Many-body Correlation Effects in the Ultrafast Nonlinear Optical Response of Confined Fermi Seas

The dynamics of electrons and atoms interacting with intense and ultrashort optical pulses presents an important problem in physics that cuts across different materials such as semiconductors and metals. The currently available laser pulses, as short as 5 fs, provide a time resolution shorter than the dephasing and relaxation times in many materials. This allows for a systematic study of many-body effects using nonlinear optical spectroscopy. In this review article, we discuss the role of Coulomb correlations in the ultrafast dynamics of modulation-doped quantum wells and metal nanoparticles. We focus in particular on the manifestations of non-Markovian memory effects induced by strong electron-hole and electron-plasmon correlations.Comment: 107 pages including 15 figures. Review article to appear in Surf. Sci. Report

Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion

With a variational three--body calculation we study the role of the interplay between the onsite Coulomb, Hund's rule, and superexchange interactions on the spinwave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron--hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spinwave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid communication

Spin dynamics in nonlinear optical spectroscopy of Fermi sea systems

We discuss the role of many-body spin correlations in nonlinear optical response of a Fermi sea system with a deep impurity level. Due to the Hubbard repulsion between electrons at the impurity, the optical transitions between the impurity level and the Fermi sea states lead to an optically-induced Kondo effect. In particular, the third-order nonlinear optical susceptibility logarithmiclly diverges at the absorption threshold. The shape of the pump-probe spectrum is governed by the light-induced Kondo temperature, which can be tuned by varying the intensity and frequency of the pump optical field. In the Kondo limit, corresponding to off-resonant pump excitation, the nonlinear absorption spectrum exhibits a narrow peak below the linear absorption onset.Comment: 7 pages inluding 2 figures. Invited paper for SPIE's Optoelectronics 2001 Conferenc

Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors

We develop a theory describing the effects of many-particle Coulomb correlations on the coherent ultrafast nonlinear optical response of semiconductors and metals. Our approach is based on a mapping of the nonlinear optical response of the ``bare'' system onto the linear response of a ``dressed'' system. The latter is characterized by effective time-dependent optical transition matrix elements, electron/hole dispersions, and interaction potentials, which in undoped semiconductors are determined by the single-exciton and two-exciton Green functions in the absence of optical fields. This mapping is achieved by eliminating the optically-induced charge fluctuations from the Hamiltonian using a Van Vleck canonical transformation. It takes into account all many-body contributions up to a given order in the optical fields as well as important Coulomb-induced quantum dynamics to all orders in the optical field. Our approach allows us to distinguish between optical nonlinearities of different origins and provides a physically-intuitive interpretation of their manifestations in ultrafast coherent nonlinear optical spectroscopy.Comment: 24 page

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.Comment: 41 pages including 11 figs. Final version to appear in Phys. Rev.

Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets

We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.Comment: 14 pages, 11 figures, published in Phys. Rev. B (1 figure added, references added

Femtosecond Coherent Control of Spin with Light in (Ga,Mn)As ferromagnets

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near--ultraviolet ($\sim$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.Comment: 4 pages, 3 figures, published in Physical Review Letter