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1.
ACS Nano ; 2020 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-33232119

RESUMO

Plasmonically enhanced optical dichroism has attracted substantial interest for its application in optical sensing, where the interplay between chirality emanating from both molecules and plasmon-supporting structures has been regarded as a critical ingredient. Here, we experimentally demonstrate that suitably self-assembled achiral plasmonic nanostructures produce a high degree of enhancement in the optical dichroism observed from chiral molecules placed in their vicinity. Specifically, we identify a near-field enhancement associated with plasmonic hotpots as the mechanism enabling our observation of visible-NIR circular dichroism emanating from small amounts of chiral molecules. Our structures consist of linear arrays of gold nanorods obtained by introducing chiral anionic surfactants, such as modified bile salts, which lead to selective destabilization of a cetyltrimethylammonium bromide coating layer on Au nanorods, thereby promoting a tip-to-tip oriented assembly. The proposed mechanism of plasmonically-enhanced circular dichroism is supported by deriving a simple, yet general theoretical formalism that confirms the observed results, revealing the role of optical hotspots at the gaps of linear tip-to-tip nanorod assemblies as the origin of enhancement in the dichroism from chiral molecules. Importantly, it is the refractive rather than the absorption-mediated chiral response of the molecules that produces dichroism in the visible-NIR plasmonic regime, far from their UV absorption resonances. The observed self-assembly mechanism suggests that chiral analytes not directly interacting with the nanorod surfaces, but just able to induce tip-to-tip aggregation, can be revealed by a CD signature in the plasmonic region, thereby supporting potential applications in ultrasensitive analysis.

2.
Phys Rev Lett ; 125(17): 176802, 2020 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-33156664

RESUMO

We reveal a dramatic departure of electron thermodiffusion in solids relative to the commonly accepted picture of the ideal free-electron gas model. In particular, we show that the interaction with the lattice and impurities, combined with a strong material dependence of the electron dispersion relation, leads to counterintuitive diffusion behavior, which we identify by comparing a two-dimensional electron gas (2DEG) and single-layer graphene. When subject to a temperature gradient ∇T, thermodiffusion of massless Dirac fermions in graphene exhibits an anomalous behavior with electrons moving along ∇T and accumulating in hot regions, in contrast to normal electron diffusion in a 2DEG with parabolic dispersion, where net motion against ∇T is observed, accompanied by electron depletion in hot regions. These findings bear fundamental importance for the understanding of the spatial electron dynamics in emerging materials, establishing close relations with other branches of physics dealing with electron systems under nonuniform temperature conditions.

3.
ACS Nano ; 2020 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-32845613

RESUMO

Circularly polarized light (CPL) is currently receiving much attention as a key ingredient for next-generation information technologies, such as quantum communication and encryption. CPL photon generation used in those applications is commonly realized by coupling achiral optical quantum emitters to chiral nanoantennas. Here, we explore a different strategy consisting in exciting a nanosphere-the ultimate symmetric structure-to produce CPL emission along an arbitrary direction. Specifically, we demonstrate chiral emission from a silicon nanosphere induced by an electron beam based on two different strategies: either shifting the relative phase of degenerate orthogonal dipole modes or interfering electric and magnetic modes. We prove these concepts both theoretically and experimentally by visualizing the phase and polarization using a fully polarimetric four-dimensional cathodoluminescence method. Besides their fundamental interest, our results support the use of free-electron-induced light emission from spherically symmetric systems as a versatile platform for the generation of chiral light with on-demand control over the phase and degree of polarization.

4.
Nat Mater ; 2020 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-32807920

RESUMO

Phonon polaritons enable light confinement at deep subwavelength scales, with potential technological applications, such as subdiffraction imaging, sensing and engineering of spontaneous emission. However, the trade-off between the degree of confinement and the excitation efficiency of phonon polaritons prevents direct observation of these modes in monolayer hexagonal boron nitride (h-BN), where they are expected to reach ultrahigh confinement. Here, we use monochromatic electron energy-loss spectroscopy (about 7.5 meV energy resolution) in a scanning transmission electron microscope to measure phonon polaritons in monolayer h-BN, directly demonstrating the existence of these modes as the phonon Reststrahlen band (RS) disappears. We find phonon polaritons in monolayer h-BN to exhibit high confinement (>487 times smaller wavelength than that of light in free space) and ultraslow group velocity down to about 10-5c. The large momentum compensation provided by electron beams additionally allows us to excite phonon polaritons over nearly the entire RS band of multilayer h-BN. These results open up a broad range of opportunities for the engineering of metasurfaces and strongly enhanced light-matter interactions.

5.
Nat Mater ; 19(8): 830-837, 2020 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-32632282

RESUMO

Semimetals are being explored for their unique advantages in low-energy high-speed photodetection, although they suffer from serious drawbacks such as an intrinsically high dark current. In this Perspective, we envision the exploitation of topological effects in the photoresponse of these materials as a promising route to circumvent these problems. We overview recent studies on photodetection based on graphene and other semimetals, and further discuss the opportunities created by topological effects, along with the additional challenges that they impose on photodetector designs.

6.
ACS Nano ; 2020 Jun 18.
Artigo em Inglês | MEDLINE | ID: covidwho-606639

RESUMO

We advocate the widespread use of UV-C light as a short-term, easily deployable, and affordable way to limit virus spread in the current SARS-CoV-2 pandemic. Radical social distancing with the associated shutdown of schools, restaurants, sport clubs, workplaces, and traveling has been shown to be effective in reducing virus spread, but its economic and social costs are unsustainable in the medium term. Simple measures like frequent handwashing, facial masks, and other physical barriers are being commonly adopted to prevent virus transmission. However, their efficacy may be limited, particularly in shared indoor spaces, where, in addition to airborne transmission, elements with small surface areas such as elevator buttons, door handles, and handrails are frequently used and can also mediate transmission. We argue that additional measures are necessary to reduce virus transmission when people resume attending schools and jobs that require proximity or some degree of physical contact. Among the available alternatives, UV-C light satisfies the requirements of rapid, widespread, and economically viable deployment. Its implementation is only limited by current production capacities, an increase of which requires swift intervention by industry and authorities.

7.
ACS Nano ; 14(7): 7704-7713, 2020 07 28.
Artigo em Inglês | MEDLINE | ID: mdl-32551537

RESUMO

We advocate the widespread use of UV-C light as a short-term, easily deployable, and affordable way to limit virus spread in the current SARS-CoV-2 pandemic. Radical social distancing with the associated shutdown of schools, restaurants, sport clubs, workplaces, and traveling has been shown to be effective in reducing virus spread, but its economic and social costs are unsustainable in the medium term. Simple measures like frequent handwashing, facial masks, and other physical barriers are being commonly adopted to prevent virus transmission. However, their efficacy may be limited, particularly in shared indoor spaces, where, in addition to airborne transmission, elements with small surface areas such as elevator buttons, door handles, and handrails are frequently used and can also mediate transmission. We argue that additional measures are necessary to reduce virus transmission when people resume attending schools and jobs that require proximity or some degree of physical contact. Among the available alternatives, UV-C light satisfies the requirements of rapid, widespread, and economically viable deployment. Its implementation is only limited by current production capacities, an increase of which requires swift intervention by industry and authorities.


Assuntos
Ar Condicionado/normas , Infecções por Coronavirus/transmissão , Transmissão de Doença Infecciosa/prevenção & controle , Desinfecção/métodos , Calefação/normas , Pneumonia Viral/transmissão , Ventilação/normas , Ar Condicionado/efeitos adversos , Espaços Confinados , Infecções por Coronavirus/epidemiologia , Calefação/efeitos adversos , Humanos , Pandemias , Pneumonia Viral/epidemiologia , Transportes/normas , Raios Ultravioleta
8.
Artigo em Inglês | MEDLINE | ID: mdl-32383890

RESUMO

Circular dichroism spectroscopy is an essential technique for understanding molecular structure and magnetic materials; however, spatial resolution is limited by the wavelength of light, and sensitivity sufficient for single-molecule spectroscopy is challenging. We demonstrate that electrons can efficiently measure the interaction between circularly polarized light and chiral materials with deeply subwavelength resolution. By scanning a nanometer-sized focused electron beam across an optically excited chiral nanostructure and measuring the electron energy spectrum at each probe position, we produce a high-spatial-resolution map of near-field dichroism. This technique offers a nanoscale view of a fundamental symmetry and could be employed as "photon staining" to increase biomolecular material contrast in electron microscopy.

9.
Nano Lett ; 2020 May 27.
Artigo em Inglês | MEDLINE | ID: mdl-32401522

RESUMO

Free electrons act as a source of highly confined, spectrally broad optical fields that are widely used to map photonic modes with nanometer/millielectronvolt space/energy resolution through currently available electron energy-loss and cathodoluminescence spectroscopies. These techniques are understood as probes of the linear optical response, while nonlinear dynamics has escaped observation with similar degree of spatial detail, despite the strong enhancement of the electron evanescent field with decreasing electron energy. Here, we show that the field accompanying low-energy electrons can trigger anharmonic response in strongly nonlinear materials. Specifically, through realistic quantum-mechanical simulations, we find that the interaction between ≲100 eV electrons and plasmons in graphene nanostructures gives rise to substantial optical nonlinearities that are discernible as saturation and spectral shifts in the plasmonic features revealed in the cathodoluminescence emission and electron energy-loss spectra. Our results support the use of low-energy electron-beam spectroscopies for the exploration of nonlinear optical processes in nanostructures.

10.
Nano Lett ; 2020 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-31967839

RESUMO

Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques. Here, we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. We finely tune the energy of surface plasmons in metallic nanowires in the vicinity of hexagonal boron nitride, making it possible to monitor and disentangle both strong phonon-plasmon coupling and plasmon-driven phonon enhancement at the nanometer scale. Because of the near-field character of the electron beam-phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.

11.
ACS Nano ; 14(1): 28-117, 2020 01 28.
Artigo em Inglês | MEDLINE | ID: mdl-31478375

RESUMO

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

12.
Nano Lett ; 20(1): 592-598, 2020 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-31855432

RESUMO

Nanoscale gaps between metals can strongly confine electromagnetic fields that enable efficient electromagnetic energy conversion and coupling to nanophotonic structures. In particular, the gap formed by depositing a metallic particle on a metallic substrate produces coupling of localized particle plasmons to propagating surface plasmon polaritons (SPPs). Understanding and controlling the phase of such coupling is essential for the design of devices relying on nanoparticles coupled through SPPs. Here we demonstrate the experimental visualization of the phase associated with the plasmonic field of metallic particle-surface composites through nanoscopically and spectroscopically resolved cathodoluminescence using a scanning transmission electron microscope. Specifically, we study the interference between the substrate transition radiation and the field resulting from out-coupling of SPP excitation, therefore giving rise to angle-, polarization-, and energy-dependent photon emission fringe patterns from which we extract phase information. Our methods should be readily applicable to more complex nanostructures, thus providing direct experimental insight into nanoplasmonic near-fields with potential applications in improving plasmon-based devices.

13.
Phys Rev Lett ; 123(6): 066803, 2019 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-31491154

RESUMO

Light interaction with rotating nanostructures gives rise to phenemona as varied as optical torques and quantum friction. Surprisingly, the most basic optical response function of nanostructures undergoing rotation has not been clearly addressed so far. Here we reveal that mechanical rotation results in circular dichroism in optically isotropic particles, which show an unexpectedly strong dependence on the particle internal geometry. More precisely, particles with one-dimensionally confined electron motion in the plane perpendicular to the rotation axis, such as nanorings and nanocrosses, exhibit a splitting of 2Ω in the particle optical resonances, while compact particles, such as nanodisks and nanospheres, display weak circular dichroism. We base our findings on a quantum-mechanical description of the polarizability of rotating particles, incorporating the mechanical rotation by populating the particle electronic states according to the principle that they are thermally equilibrated in the rotating frame. We further provide insight into the rotational superradience effect and the ensuing optical gain, originating in population inversion as regarded from the lab frame, in which the particle is out of equilibrium. Surprisingly, we find the optical frequency cutoff for superradiance to deviate from the rotation frequency Ω. Our results unveil a rich, unexplored phenomenology of light interaction with rotating objects, which might find applications in various fields, such as optical trapping and sensing.

14.
Acc Chem Res ; 52(9): 2536-2547, 2019 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-31448890

RESUMO

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.

15.
Nat Mater ; 18(11): 1158-1171, 2019 11.
Artigo em Inglês | MEDLINE | ID: mdl-31308514

RESUMO

Progress in electron-beam spectroscopies has recently enabled the study of optical excitations with combined space, energy and time resolution in the nanometre, millielectronvolt and femtosecond domain, thus providing unique access into nanophotonic structures and their detailed optical responses. These techniques rely on ~1-300 keV electron beams focused at the sample down to sub-nanometre spots, temporally compressed in wavepackets a few femtoseconds long, and in some cases controlled by ultrafast light pulses. The electrons undergo energy losses and gains (also giving rise to cathodoluminescence light emission), which are recorded to reveal the optical landscape along the beam path. This Review portraits these advances, with a focus on coherent excitations, emphasizing the increasing level of control over the electron wavefunctions and ensuing applications in the study and technological use of optically resonant modes and polaritons in nanoparticles, 2D materials and engineered nanostructures.

16.
ACS Nano ; 13(7): 7771-7779, 2019 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-31188552

RESUMO

Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons, the collective electron oscillations in conducting materials, are important because they allow the manipulation of optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant near-infrared regime. Here, we demonstrate sharp near-infrared plasmons in lithographically patterned wafer-scale atomically thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor of ∼4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications.

17.
Nano Lett ; 19(6): 3743-3750, 2019 06 12.
Artigo em Inglês | MEDLINE | ID: mdl-31117754

RESUMO

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

18.
Proc Natl Acad Sci U S A ; 116(17): 8173-8177, 2019 04 23.
Artigo em Inglês | MEDLINE | ID: mdl-30952783

RESUMO

Structural information on electronically excited neutral molecules can be indirectly retrieved, largely through pump-probe and rotational spectroscopy measurements with the aid of calculations. Here, we demonstrate the direct structural retrieval of neutral carbonyl disulfide (CS2) in the [Formula: see text] excited electronic state using laser-induced electron diffraction (LIED). We unambiguously identify the ultrafast symmetric stretching and bending of the field-dressed neutral CS2 molecule with combined picometer and attosecond resolution using intrapulse pump-probe excitation and measurement. We invoke the Renner-Teller effect to populate the [Formula: see text] excited state in neutral CS2, leading to bending and stretching of the molecule. Our results demonstrate the sensitivity of LIED in retrieving the geometric structure of CS2, which is known to appear as a two-center scatterer.

19.
ACS Nano ; 13(5): 5184-5197, 2019 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-30916551

RESUMO

Polaritonic modes in two-dimensional van der Waals materials display short in-plane wavelengths compared with light in free space. As interesting as this may look from both fundamental and applied viewpoints, such large confinement is accompanied by poor in/out optical coupling, which severely limits the application of polaritons in practical devices. Here, we quantify the coupling strength between light and 2D polaritons in both homogeneous and anisotropic films using accurate rigorous analytical methods. In particular, we obtain universal expressions for the cross sections associated with photon-polariton coupling by point and line defects, as well as with polariton extinction and scattering processes. Additionally, we find closed-form constraints that limit the maximum possible values of these cross sections. Specifically, the maximum photon-to-plasmon conversion efficiency in graphene is ∼10-6 and ∼10-4 for point and line scatterers sitting at its surface, respectively, when the plasmon and Fermi energies are comparable in magnitude. We further show that resonant particles placed at an optimum distance from the film can boost light-to-polariton coupling to order unity. Our results bear fundamental interest for the development of 2D polaritonics and the design of applications based on these excitations.

20.
Nat Commun ; 10(1): 1131, 2019 03 08.
Artigo em Inglês | MEDLINE | ID: mdl-30850594

RESUMO

Identification of gas molecules plays a key role a wide range of applications extending from healthcare to security. However, the most widely used gas nano-sensors are based on electrical approaches or refractive index sensing, which typically are unable to identify molecular species. Here, we report label-free identification of gas molecules SO2, NO2, N2O, and NO by detecting their rotational-vibrational modes using graphene plasmon. The detected signal corresponds to a gas molecule layer adsorbed on the graphene surface with a concentration of 800 zeptomole per µm2, which is made possible by the strong field confinement of graphene plasmons and high physisorption of gas molecules on the graphene nanoribbons. We further demonstrate a fast response time (<1 min) of our devices, which enables real-time monitoring of gaseous chemical reactions. The demonstration and understanding of gas molecule identification using graphene plasmonic nanostructures open the door to various emerging applications, including in-breath diagnostics and monitoring of volatile organic compounds.

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