Square-net topological materials exhibit various interesting properties arising from the interplay of their structural diversity, topologically protected bands and different possible electronic features, including intrinsic magnetic ordering and superconductivity. However, little is known about the low-dimensional structures of these materials and their properties. Realization of low-dimensional topological square-net materials has the potential of enabling enhancement of their quantum behavior arising from quasi-compact 1D geometry and high surface-to-volume ratio, and their integration into functional devices. This work reports on the synthesis and properties of high-quality, single-crystal nanowires of Si square-net material TaSiAs. The chemical-vapor-transport produces TaSiAs nanowires that are chemically encapsulated with a thin dielectric shell of SiO2, enabling remarkable ambient stability and a pristine surface, which are critical for observing robust topologically protected surface states. Atomic-resolution structural analysis reveals a sharp core-shell interface, and a Si square-net lattice extending along the nanowire axis. These chemically protected nanowires allowed us to observe rich electrical and magnetotransport features, including 4 to 11 times lower room-temperature resistivity than the closest bulk analogues and non-saturating linear magnetoresistance, revealing topologically protected coherent surface transport. First-principles calculations show a wide-range (4 eV) linear band dispersion, alongside different Dirac cones that are protected by either symmorphic (C4) or non-symmorphic symmetries, predicting transport features consistent with our results. The findings demonstrate the unique properties of low-dimensional square-net topological materials and their potential applications, including next-generation interconnects, spintronic devices and quantum computing.

Plasmonic bowtie-structured nanocavities are valuable structures for applications due to the large confinement of light at their center and the associated large enhancement of the electric field. Plasmonic bowties suffer, though, from broad spectral lines that limit their interaction with quantum emitters. We report here a significant narrowing of the longitudinal plasmon resonance that couples the two prisms of silver bowties, induced by the deposition of a thin (1 nm) Ti adhesion layer during fabrication. The quality factor of the longitudinal mode of many of these bowties is much larger than that of bowties without an adhesion layer, even while their transverse mode remains unmodified. Cross-sectional electron microscopy analysis reveals that the Ti adhesion layer is fully oxidized. Further, and surprisingly, the TiO2 adhesion layer is found to cause the bowties to partially embed in the silica substrate, thereby increasing the effective dielectric coefficient of their environment. Analysis of the results shows that the increase in the quality factor might be partially attributed to a reduction in radiative damping. The ability to obtain plasmon excitations of bowties with large quality factors is particularly attractive for multiple applications involving optical coupling to quantum emitters.

Nanowires (NWs) of topological materials are emerging as an exciting platform to probe and engineer new quantum phenomena that are hard to access in bulk phase. Their quasi-1D geometry and large surface-to-bulk ratio unlock new expressions of topology and highlight surface states. TaAs2, a compensated semimetal, is a topologically rich material harboring nodal-line, weak topological insulator (WTI), C2-protected topological crystalline insulator, and Zeeman field-induced Weyl semimetal phases. We report the synthesis of TaAs2 NWs in situ encapsulated in a dielectric SiO2 shell, which enable to probe rich magnetotransport phenomena, including metal-to-insulator transition and strong signatures of topologically nontrivial transport at remarkably high temperatures, direction-dependent giant positive, and negative magnetoresistance, and a double pattern of Aharonov-Bohm oscillations, demonstrating coherent surface transport consistent with the two Dirac cones of a WTI surface. The SiO2-encapsulated TaAs2 NWs show room-temperature conductivity up to 15 times higher than bulk TaAs2. The coexistence and susceptibility of topological phases to external stimuli have potential applications in spintronics and nanoscale quantum technology.

We present super-resolved coherent anti-Stokes Raman scattering (CARS) microscopy by implementing phase-resolved image scanning microscopy (ISM), achieving up to two-fold resolution increase as compared with a conventional CARS microscope. Phase-sensitivity is required for the standard pixel-reassignment procedure since the scattered field is coherent, thus the point-spread function (PSF) is well-defined only for the field amplitude. We resolve the complex field by a simple add-on to the CARS setup enabling inline interferometry. Phase-sensitivity offers additional contrast which informs the spatial distribution of both resonant and nonresonant scatterers. As compared with alternative super-resolution schemes in coherent nonlinear microscopy, the proposed method is simple, requires only low-intensity excitation, and is compatible with any conventional forward-detected CARS imaging setup.

We present super-resolved coherent anti-Stokes Raman scattering (CARS) microscopy by implementing phase-resolved image scanning microscopy, achieving up to two-fold resolution increase as compared with a conventional CARS microscope. Phase-sensitivity is required for the standard pixel-reassignment procedure since the scattered field is coherent, thus the point-spread function is well-defined only for the field amplitude. We resolve the complex field by a simple add-on to the CARS setup enabling inline interferometry. Phase-sensitivity offers additional contrast which informs the spatial distribution of both resonant and nonresonant scatterers. As compared with alternative super-resolution schemes in coherent nonlinear microscopy, the proposed method is simple, requires only low-intensity excitation, and is compatible with any conventional forward-detected CARS imaging setup. The authors present super-resolved CARS microscopy using phase-sensitive image scanning, achieving nearly twice the resolution of conventional CARS. This method requires low excitation laser power and integrates easily into existing systems.

Of the many applications of whispering-gallery mode (WGM) microresonators, single-atom cavity quantum electrodynamics (cavity-QED) poses the most extreme demands on mode-volume, dimensions and quality factor (Q). Here we present the procedure for the fabrication of chip-based, small mode-volume, ultrahigh Q silica WGM microresonators, designed for single-emitter cavity-QED. We demonstrate microresonators at varying geometries, from toroidal to micro-spheres, yielding ultrahigh-qualities as high as 1.7×10⁸ at 780nm. We present a comprehensive theoretical model that allows tailoring the fabrication process to attain the desired microresonator geometry.

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.

Several nanotubular structures from chalcogenide-based misfit layer compounds (MLC) were reported in recent years. MLCs consist of a stacking of two alternating and dissimilar (2D) atomic layers, e. g. one with rocksalt structure (MX) and the other- TX2 - with hexagonal layer structure. The layers are held together by weak van der Waals forces, i. e. they can be exfoliated with scotch-tape. Furthermore, in analogy to intercalation compounds, partial charge transfer between the layers with dissimilar work function results also in polar forces between the MX and TX2 layers. The mismatch between the alternating (asymmetric) layers and the seaming of the dangling bonds at the edges drives them to form tubular (and also scroll-like) structures. New structural characterization whereby the nanotubes were bisected into lamella via focused ion beam and examined by TEM, are reported.

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.

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.

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.

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 85meV 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. Dark plasmonic modes fare better in some applications due to longer lifetimes but, being subradiant, are difficult to probe. The authors apply electron energy loss spectroscopy to demonstrate that a dark mode of a plasmonic cavity can couple with a few quantum emitters to exhibit vacuum Rabi splitting.

The complementary optical properties of surface plasmon excitations of metal nanostructures and long-lived excitations of semiconductor quantum dots (QDs) make them excellent candidates for studies of optical coupling at the nanoscale level. Plasmonic devices confine light to nanometer-sized regions of space, which turns them into effective cavities for quantum emitters. QDs possess large oscillator strengths and high photostability, making them useful for studies down to the single-particle level. Depending on structure and energy scales, QD excitons and surface plasmons (SPs) can couple either weakly or strongly, resulting in different unique optical properties. While in the weak coupling regime plasmonic cavities (PCs) mostly enhance the radiative rate of an emitter, in the strong coupling regime the energy level of the two systems mix together, forming coupled matter-light states. The interaction of QD excitons with PCs has been widely investigated experimentally as well as theoretically, with an eye on potential applications ranging from sensing to quantum information technology. In this review we provide a comprehensive introduction to this exciting field of current research, and an overview of studies of QD-plasmon systems in the weak and strong coupling regimes.

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.

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.

Creating sub-micron hotspots for applications such as heat-assisted magnetic recording (HAMR) is a challenging task. The most common approach relies on a surface-plasmon resonator (SPR), whose design dictates the size of the hotspot to always be larger than its critical dimension. Here, we present an approach which circumvents known geometrical restrictions by resorting to electric field confinement via excitation of a gap-mode (GM) between a comparatively large Gold (Au) nano-sphere (radius of 100 nm) and the magnetic medium in a grazing-incidence configuration. Operating a λ=785 nm laser, sub-200 nm hot spots have been generated and successfully used for GM-assisted magnetic switching on commercial CoCrPt perpendicular magnetic recording media at laser powers and pulse durations comparable to SPR-based HAMR. Lumerical electric field modelling confirmed that operating in the near-infrared regime presents a suitable working point where most of the light's energy is deposited in the magnetic layer, rather than in the nano-particle. Further, modelling is used for predicting the limits of our method which, in theory, can yield sub-30 nm hotspots for Au nano-sphere radii of 25–50 nm for efficient heating of FePt recording media with a gap of 5 nm.

Maxwell invented a refractive-index profile where light goes in circles and every point is focused. A device with such a profile is called the Maxwell fisheye and it is an absolute optical instrument: it has the ability to collect all rays from any emitting source and recombine them in phase at the corresponding focal point inside the device. Absolute optical instruments may find diverse applications if they can be made in integrated optics on a silicon chip for infrared light. We have fabricated the Maxwell fisheye in silicon photonics and have demonstrated its focusing properties. Our fabrication technique can also be applied to the manufacturing of other devices where smooth and sharp structures need to be made in one lithography step.