Plasmonic metal-oxide metastructures comprise arrays of Au nanoantennas embedded in Si layers, with a top ultrathin layer of Al oxide, supporting an Au/Si Schottky junction in the vicinity of a Si/Al oxide charge barrier. In such structures the hot electrons generated by nonradiative decay of plasmons can be captured by the Schottky junctions, forming an electrostatic field via charge accumulation in the Si layers. When thin layers of semiconductor quantum dots are placed on top of such structures, such a field suppresses their non-radiative decay rates, making them far more efficient emitters. We study the impact of plasmon modes of the Au nanoantennas on the exciton-plasmon coupling enhancement supported by such a material platform.
Under certain conditions periodic arrays of metallic nanoantennas can support hybridization of localized surface plasmon resonances (LSPRs) with the lattice (photonic) modes, forming surface lattice resonances (SLRs). We study in-plane farfield scattering associated SLRs in cases wherein the nanoantennas support distinctively different forms of LSPRs. For this, two different categories of Au nanoantennas are considered. In one category the nanoantennas have 130 nm width and 220 nm length (array 1) while in the other category (array 2) the widths and lengths of the nanoantennas are 240 and 1300 nm, respectively. Therefore, nanoantennas in array 2 have high degree of flatness, as their lateral dimensions are much larger their heights (40 nm). We demonstrate the impact of the multipolar nature of plasmonic edge modes in such flat nanoantennas on the formation SLRs and the spectral features of their in-plane field scattering. Our results also show array 1 with more localized plasmons, can offer more efficient in-plane scattering at narrower spectral widths. Field scattering switching between SLRs and plasmonic edge modes is studied via control of the incident light polarization.
A key application of metallic nanostructures is enhancement of the radiative decay rates of quantum emitters. In this contribution we investigate control of emission dynamics of semiconductor quantum dots (QDs) using combination of both metal oxides and plasmonic effects of gold thinfilms. Different metal oxides, including Cu, Ag and Al oxides are investigated. We show how such oxides can dramatically change the decay of QDs via photocatalytic processes in the absence and presence of plasmonic fields. The outcomes show the distinct impact of such oxides, ranging from total annihilation of QD emission to their plasmonic revival via Purcell effect.
Chemical sensing based on Localized Surface Plasmonic Resonances (LSPR) and the ultra-sharp optical features of surface lattice resonances (SLR) of arrays of metallic nanoantennas have attracted much attention. Recently we studied biosensing based on the transition between LSPR and SLR (hybridization phase), demonstrating significantly higher refractive index sensitivity than each of these resonances individually. In this contribution we study the impact of size and shape of the metallic nanoantennas on the hybridization process and the way they influence application of this process for biosensing, wherein miniscule variation of the refractive index of the environment leads to dramatic changes in the spectral properties of the arrays.
We show arrays of large gold nanodisks on glass substrates can support strong optical features with narrow spectral widths associated with their collective plasmonic-lattice modes. Our results show that these modes can offer significant sensitivity to the refractive index of the environment, far more than those of individual nanodisks. We show the visible collective modes supported by such arrays can distinctively detect a monolayer of biotin with high resolution. We use donor (CdSe/ZnS) and acceptor (CuInS/ZnS) quantum dots to investigate the field properties of these arrays after deposition of a thick layer of a silicon. We demonstrate a distinct increase of emission of CuInS/ZnS quantum dots, indicating the possibility of enhancement of energy transfer between these two types of quantum dots.
We show theoretically that when a semiconductor quantum dot and metallic nanoparticle system interacts with a laser field, quantum coherence can introduce a new landscape for the dynamics of Forster resonance energy transfer (FRET). We predict adsorption of biological molecules to such a hybrid system can trigger dramatic changes in the way energy is transferred, blocking FRET while the distance between the quantum dot and metallic nanoparticle (R) and other structural specifications remain unchanged. We study the impact of variation of R on the FRET rate in the presence of quantum coherence and its ultrafast decay, offering a characteristically different dependency than the standard 1/R6. Application of the results for quantum nanosensors is discussed.
It is well known that irradiation of colloidal quantum dots can dramatically enhance their emission efficiencies, leading to so-called photoinduced fluorescence enhancement (PFE). This process is the result of the photochemical and photophysical properties of quantum dots and the way they interact with the environment in the presence of light. It has been shown that such properties can be changed significantly using metal oxides. Using spectroscopic techniques, in this paper we investigate emission of different types of quantum dots (with and without shell) in the presence of metal oxides with opposing effects. We observed significant increase of PFE when quantum dots are deposited on about one nanometer of aluminum oxide, suggesting such oxide can profoundly increase quantum yield of such quantum dots. On the other hand, copper oxide can lead to significant suppression of emission of quantum dots, making them nearly completely dark instantly.
Conventional plasmonic sensors are based on the intrinsic resonances of metallic nanoparticles. In such sensors wavelength shift of such resonances are used to detect biological molecules. Recently we introduced ultra-sensitive timedomain nanosensors based on the way variations in the environmental conditions influence coherent dynamics of hybrid systems consisting of metallic nanoparticles and quantum dots. Such dynamics are generated via interaction of these systems with a laser field, generating quantum coherence and coherent exciton-plasmon coupling. These sensors are based on impact of variations of the refractive index of the environment on such dynamics, generating time-dependent changes in the emission of the QDs. In this paper we study the impact of material properties of the metallic nanoparticles on this process and demonstrate the key role played by the design of the quantum dots. We show that Ag nanoparticles, even in a simple spherical shape, may allow these sensors to operate at room temperature, owing to the special properties of quantum dot-metallic nanoparticle systems that may allow coherent effects utilized in such sensors happen in the presence of the ultrafast polarization dephasing of quantum dots.
When a quantum dot is in the vicinity of a metallic nanoparticle and is driven by a laser field, quantum coherence can renormalize the plasmon field of the metallic nanoparticle, forming a coherent-plasmonic field (CP field). We demonstrate that for a given form of variation of this laser field with time, the CP field around the metallic nanoparticle can offer different forms of ultrafast field dynamics, depending on the location. In other words, we show the coherent exciton-plasmon coupling in such a system allows it to act as coherent nanoantenna capable of generation position-dependent coherent-plasmonic dynamics, designating each location around the metallic nanoparticle with characteristic time-position coordinates. These investigations are carried out by demonstrating that the coherent dynamics responsible for these effects can persist in the presence of the ultrafast polarization dephasing of the quantum dots. This highlights the prospect of generation and preservation of quantum coherence effects in hybrid quantum dot-metallic nanoparticle systems at elevated temperatures. Therefore, even when the decoherence times of the quantum dots are of the order of several hundreds of femtoseconds, as observed at room temperature, such coherent dynamics can remain quite distinct and observable.
We examined the interdot energy transfer between monodisperse quantum dots under different degrees of plasmonic
effects (plasmonic field enhancement and Forster energy transfer from quantum dots to metallic nanoparticles). For this we studied emission of CdSe/ZnS quantum dots deposited on substrates containing self-organized arrays of gold
nanoislands with radially distributed sizes gradually reduced from the centers of the substrates to their sides. The results suggest how metallic nanoparticles can be used to enhance interdot energy transfer in monodisperse quantum dots and how this process can explain some of the spectral changes seen in the emission of quantum dots when they are close to the metallic nanoparticles.
We studied how deposition of a very thin layer of gold or chromium oxide on glass substrates can modify the way
irradiation changes the fluorescence of CdSe/ZnS quantum dots. We found that the gold layer tends to shield the
quantum dots from the substrate, preventing photoinduced fluorescence enhancement caused by the Coulomb blockage. In this case the emission of the quantum dots did not show also any broadening but rather a slight red shift, independent of the irradiation time. In the case of the chromium-oxide coated substrates we observed significant broadening and blue shift, indicating such oxide could enhance photo-oxidation of colloidal quantum dots significantly.
When metallic nanoparticles are put in the vicinity of semiconductor quantum dots and driven by a coherent light source,
their intrinsic plasmonic fields can be replaced with a new type of fields (coherent-plasmonic fields). These fields are
generated via coherent coupling of excitons in quantum dots and localized surface plasmon resonances (LSPRs). We
show the coherent-plasmonic field of a metallic nanoparticle can lead to a significantly larger field enhancement than
that caused by its LSPR. Utilizing this, we investigate how such a coherent field enhancement can improve the
sensitivities plasmonic nanosensors for detection single biological molecules. The results demonstrate application of
quantum coherence in quantum dot-metallic nanoparticle systems for chemical and biological sensing applications.
Optical responses of active multi-quantum well photonic band gap structures are mostly determined by the excitonic effects (superradiant excitons) and the contrast between the background refractive indices of the wells and barriers (nonresonant effects). Recently we studied coherent control of such photonic band gaps via infrared dressing of the superradiant excitons. This was done considering an infrared laser field near resonantly coupled such excitons with the excitons associated with the second conduction subbands of the quantum wells. This led to the formation of photonic electromagnetically induced transparency and disentanglement of the excitonic contributions from those associated with the nonresonant effects via destruction of the superradiant modes. Here we study how such a disentanglement process dramatically changes transmissions of the Bragg multiquantum well structures. In particular, we show that when the infrared laser intensity is high, the non-resonant effects form an incomplete passive band gap around the Bragg wavelength. Such a band gap, which is immune against the infrared laser, is flanked by two non-photonic gaps (absorption peaks). These peaks are associated with the large absorption of two dressed exciton states, i.e., Aulter-Townes doublet. Any variation in the intensity of the infrared laser changes the wavelengths of these peaks, making them closer or farther to the passive photonic band gap.
In an active photonic band gap structure, a control laser can manipulate the probe signal coherently if the
probe field satisfies the Bragg condition and is resonant or near resonant with the electronic or excitonic
transitions of the constituting material structure. Using coherent effects in the conduction intersubband
transitions of an n-doped quantum well, recently, we showed that one could convert a fully transparent
waveguide into an active photonic band gap. Such an active photonic band gap was different from those based
on superradiant excitons in two main issues: (i) the probe field was near resonant with the conduction
intersubband transitions of the quantum well, and (ii) one needed a control field to generate the coherent
effects and, thereafter, the band gap. Here we use such coherent processes, which include electromagnetically
induced transparency and coherent enhancement of refractive index, to study a one-dimensional functional
photonic band gap structure. In the absence of a control laser field such a structure acts as a conventional
photonic band gap created by an off-resonant (background) refractive index perturbation. In other words, the
probe field does not feel any resonance in this structure and the photonic band gap is passive. In the presence
of the control field, the structure is activated and transformed into a resonant structure. Under this condition, the probe field becomes near resonant with the intersubband transitions while still satisfies the Bragg condition. We study how the coherent effects in such transitions can lead to destruction and enhancement of the photonic band gap in a waveguide structure.
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