M. Skolnick - Academia.edu (original) (raw)

Papers by M. Skolnick

Research paper thumbnail of Spin mixing for one and two electron states in quantum dots

AIP Conference Proceedings, 2007

We consider the role of one-electron spin-dependent couplings on the energy relaxation triggered ... more We consider the role of one-electron spin-dependent couplings on the energy relaxation triggered by polaron disintegration in quantum dots charged with two electrons.

Research paper thumbnail of Asymmetric photoluminescence spectra from excitons in a coupled microcavity

Physical Review B, 2000

Asymmetric photoluminescence ͑PL͒ emission from a coupled semiconductor microcavity, containing q... more Asymmetric photoluminescence ͑PL͒ emission from a coupled semiconductor microcavity, containing quantum wells in the upper cavity only, is reported. On resonance, the central polariton mode is found to emit strongly in the direction towards the empty lower cavity, whereas in the opposite direction, towards the cavity containing the quantum wells, no central-mode PL is observed. At the same time, the spectra from the other modes in the system are independent of direction. The optical field resulting from the central mode is shown to be only strong at macroscopic distances away from the optically active excitons. A good fit to the spectra is obtained assuming elastic scattering of uncoupled excitons into the optically active polariton states.

Research paper thumbnail of Interplay of order and disorder in the optical properties of opal photonic crystals

Physical Review B, 2002

It is shown that the polycrystalline structure of self-assembled synthetic opals leads to an inte... more It is shown that the polycrystalline structure of self-assembled synthetic opals leads to an interplay of properties determined by order and disorder in the vicinity of the optical stop band. We analyze the balance of photon fluxes by studying angle-resolved spectra of diffracted and scattered light for all directions in space. It is shown that the shape of the stop band features in different types of optical spectra ͑diffraction, transmission, and scattering͒ is interlinked and must be studied jointly to understand optical phenomena in these materials. The principal effects are ͑i͒ angular dispersion of the photonic stop bands in diffraction spectra, ͑ii͒ inhomogeneous broadening of the stop band in zero-order transmission, and ͑iii͒ appearance of very strong peaks in the spectra of scattered light, with resonant enhancements observed with intensity up to ϳ10 times greater than background scattering levels. It is shown that the resonant enhancements arise from multiple incoherent backward/forward reflections between the microcrystallites. It is shown that the spatial pattern and spectral form of the scattering spectra can be deduced from the analysis of angle-resolved zero-order transmission spectra under conditions where the attenuation length of light within the stop band is comparable to the thickness of the sample. The methodology of the studies developed in this paper is applicable to a wide class of disordered photonic crystal structures.

Research paper thumbnail of Excited states and selection rules in self-assembled InAs/GaAs quantum dots

Physical Review B, 1999

High-pumping-intensity photoluminescence ͑PL͒ studies of InAs self-assembled quantum dots ͑SAQD͒ ... more High-pumping-intensity photoluminescence ͑PL͒ studies of InAs self-assembled quantum dots ͑SAQD͒ have been performed under high pressure P up to 70 kbar. The origin of the higher-energy PL lines that appear in the spectra with increasing pumping intensity is determined by using the ⌫-X crossover effect in the conduction band. With increasing P, these lines are sequentially quenched at particular values of P. From this we unambiguously conclude that the lines correspond to the transitions from different excited conduction electron states in the SAQD. This indicates the existence of strong selection rules for the electron-hole recombination in the SAQD. ͓S0163-1829͑99͒51428-8͔ RAPID COMMUNICATIONS

Research paper thumbnail of Dependence of stimulated scattering in semiconductor microcavities on pump power, angle, and energy

Physical Review B, 2003

The dependence of stimulated polariton-polariton scattering in InGaAs/GaAs/AlGaAs microcavities o... more The dependence of stimulated polariton-polariton scattering in InGaAs/GaAs/AlGaAs microcavities on pump power, angle, and energy is investigated. We show that contrary to much previous belief there is no ''magic angle'' for stimulated scattering under continuous-wave conditions, the process being shown to be a rather general phenomenon, occurring over a wide range of pump angles. Close analogies with optical parametric oscillator behavior are found, but also aspects which can only be explained by inclusion of the polariton nature of the quasiparticles. The first physical explanation of why the optical parametric oscillator picture leads to the observed square root dependence of signal power on laser intensity is provided. The asymmetric nature of the tuning curve for excitation of the stimulated scattering process is explained in the framework of the renormalization of the lower polariton branch.

Research paper thumbnail of Hole-quasiparticle resonant polaron coupling in quantum wells containing high densities of free carriers

Research paper thumbnail of Fermi-energy edge singularity and excitonic enhancement associated with the second subband in asymmetric modulation-doped quantum wells

Research paper thumbnail of Mapping of Alport syndrome to the long arm of the X chromosome

American journal of human genetics, 1988

Five X-chromosome DNA markers were typed on 261 members of three large kindreds with Alport syndr... more Five X-chromosome DNA markers were typed on 261 members of three large kindreds with Alport syndrome (hereditary glomerulonephritis). Lod scores greater than 3.0 for linkage between the disease locus and two of the markers confirmed X-linked inheritance of the disease. A decreasing gradient in the estimated recombination fractions observed when the markers were ordered on the basis of their map locations suggested that the disease locus is on the long arm distal to all the markers typed in this study. Using three-locus analysis we rejected all but three map orders for the six loci (the disease locus and five markers). In all three the Alport syndrome locus was on the long arm of the X chromosome distal to all the markers. Two types of Alport syndrome were represented in the three kindreds. Affected males in one kindred developed deafness in addition to nephritis; deafness did not occur in members of the other two kindreds. Although larger recombination-fraction estimates were obtain...

Research paper thumbnail of Two-Dimensional Metal–Chalcogenide Films in Tunable Optical Microcavities

Nano Letters, 2014

Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic pr... more Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic properties. However, photonic applications of thin films may be limited owing to weak light absorption and surface effects leading to reduced quantum yield. Integration of 2D films in optical microcavities will permit these limitations to be overcome owing to modified light coupling with the films. Here we present tunable microcavities with embedded monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; PL lifetime shortening by a factor of 10 is achieved, a consequence of Purcell enhancement of the spontaneous emission rate. This work has potential to pave the way to microcavity-enhanced light-emitting devices based on layered 2D materials and their heterostructures, and also opens possibilities for cavity QED in a new material system of van der Waals crystals.

Research paper thumbnail of Ring emission and exciton-pair scattering in semiconductor microcavities

Physical Review B, 2002

Polariton microemitters are semiconductor devices that take advantage of the strong coupling of l... more Polariton microemitters are semiconductor devices that take advantage of the strong coupling of light and matter to produce efficient optical sources. Injecting an exciton gas into such a semiconductor microcavity can produce annular optical emission of a specific cone angle. We demonstrate that at higher exciton densities or temperatures, this ring continuously decreases in diameter due to the greater energy that can be carried off in exciton-pair scattering processes. These Coulomb-mediated exciton-exciton scatterings are shown to dominate the polariton relaxation and optical emission.

Research paper thumbnail of Whispering gallery modes in quantum dot micropillar cavities

2008 Conference on Quantum Electronics and Laser Science Conference on Lasers and Electro-Optics, CLEO/QELS, 2008

ABSTRACT

Research paper thumbnail of Measurement of the direct energy gap ofAl0.5In0.5P: Implications for the band discontinuity atGa1−xInxP/AlyIn1−yP heterojunctions

Physical Review B, 1994

], Patel and co-workers described hydrostaticpressure-dependent photoluminescence measurements pe... more ], Patel and co-workers described hydrostaticpressure-dependent photoluminescence measurements performed on Gao 47Inp 53P/Alo 5Ino 5P multiple quantum wells. They determined directly a valence-band offset of 0.24+0.05 eV from the energy difference between the indirect transition in the barrier and the transition from X states in the barrier to hh1 valence-band states in the quantum well, when extrapolated to zero pressure. The conduction-band offset was then indirectly determined to be 0.26 eV from the total-band discontinuity of the system, assuming a value of 2.45 eV for the (low temperature) lowest-energy direct gap of Alo 5Ino &P. This yields a band-offset ratio, EE,:AE", of 52:48. Here, we present evidence that the (5 K) direct gap of the Ala 5Ino 5P barrier is-2.685 eV, which results in a revised band-offset ratio of 67:33, similar to recent values determined for Gal "In,P/(Al"Ga&~)0 5Ino 5P heterojunctions.

Research paper thumbnail of Magnetic field induced indirect gap in a modulation doped quantum well

Research paper thumbnail of Temperature-induced carrier escape processes studied in absorption of individual InxGa1-xAs quantum dots

Physical Review B, 2004

... R. Oulton,1 AI Tartakovskii,1 A. Ebbens,1 J. Cahill,1 JJ Finley,1,3 DJ Mowbray,1 MS Skolnick,... more ... R. Oulton,1 AI Tartakovskii,1 A. Ebbens,1 J. Cahill,1 JJ Finley,1,3 DJ Mowbray,1 MS Skolnick,1 and M. Hopkinson2 1Department of ... laser power is observed at low temperature whereas a nearly linear power dependence is measured at T 80 K in a wide range of incident ...

Research paper thumbnail of Optical orientation and control of spin memory in individual InGaAs quantum dots

Physical Review B, 2005

A high degree of spin polarization ͑or spin memory͒ is achieved using quasiresonant optical excit... more A high degree of spin polarization ͑or spin memory͒ is achieved using quasiresonant optical excitation at zero magnetic field ͑B =0͒ for singly positively charged excitons ͑X + ͒ in individual quantum dots embedded in a Schottky diode. The high degree of spin memory indicates highly efficient optical excitation ͑"writing"͒ of long-lived spin-polarized electrons, determining the X + spin orientation. We demonstrate control of the degree of spin polarization by the applied bias, controlling carrier tunneling rates in the device. In addition, efficient spin-selective optical excitation of neutral excitons is achieved for B Ͼ 1 T.

Research paper thumbnail of Resolution of discrete excited states in InxGa1−xN multiple quantum wells using degenerate four-wave mixing

Physical Review B, 2006

We report on two pulse, degenerate four-wave mixing ͑DFWM͒ measurements on shallow In x Ga 1−x N ... more We report on two pulse, degenerate four-wave mixing ͑DFWM͒ measurements on shallow In x Ga 1−x N / GaN multiquantum wells ͑MQWs͒ grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k • p model calculation for the quantum well ͑QW͒ energy levels and optical transition matrix elements. In x Ga 1−x N / GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.

Research paper thumbnail of Spatial structure and stability of the macroscopically occupied polariton state in the microcavity optical parametric oscillator

Physical Review B, 2006

Transverse spatial images of the signal generated in semiconductor microcavities under conditions... more Transverse spatial images of the signal generated in semiconductor microcavities under conditions of resonant parametric scattering are obtained. When the system is pumped significantly above the threshold, the signal image is found to be highly structured, consisting of a set of localized bright spots. By comparison with a numerical model, it is shown that the structure arises from the blueshift generated by the nonlinear polaritonpolariton interaction which drives the microcavity optical parametric oscillator, in combination with partially ordered spatial fluctuations in the cavity mode energy. An important outcome is that the form and stability of the signal is strongly influenced by the local cavity disorder.

Research paper thumbnail of Splitting and lasing of whispering gallery modes in quantum dot micropillars

Optics Express, 2010

We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars c... more We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars containing InAs quantum dots. High quality factor WGMs are observed from a wide range of pillars with diameters from 1.2 to 50 µm. Multimode lasing with sub-milliwatt thresholds and high beta-factors approaching unity is observed under optical pumping in a 4 µm diameter pillar. Mode splitting is observed in WGMs from pillars with diameters of 5 µm, 20 µm and 50 µm. We develop a model in which the mode splitting in the larger pillars is caused by resonant scattering from the quantum dots themselves. The model explains why splittings are observed in all of the larger pillars and that the splitting decreases with increasing wavelength. Numerical simulations by COMSOL confirm that the model is plausible. This mechanism of splitting should be general for all circular resonant structures containing quantum dots such as microdisks, rings, toroids, and microspheres.

Research paper thumbnail of Singlet and triplet polaron relaxation in doubly charged self-assembled quantum dots

New Journal of Physics, 2007

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot... more Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, timeresolved pump-probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin-flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (∼30 ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (∼5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin-orbit interaction, which induces spin-flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin-flip decay times, and good agreement is obtained between the experiment and the simulation of the pump-probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.

Research paper thumbnail of Confirmation of FWT1 as a Wilms’ tumour susceptibility gene and phenotypic characteristics of Wilms’ tumour attributable to FWT1

Human Genetics, 1998

A susceptibility gene for Wilms' tumour (WT), designated FWT1, was previously mapped to chromosom... more A susceptibility gene for Wilms' tumour (WT), designated FWT1, was previously mapped to chromosome 17q12-q21 by linkage analysis of a single family. We now confirm the existence of this gene by analysis of additional cases in the original family (3-point LOD score=5.69), and by detecting strong evidence of linkage to this region in an unrelated pedigree with seven cases of WT (3-point LOD score=2.56). Analysis of 11 smaller WT families confirms that there is genetic heterogeneity in familial WT, as three families exhibit strong evidence against linkage to FWT1. One of these was subsequently found to have a predisposing WT1 mutation. However, the other two families show evidence against both FWT1 and WT1, suggesting that at least one further familial WT gene exists. Analysis of the phenotype of 16 WT cases from the families linked to FWT1 demonstrates that they present at a significantly older age and a significantly later stage than both sporadic WT and the six cases from two families unlinked to either FWT1 or WT1. The results confirm the role of FWT1 in susceptibility to WT, provide strong evidence for genetic heterogeneity in familial WT and suggest there are phenotypic differences between familial WT due to FWT1, familial WT due to other genes and non-familial WT.

Research paper thumbnail of Spin mixing for one and two electron states in quantum dots

AIP Conference Proceedings, 2007

We consider the role of one-electron spin-dependent couplings on the energy relaxation triggered ... more We consider the role of one-electron spin-dependent couplings on the energy relaxation triggered by polaron disintegration in quantum dots charged with two electrons.

Research paper thumbnail of Asymmetric photoluminescence spectra from excitons in a coupled microcavity

Physical Review B, 2000

Asymmetric photoluminescence ͑PL͒ emission from a coupled semiconductor microcavity, containing q... more Asymmetric photoluminescence ͑PL͒ emission from a coupled semiconductor microcavity, containing quantum wells in the upper cavity only, is reported. On resonance, the central polariton mode is found to emit strongly in the direction towards the empty lower cavity, whereas in the opposite direction, towards the cavity containing the quantum wells, no central-mode PL is observed. At the same time, the spectra from the other modes in the system are independent of direction. The optical field resulting from the central mode is shown to be only strong at macroscopic distances away from the optically active excitons. A good fit to the spectra is obtained assuming elastic scattering of uncoupled excitons into the optically active polariton states.

Research paper thumbnail of Interplay of order and disorder in the optical properties of opal photonic crystals

Physical Review B, 2002

It is shown that the polycrystalline structure of self-assembled synthetic opals leads to an inte... more It is shown that the polycrystalline structure of self-assembled synthetic opals leads to an interplay of properties determined by order and disorder in the vicinity of the optical stop band. We analyze the balance of photon fluxes by studying angle-resolved spectra of diffracted and scattered light for all directions in space. It is shown that the shape of the stop band features in different types of optical spectra ͑diffraction, transmission, and scattering͒ is interlinked and must be studied jointly to understand optical phenomena in these materials. The principal effects are ͑i͒ angular dispersion of the photonic stop bands in diffraction spectra, ͑ii͒ inhomogeneous broadening of the stop band in zero-order transmission, and ͑iii͒ appearance of very strong peaks in the spectra of scattered light, with resonant enhancements observed with intensity up to ϳ10 times greater than background scattering levels. It is shown that the resonant enhancements arise from multiple incoherent backward/forward reflections between the microcrystallites. It is shown that the spatial pattern and spectral form of the scattering spectra can be deduced from the analysis of angle-resolved zero-order transmission spectra under conditions where the attenuation length of light within the stop band is comparable to the thickness of the sample. The methodology of the studies developed in this paper is applicable to a wide class of disordered photonic crystal structures.

Research paper thumbnail of Excited states and selection rules in self-assembled InAs/GaAs quantum dots

Physical Review B, 1999

High-pumping-intensity photoluminescence ͑PL͒ studies of InAs self-assembled quantum dots ͑SAQD͒ ... more High-pumping-intensity photoluminescence ͑PL͒ studies of InAs self-assembled quantum dots ͑SAQD͒ have been performed under high pressure P up to 70 kbar. The origin of the higher-energy PL lines that appear in the spectra with increasing pumping intensity is determined by using the ⌫-X crossover effect in the conduction band. With increasing P, these lines are sequentially quenched at particular values of P. From this we unambiguously conclude that the lines correspond to the transitions from different excited conduction electron states in the SAQD. This indicates the existence of strong selection rules for the electron-hole recombination in the SAQD. ͓S0163-1829͑99͒51428-8͔ RAPID COMMUNICATIONS

Research paper thumbnail of Dependence of stimulated scattering in semiconductor microcavities on pump power, angle, and energy

Physical Review B, 2003

The dependence of stimulated polariton-polariton scattering in InGaAs/GaAs/AlGaAs microcavities o... more The dependence of stimulated polariton-polariton scattering in InGaAs/GaAs/AlGaAs microcavities on pump power, angle, and energy is investigated. We show that contrary to much previous belief there is no ''magic angle'' for stimulated scattering under continuous-wave conditions, the process being shown to be a rather general phenomenon, occurring over a wide range of pump angles. Close analogies with optical parametric oscillator behavior are found, but also aspects which can only be explained by inclusion of the polariton nature of the quasiparticles. The first physical explanation of why the optical parametric oscillator picture leads to the observed square root dependence of signal power on laser intensity is provided. The asymmetric nature of the tuning curve for excitation of the stimulated scattering process is explained in the framework of the renormalization of the lower polariton branch.

Research paper thumbnail of Hole-quasiparticle resonant polaron coupling in quantum wells containing high densities of free carriers

Research paper thumbnail of Fermi-energy edge singularity and excitonic enhancement associated with the second subband in asymmetric modulation-doped quantum wells

Research paper thumbnail of Mapping of Alport syndrome to the long arm of the X chromosome

American journal of human genetics, 1988

Five X-chromosome DNA markers were typed on 261 members of three large kindreds with Alport syndr... more Five X-chromosome DNA markers were typed on 261 members of three large kindreds with Alport syndrome (hereditary glomerulonephritis). Lod scores greater than 3.0 for linkage between the disease locus and two of the markers confirmed X-linked inheritance of the disease. A decreasing gradient in the estimated recombination fractions observed when the markers were ordered on the basis of their map locations suggested that the disease locus is on the long arm distal to all the markers typed in this study. Using three-locus analysis we rejected all but three map orders for the six loci (the disease locus and five markers). In all three the Alport syndrome locus was on the long arm of the X chromosome distal to all the markers. Two types of Alport syndrome were represented in the three kindreds. Affected males in one kindred developed deafness in addition to nephritis; deafness did not occur in members of the other two kindreds. Although larger recombination-fraction estimates were obtain...

Research paper thumbnail of Two-Dimensional Metal–Chalcogenide Films in Tunable Optical Microcavities

Nano Letters, 2014

Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic pr... more Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic properties. However, photonic applications of thin films may be limited owing to weak light absorption and surface effects leading to reduced quantum yield. Integration of 2D films in optical microcavities will permit these limitations to be overcome owing to modified light coupling with the films. Here we present tunable microcavities with embedded monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; PL lifetime shortening by a factor of 10 is achieved, a consequence of Purcell enhancement of the spontaneous emission rate. This work has potential to pave the way to microcavity-enhanced light-emitting devices based on layered 2D materials and their heterostructures, and also opens possibilities for cavity QED in a new material system of van der Waals crystals.

Research paper thumbnail of Ring emission and exciton-pair scattering in semiconductor microcavities

Physical Review B, 2002

Polariton microemitters are semiconductor devices that take advantage of the strong coupling of l... more Polariton microemitters are semiconductor devices that take advantage of the strong coupling of light and matter to produce efficient optical sources. Injecting an exciton gas into such a semiconductor microcavity can produce annular optical emission of a specific cone angle. We demonstrate that at higher exciton densities or temperatures, this ring continuously decreases in diameter due to the greater energy that can be carried off in exciton-pair scattering processes. These Coulomb-mediated exciton-exciton scatterings are shown to dominate the polariton relaxation and optical emission.

Research paper thumbnail of Whispering gallery modes in quantum dot micropillar cavities

2008 Conference on Quantum Electronics and Laser Science Conference on Lasers and Electro-Optics, CLEO/QELS, 2008

ABSTRACT

Research paper thumbnail of Measurement of the direct energy gap ofAl0.5In0.5P: Implications for the band discontinuity atGa1−xInxP/AlyIn1−yP heterojunctions

Physical Review B, 1994

], Patel and co-workers described hydrostaticpressure-dependent photoluminescence measurements pe... more ], Patel and co-workers described hydrostaticpressure-dependent photoluminescence measurements performed on Gao 47Inp 53P/Alo 5Ino 5P multiple quantum wells. They determined directly a valence-band offset of 0.24+0.05 eV from the energy difference between the indirect transition in the barrier and the transition from X states in the barrier to hh1 valence-band states in the quantum well, when extrapolated to zero pressure. The conduction-band offset was then indirectly determined to be 0.26 eV from the total-band discontinuity of the system, assuming a value of 2.45 eV for the (low temperature) lowest-energy direct gap of Alo 5Ino &P. This yields a band-offset ratio, EE,:AE", of 52:48. Here, we present evidence that the (5 K) direct gap of the Ala 5Ino 5P barrier is-2.685 eV, which results in a revised band-offset ratio of 67:33, similar to recent values determined for Gal "In,P/(Al"Ga&~)0 5Ino 5P heterojunctions.

Research paper thumbnail of Magnetic field induced indirect gap in a modulation doped quantum well

Research paper thumbnail of Temperature-induced carrier escape processes studied in absorption of individual InxGa1-xAs quantum dots

Physical Review B, 2004

... R. Oulton,1 AI Tartakovskii,1 A. Ebbens,1 J. Cahill,1 JJ Finley,1,3 DJ Mowbray,1 MS Skolnick,... more ... R. Oulton,1 AI Tartakovskii,1 A. Ebbens,1 J. Cahill,1 JJ Finley,1,3 DJ Mowbray,1 MS Skolnick,1 and M. Hopkinson2 1Department of ... laser power is observed at low temperature whereas a nearly linear power dependence is measured at T 80 K in a wide range of incident ...

Research paper thumbnail of Optical orientation and control of spin memory in individual InGaAs quantum dots

Physical Review B, 2005

A high degree of spin polarization ͑or spin memory͒ is achieved using quasiresonant optical excit... more A high degree of spin polarization ͑or spin memory͒ is achieved using quasiresonant optical excitation at zero magnetic field ͑B =0͒ for singly positively charged excitons ͑X + ͒ in individual quantum dots embedded in a Schottky diode. The high degree of spin memory indicates highly efficient optical excitation ͑"writing"͒ of long-lived spin-polarized electrons, determining the X + spin orientation. We demonstrate control of the degree of spin polarization by the applied bias, controlling carrier tunneling rates in the device. In addition, efficient spin-selective optical excitation of neutral excitons is achieved for B Ͼ 1 T.

Research paper thumbnail of Resolution of discrete excited states in InxGa1−xN multiple quantum wells using degenerate four-wave mixing

Physical Review B, 2006

We report on two pulse, degenerate four-wave mixing ͑DFWM͒ measurements on shallow In x Ga 1−x N ... more We report on two pulse, degenerate four-wave mixing ͑DFWM͒ measurements on shallow In x Ga 1−x N / GaN multiquantum wells ͑MQWs͒ grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k • p model calculation for the quantum well ͑QW͒ energy levels and optical transition matrix elements. In x Ga 1−x N / GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.

Research paper thumbnail of Spatial structure and stability of the macroscopically occupied polariton state in the microcavity optical parametric oscillator

Physical Review B, 2006

Transverse spatial images of the signal generated in semiconductor microcavities under conditions... more Transverse spatial images of the signal generated in semiconductor microcavities under conditions of resonant parametric scattering are obtained. When the system is pumped significantly above the threshold, the signal image is found to be highly structured, consisting of a set of localized bright spots. By comparison with a numerical model, it is shown that the structure arises from the blueshift generated by the nonlinear polaritonpolariton interaction which drives the microcavity optical parametric oscillator, in combination with partially ordered spatial fluctuations in the cavity mode energy. An important outcome is that the form and stability of the signal is strongly influenced by the local cavity disorder.

Research paper thumbnail of Splitting and lasing of whispering gallery modes in quantum dot micropillars

Optics Express, 2010

We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars c... more We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars containing InAs quantum dots. High quality factor WGMs are observed from a wide range of pillars with diameters from 1.2 to 50 µm. Multimode lasing with sub-milliwatt thresholds and high beta-factors approaching unity is observed under optical pumping in a 4 µm diameter pillar. Mode splitting is observed in WGMs from pillars with diameters of 5 µm, 20 µm and 50 µm. We develop a model in which the mode splitting in the larger pillars is caused by resonant scattering from the quantum dots themselves. The model explains why splittings are observed in all of the larger pillars and that the splitting decreases with increasing wavelength. Numerical simulations by COMSOL confirm that the model is plausible. This mechanism of splitting should be general for all circular resonant structures containing quantum dots such as microdisks, rings, toroids, and microspheres.

Research paper thumbnail of Singlet and triplet polaron relaxation in doubly charged self-assembled quantum dots

New Journal of Physics, 2007

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot... more Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, timeresolved pump-probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin-flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (∼30 ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (∼5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin-orbit interaction, which induces spin-flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin-flip decay times, and good agreement is obtained between the experiment and the simulation of the pump-probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.

Research paper thumbnail of Confirmation of FWT1 as a Wilms’ tumour susceptibility gene and phenotypic characteristics of Wilms’ tumour attributable to FWT1

Human Genetics, 1998

A susceptibility gene for Wilms' tumour (WT), designated FWT1, was previously mapped to chromosom... more A susceptibility gene for Wilms' tumour (WT), designated FWT1, was previously mapped to chromosome 17q12-q21 by linkage analysis of a single family. We now confirm the existence of this gene by analysis of additional cases in the original family (3-point LOD score=5.69), and by detecting strong evidence of linkage to this region in an unrelated pedigree with seven cases of WT (3-point LOD score=2.56). Analysis of 11 smaller WT families confirms that there is genetic heterogeneity in familial WT, as three families exhibit strong evidence against linkage to FWT1. One of these was subsequently found to have a predisposing WT1 mutation. However, the other two families show evidence against both FWT1 and WT1, suggesting that at least one further familial WT gene exists. Analysis of the phenotype of 16 WT cases from the families linked to FWT1 demonstrates that they present at a significantly older age and a significantly later stage than both sporadic WT and the six cases from two families unlinked to either FWT1 or WT1. The results confirm the role of FWT1 in susceptibility to WT, provide strong evidence for genetic heterogeneity in familial WT and suggest there are phenotypic differences between familial WT due to FWT1, familial WT due to other genes and non-familial WT.