Wednesday 3 September Session 3: Quantum dots, nanocrystals and low dimensional materials (16:15 16:45, Huxley LT311)

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1 Session 3: Quantum dots, nanocrystals and low dimensional materials (16:15 16:45, (invited) Engineering entangled photon emission from site controlled quantum dots E Pelucchi, G Juska, V Dimastrodonato, T H Chung, and A Gocalinska Tyndall National Institute, Ireland The process which allows emitting entangled photons from quantum dots (QDs) is in many senses a good example of their similarity to atomic structures. Like for atoms, it is the result of textbook particle indistinguishability, which forces the (atomic like) system description to be antisymmetric in respect to particle exchange: the photon cascade merely reproducing the electronic entanglement. Nevertheless a QD suffers from a number of non-ideal features, which limit their exploitation in the quest for optical, integrated quantum computers. In our contribution we will first review the state of the art for metalorganic vapour phase epitaxy (MOVPE) grown, highly symmetric (111)B pyramidal quantum dots, which recently raised to the attention of the scientific community for their capability of emitting entangled photons with a relatively high density of emitters (best case ~ 15% of good emitters over the patterned area, exploiting surfactant effects [1]). We will then discuss how the high symmetry can be obtained over a variety of growth conditions, point to additional factors limiting the quality of the entangled cascade process (and other open issues) and finally discuss their potentiality for quantum integration. [1] G. Juska, V. Dimastrodonato, L. O. Mereni, A. Gocalinska, and E. Pelucchi, Nature Phot. 7, 527 (2013).

2 Session 3: Quantum dots, nanocrystals and low dimensional materials (16:45 17:15, (invited) Core/Shell quantum dot photovoltaics D C J Neo, C Cheng, C A Cattley, H E Assender and A A R Watt University of Oxford, UK Improvements in the power conversion efficiency of colloidal quantum dot (CQD) solar cells is currently propelled by both improving the electronic properties of materials and designing novel device architectures. Some of the best performing CQD solar cells reported to date have utilized an interpenetrated donor-acceptor bulk heterojunction structure which simultaneously gives good light absorption and charge extraction. In this talk I will discuss our evolution in CQD device design and describe our latest results. Firstly I will discuss how we template titanium dioxide electrode to create a macroporous structure which can be easily filled with QDs to maximize the donoracceptor interface and enhance device efficiency through improved absorption of light and charge collection pathways. Secondly I will outline how we use cation-exchange to synthesize PbS/CdS core/shell colloidal quantum dots from PbS starting cores. Incorporating these as the absorber we show that core/shell colloidal quantum dots can replace their unshelled counterparts with a similar band-gap as the active layer in a solar cell device, leading to an improvement in open circuit voltage from 0.42 V to 0.66 V.

3 Session 3: Quantum dots, nanocrystals and low dimensional materials (17:15 17:30, Size-dependence of ultrafast charge dynamics in monodisperse Au nanoparticles on colloidal TiO 2 spheres A Al-Otaify 1, M A Leontiadou 1*, P H C Camargo 2 and D J Binks 1 1 University of Manchester, UK, 2 niversidade de a o Paulo, Brazil TiO 2 has been widely investigated as a photocatalyst for water splitting and degradation of organic pollutants, and in the development of dye sensitized solar cells. On its own, its performance is limited by a relatively large band gap and a high recombination rate for photo-generated charges. This can be improved significantly, however, by the deposition of noble metal nanoparticles onto the TiO 2 surface. These can act as electron traps, aiding charge separation, and also enhance photogeneration in the visible part of the spectrum via a localized surface plasmon resonance. However, the synthesis of samples containing TiO 2 decorated with metal NPs of well-controlled sizes remains challenging, restricting the systematic investigation of their properties. Here, we present a study of sub-nanosecond charge dynamics in monodisperse Au nanoparticles (NPs) on TiO 2 colloidal spheres as a function of NP diameter using ultrafast transient absorption spectroscopy. The transmittance changes observed in the region of the plasmon resonance of the Au NPs following photo-excitation of the TiO 2 spheres are well-described by a bi-exponential function consisting of a fast component of 2ps duration associated with electron-phonon scattering, followed by a slow component associated with phonon-phonon scattering. The decay constant characterising the latter component was found to be dependent on the size of the Au NPs, increasing from 49±3 to 128±6 ps as the diameter of the Au NPs rose from 12.2±2.2 nm to 24.5±2.8 nm.

Session 3: Quantum dots, nanocrystals and low dimensional materials (17:30 17:45, Anomalous anticrossing of neutral exciton states in GaAs/AlGaAs quantum dots S Kumar 1,3, E Zallo 1, Y H Liao 2, P Y

4 Session 3: Quantum dots, nanocrystals and low dimensional materials (17:30 17:45, Anomalous anticrossing of neutral exciton states in GaAs/AlGaAs quantum dots S Kumar 1,3, E Zallo 1, Y H Liao 2, P Y Lin 2, R. Trotta 1,4, P Atkinson 1, J D Plumhof 1, F Ding 1, B D Gerardot 3, S J Cheng 2, A Rastelli 1,4, and O G Schmidt 1 1 Institute for Integrative Nanosciences, Germany, 2 National Chiao Tung University, Republic of China, 3 Heriot-Watt University, UK, 4 Johannes Kepler University, Austria Self-assembled InAs/GaAs quantum dots (QDs) have been successfully used for applications like generation of ondemand single-photons and polarization-entangled photon-pairs, and control and manipulation of spins of confined single particles. Despite showing several interesting features, these dots are associated with intrinsic strain and composition fluctuations which present difficulties to understand the effects of external perturbations required to functionalize their properties. On the other hand, GaAs/AlGaAs QDs are now attracting increasing interest as they are strain-free and the growth process allows strong reduction in the composition fluctuations and assure competitive optical qualities [1]. The fundamental excitation, a neutral-exciton (X 0 ), when confined in these QDs usually has a heavy-hole (HH) character as the quantumconfinement pushes down the light-hole (LH) state by several tens of mev. Very recently, it was shown that the character of such excitation can be switched to LH by embedding the QDs into a prestressed membrane [2]. Fig.1 Polar plots of the intensities of the low- (triangles) and high-energy (squares) X 0 lines of a single GaAs/AlGaAs QD at (a) ~zero and (b) ~60% HH-LH mixings. The gray lines are the cosine fits. Plots of (c) FSSs and (b) polarization angles of low- (triangles) and high energy (squares) X 0 lines versus electric-field applied to the piezoelectric actuator. Here we investigate the optical properties of the X 0 in the HH-LH mixed regime and find these properties to be very different compared to pure HH or LH character. We access the mixing regime of 0 70% (see Fig. 1a for ~ zero and 1b for ~ 60% mixing) by applying anisotropic variable stress to the QDs. This is done by transferring free standing AlGaAs membranes containing QDs onto piezoelectric actuators. Usually the X 0 emission of the dots splits into two linearly polarized lines due to electron-hole spin exchange interaction. The energetic splitting between these lines is called fine structure splitting (FSS). The detailed analysis of micro-photoluminescence at large and variable stresses leads to two major outcomes. Firstly, we observe anomalous anticrossing behaviour of bright X 0 states due to HH-LH mixing (the term anomalous here means that though we observe FSS minima (see Fig. 1c), the polarization directions of each FS-split X 0 lines do not show usually observed sigmoidal-type variations (see Fig. 1d for these deviations)) and secondly, the measurement of stress inside the QDs using the bulk-environment surrounding the QDs allows us (a) to correlate the observed optical properties of X 0 emission of the QDs with the stress and (b) to estimate HH-LH splitting ( E qc ) of as-grown QDs. These results are important for understanding the fundamental optical properties of QDs under elastic field and the hole-spin dynamics as a function of HH-LH mixing. [1] Kumar S, Trotta R, Zallo E, Plumhof J D, Atkinson P, Rastelli A and Schmidt O G 2011 Appl. Phys. Lett [2] Huo Y H, Witek B J, Kumar S, Cardenas J R, et al Nature Phys

Session 3: Quantum dots, nanocrystals and low dimensional materials (17:45 18:00, Phonon-assisted neutral exciton state preparation for a single quantum dot A J Brash 1, J H Quilter 1, F Liu 1, M

5 Session 3: Quantum dots, nanocrystals and low dimensional materials (17:45 18:00, Phonon-assisted neutral exciton state preparation for a single quantum dot A J Brash 1, J H Quilter 1, F Liu 1, M Glässl 2, A M Barth 2, V M Axt 2, A J Ramsay 3, M S Skolnick 1 and A M Fox 1 1 University of Sheffield, UK, 2 Universität Bayreuth, Germany, 3 University of Cambridge, UK In this work we demonstrate the creation of a neutral exciton state by pulsed optical excitation into the acoustic phonon sideband of a single quantum dot. This scheme [1] enables creation of a population inversion which is robust against fluctuations in pulse area compared to resonant excitation. The phonon-assisted process is illustrated by the strong modification of the exciton lineshape in photocurrent absorption spectra at high pulse areas due to the increase of effective carrier-phonon coupling. Single-pulse measurements show driven Rabi oscillations for a range of laser detunings as shown in figure 1. A significant neutral exciton population is observed when the laser is positively detuned in agreement with theory [1]. Figure 1: Experimental & theoretical plot of Rabi rotation vs. detuning illustrating the strong modification of the lineshape by phonon processes. Figure 2: Co-polarised pump-probe spectra showing increasing X 0 population (inferred from the absorption of a resonant π-pulse probe) with increased pulse area of a fixed +1 mev detuned pump. Pump-probe measurements confirm that the photocurrent measured off-resonance originates from the quantum dot neutral exciton. The absorption of a resonant π -pulse probe is modulated proportionally to the off-resonantly created exciton population as illustrated figure 2. We observe similar behaviour for a range of positive pump detunings up to the cut-off frequency for LA phonon coupling. Further measurements show that the phonon-assisted channel has excellent spin fidelity and allow us to probe the time-dynamics and the biexciton transition. We note that this scheme may be of use for spin qubit initialisation, single photon sources or as part of an architecture for quantum networks as it allows spectral separation of the fluorescence. [1] M. Glässl et al. Phys. Rev. Lett. 110, (2013)

6 Session 3: Quantum dots, nanocrystals and low dimensional materials (18:00 18:15, Phonon inuences in cavity quantum dot systems J Iles-Smith 1,2 and A Nazir 2 1 Imperial College London, UK, 2 The University of Manchester, UK Semiconductor quantum dots (QDs) in combination with high-q semi-conductor microcavities are central to many proposals for quantum technologies, examples of which include single photon sources and optical switches, to name a few. Many of these proposals rely both on the two level nature of QDs and their atomic like' emission characteristics. Recently, however, theoretical studies have suggested that in the abscence of a cavity, non-trivial phonon interactions can lead to a striking departure from the accepted atomic behavior, predicting enhanced coherent scattering well beyond saturation, behavior which can be attributed to competing phonon and photon processes [1]. Here we examine the effect of phonon interactions on the emission spectrum of a quantum dot in a high-q optical cavity, focussing in particular on a micropillar type setup. By making use of the master equation formalism we show that phonon interactions actively alter the emission properties of the cavity in the Purcell regime. Furthermore, we show that a phenomenological pure dephasing model is unable to capture the underlying physical processes responsible for this behavior, thus requiring us to respect the full eigenstructure of the cavity quantum dot system when deriving the appropriate master equation. [1] D. P. S. McCutcheon and A. Nazir, Model of the optical emission of a driven semiconductor quantum dot: Phonon-enhanced coherent scattering and off-resonant sideband narrowing," Phys. Rev. Lett., vol. 110, p , May 2013.

SUPPLEMENTARY INFORMATION

SUPPLEMENTARY INFORMATION SUPPLEMENTARY MATERIAL Towards quantum dot arrays of entangled photon emitters Gediminas Juska *1, Valeria Dimastrodonato 1, Lorenzo O. Mereni 1, Agnieszka Gocalinska 1 and Emanuele Pelucchi 1 1 Tyndall