Quantum Dot Nanostructures Research Articles

Optical bistability in a single-sided cavity coupled to a quantum channel

In this paper, we discuss the long wavelength optical reflection and bistable behavior of an InGaN/GaN quantum dot nanostructure coupled to a single-sided cavity. It is found that due to the presence of a strong coupling field, the reflection coefficient can be controlled at long wavelength, which is essential for adjusting the threshold of reflected optical bistability. Moreover, the phase shift features of the reflection pulse inside an electromagnetically induced transparency window are also discussed.

Absorption and dispersion management of near-infrared probe light in the carbon nanotube quantum dot nanostructure with spin–orbit coupling

We investigated the absorption–dispersion properties of near-infrared probe light in the carbon nanotube quantum dot (CNT-QD) nanostructure with spin–orbit (SO) coupling parameter based on quantum mechanical density matrix formalisms. We found that in the presence of SO parameter, the amplification of probe light can be achieved in the presence and lack of population inversion (lasing with and without inversion). Moreover, we realized that the normal and anomalous dispersion of probe light can occur by redacting the incoherent pumping and transverse magnetic fields.

Optical multistability in 1D photonic crystal doped with carbon-nanotube quantum dot nanostructures

The optical bistability (OB) and multistability (OM) in one-dimensional photonic crystals doped with carbon-nanotube quantum dots (CNT QDs) is theoretically investigated. Our investigations show that due to spin–orbit coupling in CNT QDs the threshold of optical bistability can be adjusted. Also, the switching from OB to OM, or vice versa, can be achieved by controlling the transverse magnetic field. Our proposed model can be used as a suitable structure based on CNT QDs for realizing all-optical switching devices. Moreover, we realize that the controlling of OB and OM can be done without changing the thickness of the layers.

Electrochemical quantification of some water soluble vitamins in commercial multi-vitamin using poly-amino acid caped by graphene quantum dots nanocomposite as dual signal amplification elements

Rapid analyses of some water soluble vitamins (Vitamin B2, B9, and C) in commercial multi vitamins could be routinely performed in analytical laboratories. This study reports on the electropolymerization of a low toxic and biocompatible polymer “poly aspartic acid-graphene quantum dots” as a novel strategy for surface modification of glassy carbon electrode and preparation a new interface for measurement of selected vitamins in commercial multi vitamins. Electrochemical deposition, as a well-controlled synthesis procedure, has been used for subsequently layer-by-layer preparation of graphene quantum dots nanostructures on a poly aspartic acid using cyclic voltammetry techniques in the regime of −1.5 to 2 V. The field emission scanning electron microscopy indicated immobilization of graphene quantum dots onto poly aspartic acid film. The modified electrode possessed as an effective electroactivity for detection of water soluble vitamins by using cyclic voltammetry, chronoamperometry and differential pulse voltammetry. Enhancement of peak currents is ascribed to the fast heterogeneous electron transfer kinetics that arise from the synergistic coupling between the excellent properties of poly aspartic acid as semiconducting polymer, graphene quantum dots as high density of edge plane sites and chemical modification. Under the optimized analysis conditions, the prepared sensor for detection of VB2, VB9, and VC showed a low limit of quantification 0.22, 0.1, 0.1 μM, respectively.

Utilizing electromagnetically induced transparency in InAs quantum dots for all-optical transistor design.

We propose a multilayer medium with semiconductor quantum dot nano-structures as a defect layer for all-optical control of the 1.55μm probe beam. The effect of the coupling field and incoherent pump on absorption and dispersion properties of the quantum dot defect layer is investigated. Depending on the intensity of the coupling field and rate of the incoherent pump field, the possibility of the absorption cancellation and even amplification are demonstrated. The optimum values of the coupling field intensity and incoherent pump field for complete transmission or amplification are obtained. The dynamical behavior of the structure is investigated, and the estimated switching time scales are about tens of picoseconds.

Theoretical Investigation of Light Transmission in a Slab Cavity via Kerr Nonlinearity of Carbon Nanotube Quantum Dot Nanostructure

In this paper, we discuss the transmission properties of weak probe laser field propagate through slab cavity with defect layer of carbon-nanotube quantum dot (CNT-QD) nanostructure. We show that due to spin-orbit coupling, the double electromagnetically induced transparency (EIT) windows appear and the giant Kerr nonlinearity of the intracavity medium can lead to manipulating of transmission coefficient of weak probe light. The thickness effect of defect layer medium has also been analyzed on transmission properties of probe laser field. Our proposed model may be useful for integrated photonics devices based on CNT-QD for applications in all-optical systems which require multiple EIT effect.

Type-I to Type-II Transformation of Hybrid Quantum Nanostructures

Hybrid nanostructures of type-I InAs quantum dots (QDs) and type-II GaSb quantum rings (QRs) separated by a GaAs spacer layer is experimentally demonstrated in this paper. As the thickness of GaAs spacer layer decreases, the dot density increases with the reduced dot height, indicating that the smaller InAs QDs tend to grow atop and along the rims of GaSb QRs. With a thin GaAs spacer of 2 nm, the photoluminescence of hybrid nanostructures at room temperature is redshifted to 1.4 μm and remains bright at room temperature. This phenomenon suggests that the electrons in InAs QDs and holes in GaSb QRs recombine radiatively with high quantum efficiencies as the two nanostructures are nearby. The longer carrier lifetime is also observed from the hybrid nanostructures which keep electrons and holes farther, confirming the nature of type-II transitions in these hybrid nanostructures.

Controlling Goos–Hänchen shift in a negative-refractive-index quantum-dot nanostructure

The Goos–Hänchen (GH) shifts due to a negative refractive index in double quantum-dot nanostructures are discussed in reflected and transmitted light beams. It is realized that positive and negative GH shifts can be attained in the negative-refraction medium due to the presence of the indirect incoherent pumping field and electron tunneling. We found that the lateral shift at the fixed incident angle can be enhanced (positive or negative) under suitable conditions on the control field, without changing the structure of the cavity. We hope that our proposed configuration may be helpful for future all-optical sensor devices based on quantum-dot nanostructures.

Novel aspects of application of cadmium telluride quantum dots nanostructures in radiation oncology

In the last two decades, quantum dots nanomaterials have garnered a great deal of scientific interest because of their unique properties. Quantum dots (QDs) are inorganic fluorescent nanocrystals in the size range between 1 and 20 nm. Due to their structural properties, they possess distinctive properties and behave in different way from crystals in macro scale, in many branches of human life. Cadmium telluride quantum dots (CdTe QDs) were labeled with 68Ga radio nuclide for fast in vivo targeting and coincidence imaging of tumors. Using instant paper chromatography, the physicochemical properties of the Cadmium telluride quantum dots labeled with 68Ga NPs (68Ga@ CdTe QDs) were found high enough stable in organic phases, e.g., a human serum, to be reliably used in bioapplications. In vivo biodistribution of the 68Ga@ CdTe QDs nanoconposite was investigated in rats bearing fibro sarcoma tumor after various post-injection periods of time. The 68Ga NPs exhibited a rapid as well as high tumor uptake in a very short period of time (less than 10 min), resulting in an efficient tumor targeting/imaging agent. Meantime, the low lipophilicity of the 68Ga NPs caused to their fast excretion throughout the body by kidneys (as also confirmed by the urinary tract). Because of the short half-life of 68Ga radionuclide, the 68Ga@ CdTe QDs with an excellent tumor targeting/imaging and fast washing out from the body can be suggested as one of the most effective and promising nanomaterials in nanotechnology-based cancer diagnosis and therapy.

Excitation-dependent photoluminescence from WS2 nanostructures synthesized via top-down approach

Crystalline and hetero-dimensional nanostructures of WS2 quantum dots and a few layered sheets have been successfully synthesized in a single-step process using liquid exfoliation in deionized water. The X-rays diffraction analysis confirmed the presence of hexagonal as well as tetragonal phase of WS2 in quantum dots. The nanostructures so obtained exhibit strong photoluminescence, which is otherwise difficult to detect (due to low quantum yield) when WS2 is in bulk form. The UV–Vis spectra from these nanostructures display strong excitonic absorptions together with band edge absorption at ≈5.1 eV, sufficiently larger than the indirect band gap (≈1.3 eV) of bulk WS2. The average statistical size distribution of quantum dots was found to be 3–4.5 nm and corresponds to a band gap of 6 and 3.4 eV, respectively, under effective mass approximation. Further, the photoluminescence spectra shifts with the excitation wavelength, a signature of size-dependent photoluminescence emission from these nanostructures. These results suggest that quantum confinement effects together with band gap transformation from indirect to direct, not only shift the band edge in the ultraviolet region but also give rise to enhancement in the photoluminescence emission.

Phase control of Goos-Hänchen shifts in a fixed structure with carbon-nanotube quantum dot nanostructure

In this article we proposed a new model based on carbon-nanotube quantum dot (CNT QD) nanostructure for controlling the Goos-Hanchen (GH) shifts in reflected and transmitted light beams. In our calculation we introduce the spin-orbit coupling parameter in the CNT QD nanostructure. Our results show that owing to the presence of spin-orbit coupling, Rabi frequency of magnetic field, coupling laser field and relative phase between applied fields, the enhanced GH shifts in reflected and transmitted light beams can be achieved. Moreover, we demonstrate that such enhanced GH shifts are simultaneously obtained for both reflected and transmitted light beams. At the end, we discuss the impact of intracavity thickness on the GH shift properties of reflected and transmitted light beams. We hope that our proposed model may be used for future developments based on CNT QD nanostructures.

Dynamical and Steady State Processes of Probe Absorption-Dispersion in a Four-Level Quantum Dot Nanostructure

Dynamical and Steady State Processes of Probe Absorption-Dispersion in a Four-Level Quantum Dot Nanostructure

Electron spin relaxation in a transition-metal dichalcogenide quantum dot

We study the relaxation of a single electron spin in a circular quantum dot in a transition-metal dichalcogenide monolayer defined by electrostatic gating. Transition-metal dichalcogenides provide an interesting and promising arena for quantum dot nano-structures due to the combination of a band gap, spin-valley physics and strong spin–orbit coupling. First we will discuss which bound state solutions in different B-field regimes can be used as the basis for qubits states. We find that at low B-fields combined spin-valley Kramers qubits to be suitable, while at large magnetic fields pure spin or valley qubits can be envisioned. Then we present a discussion of the relaxation of a single electron spin mediated by electron–phonon interaction via various different relaxation channels. In the low B-field regime we consider the spin-valley Kramers qubits and include impurity mediated valley mixing which will arise in disordered quantum dots. Rashba spin–orbit admixture mechanisms allow for relaxation by in-plane phonons either via the deformation potential or by piezoelectric coupling, additionally direct spin-phonon mechanisms involving out-of-plane phonons give rise to relaxation. We find that the relaxation rates scale as B6 for both in-plane phonons coupling via deformation potential and the piezoelectric effect, while relaxation due to the direct spin-phonon coupling scales independant to B-field to lowest order but depends strongly on device mechanical tension. We will also discuss the relaxation mechanisms for pure spin or valley qubits formed in the large B-field regime.

Finite Element Modeling Piezoelectric Materials under Thermal Loads with Application for Quantum Dots

Significant research has been done in the analysis of properties of quantum dots over the previous decade. A 3D finite element model is developed to analyze quantum dots (QD) under static thermal loads. The lattice mismatch between the quantum dot and the piezoelectric matrix is created by different thermal properties of materials at enhanced temperature of this electronic structure. The fully coupled thermo-piezoelectricity is applied to the analysis of the problem. Commercial FEM software ANSYS was used for analysis. Finite element numerical results are given for the dot with a cubic shape. Numerical results for the InAs/GaAs QD nanostructure show that the elastic and electric fields are strongly influenced by the differences between the material properties of the piezoelectric QD and matrix.

Goos-Hänchen shifts due to spin-orbit coupling in the carbon nanotube quantum dot nanostructures.

The properties of Goos-Hänchen (GH) shifts for transmitted and reflected light pulses in a cavity with an intracavity medium consist of carbon nanotube quantum dot nanostructures, which have been discussed theoretically by using the stationary phase theory. Our findings show that due to the presence of spin-orbit coupling, the maximum negative and positive shifts can be realized by modifying the absorption and dispersion properties of the intracavity medium. Moreover, the effect of the transverse magnetic field has been also considered as a new parameter for controlling the GH shifts in reflected and transmitted light beams. We hope that our proposed structure may be suitable for the generation of future all-optical system devices based on carbon nanotube quantum dot nanostructures.

Interband Optical Transition Energies in Type-II PbSe/PbS Core/Shell Quantum Dots

Electronic and optical properties of Type-II lead based core/shell semiconducting quantum dots are reported. Binding energies of electron–hole pair, optical transition energies and the absorption coefficients are investigated taking into account the geometrical confinement in PbSe/PbS core/shell quantum dot nanostructure. The energies are obtained with the increase of shell thickness for various inner core radii. The probability densities of electron and hole wave functions of radial coordinate of the core PbSe and PbS shell quantum dots are presented. The optical transition energy with the spatial confinement is brought out. The electronic properties are obtained using variational approach whereas the compact density matrix method is employed for the nonlinear optical properties. The results show that (i) a decrease in binding energy is obtained when the shell thickness increases due to more separation of electron–hole pair and (ii) the energy band gap decreases with the increase in the shell thickness resulting in the reduction of the higher energy interband transitions.

Incoherent control of optical bistability in an InGaN/GaN quantum dot nanostructure

The manipulation of bistable curve in the infrared (IR) region has been investigated theoretically in a unidirectional ring cavity doped by a four-level InGaN/GaN quantum dot nanostructure. The four-level quantum dot nanostructure is designed numerically by using the Schrödinger and Poisson equations. By controlling the size of the quantum dot and external voltage, one can design a four-level quantum dot with appropriate energy levels which can be suitable for interaction with IR signals. It is realized that the incoherent pumping fields play an essential role in controlling the intensity threshold of optical bistability. Decoherence effects such as the dephasing rate and electron density of the quantum dot are also analyzed at the threshold of optical bistability. Our proposed model due to its important application in all-optical systems may be favorable for real experimental evolution in infrared regions.

Experimental investigation of energy transfer between CdSe/ZnS quantum dots and different-sized gold nanoparticles

Hybrid nanostructures of quantum dots(QDs) and metallic nanostructure are attractive for future use in a variety of optoelectronic devices. For photodetection applications, it is important that the photoluminescence (PL) of QDs is quenched by the metallic nanostructures. Here, the quenching efficiency of CdSe/ZnS core-shell quantum dots (QDs) with different sized gold nanoparticles (NPs) films through energy transfer is investigated by measuring the PL intensity of the hybrid nanostructures. In our research, the gold NPs films are formed by the post-annealing of the deposited Au films on the quartz substrate. We find that the energy transfer from the QDs to the Au NPs strongly depends on the sizes of the Au NPs. For CdSe/ZnS QDs direct contact with the Au NPs films, the largest energy transfer efficiency are detected when the resonance absorption peak of the Au NPs is nearest to the emission peak of the CdSe/ZnS QDs. However, when there is a PMMA spacer between the QDs layer and the Au NPs films, firstly, we find that the energy transfer efficiency is weakened, and the largest energy transfer efficiency is obtained when the resonant absorption peak of the Au NPs is farthest to the emission peak wavelength of CdSe/ZnS QDs. These results will be useful for the potential design of the high efficiency QDs optoelectronic devices.

Manipulating transmission and reflection properties of a photonic crystal doped with quantum dot nanostructures

The transmission and reflection properties of incident light in a defect dielectric structure is studied theoretically. The defect structure consists of donor and acceptor quantum dot nanostructures embedded in a photonic crystal. It is shown that the transmission and reflection properties of incident light can be controlled by adjusting the corresponding parameters of the system. The role of dipole–dipole interaction is considered as a new parameter in our calculations. It is noted that the features of transmission and reflection curves can be adjusted in the presence of dipole–dipole interaction. It is found that the absorption of weak probe light can be converted to the probe amplification in the presence of dipole–dipole interaction. Moreover, the group velocity of transmitted and reflected probe light is discussed in detail in the absence and presence of dipole–dipole interaction. Our proposed model can be used as a new all-optical devices based on photonic materials doped with nanoparticles.

Phase control of Goos-Hänchen shifts in a four-level quantum dot nanostructure

In this letter, phase control of the Goos-Hänchen shifts of the reflected and transmitted probe light beams through a cavity containing four-level InGaN/GaN quantum dot nanostructure is theoretically discussed. In order to achieve the wave functions and their corresponding energy levels of the mentioned quantum dot nanostructure, Schrödinger and Poisson equations must be solved in a self consistent manner for carriers (here electron) in quantum dot. It is found that the coupling field, the pumping field as well as the cycling field can enhance the GH shifts of the reflected and transmitted probe beams. The effect of relative phase and the detuning of the probe light on the GH shifts of the reflected and transmitted probe beams are also investigated. We find that the GH shifts can be switched between the large positive and negative values by adjusting the controllable parameters.