Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X25: Quantum Structures |
Hide Abstracts |
Sponsoring Units: DCMP FIAP Chair: Gerard Martinez, LNCMI, CNRS, Grenoble Room: D135 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X25.00001: Electron-Phonon Interactions in a single modulation doped Ga$_{0.24}$In$_{0.76}$As/InP Quantum Well Gerard Martinez, Milan Orlita, Clement Faugeras, Sergei Studenikin, Philip Poole, Geof Aers A series of Ga$_{0.24}$In$_{0.76}$As/InP modulation doped single quantum well (QW) structures have been investigated using cyclotron resonance experiments. The far-infrared magneto-transmission experiments are analyzed with a multidielectric model allowing the extraction of the imaginary part of the response function, revealing strong interactions with the different phonon energies of the mixed compound. For carrier densities n$_{S}$ higher than about 3.4x10$^{11}$ cm$^{-2 }$the only observed interaction is with the TO modes of the system whereas for lower densities, in addition to this interaction, a clear polaronic interaction with the LO phonons develops and increases as n$_{S}$ decreases. Due to the specific dielectric character of this compound these different types of interaction can be clearly indentified and even quantified. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X25.00002: Extraction of many-body excitation configurations from nonlinear absorption in semiconductor quantum wells Ryan Smith, Andrew Funk, Jared Wahlstrand, Richard Mirin, Steven Cundiff, Johannes Steiner, Martin Schafer, Mack Kira, Stephan Koch We extract detailed electronic many-body configurations by analyzing quantitatively measured, time-resolved nonlinear absorption spectra of resonantly excited GaAs quantum wells with our fully consistent microscopic theory. Nonlinear spectral changes observed in the probe absorption are attributed by our theory to a unique mixture of the effects of electron-hole plasma, exciton populations, and polarization. Studies of these effects include quantitative comparison of co-linear- and co-circular-polarization pump-probe excitation schemes that reveal the consequences of spin-selection rules on scattering. For co-circular excitation conditions, we observe strong transient gain. We attribute this gain to the transfer of pump-induced coherences to the probe. Unexpectedly, we also find that true exciton populations do not significantly contribute to spectral broadening or shifting; rather, the nonlinear modifications are dominated by the excited carrier densities. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X25.00003: Absence of Excitonic Mott Transition in InGaAs Quantum Wells in High Magnetic Fields G.T. Noe, J. Kono, J. Lee, D.H. Reitze, C.J. Stanton, A.A. Belyanin, G.S. Solomon Optically produced electron-hole pairs (excitons) provide a rich system to study carrier interaction in a highly controllable environment. Excitons are only stable in the dilute limit when the Bohr radius is much smaller than the interexciton distance. As the density of excitons increases, the Mott transition is expected to transform the insulating excitonic gas into a metallic electron-hole plasma. Here, we present emission and absorption properties of undoped In$_{0.2}$Ga$_{0.8}$As quantum wells at low temperatures and in high magnetic fields for both the high and low density regimes. When a magnetic field is applied, each ns-state of a 2D exciton (n = 1,2, {\ldots}) experiences the diamagnetic shift and approaches the Nth Landau level (N = n-1) with increasing magnetic field. In the dilute limit, single-exciton states are clearly distinguishable in the PL spectra. In contrast, for high exciton density, all ns-states converge to a common value at zero field, which indicates complete screening of the exciton binding energy. At the same time, the dependence of the lowest Landau level as a function of field still shows a clear excitonic 1s-state character at all fields. These results indicate that there is no Mott transition to the metallic state in the high density regime when the magnetic field is applied. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X25.00004: Transient optical response of quantum well excitons to intense narrowband terahertz pulses Andrew Jameson, Joseph Tomaino, Yun-Shik Lee We demonstrate experimental observations and theoretical calculations of strong interactions between THz light and the 1$s$-to-2$p $transition of the excitonic polarization in resonantly driven GaAs/AlGaAs QWs, using our novel tabletop THz source of high intensity and continuous tunability. The strong narrowband THz pulses are used to modify excitonic transitions in the semiconductor QWs and to study the dephasing properties of the optically dark 2$p $states. Time-resolved THz-pump and optical-probe measurements exhibit strong nonlinear optical transients of the 1$s $heavy-hole and light-hole exciton resonances when the THz radiation is tuned near the 1$s $to 2$p $intraexciton transition. A microscopic theory attributes the observed nonlinearities to Rabi sidebands showing that the 2$p$-dephasing time is three times that of the 1$s$-state. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X25.00005: Coherent Phonons Spectroscopy in Si/SiGe superlattices Helene Michel, Younes Ezzahri, Ali Shakouri, Gilles Pernot, Jean-Michel Rampnoux, Stefan Dilhaire Ultrafast pump-probe experiments have been extensively used for coherent zone-folded acoustic phonon spectroscopy in semiconductor superlattices (SL). Most of the spectroscopy studies have been realized via impulsive stimulated Raman scattering (ISRS). More recently some studies, focused on Si/Si$_{x}$Ge$_{1-x }$SL, have combined the spectroscopy via ISRS with the spectroscopy of phonons Bragg reflected via picosecond acoustic experiment. In the latter case, sample needs to be covered by a metallic film which serves as a transducer to convert the optical energy into an impulse heating and thermal expansion. This launches coherent acoustic phonons into the SL structure. Here we present a systematic study of coherent phonons in different Si/Si$_{x}$Ge$_{1-x }$SL structures with two different superlattice periods and transducer thicknesses. The measured acoustic spectrums show that the thickness of the transducer should be chosen as function as the SL period to be able to generate and detect both phonons Bragg reflected and excited by ISRS. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X25.00006: Experimental Determination of Space-Charge Electric Field in Photorefractive Multiple Quantum Wells Xiangxue Zhang, Mohammad Samiullah, Ping Yu We present a method for deducing the space-charge electric field in a photorefractive multiple quantum wells (PRQW) illuminated by two coherent beams under drift-current dominant conditions where the phase difference between the space-charge electric field and light intensity is predicted to be $\pi $ radians. Firstly, we determine a relation J$_{0}$(E$_{0}$,I$_{0})$ for the device between the total current J$_{0}$, constant applied electric field E$_{0}$ and uniform intensity I$_{0}$. When the intensity inside the PRQW varies sinusoidally, substituting current for J$_{0}$ and maximum intensity for I$_{0}$ in J$_{0}$(E$_{0}$,I$_{0})$ yields E$_{0}$, which corresponds to the minimum of the net electric field under the assumption that intensity is uniform near maxima and minima. A similar procedure is used to deduce the maximum value of the net electric field inside PRQW. Subtracting the applied electric field from the net electric field yields the space-charge field. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X25.00007: Phase-coherent multi-orbital bands of microcavity exciton-polaritons in a lattice Na Young Kim, Kenichiro Kusudo, Congjun Wu, Naoyuki Masumoto, Sven Hoefling, Alfred Forchel, Yoshihisa Yamamoto Microcavity exciton-polaritons are quantum bose particles arising from the strong light-matter coupling between cavity photons and quantum well excitons. They have been attractive entities to explore fundamental quantum nature phenomena in solid-state systems including Bose-Einstein condensation and superfluidity. Here we investigate phase-coherent multi-orbital bands of microcavity exciton-polaritons which are trapped in an artificial periodic lattice potential. Optical properties of the system are studied via photoluminescence imaging and spectroscopy at low temperatures. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X25.00008: Intersubband absorption by electrons in InSb quantum wells with an in-plane magnetic field M.B. Santos, S.D. Lowe, T.D. Mishima, R.E. Doezema, L.C. Tung, Y.-J. Wang We performed magneto-transmission experiments at 4.2K on $n$-type InSb multiple quantum wells (MQWs) in the Voigt geometry. The direction normal to the plane of the MQWs was perpendicular to the direction of the applied magnetic field and parallel to the propagation direction for the incident radiation. Electrons at densities of 1 to 8$\times $10$^{11}$cm$^{-2}$ were confined to InSb QWs (10 to 30 nm thick) by Al$_{0.09}$In$_{0.91}$Sb barrier layers. Intersubband transitions within the conduction band were observed at magnetic fields up to B=17T and photon energies up to 100 meV. A single intersubband transition observed at low B ($<$7T) branches into two well-resolved transitions at high B. The in-plane magnetic field is expected to mediate the spin-conserving transitions, whereas the bulk inversion asymmetry is expected to induce spin-flip excitations. A depolarization shift is expected for the spin-conserving transitions. The large g-factor (-51 in bulk InSb) and small effective mass (0.014 m$_{e}$ in bulk InSb) for electrons in InSb QWs enables the study of both spin-conserving and spin-flip intersubband transitions. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X25.00009: Magneto-absorption in Narrow Gap InSb/AlInSb Parabolic Quantum Wells D. Saha, G. D. Sanders, C. J. Stanton, T. Kasturiarachchi, W. Gempel, M. Edirisooriya, T. D. Mishima, R. E. Doezema, M. B. Santos Because of its narrow gap, InSb has considerable promise as a quantum well material because its small conduction-band mass gives it the highest room temperature electron mobility among the III-V materials. We present experiments and calculations for the magneto-absorption spectra in a strained, narrow gap InSb/AlInSb parabolic quantum well. Our calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the parabolic confinement potential as well as pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with experiments. The magneto-optical absorption spectrum is calculated for magnetic fields from 1 to 8 T for x-linear, e-active and h-active polarizations. Comparison to experiment allows one to accurately determine the quantum confined, spin-split conduction and valence band energies. Results show a sensitive dependence on the strain at the pseudomorphic interfaces. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X25.00010: Quantum well subbands in high conduction band offset heterostructures Yong-Hee Cho, Alexey Belyanin High conduction band offset heterostructures are becoming the material of choice in quantum cascade lasers and other intersubband devices operating in the short-wavelength range below $\sim $ 3 $\mu $m. To design these devices one needs to determine the position of highly excited subbands and lateral valleys located at energies comparable to band gap above the bottom of the conduction band in the quantum wells (QWs). In this case standard methods of band structure calculation based on the one band model with the $\Gamma $ edge effective mass and non-parabolicity correction or on simplified k.p models become inadequate. We calculate the confinement energies in the conduction band of high band offset QWs, such as InGaAs/AlAsSb or InAs/AlSb, using 30-band k.p band structures over the entire first Brillouin zone. We discuss the effect of lowest lateral valleys of the conduction band on the QW confinement energies. We compare the results of the rigorous treatment with that obtained using various versions of the effective mass approximation with the energy-dependent electron mass. Implications for the device design and performance are discussed. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X25.00011: Interband Relaxation Dynamics of Photo-Excited Carriers in InSb Quantum Wells Giti A. Khodaparast, M. Bhowmick, R.N. Kini, K. Nontapot, M. Frazier, T.D. Mishima, M.B. Santos As the switching rates in electronic and optoelectronic devices are pushed to higher frequencies, it is important to understand carrier dynamic phenomena in semiconductors on femtosecond time- scales. Here we present carrier dynamics, using several pump/probe schemes, in doped and undoped InSb quantum wells (QWs) with Al$_{x}$In$_{1-x}$Sb barrier layers. In one scheme, the carriers were created by NIR pulses fixed at 800 nm, above the band gap of Al$_{x}$In$_{1-x}$Sb and InSb, and probed by MIR pulses, tuned in the vicinity of several interband transitions. We observed that the carriers were captured in the QWs in a time scale of $\sim$ 800 fs and not fully relaxed in a time scale longer than 20 ps. Electrons that are sufficiently energetic have the possibility to scatter between the X, L, and $\Gamma$ valleys in the conduction band, resulting in more complex dynamics. We probed the influence of the initial distribution function by employing a degenerate pump/probe scheme close to several interband transitions. We will discuss several mechanisms describing the observed dynamics, important for developing long-wavelength optoelectronic devices. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X25.00012: Optical properties and band structure of atomically thin MoS2 Jie Shan, Kin Fai Mak, Changgu Lee, James Hone, Tony Heinz Atomically thin layers of materials can be expected to exhibit distinct electronic structure and novel properties compared to their bulk counterparts. Layered compounds, for which stable atomically thin samples can be produced, are ideal candidates for such studies. Graphene, a monolayer slice of the graphite crystal, is an illustrative example of both the stability and of the interest and importance of such materials. Here we report a study of thin layers of MoS$_{2}$, a hexagonal layered bulk semiconductor with an indirect band gap of 1.3 eV. MoS$_{2}$ samples with layer thickness N down to a monolayer were obtained by mechanical exfoliation. We observed an enhancement of the luminescence quantum yield by more than a factor of 100 in monolayer MoS$_{2}$ compared to the bulk material. The combination of absorption, photoluminescence, and photoconductivity measurements indicates that a transition to a direct-gap material occurs in the limit of the single MoS$_{2}$ layer. This result is supported by an earlier first-principles calculation [J. Phys. Chem. C \textbf{2007}, 111, 16192]. Further, by varying the thickness of the samples, we were able to probe the evolution of the electronic structure for N = 1 -- 6 layers. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X25.00013: Unusual Photoluminescence in Ultrathin MoS$_{2}$ Liang Sun, Andrea Splendiani, Yuanbo Zhang, Tianshu Li, Jonghwan Kim, Chi-Yung Chim, Giulia Galli, Feng Wang In this talk we will report optical studies on ultrathin MoS$_{2}$ layers through optical reflection, Raman scattering, and photoluminescence spectroscopy. Bulk MoS$_{2}$, a layered transition metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. Surprisingly, when the thickness of MoS$_{2}$ is reduced to a few unit cell thickness, a strong photoluminescence emerges. Further this photoluminescence increases with reduced MoS$_{2}$ layer thickness, although available materials amount is reduced. We will discuss possible mechanism that can give rise to this surprising photoluminescence behavior in MoS$_{2}$. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X25.00014: Calculations the Dresselhaus effect of the electron spin relaxation momentum time in a GaAs cylindrical quantum dots: Including ionic strain Yung-Sheng Huang The electron spin relaxation momentum time (SRT) of a GaAs quantum dots varying with some physical parameters is presented. We propose the model of the GaAs cylindrical quantum dots which is embedded in AlGaAs bulk material. We deal with the polar coordinate ionic strain displacement field (IDF) and piezoelectric effect to calculate Dresselhaus effect of the electron SRT. The scattering between electron and acoustic models via the piezoelectric interaction. Since the system is under very low temperatures and high magnetic fields, the assumptions of equipartition are failed. The fact that piezoelectric scattering is important for low energies makes a parabolic approximation a good one. Besides, we assume that GaAs material is a very pure semiconductor, so the neutrial-impurity scattering is ignored. One can find that SRT decreases while the four parameters: external magnetic field, surrounding temperatures, both quantum wire width and thickness increase. The reason is that more and more phonons result in a higher scattering probability between electrons and phonons. The theoretical results used for comparing with experiments. [Preview Abstract] |
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