Carrier relaxation mechanisms in self-assembled (In,Ga)As/GaAs quantum dots: Efficient P -> S Auger relaxation of electrons

We calculate the P-shell--to-S-shell decay lifetime \tau(P->S) of electrons in lens-shaped self-assembled (In,Ga)As/GaAs dots due to Auger electron-hole scattering within an atomistic pseudopotential-based approach. We find that this relaxation mechanism leads to fast decay of \tau(P->S)~1-7 ps for dots of different sizes. Our calculated Auger-type P-shell--to-S-shell decay lifetimes \tau(P->S) compare well to data in (In,Ga)As/GaAs dots, showing that as long as holes are present there is no need for an alternative polaron mechanism.Comment: Version published in Phys. Rev.

Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay.Comment: 10 pages, 4 figures, submitted to Phys Rev Let

Controlling electronic and adiabatic isolation of quantum dots from the substrate: An ionization-energy theoretic study

Recent controversy on the quantum dots dephasing mechanisms (between pure and inelastic) is re-examined by isolating the quantum dots from their substrate by using the appropriate limits of the ionization energy theory and the quantum adiabatic theorem. When the phonons in the quantum dots are isolated adiabatically from the phonons in the substrate, the elastic or pure dephasing becomes the dominant mechanism. On the other hand, for the case where the phonons from the substrate are non-adiabatically coupled to the quantum dots, the inelastic dephasing process takes over. This switch-over is due to different elemental composition in quantum dots as compared to its substrate. We also provide unambiguous analyses as to understand why GaAs/AlGaAs quantum dots may only have pure dephasing while InAs/GaAs quantum dots give rise to the inelastic dephasing as the dominant mechanism. Our study accentuates the importance of the elemental composition (of both quantum dots and substrate) in evaluating the dephasing mechanisms of quantum dots.Comment: 11 pages, Rewritten with simplified explanation

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano3 software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV

Microscopic Aspects of Stretched Exponential Relaxation (SER) in Homogeneous Molecular and Network Glasses and Polymers

Because the theory of SER is still a work in progress, the phenomenon itself can be said to be the oldest unsolved problem in science, as it started with Kohlrausch in 1847. Many electrical and optical phenomena exhibit SER with probe relaxation I(t) ~ exp[-(t/{\tau}){\beta}], with 0 < {\beta} < 1. Here {\tau} is a material-sensitive parameter, useful for discussing chemical trends. The "shape" parameter {\beta} is dimensionless and plays the role of a non-equilibrium scaling exponent; its value, especially in glasses, is both practically useful and theoretically significant. The mathematical complexity of SER is such that rigorous derivations of this peculiar function were not achieved until the 1970's. The focus of much of the 1970's pioneering work was spatial relaxation of electronic charge, but SER is a universal phenomenon, and today atomic and molecular relaxation of glasses and deeply supercooled liquids provide the most reliable data. As the data base grew, the need for a quantitative theory increased; this need was finally met by the diffusion-to-traps topological model, which yields a remarkably simple expression for the shape parameter {\beta}, given by d*/(d* + 2). At first sight this expression appears to be identical to d/(d + 2), where d is the actual spatial dimensionality, as originally derived. The original model, however, failed to explain much of the data base. Here the theme of earlier reviews, based on the observation that in the presence of short-range forces only d* = d = 3 is the actual spatial dimensionality, while for mixed short- and long-range forces, d* = fd = d/2, is applied to four new spectacular examples, where it turns out that SER is useful not only for purposes of quality control, but also for defining what is meant by a glass in novel contexts. (Please see full abstract in main text

Design strategy for terahertz quantum dot cascade lasers

The development of quantum dot cascade lasers has been proposed as a path to obtain terahertz semiconductor lasers that operate at room temperature. The expected benefit is due to the suppression of nonradiative electron-phonon scattering and reduced dephasing that accompanies discretization of the electronic energy spectrum. We present numerical modeling which predicts that simple scaling of conventional quantum well based designs to the quantum dot regime will likely fail due to electrical instability associated with high-field domain formation. A design strategy adapted for terahertz quantum dot cascade lasers is presented which avoids these problems. Counterintuitively, this involves the resonant depopulation of the laser's upper state with the LO-phonon energy. The strategy is tested theoretically using a density matrix model of transport and gain, which predicts sufficient gain for lasing at stable operating points. Finally, the effect of quantum dot size inhomogeneity on the optical lineshape is explored, suggesting that the design concept is robust to a moderate amount of statistical variation

Non-square-well potential profile and non-blinking effect in graded CdZnSe/ZnSe nanocrystals: An ionization-energy theoretic study

Randomly blinking nanocrystals have given rise to numerous and intense theoretical and experimental investigations recently. An experimental breakthrough was finally made by fabricating the non-blinking Cd$_{1-x}$Zn$_x$Se/ZnSe graded nanocrystal [Nature 459 (2009) 686]. Here, we report (1) an unequivocal and detailed theoretical investigation to understand the properties of the potential-well and the distribution of Zn content with respect to the nanocrystal radius and (2) develop a strategy to find the relationship between the photoluminescence (PL) energy peaks and the potential-well due to Zn distribution in nanocrystals. We found that the non-square-well potential can be varied in such a way that one can indeed control the PL intensity and the energy-level difference (PL energy peaks) in any non-free-electron nanocrystals accurately. This implies that we can either suppress the blinking altogether, or alternatively, manipulate the PL energy peaks and intensities systematically to give non-random blinking. Our theoretical strategy developed here is generic and can be applied to any non-free-electron nanocrystals.Comment: Published online in Nanoscale (2010). Different from published versio

High Purcell factor generation of indistinguishable on-chip single photons

On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission, but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is to use the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7 ps) in a waveguide-coupled quantum dot–photonic crystal cavity system. This leads to near-lifetime-limited single-photon emission that retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under π-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates

Intermediate-band dynamics of quantum dots solar cell in concentrator photovoltaic modules

We report for the first time a successful fabrication and operation of an InAs/GaAs quantum dot based intermediate band solar cell concentrator photovoltaic (QD-IBSC-CPV) module to the IEC62108 standard with recorded power conversion efficiency of 15.3%. Combining the measured experimental results at Underwriters Laboratory (ULH) licensed testing laboratory with theoretical simulations, we confirmed that the operational characteristics of the QD-IBSC-CPV module are a consequence of the carrier dynamics via the intermediate-band at room temperature