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Biomedical subjects

M Bayer

Publications and source records attributed to M Bayer.

At least 19 recordsLinked to original sources
PubMed2006

Mode locking of electron spin coherences in singly charged quantum dots.

The fast dephasing of electron spins in an ensemble of quantum dots is detrimental for applications in quantum information processing. We show here that dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins, and we demonstrate this effect in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. This mode locking leads to constructive interference of contributions to Faraday rotation and presents potential applications based on robust quantum coherence within an ensemble of dots.

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PubMed2006

Ultrafast band-gap shift induced by a strain pulse in semiconductor heterostructures.

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of approximately 10 ps are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the GaAs/(Al,Ga)As heterostructure over a distance of 100 microm and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.

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PubMed2006

Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots.

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.

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PubMed2006

Quantitative ultrasonometry of the calcaneus in children with juvenile idiopathic arthritis.

OBJECTIVES: To evaluate bone quality by means of quantitative ultrasonometry (QUS) in children with juvenile idiopathic arthritis (JIA). METHODS: Seventy children [37 with oligoarticular JIA, mean age (+/-s.d.) 10.54 +/- 3.42 yr; and 33 with polyarticular rheumatoid factor negative JIA, mean age (+/- s.d.) 11.33 +/- 2.88 yr] were enrolled. Quantitative ultrasonometry was measured on both heels with a Cuba Clinical portable device. Body height, weight and body mass index were recorded together with disease duration and cumulative dose of prednisone. RESULTS: The lowest QUS parameters were observed in children with polyarticular JIA (P< 0.001 and 0.01 when compared with reference data and oligoarticular JIA, respectively). In children with oligoarticular JIA, the QUS values were also significantly lower in comparison with the reference data (P< 0.002). The QUS parameters were strongly influenced by body height, and to a lesser degree by body weight. In children with polyarticular JIA, there were significant inverse correlations between QUS parameters and disease duration [r=-0.57, P< 0.01 for broadband ultrasound attenuation (BUA) and r = - 0.67, P< 0.01 for velocity of sound (VOS)]. Similarly, there were inverse correlations between QUS and cumulative dose of prednisone (r = - 0.48, P< 0.05 for BUA and r =- 0.50, P < 0.01 for VOS, respectively). Similar results were obtained when BUA and VOS were adjusted for height. CONCLUSIONS: Disease duration and cumulative dose of prednisone in children with polyarticular JIA are risk factors of stunted growth and decreased QUS values of bone quality.

Adolescent↗
PubMed2006

Spin and orbital quantization of electronic states as origins of second harmonic generation in semiconductors.

Basically different mechanisms of optical second harmonic generation (SHG) in semiconductors, induced by an external magnetic field H, have been identified experimentally by studying the diluted magnetic semiconductor (Cd,Mn)Te. For paramagnetic (Cd,Mn)Te the SHG response is governed by spin quantization of electronic states, in contrast with diamagnetic CdTe with its dominating orbital quantization. The mechanisms can be identified by the distinct magnetic field dependence of the SHG intensity which scales with the spin splitting in the paramagnetic case as compared to the H2 dependence observed for the diamagnetic case.

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PubMed2005

Magnetic-field-induced second-harmonic generation in semiconductor GaAs.

We show that application of a magnetic field induces optical second-harmonic generation (SHG) in GaAs. This phenomenon arises from field-induced symmetry breaking causing new optical nonlinearities. A series of narrow SHG lines is observed in the spectral range from 1.52 to 1.77 eV that we attribute to Landau-level quantization of the band energy spectrum. The rotational anisotropy of the SHG signal distinctly differs from that of the electric-dipole approximation. Model calculations reveal that nonlinear magneto-optical spatial dispersion that comes together with the electric-dipole term is the dominant mechanism for this nonlinearity.

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PubMed2005

Control of vertically coupled InGaAs/GaAs quantum dots with electric fields.

Controllable interactions that couple quantum dots are a key requirement in the search for scalable solid state implementations for quantum information technology. From optical studies of excitons and corresponding calculations, we demonstrate that an electric field on vertically coupled pairs of In(0.6)Ga(0.4)As/GaAs quantum dots controls the mixing of the exciton states on the two dots and also provides controllable coupling between carriers in the dots.

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PubMed2004

Excitonic energy shell structure of self-assembled InGaAs/GaAs quantum dots.

Performing optical spectroscopy of highly homogeneous quantum dot arrays in ultrahigh magnetic fields, an unprecedently well resolved Fock-Darwin spectrum is observed. The existence of up to four degenerate electronic shells is demonstrated where the magnetic field lifts the initial degeneracies, which reappear when levels with different angular momenta come into resonance. The resulting level shifting and crossing pattern also show evidence of many-body effects such as the mixing of configurations and exciton condensation at the resonances.

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PubMed2003

Exciton dephasing in quantum dot molecules.

We have measured the exciton dephasing time in InAs/GaAs quantum dot molecules having different interdot barrier thicknesses in the temperature range from 5 to 60 K, using a highly sensitive four-wave mixing heterodyne technique. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed. These results show how the quantum-mechanical coupling of the electronic wave functions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.

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PubMed2003

Wave-vector-dependent exciton exchange interaction.

The exchange interaction for the yellow 1S orthoexciton in Cu2O is derived up to the order K2. The resulting exchange splittings are verified experimentally using high resolution spectroscopy. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector.

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PubMed2003

Optical detection of the Aharonov-Bohm effect on a charged particle in a nanoscale quantum ring.

We study spectroscopically the current produced by a charged particle moving in a nanosize semiconductor quantum ring subject to a perpendicular magnetic field. Several Aharonov-Bohm oscillations are observed in the emission of a charged exciton confined in a single ring structure. The magnetic field period of the oscillations correlates well with the size of the rings.

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PubMed2003

Fine structure of excitons in InAs/GaAs coupled auantum dots: a sensitive test of electronic coupling.

Exciton fine structure in InAs/GaAs coupled quantum dots has been studied by photoluminescence spectroscopy in magnetic fields up to 8 T. Pronounced anticrossings and mixings of optically bright and dark states as functions of magnetic field are seen. A theoretical treatment of the mixing of the excitonic states has been developed, and it traces observed features to structural asymmetries. These results provide direct evidence for coherent coupling of excitons in quantum dot molecules.

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PubMed2001

Inhibition and enhancement of the spontaneous emission of quantum dots in structured microresonators.

Spontaneous emission of quantum dot systems in laterally structured microcavities that exhibit photon confinement in all three directions has been studied by time-resolved photoluminescence spectroscopy. For on-resonance conditions, we find that the dot emission rate is increased substantially over that of the unstructured planar cavity. For off-resonance conditions, we are able to suppress the emission rate by an order of magnitude by using cavities with metal coatings, which we attribute to the suppression of leaky optical modes in these structures.

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PubMed2001

Isomeric photonic molecules formed from coupled microresonators.

Isomeric photonic molecules were formed by connecting four identical cavities in different geometries: a chain, a square, and a T shape. The optical mode spectrum in these structures exhibits three-dimensionally confined photonic states, which have been studied by photoluminescence spectroscopy. The energies of the optical modes depend strongly on the molecule geometry. The experimental data are in good agreement with detailed calculations of the fields in the cavities.

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PubMed2001

Coupling and entangling of quantum states in quantum dot molecules.

We demonstrate coupling and entangling of quantum states in a pair of vertically aligned, self-assembled quantum dots by studying the emission of an interacting electron-hole pair (exciton) in a single dot molecule as a function of the separation between the dots. An interaction-induced energy splitting of the exciton is observed that exceeds 30 millielectron volts for a dot layer separation of 4 nanometers. The results are interpreted by mapping the tunneling of a particle in a double dot to the problem of a single spin. The electron-hole complex is shown to be equivalent to entangled states of two interacting spins.

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PubMed2001

Ultrasound parameters of calcaneal bone density in girls with anorexia nervosa.

Osteoporosis is common in patients with anorexia nervosa (AN), but ultrasound has so far been scarcely used to detect it We measured calcaneal broadband ultrasound attenuation (BUA) and velocity of sound (VOS) in 26 AN girls (mean age 15.1+/- 1.5 years) using a Cuba Clinical device (McCue Ultrasonics, UK). Basic anthropometric (body weight, height and body mass index--BMI) and clinical data (mean duration of AN, number of absent cycles, weight loss) were collected. All of the girls reported that they did at least one hour's vigorous exercise a day. BUA was significantly lower (p<0.004) and VOS significantly higher (p<0.0001) in comparison with reference data. Body weight and BMI at the time of the measurements were significantly lower than the reference data (p<0.0001). There were no correlations between body weight or height and BUA or VOS, but there was a slight correlation between BUA and BMI (r=0.4, p<0.05) and a slight inverse correlation between VOS, body weight and BMI (r=-0.48 and r=-0.43, p<0.01). VOS slightly correlated with weight loss (r=0.4, p<0.05), significantly with the weekly number of exercise hours (r=0.48, p<0.01). The duration of AN, the number of missed cycles and the percentage of weight loss did not correlate with BUA, and neither the duration of AN nor the number of missed cycles correlated with VOS. The low BUA value could be attributed to poor nutrition, and substantial physical activity may lead to the increase in VOS. In conclusion, girls with AN have low BUA and high VOS values, neither of which correlate with the duration of AN or the number of missed cycles.

Adolescent↗