We begin our NSF project: New Plasmonic Platforms for Nanophotonics: PT-symmetry, Geometry, and Dimensionality.
The overarching goal of this proposal is to open new research paths in plasmonics that can lead to the development of new applications in nanophotonics. To achieve that goal, a range of unexplored concepts affecting the composition, geometrical arrangement, and dimensionality of metallic nanostructures will be explored. The motivation is twofold: first, to understand the fundamentals of these new physical phenomena and, second, to exploit that knowledge to develop plasmonic systems with capabilities beyond those of conventional structures that can be used to manipulate light below the diffraction limit. The investigation will be structured in three parallel research paths that will address the following specific goals: (1) investigate parity-time symmetric plasmonic nanostructures to achieve strongly asymmetric responses that can be used to gain new levels of control over the electromagnetic field, (2) understand how the geometry of complex arrangements of plasmonic nanostructures can produce strongly localized, long-lived plasmonic resonances with enhanced near- and far-field responses, and (3) study the unique characteristics of the response of low-dimensional nanostructures and exploit them to create ultracompact plasmonic platforms.
Our last publication - How to identify plasmons from the optical response of nanostructures - has appeared in ACS Nano
A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light−matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons. Our study provides a rigorous foundation for the further development of ultrasmall nanostructures based on molecular plasmonics.
Our last publication - Unidirectional evanescent-wave coupling from circularly polarized electric and magnetic dipoles: An angular spectrum approach - has appeared in Physical Review B
Unidirectional evanescent-wave coupling from circularly polarized dipole sources is one of the most striking types of evidence of spin-orbit interactions of light and an inherent property of circularly polarized dipoles. Polarization handedness self-determines propagation direction of guided modes. In this paper, we compare two different approaches currently used to describe this phenomenon: the first requires the evaluation of the coupling amplitude between dipole and waveguide modes, while the second is based on the calculation of the angular spectrum of the dipole. We present an analytical expression of the angular spectrum of dipole radiation, unifying the description for both electric and magnetic dipoles. The symmetries unraveled by the implemented formalism show the existence of specific terms in the dipole spectrum which can be recognized as being directly responsible for directional evanescent-wave coupling. This provides a versatile tool for both a comprehensive understanding of the phenomenon and a fully controllable engineering of directionality of guided modes.
Our last publication - Spatially resolved optical sensing using graphene Nanodisk Arrays - has appeared in ACS Photonics
The ability of graphene nanostructures to support strong plasmonic resonances in the infrared part of the spectrum makes them an ideal platform for plasmon-enhanced spectroscopy techniques. Here we propose to exploit the exceptional tunability of graphene plasmons to perform infrared detection of molecules with subwavelength spatial resolution. To that end, we investigate the optical response of finite arrays of graphene nanodisks that are divided into a number of identical subarrays, or pixels, each of them with a uniform level of doping. Using realistic conditions, we show that, by adjusting individually the doping level of each of these pixels, it is possible to bring them sequentially into resonance with the vibrational spectrum of the analyte. This enables the identification of the analyte and the simultaneous detection of its spatial location with a resolution determined by the size of the pixels. Our work brings new possibilities to plasmon-enhanced infrared sensing by combining the already demonstrated sensing abilities of graphene nanostructures with subwavelength spatial resolution. This could be exploited to develop actively tunable substrates for multiplexed sensing, which could be used to analyze the chemical composition of complex biological systems and to follow their temporal evolution with spatial resolution.
Our last publication - Plasmonic coupling of multipolar edge modes and the formation of gap modes - has appeared in ACS Photonics
The coupling of plasmonic resonances is an effective tool to tailor the optical properties of nanostructures. However, the coupling of higher order plasmonic resonances has not received much attention, with most studies focusing on the interaction of dipolar modes. Taking advantage of the high spatial and energy resolution of modern scanning transmission electron microscopes equipped with electron energy loss spectroscopy, we analyze the coupling of edge modes in planar nanostructures with emphasis on the interaction of high order modes and the formation of gap modes. We show that coupling of edge modes can be understood by a simple and intuitive scheme, with three regimes: First, a strong coupling through the edge of the structure resulting in bonding and antibonding gap edge modes; second, coupling through the corners of the structures resulting in bonding and antibonding corner edge modes; and a third behavior where the edge modes do not couple and behave independently of the rest of the structure. The formation of gap modes through the coupling of edge modes is analyzed and compared to the modes found in planar slot waveguides, finding that the properties of the symmetric and asymmetric modes on slot waveguides are equivalent to the antibonding and bonding gap edge modes, respectively. Our experimental and numerical analysis of the plasmon resonances in nanosquares and waveguides shows that our scheme of plasmonic coupling of edge modes can be generalized to other planar structures with straight edges and might inspire the design of more complex planar plasmonic devices based on the coupling of edge modes.
Our last publication - Hot hole photoelectrochemistry on Au@SiO2@Au nanoparticles - has appeared in Journal of Physical Chemistry Letters
There is currently a worldwide need to develop efficient photocatalytic materials that can reduce the high-energy cost of common industrial chemical processes. One possible solution focuses on metallic nanoparticles (NPs) that can act as efficient absorbers of light due to their surface plasmon resonance. Recent work indicates that small NPs, when photoexcited, may allow for efficient electron or hole transfer necessary for photocatalysis. Here we investigate the mechanisms behind hot hole carrier dynamics by studying the photodriven oxidation of citrate ions on Au@SiO2@Au core−shell NPs. We find that charge transfer to adsorbed molecules is most efficient at higher photon energies but still present with lower plasmon energy. On the basis of these experimental results, we develop a simple theoretical model for the probability of hot carrier−adsorbate interactions across the NP surface. These results provide a foundation for understanding charge transfer in plasmonic photocatalytic materials, which could allow for further design and optimization of photocatalytic processes.
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Our last publication - Lateral Casimir force on a rotating particle near a planar surface - has appeared in Physical Review Letters
We study the lateral Casimir force experienced by a particle that rotates near a planar surface. The origin of this force lies in the symmetry breaking induced by the particle rotation in the vacuum and thermal fluctuations of its dipole moment, and therefore, in contrast to lateral Casimir forces previously described in the literature for corrugated surfaces, it exists despite the translational invariance of the planar surface. Working within the framework of fluctuational electrodynamics, we derive analytical expressions for the lateral force and analyze its dependence on the geometrical and material properties of the system. In particular, we show that the direction of the force can be controlled by adjusting the particle-surface distance, which may be exploited as a new mechanism to manipulate nanoscale objects.
Our last publication - Ultrafast radiative heat transfer - has appeared in Nature Communications
Light absorption in conducting materials produces heating of their conduction electrons, followed by relaxation into phonons within picoseconds, and subsequent diffusion into the surrounding media over longer timescales. This conventional picture of optical heating is supplemented by radiative cooling, which typically takes place at an even lower pace, only becoming relevant for structures held in vacuum or under extreme thermal isolation. Here, we reveal an ultrafast radiative cooling regime between neighboring plasmon-supporting graphene nanostructures in which noncontact heat transfer becomes a dominant channel. We predict that more than 50% of the electronic heat energy deposited on a graphene disk can be transferred to a neighboring nanoisland within a femtosecond timescale. This phenomenon is facilitated by the combination of low electronic heat capacity and large plasmonic field concentration in doped graphene. Similar effects should occur in other van der Waals materials, thus opening an unexplored avenue toward efficient heat management.
Our last publication - Basis for paraxial surface-plasmon-polariton packets - has appeared in Physical Review A
We present a theoretical framework for the study of surface-plasmon polariton (SPP) packets propagating along a lossy metal-dielectric interface within the paraxial approximation. Using a rigorous formulation based on the plane-wave spectrum formalism, we introduce a set of modes that constitute a complete basis set for the solutions of Maxwell's equations for a metal-dielectric interface in the paraxial approximation. The use of this set of modes allows us to fully analyze the evolution of the transversal structure of SPP packets beyond the single plane-wave approximation. As a paradigmatic example, we analyze the case of a Gaussian SPP mode, for which, exploiting the analogy with paraxial optical beams, we introduce a set of parameters that characterize its propagation.
Alejandro Manjavacas has been awarded the prestigious Royal Spanish Society of Physics - BBVA Foundation Award for Physics in the category of Young Researcher in Theoretical Physics
This distinction is awarded to investigators under 35 whose research has achieved great scientific value at the time of the announcement of the prize. The Awards of the Royal Spanish Society of Physics (RSEF) and the BBVA Foundation include categories aimed at junior researchers, as well as teaching and dissemination of physics. Its purpose is to recognize high-quality research, encouraging younger researchers and fostering innovation.
The award cites Manjavacas work in "The study of the interaction of light with physical structures of dimensions in the nanometer scale, and particularly metal and graphene nanostructures. His theoretical predictions have inspired new lines of experimental research in nanophotonics."
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Our last publication - Molecular plasmon-phonon coupling - has appeared in Nano Letters
Charged polycyclic aromatic hydrocarbons (PAHs), ultrasmall analogs of hydrogen-terminated graphene consisting of only a few fused aromatic carbon rings, have been shown to possess molecular plasmon resonances in the visible region of the spectrum. Unlike larger nanostructures, the PAH absorption spectra reveal rich, highly structured spectral features due to the coupling of the molecular plasmons with the vibrations of the molecule. Here, we examine this molecular plasmon-phonon interaction using a quantum mechanical approach based on the Franck-Condon approximation. We show that an independent boson model can be used to describe the complex features of the PAH absorption spectra, yielding an analytical and semiquantitative description of their spectral features. This investigation provides an initial insight into the coupling of fundamental excitations - plasmons and phonons - in molecules.
Our last publication - Anisotropic optical response of nanostructures with balanced gain and loss - has appeared in ACS Photonics
Photonic systems containing active and passive elements with balanced gain and loss are attracting increased attention due to their extraordinary properties. These structures, usually known as PT-symmetric systems, display strongly asymmetric behaviors. Here we study the optical response of finite nanostructures composed of pairs of active and passive nanospheres operating close to the PT-symmetry condition. We find that, despite their highly regular geometry, these systems scatter light predominantly toward the gain side of the structure when illuminated perpendicularly to their axis.Furthermore, the backscattering intensity for illumination parallel to the axis depends strongly on the side of incidence, being several times larger for light coming along the gain side. Interestingly, under the same conditions, the forward scattering and, consequently, the extinction cross-section remain independent of the side of incidence. This leads to an asymmetric absorption cross-section that can be made arbitrarily small for light impinging on the gain side of the structure. These results contribute to the basic understanding of the optical properties of active-passive finite nanostructures with potential applications for the design of novel nanostructures displaying asymmetric and tunable responses.
Alejandro Manjavacas has been awarded the 2016 Physics and Astronomy Department Excellence in Teaching Award
Our last publication - Toward Surface Plasmon-Enhanced Optical Parametric Amplification (SPOPA) with engineered nanoparticles: A nanoscale tunable infrared source - has appeared in Nano Letters
Active optical processes such as amplification and stimulated emission promise to play just as important a role in nanoscale optics as they have in mainstream modern optics. The ability of metallic nanostructures to enhance optical nonlinearities at the nanoscale has been shown for a number of nonlinear and active processes; however, one important process yet to be seen is optical parametric amplification. Here, we report the demonstration of surface plasmon-enhanced difference frequency generation by integration of a nonlinear optical medium, BaTiO3, in nanocrystalline form within a plasmonic nanocavity. These nanoengineered composite structures support resonances at pump, signal, and idler frequencies, providing large enhancements of the confined fields and efficient coupling of the wavelength-converted idler radiation to the far-field. This nanocomplex works as a nanoscale tunable infrared light source and paves the way for the design and fabrication of a surface plasmon-enhanced optical parametric amplifier.
Our last publication - Extraordinary light-induced local angular momentum near metallic nanoparticles - has appeared in ACS Nano
The intense local field induced near metallic nanostructures provides strong enhancements for surface-enhanced spectroscopies, a major focus of plasmonics research over the past decade. Here we consider that plasmonic nanoparticles can also induce remarkably large electromagnetic field gradients near their surfaces. Sizeable field gradients can excite dipole-forbidden transitions in nearby atoms or molecules and provide unique spectroscopic fingerprinting for chemical and bimolecular sensing. Specifically, we investigate how the local field gradients near metallic nanostructures depend on geometry, polarization, and wavelength. We introduce the concept of the local angular momentum (LAM) vector as a useful figure of merit for the design of nanostructures that provide large field gradients. This quantity, based on integrated fields rather than field gradients, is particularly well-suited for optimization using numerical grid-based full wave electromagnetic simulations. The LAM vector has a more compact structure than the gradient matrix and can be straightforwardly associated with the angular momentum of the electromagnetic field incident on the plasmonic structures.
Our last publication - Electron Energy-Loss spectroscopy of multipolar edge and cavity modes in silver nanosquares - has appeared in ACS Photonics
The characterization of surface plasmon resonances supported by metallic nanostructures requires high spatial and energy resolution. In the past few years, electron energy loss spectroscopy (EELS) has emerged as a very powerful tool to accomplish this task. In this work, we demonstrate the power of this technique for probing and imaging resonances of metallic nanostructures by analyzing the plasmonic response of silver nanosquares of sizes ranging from 230 nm up to 1 μm. Because of the relatively large size of these structures, we find that, despite their simple geometry, these systems can support a large variety of multipolar modes, which can only be detected and imaged thanks to the high spatial and energy resolution achieved by pushing EELS to its limits. The experimental results are supported by rigorous theoretical calculations that allow a detailed interpretation of the EELS measurements. In particular, we were able to map, with high level of detail, edge and high-order cavity modes. Furthermore, by calculating the scattering cross-section of these nanostructures, we confirm that most of the observed modes are dark and thus remain hidden in optical measurements, thus demonstrating the power of EELS as a unique tool for probing and imaging a large range and variety of plasmonic resonances of metallic nanostructures.
Our last publication - Aluminum nanocrystal as a plasmonic photocatalyst for hydrogen dissociation - has appeared in Nano Letters
Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier toward hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules facilitates their dissociation. Hot electrons generated from surface plasmon decay and from direct photoexcitation of the interband transitions of Al both contribute to this process. Our results pave the way for the use of aluminum, an earth-abundant, nonprecious metal, for photocatalysis.
Our last publication - High chromaticity aluminum plasmonic pixels for active liquid crystal displays - has appeared in ACS Nano
Chromatic devices such as flat panel displays could, in principle, be substantially improved by incorporating aluminum plasmonic nanostructures instead of conventional chromophores that are susceptible to photobleaching. In nanostructure form, aluminum is capable of producing colors that span the visible region of the spectrum while contributing exceptional robustness, low cost, and streamlined manufacturability compatible with semiconductor manufacturing technology. However, individual aluminum nanostructures alone lack the vivid chromaticity of currently available chromophores because of the strong damping of the aluminum plasmon resonance in the visible region of the spectrum. In recent work, we showed that pixels formed by periodic arrays of Al nanostructures yield far more vivid coloration than the individual nanostructures. This progress was achieved by exploiting far-field diffractive coupling, which significantly suppresses the scattering response on the long-wavelength side of plasmonic pixel resonances. In the present work, we show that by utilizing another collective coupling effect, Fano interference, it is possible to substantially narrow the short-wavelength side of the pixel spectral response. Together, these two complementary effects provide unprecedented control of plasmonic pixel spectral line shape, resulting in aluminum pixels with far more vivid, monochromatic coloration across the entire RGB color gamut than previously attainable. We further demonstrate that pixels designed in this manner can be used directly as switchable elements in liquid crystal displays and determine the minimum and optimal numbers of nanorods required in an array to achieve good color quality and intensity.
Our last publication - Propagation and localization of quantum dot emission along a gap-plasmonic transmission line - has appeared in Optics Express
Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 um. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.
Our last publication - Parametric characterization of surface plasmon polaritons at a lossy interface - has appeared in Optics Express
Using exact solutions of Maxwell's equations, we investigate the evolution of the transversal profile of a surface plasmon polariton (SPP) packet propagating along a planar interface between a dielectric and a lossy metal. We introduce a parameter to measure the propagation length of the SPP packet and analyze its behavior with respect to the shape of the packet and the dielectric characteristics of the interface. Furthermore, we study the polarization properties of the SPP packet and define two parameters to quantify the fraction of the irradiance contained in the s- and p-polarization components of the associated field. Our results help to advance in the understanding of the SPP optics beyond the single-mode description.
Our last publication - Laser-induced spectral hole-burning through a broadband distribution of Au nanorods - has appeared in The Journal of Physical Chemistry C
Nanorods are amenable to laser-induced reshaping, a process that can dramatically modify their shape and therefore their plasmonic properties. Here we show that when a broadband spectral distribution of nanorods is irradiated with a femtosecond-pulsed laser, an optical transmission window is formed in the extinction spectrum. Surprisingly, the transmission window that is created does not occur at the laser wavelength but rather is consistently shifted to longer wavelengths, and the optical extinction on the short-wavelength side of the transmission window is increased by the hole-burning process. The laser irradiation results in a wavelength-dependent partial reshaping of the nanorods, creating a range of unusual nanoparticle morphologies. We develop a straightforward theoretical model that explains how the spectral position, depth, and width of the laser-induced transmission window are controlled by laser irradiation conditions. This work serves as an initial example of laser-based processing of specially designed nanocomposite media to create new materials with "written-in" optical transmission characteristics.
Our last publication - Pronounced linewidth narrowing of an aluminum nanoparticle plasmon resonance by interaction with an aluminum metallic film - has appeared in Nano Letters
Aluminum nanocrystals and fabricated nanostructures are emerging as highly promising building blocks for plasmonics in the visible region of the spectrum. Even at the individual nanocrystal level, however, the localized plasmons supported by Al nanostructures possess a surprisingly broad spectral response. We have observed that when an Al nanocrystal is coupled to an underlying Al film, its dipolar plasmon resonance linewidth narrows remarkably and shows an enhanced scattering efficiency. This behavior is observable in other plasmonic metals, such as gold; however, it is far more dramatic in the aluminum nanoparticle–film system, reducing the dipolar plasmon linewidth by more than half. A substrate-mediated hybridization of the dipolar and quadrupolar plasmons of the nanoparticle reduces the radiative losses of the dipolar plasmon. While this is a general effect that applies to all metallic nanoparticle–film systems, this finding specifically provides a new mechanism for narrowing plasmon resonances in aluminum-based systems, quite possibly expanding the potential of Al-based plasmonics in real-world applications.
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