S. M. Sadeghi

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.

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.

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

We study the variation of the energy absorption rate in a hybrid semiconductor quantum dot-metallic nanoparticle system doped in a photonic crystal. The quantum dot is taken as a three-level V-configuration system and is driven by two applied fields (probe and control). We consider that one of the excitonic resonance frequencies is near to the plasmonic resonance frequency of the metallic nanoparticle, and is driven by the probe field. The other excitonic resonance frequency is far from both the plasmonic resonance frequency and the photonic bandgap edge, and is driven by the control field. In the absence of the photonic crystal we found that the system supports three excitonic-induced transparencies in the energy absorption spectrum of the metallic nanoparticle. We show that the photonic crystal allows us to manipulate the frequencies of such excitonic-induced transparencies and the amplitude of the energy absorption rate.

Optical response of a quantum dot?metal nanoparticle hybrid interacting with a weak probe field

We study optical effects in a hybrid system composed of a semiconductor quantum dot and a spherical metal nanoparticle that interacts with a weak probe electromagnetic field. We use modified nonlinear density matrix equations for the description of the optical properties of the system and obtain a closed-form expression for the linear susceptibilities of the quantum dot, the metal nanoparticle, and the total system. We then investigate the dependence of the susceptibility on the interparticle distance as well as on the material parameters of the hybrid system. We find that the susceptibility of the quantum dot exhibits optical transparency for specific frequencies. In addition, we show that there is a range of frequencies of the applied field for which the susceptibility of the semiconductor quantum dot leads to gain. This suggests that in such a hybrid system quantum coherence can reverse the course of energy transfer, allowing flow of energy from the metallic nanoparticle to the quantum dot. We also explore the susceptibility of the metal nanoparticle and show that it is strongly influenced by the presence of the quantum dot.

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.

Quantum dot-metallic nanorod sensors via exciton-plasmon interaction.

We investigate quantum nanosensors based on hybrid systems consisting of semiconductor quantum dots and metallic nanorods in the near-infrared regime. These sensors can detect biological and chemical substances based on their impact on the coherent exciton-plasmon coupling and molecular resonances supported by such systems when they interact with a laser field. We demonstrate that the ultrahigh sensitivity of such molecular resonances on environmental conditions allows dramatic and nearly instantaneous changes in the total field experienced by the semiconductor quantum dot via minuscule variations of the local refractive indices of the quantum dot or nanorod. The proposed nanosensors can utilize quantum effects to control the sense (or direction) of the changes in the quantum dot emission, allowing us to have bistable switching from dark to bright states or vice versa via adsorption (or detachment) of biomolecules. These sensors can also offer detection of ultra-small variations in the local dielectric constant of the quantum dots or metallic nanorods via coherent induction of time delays in the effective field experienced by the quantum dots when the hybrid systems interact with time-dependent laser fields. This leads to unprecedented bulk refractive index sensitivities. Our results show that one can utilize quantum phase to control the coherent exciton-plasmon dynamics in these sensors such that introduction of a biomolecule can increase or decrease the time delay. These results offer novel ways to detect single biomolecules via application of quantum coherence to convert their impact into spectacular optical events.

Plasmonic Metaresonance Nanosensors: Ultrasensitive Tunable Optical Sensors Based on Nanoparticle Molecules

We introduce novel ultra-sensitive tunable optical nanosensors based on conceptually new physical phenomena and techniques. The core foundation of these sensors is based on molecular-like behavior of hybrid systems consisting of metallic nanoparticles and semiconductor quantum dots when they interact with a laser field (activating field). Therefore, instead of using plasmons or excitons as in conventional sensors, in these nanosensors we utilize the characteristic resonances of such metamolecules (plasmonic metaresonances) caused by coherent exciton-plasmon coupling. It is shown that such resonances can be tuned by changing the intensity of the laser field responsible for activating the nanoparticle-hybrid systems. Therefore, these nanosensors are tunable with significant dynamic range and ultrahigh sensitivity. The proposed optical nanosensors can be used for real-time detection of chemical and biological substances and variation of the physical parameters of nanoscale systems (temperature, conformational changes, etc.).

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.

Quantum coherence effects in hybrid nanoparticle molecules in the presence of ultra-short dephasing times

We study quantum coherence effects in single nanoparticle systems consisting of a semiconductor quantum dot and a metallic nanoparticle in the presence of the ultra-short dephasing times of the quantum dots. The results suggest that coherent exciton-plasmon coupling can sustain the collective molecular resonances (plasmonic meta-resonances) of these systems at about room temperature. We investigate quantum optical properties of the quantum dots under this condition, demonstrating formation of ultranarrow gain and absorption spectral lines. These results are discussed in terms of plasmonic normalization of coherent population oscillation and the collective states of the nanoparticle systems.

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.