Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J16: Focus Session: Nano-spectroscopy of Quantum Dots |
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Sponsoring Units: DMP Chair: Sergio E. Ulloa, Ohio University Room: LACC 404A |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J16.00001: Learning the Rules of Extreme Quantum Confinement: Ultrafast Photophysics of Semiconductor Nanocrystals Invited Speaker: Zero-dimensional (0D) semiconductor nanocrystals (NCs) allow the realization of new types of strongly interacting multiexciton states that do not occur in bulk semiconductors. Using sub-10 nm colloidal nanoparticles one can generate states, in which several excitons occupy a volume comparable to or smaller than the volume of a bulk exciton. Such ``squeezed'' exciton states are characterized by greatly enhanced mutiparticle interactions resulting from a forced overlap of electronic wavefunctions and a reduced dielectric screening. In this paper, we utilize various ultrafast optical technique in order to study spectroscopic and dynamical signatures of multiexciton states in size- and shape-controlled CdSe and PbSe NCs. Specifically, by using a series of elongated CdSe nanoparticles (quantum rods), we investigate the effect of the 0D-to-1D transition on the efficiency of multiparticle Auger recombination. Further, by applying a femtosecond photoluminescence up-conversion technique, we detect the emission from neutral and charged biexcitons and directly measure their interaction energy. Finally, we use PbSe NCs to study the effect of carrier muliplication, in which relaxation of a single, high-energy exciton produces multiple excitons. In addition to providing new insights into the physics of exciton-exciton interactions in the regime of extreme quantum confinement, our studies have a direct relevance to the number of emerging applications of NCs in such areas as optical amplification and lasing, nonlinear-optical switching, and photovoltaics. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J16.00002: Ultrafast carrier capture dynamics in laterally ordered InGaAs/GaAs quantum wires F.A. Hegmann, D.G. Cooke, Yu.I. Mazur, W.Q. Ma, X. Wang, Z.M. Wang, G.J. Salamo, M. Xiao, G.D. Lian, J. Kaey, M.B. Johnson Carrier capture into semiconductor nanostructures has been a popular research area in recent years, in part due to efforts to improve the efficiency of nanoscale devices which require carrier capture into an active region after optical or electrical injection. Terahertz (THz) pulse spectroscopy is a powerful tool for investigating this capture process due to its sensitivity to free carriers, sub-picosecond time resolution and non-contact nature. In this talk, we present results of recent time-resolved THz pulse spectroscopy experiments investigating carrier capture into a single layer of strain-induced laterally-ordered InGaAs quantum wires on a [311] GaAs substrate after photoexcitation by 400 nm, 100 fs pump pulses. The temperature and fluence dependence of the carrier capture process is examined both perpendicular and parallel to the wires by using the polarization of the THz probe pulse. The authors acknowledge financial support from NSERC, CIPI, iCORE, NSF, and DMR-0080054 (C-SPIN). [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J16.00003: Polarization and Pumping Intensity Effects on the Energy Transfer Rate in Quantum Dots Ameenah N. Al-Ahmadi, Sergio E. Ulloa We study the dependence of the excitation transfer rate between coupled quantum dots (QDs) on the polarization and the intensity of the exciting light. We use the density matrix to study the dynamics of the luminescence polarization of the QDs [1]. We consider a detailed description of the band edge fine structure of the exciton in the QDs based on an effective mass description with eight exciton levels [2]. The QDs are coupled via the dipole-dipole Forster-like interaction with realistic, experiment-relevant parameters. We investigate the dependence of the luminescence polarization on the polarization of the exciting light and how such measurement give us information about the angular orientation of the excited dipole in the donor dot and the transferred dipole in the acceptor dot. We also study the dynamics of the QD system in the case of multiple-excitons, obtained by increasing the intensity of the exciting light. In this case, the coupling between the QDs includes all the possible interactions between the excitons. The exchange interactions are found to strongly influence the energy transfer rate and affect the resulting polarization. Supported by The Indiana 21$^{st}$ Century Research and Technology Fund. [1] A. N. Al-Ahmadi and S. E. Ulloa, Phys. Rev. B \textbf{70}, 201302(R) (2004). [2] Al. L. Efros and M. Rosen, Annu. Rev. Mater. Sci. \textbf {30}, 475 (2000). Al. L. Efros, phys.rev. B 46, 7448 (1991). [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J16.00004: Spin memory effect in single magnetic quantum dots Tak Gurung, Sebastian Mackowski, Howard E. Jackson, Leigh M. Smith, Jacek Kossut, Grzegorz Karczewski We image zero-field spin polarization of single CdMnTe quantum dots (QDs) using a slit-confocal microscopy and a solid immersion lens. For non-resonant excitation, where excitons randomize their spins before being captured by QDs, both $\sigma \quad ^{+}$ and $\sigma \quad ^{-}$ - polarized PL emission lines are observed. This strongly ($>$25{\%}) polarized emission implies a preferred direction of spin alignment of magnetic impurities in single QDs, and thus a preferred orientation of exciton magnetic polarons (EMPs). Since the single dot emission lines include accumulation of $\sim $ 10$^{4}$ photons this magnetization must persist through many recombination events. Therefore, we conclude that the EMP formed in a QD by a subsequent exciton often follows the spin alignment of the EMP formed through the previous exciton occupation. We attribute the zero-field spin memory effect observed for single CdMnTe QDs to the decay time of the Mn magnetization being significantly longer than the time intervals between consecutive exciton occupations. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J16.00005: Time-Resolved Spectroscopy of Single Excitons Bound to Te Isoelectronic Pairs in ZnSe Andreas Muller, Pablo Bianucci, Carlo Piermarocchi, Marco Fornari, Ivan-Christophe Robin, Regis Andre, Chih-Kang Shih Single Te impurity centers in ZnSe were probed with time-resolved photoluminescence spectroscopy. Resolution-limited peaks with an ultra-low spatial density originate in the recombination of excitons deeply bound to single nearest-neighbor isoelectronic Te pairs. This interpretation is confirmed by ab-initio calculations. The peaks reveal anti-bunched photon emission and a doublet structure polarized along $[110]$ and $[\bar {1}10]$. We analyze the time-resolved PL decay to clarify the role of the dark states in the spin relaxation and radiative recombination of single fine-structure split excitons. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J16.00006: Population oscillations of two orthogonal states in a single quantum dot Q.Q. Wang, A. Muller, H.J. Zhou, M.T. Cheng, Q.K. Xue, P. Bianucci, C.K. Shih We investigated the exciton dynamics in a single self-assembled quantum dot with a V-type three-level structure. Using tailored pulse pairs we generated population oscillations between two orthogonal excitonic states without a direct transition. We found good agreement between measured data and theoretical calculations. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J16.00007: Tunneling control and phase dependent phenomena in a two-level system Nelson Studart, Jose M. Villas-Boas, Sergio Ulloa The search for an ideal qubit has resulted in the study of structures composed of effective two level systems, including double quantum dots, small molecules, and superconducting junctions. The dynamical manipulation of such systems is then of great broad interest. One common approach is to drive the system with an oscillating field, which is known to produce the interesting phenomenon of coherent destruction of tunneling (CDT) [1]. We have reported previously on the degree of localization (or the ability to block the tunneling) in this system, and how it decreases by lowering the frequency of the driving field [2]. In this work we show that this localization is in fact highly dependent on the {\em phase} of the drive, which makes this phenomenon even richer and more interesting, with possible application to quantum information. We report here how to use this effect to control the rotation of the equivalent qubit on the Bloch sphere and how the phase can produce a well- controlled `revival’ of the localization. Supported by the Indiana 21st Century Research and Technology Fund, and FAPESP-Brazil. [1] M. Grifoni and P. H\"{a}nggi, Phys. Rep. \textbf{304}, 229 (1998). [2] J. M. VillasÂ-B\^{o}as, W. Zhang, S. E. Ulloa, P. H. Rivera, and N. Studart, Phys. Rev. B \textbf{66}, 085325 (2002). [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J16.00008: Interdot coupling through common excited state in II-VI self-assembled quantum dots Tuan A. Nguyen, Thang B. Hoang, Sebastian Mackowski, Howard E. Jackson, Leigh M. Smith, Jacek Kossut, Grzegorz Karczewski We study the excitation coupling in CdTe/ZnTe self-assembled quantum dots (QDs) by means of photoluminescence excitation (PLE) imaging. We use a solid immersion lens in combination with slit-confocal microscope and a multi-channel CCD detector to simultaneously resolve the single dot emission energy, excitation energy and position. The PLE spectra feature sets of several different single dot emission lines coupled to identical excitation resonances with energies about 100 meV above the QD emissions. This result is a signature of an excitation coupling through a common excited state between QDs. A very narrow linewidth of these high-energy resonances of 0.5 meV suggests a very efficient relaxation from these excited states. Using spatially resolved PLE imaging spectroscopy we show that these coupled QDs occur within a 500 nm spatial cluster. This suggests that there is a quasi zero-dimensional electronic state which extends across 500 nm and is coupled directly to the cluster of strongly confined CdTe dots. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J16.00009: Optical properties of quantum dot systems Victor Bondarenko, Yang Zhao Interlevel electromagnetic response of different quantum dot (QD) systems is theoretically investigated within the self- consistent field approach. It is shown that the Coulomb coupling must be taken into account for correct description of optical spectra of the systems. Fundamental importance of the problem of the electron self-interaction in QD systems is established. It is shown that the shape of QD can dramatically affect the spectra, in particular, depending on the polarization of incident radiation and number of electrons in the dot. It is found that the effects of the intradot and interdot Coulomb interactions on the response can be analyzed separately. It is established that the approximation of the point dipole-dipole interaction can be used for adequate representation of the dynamic interdot electron-electron interaction in the lattice. Also it is shown that the approach of the modified oscillator strength very well reproduces the absorption spectra of the considered systems with interacting modes of the collective excitation. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J16.00010: Optical spectroscopy of single impurity centers in semiconductors Sebastien Francoeur, J.F. Klem, A. Mascarenhas Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J16.00011: A theoretical study of the structural and electronic properties of CdSe/CdS and CdS/CdSe core/shell nanoparticles Michael Springborg, Pranab Sarkar, Gotthard Seifert We present a theoretical study of the structural and electronic properties of CdSe/CdS and CdS/CdSe core/shell nanoparticles. The results are relevant not only for understanding the properties of these nanoparticles but also for understanding those of quantum dots. We have considered nanoparticles whose structures were obtained as relaxed structures of essentially spherical parts of the zincblende crystal structure and with one semiconductor compound outside the other one. The electronic properties and the total energy for a given structure were calculated using a parameterized density-functional tight-binding method. The results give information on structure, electronic properties, the HOMO and LUMO orbitals, the charge distribution, and the stability of these core/shell nanoparticles. The results depend critically on the size of both core and shell, and only in one single case we find a charge separation upon excitation. We have also investigated the energetics related to the interchange of a S and a Se atoms between the core and the shell. Although the total energy may be lowered upon interchange, the energy barrier for this process is so large that the systems should be stable against degradation. [Preview Abstract] |
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J16.00012: Exciton radiative lifetime and the ``dead-layer'' effect in ZnO quantum dots Vladimir A. Fonoberov, Alexander A. Balandin ZnO quantum dots (QDs) have recently attracted significant attention due to the proposed optoelectronic and nanoelectronic applications. We theoretically investigated the exciton radiative lifetime and the thickness of the ``dead-layer'' (layer where exciton does not penetrate) for ZnO QDs with radii from 1 to 3 nm [1]. Our calculations show that the dead layer formed near the QD surface is rather thick for ZnO QDs, what is attributed to the large hole-electron effective mass ratio in ZnO. The excited exciton states are also investigated as a function of the ZnO QD size. The small radiative lifetime and thick dead layer found in ZnO QDs can be beneficial for device applications owing to better luminescence and isolation of the exciton from surface defects [2]. The obtained results can be used for the optimization of ZnO QD arrays for optoelectronic applications. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] V.A. Fonoberov and A.A. Balandin, Phys. Rev. B 70, 195410 (2004). [2] V.A. Fonoberov and A.A. Balandin, Appl. Phys. Lett. 85, in press (Dec. 20, 2004). [Preview Abstract] |
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