Control of spontaneous emission of quantum dots using correlated effects of metal oxides and dielectric materials.

We study the emission dynamics of semiconductor quantum dots in the presence of the correlated impact of metal oxides and dielectric materials. For this we used layered material structures consisting of a base substrate, a dielectric layer, and an ultrathin layer of a metal oxide. After depositing colloidal CdSe/ZnS quantum dots on the top of the metal oxide, we used spectral and time-resolved techniques to show that, depending on the type and thickness of the dielectric material, the metal oxide can characteristically change the interplay between intrinsic excitons, defect states, and the environment, offering new material properties. Our results show that aluminum oxide, in particular, can strongly change the impact of amorphous silicon on the emission dynamics of quantum dots by balancing the intrinsic near band emission and fast trapping of carriers. In such a system the silicon/aluminum oxide charge barrier can lead to large variation of the radiative lifetime of quantum dots and control of the photo-ejection rate of electrons in quantum dots. The results provide unique techniques to investigate and modify physical properties of dielectrics and manage optical and electrical properties of quantum dots.

Undamped ultrafast pulsation of plasmonic fields via coherent exciton-plasmon coupling.

We show that under certain conditions the plasmonic field of a hybrid system consisting of a metallic nanoparticle and a semiconductor quantum dot can be converted into ultrashort stationary pulses with temporal widths as short as 300 ps. This happens as this system interacts with an infrared and visible laser fields, both with time-independent amplitudes. These fields generate quantum coherence via simultaneous interband and intersubband transitions of the quantum dot, forcing the polarization dephasing rate of the quantum dot to become negative during the plasmon pulses. This makes the amplitudes of such pulses time-independent (undamped), indicating total suppression of quantum decoherence of the quantum dot. These results suggest that hybrid quantum dot-metallic nanoparticle systems can act as undamped coherent-plasmonic oscillators.

Dynamics of plasmonic field polarization induced by quantum coherence in quantum dot-metallic nanoshell structures.

When a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle interacts with a laser field, the plasmonic field of the metallic nanoparticle can be normalized by the quantum coherence generated in the quantum dot. In this Letter, we study the states of polarization of such a coherent-plasmonic field and demonstrate how these states can reveal unique aspects of the collective molecular properties of the hybrid system formed via coherent exciton-plasmon coupling. We show that transition between the molecular states of this system can lead to ultrafast polarization dynamics, including sudden reversal of the sense of variations of the plasmonic field and formation of circular and elliptical polarization.

Enhancement of emission efficiency of colloidal CdSe quantum dots on silicon substrate via an ultra-thin layer of aluminum oxide.

We demonstrate that an ultra-thin layer of aluminum oxide can significantly enhance the emission efficiency of colloidal quantum dots on a Si substrate. For an ensemble of single quantum dots, our results show that this super brightening process can increase the fluorescence of CdSe quantum dots, forming well-resolved spectra, while in the absence of this layer the emission remains mostly at the noise level. We demonstrate that this process can be further enhanced with irradiation of the quantum dots, suggesting a significant photo-induced fluorescence enhancement via considerable suppression of non-radiative decay channels of the quantum dots. We study the impact of the Al oxide thickness on Si and interdot interactions, and discuss the results in terms of photo-induced catalytic properties of the Al oxide and the effects of such an oxide on the Coulomb blockade responsible for suppression of photo-ionization of the quantum dots.

Theoretical Investigation of Optical Detection and Recognition of Single Biological Molecules Using Coherent Dynamics of Exciton-Plasmon Coupling.

We use quantum coherence in a system consisting of one metallic nanorod and one semi-conductor quantum dot to investigate a plasmonic nanosensor capable of digital optical detection and recognition of single biological molecules. In such a sensor the adsorption of a specific molecule to the nanorod turns off the emission of the system when it interacts with an optical pulse having a certain intensity and temporal width. The proposed quantum sensors can count the number of molecules of the same type or differentiate between molecule types with digital optical signals that can be measured with high certainty. We show that these sensors are based on the ultrafast upheaval of coherent dynamics of the system and the removal of coherent blockage of energy transfer from the quantum dot to the nanorod once the adsorption process has occurred.

Coherent confinement of plasmonic field in quantum dot-metallic nanoparticle molecules.

Interaction of a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle (MNP) with a laser beam can replace the intrinsic plasmonic field of the MNP with a coherently normalized field (coherent-plasmonic or CP field). In this paper we show how quantum coherence effects in such a hybrid system can form a coherent barrier (quantum cage) that spatially confines the CP field. This allows us to coherently control the modal volume of this field, making it significantly smaller or larger than that of the intrinsic plasmonic field of the MNP. We investigate the spatial profiles of the CP field and discuss how the field barrier depends on the collective states of the hybrid system.

Identification of chemical constituents and larvicidal activity of Kelussia odoratissima Mozaffarian essential oil against two mosquito vectors Anopheles stephensi and Culex pipiens (Diptera: Culicidae).

The larvicidal activity of essential oil extracted from an indigenous plant, Kelussia odoratissima Mozaffarian was evaluated against two mosquito species, Anopheles stephensi and Culex pipiens. The chemical composition of the essential oil obtained by hydrodistillation from branch tips and leaf of this plant was determined by GC and GC/MS analysis. Forty-nine constituents were identified in the oil. The main constituents of the oil were Z-ligustilide (77.73%), 2-octen-1-ol acetate (6.27%), E-ligustilide (2.27%) and butylidene phthalide (1.97%). Five different logarithmic concentrations of essential oil were evaluated against the 4th instar larvae of An. Stephensi and Cx. pipiens. The LC(50) and LC(90) values against An. stephensi larvae were 4.88 and 9.60 ppm and for Cx. pipiens were 2.69 and 7.90 ppm, respectively. These properties suggest that K. odoratissima oil has potential source of valuable larvicidal compounds for mosquito larval control. This plant which causes high mortality at lower dose could be considered as a highly active plant. In this paper a guideline suggested for larvicidal activity of plant essential oils.

Coherent molecular resonances in quantum dot-metallic nanoparticle systems: coherent self-renormalization and structural effects.

It is known that surface-plasmon resonances of metallic nanoparticles can significantly enhance the field experienced by semiconductor quantum dots. In this paper we show that, when quantum dots are in the vicinity of metallic nanoparticles and interact with coherent light sources (laser fields), coherent exciton-plasmon coupling (quantum coherence effects) can increase the amount of the plasmonic field enhancement significantly. We also study how the coherent molecular resonances generated by such a coupling process are influenced by the self-renormalization of the plasmonic fields and the structural parameters of the systems, particularly the size and shape of the metallic nanoparticle. The renormalization process happens via mutual impacts of the radiative decay rate of excitons and the coherent exciton-plasmon coupling on each other. Our results highlight the conditions where the molecular resonances become very sharp, offering optical switching processes with high extinction ratio and wide ranging device applications.

Coherent control of Forster energy transfer in nanoparticle molecules: energy nanogates and plasmonic heat pulses.

We study how Forster energy transfer from a semiconductor quantum dot to a metallic nanoparticle can be gated using quantum coherence in quantum dots. We show this allows us to use a laser field to open the flow of the energy transfer for a given period of time (on-state) before it is switched off to about zero. Utilizing such an energy gating process it is shown that quantum-dot-metallic-nanoparticle systems (meta-molecules) can act as functional nanoheaters capable of generating heat pulses with temporal widths determined by their environmental and physical parameters. We discuss the physics behind the energy nanogates using molecular states of such meta-molecules and the resonance fluorescence of the quantum dots.

Photo-induced suppression of plasmonic emission enhancement of CdSe/ZnS quantum dots.

Emission of semiconductor quantum dots can be increased via two fundamentally different processes: (i) surface plasmon resonances (plasmonic emission enhancement) and (ii) irradiation with light (photo-induced fluorescence enhancement). In this paper we theoretically and experimentally study the mutual impacts of these processes on each other in quantum dot solids. We show that when thin films of colloidal quantum dots are placed in the vicinity of Au nano-islands, the plasmonic enhancement of the radiative decay rates of quantum dots and Forster energy transfer can hinder the photo-induced fluorescence enhancement of these films. This in turn leads to significant suppression of their plasmonic emission enhancement when they are irradiated with a laser beam. We investigate the impact of the sizes and shapes of the metallic nanoparticles in this process and theoretically analyze how plasmons and energy transfer can hinder the electrostatic barrier responsible for photo-induced fluorescence enhancement.

Gain without inversion in hybrid quantum dot-metallic nanoparticle systems.

We study the generation of tunable gain without inversion in semiconductor quantum dots using plasmonic effects. For this we investigate the impact of localized surface plasmons on coherent nonlinear exciton effects in a quantum dot when it is located in the vicinity of a metallic nanoparticle. It is shown that when such a system is exposed to a laser field and the distance between the quantum dot and the metallic nanoparticle is reduced the initial impact of plasmons is enhancement of the ac-Stark shift in the quantum dot. When this distance is reduced beyond a critical value, the results show abrupt formation of a significant of amount of gain without inversion in the quantum dot. We show that such a 'molecular' gain is associated with the plasmonic metaresonance (PMR) formed via combined effects of laser-induced coherence in the quantum dot and plasmons.

Tunable nanoswitches based on nanoparticle meta-molecules.

We introduce ultra-fast tunable nanoswitches based on the transition between states of nanoparticle meta-molecules. These molecules are formed (activated) when hybrid systems consisting of metallic nanoparticles and semiconductor quantum dots interact with coherent light sources (laser fields). The switching process occurs via minuscule changes of the refractive index of the environment or the distance between the quantum dots and metallic nanoparticles. These changes stimulate the transition between the states of the meta-molecules in nanosecond timescales, setting up dramatic optical events that can be observed easily. These nanoswitches can be tuned by varying the intensity of the activating laser field, allowing us to adjust the switching process to occur at different values of refractive indices. The results open a new horizon for chemically, biologically, or physically triggered optical nanoswitches and nanosensors that are sensitive to ultra-small changes in the environment.

Plasmonic (thermal) electromagnetically induced transparency in metallic nanoparticle-quantum dot hybrid systems.

We study the application of an infrared laser to control heat dissipation in a metallic nanoparticle when it is in the vicinity of a semiconductor quantum dot. The infrared laser is considered to be near-resonant with two of the conduction states of the quantum dot, coherently mixing them together. Via exciton-plasmon coupling, this process normalizes the internal field of the metallic nanoparticle, forming a plasmonic (thermal) electromagnetically induced transparency. When this process happens the metallic nanoparticle becomes nearly completely non-dissipative around its plasmon frequency, while it remains strongly dissipative at other frequencies. We show that, by adjusting the intensity of the infrared laser, one can control the transparency window width and optical Stark shift associated with such a process.

The inhibition of optical excitations and enhancement of Rabi flopping in hybrid quantum dot-metallic nanoparticle systems.

We study the inhibition of optical excitation and enhancement of Rabi flopping and frequency in semiconductor quantum dots via plasmonic effects. This is done by demonstrating that the interaction of a quantum dot with a laser field in the vicinity of a metallic nanoparticle can be described in terms of optical Bloch equations with a plasmically normalized Rabi frequency. We show that in the weak-field regime plasmonic effects can suppress the interband transitions, inhibiting exciton generation. In the strong-field regime these effects delay the response of the quantum dot to the laser field and enhance Rabi flopping. We relate these to the conversion of Rabi frequency from a real quantity into a complex and strongly frequency-dependent quantity as plasmonic effects become significant. We show that, within the strong-field regime, in the wavelength range where real and imaginary parts of this frequency reach their maxima, a strongly frequency-dependent enhancement of carrier excitation can happen.

Infrared switching from resonant to passive photonic bandgaps: transition from purely photonic to hybrid electronic/photonic systems.

Coherently controlled optical processes have been extensively studied in various systems including atoms, quantum wells and quantum dots. In this study we investigate such processes in Bragg multi-quantum well resonant (active) photonic bandgaps, wherein the dipole-dipole interwell interaction couples different quantum wells together, forming supperradiant exciton modes. Our results show that in such systems one can use an infrared laser beam to replace the collective superradiant mode with an electromagnetically induced transparency mode, demonstrating infrared switching of an active photonic bandgap to a hybrid system. Such a hybrid system consists of a passive photonic bandgap and an electromagnetically induced transparency band window superimposed on each other. A detailed study of the interplay between the collective and infrared-induced exciton excitations of quantum wells in Bragg multi-quantum well resonant photonic bandgap structures is presented.

Semiconductor quantum templates: bottom-up design of optical devices and photonic circuits using sub-nanoscale high monolayer features and quantum optics.

We propose an alternative bottom-up technique for designing, fabricating and monolithically integrating optical components, including functional distributed feedback lasers, modulators, waveguides, etc in a single semiconductor wafer without any need for etching or post-processing epitaxial growth. The proposed technique is based on the formation of semiconductor quantum templates at the well/barrier interfaces of quantum well structures. Such templates are responsible for changing the thickness of a quantum well in designated regions by adding one extra monolayer of the well material in those regions during the growth process. We show that, using a control laser field, these templates or sub-nanoscale high monolayer features allow us to spatially control the formation of electromagnetically induced transparency, gain without inversion, and coherent enhancement/suppression of the refractive index along the plane of the quantum well. We demonstrate that this can lead to bottom-up design capabilities for functional optical devices and their monolithic integration using a single epitaxial growth process.