arXiv subjects

D. S. Chemla

Publications and source records attributed to D. S. Chemla.

17 recordsLinked to original sources

arXiv2009

Optically induced softening of the charge-transfer gap in Sr2CuO2Cl2

Energy- and time-resolved spectroscopy reveals a photoinduced softening of the charge-transfer gap in the insulating copper oxide Sr2CuO2Cl2 that indicates rapid and efficient photoproduction of optical phonons. By relating the pump-probe signal amplitude to the thermal difference spectrum, we estimate that eleven to twenty optical phonons are created for every one 3 eV photon. Assuming relaxation to the optical absorption edge at 1.5 eV, this corresponds to 70-130 meV per boson. While the lower limit is consistent with relaxation exclusively through optical phonons, the upper limit suggests a significant role for magnetic excitations. We observe a photoinduced bleaching of the gap excitation that we associate with phase space filling, and estimate the excluded area of the photoexcited state to be about nine copper oxide plaquettes. The temporal decay of the pump-probe signal is consistent with anharmonic phonon decay.

cond-mat.str-el↗

arXiv2009

Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases

We report a comprehensive experimental study and detailed model analysis of the terahertz dielectric response and density kinetics of excitons and unbound electron-hole pairs in GaAs quantum wells. A compact expression is given, in absolute units, for the complex-valued terahertz dielectric function of intra-excitonic transitions between the 1s and higher-energy exciton and continuum levels. It closely describes the terahertz spectra of resonantly generated excitons. Exciton ionization and formation are further explored, where the terahertz response exhibits both intra-excitonic and Drude features. Utilizing a two-component dielectric function, we derive the underlying exciton and unbound pair densities. In the ionized state, excellent agreement is found with the Saha thermodynamic equilibrium, which provides experimental verification of the two-component analysis and density scaling. During exciton formation, in turn, the pair kinetics is quantitatively described by a Saha equilibrium that follows the carrier cooling dynamics. The terahertz-derived kinetics is, moreover, consistent with time-resolved luminescence measured for comparison. Our study establishes a basis for tracking pair densities via transient terahertz spectroscopy of photoexcited quasi-two-dimensional electron-hole gases.

cond-mat.mes-hall↗

arXiv2008

Observation of inter-Landau-level quantum coherence in semiconductor quantum wells

Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a non-radiative coherence between the discrete Landau levels of an undoped quantum well and study its dynamics. We observe quantum beats that reflect the time evolution of the coherence between the two lowest Landau level magnetoexcitons. We interpret our observations using a many-body theory and find that the inter Landau level coherence decays with a new time constant, substantially longer than the corresponding interband magnetoexciton dephasing times. Our results indicate a new intraband excitation dynamics that cannot be described in terms of uncorrelated interband excitations.

cond-mat.mes-hall↗

arXiv2008

Memory Effect in the Photoinduced Femtosecond Rotation of Magnetization in the Ferromagnetic Semiconductor GaMnAs

We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.

cond-mat.mtrl-sci↗

arXiv2007

Ultrafast Enhancement of Ferromagnetism via Photoexcited Holes in GaMnAs

We report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature Tc and dominates the demagnetization component when approaching Tc. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20K and a transient increase in Tc.

cond-mat.stat-mech↗

arXiv2006

Nonlinear Optical studies of the Transient Coherence in the Quantum Hall System

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors $ν\sim 1$. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical responce. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.

cond-mat.mes-hall↗

arXiv2006

Ultrafast dynamics of coherences in the quantum Hall system

Using three-pulse four-wave-mixing optical spectroscopy, we study the ultrafast dynamics of the quantum Hall system. We observe striking differences as compared to an undoped system, where the 2D electron gas is absent. In particular, we observe a large off-resonant signal with strong oscillations. Using a microscopic theory, we show that these are due to many-particle coherences created by interactions between photoexcited carriers and collective excitations of the 2D electron gas. We extract quantitative information about the dephasing and interference of these coherences.

cond-mat.mes-hall↗

arXiv2003

Charge transport and phase transition in exciton rings

The macroscopic exciton rings observed in the photoluminescence (PL) patterns of excitons in coupled quantum wells (CQWs) are explained by a series of experiments and a theory based on the idea of carrier imbalance, transport and recombination. The rings are found to be a source of cold excitons with temperature close to that of the lattice. We explored states of excitons in the ring over a range of temperatures down to 380 mK. These studies reveal a sharp, albeit continuous, second order phase transition to a low-temperature ordered exciton state, characterized by ring fragmentation into a periodic array of aggregates. An instability at the onset of degeneracy in the cold exciton system, due to stimulated exciton formation, is proposed as the transition mechanism.

cond-mat↗

arXiv2002

Ultrafast Nonlinear Optical Response of Strongly Correlated Systems: Dynamics in the Quantum Hall Effect Regime

We present a theoretical formulation of the coherent ultrafast nonlinear optical response of a strongly correlated system and discuss an example where the Coulomb correlations dominate. We separate out the correlated contributions to the third-order nonlinear polarization, and identify non-Markovian dephasing effects coming from the non-instantaneous interactions and propagation in time of the collective excitations of the many-body system. We discuss the signatures, in the time and frequency dependence of the four-wave-mixing (FWM) spectrum, of the inter-Landau level magnetoplasmon (MP) excitations of the two-dimensional electron gas (2DEG) in a perpendicular magnetic field. We predict a resonant enhancement of the lowest Landau level (LL) FWM signal, a strong non-Markovian dephasing of the next LL magnetoexciton (X), a symmetric FWM temporal profile, and strong oscillations as function of time delay, of quantum kinetic origin. We show that the correlation effects can be controlled experimentally by tuning the central frequency of the optical excitation between the two lowest LLs.

cond-mat.mes-hall↗

arXiv2002

Macroscopically ordered state in exciton system

Macroscopically ordered arrays of vortices in quantum liquids, such as superconductors, He-II, and atom Bose-Einstein Condensates (BEC), demonstrate macroscopic coherence in flowing superfluids [1-4]. Despite of the rich variety of systems where quantum liquids reveal macroscopic ordering, experimental observation of a macroscopically ordered electronic state in semiconductors has remained a challenging unexplored problem. A system of excitons is a promising candidate for the realization of macroscopic ordering in a quantum liquid in semiconductors. An exciton is a bound pair of an electron and a hole. At low densities, it is a Bose quasi-particle. At low temperatures, of the order of a few Kelvins, excitons can form a quantum liquid, i.e., a statistically degenerate Bose gas and eventually BEC [5-9]. Here, we report the experimental observation of a macroscopically ordered state in an exciton system.

cond-mat↗

arXiv2001

Far-infrared optical conductivity gap in superconducting MgB2 films

We report the first study of the optical conductivity of MgB2 covering the range of its superconducting energy gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity of thin films. The imaginary part reveals an inductive reponse due to the emergence of the superconducting condensate. The real part exhibits a strong depletion of oscillator strength near 5 meV resulting from the opening of a superconducting energy gap. The gap ratio of 2Delta/kTc = 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor

cond-mat.supr-con↗

arXiv2001

Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy

We present the first study of nonlinear optical third harmonic generation in the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental excitation in the near-infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity excitation at 2 eV, with a_1g symmetry, participates in the third harmonic susceptibility.

cond-mat.str-el↗

arXiv2001

Evidence of phonon mediated coupling between charge transfer and ligand field excitons in Sr$_2$CuO$_2$Cl$_2$

We present a comparative experimental and theoretical investigation of the two--dimensional charge-transfer gap in the strongly correlated material Sr$_2$CuO$_2$Cl$_2$. We observe an Urbach behaviour in the absorption profile over a surprisingly wide range of energies and temperatures. We present a model that accounts for phonon scattering to infinite order and which allows us to explain the data accurately by assuming coupling of the charge transfer gap exciton to lower energy electronic excitations.

cond-mat.str-el↗

arXiv1999

Time-resolved optical observation of spin-wave dynamics

We have created a nonequilibrium population of antiferromagnetic spin-waves in Cr2O3, and characterized its dynamics, using frequency- and time-resolved nonlinear optical spectroscopy of the exciton-magnon transition. We observe a time-dependent pump-probe line shape, which results from excitation induced renormalization of the spin-wave band structure. We present a model that reproduces the basic characteristics of the data, in which we postulate the optical nonlinearity to be dominated by interactions with long-wavelength spin-waves, and the dynamics to be due to spin-wave thermalization.

cond-mat.str-el↗

arXiv1999

Electronic Dephasing in the Quantum Hall Regime

By means of degenerate four-wave mixing (FWM), we investigate the quantum coherence of electron-hole pairs in the presence of a two-dimensional electron gas in modulation-doped GaAs-AlGaAs quantum wells in the quantum Hall effect regime. With increasing magnetic field, we observe a crossover from Markovian to non-Markovian behavior, as well as large jumps in the decay time of the FWM signal at even Landau level filling factors. The main observations can be qualitatively reproduced by a model which takes into account scattering by the collective excitations of the two-dimensional electron gas.

cond-mat↗

arXiv1999

Femtosecond Coherent Dynamics of the Fermi Edge Singularity and Exciton Hybrid

We study theoretically the coherent nonlinear optical response of doped semiconductors with partially occupied subbands. When the Fermi energy of an occupied subband approaches the exciton level of an upper subband, the absorption spectrum acquires a characteristic double-peak shape originating from the interference between the Fermi-edge singularity and the exciton resonance. We demonstrate that, for the off-resonant pump excitation, the pump/probe spectrum undergoes a dramatic transformation in the coherent regime, with a strong time-dependent exchange of oscillator strength between Fermi edge singularity and exciton peaks. We show that this effect originates from the many-body electron-hole correlations which determine the dynamical response of the Fermi see. To calculate the nonlinear absorption spectrum, we use the coupled cluster method combined with time-dependent Schrieffer-Wolff transformation. Possible experimental applications are discussed.

cond-mat↗

arXiv1999

Coherent Ultrafast Optical Dynamics of the Fermi Edge Singularity

We develop a non-equilibrium many-body theory of the coherent femtosecond nonlinear optical response of the Fermi edge singularity. We study the role of the dynamical Fermi sea response in the time-evolution of the pump-probe spectra. The electron-hole correlations are treated nonperturbatively with the time-dependent coupled cluster cxpansion combined with the effective Hamiltonian approach. For short pulse durations, we find a non-exponential decay of the differential transmission during negative time delays, which is governed by the interactions. This is in contrast to the results obtained within the Hartree-Fock approximation, which predicts an exponential decay governed by the dephasing time. We discuss the role of the optically-induced dephasing effects in the coherent regime.

cond-mat↗