Search query: Polaron coherent optical phonon

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abstract: When an electron hole pair is optically excited in a semiconductor quantum dot, the host crystal lattice adapts to the presence of the generated charge distribution. Therefore, the coupled exciton phonon system has to establish a new equilibrium, which is reached in the form of a quasiparticle called a polaron. Especially, when the exciton is abruptly generated on a timescale faster than the typical lattice dynamics, the lattice cannot follow adiabatically. Consequently, rich dynamics on the picosecond timescale of the coupled system is expected. In this study, we combine simulations and measurements of the ultrafast, coherent, nonlinear optical response, obtained by four wave mixing (FWM) spectroscopy, to resolve the formation of this polaron. By detecting and investigating the phonon sidebands in the FWM spectra for varying pulse delays and different temperatures, we have access to the influence of phonon emission and absorption processes, which finally result in the emission of an acoustic wave packet.
author_names: Daniel Wigger

Vage Karakhanyan

Christian Schneider

Martin Kamp

Sven Hofling

Pawel Machnikowski

Tilmann Kuhn

Jacek Kasprzak
title: Acoustic phonon sideband dynamics during polaron formation in a single quantum dot.
venue: Optics letters
year: 2020

abstract: A long lived hot carrier in organic inorganic hybrid perovskite materials is possibly assisted by the large polaron and hot phonon bottleneck. The phonon plays a significant role in the properties of hybrid perovskites. Coherent phonon refers to the in phase vibration of the crystal lattice. Here, temperature dependent, fluence dependent, and polarization dependent transient absorption spectroscopy is utilized to study the coherent phonon in methylammonium lead iodide. A coherent optical phonon is observed due to the existence of Pb I Pb angular distortion at 24 33 cm 1 in both tetragonal and orthorhombic phases. Oscillation from the excited state absorption is also observed, suggesting coupling between the coherent phonon and photoexcited state. Oscillation of the coherent phonon attenuates with increasing environmental temperature. The dependence of the coherent phonon on polarization of the pulse and quantitative analysis indicates that the generation mechanism is impulsive stimulated Raman scattering
author_names: Chengbin Fei

Julio S Sarmiento

He Wang
title: Generation of Coherent Optical Phonons in Methylammonium Lead Iodide Thin Films
year: 2018

abstract: Femtosecond optical pump probe spectroscopy is used to reveal the influence of charge and magnetic order on polaron dynamics and coherent acoustic phonon oscillations in single crystals of charge ordered, ferrimagnetic LuFe2O4. We experimentally observed the influence of magnetic order on polaron dynamics. We also observed a correlation between charge order and the amplitude of the acoustic phonon oscillations, due to photoinduced changes in the lattice constant that originate from the photoexcited electrons. This provides insight into the general behavior of coherent acoustic phonon oscillations in charge ordered materials.
author_names: Jihye Lee

S A Trugman

Chenglin Zhang

Diyar Talbayev

Xiaoshan Xu

Sang-Wook Cheong

Dmitry Anatolievitch Yarotski

Antoinette J Taylor

Rohit Prativadi Prasankumar
title: The influence of charge and magnetic order on polaron and acoustic phonon dynamics in LuFe2O4
year: 2015

abstract: Hematite (a Fe2O3) is the most studied artificial oxygen evolving photo anode and yet its efficiency limitations and their origin remain unknown. A sub picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device based optical control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes <100 cm 1) Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly studied hematite devices. The efficiency of Hematite (a Fe2O3) photo anodes is thought to be limited by ultrafast lattice distortions or polarons. Here, we use an optical control method with photocurrent detection to track small polarons in real time and demonstrate that they impact photoelectrochemical cell activity
author_names: Ernest Pastor

Ji-Sang Park

Ludmilla Steier

Sunghyun Kim

Michael Gratzel

James R Durrant

Aron Walsh

Artem A Bakulin
title: In situ observation of picosecond polaron self localisation in a Fe2O3 photoelectrochemical cells
venue: Nature Communications
year: 2019

abstract: The long carrier lifetime and defect tolerance in metal halide perovskites (MHPs) are major contributors to the superb performance of MHP optoelectronic devices. Large polarons were reported to be responsible for the long carrier lifetime. Yet microscopic mechanisms of the large polaron formation including the so called phonon melting, are still under debate. Here, time of flight (TOF) inelastic neutron scattering (INS) experiments and first principles density functional theory (DFT) calculations were employed to investigate the lattice vibrations (or phonon dynamics) in methylammonium lead iodide \rm{MAPbI_3} a prototypical example of MHPs. Our findings are that optical phonons lose temporal coherence gradually with increasing temperature which vanishes at the orthorhombic to tetragonal structural phase transition. Surprisingly, however, we found that the spatial coherence is still retained throughout the decoherence process. We argue that the temporally decoherent and spatially coherent vibrations contribute to the formation of large polarons in this metal halide perovskite.
author_names: Depei Zhang

Xiao Jun Hu

Tianran Chen

Douglas L Abernathy

Ryoichi Kajimoto

Mitsutaka Nakamura

Maiko Kofu

Benjamin J Foley

Mina Yoon

Joshua J Choi

Seung-Hun Lee
title: Temporally decoherent and spatially coherent vibrations in metal halide perovskite.
year: 2020

abstract: Abstract We investigated the optical response of La0.67Sr0.33MnO3 (LSMO) films which show the metal insulator transition (MIT) with the film thickness dependence. By using spectroscopic ellipsometric technique we found that the optical spectra below the charge transfer gap exhibited the coherent incoherent crossover behavior through MIT. The formation of the incoherent mode near 1.5 eV was reminiscent of the polaron absorption which had been widely observed in various manganites. We suggest that the electron phonon coupling could be enhanced due to low dimensionality in the ultrathin LSMO film in consideration of orbital polarization.
author_names: Il Wan Seo

Yunsang Lee

Ji-Won Seo

Seung Hyub Baek
title: Optical investigation of the metal insulator transition in the manganite films with the thickness dependence
venue: Current Applied Physics
year: 2019

abstract: We investigate the effects of a nearly uniform Bose Einstein condensate (BEC) on the properties of immersed trapped impurity atoms. Using a weak coupling expansion in the BEC impurity interaction strength, we derive a model describing polarons, i.e. impurities dressed by a coherent state of Bogoliubov phonons, and apply it to ultracold bosonic atoms in an optical lattice. We show that, with increasing BEC temperature, the transport properties of the impurities change from coherent to diffusive. Furthermore, stable polaron clusters are formed via a phonon mediated off site attraction.
author_names: Martin Bruderer

Alexander Klein

Stephen R Clark

Dieter Jaksch
title: Polaron Physics in Optical Lattices
year: 2007

abstract: Mixed valence O doped UO2+x and photoexcited UO2 containing transitory U3+ and U5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Frohlich type, nonequilibrium, phonon coupled Bose Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO2, U4O9, and U3O7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f states of the valence band by ~0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2 2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U5f states and give a continuous density of states that increases rapidly above its constricted intersection. Femtosecond resolved photoemission measurements of UO2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO2 and no hot electrons. 3.1 eV excitation, however, complements the O doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO2 therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5 10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived >3 10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating UO2 excited state. This heterogeneous, nonequilibrium, Frohlich BEC stabilized by a Fano Feshbach resonance therefore continues to exhibit unique properties.
author_names: Steven D Conradson

David A Andersson

Kevin S Boland

J A Bradley

Darrin D Byler

Tomasz Durakiewicz

Steve M Gilbertson

Stosh Anthony Kozimor

Kristina O Kvashnina

Dennis Nordlund

George Rodriguez

Gerald T Seidler

Paul S Bagus

Sergei M Butorin

Dylan R Conradson

F J Espinosa-Faller

Nancy J Hess

Joshua J Kas

Juan S Lezama-Pacheco

Philippe M Martin

Mary B Martucci

John J Rehr

James Anthony Valdez

Alan R Bishop

Gianguido Baldinozzi

David L Clark

Akhil Tayal
title: Closure of the Mott gap and formation of a superthermal metal in the Frohlich type nonequilibrium polaron Bose Einstein condensate in U O 2 x
year: 2017

abstract: Electron transport in periodic quantum dot arrays in the presence of interactions with phonons was investigated using the formalism of nonequilibrium Green's functions. The self consistent Born approximation was used to model the self energies. Its validity was checked by comparison with the results obtained by direct diagonalization of the Hamiltonian of interacting electrons and longitudinal optical phonons. The nature of charge transport at electron phonon resonances was investigated in detail and contributions from scattering and coherent tunneling to the current were identified. It was found that at larger values of the structure period, the main peak in the current field characteristics exhibits a doublet structure which was shown to be a transport signature of polaron effects. At smaller values of the period, electron phonon resonances cause multiple peaks in the characteristics. A phenomenological model for treatment of nonuniformities of a realistic quantum dot ensemble was also introduced to estimate the influence of nonuniformities on current field characteristics.
author_names: Nenad Vukmirovi'c

Zoran Ikoni'c

Dragan Indjin

Paul Harrison
title: Quantum transport in semiconductor quantum dot superlattices: Electron phonon resonances and polaron effects
year: 2007

abstract: We present a theory of the optical line shape of coherent intersubband transitions in a semiconductor quantum well, considering non Markovian LO phonon scattering as major broadening mechanism. We show that a quantum kinetic approach leads to additional polaron resonances and a resonance enhancement for gap energies close to the phonon energy.
author_names: Stefan Butscher

Jens Forstner

I Waldmuller

Andreas Knorr
title: Polaron signatures in the line shape of semiconductor ;intersubband transitions: quantum kinetics of the electron phonon interaction
year: 2004