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An automated multi-pumping flow system is proposed for the chemiluminometric determination of ascorbic acid in pharmaceutical formulations, relying on the ability of semiconductor nanocrystals to generate short-lived reactive species upon photo-irradiation. A photo-unit based on visible-light-emitting diodes is used to photo-excite cadmium telluride (CdTe) quantum dots capped with glutathione, leading to the generation of radicals that react with luminol under alkaline conditions, yielding the chemiluminescence. Ascorbic acid acts as a radical scavenger, preventing the oxidation of luminol, thus ensuring a concentration-dependent chemiluminescence quenching. After system optimization, a linear working range of 5.0 × 10(-7) to 5.0 × 10(-6) mol/L ascorbic acid (r = 0.9967, n = 5) was attained, with a detection limit of 3.05 × 10(-7) mol/L and a sampling rate of 200/h. The flow system was applied to the analysis of pharmaceutical formulations and the results were in good agreement with those obtained by the reference titrimetric procedure (RD < ± 4.3%, n = 7)., (Copyright © 2014 John Wiley & Sons, Ltd.)

We studied InAs quantum dots (QDs) where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. With time-resolved luminescence measurements at the wavelength around 1550 nm, we observed luminescence enhancement and reduction of luminescence decay time constants at temperature below the superconducting critical temperature (TC) of Nb. On the basis of these measurements, we propose a method to determine the contribution of Cooper-pair recombination in InAs QDs. We show that the luminescence enhancement measured below TC is well explained with our theory including Cooper-pair recombination. [ABSTRACT FROM AUTHOR]

An automated multi-pumping flow system is proposed for the chemiluminometric determination of ascorbic acid in pharmaceutical formulations, relying on the ability of semiconductor nanocrystals to generate short-lived reactive species upon photo-irradiation. A photo-unit based on visible-light-emitting diodes is used to photo-excite cadmium telluride (CdTe) quantum dots capped with glutathione, leading to the generation of radicals that react with luminol under alkaline conditions, yielding the chemiluminescence. Ascorbic acid acts as a radical scavenger, preventing the oxidation of luminol, thus ensuring a concentration-dependent chemiluminescence quenching. After system optimization, a linear working range of 5.0 × 10-7 to 5.0 × 10-6 mol/L ascorbic acid (r = 0.9967, n = 5) was attained, with a detection limit of 3.05 × 10-7 mol/L and a sampling rate of 200/h. The flow system was applied to the analysis of pharmaceutical formulations and the results were in good agreement with those obtained by the reference titrimetric procedure (RD< ± 4.3%, n = 7). [ABSTRACT FROM AUTHOR]

The extremely long dephasing time of excitons in strain-compensated quantum dots at telecommunications wavelengths was measured using a polarization-dependent four-wave mixing technique. The use of a 150-layer-stacked structure enabled them to measure a four-wave mixing signal with a high signal-to-noise ratio, in spite of the fact that a high-sensitive heterodyne detection was not used. The large anisotropy of the dephasing time indicates the dominance of the radiative recombination process on dephasing. By simultaneously measuring the radiative lifetime using a pump-probe technique, they could directly estimate pure dephasing with an accuracy of better than 0.1 μeV. [ABSTRACT FROM AUTHOR]

Excitonic dephasing is investigated in InAs self-assembled quantum dots fabricated by the strain-compensation technique. The exciton ground-state emission is centered at the wavelength of 1420 nm at 5 K. Transient four-wave mixing measurements under resonant excitation clearly demonstrate a long dephasing time of 1.09 ns at 5 K, corresponding to the homogeneous broadening of 1.2 μeV. The extrapolated zero-temperature homogeneous broadening is limited only by the population lifetime of the exciton ground state. At slightly increased temperatures, the acoustic-phonon broadening becomes dominant on dephasing. [ABSTRACT FROM AUTHOR]

We report the generation of undamped exciton-biexciton beats for coherent dynamics of excitons in InAs quantum dots at telecommunication wavelengths. The exciton-biexciton beat is damped rapidly in four-wave mixing measurements, but a six-wave mixing technique with three excitation pulses enables us to regenerate and control the exciton-biexciton beat perfectly. We have demonstrated the complete regeneration of the exciton-biexciton beat by adjusting the delay time between the first and second excitation pulses. This approach is different from conventional coherent control in quantum dots. Furthermore, the initial phase of the exciton-biexciton beat is controlled by using this technique. We show that the generation of undamped beats is a powerful tool for controlling quantum states. [ABSTRACT FROM AUTHOR]

We report the optical anisotropy of excitons in self-assembled InAs quantum dots (QDs), the strain of which is controlled by tuning the lattice constant of the spacer layers between stacked QD layers. The strain dependence of the energy structure for excitons is investigated using the four-wave mixing technique. The fine-structure beat and the exciton-biexciton beat in four-wave mixing measurements clearly show that the fine-structure splitting of exciton states increases with increasing strain in the growth plane. The strain also causes significant optical anisotropy, which is observed by the angular-dependent measurements of the collinear polarization of excitation pulses. The strain dependence of the optical anisotropy is accurately reproduced by a model calculation in which the valence band mixing between heavy- and light-hole bands is taken into account. We demonstrate that the optical properties of excitons in QD ensembles are successfully controlled by tuning the lattice constant of the spacer layers. [ABSTRACT FROM AUTHOR]

Abstract: We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree–Fock-type Coulomb interactions between excitons. [Copyright &y& Elsevier]

Abstract: We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump–probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2ns for orthogonally polarized exciton ground states. [Copyright &y& Elsevier]

We study luminescence spectra observed from InAs quantum dots (QDs) embedded in an n-type InGaAs-based heterostructure, where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. Below the superconducting (SC) critical temperature of Nb, we observe substantial luminescence intensity enhancement and a sharp edge in luminescence spectra of InAs QDs. We explain the observed sharp edge in the luminescence spectra with the proximity effect, that is, with the consideration of opening of SC gap and modification of density of states (DOS) near the electron Fermi level in the n-type semiconductor heterostructure. We demonstrate that the sharp edge luminescence spectra are well reproduced by the SC DOS, with quasiparticle lifetime broadening and a Gaussian distribution of lowest QD state emission lines. We discuss the reason why it has been difficult to observe the sharp edge luminescence spectra in the previous quantum well-based SC light emitting diodes. [ABSTRACT FROM AUTHOR]