Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D11: Optical Properties of Dots, Wires and Superlattices |
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Sponsoring Units: DCMP Chair: Michael Scheibner, Naval Research Laboratory Room: 305 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D11.00001: Robust Ferromagnetism in type-II (ZnMn)Te Quantum Dots I.R. Sellers, V.R. Whiteside, M. Eginligil, R. Oszwaldowski, I. Zutic, A. Petrou, B.D. McCombe, W.C. Chou Temperature dependant magneto-photoluminescence studies of type-II diluted magnetic semiconductor (ZnMn)Te/ZnSe quantum dots (QDs) will be presented. As expected, the exchange interaction between the Mn spins and charge carriers results in a strong optical polarization of the luminescence at low temperature in a magnetic field. In addition, however, a \underline {zero magnetic field} optical polarization degree of 7{\%} is observed in the photoluminescence (PL). This polarization is shown to be independent of temperature until 180 K, and is only quenched by the loss of PL intensity as the type-II QDs are ionized. In this submission, we will present continuous wave and temporal PL measurements which indicate that the finite polarization, at zero magnetic field results from the formation of exciton magneto polarons (EMPs). Furthermore, we will show that these EMPs are remarkably robust with binding energies in excess of 40 meV, far larger than any previously studied EMP system. The origin of this behavior will be discussed. Work supported in part by CSEQuIN and the Office of the Provost at the U. Buffalo. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D11.00002: Low threshold amplified spontaneous emission and Auger recombination suppression in giant nanocrystal quantum dots Istvan Robel, Florencio Garcia-Santamaria, Richard D. Schaller, Yongfen Chen, Jennifer A. Hollingsworth, Victor I. Klimov Nanocrystals quantum dots (NQDs) are attractive materials for various light-emitting applications including optical amplification and lasing. A complication associated with the multiexcitonic nature of light amplification is NQDs is the picosecond optical-gain decay induced by nonradiative Auger recombination, in which one exciton recombines by transferring the energy to the other. Here, we present new results on a novel type of nanocrystals dubbed ``giant'' quantum dots (g-NQDs). These g-NQDs comprise an emitting core particle of CdSe overcoated with a thick shell (up to 20 monolayers) of wider-gap CdS. We report that biexciton and gain lifetimes are greatly augmented and the ASE threshold drops down to just a few $\mu $J/cm2. We explain this result by a significant increase in the absorption cross-section of g-NQDs compared to traditional nanocrystals and lengthening of biexciton lifetimes. We also observe other unusual optical-gain behaviors for these structures such as multi-band ASE, the spectra spectral range of optical amplification extends over more than 500 meV. These results demonstrate that g-NQDs are very promising materials for applications in practical lasing technologies. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D11.00003: Investigation of group-III-nitride semiconductor nanostructures using an eight-band $\mathbf{k}\cdot\mathbf{p}$ formalism Oliver Marquardt, Tilmann Hickel, Joerg Neugebauer Despite its approximative nature, the $\mathbf{k}\cdot\mathbf{p}$-formalism provides a numerically efficient and accurate description of the electronic structure of group-III nitride semiconductor nanostructures with characteristic dimensions of few nanometers. [1] With the computational effort of this method being independent of the system size, it is possible to study an extensive set of zero-, one- and two-dimensional semiconductor nanostructures. We applied a plane-wave implementation of the 8-band $\mathbf{k}\cdot\mathbf{p}$ formalism and second-order continuum elasticity theory to various III-nitride nanostructures such as InGaN/GaN or GaN/AlN quantum dots in the characteristic wurtzite and zincblende crystal structures. We investigated the effect of strain and polarization effects on the charge carrier localization which typically leads to a spatial separation of electrons and holes in wurtzite nanostructures. Additionally, studies have been performed in order to evaluate trends when varying the alloy composition in InGaN/GaN nanostructures in order to understand light emission processes in realistic nanostructures.\\ \\ 1: Marquardt, Mourad, Schulz, Hickel, Czycholl, Neugebauer, Phys. Rev. B, in print (2008) [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D11.00004: Infrared Photodetectors based on PbSe and PbS Nanoparticles. Don-Hyung Ha, Christopher Murray There is growing interest in developing nanoparticles for photodetectors, due to the potential to selectively tune the wavelengths detected by varying the size of the nanoparticles. Also, by virtue of the solution processibility of nanoparticles, photodetectors based on nanoparticles provide a low cost, easily processed opportunity for photodetection on flexible substrates. Especially for the near-infrared (NIR) region, PbS and PbSe nanoparticles are ideal candidates since their absorption windows fall between 900-1500nm and 1400-2500nm, respectively, covering a wide range of the IR region. This presentation reports the synthesis, structural characterization, and photoconductivity of colloidal PbSe and PbS nanoparticles. Photocurrent and normalized detectivity are measured from the nanoparticle photodiode under the illumination of NIR light (800-2500 nm) and under dark conditions. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D11.00005: Single photon to single electron conversion using a quantum dot Haruki Kiyama, Takafumi Fujita, Tetsuya Asayama, Akira Oiwa, Seigo Tarucha Photons, and electron spins are leading candidates for implementing qubits useful in information transmission, and computing, respectively. Therefore, quantum media conversion (QMD) between them is a key technology for a comprehensive quantum network. In this work, as a first step toward QMD, we demonstrate single photon to single electron (charge) conversion using a GaAs based lateral QD equipped with a quantum point contact (QPC) as a charge sensor. A distinctive step, which is quite similar to those observed for single electron tunneling onto the QD from the leads, is observed in the single-shot time evolution of the QPC current immediately after the incidence of single photons. From detailed measurements of the light intensity dependence and the QD-lead tunnel rate dependence we confirm that the observed steps are due to single electron generation in the QD just after single photon irradiation. The minimum time resolution of this single photon to single electron conversion is 50 $\mu $s. This is short enough to demonstrate the angular momentum transfer between photon polarization and electron spin in the QD as the next step. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D11.00006: Scalable Single Photon Detector for Terahertz and Infrared Applications. Bernard Matis, Dong Ho Wu Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) open up an exciting opportunity for the development of nanostructure devices. Of the various devices, our attention is drawn in particular to detectors, which can respond to a single photon over a broad frequency spectrum, namely, microwave to infrared (IR) frequencies. Here we report on transport measurements of parallel quantum dots, fabricated on a GaAs/AlGaAs 2-dimensional electron gas (2DEG) substrate, under the influence of external fields associated with 110GHz and 1 THz signals. We further investigate the scalability of our detector in addition to its temperature dependence up to 4.2K. We will discuss experimental results, and their dependence on quantum dot size, and fabrication techniques, as well as the limitations in developing a QD photon detector for microwave and IR frequencies, whose noise equivalent power can be as high as 10$^{-22}$ W/Hz$^{1/2}$. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D11.00007: Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystallite quantum dots Farbod Shafiei, Ricardo S. Decca An apparatus has been built to study the separation dependence of the interaction between small and large resonant groups of CdSe/ZnS nanocrystallite quantum dots (NQDs). A near-field scanning optical microscope (NSOM) is used to bring a group of mono-disperse 6 nm dots close (near-field range) to an 8 nm group of dots which are deposited on a solid immersion lens. Combination of spectral and positional filtering allows us to measure the interaction between small numbers of NQDs, with the ultimate goal of identifying the interaction between individual dots. Quenching of the small NQDs photoluminescence signal yields the transition probability between these two groups of NQDs which is obtained to be $(4.5\times10^{-47}$ m$^6$)/R$^6$, matching the theoretical calculation. F\"{o}rster radius as a signature of energy transfer efficiency is extracted from experimental data to be 17 nm. Separation between two groups of the NQDs was increasing up to 40nm during the experiment. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D11.00008: Resonant creation of positive trion in coupled InAs/GaAs quantum dots Swati Ramanathan, Kushal C. Wijesundara, Mauricio Garrido, Eric A. Stinaff, A. S. Bracker, D. Gammon Recent photoluminescence excitation (PLE) experiments have revealed the unexpected resonant creation of a positive trion in a coupled InAs/GaAs quantum dot system. Positive trion creation is a two photon process requiring the second photon to have a different energy from the first due to the presence of the photogenerated hole. This leads us to conclude that the positive trion may be created through two indirect absorptions, along with two tunneling events. To verify this scenario, experiments using circularly polarized excitation should result in hole spin states with either spin -3/2 or +3/2. This should lead to Pauli blocking of spins, resulting in a reduced intensity of positive trion emission under excitation with circularly polarized light when compared to unpolarized light. High resolution PLE will also provide additional insight into the details of this mechanism. It may be possible to use resonant excitation processes such as this to create defined hole spin states in coupled quantum dot systems. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D11.00009: Exchange interactions in coupled quantum dots observed through polarized photoluminescence Kushal C. Wijesundara, Mauricio Garrido, Swati Ramanathan, Eric Stinaff, Allan Bracker, Dan Gammon Identification and manipulation of the exchange interaction between different spin configurations may be useful for implementing quantum logic operations. Coupled quantum dots offer the possibility of controlling the exchange interaction by continuously tuning between direct and indirect excitonic configurations. The effect of the anisotropic part of the exchange energy was clearly identified from polarization dependent photoluminescence (PL) results arising from the direct and indirect configurations of the neutral exciton. We also observe direct experimental evidence of the isotropic exchange interaction via PL measurements from positive trion configurations. The isotropic exchange interactions observed to be an order of magnitude larger than the anisotropic splitting as expected. High resolution measurements of this charged exciton configurations are expected to reveal additional insight into the details of the exchange interaction. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D11.00010: Hybrid Exciton in a Semiconductor Nanorod Coated By an Organic Shell Daniel Velazquez, Huong Nguyen We study the Wannier Mott-Frenkel hybrid exciton in a nanorod coated by a thin organic shell. Using the wavefunctions of the 1D Wannier-Mott and the Frenkel exciton, we obtain the wavefunctions and energy of the hybridization state. The new exciton state is a linear combination of the basic exciton states and is smoothly distributed over the whole system. The hybridization depends strongly on the coupling (hybridization) parameter as well as the shape of the nanorod and the thickness of the organic layer. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D11.00011: Stimulated Raman Scattering from Short GaP Nanowires Jian Wu, Awnish Gupta, Peter Eklund We report an interesting discovery of very strong non-linear optical behavior in short GaP nanowire segments. They were formed by cutting a $\sim $40 $\mu $m long and 210 nm diameter GaP nanowire into various lengths using a focused ion beam. This approach allows us to study length as the variable in the non-linear behavior. A giant nonlinear Raman amplification has been observed in these segments with length L$<$ 1.2$\mu $m for the first time. The nonlinear Raman effect has been demonstrated to increase as the lengths of nanowire segments decreases. As far as the relationship between Raman scattering intensity and laser pump power, we also observed that there exists a threshold pump laser power which separates the linear and super-linear regions. The effective pump power can be as low as 200 $\mu $W that is 1000 times smaller than bulk values. We attribute this giant nonlinear Raman effect to stimulated Raman scattering (SRS) from nanocavities formed by these short GaP nanowires. The quality factor Q of these short segments was estimated to be 10$^{3}$ to 10$^{4}$. We believe our observation suggests the possibility to make a new type of SRS semiconductor laser. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D11.00012: Polarized Rayleigh and Raman Study of single CuO Nanowire Qiujie Lu, Jian Wu, Humberto Gutierrez, Timothy Russin, Peter Eklund Crystalline CuO is an interesting Ferroelectric and Ferromagnetic system which we have recently grown in nanowire (NW) form. In this paper, we present results of Raman and Rayleigh scattering studies of individual CuO NWs to probe optical antenna effects that we first discovered in GaP NWs. We have shown that these antenna effects can, in general, strongly mask Raman selection rules in semiconducting nanowires[1]. Using a microRaman spectrometer, polarized light scattering experiments (backscattering geometry) were carried on NWs suspended over holes in a TEM grid. TEM was therefore also used to identify the growth axis and determine the NW diameter. As a function of the angle q between the NW axis and the incident laser field, we collect the Rayleigh scattering intensity as well as the Raman LO and TO optical phonon scattering intensity. These results can then be used to quantify the optical antenna effects in the CuO system. NWs of different diameters, from 70nm to 200 nm were studied; the results depend dramatically on the NW diameter. Our results will be compared to EM calculations based on the DDA approximation. This work is supported by NSF NIRT, grant DMR-0304178. [1] Chen G. Jian Wu, etc., Nano Lett. 2008 Vol.8 pp. 1341-1346. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D11.00013: Polarized Rayleigh and Raman Back-scattering from Individual GaP Nanowires Duming Zhang, Jian Wu, Qiujie Lu, Humberto Gutierrez, Peter Eklund Results of polarized Rayleigh and Raman back-scattering studies are reported for individual $\sim $20 $\mu $m long crystalline GaP Nanowires (NWs) using 514.5 nm excitation. The NWs were supported over holes in TEM grids. The diameters and growth directions of the NWs were thereby determined by TEM and the same NW could be studied optically. Peak positions of characteristic LO, TO phonon Raman bands were found to agree with bulk GaP. Both the Rayleigh and Raman back-scattering intensity polar patterns $I$(\textit{$\theta $}) were measured at room temperature, where \textit{$\theta $} is the angle between the incident electric field and the NW axis. The scattered radiation was polarized parallel to the incident electric field. From the Rayleigh back-scattering intensity polar patterns, the factor in the scattered radiation was obtained. Together with the Raman tensor determined from the growth direction of the NWs, the Raman back-scattering intensity polar pattern was calculated for each case and correlated with the experimental data. Our measurement on the Rayleigh and Raman back-scattering intensity polar patterns reveled different patterns ranging from dipole-like to symmetric (circular or elliptical) depending on the NWs growth directions and diameters. This work is supported by NSF- NIRT, grant DMR-0304178. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D11.00014: Optical properties of Nb:SrTiO$_{3}$/SrTiO$_{3}$ superlattices Woo Seok Choi, Soon Jae Moon, Hiromichi Ohta, Byung Cheol Jeon, Jong Hoon Shin, Yun Sang Lee, Tae Won Noh Recently, oxide heterostructures are being extensively investigated as an effort to understand unusual physics occurring at the oxide interfaces. For example, formation of 2DEG or strongly modified electronic structures at the interface between two different oxide constituents have been examined carefully and understood to some extent. In this contribution, we studied the electronic properties and phonon dynamics of Nb:SrTiO$_{3}$/SrTiO$_{3}$ superlattices. Using optical spectroscopy, we obtained the optical conductivity for a wide range of photon energy (3.7 meV -- 6 eV). We could separately identify the free carrier response and the phonon dynamics of the superlattice. The carrier density and unusually small scattering rate were obtained varying temperature as well as the superlattice period. In addition, we could reveal a strong electron-phonon coupling from the existence of the spectral weight in mid-IR photon range. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D11.00015: Effect of Magnetic Field on Broadening of Excitonic Spectra in Superlattices Yuri Khavin, Nikolai Sibeldin, Mikhail Skorikov, Vitaliy Tsvetkov, Daniel Oberli, Alok Rudra, Romain Carron, Eli Kapon We present a comprehensive study of optical properties of GaAs/AlGaAs superlattices (SLs) with different tunneling coupling between wells in magnetic fields in a wide range of excitation levels. Photoluminescence (PL) and PL excitation spectra demonstrate significant broadening of the exciton lines with increasing excitation power. Several features indicate that the exciton line widths are mainly determined by excitonic scattering. In particular, in zero magnetic field, the heavy hole (hh) PL line broadens symmetrically with increasing excitation power. In nonzero in-plane magnetic field, the exciton binding energy strongly increases (by a factor of 2 in 14 T), and the dependence of the line width on excitation power becomes much weaker. In strong in-plane fields, significant shift of the hh PL line towards lower energies is observed under high excitation levels. It is possible that this shift is a manifestation of interparticle interactions in an electron-hole system. [Preview Abstract] |
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