Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V20: Focus Session: Semiconductor Spin Dynamics: Optics |
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Sponsoring Units: GMAG DMP Chair: Dimitri Basov, University of California, San Diego Room: Baltimore Convention Center 317 |
Thursday, March 16, 2006 11:15AM - 11:27AM |
V20.00001: Unusual Nuclear Spin Alignment in GaAs James McGuire, Tom Silva Time-resolved Faraday rotation experiments are used to measure nuclear spin polarization in GaAs due to hyperfine coupling with pumped electron spins. Both bulk and epilayer samples are examined. We find unusual dependence of the induced nuclear field on the cw pump power, the cw pump helicity, and the external magnetic field. Our results are compared with existing theories and experiments of the coupling of the electron and nuclear spin systems. [Preview Abstract] |
Thursday, March 16, 2006 11:27AM - 11:39AM |
V20.00002: $^{69}$Ga Knight shift in semi-insulating GaAs observed by optically polarized NMR Kannan Ramaswamy, Stacy Mui, Sophia Hayes Optical orientation in semiconductors continues to capture the attention of many researchers all over the world$^{1,2}$. This is especially due to the emergence of a new area called ``spintronics'' where it is proposed that the spin of the electrons instead of its charge will be manipulated to create various devices. We report $^{69}$Ga optically polarized NMR investigations in semi-insulating GaAs at 6K. In our investigations, we have observed $^{69}$Ga Knight shifts for photon energies above and below the band gap. This observation is important in understanding the mechanism of NMR signal enhancement in semiconductors by polarized light. Our investigations also indicate that nuclear spin-diffusion plays a role in the NMR signal intensity for illumination times of the order of 100s and above. Furthermore, an estimate of $\sim $0.067 has been obtained for the average electron spin polarization which is 26{\%} of the maximum achievable polarization in bulk GaAs. The cause for the reduction is under investigation. 1. Meier and B. P. Zakharchenya , Modern Problems in Condensed Matter Sciences, Vol 8, F. (1984) \begin{enumerate} \item D.D.Awschalom, D.Loss and N.Samarth, Semiconductor Spintronics and Quantum Computation, (2002) \end{enumerate} [Preview Abstract] |
Thursday, March 16, 2006 11:39AM - 11:51AM |
V20.00003: Electrically induced changes to optical nuclear polarizations in bulk GaAs Patrick Coles, Anant Paravastu, Jeffrey Reimer The semiconductor crystal contains a spin refrigeration system, which turns on in the presence of near-band-gap laser light. Simultaneous application of a DC electric field to bulk, semi-insulating GaAs was observed to significantly alter the nuclear polarization process. At certain photon energies, the nuclear polarization inverted. Changes to nuclear polarization occurred upon the onset of nonlinear photoconductivity. Polarization in this regime could originate from electron spins that are perturbed first by the light and subsequently by relaxation with their (hot) momentum reservoir, while rapid spin-exchange equilibrates the electron spin reservoirs. The results suggest that a pathway towards localized optical and electric field control of nuclear spin may exist within bulk semiconductors. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:27PM |
V20.00004: Bias-dependent spin lifetimes in quantum wells Invited Speaker: The prospect of novel high-performance spin-based semiconductor technologies has lead to new research in spintronics, in which the fields of electronics, photonics, and magnetics merge with the promise of applications in ultra-low-power logic architectures, non-volatile reprogrammable gate arrays, and optoelectronic technologies. Innovation in these areas requires the development of efficient methods for spin manipulation in semiconductor materials. Spintronic device architectures that do not require external magnetic fields or magnetic contacts are especially attractive as they would provide seamless integration with the materials and processing techniques of existing semiconductor devices, while avoiding undesirable stray magnetic fields that may hinder device performance. InAs is an excellent candidate for non-magnetic spintronic device applications due to its strong spin-orbit effects, which lead to gate-controllable pseudomagnetic fields in excess of 1 Tesla.[1] We report the demonstration of room temperature gate control over the electron spin dynamics using the Rashba effect in a (110) InAs/AlSb two-dimensional electron gas. Using the large pseudomagnetic fields in this system, we demonstrate spin manipulation on a picosecond time scale with a low threshold voltage.[1] Our findings are promising for the prospect of nonmagnetic low-power, high-speed spintronics.[2] This research is supported by DARPA MDA972-01-C-0002, DARPA/ARO DAAD19-01-1-0490, NSF ECS 03-22021, and NSERC. [1] K.C. Hall et al., Appl. Phys. Lett. 86, 202114 (2005). [2] K.C. Hall et al., Appl. Phys. Lett. 83, 2937 (2003). [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V20.00005: Electron dephasing and decoherence of neutral donor bound electrons in GaAs Kai-Mei Fu, Susan Clark, Charles Santori, Bingyang Zhang, Colin Stanley, M.C. Holland, Yoshihisa Yamamoto Strong oscillator strengths, small inhomogeneous broadenings of the optical transitions, and semiconductor device integration possibilities make the GaAs donor-bound exciton (D0X) system an attractive candidate for electromagnetically induced transparency based applications. However, the recent observation of coherent population trapping in the GaAs D0X system indicates a fast (1-2 ns) dephasing rate of the bound- electron spin states which severely limits the achievable transparency. Theoretical and experimental research in other groups indicate the fast dephasing is due to the random nuclear spin environment in the GaAs lattice. We perform measurements of the electron Zeeman Raman transition linewidth which confirm the 1-2 ns dephasing rate. Using this technique, we study the effect of doping density and magnetic field on the Raman linewidth. Both variables can theoretically affect the inhomogeneous broadening due to the nuclear spin environment. If the inhomogeneous broadening can be decreased, spin-echo techniques should be possible to further increase the spin dephasing time to the homogeneous microsecond regime. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V20.00006: Temperature and doping dependence of spin-flip times in n-GaAs John Colton, Lee Wienkes, Michael Heeb Previously-reported time resolved photoluminescence experiments [1] in lightly-doped n-GaAs have been extended to provide additional measurements of T$_{1}$ spin-flip times as a function of temperature and of doping density. The samples studied were MBE-grown 1 micron thick layers of doping densities from 3E14 cm$^{-3}$ to 3E15 cm$^{-3}$. The technique was to use a pump pulse to inject spin polarized electrons and a probe pulse to read out the polarization at some later time; spin flips caused the polarization to decrease exponentially with pump-probe delay. Some measured T$_{1}$ spin-flip times (at low temperature, at low doping densities) were even longer than the previously-reported 1 microsecond value. Work supported by NSF, ACS/PRF, and Research Corporation. [1] J.S. Colton et al., Phys Rev B 69, 121307(R) (2004). [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V20.00007: Long-lived precession of spin gratings in $n$-doped GaAs Sam Carter, Zhigang Chen, Steven Cundiff Transient grating experiments have been performed in lightly $n$-doped GaAs to measure population and spin dynamics. In the presence of a magnetic field perpendicular to the optical axis (Voigt geometry), the spin grating precesses, leading to a diffracted probe signal that oscillates at twice the precession frequency. In contrast to previous experiments in undoped [1] and heavily doped [2] samples, the spin gratings last up to $\sim $1 ns at low temperatures, much longer than the lifetime of the photo-excited carriers. These results indicate that a spin grating is formed in the itinerant electrons, which decays due to spin relaxation and diffusion. By measuring changes in the grating decay rate with the grating period, the spin and population diffusion rates have been determined. Attempts to generate a spin grating without generating photo-excited carriers using Raman excitation will also be discussed. [1] A. R. Cameron, P. Riblet, and A. Miller, \textit{Phys. Rev. Lett}. \textbf{76}, 4793 (1996). [2] C. P. Weber, N. Gedik, J. E. Moore, J. Orenstein, J. Stephens, and D. D. Awschalom, \textit{Nature} \textbf{437}, 1330 (2005). [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V20.00008: Spin dynamics in a GaAs quantum well with optically controlled charge density: localization vs. delocalization Zhigang Chen, Sam Carter, Rudolf Bratschitsch, Steven Cundiff, Phil Dawson We have studied electron spin dynamics in a mixed type I/type II quantum well (QW) structure, which consists of narrow and wide GaAs QWs separated by AlAs barriers. The electron (hole) densities in the wide (narrow) QW can be varied continuously over a wide range by low power photoexcitation at an energy above the narrow QW bandgap. We have performed time-resolved Kerr rotation measurements in the Voigt geometry to probe the electron spin dynamics in the wide QW. We measure two $g$-factors for electron spin precession, which we ascribe to the localized and delocalized electrons in the wide QW. Photoexcited holes, localized by the well-width fluctuations in the narrow QW, bind and localize the electrons in the wide QW. A study of the spectral dependence shows strong inhomogeneity of the localized electron $g$-factor, while the delocalized electrons have a constant $g$-factor. With increasing carrier density, the precession amplitude of the delocalized (localized) electrons increases (decreases) until only the delocalized electron precession can be observed at high density. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V20.00009: Coupling single bright spins with channels of intermediate dark spins in diamond Felix M. Mendoza, Ronald Hanson, Ryan J. Epstein, David D. Awschalom The nitrogen-vacancy (N-V) center in diamond has garnered interest as a room-temperature solid-state system not only for exploring electronic and nuclear spin phenomena but also as a candidate for spin-based quantum information processing. Recent experiments reveal the coupling of a single bright electron spin of an N-V center to small numbers of dark electron spins of nitrogen defects in its immediate vicinity, not otherwise detected in luminescence \footnote{R.J. Epstein, F.M. Mendoza, Y.K. Kato and D.D. Awschalom, \textit{Nature Physics} \textbf{1}, 94 (2005)}. We explore the possibility of utilizing this magnetic dipole coupling between bright and dark spins to couple two spatially separated single N-V center spins by means of intermediate dark nitrogen spins. The angle-resolved magneto-photoluminescence microscopy technique is extended to simultaneously detect a pair of single N-V centers a few microns apart. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V20.00010: Anisotropic Interactions of a Single Spin and Dark-Spin Spectroscopy in Diamond R.J. Epstein, F.M. Mendoza, Y.K. Kato, D.D. Awschalom Anisotropic spin interactions of single nitrogen-vacancy color centers in diamond are investigated at room temperature using angle-resolved magneto-photoluminescence microscopy.\footnote{R. J. Epstein \emph{et al.}, Nature Physics \textbf{1}, 94 (2005); cond-mat/0507706.} Negative peaks in the photoluminescence intensity are observed as a function of both magnetic field magnitude and angle, and are modeled by coherent spin precession and anisotropic relaxation at spin-level anti-crossings. In addition, precise field alignment reveals the resonant magnetic dipolar coupling to nearby ‘dark’ nitrogen spins, otherwise undetected by photoluminescence. The results present an avenue for transferring spin information between bright spins through the intermediate dark spins. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V20.00011: External strain engineering of the optical response of (Al,Ga)As/GaAs microdisk lasers X. Li, M. H. Mikkelsen, S. Ghosh, D. D. Awschalom, N. Samarth Recent studies show that semiconductor microcavities provide unexpected ways of controlling electron spin coherence via light-matter interactions in confined geometries [S. Ghosh et al., cond-mat/ 0509500], suggesting new routes towards spin-based quantum information processing. Here, we use optical spectroscopy to study the effects of external strain on lattice-matched (Al,Ga)As/GaAs microdisk lasers. We demonstrate that the encapsulation of such lasers with SiN$_x$ allows systematic strain engineering of steady state optical characteristics, where measurements of spontaneous emission show that GaAs quantum wells in the active region of the microdisks experience strain from both the SiN$_x$ and the free standing disk shape, and measurements of stimulated emission show that the laser threshold decreases with the magnitude of net compressive strain. Further, we also explore related modifications in the electron spin coherence time. Work supported by DARPA/QUIST and NSF. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V20.00012: Polarized luminescence in silicon Frederic Roux, Georges Lampel, Yves Lassailly, Jacques Peretti Although silicon is a widely used semiconductor in the microelectronic industry, few studies habe been performed so far about its spin-transport properties. Creating and manipulating both spin and charge of electrons in silicon could be very promising in the research of convenient spintronic devices. One way to create and detect spin orientation of the conduction electrons is to perform a photoluminescence experiment under optical pumping conditions. We will present here a stationary photoluminescence experiment. Silicon samples of various doping concentrations, at nitrogen or room temperature, are optically pumped by a Ti:saphir laser. In silicon, the spin relaxation time is much shorter than the carrier life-time, thus yielding to a weak polarization of the recombination radiation. Polarization spectra under either modulated excitation or modulated reception have shown spectral regions where the degree of polarization of the luminescence can be as high as 5 percents. The possibility of a depolarization effect by a transverse magnetic field will be discussed (Hanle effect). [Preview Abstract] |
Thursday, March 16, 2006 2:03PM - 2:15PM |
V20.00013: An Interferometric Approach to Time Resolved Faraday Rotation Measurements J.M. LaForge, G.M. Steeves Time Resolved Faraday Rotation (TRFR) is an optical pump-probe technique used to detect electron spin precession about an external magnetic field in semiconductors. In TRFR a net electron magnetic moment can induce small rotations in the polarization of the probe beam, which are detected through a polarizing cube beam splitter and two balanced photodiodes. We present a new approach that divides the probe beam into two orthogonally polarized arms of a free-space Mach-Zender interferometer where one arm contains the sample being studied. Recombination of the probe beams optically amplifies the Faraday Rotation (FR) signal. There are two benefits to this approach; traditional samples can be probed at lower intensities while maintaining signal quality, and weak FR signals, such as those found in the spintronic studies of quantum dot materials, can be optically amplified before detection. Vibrational isolation and thermal stability are important operational factors since the optical gain can be affected by optical path length difference between the two arms of the interferometer. [Preview Abstract] |
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