A-Control of photophysical and photochemical properties of semiconductor quantum dots:

Nanoscale semiconductor and metallic structures support very different materials and optical properties than their bulk counterparts. Additionally, because of their large surface to volume ratios, th

ey can interact strongly with their environments. We study how the physical/optical properties of semiconductor quantum dots (QDs) and metallic nanoantennas are influenced their defect environment, air molecules, and substrate as they are irradiated with a laser beam. Our research activities include application of metal oxides such as Al, Cr, Ag, and Cu oxide to control the photophysical and photochemical properties of quantum dots. Our goals are to make QDs brighter, understand how their structures are changed as they interact with light, use them for sensor application.

Enhancement of single QD emission using Al oxide: 3D renderings of photoluminescence images of single QDs on (a) 0.5 nm Al/Si and (b) Si after 30 min irradiation at 5.2 W cm−2. Frames in (a) and (b) are 40 μm × 40 μm in size. Inset frame showing contour view of a peak in (a) is 4.2 μm × 4.2 μm in size. Image resolution is approximately 1 μm.

Patty, Sadeghi, Nejat, and Mao "Enhancement of emission efficiency of colloidal CdSe quantum dots on silicon substrate via an ultra-thin layer of aluminum oxide" Nanotechnology, vol. 15, 2014

Augmentation of photo-induced fluorescent enhancement of QDs using Al oxide: (a) Normalized emission intensity of CdSe QDs on various thicknesses of Al/Si substrates and c-Si over 30 min of irradiation. Intensities are normalized to the initial intensity measured on the reference (I0). (b) Emission enhancement factor, Eenh, versus time for the data presented in (a). Legend: 0.5 nm Al/Si thick blue line (label 0.5); 1.0 nm Al/Si green line (label 1.0); 1.5 nm Al/Si dot-dashed red line (label 1.5); 2.0 nm Al/Si dotted red line (label 2.0); Si black dashed line (in (a) only).

Patty, Sadeghi, Nejat, and Mao "Enhancement of emission efficiency of colloidal CdSe quantum dots on silicon substrate via an ultra-thin layer of aluminum oxide" Nanotechnology, vol. 15, 2014

B. Material design for control of spontaneous emission of semiconductor quantum dots and energy transfer:

We investigate novel material structures that allows us to control the rates of spontaneous decay of QDs and enhance their emission intensity. Materials are also designed to control energy transfer from QD to another (FRET). This part of the research coupling between fundamental resonances of QDs (excitons and plasmons), changing the normal tradeoff between plasmonic field enhancement of transfer of energy from QDs to metallic nanoantennas. The overall goals are to use plasmonic effect and dielectric materials to control of optics of QDs, forming polarization-dependent fluorescence enhancement, and change the decay in favorable way.

Impact of heat on enhancement of emission intensity of colloidal quantum dots on metallic nanoparticles:

Variation of emission intensity of QDs deposited on metallic nanoisland with 15 nm of silica spacer as a function of the applied laser intensity. The sharp decline in emission intensity of on QDs on metallic nanoislands is caused by activation of Auger decay via heat.

S. M. Sadeghi and A. Nejat, " Abrupt plasmonic activation of photoionization rates in quantum dot solids" , Journal of Physical Chemistry C, vol. 115 (2011)

Plasmonic-dielectric metasubstrate: Such metasubstrate can keep the emission intensity of the QDs nearly unchanged but reduce their decay lifetime by more than one order of magnitude. These metasubsrates ware designed based on Au/am:Si/AuD/Al oxide multilayered structures. The competition between energy transfer from QDs to metallic nanostructures and plasmonic field enhancement is controlled via Au thin film/dielectric/metallic nanostructures.

Emission of QDs on plasmonic-dielectric metasubstrate (a). The emission of QDs on the am:Si (line 1), am:Si/Au0.5 (line 2), and Au/am:Si/Au0.5 (line 3) regions is shown in (a), while the decay of these regions is shown in (b). For all plots, the am:Si thickness is 25 nm.

W. Wing and S. M. Sadeghi, "Ultrafast emission decay with high emission efficiency of quantum dots in plasmonic–dielectric metasubstrates" Journal of Phys: Cond. Mat. Vol. 29 (2017)

C. Quantum devices:

We study collective properties of hybrid systems consisting of one QD and one metallic nanoantenna. Particularly we study how these systems can act as sort of molecules (meta-molecules) with their own characteristic states (meta-states) and resonances (plasmonic meta-resonances or PMR). We study various fundamental properties of such molecules, including their optical and dynamics characteristic. Of particular interest is application of coherently controlled processes in such hybrid systems for quantum device applications. The key aspect of this part our research is suppression of quantum decoherence in such hybrid systems. Ultrafast quantum-plasmonic oscillator: We study how hybrid systems consisting of single QDs and metallic nanoantennas can act as oscillator, wherein oscillation is caused by quantum coherence. We study how systems allow us to pass by the ultrafast decoherence of plasmon and QDs, to offer undamped oscillations of plasmonic field and Rabi flopping when these systems interact with CW laser fields.

Oscillation of plasmon field enhancement for a point close to the nanoantenna (P) when the system interacting with a visible and infrared laser.

S M Sadeghi, W J Wing and R R Gutha" Undamped ultrafast pulsation of plasmonic fields via coherent exciton-plasmon coupling", Nanotechnology, vol. 26, 085202 (2015)

S M Sadeghi "Exciton-plasmon quantum metastates: self-induced oscillations of plasmon fields in the absence of decoherence in nanoparticle molecules" Journal of Nanoparticle Research (2016) 18:46

S M Sadeghi, "Ultrafast plasmonic field oscillations and optics of molecular resonances caused by coherent exciton-plasmon coupling" Phys. Rev. A 88, 01383(2013)

Coherent nano logic gates: We study irreversible ultrafast dynamics caused by interaction of a semiconductor quantum-dot–metallic-nanorod system with an infrared laser field. We show that when this system supports exciton-plasmon coupling, by just varying the amplitude of this laser for a short period of time (several nanoseconds), one can decide the instance when the plasmon field of the nanorod becomes significant and its duration. This is done by showing that a sudden rise in the amplitude of the infrared laser (positive pulse) can induce irreversible transition from one of the collective molecular states of this system to another, making the plasmon field significant. When this amplitude reduces for a short period of time (negative pulse), the system returns back to its initial state, suppressing this field. Using these we show that a quantum-dot–metallic-nanoparticle system can act as an all-optical and logic gate.

Schematic illustration of the and logic gate based on plasmonic metaresonances in a QD-NR system. From the left, two trains of visible and MIR laser pulses are reaching this system. When the visible and MIR pulses interact with the QD-NR system simultaneously, the system is transferred to the B state (1), otherwise it stays in the D state (0). When the system is in 1 the QD emits efficiently (right-side peaks).

S. M. Sadeghi, W. J. Wing, and R. R. Gutha "Control of plasmon fields via irreversible ultrafast dynamics caused by interaction of infrared laser pulses with quantum-dot–metallic-nanoparticle molecules"Phys. Rev. A 92, 023808 (2015)

Quantum nanosensors: We study application of quantum coherence in a system consisting of one metallic nanorod and one semiconductor quantum dot for plasmonic nanosensors 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.

Schematic illustration of quantum nanosensors based on a QD-NR system. A, B, and C are three incoming optical pulses (biologically designated optical pulses) interacting with this system. a, b, and c are the corresponding emission pulses of the QD caused by such interaction. In part b one biological molecule is adsorbed to the functionalized NR, removing a. In part c two of such molecules are absorbed, leading to the suppression of a and b.

S. M. Sadeghi, B. Hood, K. Patty, and C-B Mao "Theoretical Investigation of Optical Detection and Recognition of Single Biological Molecules Using Coherent Dynamics of Exciton- Plasmon Coupling" J. Phys. Chem. C, 2013, 117 (33), pp 17344–17351

S. M. Sadeghi "Plasmonic Metaresonance Nanosensors: Ultrasensitive Tunable Optical Sensors Based on Nanoparticle Molecules" IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 10, 566 (2011)

D. Control of emission lifetime and defect environments of semiconductor quantum dots using metal oxide plasmonic metastructures:

E. Plasmonic and surface lattice resonances:

S.M. Sadeghi, W. Wing, R. Gutha, C. Sharp, and Ali Hatef, " Optically saturated and unsaturated collective resonances of flat metallic nanoantenna arrays" Journal of Applied Physics, Vol. 122, 063102(2017)

F. Control of emission of single quantum dots:

G. Biological metallic nanoantenna-quantum dot hybrid systems:

H. Biochemical sensing based collective resonances and quantum coherence processes:

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