I. E. Perakis

SIZE-DEPENDENT SURFACE PLASMON DYNAMICS IN METAL NANOPARTICLES

We study the effect of Coulomb correlations on the ultrafast optical dynamics of small metal particles. We demonstrate that a surface-induced dynamical screening of the electron-electron interactions leads to quasiparticle scattering with collective surface excitations. In noble-metal nanoparticles, it results in an interband resonant scattering of d-holes with surface plasmons. We show that this size-dependent many-body effect manifests itself in the differential absorption dynamics for frequencies close to the surface plasmon resonance. In particular, our self-consistent calculations reveal a strong frequency dependence of the relaxation, in agreement with recent femtosecond pump-probe experiments.

Femtosecond switching of magnetism via strongly correlated spin-charge quantum excitations

The technological demand to push the gigahertz (109 hertz) switching speed limit of today’s magnetic memory and logic devices into the terahertz (1012 hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics—in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states—femtosecond-laser-excited coherence between electronic states can switch magnetic order by ‘suddenly’ breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10−15 seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10−12 seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators, and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.

Non-thermal separation of electronic and structural orders in a persisting charge density wave

The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter--excitonic correlations and a periodic lattice distortion (PLD)--respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼40K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼TC∕2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.

Near-bandgap wavelength dependence of long-lived traveling coherent longitudinal acoustic phonons in GaSb-GaAs heterostructures

We report first studies of long-lived oscillations in optical pump-probe measurements on GaSb-GaAs heterostructures. The oscillations arise from a photogenerated coherent longitudinal acoustic phonon wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs.

Electronic dephasing in the quantum Hall regime

Summary form only given. We present the first investigation of electronic dephasing in the presence of two dimensional electron gas (2DEG) in the Quantum Hall regime using four wave mixing (FWM). We observe a transition from Markovian to non-Markovian behavior and abrupt variation of the decay time with filling factor, which we interpret in terms of photoexcited electrons scattering with collective excitations of the 2DE-liquid in the lowest Landau level.

Memory effects in photoinduced femtosecond magnetization rotation in ferromagnetic GaMnAs

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

Ultrafast dynamics of interfacial electric fields in semiconductor heterostructures monitored by pump-probe second-harmonic generation

We report measurements of the ultrafast dynamics of interfacial electric fields in semiconductor multilayers using pump-probe second-harmonic generation (SHG). A pump beam was tuned to excite carriers in all the layers in GaAs/GaSb and GaAs/GaSb/InAs heterostructures. The resulting carrier dynamics manifests itself via electric fields created by charge separation due to carrier redistribution at the interfaces. The evolution of interfacial fields is monitored by a probe beam through an eletric-field-induced SHG signal. We distinguish between several stages of dynamics originating from redistribution of carriers between the layers. We also find a strong enhancement of the induced electric field caused by hybridization of the conduction and valence bands at the GaSb/InAs interface.

Femtosecond coherent control of spins in (Ga,Mn)As ferromagnetic semiconductors using light.

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near–ultraviolet (∼3eV), coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the {111} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. We predict nonthermal magnetization control by tuning the laser frequency and polarization direction. Our mechanism explains recent ultrafast pump–probe experiments.

Ultrafast Coulomb-induced dynamics of quantum well magnetoexcitons

We study theoretically the ultrafast nonlinear optical response of quantum well excitons in a perpendicular magnetic field. We show that for magnetoexcitons confined to the lowest Landau levels, the third-order four-wave-mixing (FWM) polarization is dominated by the exciton-exciton interaction effects. For repulsive interactions, we identify two regimes in the time evolution of the optical polarization characterized by exponential and power law decay of the FWM signal. We describe these regimes by deriving an analytical solution for the memory kernel of the two-exciton wave function in a strong magnetic field. For strong exciton-exciton interactions, the decay of the FWM signal is governed by an antibound resonance with an interaction-dependent decay rate. For weak interactions, the continuum of exciton-exciton scattering states leads to a long tail of the time-integrated FWM signal for negative time delays, which is described by the product of a power law and a logarithmic factor. By combining this analytic solution with numerical calculations, we study the crossover between the exponential and nonexponential regimes as a function of magnetic field. For attractive exciton-exciton interactions, we show that the time evolution of the FWM signal is dominated by biexcitonic effects.