Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C15: Exciton and Photo-induced Charge DynamicsFocus
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Sponsoring Units: DMP Chair: Sahar Sharifzadeh, Boston University Room: LACC 304C |
Monday, March 5, 2018 2:30PM - 3:06PM |
C15.00001: Coherent exciton dynamics in lead halide perovskites probed via two-dimensional electronic spectroscopy Invited Speaker: Carlos Silva Two-dimensional lead halide perovskites have recently emerged as promising optoelectronic materials and are often referred to as quantum-well-like structures. They exhibit narrow and well-defined excitonic transitions in linear absorption, with large exciton binding energies (200-300 meV). Whilst such a spectral structure can be rationalized with a 2D Elliott model accounting for the electronic confinement within the lead halide layers, we also observe a pronounced fine structure within the exciton line, which cannot be explained by such a model. Here, we investigate its origin via optical and vibrational spectroscopy of a series of 2D perovskites templated by different organic cations. We establish the role of octahedral distortions in excitonic correlations, which lift the orbital degeneracies via Jahn-Teller like mechanism and subsequently result in four distinct excitonic states. These transitions are also seen in the two-dimensional correlation maps obtained via multi-dimensional spectroscopy, where we identify spectral correlations. We observe multi-particle interactions using two-quantum coherence spectroscopy, where we identify bi-exciton resonances and extract biexciton binding energies of around 50 meV, one of the highest reported values for two-dimensional materials. |
Monday, March 5, 2018 3:06PM - 3:42PM |
C15.00002: Dissociation of triplet pair states in intramolecular singlet fission materials Invited Speaker: Matthew Sfeir We have recently demonstrated efficient intramolecular singlet fission (iSF) materials that achieve multiple exciton generation in isolated (i.e., in dilute solution) polymers and small molecules with multichromophore character. In these materials, singlet fission is a two-step process in which photoexcited singlet excitons rapidly decay into triplet pair states with strong exchange coupling (multiexciton), followed by dissociation into free triplets. Still, fundamental questions concerning the electronic and mechanistic requirements that drive both the formation and decay of the triplet pair state remain. Using correlated ultrafast optical and electron spin resonance measurements, we identify long lived triplet pairs that exist in net singlet and quintet spin multiplicities. By exploiting both intra- and intermolecular interfacial interactions, we are able to promote the dissociation of triplet pairs into free triplets. These studies establish the efficacy of iSF compounds as viable candidates for singlet fission based optoelectronic devices. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C15.00003: Formation and Evolution of Nonlinear Excitations in Low Dimensional Materials Jason Leicht, Sarah Kim, Susan Dexheimer We present femtosecond time-resolved optical measurements of the formation and evolution of excitons, polarons, and solitons in mixed-valence halide-bridged platinum chain materials in which the electronic excitations are confined to the one-dimensional geometry defined by the chain. Interpretation of the dynamics is aided by global analysis, a signal processing technique that extracts time-dependent spectral components, allowing the overlapping spectra associated with each type of electronic excitation to be identified and individually tracked. In the platinum chain materials, the nature of the photogenerated excitations can be controlled via the optical excitation energy, with excitation well above the optical gap energy resulting in the formation of charged polarons in addition to the self-trapped excitons that form following excitation near the band edge. For low-energy excitation, our results show the evolution of initially generated excitons into self-trapped excitons, followed by dissociation into soliton pairs. For high-energy excitation, we find that polarons are generated as primary photoexcitations, with branching between polaron and exciton formation. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C15.00004: Anisotropic exciton transport in transition-metal dichalcogenides Pouyan Ghaemi Mohammadi, Areg Ghazaryan Due to the intervalley exchange interaction exciton eignestates in monolayer transitional metal dichalcogenides are coherent superposition of two valleys. The branch which couples with transverse electric linearly polarized light has parabolic dispersion in terms of it center of mass momentum, whereas the one which couples to transverse magnetic has both parabolic and linear components. While the dispersion of excitons is well established theoretically there is no experimental verification of the linear component of the dispersion up to date. In this talk we present experimentally realizable method for unambiguous observation of the signature of linear component of the dispersion. In particular, we will demonstrate that by pumping the system with linearly polarized light the exciton transport is anisotropic compared to circularly polarized pump and this feature is special for this specific type of dispersion. This signature is present also for positively detuned exciton-polaritons and for reasonable level of disorder. Therefore, this opens the possibility for experimental identification of the peculiar structure of exciton dispersion of transition metal dichalcogenides. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C15.00005: Theory of Optical Absorption by Interlayer Excitons in Transition Metal Dichalcogenide Heterobilayers Fengcheng Wu, Timothy Lovorn, Allan MacDonald We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moire pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moire Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moire potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moire pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C15.00006: Charging Dynamics of Single InGaAs Quantum Dots under Resonant Excitation Gary Lander, Samantha Isaac, Disheng Chen, Samet Demircan, Glenn Solomon, Edward Flagg Semiconductor quantum dots can act as sources of single photons, but certain experimental factors can complicate their single photon emission. Resonant excitation of either a neutral or charged quantum dot can cause a transition to the opposite charge state, which greatly diminishes the fluorescence and reduces a dot’s suitability to act as an efficient photon source. A counter to this effect is application of a low-power above-band laser that supplies the local charge environment with extra electrons and holes in the bulk material. These charge carriers can be captured by either a charged quantum dot, resulting in neutralization and allowing resonant excitation of the exciton state, or a neutral quantum dot, allowing resonant excitation of the trion state. We characterize as a function of laser power the steady-state regime and the time-dependent dynamics of a charged quantum dot by modulating the above-band laser power. The time-resolved fluorescence is recorded and phenomenological fits are used to extract the charging and discharging rates associated with the different processes that move charge carriers into and out of the quantum dot. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C15.00007: Excitonic Effects and Mott Transition in Semiconductor Heterostructures Sergey Rudin, Greg Rupper, Gregory Garrett, Chelsea Haughn, Mahesh Neupane We present a study of excitonic effects and Mott transition in two different types of heterostructures: polar AlGaN/AlN quantum wells and MoS2 bilayers. Narrow AlGaN quantum wells are important elements of ultra-violet light emitting devices and the excitonic effects in radiative lifetimes were found to be significant. We included the density dependent screening of polarization fields in our theory of luminescence, and employed Green’s function formalism with self-consistent T-matrix approximation. This allows to include effects of exciton formation and dissociation. Exciton formation was found to have a strong effect on the density dependence of the radiative recombination rates. We evaluated the ionization ratio as a function of carrier density for different values of well width, from low to high densities, through the excitonic Mott transition where the Pauli blocking and screening raise the ionization rate to unity. In case of MoS2 bilayer we have n and p type conduction channels separated by a thin dielectric barrier allowing strong electron-hole Coulomb correlation and formation of indirect excitons. For the case of gated MoS2 monolayers separated by a few-monolayer BN we evaluated the transition from insulating excitonic state to the conducting electro-hole plasma state. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C15.00008: Quasi-particle band gap and excitonic effects in 1-Dimensional (1D) Selenium and Tellurium nano-wires Eesha Andharia, Salvador Barraza-Lopez 1-Dimensional(1D) vdW’s structures represent an exotic class of materials for opto-electronic applications due to high excitonic binding energy arising from reduced screening and quantum confinement effects. Theoretical investigation is crucial to guide incipient experimental efforts[1]. Many-body interactions have a prominent effect on the electronic and optical properties of reduced-dimensional systems, and we will report the electronic & optical properties of selenium and tellurium atomic chains with quasi-particle self-energy corrections within the GW approximation[2], on structures obtained without and with van der Waals corrections here. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C15.00009: Polarized photocurrent spectroscopy of wurtzite InAs nanowires Seyyedesadaf Pournia, Giriraj Jnawali, Howard Jackson, Leigh Smith, Hoe Tan, Chennupati Jagadish Polarized photocurrent measurements on devices fabricated from wurtzite InAs nanowires reveal the energy band structure and optical transitions in this hexagonal material. InAs nanowires were grown by MOCVD and dispersed onto a Si++/SiO2 substrate. Aluminum contacts were placed at the two ends of nanowire using photolithography. Experiments were performed with light polarized along the long axis of the nanowire (the c-axis), as well as perpendicular to the c-axis. The data displays a 0.43 eV band gap energy at room temperature, which is ~70 meV larger than observed in bulk zinc blende material. Valence band splittings are in a good agreement with theory. The transition from the A hole band to the conduction band is only seen for perpendicular polarized light. The transitions from B and C to the conduction band are seen for both polarizations in accordance with group theory. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C15.00010: Carrier Dynamics in Single GaAs0.7 Sb0.3 / InP Core-Shell Nanowires with Type II Band Alignment Iraj Abbasian Shojaei, Samuel Linser, Giriraj Jnawali, Howard Jackson, Leigh Smith, Xiaoming Yuan, Philippe Caroff, Hoe Tan, Chennupati Jagadish Time-resolved optical measurements of single zinc-blende GaAs0.7Sb0.3 core-only and GaAs0.7Sb0.3/InP core-shell nanowires at 10K are obtained from pump-probe measurements. The core-shell nanowires exhibit a remarkably longer lifetime (≈ 600 times) compared with core-only nanowires. Measurements at 300K show only a factor of 6 enhancement. These results suggest that band alignment of the core-shell nanowire may be Type II with less than a 30meV off-set. Carrier thermalization and carrier density evolution obtained from fitting the nanowire spectra using Transient Rayleigh scattering (TRS) modeling provide supporting evidence for Type II band alignment. By modeling the energy loss mechanism of carriers based on longitudinal optical and acoustic phonon emission, we are able to simulate carrier thermalization. These results show that at early times optical phonon emission dominates the carrier energy loss rate. For core-shell nanowires at 10 K, the acoustic phonon emission dominates after 300 ps. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C15.00011: Pump-probe Spectroscopy of Single InGaAs and InGaAs/InP Nanowires: Shedding Light on the Wurtzite Band Structure and Carrier Dynamics Samuel Linser, Iraj Abbasian Shojaei, Giriraj Jnawali, Howard Jackson, Leigh Smith, Amira Ameruddin, Philippe Caroff, Hoe Tan, Chennupati Jagadish We use transient Rayleigh scattering to optically characterize the band structure and carrier dynamics of single wurtzite nanowires. We employ a 1.51 eV pump and a tunable near-infrared probe (0.79 to 1.16 eV) to study core-only InGaAs nanowires as well as strained core-shell InGaAs-InP heterostructures at 300 K and 10 K. We observe an electronic transition in low temperature core-only spectra at 0.86 eV, a higher energy than the fundamental bandgap of bulk zincblende InGaAs of similar composition. We observe a corresponding transition in core-shell spectra at 0.97 eV. The blue-shift of the transition energy for the core-shell nanowire relative to the core-only nanowire is consistent with compressive strain in the InGaAs core. We report an order of magnitude enhancement in the 10 K carrier lifetimes of the core-shell nanowires (2100 ps) compared to the core-only nanowires (90 ps). Numerical modeling of our TRS spectra provides further insight into the thermalization of charge carriers after photoexcitation. |
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