Enhanced Nonlinear Emission from Single Multilayered Metal−Dielectric Nanocavities Resonating in the Near-Infrared

in ACS Photonics (2021), 8(2), 512-520

Harmonic generation mechanisms are of great interest in nanoscience and nanotechnology, since they allow generating visible light by using near-infrared radiation, which is particularly suitable for its ... [more ▼]

Harmonic generation mechanisms are of great interest in nanoscience and nanotechnology, since they allow generating visible light by using near-infrared radiation, which is particularly suitable for its countless applications in bionanophotonics and optoelectronics. In this context, multilayer metal−dielectric nanocavities are widely used for light confinement and waveguiding at the nanoscale. They exhibit intense and localized resonances that can be conveniently tuned in the near-infrared and are therefore ideal for enhancing nonlinear effects in this spectral range. In this work, we experimentally investigate the nonlinear emission properties of multilayer metal−dielectric nanocavities. By engineering their absorption efficiency and exploiting their intrinsic interface-induced symmetry breaking, we achieve an almost 2 orders of magnitude higher second-harmonic generation efficiency compared to gold nanostructures featuring the same geometry and optical resonant behavior. In particular, while both the third-order nonlinear susceptibility and conversion efficiency are comparable with those of the Au nanoresonators, we estimate a second-order nonlinear susceptibility of the order of 1 pm/V, which is comparable with that of typical nonlinear crystals. We envision that our system, which combines the advantages of both plasmonic and dielectric materials, might enable the realization of composite and multifunctional nanosystems for the efficient manipulation of nonlinear optical processes at the nanoscale. [less ▲]

Hyperbolic dispersion metasurfaces for molecular biosensing

in Nanophotonics (2021), 10(1), 295314

Sensor technology has become increasingly crucial in medical research and clinical diagnostics to directly detect small numbers of low-molecular-weight biomolecules relevant for lethal diseases. In recent ... [more ▼]

Sensor technology has become increasingly crucial in medical research and clinical diagnostics to directly detect small numbers of low-molecular-weight biomolecules relevant for lethal diseases. In recent years, various technologies have been developed, a number of them becoming core label-free technologies for detection of cancer biomarkers and viruses. However, to radically improve early disease diagnostics, tracking of disease progression and evaluation of treatments, today’s biosensing techniques still require a radical innovation to deliver high sensitivity, specificity, diffusion-limited transport, and accuracy for both nucleic acids and proteins. In this review, we discuss both scientific and technological aspects of hyperbolic dispersion metasurfaces for molecular biosensing. Optical metasurfaces have offered the tantalizing opportunity to engineer wavefronts while its intrinsic nanoscale patterns promote tremendous molecular interactions and selective binding. Hyperbolic dispersion metasurfaces support high-k modes that proved to be extremely sensitive to minute concentrations of ultralow-molecular-weight proteins and nucleic acids. [less ▲]

Recent advances on plasmonic nanocavities for single-molecule spectroscopy

in Nanoscale Advances (2021), 3

Plasmonic nanocavities are able to engineer and confine electromagnetic fields into subwavelength volumes. In the past decade, they have enabled a large set of applications, in particular for sensing ... [more ▼]

Plasmonic nanocavities are able to engineer and confine electromagnetic fields into subwavelength volumes. In the past decade, they have enabled a large set of applications, in particular for sensing, optical trapping, as well as the investigation of physical and chemical phenomena at a few or single-molecule levels. This extreme sensitivity is possible thanks to the highly confined local field intensity enhancement, which depends on the geometry of the plasmonic nanocavities. Indeed, properly designed structures providing engineered local optical fields lead to enhanced optical sensing based on different phenomena like surface enhanced Raman scattering, fluorescence, and Förster Resonant Energy Transfer. In this mini-review, we illustrate the most recent results on plasmonic nanocavities, with specific emphasis on the detection of single molecules. [less ▲]

Field-resolved detection of the temporal response of a single plasmonic antenna in the mid infrared

in Optica (2021), 8(6), 898-903

Unveiling the spatial and temporal dynamics of a light pulse interacting with nanosized objects is of extreme importance to widen our understanding of how photons interact with matter at the nanoscale and ... [more ▼]

Unveiling the spatial and temporal dynamics of a light pulse interacting with nanosized objects is of extreme importance to widen our understanding of how photons interact with matter at the nanoscale and trigger physical and photochemical phenomena. An ideal platform to study light–matter interactions with an unprecedented spatial resolution is represented by plasmonics, which enables an extreme confinement of optical energy into sub-wavelength volumes. The ability to resolve and control the dynamics of this energy confinement on the time scale of a single optical cycle is at the ultimate frontier towards a full control of nanoscale phenomena. Here, we resolve in the time domain the linear behavior of a single germanium plasmonic antenna in the mid-infrared by measuring the complex optical field response in amplitude and phase with sub-optical-cycle precision, with the promise to extend the observation of light–matter interactions in the time domain to single quantum objects. Accessing this fundamental information opens a plethora of opportunities in a variety of research areas based on plasmon-mediated photonic processes and their coherent control, such as plasmon-enhanced chemical reactions and energy harvesting. [less ▲]

Directional Plasmonic Excitation by Helical Nanotips

in Nanomaterials (2021), 11(5), 1333

The phenomenon of coupling between light and surface plasmon polaritons requires specific momentum matching conditions. In the case of a single scattering object on a metallic surface, such as a ... [more ▼]

The phenomenon of coupling between light and surface plasmon polaritons requires specific momentum matching conditions. In the case of a single scattering object on a metallic surface, such as a nanoparticle or a nanohole, the coupling between a broadband effect, i.e., scattering, and a discrete one, such as surface plasmon excitation, leads to Fano-like resonance lineshapes. The necessary phase matching requirements can be used to engineer the light–plasmon coupling and to achieve a directional plasmonic excitation. Here, we investigate this effect by using a chiral nanotip to excite surface plasmons with a strong spin-dependent azimuthal variation. This effect can be described by a Fano-like interference with a complex coupling factor that can be modified thanks to a symmetry breaking of the nanostructure. [less ▲]

Near- and Mid-Infrared Graphene-Based Photonic Architectures for Ultrafast and Low-Power Electro-Optical Switching and Ultra-High Resolution Imaging

in ACS Applied Nano Materials (2020), 3(12), 1221812230

Confining near-infrared (NIR) and mid-infrared (MIR) radiation (1–10 μm) at the nanoscale is one of the main challenges in photonics. Thanks to the transparency of silicon in the NIR-MIR range ... [more ▼]

Confining near-infrared (NIR) and mid-infrared (MIR) radiation (1–10 μm) at the nanoscale is one of the main challenges in photonics. Thanks to the transparency of silicon in the NIR-MIR range, optoelectronic systems like electro-optical modulators have been broadly designed in this range. However, the trade-off between energy-per-bit consumption and speed still constitutes a significant bottleneck, preventing such a technology to express its full potentialities. Moreover, the harmless nature of NIR radiation makes it ideal for bio-photonic applications. In this work, we theoretically showcase a new kind of electro-optical modulators in the NIR-MIR range that optimize the trade-off between power consumption, switching speed, and light confinement, leveraging on the interplay between graphene and metamaterials. We investigate several configurations among which the one consisting in a SiO2/graphene hyperbolic metamaterial (HMM) outstands. The peculiar multilayered configuration of the HMM allowed one also to minimize the equivalent electrical capacitance to achieve attoJoule electro/optical modulation at about 500 MHz switching speed. This system manifests the so-called dielectric singularity, in correspondence to which an HMM lens with resolving power of λ/1660 has been designed, allowing to resolve 3 nm-wide objects placed at an interdistance of 3 nm and to overcome the diffraction limit by 3 orders of magnitude. The imaging possibilities opened by such technologies are evident especially in bio-photonic applications, where the investigation of biological entities with tailored/broadband-wavelength radiation and nanometer precision is necessary. Moreover, the modulation performances demonstrated by the graphene-based HMM configure it as a promise for ultrafast and low-power opto-electronics applications. [less ▲]

λ-DNA Through porous materials – Surface Enhanced Raman Scattering in a simple plasmonic nanopore

in Journal of Physical Chemistry. C, Nanomaterials and interfaces (2020), 124(41), 22663-22670

Engineered electromagnetic fields in plasmonic nanopores enable enhanced optical detection for single molecule sensing and sequencing. Here, a plasmonic nanopore prepared in a thick nanoporous film is ... [more ▼]

Engineered electromagnetic fields in plasmonic nanopores enable enhanced optical detection for single molecule sensing and sequencing. Here, a plasmonic nanopore prepared in a thick nanoporous film is used to investigate, by means of surface enhanced Raman spectroscopy, the interaction between the metallic surface of the pore and a long-chain double strand DNA molecule free to diffuse through the pore. We discuss how the matrix of the porous material can interact with the molecule thanks to: i) transient aspecific interactions between the porous surface and DNA; ii) diffusion; iii) thermal and optical forces exerted by the localized field in a metallic nanostructure on the DNA molecule. An interaction time up to tens of milliseconds enables to collect high signal-to-noise Raman signatures, allowing an easy label-free reading of information from the DNA molecule. Moreover, to increase the rate of detection, we tested a polymeric porous hydrogel placed beneath the solid-state membrane. The hydrogel enables a slowdown of the molecule diffusion time, thus increasing the number of detected interaction events by a factor 20. The analysis of the observed Raman peaks and their relative intensities, combined with theoretical simulations, allow to get further information on the process of translocation and on the folding state and orientation of the translocating molecule. Our results demonstrate temporary adsorption of the DNA molecule on the porous material during the translocation due to the diffusion force. Finally, we provide a qualitative evaluation of the nucleotides’ contents in the different groups of collected signal. The proposed approach can find interesting applications not only in DNA sensing and sequencing, but also on generic nanopore spectroscopy. [less ▲]

Two-state switchable plasmonic tweezers for dynamic manipulation of nano-objects

in Nanoscale (2020), 12

In this work, we present a plasmonic platform capable of trapping nano-objects in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer ... [more ▼]

In this work, we present a plasmonic platform capable of trapping nano-objects in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer wall of a vertical gold nanochannel, can be activated by varying the focusing position of the excitation laser along the main axis of the nanotube. We show that the switching of the trapping site is induced by changes in the distribution of the electromagnetic field and of the trapping force. The “inner” and “outer” trapping states are characterized by a static and a dynamic behavior respectively, and their stiffness is measured by analyzing the positions of the trapped specimens as a function of time. In addition, we demonstrate that the stiffness of the static state is high enough to trap particles with diameter as small as 40 nm. These results show a simple, controllable way to generate a switchable two-state trapping regime, which could be used as a model for the study of dynamic trapping or as a mechanism for the development of nanofluidic devices. [less ▲]

Plasmon Hybridization in Compressible Metal-Insulator-Metal Nano-Cavities: an Optical Approach for Sensing Deep Sub-Wavelength Deformation

in Advanced Optical Materials (2020), 8(18), 2000609

We present a pressure-induced deformation-sensitive device based on 2D matrices of plasmonic gold nanodisks coupled to a metal thin layer through a compressible dielectric spacer, namely a deformable ... [more ▼]

We present a pressure-induced deformation-sensitive device based on 2D matrices of plasmonic gold nanodisks coupled to a metal thin layer through a compressible dielectric spacer, namely a deformable metal-insulator-metal (MIM) nanocavity, to report deep sub-wavelength size variations (< λ/200). The system is characterized by two hybrid branches, which are resonant in the visible/near infrared spectral region. The fundamental mode, owing to the near-field interaction between the plasmonic nanostructures and the metal film, exhibits a remarkable sensitivity to the gap size, exceeding that of a planar “macroscopic” optical cavity and extending its operational domain to the sub-wavelength range, where excellent opportunities towards truly multiscale MIMs-based pressure sensors can be envisioned. Concurrently, its intrinsic plasmonic nature synergistically combines into a single platform multi-purpose functionalities, such as ultrasensitive detection, remote temperature readout etc., with practical perspectives in ultra-compact inspection tools for structural and functional information at the nanoscale. [less ▲]

Speeding up nanoscience and nanotechnology with ultrafast plasmonics

in Nano Letters (2020)

Surface plasmons are collective oscillations of free electrons at the interface between a conducting material and the dielectric environment. These excitations support the formation of strongly enhanced ... [more ▼]

Surface plasmons are collective oscillations of free electrons at the interface between a conducting material and the dielectric environment. These excitations support the formation of strongly enhanced and confined electromagnetic fields. As well, they display fast dynamics lasting tens of femtoseconds and can lead to a strong nonlinear optical response at the nanoscale. Thus, they represent the perfect tool to drive and control fast optical processes, such as ultrafast optical switching, single photon emission, as well as strong coupling interactions to explore and tailor photochemical reactions. In this Virtual Issue, we gather several important papers published in Nano Letters in the past decade reporting studies on the ultrafast dynamics of surface plasmons. [less ▲]