Integrated Sideband-Resolved SERS with a Dimer on a Nanobeam Hybrid.

In analogy to cavity optomechanics, enhancing specific sidebands of a Raman process with narrowband optical resonators would allow for parametric amplification, entanglement of light and molecular vibrations, and reduced transduction noise. We report on the demonstration of waveguide-addressable sideband-resolved surface-enhanced Raman scattering (SERS). We realized a hybrid plasmonic-photonic resonator consisting of a 1D photonic crystal cavity decorated with a sub-20 nm gap dimer nanoantenna. Hybrid resonances in the near-IR provide designer Q factors of 1000, and Q/V=(λ^{3}/10^{6})^{-1}, with SERS signal strength on par with levels found in state-of-the-art purely plasmonic systems. We evidence Fano line shapes in the SERS enhancement of organic molecules, and quantitatively separate out the pump enhancement and optical reservoir contributions.

Extreme-Ultraviolet Shaping and Imaging by High-Harmonic Generation from Nanostructured Silica.

Coherent extreme-ultraviolet pulses from high-harmonic generation have ample applications in attosecond science, lensless imaging, and industrial metrology. However, tailoring complex spatial amplitude, phase, and polarization properties of extreme-ultraviolet pulses is made nontrivial by the lack of efficient optical elements. Here, we have overcome this limitation through nanoengineered solid samples, which enable direct control over amplitude and phase patterns of nonlinearly generated extreme-ultraviolet pulses. We demonstrate experimental configurations and emitting structures that yield spatially patterned beam profiles, increased conversion efficiencies, and tailored polarization states. Furthermore, we use the emitted patterns to reconstruct height profiles, probe the near-field confinement in nanostructures below the diffraction limit of the fundamental radiation, and to image complex structures through coherent diffractive emission from these structures. Our results pave the way for introducing sub-fundamental-wavelength resolution imaging, direct manipulation of beams through nanoengineered samples, and metrology of nanostructures into the extreme-ultraviolet spectral range.

High-Index Dielectric Metasurfaces Performing Mathematical Operations.

Image processing and edge detection are at the core of several newly emerging technologies, such as augmented reality, autonomous driving, and more generally object recognition. Image processing is typically performed digitally using integrated electronic circuits and algorithms, implying fundamental size and speed limitations, as well as significant power needs. On the other hand, it can also be performed in a low-power analog fashion using Fourier optics, requiring, however, bulky optical components. Here, we introduce dielectric metasurfaces that perform optical image edge detection in the analog domain using a subwavelength geometry that can be readily integrated with detectors. The metasurface is composed of a suitably engineered array of nanobeams designed to perform either first- or second-order spatial differentiation. We experimentally demonstrate the second-derivative operation on an input image, showing the potential of all-optical edge detection using a silicon metasurface geometry working at a numerical aperture as large as 0.35.

Controlling Nanoantenna Polarizability through Backaction via a Single Cavity Mode.

The polarizability α determines the absorption, extinction, and scattering by small particles. Beyond being purely set by scatterer size and material, in fact polarizability can be affected by backaction: the influence of the photonic environment on the scatterer. As such, controlling the strength of backaction provides a tool to tailor the (radiative) properties of nanoparticles. Here, we control the backaction between broadband scatterers and a single mode of a high-quality cavity. We demonstrate that backaction from a microtoroid ring resonator significantly alters the polarizability of an array of nanorods: the polarizability is renormalized as fields scattered from-and returning to-the nanorods via the ring resonator depolarize the rods. Moreover, we show that it is possible to control the strength of the backaction by exploiting the diffractive properties of the array. This perturbation of a strong scatterer by a nearby cavity has important implications for hybrid plasmonic-photonic resonators and the understanding of coupled optical resonators in general.

Directional sideward emission from luminescent plasmonic nanostructures.

Periodic arrays of metallic nanoparticles can be used to enhance the emission of light in certain directions. We fabricated hexagonal arrays of aluminium nanoparticles combined with thin layers of luminescent material and optimized period (275 nm) and thickness (1500 nm) to obtain sideward directional emission into glass for a wavelength band around 620 nm. The key physics is that the luminescent layer acts as a waveguide, from which light is emitted at preferential angles using diffractive effects. This phenomenon has applications in the field of solid-state lighting, where there is a desire for small, bright and directional sources.

Drexhage's Experiment for Sound.

Drexhage's seminal observation that spontaneous emission rates of fluorophores vary with distance from a mirror uncovered the fundamental notion that a source's environment determines radiative linewidths and shifts. Further, this observation established a powerful tool to determine fluorescence quantum yields. We present the direct analogue for sound. We demonstrate that a Chinese gong at a hard wall experiences radiative corrections to linewidth and line shift, and extract its intrinsic radiation efficiency. Beyond acoustics, our experiment opens new ideas to extend the Drexhage experiment to metamaterials, nanoantennas, and multipolar transitions.

Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots.

The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by "blinking" on the single-nanocrystal level, that is, random alternation of bright and dark periods. The time scales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work, we investigate photoluminescence from QDs delayed over microseconds to milliseconds. Our results prove the existence of long-lived charge-separated states in QDs. We study the properties of delayed emission as a direct way to learn about charge carrier separation and recovery of the exciton state. A new microscopic model is developed to connect delayed emission to exciton recombination and blinking from which we conclude that bright periods in blinking are in fact not characterized by uninterrupted optical cycling as often assumed.

Perturbing Open Cavities: Anomalous Resonance Frequency Shifts in a Hybrid Cavity-Nanoantenna System.

The influence of a small perturbation on a cavity mode plays an important role in fields like optical sensing, cavity quantum electrodynamics, and cavity optomechanics. Typically, the resulting cavity frequency shift directly relates to the polarizability of the perturbation. Here, we demonstrate that particles perturbing a radiating cavity can induce strong frequency shifts that are opposite to, and even exceed, the effects based on the particles' polarizability. A full electrodynamic theory reveals that these anomalous results rely on a nontrivial phase relation between cavity and nanoparticle radiation, allowing backaction via the radiation continuum. In addition, an intuitive model based on coupled mode theory is presented that relates the phenomenon to retardation. Because of the ubiquity of dissipation, we expect these findings to benefit the understanding and engineering of a wide class of systems.

Reduced Auger recombination in single CdSe/CdS nanorods by one-dimensional electron delocalization.

Progress to reduce nonradiative Auger decay in colloidal nanocrystals has recently been made by growing thick shells. However, the physics of Auger suppression is not yet fully understood. Here, we examine the dynamics and spectral characteristics of single CdSe-dot-in-CdS-rod nanocrystals. These exhibit blinking due to charging/discharging, as well as trap-related blinking. We show that one-dimensional electron delocalization into the rod-shaped shell can be as effective as a thick spherical shell at reducing Auger recombination of the negative trion state.

Temporal Dynamics of Collective Resonances in Periodic Metasurfaces.

Temporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems.

Spontaneous emission control in a tunable hybrid photonic system.

We experimentally demonstrate control of the rate of spontaneous emission in a tunable hybrid photonic system that consists of two canonical building blocks for spontaneous emission control, an optical antenna and a mirror, each providing a modification of the local density of optical states (LDOS). We couple fluorophores to a plasmonic antenna to create a superemitter with an enhanced decay rate. In a superemitter analog of the seminal Drexhage experiment we probe the LDOS of a nanomechanically approached mirror. Because of the electrodynamic interaction of the antenna with its own mirror image, the superemitter traces the inverse of the LDOS enhancement provided by the mirror, in stark contrast to a bare source, whose decay rate is proportional to the mirror LDOS.

Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement.

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

Signature of a Fano resonance in a plasmonic metamolecule's local density of optical states.

We present measurements on plasmonic metamolecules under local excitation using cathodoluminescence which show a spatial redistribution of the local density of optical states at the same frequency where a sharp spectral Fano feature in extinction has been observed. Our analytical model shows that both near- and far-field effects arise due to interference of the same two eigenmodes of the system. We present quantitative insights both in a bare state, and in a dressed state picture that describe Fano interference either as near-field amplitude transfer between coupled bare states, or as interference of uncoupled eigenmodes in the far field. We identify the same eigenmode causing a dip in extinction to strongly enhance the radiative local density of optical states, making it a promising candidate for spontaneous emission control.

Ubiquity of optical activity in planar metamaterial scatterers.

Recently it was discovered that periodic lattices of metamaterial scatterers show optical activity, even if the scatterers or lattice show no 2D or 3D chirality, if the illumination breaks symmetry. We demonstrate that such "pseudochirality" is intrinsic to any single planar metamaterial scatterer and in fact has a well-defined value at a universal bound. We argue that in any circuit model, a nonzero electric and magnetic polarizability derived from a single resonance automatically imply strong bi-anisotropy, i.e., magnetoelectric cross polarizability at the universal bound set by energy conservation. We confirm our claim by extracting polarizability tensors and cross sections for handed excitation from transmission measurements on near-infrared split ring arrays, and electrodynamic simulations for diverse metamaterial scatterers.

Directional emission from plasmonic Yagi-Uda antennas probed by angle-resolved cathodoluminescence spectroscopy.

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.

Over 65% Sunlight Absorption in a 1 µm Si Slab with Hyperuniform Texture.

Thin, flexible, and invisible solar cells will be a ubiquitous technology in the near future. Ultrathin crystalline silicon (c-Si) cells capitalize on the success of bulk silicon cells while being lightweight and mechanically flexible, but suffer from poor absorption and efficiency. Here we present a new family of surface texturing, based on correlated disordered hyperuniform patterns, capable of efficiently coupling the incident spectrum into the silicon slab optical modes. We experimentally demonstrate 66.5% solar light absorption in free-standing 1 µm c-Si layers by hyperuniform nanostructuring for the spectral range of 400 to 1050 nm. The absorption equivalent photocurrent derived from our measurements is 26.3 mA/cm2, which is far above the highest found in literature for Si of similar thickness. Considering state-of-the-art Si PV technologies, we estimate that the enhanced light trapping can result in a cell efficiency above 15%. The light absorption can potentially be increased up to 33.8 mA/cm2 by incorporating a back-reflector and improved antireflection, for which we estimate a photovoltaic efficiency above 21% for 1 µm thick Si cells.

Nanopatterning of Perovskite Thin Films for Enhanced and Directional Light Emission.

Lead-halide perovskites offer excellent properties for lighting and display applications. Nanopatterning perovskite films could enable perovskite-based devices with designer properties, increasing their performance and adding novel functionalities. We demonstrate the potential of nanopatterning for achieving light emission of a perovskite film into a specific angular range by introducing periodic sol-gel structures between the injection and emissive layer by using substrate conformal imprint lithography (SCIL). Structural and optical characterization reveals that the emission is funnelled into a well-defined angular range by optical resonances, while the emission wavelength and the structural properties of the perovskite film are preserved. The results demonstrate a flexible and scalable approach to the patterning of perovskite layers, paving the way toward perovskite LEDs with designer angular emission patterns.

Wavelength-selective addressing of visible and near-infrared plasmon resonances for SU8 nanolithography.

We imprint plasmonic near field enhancements as nanoscale topography in SU8 photoresist using two-photon absorption from a spectrally filtered broadband supercontinuum light source. Imprinted patterns smaller than 50 nm across are obtained localized at positions of high local field enhancements in gold bow tie antennas, and gold split rings resonant in the visible and near-infrared. Enhanced exposure only occurs at wavelengths and polarizations that exactly match the plasmonic resonances. Hence our work demonstrates that wavelength selective addressing of hot spots for nanolithography using an inexpensive, low peak-power picosecond pulsed source is freely tunable throughout the visible and infrared to match any desired plasmon resonance.

Scanning emitter lifetime imaging microscopy for spontaneous emission control.

We report an experimental technique to map and exploit the local density of optical states of arbitrary planar nanophotonic structures. The method relies on positioning a spontaneous emitter attached to a scanning probe deterministically and reversibly with respect to its photonic environment while measuring its lifetime. We demonstrate the method by imaging the enhancement of the local density of optical states around metal nanowires. By nanopositioning, the decay rate of a pointlike source of fluorescence can be reversibly and repeatedly changed by a factor of 2 by coupling it to the guided plasmonic mode of the wire.

A Python Toolbox for Unbiased Statistical Analysis of Fluorescence Intermittency of Multilevel Emitters.

We report on a Python toolbox for unbiased statistical analysis of fluorescence intermittency properties of single emitters. Intermittency, that is, step-wise temporal variations in the instantaneous emission intensity and fluorescence decay rate properties, is common to organic fluorophores, II-VI quantum dots, and perovskite quantum dots alike. Unbiased statistical analysis of intermittency switching time distributions, involved levels, and lifetimes are important to avoid interpretation artifacts. This work provides an implementation of Bayesian changepoint analysis and level clustering applicable to time-tagged single-photon detection data of single emitters that can be applied to real experimental data and as a tool to verify the ramifications of hypothesized mechanistic intermittency models. We provide a detailed Monte Carlo analysis to illustrate these statistics tools and to benchmark the extent to which conclusions can be drawn on the photophysics of highly complex systems, such as perovskite quantum dots that switch between a plethora of states instead of just two.