Quantum Dot Nanostructures Research Articles

Phase control of optical bistability in an InGaN∕GaN quantum dot nanostructure

In this paper, we propose a model for controlling the optical bistability in four-level [Formula: see text] quantum dot nanostructure which is embedded in a unidirectional ring cavity. [Formula: see text] quantum dot nanostructure is based on our recent paper [S. H. Asadpour, Z. Golsanamlou and H. R. Soleimani, Physica E 54 (2013) 45]. It is found that intensity threshold of optical bistability can be manipulated by signal intensity of applied fields. Moreover, we find that phase variation of terahertz signal field can also affect the behaviors of optical bistability and hysteresis loop.

Influence of incoherent pumping field on spatial evolution of gain without inversion in a four-level quantum dot nanostructure

We investigated the propagation effect on gain without inversion (GWI) phenomena in an open four level quantum dot nanostructure in the presence and absence of incoherent pumping field. The simulation results show that, the ratio of the injection rates and strength of incoherent pumping field has remarkable effect on spatial evolution of GWI and output. We can obtain the optimal GWI and output by choosing appropriate values of parameters. The theoretical results show that, in the open system the value of gain (output) in the absence of incoherent pumping field is much larger than that in the presence of incoherent pumping field.

Effect of position-dependent effective mass on optical properties of spherical nanostructures

The effects of position-dependent effective mass on the absorption coefficients (ACs) of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. We show that the radial position-dependent effective mass Schrodinger equation can be reduced into ordinary one-dimensional Schrodinger equation with some suitable transformations. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. The comparative approach is used for presenting the results of both QD and QAD models. It is found that not only for the effective constant mass but also for the position-dependent effective mass, the ACs curves corresponding to QD and QAD are considerably different in shapes and behaviors. Our obtained results show that the spatial variation of the electron effective mass can not be neglected in the optical ACs of both QD and QAD nanostructures.

Raman scattering study on Sb spray InAs/GaAs quantum dot nanostructure systems.

The effect of Sb spray time on the structure of InAs/GaAs quantum dot (QD) systems with Sb spray prior to the capping of a GaAs layer was determined by a Raman scattering study. The Raman spectra of the InAs/GaAs system show two phonon signal bands related to interface (IF) defects, located at the low-energy side of InAs QDs and GaAs cap layer main phonon peaks, respectively. The intensity ratio of the IF defect relative phonon signal to its corresponding main peak shows a significant decrease with the Sb spray time increasing from 0 to 15 s, but increases for spray times larger than 15 s. In addition, the InAs QD phonon peaks appear to be resolved with improved symmetry for 15 s of spray time. Finally, the GaAs transverse optical (TO) phonon peak is seen to vary with Sb spray time, both in terms of the intensity and the peak position, in a similar manner to the other results. Taken together, these results suggest the InAs/GaAs QDs with a 15-s Sb spray lead to a GaAs capping layer with less strain at the IF with the QDs and a lower density of crystalline defects.PACS: 81.05.Ea; 81.07.-b; 81.07.Ta

Electronic and optical properties of exciton, trions and biexciton in II–VI parabolic quantum dot

Binding energies of exciton, trions and biexciton and their interband optical transition energies are studied in a CdTe/ZnTe quantum dot nanostructure taking into consideration the geometrical confinement effect. The radial spread of the wavefunctions, binding energies, optical transition energies, oscillator strength, radiative life time and the absorption coefficients of exciton, positively and negatively charged excitons and biexciton are carried out. It is found that the ratio of the radiative life time of exciton with the trions and biexciton enhances with the reduction of geometrical confinement. The results show that (i) the binding energies of exciton, positive and negative trions and the biexciton have strong influence on the reduction of geometrical confinement effect, (ii) the binding energy is found to decrease from the binding energies of exciton to positive trion through biexciton and negative trion binding energies, (iii) the oscillator strength of trions is found to be lesser than exciton and the biexciton and (iv) the electronic and optical properties of exciton, trions and the biexciton are considerably dependent on the spatial confinement, incident photon energy and the radiative life time. The obtained results are in good agreement with the other existing literature.

Numerical subgap spectroscopy of double quantum dots coupled to superconductors

Double quantum dot nanostructures embedded between two superconducting leads or in a superconducting ring have complex excitation spectra inside the gap which reveal the competition between different many-body phenomena. We study the corresponding two-impurity Anderson model using the non-perturbative numerical renormalization group (NRG) technique and identify the characteristic features in the spectral function in various parameter regimes. At half-filling, the system always has a singlet ground state. For large hybridization, we observe an inversion of excited inter-dot triplet and singlet states due to the level-repulsion between two sub-gap singlet states. The Shiba doublet states split in two cases: a) at non-zero superconducting phase difference and b) away from half-filling. The most complex structure of sub-gap states is found when one or both dots are in the valence fluctuation regime. Doublet splitting can lead to a parity-changing quantum phase transition to a doublet ground state in some circumstances. In such cases, we observe very different spectral weights for the transitions to singlet or triplet excited Shiba states: the triplet state is best visible on the valence-fluctuating dot, while the singlets are more pronounced on the half-filled dot.

Optical properties of type-I PbSe/CdSe core/shell quantum dot

Electronic properties and optical properties of exciton in a PbSe/CdSe core/shell quantum dot are investigated taking into account the spatial confinement effect. The present model is based on the PbSe/CdSe quantum dot for type-I confinement regime. The dielectric mismatch effect and the self polarization potential are taken into consideration in the PbSe/CdSe quantum dot nanostructure. Polarization charges are incorporated at the interface of the core/shell materials. Numerical calculations on the electronic and optical properties are found with the ratio of radius of inner and outer shell materials for various shell radii. The exciton binding energy and the interband optical transition energies are computed using variational formulism within the single band effective mass approximation. The oscillator strength and the recombination life time are determined with the ratio of radius of core to shell materials taking into account the dielectric mismatch between the materials. The nonlinear absorption coefficients and the changes of refractive index are computed for the ground and first excited state using compact density matrix method. The obtained results are found to be in good agreement with those reported by other investigators.

Investigation of optical bistability in a double In(x)Ga(1-x)N/GaN quantum-dot nanostructure via inter-dot tunneling effect.

In this paper, our aim is to control optical bistability (OB) and optical multistability (OM) in a five-level system designed in a double quantum dot (QD) nanostructure. In a realistic example, this atomic system is created in two semiconductor QDs (In(x)Ga(1-x)N/GaN), owning transfer of carriers via tunneling effect. OB behavior is controlled not only by the inter-dot tunneling effect but also by variation of probe detuning and intensity of the control field. It is demonstrated that voltage-controlled detuning can significantly affect the behavior of OB and OM; therefore, the OM converts to OB by probe detuning and intensity of the control field.

Preparation of chiral quantum dots.

Chiral quantum dots (QDs) are expected to have a range of potential applications in photocatalysis, as specific antibacterial and cytotoxic drug-delivery agents, in assays, as sensors in asymmetric synthesis and enantioseparation, and as fluorescent chiral nanoprobes in biomedical and analytical technologies. In this protocol, we present procedures for the synthesis of chiral optically active QD nanostructures and their quality control using spectroscopic studies and transmission electron microscopy imaging. We closely examine various synthetic routes for the preparation of chiral CdS, CdSe, CdTe and doped ZnS QDs, as well as of chiral CdS nanotetrapods. Most of these nanomaterials can be produced by a very fast (70 s) microwave-induced heating of the corresponding precursors in the presence of D- or L-chiral stabilizing coating ligands (stabilizers), which are crucial to generating optically active chiral QDs. Alternatively, chiral QDs can also be produced via the conventional hot injection technique, followed by a phase transfer in the presence of an appropriate chiral stabilizer. We demonstrate that the properties, structure and behavior of chiral QD nanostructures, as determined by various spectroscopic techniques, strongly depend on chiral stabilizers and that the chiral effects induced by them can be controlled via synthetic procedures.

Raman, AFM, and TEM profiling of QD multilayer structures

We report on a combined atomic force microscopy, Raman spectroscopy, and transmission electron microscopy study of InAs/Al(Ga)As and AlAs/InAs quantum dot nanostructures grown by molecular beam epitaxy in the Stranski–Krastanov growth mode on (001)-oriented GaAs substrates. Structure periods of 50 nm and 100 nm were determined by atomic force microscopy of cleaved as well as ion-milled sample surfaces and were confirmed by high resolution transmission electron microscopy. Spatially resolved Raman spectroscopy analysis of a beveled surface allows profiling the Raman intensity of phonon modes. An optimal access to the spatially dependent changes in the phonon spectra is achieved by polishing the samples at an angle of 5–7 degrees with respect to the original sample surface plane. Such analysis allowed the localization of individual quantum dot layers. This opens a way to the structural characterization of quantum dot multilayers by the combination of atomic force microscopy and Raman spectroscopy.

Infrared control of slow light in a slab doped by InGaN/GaN quantum dot nanostructure

In this paper, we proposed a model for controlling the group velocity of the transmitted and reflected pulses in a slab medium doped by four-level quantum dot nanostructure. Here, an infrared signal field interacted by quantum dot nanostructure can affect the behavior of reflected and transmitted pulses. We show that in the presence and absence of infrared pulses, the other controllable parameters have essential roles for controlling the slow and fast light propagation through the medium. Moreover, we found that the simultaneous slow and fast light can be obtained for the transmitted and reflected pulses by infrared signal field. Our proposed model may be useful for ultrahigh density optical memories in quantum communication systems or in various fields of all-optical systems.

Diffusion dynamics and concentration of toxic materials from quantum dots-based nanotechnologies: an agent-based modeling simulation framework

Due to their favorable electrical and optical properties, quantum dots (QDs) nanostructures have found numerous applications including nanomedicine and photovoltaic cells. However, increased future production, use, and disposal of engineered QD products also raise concerns about their potential environmental impacts. The objective of this work is to establish a modeling framework for predicting the diffusion dynamics and concentration of toxic materials released from Trioctylphosphine oxide-capped CdSe. To this end, an agent-based model simulation with reaction kinetics and Brownian motion dynamics was developed. Reaction kinetics is used to model the stability of surface capping agent particularly due to oxidation process. The diffusion of toxic Cd2+ ions in aquatic environment was simulated using an adapted Brownian motion algorithm. A calibrated parameter to reflect sensitivity to reaction rate is proposed. The model output demonstrates the stochastic spatial distribution of toxic Cd2+ ions under different values of proxy environmental factor parameters. With the only chemistry considered was oxidation, the simulation was able to replicate Cd2+ ion release from Thiol-capped QDs in aerated water. The agent-based method is the first to be developed in the QDs application domain. It adds both simplicity of the solubility and rate of release of Cd2+ ions and complexity of tracking of individual atoms of Cd at the same time.

Phase Control of Transient Optical Properties of Quantum-Dot Nanostructure via Terahertz Signal Radiation

We theoretically analyze the transient properties of a probe field absorption dispersion in a quantum-dot nanostructure driven by terahertz signal radiation. Here a four-level quantum-dot nanostructure is designed numerically by Schrödinger and Poisson's equations. We show that in the presence of the THz signal, the medium becomes phase dependent and the absorption dispersion can be dramatically influenced by the relative phase between applied fields.

Optical studies of an exciton and a biexciton in a CdTe/ZnTe quantum dot nanostructure

Electronic and some optical properties of exciton and biexciton in a CdTe/ZnTe quantum dot nanostructure are dealt taking into consideration of geometrical confinement effect. The binding energies, optical transition energies, recombination of oscillator strength, radiative life time and the absorption coefficients are carried out. Numerical calculations are performed using Hartree potential in order to obtain the Coulomb interaction energy and self-consistent technique is used to calculate the energy eigenvalue and wave functions of exciton and the biexciton. The electron and hole potentials are calculated from the Poisson equation. The dielectric mismatch on the electronic and optical properties is taken into account throughout the calculations. The linear and the nonlinear optical absorption coefficients for the heavy hole exciton and the biexciton as a function of incident photon energy for CdTe/ZnTe quantum dot are discussed. The results show that the electronic and optical properties strongly depend on the incident photon energy, recombination life time and the geometrical confinement.

Polarization-entangled photon generation in a semiconductor quantum dot coupled to a cavity interacting with external fields

We theoretically investigate polarization-entangled photon generation by using a semiconductor quantum dot embedded in a microcavity. The entangled states can be produced by the application of two cross-circularly polarized laser fields. The quantum dot nanostructure is considered as a four-level system (ground, two excitons and bi-exciton states) and the theoretical study relies on the dressed states scheme. The quantum correlations, reported in terms of the entanglement of formation, are extensively studied for several values of the important parameters of the quantum dot system as the bi-exciton binding energy, {the decoherence times of the characteristic transitions, the quality factor of the cavity} and the intensities of the applied fields.

Controllable optical bistability in triple quantum dot nanostructure via double tunnel coupling

The behavior of optical bistability in triple quantum dot nanostructure using double tunnel coupling inside a unidirectional ring cavity is investigated. Also, the linear and nonlinear absorption of the system are investigated. The double tunneling between the quantum dots can change the absorption of the system. The threshold of OB can be controlled by the intensity of the double tunneling and the detuning of probe field. This may be employed for the development of new types of nanoelectronic devices for realizing switching process.

Technical Tasks and Development Current Status of Organic Solar Cells

Serious environmental problems have been caused by the greenhouse effect due to carbon dioxide(CO2) or nitrogen oxides(NOx) generated by the use of fossil fuels, including oil and liquefied natural gas. Many countries, including our own, the United States, those of the European Union and other developed countries around the world; have shown growing interest in clean energy, and have been concentrating on the development of new energy-saving materials and devices. Typical non-fossil-fuel sources include solar cells, wind power, tidal power, nuclear power, and fuel cells. In particular, organic solar cells(OSCs) have relatively low power-conversion efficiency(PCE) in comparison with inorganic(silicon) based solar cells, compound semiconductor solar cells and the CIGS [Cu(In1-xGax)Se2] thin film solar cells. Recently, organic cell efficiencies greater than 10 % have been obtained by means of the development of new organic semiconducting materials, which feature improvements in crystalline properties, as well as in the quantum-dot nano-structure of the active layers. In this paper, a brief overview of solar cells in general is presented. In particular, the current development status of the next-generation OSCs including their operation principle, device-manufacturing processes, and improvements in the PCE are described.

Controlling the optical bistability and multistability via tunneling-induced and incoherent pumping field in a triple coupled quantum dots at a wavelength of λ = 1.550 μm

The behavior of the optical bistability (OB), optical multistability (OM) and transition between them at a wavelength of λ=1.550μm, are investigate in a triple coupled quantum dots nanostructure with two consecutive tunneling-induced and an incoherent pumping field. It is found that OB, OM and transition between them can be accomplished by adjusting different values of intensity tunneling-induced and incoherent pumping field. We observed that with increasing intensity tunneling-induced and incoherent pumping field and probe wavelength detuning λ0 the bistable hysteresis loop becomes narrower which makes the cavity field easier to reach saturation. Switching OB and OM and vice versa is calculated by discussing the dependency of optical properties on the incoherent pumping and inter-dot tunneling rates between QD1, QD2 and QD2, QD3. This system with wavelength 1.550μm, can be useful for provides a new experimental method for the development of new types of nano-optoelectronic devices for the realizing switching process.

Impact of the incoherent pumping on the optical properties of a dielectric slab doped with quantum dot nanostructure

In this paper, propagation of an electromagnetic pulse through a dielectric slab doped with a quantum dot nanostructure via electron tunneling has been discussed; moreover the effect of incoherent pump field on the transmission and reflection of pulse in a quantum dot has been theoretically investigated. It is shown that with applying these parameters, the proposed model can be controlled as an all-optical filter which is of great interest in optical communication and quantum information processing.

Complex optical signatures from quantum dot nanostructures and behavior in inverted pyramidal recesses

A study of previously overlooked structural and optical properties of InGaAs heterostructures grown on (111)$B$ oriented GaAs substrates patterned with inverted 7.5-\ensuremath{\mu}m pitch pyramidal recesses is presented. First, the composition of the confinement barrier material (GaAs in this work) and its growth temperature are shown as some of the key parameters that determine the main quantum dot properties, including nontrivial emission energy dependence, excitonic pattern, and unusual photoluminescence energetic ordering of the InGaAs ensemble nanostructures. Second, the formation of a formerly unidentified type of InGaAs nanostructures---three corner quantum dots---is demonstrated in our structures next to the well-known ones (a quantum dot and three lateral quantum wires and quantum wells). The findings show the complexity of the pyramidal quantum dot system which strongly depends on the sample design and which should be considered when selecting highly symmetric (central) quantum dots in newly designed experimental projects.