Session D44: Focus Session: Spin and Nonlinear Dynamics in Optical Nanostructures

Optical control and determination of charge in self-assembled quantum dots

We present a theory and experiment allowing for optical control of charge in a single InAs/GaAs quantum dot (QD) in magnetic fields up to 23 T [1]. The charge is controlled by excitation energy and power and is determined by comparing the experimental PL spectra of the QD to the ones calculated for N electrons and one hole using the parabolic confinement and the CI technique for many-carrier states. The number N is determined from the characteristic features in PL [2]. For N=4 electrons in low fields the degenerate p shell is half-filled and the system is in a triplet state. At larger fields the degeneracy is removed and a triplet-singlet transition occurs. This transition is seen as a discontinuity in the magnetic-field dependence of PL lines. In even higher fields, electrons increase their polarization through spin-flip transitions, which also leads to discontinuities of the PL spectra. Also, as the magnetic moment of electrons increases, the electron-hole exchange leads to the appearance of multiple PL lines. [1] A. Babinski et al, Physica E 26, 190 (2005) [2] A. Wojs and P. Hawrylak, Phys. Rev. B 55, 13066 (1997)   

Spin effects in coupled quantum dots under ac electric fields

Spin control has recently attracted attention for applications in spin-based devices. Different effects and applied fields have been suggested to accomplish the goal. We explore the time evolution of electronic spin in coupled quantum dots under harmonic electric fields. Using the Floquet formalism, we obtain the time dependent wave function in terms of the Floquet states and the quasi-energy spectrum for a single electron in double InSb dots. The spatial part of the wave function includes the SIA and BIA spin-orbit effects. The spectral force is analyzed at anti-crossings of the quasi-energy bands as a function of the field strength. The resulting dynamical symmetries and the way they reflect in the time evolution of the spin clouds will be discussed.   

Qubit identification and entanglement in tunneling and F\"{o}rster coupled quantum dots

We investigate the possibility of qubit coherent manipulation using the multi-excitonic optical spectrum features of a quantum dot molecule (QDM), a system of two vertically coupled InAs/GaAs self-assembled quantum dots. The spectrum is modeled using a Hamiltonian that incorporates coupling dependence on several experimental parameters, such as gate voltage, optical excitation intensity and its detuning. We use realistic structure parameters to describe the important coupling constants, including electron and hole tunneling, and Coulomb correlations that depend on the QDM strain field, and interdot distance [1]. We also incorporate the role of the F\"{o}rster-Dexter resonant energy transfer processes, as well as, exciton oscillator strengths extracted from available PL spectroscopy data. The dynamics given by the time evolution of the density matrix and the qubit-qubit entangling interaction is monitored by calculations of the entanglement of formation [2] for the suitable excitonic molecular states. We discuss how to optimize Rabi flops and entanglement via gate-controlled adiabatic passage through a level anticrossing [3]. [1] G. Bester, A. Zunger, PRB 71 075325 (2005) [2] W.K. Wootters, PRL 80(10) 2245 (1998) [3] K. Bergmann, Rev. Mod. Phys. 70(3) (1998)   

Spin Interactions in Optically Excited Quantum Dot Molecules

Recently we have demonstrated controlled interaction between QDs [1] -- a key requirement for the use of QDs as basic building blocks in novel information processing technologies, as e.g. in quantum computation or spintronics. Here we delineate for the first time the origin of the exchange coupling between spins [2] in optically excited QD molecules, and we trace its atomic to molecular evolution. We have performed photoluminescence spectroscopy on single InAs/GaAs QDMs. The QD molecules were formed by the subsequent growth of two closely spaced layers of self-assembled QDs. The two QD layers were embedded in a diode structure in order to controllably tune different excitonic charge states through molecular resonances. The resulting optical spectra of double QDs exhibit a rich variety of fascinating features. Distinct patterns of anticrossings formed by the various excitonic and biexcitonic charge states allow us to determine the rules governing the state energies and quantum mechanical coupling between two QDs. Prominent spin fine structure in the molecular spectra is understood in terms of the interplay between h-h, e-h, and e-e exchange interactions. This work sets the stage for using laser fields to execute two-qubit operations in quantum dots. \newline \newline [1] E. A. Stinaff, et al., Science \textbf{311}, 636 (2006) \newline [2] M. Scheibner, et al., cond-mat/0607241   

Theory of Spin States in Coupled Quantum Dots.

The system of vertically coupled self-assembled quantum dots (CQDs) tuned by external electric field is a promising candidate as a basis for coherent optical spin manipulation in quantum information applications and spintronics [1]. We have developed a theoretical model that describes spin states of neutral and charged excitons in CQDs [2]. In this approach the electric field induced resonant tunneling of the electron and hole states occurs at different biases due to the inherent asymmetry of CQDs. The truncated many-body basis configurations for each molecule are constructed from antisymmetrized products of single-particle states. The interplay between tunneling, electron-electron, hole-hole and electron-hole exchange interactions splits the states with different spin-projections. The model explains a rich diversity of spectral line patterns in photoluminescence spectra observed in recent experiments. [1] E.A.Stinaff et al., Science 311, 636 (2006). [2] I.V. Ponomarev et al., Phys. Stat. Sol. (b), 243, 3869. (2006)   

Polarized stimulated emission from photonic molecule states in coupled microdisk lasers

Recent studies have demonstrated the engineering of spin coherence via photon-spin interactions in microdisk lasers [S. Ghosh {\it et al.}, Nature (Materials) {\bf 5}, 261 (2006)], motivating the extension of such measurements to pairs of microdisks coupled through the evanescent electromagnetic field. Such coupled microdisks behave like ``photonic molecules'' (PMs) with bonding and antibonding states for the confined photon modes [A. Nakagawa {\it et al.}, Appl. Phys. Lett. {\bf 86}, 04112 (2005)]. We describe the fabrication and optical characterization of different PM geometries, consisting of laterally coupled GaAs/GaAlAs microdisks of both circular and elliptical shape. Steady state photoluminescence measurements reveal bonding and antibonding modes with distinct geometry-dependent polarization characteristics that are consistent with finite-difference time-domain simulations. We also discuss time-resolved optical measurements that probe both carrier and spin dynamics in these PMs.   

Spin Multiphoton Antiresonance at Finite Temperatures

Weakly anisotropic $S>1$ spin systems display multiphoton antiresonance. It occurs when an Nth overtone of the radiation frequency coincides with the distance between the ground and the Nth excited energy level (divided by $\hbar$). The coherent response of the spin displays a sharp minimum or maximum as a function of frequency, depending on which state was initially occupied. We find the spectral shape of the response dips/peaks. We also study the stationary response for zero and finite temperatures. The response changes dramatically with increasing temperature, when excited states become occupied even in the absence of radiation. The change is due primarily to the increasing role of single-photon resonances between excited states, which occur at the same frequencies as multiphoton resonances. Single-photon resonances are broad, because the single-photon Rabi frequencies largely exceed the multi-photon ones. This allows us to separate different resonances and to study their spectral shape. We also study the change of the spectrum due to relaxational broadening of the peaks, with account taken of both decay and phase modulation.   

Nonlinear interlevel optical phenomena in quantum dots

Nonlinear interlevel optical phenomena caused by the electron-electron interaction in quantum dots are investigated theoretically within the semiclassical density matrix formalism. A special attention is paid to the intrinsic optical bistability. Obtained analytical relations and results of numerical simulations reveal role of driving charasteristic parameters of quantum dot systems as well as of the incident radiation in the phenomena. Self-consistent treatment of the electron-electron interaction is shown to be of crucial importance. A proper microscopical treatment is shown to be needed for accurate description of the phenomena.   

Origin of second-harmonic generation of Si nanoinclusions in glass

We applied our anisotropic bond model (ABM) to clarify the origin of the second-harmonic-generation (SHG) signals observed by Figliozzi et al.[1] for Si nanoinclusions in glass. The ABM describes nonlinear-optic (NLO) responses in terms of radiation from anisotropically and anharmonically bound bond charges, and differs from conventional force formulations by (1) incorporating anisotropy at the bond level and (2) describing observed NLO intensities as a coherent superposition of radiation from these charges accelerated by the driving field. It therefore provides specific information about the origins of NLO signals at the atomic level. Here, SHG signals from the glass and bulk of the Si inclusions are found to be essentially nonexistent, as expected,[2] in the former case as a result of cancellation of radiation fields of bonds oriented in random directions, and in the latter case due to dielectric screening. Our calculations show that SHG is dominated by gradient effects, specifically from the variation in field across the inclusion (spatial-dispersion and crossed-beam effects), consistent with experiment. The large interface field gradient contributes a weak signal from charge motion transverse to the bond direction. [1] P. Figliozzi et al. Phys Rev Lett 94 (2005). [2] V. L. Brudny et al. Phys Rev B 62 (2000).   

Rabi Coupling Between IR-active Phonon and Cavity-resonant Electromagnetic Modes

We predict an approximately 200 micron Rabi coupling between a cavity-resonant electromagnetic mode and the infrared-active phonon of an enclosed GaAs sample. This prediction follows from our quantized description of the electromagnetic field, the phonon field, and their interaction. We believe the prediction to be supported by recent observations of geometry-enhanced terahertz emission, and boundary-condition dependent phonon-polariton spectra in pump-probe optical studies.   

Electron-Photon interaction associated Uncertaity Relation based Tunneling in a Parallel Double Quantum Dot System

A new mechanism of electron-photon interaction in a parallel double quantum dot (DQD) system is studied. The electron is allowed to transit between dots due to the electron-photon interaction. When the electron in quantum dot m (QDm) transits to the adjoining QDm(m,m1,2 and mm), it is allowed to tunnel into leadm , which is connected to QDm , via energy-time uncertainty relation in a very short time interval. Like the Kondo resonant peak in Anderson model, the new mechanism of the electron-photon interaction exhibits peaks which depends logarithmically on temperature. The character temperature obtained is found to be higher than the Kondo temperature in some situations. Unlike the Kondo effect, the quantum mechanical tunneling associated the electron-photon interaction is not always on resonance.