Plasmonic Cavities and Individual Quantum Emitters in the Strong Coupling Limit.

ConspectusThe interaction of emitters with plasmonic cavities (PCs) has been studied extensively during the past decade. Much of the experimental work has focused on the weak coupling regime, manifested most importantly by the celebrated Purcell effect, which involves a modulation of the spontaneous emission rate of the emitter due to interaction with the local electromagnetic density of states. Recently, there has been a growing interest in studying hybrid emitter-PC systems in the strong-coupling (SC) regime, in which the excited state of an emitter hybridizes with that of the PC to generate new states termed polaritons. This phenomenon is termed vacuum Rabi splitting (VRS) and is manifested in the spectrum through splitting into two bands.In this Account, we discuss SC with PCs and focus particularly on work from our lab on the SC of quantum dots (QDs) and plasmonic silver bowtie cavities. As bowtie structures demonstrate strong electric field enhancement in their gaps, they facilitate approaching the SC regime and even reaching it with just one to a few emitters placed there. QDs are particularly advantageous for such studies, due to their significant brightness and long lifetime under illumination. VRS was observed in our lab by optical dark-field microspectroscopy even in the limit of individual QDs. We further used electron energy loss spectroscopy, a near-field spectroscopic technique, to facilitate measuring SC not only in bright modes but also in subradiant, dark plasmonic modes. Dark modes are expected to live longer than bright modes and therefore should be able to store electromagnetic energy for longer times.Photoluminescence (PL) is another useful observable for probing the SC regime at the single-emitter limit, as shown by several laboratories. We recently used Hanbury Brown and Twiss interferometry to demonstrate the quantum nature of PL from QDs within PCs, verifying that the measurements are indeed from one to three QDs. Further spectroscopic studies of QD-PC systems in fact manifested several surprising features, indicating discrepancies between scattering and PL spectra. These observations pointed to the contribution of multiple excited states. Indeed, using model simulations based on an extended Jaynes-Cummings Hamiltonian, it was found that the involvement of a dark state of the QDs can explain the experimental findings. Given that bright and dark states couple to the cavity with different degrees of coupling strength, the PC affects in a different manner each excitonic state. This yields complex relaxation pathways and interesting dynamics.Future work should allow us to increase the QD-PC coupling deeper into the SC regime. This will pave the way to exciting applications including the generation of single-photon sources and studies of cavity-induced coherent interactions between emitters.

Improving the quality factors of plasmonic silver cavities for strong coupling with quantum emitters.

Plasmonic cavities (PCs) made of metallic nanostructures can concentrate electromagnetic radiation into an ultrasmall volume, where it might strongly interact with quantum emitters. In recent years, there has been much interest in studying such a strong coupling in the limit of single emitters. However, the lossy nature of PCs, reflected in their broad spectra, limits their quality factors and hence their performance as cavities. Here, we study the effect of the adhesion layer used in the fabrication of metal nanostructures on the spectral linewidths of bowtie-structured PCs. Using dark-field microspectroscopy, as well as electron energy loss spectroscopy, it is found that a reduction in the thickness of the chromium adhesion layer we use from 3 nm to 0.1 nm decreases the linewidths of both bright and dark plasmonic modes. We further show that it is possible to fabricate bowtie PCs without any adhesion layer, in which case the linewidth may be narrowed by as much as a factor of 2. Linewidth reduction increases the quality factor of these PCs accordingly, and it is shown to facilitate reaching the strong-coupling regime with semiconductor quantum dots.

Enhanced Magnetoresistance in Molecular Junctions by Geometrical Optimization of Spin-Selective Orbital Hybridization.

Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their ferromagnet-molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to ∼200% near the conductance quantum G0. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal-molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal-molecule interfaces.

Defect-Free Carbon Nanotube Coils.

Carbon nanotubes are promising building blocks for various nanoelectronic components. A highly desirable geometry for such applications is a coil. However, coiled nanotube structures reported so far were inherently defective or had no free ends accessible for contacting. Here we demonstrate the spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils of up to more than 70 turns with identical diameter and chirality, and free ends. We characterize the structure, formation mechanism, and electrical properties of these coils by different microscopies, molecular dynamics simulations, Raman spectroscopy, and electrical and magnetic measurements. The coils are highly conductive, as expected for defect-free carbon nanotubes, but adjacent nanotube segments in the coil are more highly coupled than in regular bundles of single-wall carbon nanotubes, owing to their perfect crystal momentum matching, which enables tunneling between the turns. Although this behavior does not yet enable the performance of these nanotube coils as inductive devices, it does point a clear path for their realization. Hence, this study represents a major step toward the production of many different nanotube coil devices, including inductors, electromagnets, transformers, and dynamos.

Field-effect transistors based on WS2 nanotubes with high current-carrying capacity.

We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm(2) V(-1) s(-1) for an individual WS2 nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 10(8) A cm(-2). These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications.

Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity.

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

Vortex beams of atoms and molecules.

Angular momentum plays a central role in quantum mechanics, recurring in every length scale from the microscopic interactions of light and matter to the macroscopic behavior of superfluids. Vortex beams, carrying intrinsic orbital angular momentum (OAM), are now regularly generated with elementary particles such as photons and electrons. Thus far, the creation of a vortex beam of a nonelementary particle has never been demonstrated experimentally. We present vortex beams of atoms and molecules, formed by diffracting supersonic beams of helium atoms and dimers off transmission gratings. This method is general and could be applied to most atomic and molecular gases. Our results may open new frontiers in atomic physics, using the additional degree of freedom of OAM to probe collisions and alter fundamental interactions.

Vacuum Rabi splitting of a dark plasmonic cavity mode revealed by fast electrons.

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the periphery of the bowtie gaps, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes.

In-Plane Nanowires with Arbitrary Shapes on Amorphous Substrates by Artificial Epitaxy.

The challenge of nanowire assembly is still one of the major obstacles toward their efficient integration into functional systems. One strategy to overcome this obstacle is the guided growth approach, in which the growth of in-plane nanowires is guided by epitaxial and graphoepitaxial relations with the substrate to yield dense arrays of aligned nanowires. This method relies on crystalline substrates which are generally expensive and incompatible with silicon-based technologies. In this work, we expand the guided growth approach into noncrystalline substrates and demonstrate the guided growth of horizontal nanowires along straight and arbitrarily shaped amorphous nanolithographic open guides on silicon wafers. Nanoimprint lithography is used as a high-throughput method for the fabrication of the high-resolution guiding features. We first grow five different semiconductor materials (GaN, ZnSe, CdS, ZnTe, and ZnO) along straight ridges and trenches, demonstrating the generality of this method. Through crystallographic analysis we find that despite the absence of any epitaxial relations with the substrate, the nanowires grow as single crystals in preferred crystallographic orientations. To further expand the guided growth approach beyond straight nanowires, GaN and ZnSe were grown also along curved and kinked configurations to form different shapes, including sinusoidal and zigzag-shaped nanowires. Photoluminescence and cathodoluminescence were used as noninvasive tools to characterize the sine wave-shaped nanowires. We discuss the similarities and differences between in-plane nanowires grown by epitaxy/graphoepitaxy and artificial epitaxy in terms of generality, morphology, crystallinity, and optical properties.

Interfacial Electron Beam Lithography: Chemical Monolayer Nanopatterning via Electron-Beam-Induced Interfacial Solid-Phase Oxidation.

Chemical nanopatterning-the deliberate nanoscale modification of the chemical nature of a solid surface-is conveniently realized using organic monolayer coatings to impart well-defined chemical functionalities to selected surface regions of the coated solid. Most monolayer patterning methods, however, exploit destructive processes that introduce topographic as well as other undesired structural and chemical transformations along with the desired surface chemical modification. In particular in electron beam lithography (EBL), organic monolayers have been used mainly as ultrathin resists capable of improving the resolution of patterning via local deposition or removal of material. On the basis of the recent discovery of a class of radiation-induced interfacial chemical transformations confined to the contact surface between two solids, we have advanced a direct, nondestructive EBL approach to chemical nanopatterning-interfacial electron beam lithography (IEBL)-demonstrated here by the e-beam-induced local oxidation of the -CH3 surface moieties of a highly ordered self-assembled n-alkylsilane monolayer to -COOH while fully preserving the monolayer structural integrity and molecular organization. In this conceptually different EBL process, the traditional resist is replaced by a thin film coating that acts as a site-activated reagent/catalyst in the chemical modification of the coated surface, here the top surface of the to-be-patterned monolayer. Structural and chemical transformations induced in the thin film coating and the underlying monolayer upon exposure to the electron beam were elucidated using a semiquantitative surface characterization methodology that combines multimode AFM imaging with postpatterning surface chemical modifications and quantitative micro-FTIR measurements. IEBL offers attractive opportunities in chemical nanopatterning, for example, by enabling the application of the advanced EBL technology to the straightforward nanoscale functionalization of the simplest commonly used organosilane monolayers.

Grazing-incidence optical magnetic recording with super-resolution.

Heat-assisted magnetic recording (HAMR) is often considered the next major step in the storage industry: it is predicted to increase the storage capacity, the read/write speed and the data lifetime of future hard disk drives. However, despite more than a decade of development work, the reliability is still a prime concern. Featuring an inherently fragile surface-plasmon resonator as a highly localized heat source, as part of a near-field transducer (NFT), the current industry concepts still fail to deliver drives with sufficient lifetime. This study presents a method to aid conventional NFT-designs by additional grazing-incidence laser illumination, which may open an alternative route to high-durability HAMR. Magnetic switching is demonstrated on consumer-grade CoCrPt perpendicular magnetic recording media using a green and a near-infrared diode laser. Sub-500 nm magnetic features are written in the absence of a NFT in a moderate bias field of only µ0H = 0.3 T with individual laser pulses of 40 mW power and 50 ns duration with a laser spot size of 3 µm (short axis) at the sample surface - six times larger than the magnetic features. Herein, the presence of a nanoscopic object, i.e., the tip of an atomic force microscope in the focus of the laser at the sample surface, has no impact on the recorded magnetic features - thus suggesting full compatibility with NFT-HAMR.

Nonlinear metamaterials for holography.

A hologram is an optical element storing phase and possibly amplitude information enabling the reconstruction of a three-dimensional image of an object by illumination and scattering of a coherent beam of light, and the image is generated at the same wavelength as the input laser beam. In recent years, it was shown that information can be stored in nanometric antennas giving rise to ultrathin components. Here we demonstrate nonlinear multilayer metamaterial holograms. A background free image is formed at a new frequency-the third harmonic of the illuminating beam. Using e-beam lithography of multilayer plasmonic nanoantennas, we fabricate polarization-sensitive nonlinear elements such as blazed gratings, lenses and other computer-generated holograms. These holograms are analysed and prospects for future device applications are discussed.

Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit.

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light-matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.