September 8, 2020

Our last publication on the super- and subradiant lattice resonances supported by bipartite nanoparticle arrays has been highlighted in different news media.

August 14, 2020

Our last publication Super- and subradiant lattice resonances in bipartite nanoparticle arrays has appeared in ACS Nano

Lattice resonances, the collective modes supported by periodic arrays of metallic nanoparticles, give rise to very strong and spectrally narrow optical responses. Thanks to these properties, which emerge from the coherent multiple scattering enabled by the periodic ordering of the array, lattice resonances are used in a variety of applications such as nanoscale lasing and biosensing. Here, we investigate the lattice resonances supported by bipartite nanoparticle arrays. We find that, depending on the relative position of the two particles within the unit cell, these arrays can support lattice resonances with a super- or subradiant character. While the former result in large values of reflectance with broad lineshapes due to the increased radiative losses, the latter give rise to very small linewidths and maximum absorbance, consistent with a reduction of the radiative losses. Furthermore, by analyzing the response of arrays with finite dimensions, we demonstrate that the subradiant lattice resonances of bipartite arrays require a much smaller number of elements to reach a given quality factor than the lattice resonances of arrays with single-particle unit cells. The results of this work, in addition to advancing our knowledge of the optical response of periodic arrays of nanostructures, provide an efficient approach to obtain narrow lattice resonances that are robust to fabrication imperfections.

July 15, 2020

Our last publication Two-photon spontaneous emission in atomically thin plasmonic nanostructures has appeared in Physical Review Letters.

The ability to harness light-matter interactions at the few-photon level plays a pivotal role in quantum technologies. Single photons—the most elementary states of light—can be generated on demand in atomic and solid state emitters. Two-photon states are also key quantum assets, but achieving them in individual emitters is challenging because their generation rate is much slower than competing one-photon processes. We demonstrate that atomically thin plasmonic nanostructures can harness two-photon spontaneous emission, resulting in giant far field two-photon production, a wealth of resonant modes enabling tailored photonic and plasmonic entangled states, and plasmon-assisted single-photon creation orders of magnitude more efficient than standard one-photon emission. We unravel the two-photon spontaneous emission channels and show that their spectral line shapes emerge from an intricate interplay between Fano and Lorentzian resonances. Enhanced two-photon spontaneous emission in two-dimensional nanostructures paves the way to an alternative efficient source of light-matter entanglement for on-chip quantum information processing and free-space quantum communications.

May 21, 2020

Our last publication Active temporal control of radiative heat transfer with graphene nanodisks has appeared in Physical Review Applied.

The ability to dynamically control the radiative transfer of heat at the nanoscale holds the key to the development of a diverse number of technologies, ranging from nanoscale thermal-management systems to improved thermophotovoltaic devices. Recently, graphene has emerged as an ideal material to achieve this goal, since it can be electrically doped to support surface plasmons, collective oscillations of the conduction electrons. These resonances produce large and spectrally narrow optical cross sections, which dictate the emission and absorption properties of the graphene nanostructure and, thus, the heat that it radiatively exchanges with other objects and the environment. For attainable levels of doping, the plasmons supported by graphene nanostructures naturally lie in the midinfrared part of the spectrum, which is the most relevant wavelength range for radiative heat transfer under realistic temperatures. Furthermore, these resonances are actively tunable, thus providing full dynamic control over the heat transfer. Motivated by this great potential, we present a comprehensive analysis of the temporal evolution of the radiative heat transfer between arrangements of graphene nanodisks, showing that it is possible to exploit the tunability of these structures to obtain actively controlled heat transfer scenarios not possible with conventional passive nanostructures. The results of this work provide a framework for achieving fully dynamical control over nanoscale radiative heat transfer and thus provide fundamental insights into this process.

April 8, 2020

We are very happy to receive an NSF Career Award for a project entitled: Transfer of Momentum and Energy in the Nanoscale Using Quantum and Thermal Fluctuations.

The interaction between light and matter in the nanoscale can be very different from our daily macroscopic experience. When the dimensions of material structures, or the space separating them, reach the range of nanometers, the quantum nature of light and matter emerges, giving rise to new phenomena. In that limit, Casimir interactions, which arise from quantum and thermal fluctuations of the electromagnetic field, play a dominant role and can overcome other interactions, such as gravitational forces, thus conditioning the dynamics of nanoscale objects. The fluctuations of the electromagnetic field are also at the origin of the radiative transfer of energy between bodies at different temperatures. In this context, and thanks to the enormous advances in nanofabrication technologies, we have reached the limit in which the effects caused by the quantum and thermal fluctuations of the electromagnetic field have important consequences for the mechanical and thermal dynamics of nanostructures. This has posed new challenges for the development of applications in nanotechnology. However, it also constitutes a unique opportunity to develop new approaches to manipulate the mechanical and thermal dynamics of nanostructures. In this project, we will tackle this research challenge by investigating the transfer of momentum and energy between nanoscale objects within the context of two novel concepts that have recently emerged in nanophotonics: structures with atomic thickness and spin-orbit interactions of light. The investigation of these phenomena within a common theoretical framework will allow us to establish the foundations for new paradigms enabling noncontact transfer of momentum and energy in the nanoscale, which can help to develop novel approaches to manipulate nanoscale objects, including biologically relevant structures. Furthermore, the results on the energy transfer will have an impact on the improvement of thermophotovoltaic devices and heat management strategies in nanoelectronics. At the same time, this project will be an opportunity to improve the recruitment and retention of STEM students, which is one of the most important structural problems that education in New Mexico currently faces, with a special emphasis on targeting first-generation and low-income students from underrepresented minorities. To that end, we will implement a range of activities targeting students from middle school to the graduate level, which aim to build interest in STEM disciplines, preserve that interest, and mold it into essential skills and experience

December 24, 2019

Our last publication Nanoantennas with balanced gain and loss has appeared in Nanophotonics.

The large cross sections and strong confinement provided by the plasmon resonances of metallic nanostructures make these systems an ideal platform to implement nanoantennas. Like their macroscopic counterparts, nanoantennas enhance the coupling between deep subwavelength emitters and free radiation, providing, at the same time, an increased directionality. Here, inspired by the recent works in parity-time symmetric plasmonics, we investigate how the combination of conventional plasmonic nanostructures with active materials, which display optical gain when externally pumped, can serve to enhance the performance of metallic nanoantennas. We find that the presence of gain, in addition to mitigating the losses and therefore increasing the power radiated or absorbed by an emitter, introduces a phase difference between the elements of the nanoantenna that makes the optical response of the system directional, even in the absence of geometrical asymmetry. Exploiting these properties, we analyse how a pair of nanoantennas with balanced gain and loss can enhance the far-field interaction between two dipole emitters. The results of this work provide valuable insight into the optical response of nanoantennas made of active and passive plasmonic nanostructures, with potential applications for the design of optical devices capable of actively controlling light at the nanoscale.

November 7, 2019

Lauren, Paul, Keith, and Lucas have presented their research at the 2019 Shared Knowledge Symposium.

October 11, 2019

Lauren, Paul, Keith, and Lucas have presented their research at the APS 4 Corners Meeting held at Embry-Riddle Aeronautical University. Paul and Lauren won awards for their contributions!

October 10, 2019

Our last publication on the analysis of the limits of the near-field produced by nanoparticle arrays has been highlighted in different news media.

September 11, 2019

Our last publication Analysis of the limits of the near-field produced by nanoparticle arrays has appeared in ACS Nano.

Periodic arrays are an exceptionally interesting arrangement for metallic nanostructures because of their ability to support collective lattice resonances. These modes, which arise from the coherent multiple scattering enabled by the lattice periodicity, give rise to very strong and spectrally narrow optical responses. Here, we investigate the enhancement of the near-field produced by the lattice resonances of arrays of metallic nanoparticles when illuminated with a plane wave. We find that, for infinite arrays, this enhancement can be made arbitrarily large by appropriately designing the geometrical characteristics of the array. On the other hand, in the case of finite arrays, the near-field enhancement is limited by the number of elements of the array that interact coherently. Furthermore, we show that, as the near-field enhancement increases, the length scale over which it extends above and below the array becomes larger and its spectral linewidth narrows. We also analyze the impact that material losses have on these behaviors. As a direct application of our results, we investigate the interaction between a nanoparticle array and a dielectric slab placed a certain distance above it and show that the extraordinary near-field enhancement produced by the lattice resonance can lead to very strong interactions, even at significantly large separations. This work provides a detailed characterization of the limits of the near-field produced by lattice resonances and, therefore, advances our knowledge of the optical response of periodic arrays of nanostructures, which can be used to design and develop applications exploiting the extraordinary properties of these systems.

August 15, 2019

Our last publication on high-harmonic generation using epsilon-near-zero materials has been highlighted in the news.

July 15, 2019

Our last publication High-harmonic generation from an epsilon-near-zero material has appeared in Nature Physics.

High-harmonic generation (HHG) is a signature optical phenomenon of strongly driven, nonlinear optical systems. Specifically, the understanding of the HHG process in rare gases has played a key role in the development of attosecond science1. Recently, HHG has also been reported in solids, providing novel opportunities such as controlling strong-field and attosecond processes in dense optical media down to the nanoscale. Here, we report HHG from a low-loss, indium-doped cadmium oxide thin film by leveraging the epsilon-near-zero (ENZ) effect, whereby the real part of the material's permittivity in certain spectral ranges vanishes, as well as the associated large resonant enhancement of the driving laser field. We find that ENZ-assisted harmonics exhibit a pronounced spectral redshift as well as linewidth broadening, resulting from the photo induced electron heating and the consequent time-dependent ENZ wavelength of the material. Our results provide a new platform to study strong-field and ultrafast electron dynamics in ENZ materials, reveal new degrees of freedom for spectral and temporal control of HHG, and open up the possibilities of compact solid-state attosecond light sources.

June 25, 2019

Our last publication Nanoscale transfer of angular momentum mediated by the Casimir torque has appeared in Communication Physics.

Casimir interactions play an important role in the dynamics of nanoscale objects. Here, we investigate the noncontact transfer of angular momentum at the nanoscale through the analysis of the Casimir torque acting on a chain of rotating nanoparticles. We show that this interaction, which arises from the vacuum and thermal fluctuations of the electromagnetic field, enables an efficient transfer of angular momentum between the elements of the chain. Working within the framework of fluctuational electrodynamics, we derive analytical expressions for the Casimir torque acting on each nanoparticle in the chain, which we use to study the synchronization of chains with different geometries and to predict unexpected dynamics, including a rattleback-like behavior. Our results provide insights into the Casimir torque and how it can be exploited to achieve efficient noncontact transfer of angular momentum at the nanoscale, and therefore have important implications for the control and manipulation of nanomechanical devices.

June 20, 2019

Our last publication on the plasmonic properties of daguerreotypes has been highlighted in different news media.

June 10, 2019

Our last publication Nineteenth-century nanotechnology: The plasmonic properties of daguerreotypes has appeared inThe Proceedings of the National Academy of Sciences.

Plasmons, the collective oscillations of mobile electrons in metallic nanostructures, interact strongly with light and produce vivid colors, thus offering a new route to develop color printing technologies with improved durability and material simplicity compared with conventional pigments. Over the last decades, researchers in plasmonics have been devoted to manipulating the characteristics of metallic nanostructures to achieve unique and controlled optical effects. However, before plasmonic nanostructures became a science, they were an art. The invention of the daguerreotype was publicly announced in 1839 and is recognized as the earliest photographic technology that successfully captured an image from a camera, with resolution and clarity that remain impressive even by today’s standards. Here, using a unique combination of daguerreotype artistry and expertise, experimental nanoscale surface analysis, and electromagnetic simulations, we perform a comprehensive analysis of the plasmonic properties of these early photographs, which can be recognized as an example of plasmonic color printing. Despite the large variability in size, morphology, and material composition of the nanostructures on the surface of a daguerreotype, we are able to identify and characterize the general mechanisms that give rise to the optical response of daguerreotypes. Therefore, our results provide valuable knowledge to develop preservation protocols and color printing technologies inspired by past ones.

June 5, 2019

Our last publication Titanium nitride nanoparticles for the efficient photocatalysis of bicarbonate into formate has appeared in Solar Energy Materials and Solar Cells.

Metallic nanoparticles can act as efficient photocatalysts thanks to the surface plasmons that they support, which are capable of harvesting light and generating hot carriers. Recently, titanium nitride (TiN) nanostructures have emerged as promising candidates for this application due to their much lower cost, and therefore greater sustainability, than structures made of noble metals, as well as their expected long-term thermal stability. In this work, we demonstrate that, under solar illumination, TiN nanoparticles, in combination with titanium dioxide (TiO2) nanostructures, can significantly increase the photocatalytic production of formate through the simultaneous photoreduction of bicarbonate and oxidation of glycerol. Importantly, we also show that TiN nanoparticles alone can provide an enhancement of the photocatalytic efficiently when compared to TiO2 nanocatalysts. Furthermore, by characterizing the morphology and material properties of the TiN nanoparticles after the reaction, we confirm that they remain stable under reaction conditions for extended periods of solar light exposure (8 hours). The results of this work advance our understanding of TiN nanoparticles as efficient photocatalysts and their use for the production of valuable chemicals.

May 24, 2019

Lauren has been received four graduate fellowships and a Sigma Xi award! Congrats!

Lauren has received the Department of Energy Computational Science Graduate Fellowship (DOE CSGF), the National Defense Science and Engineering Graduate Fellowship (NDSEG), and the National Science Foundation Graduate Research Fellowship (NSF GRFP). She has accepted the DOE CSGF and will continue working in the group as a graduate student. In addition to these honors, Lauren has also received a graduate research fellowship from the New Mexico Space Grant Consortium, as well as the Sigma Xi Outstanding Undergraduate Award.

December 6, 2018

Our last publication Finite-size effects on periodic arrays of nanostructures has appeared in Journal of Physics: Photonics.

Arrays of nanostructures have emerged as exceptional tools for the manipulation and control of light. Oftentimes, despite the fact that real implementations of nanostructure arrays must be finite, these systems are modeled as perfectly periodic, and therefore infinite. Here, we investigate the legitimacy of this approximation by studying the evolution of the optical response of finite arrays of nanostructures as their number of elements is increased. We find that the number of elements necessary to reach the infinite array limit is determined by the strength of the coupling between them, and that, even when that limit is reached, the individual responses of the elements may still display significant variations. In addition, we show that, when retardation is negligible, the resonance frequency for the infinite array is always redshifted compared to the single particle. However, in the opposite situation, there could be either a blue- or a redshift. We also study the effects of inhomogeneity in size and position of the elements on the optical response of the array. This work advances the understanding of the behavior of finite and infinite arrays of nanostructures, and therefore provides guidance to design applications that utilize these systems.

August 30, 2018

Our last publication Robust charge transfer plasmons in metallic particle-film systems has appeared in ACS Photonics.

Understanding how the plasmonic response of a metallic nanoparticle is modified by its coupling with a metallic film is a fundamental research problem relevant for many applications including sensing, solar energy harvesting, spectroscopy, and photochemistry. Despite significant research effort on this topic, the nature of the interaction between colloidal nanoparticles and metallic films is not fully understood. Here, we investigate, both experimentally and theoretically, the optical response of surface ligand-coated gold nanorods interacting with gold films. We find that the scattering cross section of these systems is dominated by a charge transfer plasmon mode, for which charge flows between the particle and the film. The properties of this mode are dictated by the characteristics of the particle−film junction, which makes the frequency of this charge transfer plasmon far less sensitive to the nanoparticle size and geometry than a typical dipolar plasmon mode excited in similar nanorods placed directly on a purely dielectric substrate. The results of this work serve to advance our understanding of the interaction between metallic nanoparticles and metallic films, as well as provide a method for creating more robust plasmonic platforms that are less affected by changes in the size of individual nanoparticles.

June 11, 2018

Our last publication Extraordinary enhancement of quadrupolar transitions using nanostructured graphene has appeared in ACS Photonics.

Surface plasmons supported by metallic nanostructures interact strongly with light and confine it into subwavelength volumes, thus forcing the corresponding electric field to vary within nanoscale distances. This results in exceedingly large field gradients that can be exploited to enhance the quadrupolar transitions of quantum emitters lo- cated in the vicinity of the nanostructure. Graphene nanostructures are ideally suited for this task, since their plasmons can confine light into substantially smaller volumes than equivalent excitations sustained by conventional plasmonic nanostructures. Fur- thermore, in addition to their geometric tunability, graphene plasmons can also be efficiently tuned by controlling the doping level of the nanostructure, which can be accomplished either chemically or electrostatically. Here, we provide a detailed inves- tigation of the enhancement of the field gradient in the vicinity of different graphene nanostructures. Using rigorous solutions of Maxwell’s equations, as well as an analytic electrostatic approach, we analyze how this quantity is affected by the size, shape, dop- ing level, and quality of the nanostructure. We investigate, as well, the performance of arrays of nanoribbons, which constitute a suitable platform for the experimental veri- fication of our predictions. The results of this work bring new possibilities to enhance and control quadrupolar transitions of quantum emitters, which can find application in the detection of relevant chemical species, as well as in the design of novel light emitting devices.

March 30, 2018

Lauren has been awarded the prestigious Goldwater Scholarship. Congratulations!

March 14, 2018

Our last publication Analysis of the limits of the local density of photonic states near nanostructures has appeared in ACS Photonics.

Nanostructures with sizes smaller than or comparable to visible light strongly modify the decay rate of dipole emitters placed in their vicinity. Such modification is usually characterized using the local density of photonic states (LDOS), which quantifies the availability of photonic states at a certain position and frequency in the presence of a nanostructure. Here, we present a detailed analysis of the limits of this quantity through the study of a sum rule that bounds its spectral integral, taking into account both its radiative and nonradiative components. The sum rule studied here relates the integral over the spectrum of the LDOS at a certain point to the field induced by a static dipole placed at that same location. We confirm the validity of this sum rule and investigate its implications for the response of nanostructures by performing rigorous numerical calculations for a variety of systems, including nanospheres, nanodisks, and films, made of different metallic and dielectric materials, as well as graphene. Furthermore, we apply the sum rule to the cross density of photonic states (CDOS), a quantity that characterizes the spatial coherence of light in the presence of a nanostructure and determines, as well, the interaction between two dipole emitters located in its vicinity. We show how this result can be used as a guide to select the most favorable nanostructure geometries and materials to achieve strong values of the LDOS and the CDOS over desired parts of the spectrum, thus helping to engineer strong decay rates and coupling enhancements near nanostructures.

February 17, 2018

Lauren, Paul, and Keith have presented their research at the 2018 STEM Research Symposium.

February 5, 2018

Our last publication on the hybridization of lattice resonances has been highlighted in different news media.

January 18, 2018

Our last publication Hybridization of lattice resonances has appeared in ACS Nano.

Plasmon hybridization, the electromagnetic analog of molecular orbital theory, provides a simple and intuitive method to describe the plasmonic response of complex nanostructures from the combination of the responses of their individual constituents. Here, we follow this approach to investigate the optical properties of periodic arrays of plasmonic nanoparticles with multi-particle unit cells. These systems support strong collective lattice resonances, arising from the coherent multiple scattering enabled by the lattice periodicity. Due to the extended nature of these modes, the interaction between them is very different from that among localized surface plasmons supported by individual nanoparticles. This leads to a much richer hybridization scenario, which we exploit here to design periodic arrays with engineered properties. These include arrays with two-particle unit cells, in which the interaction between the individual lattice resonances can be canceled or maximized by controlling the relative position of the particles within the unit cell, as well as arrays whose response can be made either invariant to the polarization of the incident light or strongly dependent on it. Moreover, we explore systems with three- and four-particle unit cells and show that they can be designed to support lattice resonances with complex hybridization patterns in which different groups of particles in the unit cell can be selectively excited. The results of this work serve to advance our understanding of periodic arrays of nanostructures and provide a methodology to design periodic structures with engineered properties for applications in nanophotonics.

November 21, 2017

Keith has passed his candidacy examination with excellent feedback from the committee, and Lauren has been awarded the New Mexico Space Grant Undergraduate Research Scholarship. Congratulations to both!

November 8, 2017

Keith has presented a poster UNM Shared Knowledge conference.

October 26, 2017

Our last publication Magnetic light and forbidden photochemistry: the case of singlet Oxygen has appeared in The Journal of Materials Chemistry C.

Most optical processes occurring in nature are based on the well-known selection rules for optical transitions between electronic levels of atoms, molecules, and solids. Since in most situations the magnetic component of light has a negligible contribution, the dipolar electric approximation is generally assumed. However, this traditional understanding is challenged by nanostructured materials, which interact strongly with light and produce very large enhancements of the magnetic field in their surroundings. Here we report on the magnetic response of different metallic nanostructures and their influence on the spectroscopy of molecular oxygen, a paradigmatic example of dipole-forbidden optical transitions in photochemistry.

October 21, 2017

Lauren has presented two talks at the APS 4 Corners Meeting held at Colorado State University, winning the award for the best udergraduate oral presentation.

October 11, 2017

Our last publication Flat top surface plasmon polariton beams has appeared in Optics Letters.

Surface plasmon polaritons (SPPs) have emerged as powerful tools for guiding and manipulating light below the diffraction limit. In this context, the availability of flat top SPP beams displaying a constant transversal profile can allow for uniform excitation and coupling scenarios, thus opening the door to developing novel applications that cannot be achieved using conventional Gaussian SPP beams. Here, we present a rigorous theoretical description of flat top SPP beams propagating along flat metal-dielectric interfaces. This is accomplished through the use of Hermite–Gaussian SPP modes that constitute a complete basis set for the solutions of Maxwell's equations for a metal-dielectric interface in the paraxial approximation. We provide a comprehensive analysis of the evolution of the transversal profiles of these beams as they propagate, which is complemented with the study of the width and kurtosis parameters. Our results serve to enlarge the capabilities of surface plasmon polaritons to control and manipulate light below the diffraction limit.

July 31, 2017

Our last publication Controlling the heat dissipation in temperature-matched plasmonic nanostructures has appeared in Nano Letters.

Heat dissipation in a plasmonic nanostructure is generally assumed to be ruled only by its own optical response even though also the temperature should be considered for determining the actual energy-to-heat conversion. Indeed, temperature influences the optical response of the nanostructure by affecting its absorption efficiency. Here, we show both theoretically and experimentally how, by properly nanopatterning a metallic surface, it is possible to increase or decrease the light-to-heat conversion rate depending on the temperature of the system. In particular, by borrowing the concept of matching condition from the classical antenna theory, we first analytically demonstrate how the temperature sets a maximum value for the absorption efficiency and how this quantity can be tuned, thus leading to a temperature-controlled optical heat dissipation. In fact, we show how the nonlinear dependence of the absorption on the electron-phonon damping can be maximized at a specific temperature, depending on the system geometry. In this regard, experimental results supported by numerical calculations are presented, showing how geometrically different nanostructures can lead to opposite dependence of the heat dissipation on the temperature, hence suggesting the fascinating possibility of employing plasmonic nanostructures to tailor the light-to-heat conversion rate of the system.

July 15, 2017

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.

June 26, 2017

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.

June 16, 2017

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.

June 13, 2017

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.

May 18, 2017

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.

April 20, 2017

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.

April 11, 2017

Our last publication on lateral Casimir forces has been highlighted in different news media.

March 31, 2017

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.

February 23, 2017

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.

December 15, 2016

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.

December 14, 2016

Alejandro has been awarded the prestigious Royal Spanish Society of Physics - BBVA Foundation Award for Physics in the category of Young Researcher in Theoretical Physics

his 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."

September 26, 2016

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.

June 17, 2016

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.

May 14, 2016

Alejandro Manjavacas has been awarded the 2016 Physics and Astronomy Department Excellence in Teaching Award

April 18, 2016

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.

April 5, 2016

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.

February 9, 2016

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.

January 22, 2016

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.

December 5, 2015

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.

November 2, 2015

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.

October 23, 2015

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.

September 30, 2015

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.

September 18, 2015

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.