Biexciton Emission Lines Research Articles

Correlation between exciton polarized lifetime and fine structure splitting in InAs/GaAs quantum dots

Semiconductor self-assembled single quantum dots (QDs) provide a promising solid-state light source for single photons and entangled photons. The structural asymmetry in QDs results in fine structure splitting (FSS) of exciton and biexciton emission lines. Here, we propose a method to study QD symmetry by measuring the difference in two different polarized lifetimes of QD excitonic emission lines under applied stress. The method can be reasonably correlated with the direct FSS measurement. Actually, due to the limitation of the resolution of the spectrometer, the zero value of FSS is difficult to measure. Instead, the lifetime measurement is an effective method to judge the symmetry condition of single QDs.

Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals

Biexciton emission from individual silicon nanocrystals was detected at room temperature by time-resolved, single-particle luminescence measurements. The efficiency of this process, however, was found to be very low, about 10–20 times less than the single exciton emission efficiency. It decreases even further at low temperature, explaining the lack of biexciton emission line observations in silicon nanocrystal single-dot spectroscopy under high excitation. The poor efficiency of the biexciton emission is attributed to the dominant nonradiative Auger process. Corresponding measured biexciton decay times then represent Auger lifetimes, and the values obtained here, from tens to hundreds of nanoseconds, reveal strong dot-to-dot variations, while the range compares well with recent calculations taking into account the resonant nature of the Auger process in semiconductor nanocrystals.

Cascaded emission of linearly polarized single photons from positioned InP/GaInP quantum dots

We report on the optical characterization of site-controlled InP/GaInP quantum dots (QDs). Spatially resolved low temperature cathodoluminescence proves the long-range ordering of the buried emitters, revealing a yield of ∼90% of optically active, positioned QDs and a strong suppression of emitters on interstitial positions. The emission of single QDs shows a pronounced degree of linear polarization along the [0,−1,1] crystal axis with an average degree of polarization of 94%. Photon correlation measurements of the emission from a single QD indicate the single-photon character of the exciton and biexciton emission lines as well as the cascaded nature of the photon pair.

Towards Bose-Einstein Condensation of Semiconductor Excitons: The Biexciton Polarization Effect

We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells.

Engineering photon cascades from multiexciton complexes in a self-assembled quantum dot by a lateral electric field

We demonstrate theoretically that by applying an in-plane electric field it is possible to engineer and tune photon cascades originating from recombination of multiexciton complexes confined in self-assembled quantum dots. We find the multiexciton energies and states as functions of the field using the effective-mass configuration-interaction approach with the effects of electron-hole exchange treated perturbatively, and use Fermi's golden rule to compute the emission spectra. We show that the field-induced Stark shift of the energies of photons emitted by the biexciton and two linearly polarized excitons is strongly renormalized by the electron-hole interactions, which are governed by the separation of electrons and holes induced by the field. As a result, the effective Stark shifts of exciton and biexciton emission lines have opposite signs, leading to a removal of the biexciton binding energy at a finite field. This enables the cascade of a pair of polarization entangled photons in the presence of a finite exciton anisotropic exchange splitting. We compare these emission spectra to those of the charged exciton, the triplet biexciton, and the three-exciton complex. We find that the electric field and Coulomb interactions differentiate the biexciton-exciton cascade from other cascades, facilitating identification of its spectra and practical implementation.

Microphotoluminescence of exciton and biexciton around 1.5μm from a single InAs∕InP(001) quantum dot

We report on the fabrication by low-pressure metalorganic vapor phase epitaxy of InAs∕InP(001) quantum dots (QDs) emitting around 1.5μm, and on the observation of microphotoluminescence (μPL) from a single QD in this wavelength range. The QDs are diamond shaped, with a density of 6.3∙109cm−2. μPL experiments were carried out on a QD sample covered with a gold mask containing apertures (diameter of 200nm). Well defined peaks corresponding to the emission of single QDs were recorded between 1.24 and 1.6μm. The analysis of the pump power dependence of their intensity allows us to discriminate between exciton and biexciton emission lines.

Phonon sidebands in exciton and biexciton emission from single GaAs quantum dots

We report on the observation of asymmetric phonon sidebands on both the exciton and biexciton emission lines in single GaAs monolayer fluctuation quantum dots. The contribution of phonon sidebands to the emission line is larger for the biexciton than for the exciton. We model the exciton line shape by means of a nonperturbative coupling with acoustic phonons and show that energetic confinement is an important clue to understand why phonon sidebands are sometimes observed and sometimes not. Finally, we discuss the extension of our model to the biexciton case.

Self‐assembled quantum dots as a source of single photons and photon pairs

AbstractWe investigate the emission from a single self‐organised InAs/GaAs quantum dots as a potential single or pair photon source. Single photon emission is stimulated by exciting the dot with ps laser pulses and collecting either the exciton or biexciton emission line. A more convenient and practical arrangement is to excite the quantum dot electrically by growing it inside a p–i–n structure. Photon pairs are generated through collecting both the exciton and biexciton emissions. We show a strong correlation in the emission times of the exciton and biexciton photon from the dot, as well as in their linear polarisations.

Optical and theoretical investigations of small InP quantum dots inGaxIn1−xP

We have studied small InP quantum dots in a GaInP matrix theoretically and experimentally. Using low-temperature photoluminescence spectroscopy in conjunction with six band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations, including direct and exchange interactions, we show that the dot size is a crucial parameter that determines whether the dot is neutral or charged with electrons in the nominally undoped n-type host material. For a small enough quantum dot, the conduction-band ground state is positioned above the Fermi level and the dot remains neutral. However, as soon as the dot is large enough for the conduction-band ground state to be located below the Fermi level the dot is charged. Furthermore, we show that, for neutral quantum dots, the position of the bi-exciton emission line with respect to the exciton emission line depends on the size of the quantum dot and that the bi-exciton emission can be on either side of the exciton emission: for the smallest dots the bi-exciton emission is always at higher energy than the exciton emission but for larger dots the ordering is the opposite with the exciton emission line on the high-energy side.

Luminescence of excitonic molecules in AgBr

Luminescence spectra of AgBr at the temperature range 5–50 K and under pulse excitation intensity up to ∼ 0.7 MW cm −2 are investigated. Intensity and temperature dependences of biexciton emission line (2.67 eV) yield thermodynamic determination of biexciton binding energy in agreement with the spectroscopic value (6.7 meV). The behaviour of biexciton emission in the investigated temperature range is explained by the concept of “biexciton pocket”. Some questions of the electron-hole liquid recombination radiation (2.60 eV) are discussed as well.