Session F2: Focus Session: Quantum Control of Molecular, Nano, and Plasmonic Materials III

Ultrafast nanooptics: Using strong laser fields to control the motion of electrons in and around metallic nanotstructures

Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wavepackets with attosecond duration and strong directivity. The possibility to steer, control or switch such electron wavepackets by light is expected to pave the way towards real-time probing of electron motion in solid state nanostructures. Such pulses can be generated by strong-field induced tunneling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we study for the first time the effect of the carrier envelope phase of few cycle laser pulses on the motion of electrons emitted from metallic nanostructures by strong-field tunneling [1]. We illuminate very sharp, single-crystalling gold tips with CEP-stable few-cycle near-infrared pulses at 1.5 $\mu$m and record angle-resolved kinetic energy spectra of the photoemitted electrons. Our experiments give first evidence for the effect of absolute phase of the laser pulses on the emission direction and kinetic energy distribution of the photoemitted electrons. \\[4pt] [1] B. Piglosiewicz\textit{ et al.}, \textit{Nature Photonics}, doi:10.1038/NPOTON.2013.288 (2013).   

Resonant Scattering of Surface-Plasmon-Polariton Waves by a Dynamical Quantum Dot

The resonant scattering of a launched surface-plasmon-polariton wave by an embedded quantum dot above the dielectric/metal interface is explored in the strong-coupling regime. In contrast to non-resonant scattering by a localized dielectric surface defect, a strong resonant peak in the scattering-field spectrum is predicted and accompanied by the presence of two side valleys. The peak strength depends nonlinearly on the amplitude of surface-plasmon-polariton wave, reflecting the feedback dynamics from photoexcited electron-hole pairs inside the quantum dot. This unique behavior in the scattering-field peak strength is correlated with a resonant dip in the absorption spectrum of surface-plasmon-polariton wave due to interband photon-dressing effect.   

Hybrid metal-dielectric nanocavity for ultrafast quantum dot optical field interaction

Efficient light-matter interfaces for solid-state quantum emitters offering high single-photon collection efficiency as well as strong light-matter interaction are an important ingredient to a variety of quantum technologies. In this talk we introduce and demonstrate a new light-matter interface based on a hybrid metal-dielectric nanopillar cavity coupled to a single InAs quantum dot (QD). Its essential design characteristics include low quality factor Q $\approx$ 25 resonance, ultrasmall mode volume V $\approx$ 0.04 $(\lambda /n)^3$, and record-high coherent coupling g/2$\pi$ $\approx$ 150-200GHz, exceeding those offered in other light-matter interfaces including in photonic crystal cavities coupled to single QDs. We have observed that the single QD emitters are both embedded in the nanometallic devices and well-coupled to the orthogonal nanocavity modes, that our devices significantly enhance the spontaneous emission rate of the QD transitions (Purcell factor Fp $\approx$ 8 relative to bulk), and that overall single photon flux from the QD is increased by nearly two orders of magnitude relative to bulk. We conclude with an outlook for applications of this nanocavity geometry in information processing.   

Molecular controlled of quantum nano systems

A century ago quantum mechanics created a conceptual revolution whose fruits are now seen in almost any aspect of our day-to-day life. Lasers, transistors and other solid state and optical devices represent the core technology of current computers, memory devices and communication systems. However, all these examples do not exploit fully the quantum revolution as they do not take advantage of the coherent wave-like properties of the quantum wave function. Controlled coherent system and devices at ambient temperatures are challenging to realize. We are developing a novel nano tool box with control coupling between the quantum states and the environment. This tool box that combines nano particles with organic molecules enables the integration of quantum properties with classical existing devices at ambient temperatures. The nano particles generate the quantum states while the organic molecules control the coupling and therefore the energy, charge, spin, or quasi particle transfer between the layers. Coherent effects at ambient temperatures can be measured in the strong coupling regime. In the talk I will present our nano tool box and show studies of charge transfer, spin transfer and energy transfer in the hybrid layers as well as collective transfer phenomena. These enable the realization of room temperature operating quantum electro optical devices. For example I will present in details, our recent development of a new type of chiral molecules based magnetless universal memory exploiting selective spin transfer.   

Three-Dimensional Plasmonic Nanoclusters

Recent developments in the control and manipulation of electromagnetic radiation allow for the emergence of new concepts, found only in artificially engineered nanoscale media. Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. While displaying fascinating optical properties, nanostructures created by self-assembly or lithography have a major drawback; strong angular-dependent optical properties resulting from their two-dimensionality. Here, we present novel three-dimensional nanoclusters comprised of noble metal nanoparticles encapsulated in a polymer displaying interesting optical features in the visible (Fano resonances, optical isotropy,...). We investigate the nature of the optical properties and their dependence on cluster geometry. Such three-dimensional clusters show great promise as optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation.   

Long range emission enhancement and anisotropy in coupled quantum dots induced by aligned elongated and proximal gold nanoantenna

Metal nanoparticles have been shown to considerably modify the optical properties of quantum emitters like quantum dots and molecules when they are in close proximity to each other. Understanding the microscopic nature of such interactions requires studying the optical properties in the near field. Here, we discuss experimental results on non-local long range emission intensity enhancement and anisotropy in quantum dot assemblies induced by isolated and partially aligned gold nanoantennas overlaid on the quantum dots. Sub-diffraction and near field, spatially resolved, photoluminescence spectroscopy of these hybrid films, clearly demonstrate that the effect is maximum when the longitudinal surface plasmon resonance of the nanoantenna is resonant with the emission maxima of the quantum dots. Numerical simulations qualitatively captures the near field behavior of the nanorods but fails to match the experimentally observed non-local effects. We have suggested how collective excitations of quantum dots in the close packed assemblies, mediated by the nanoantennas, could lead to such observed behavior.   

Plasmon-enhanced energy transfer for improved upconversion of infrared radiation in doped-lanthanide nanocrystals

Upconversion of infrared radiation into visible light has been investigated for applications in biological imaging and photovoltaics. However, low conversion efficiency due to small absorption cross-section for infrared light (Yb$^{\mathrm{3+}})$, and slow rate of energy transfer (to Er$^{\mathrm{3+}}$ states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, we utilize resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. Our analysis indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increases the rate of resonant energy transfer from Yb$^{\mathrm{3+}}$ to Er$^{\mathrm{3+}}$ ions by 6 fold. While we do observe strong metal mediated quenching (14 fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the infrared, and hence enhances the nanocrystal UPL. This strong columbic effect on energy transfer can have important implications for other fluorescent and excitonic systems too.   

Quantum and nonlinear optics at the single photon level with quantum dots in optical nanocavities

By embedding a single InAs/GaAs quantum dot (QD) inside a nanocavity that strongly localizes optical field, it is possible to achieve a very strong light-matter interaction. The strength of this interaction is characterized by the coherent emitter-field coupling strength (g) which also sets the limit on the operational speed of such a system. While in systems consisting of a single neutral atom coupled to a cavity maximum $g/(2\pi) \sim$ 20 MHz has been demonstrated, InAs/GaAs QDs inside photonic crystal cavities have reached $g/(2\pi) \sim$ 40 GHz. Such a QD-cavity platform has also been employed in a series of quantum and nonlinear optics experiments at the single or few photon level which will be discussed in this talk, including: 1) photon blockade and photon induced tunneling (which can be employed to build high throughput sources of single or n-photons); 2) all optical switching at the single photon level and at the speed of 25GHz (which can be employed in all optical gates); 3) single quantum dot based optical modulators that operate at the sub-fJ control energies and potentially at $>$10GHz speeds; 4) single QD spin-photon interfaces that could be employed as nodes of a quantum repeater. However, considering that the speed of each of these elements is ultimately limited by g, which in turn scales as $\sim 1/\sqrt{V}$, where V is the optical mode volume, it is worthwhile building structures with V even smaller than those of photonic crystal cavities (which typically have V on the order of a cubic optical wavelength). With our recently demonstrated metal- GaAs nanocavity, V is squeezed by more than 10 times relative to photonic crystal cavities, and we demonstrate $g/(2\pi) > $ 100GHz with a single, embedded InAs/GaAs quantum dot. We are also working on extensions of this platform from two-level to multi-level quantum emitters strongly coupled to a cavity, as well as the extensions to emitters coupled to photonic molecules and cavity arrays, with applications in nonclassical light generation and quantum simulation.   

Near-field Mediated Plexcitonic Coupling and Giant Rabi Splitting in Individual Metallic Dimers

We investigate hybrid metallic dimer -- J-aggregate nanostructures that show coherent coupling between the localized surface plasmon (LSP) of the metallic disks and the exciton of the J-aggregate molecular complex. This hybrid nanostructure, combining both bottom-up and top-down approaches, is designed to probe the limitations of coherent coupling detection. Indeed, this allows us to report, for the first time, an experimental investigation of a plexcitonic coupling mechanism at the single-particle regime. By varying the diameter of the nanodisks, the LSP energies of the dimers can be systematically modified and tuned across the exciton energy of the J-aggregate. This allows for the direct measurement of the plexcitonic coupling energy of the dimer -- J-aggregate hybrid nanostructures. In this work, using single particle dark-field scattering spectroscopy as well as Finite-Difference Time-Domain (FDTD) calculations, we report giant Rabi splitting energies up to 400 meV resulting in a plexcitonic-induced transparency window observed both experimentally and theoretically at the overlap energy of the individual excitations. Furthermore, through a rigorous study of the polarization dependence and of the tunable geometric parameter effects (gap, diameter), the plexcitonic coupling mechanism has been investigated in these hybrid nanostructures, leading to the determination of the crucial role played by the plasmonic hot spots in the formation of the hybrid plexcitonic modes.   

Plasmon-assisted surface photochemistry and nanoassembly in silver nanoparticles

Bottom-up self-assembly of nanostructures into larger constructs remains a difficult proposition marred by low precision and low yield. Here we report on our effort to use optical activation to drive the assembly of particles onto silver nanospheres to form well-defined dumbells. The spheres were adsorbed onto a substrate and functionalized with a photocleavable o-nitrobenzyl-based ligand, which becomes positively charged upon photactivation. Illuminating the spheres with polarized light at either visible or ultraviolet wavelengths, plasmonic effects induce preferential photocleavage on opposite poles of the spheres, where negatively charged particles then can be adsorb. We will also discuss how this technique can be extended to enable the assembly of more complex nanostructures.   

Lightning-rod-effect-directed photo assembly of gold nanorods and spheres in a colloidal suspension

We describe a method for making colloidally stable gold nanorods that can be photo-functionalized at their ends---the plasmon hot spots---while dispersed in a fluid. Such particles could be used in supramolecular self-assembly and in developing chemical sensors. Gold nanorods---approximately 60 nm long and 20 nm in diameter---were functionalized with combination of mono-thiol PEG and a photophotocleavable {\em o}-nitrobenzyl ligand. The PEG serves to help stabilize the gold nanorods suspended in a mixture of water and alcohols so that the assembly can stably be done in suspension. The functionalized gold nanorods were then exposed to UV light that triggered photocleavage, resulting in the formation of positively charged amine groups. When these rods were mixed with negatively charged gold nanospheres, there was a red-shift in the wavelength of the longitudinal plasmon peak of more than 20-30 nm, indicating the preferential binding of gold nanospheres to the ends of the gold nanorods, which we attribute to the lightning rod effect.