Interface effects in metals-semiconductors heterojunctions are subject of intense research due to the possibility to exploit the synergy between their electronic and optical properties in next-generation opto-electronic devices. In this framework, understanding the carrier dynamics at the metal-semiconductor interface, as well as achieving a coherent control of charge and energy transfer in metal-semiconductor heterostructures, are crucial and yet quite unexplored aspects. Here, we experimentally show that thermionically injected carriers from a gold substrate can drastically affect the dynamics of excited carriers in bulk WS2. By employing a pump-push-probe scheme, where a push pulse excites direct transitions in the WS2, and another delayed pump pulse induces thermionic injection of carriers from the gold substrate into the semiconductor, we can control both the formation and annihilation of excitons. Our findings might foster the development of novel opto-electronic approaches to control charge dynamics using light at ultrafast timescales.
Ultrafast control of light-matter interactions is fundamental in view of new technological frontiers, for instance in light-driven information processing and nanoscale photochemistry. In this framework, we explore metal-dielectric nanocavities to achieve all-optical modulation of the light reflectance at a specific wavelength. Without the need of driving higher order effects, our system is based on linear absorption, provides large relative modulation exceeding 100% and switching bandwidths of few hundred GHz at moderate excitation fluence. This archetypical system becomes even more interesting if the “gain medium” is an inorganic van der Waals bonded semiconductor, like a transition metal dichalcogenide (TMD). TMDs are subject of intense research due to their electronic and optical properties which are promising for next-generation optoelectronic devices. In this context, understanding the ultrafast carrier dynamics, as well as charge and energy transfer at the interface between metals and semiconductors is crucial and yet quite unexplored. By employing a pump-push-probe scheme, we experimentally study how thermally induced ultrafast charge carrier injection affects the exciton formation dynamics in bulk WS2, opening up excellent opportunities also in nano-chemistry. In fact, if an electronic transition strongly interacts with the light modes of a resonator, we can tailor the energetics and the morphology of a molecular state. By combining quantum mechanical modelling and pump-probe spectroscopy, we shed light on the ultrafast dynamics of a hybrid system composed of photo-switchable dye molecules coupled with optically anisotropic plasmonic nanoantennas, which allow us to selectively switch between two regimes where the light-matter interaction is either weak or strong. Our synergistic approach is instrumental to devise new strategies for tailoring electronic states by using plasmons for applications in polaritonic chemistry on femtosecond timescales.
Photonics and spintronics represent a great promise to overcome the fundamental limits of electronics since light and spins are simultaneously much faster and less dissipative of electrons. In this framework, the quest for energy-efficient data processing and storage functionalities led great attention to the field of femtomagnetism, the study and control of magnetism using ultrashort light pulses. However, our knowledge of magnetic phenomena and ultrafast light-matter interactions in nanoscale magnetic materials is extremely limited. In this work, we introduce a time-resolved magnetooptical pump-probe spectroscopy scheme enabling to access both the thermal and nonthermal spin (and charge) dynamics with sub-15 fs temporal resolution. We test the capabilities of our system on archetypical magnetic and magnetoplasmonic materials, such as Ni thin films and Ni nanodisks.
In the last decade, solid-state nanopores have been intensively investigated as label-free detectors of for single biological entities, such as protein chains or DNA molecules. With this approach, single entities are typically driven through a nanopore by applying an external electrical potential. However, this method cannot enable control over the speed of translocation, thus limiting the signal integration time. The most explored approach to introduce control of the translocation speed is based on trapping. In particular, a long acquisition time can be obtained by trapping a nanoparticle tagged with molecules close to a nanopore. The trapping phenomena can be generated by means of external stimuli such as light excitation and magnetic field application, obtaining respectively the so-called optical and magnetic trapping. Magnetic trapping, in particular, has been less explored but can be a useful approach to obtain very large trapping forces without interfering with other optical exitations that can be used for spectroscopic purposes. Here, we will briefly summarize the major examples of magnetic trapping approaches reported so far in solid-state nanopore technology.
Hyperbolic nanoparticles provide a versatile platform to widely tune light-matter interactions. Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical response of hyperbolic nanoparticles across the visible and near infrared spectral range. By using a perturbative approach, we establish a model where the magneto-optical activity of the system is described in terms of the coupling of fundamental electric and magnetic dipole modes, which are induced by the hyperbolic dispersion, with a static magnetic field. Finally, an analytical model is established in the framework of Mie theory to describe the magneto-optical response and identify the contribution of electric and magnetic modes to the total spectrum.
In the framework of magneto-photonics, the optical properties of a material can controlled by an external magnetic field, providing active functionalities for applications, such as sensing and nonreciprocal optical isolation. For noble metals in particular, the inherently weak magnetooptical coupling of the bulk material can be greatly enhanced via excitation of localized surface plasmons (LSP) in nanostructured samples. Hyperbolic metamaterials therein provide the ideal platform to tune the plasmonic properties via careful design of the effective permittivity tensor. Here, we report on the magnetic circular dichroism of electric and magnetic dipole modes of a type II hyperbolic metasurface. Disk-shaped nanoparticles consist in stacks of alternating dielectric and metallic layers. Using an effective medium theory, we show that the optical properties of the system can be perfectly described by an anisotropic homogenized permittivity. Magnetic circular dichroism spectroscopy experiments are compared with plain gold disk samples and reveal a broadband magneto-optical response across the visible and near infrared spectral range. In particular, derivative-like spectral signatures at the resonances of the nanoparticles prove the induced dichroism for the two modes of the system. Results are interpreted in terms of magnetically induced spatial confinement/broadening of circular currents in the nanoparticles and are compared with a comprehensive numerical model based on the finite elements method using the real dimensions of the nanostructure. Spherical particles are employed as an analytical model system, allowing to generalize the contribution of electric and magnetic modes to the total magneto-optical response. More in detail, interaction cross sections are calculated as a weighted sum of the corresponding Mie coefficients. Utilizing a perturbative approach, we describe the magneto-optical effect in terms of linear changes in the cyclotron frequency of free charge carriers in the metal. By comparing our analytical model with full-wave numerical results, we can identify the contribution of electric and magnetic dipole modes to the spectrum and reproduce the spectral line shape we observe in the experiments for the hyperbolic nanoparticles.
We report efficient second-harmonic emission by single multilayer metal–dielectric nanocavities. Engineering the intrinsic interface-induced symmetry breaking by resonant optical absorption design, allows to achieve almost two orders of magnitude higher second-harmonic generation efficiency compared to gold nanostructures with the same geometry. We estimate a second-order nonlinear susceptibility of the order of 1 pm/V, which is comparable to widely used nonlinear crystals. We envision that our system, which combines the advantages of both plasmonic and dielectric materials, might enable the realization of composite nano-systems for an efficient multi-purpose manipulation of nonlinear optical processes at the nanoscale.
We report on ultrafast opto-acoustic modulation of light reflectance in artificial epsilon-near-zero metamaterials made of two layers of Ag separated by an Al2O3 layer. By means of non-degenerate two color pump-probe experiments we demonstrate an optically induced acoustic modulation of the reflectance up to 20% via generation of acoustic waves inside the cavity upon mechanical expansion of the metal due to hot electron-phonon coupling nonlinearity in the Ag layers. The presented architecture opens the pathway towards novel routes to exploit light-matter interactions for opto-acoustic modulation at GHz frequencies. Moreover, our system can be designed to work in transmission geometry and is very versatile in terms of shifting the presented properties along a broad range of wavelengths, from UV to mid-IR. Our approach, beyond light-driven information processing, might impact also opto-mechanics, light-driven phonon induced up conversion mechanisms, non-linear optical and acoustic properties of materials, energy harvesting, and heat-assisted ultrafast magneto-optical recording.
Here, we propose easy and robust strategies for the versatile integration 2D material flakes on plasmonic nanoholes by means of site selective deposition of MoS2. The methods can be applied both to simple metallic flat nanostructures and to complex 3D metallic structures comprising nanoholes. The deposition methods allow the decoration of large ordered arrays of plasmonic structures with single or few layers of MoS2. We show that the plasmonic field generated by the nanohole can interact significantly with the 2D layer, thus representing an ideal system for hybrid 2DMaterial/ Plasmonic investigation. The controlled/ordered integration of 2D materials on plasmonic nanostructures opens a pathway towards new investigation of the following: enhanced light emission; strong coupling from plasmonic hybrid structures; hot electron generation; and sensors in general based on 2D materials.
We fabricated hollow nanoantennas with varying inner channels sizes on a gold-covered silicon nitride membrane. Our fabrication technique allowed us to narrow the size of the inner channels down to 15nm. We managed to exclusively decorate the tips of the antennas with thiol-conjugated dyes by creating a concentration gradient through the nanoantennas. Finally, we characterized the antennas in terms of their effect on the lifetime of dyes. We used Atto 520 and Atto 590 for the experiments. We carried out experiments with the antennas decorated with Atto 520, with Atto 590 as well as with the two Atto dyes at the same time. The experiments carried out with the antennas decorated with Atto 520 only and Atto 590 only yielded a lifetime reduction with respect to the confocal case. Interestingly, their lifetime reductions were significantly different. Then, we decorated the antennas with the two dyes at the same time. Even though we could not control the distance between the two dyes, FRET effects were clearly observed. The FRET effects were found to be dependent on the size of the inner channel. We believe that our tip decorated hollow nanoantennas could find application in FRET-based single molecule nanopore technologies.
We report on the fabrication and optical characterization of hyperbolic nanoparticles on a transparent substrate. These nanoparticles enable a separation of ohmic and radiative channels in the visible and near-infrared frequency ranges. The presented architecture opens the pathway towards novel routes to exploit the light to energy conversion channels beyond what is offered by current plasmon-based nanostructures, possibly enabling applications spanning from thermal emission manipulation, theragnostic nano-devices, optical trapping and nano-manipulation, non-linear optical properties, plasmonenhanced molecular spectroscopy, photovoltaics and solar-water treatments, as well as heat-assisted ultra-dense and ultrafast magnetic recording.
A major challenge facing plasmon nanophotonics is the poor dynamic tunability. A functional nanophotonic element would feature the real-time sizeable tunability of transmission, reflection of light’s intensity or polarization over a broad range of wavelengths, and would be robust and easy to integrate. Here we devise an ultra-thin chiroptical surface, built on 2D nanoantennas, where the chiral light transmission is controlled by the externally applied magnetic field. We produced a class of highly tunable by the magnetic field macroscale bottom-up plasmonic chiroptical surfaces. The tuned parameter is the chiroptical transmission, enabled by the nanoantenna design that accommodates ferromagnetic plasmonic elements. The already significant chiroptical response of this system is further tuned up to 150% by the external magnetic field. The presented compact 2D design promises the easy integration and potentially fast operation in the broad spectral range, enabling this type of functional plasmonic surfaces entering the realm of practical optical devices. The magnetic field-induced modulation of the far-field chiroptical response with this surface exceeds 100% in the visible and near-infrared spectral ranges, opening the route for nanometer-thin magnetoplasmonic light-modulating surfaces tuned in real time and featuring a broad spectral response. For this we design a 2D composite trimer nanoantennas comprising three near-field-coupled nanosized disks of diameters close to 100 nm and identical height of 30 nm, of which one is made of a ferromagnetic material and the other two are made of a noble metal. The use of two materials breaks the 2D rotational symmetry, endowing the handedness to the trimer that results in a chiroptical response in otherwise structurally symmetric nanoantenna. We leverage on the presence of the plasmon resonances in metallic nanoferromagnets to add the magnetoplasmonic functionality to the system.
Nanoporous gold is a very promising material platform for several plasmonic applications. Nanoporous gold can be
formed by dealloying Au–Ag alloys, previously grown by means of Ag-Au co-sputtering. The optical response is
completely determined by the nanostructured film features, that only depend on the initial alloy composition. It has been
extensively used as SERS substrate both as thin film and nanofabricated fancy designs. Here we explore the potential
application of nanoporous gold as SERS substrate as it is coupled and decorated with Ag nanoparticles. Significant
enhancement has been observed in comparison with bare nanoporous film.
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