublevels, sizable blockade effects are observed on an ∼0.1 mm length scale extending blockade measurements into the near-macroscopic regime and enabling study of the dynamics of strongly coupled many-body high-n Rydberg systems under carefully controlled conditions.Item Rydberg blockade in a hot atomic beam(American Physical Society, 2017) Yoshida, S.; Burgdörfer, J.; Zhang, X.; Dunning, F.B.The dipole blockade of very-high-n, n∼300, strontium 5snf1F3 Rydberg atoms in a hot atomic beam is studied. For such high n, the blockade radius can exceed the linear dimensions of the excitation volume. Rydberg atoms formed inside the excitation volume can, upon leaving the region, continue to suppress excitation until they have moved farther away than the blockade radius. Moreover, the high density of states originating from the many magnetic sublevels associated with the F states results in a small but finite probability of excitation of L=3n1F3 atom pairs at small internuclear separations below the blockade radius. We demonstrate that these effects can be distinguished from one another by the distinct features they imprint on the Mandel Q parameter as a function of the duration of the exciting laser.