Toward next-generation nanophotonic devices

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit de CiènciesIn this thesis, we aim to explore several novel designs of nanostructures based on graphene to realize various functionalities. We briefly introduce the fundamental concepts and theoretical models used in this thesis in Chapter 1. Following the macroscopic analytical method outlined in the first chapter, in Chapter 2 we show that simple simulation methods allow us to accurately describe the optical response of plasmonic nanoparticles, including retardation effects, without the requirement of large computational resources. We then move to our proposed first type of device: optical modulators. We explore graphene sheets coupled to different kinds of optical resonators to enhance the light intensity at the graphene plane, and so also its absorption, which can be switched on/off and modulated through varying the level of doping, as explored in Chapter 3. Unity-order changes in the transmission and absorption of incident light are predicted upon electrical doping of graphene. Heat deposition via light absorption can severely degrade the performance and limit the lifetime of nano-devices (e.g., aforementioned optical modulators), which makes the manipulation of nanoscale heat sources/flows become crucial. In Chapter 4, we exploit the extraordinary optical and thermal properties of graphene to show that ultrafast radiative heat transfer can take place between neighboring nanostructures facilitated by graphene plasmons, where photothermally induced effects on graphene plasmons are taken into account. Our findings reveal a new regime for the nanoscale thermal management, in which non-contact heat transfer becomes a leading mechanism of heat dissipation. Apart from the damage caused by heat deposition, generated thermal energy can be in fact used as a tool for photodetection (e.g., silicon bolometers for infrared photodetection). In Chapter 5, we show that the excitation of a single plasmon in a graphene nanojunction produces profound modifications in its electrical properties through optical heating, which we then use to demonstrate an efficient mid-infrared photodetector working at room temperature based on theoretical predictions that are corroborated in an experimental collaboration with the group of Prof. Fengnian Xia in Yale University. Finally, in Chapter 6, we show through microscopic quantum-mechanical simulations, introduced in the first chapter, that both the linear and nonlinear optical responses of graphene nanostructures can be dramatically altered by the presence of a single neighboring molecule that carries either an elementary charge or a small permanent dipole. Based on these results, we claim that nanographenes can serve as an efficient platform for detecting charge- or dipole-carrying molecules.En esta tesis, pretendemos explorar varios diseños novedosos de nanoestructuras basadas en grafeno, con diversas funcionalidades. Tras presentar brevemente los conceptos fundamentales y los modelos teóricos utilizados en esta tesis en el Capítulo 1, en el Capítulo 2 mostramos la posibilidad de describir la respuesta de nanopartículas plasmónicas (incluyendo efectos de retardo) mediante métodos de simulación semi-analíticos sencillos y sin la necesidad de emplear grandes recursos computacionales. Posteriormente, empleamos estos modelos en el desarrollo de un primer tipo de dispositivo: moduladores ópticos. Añadiendo láminas de grafeno acopladas a diferentes tipos de resonadores ópticos, podemos mejorar la intensidad de la luz en el plano del grafeno, y por lo tanto también su nivel de absorción, la cual puede ser modulada a voluntad mediante el nivel de dopado electrostático del grafeno, como se explora en el Capítulo 3. Los modelos empleados predicen cambios en la transmisión del orden de la unidad, produciendo así la absorción total por parte del dispositivo de la luz incidente. En esta clase de dispositivos, así como en todos los dispositivos nanofotónicos, la producción de calor mediante la absorción de la luz puede degradar severamente su rendimiento, así como limitar su vida útil, lo que hace que la manipulación de la fuente y el flujo de calor en la nanoescala sea una componente crucial del desarrollo. En el Capítulo 4, empleamos las extraordinarias propiedades ópticas y térmicas del grafeno para mostrar que puede tener lugar una transferencia ultrarrápida de calor radiativo entre nanoestructuras vecinas, facilitada por los plasmones del grafeno, los cuales a su vez experimentan efectos fototérmicos asociados con este proceso de disipación. Nuestros hallazgos revelan un nuevo régimen para la energía térmica a nanoescala, en la que la transferencia de calor radiativa se convierte en el mecanismo principal de disipación de calor. Además de los daños causados por la deposición de calor, la energía térmica generada puede ser de hecho usada como herramienta para la fotodetección: tal es el caso, por ejemplo, de los bolómetros de silicona, empleados para la fotodetección por infrarrojos. En el Capítulo 5, mostramos que la excitación de un solo plasmón en una unión de grafeno altera radicalmente sus propiedades eléctricas debido al calentamiento óptico. Este hecho puede ser empleado para demostrar el funcionamiento eficaz de un fotodetector en la región media de los infrarrojos a temperatura ambiente, tanto a través de predicciones teóricas como su corroboración experimental (en colaboración con el grupo del Prof. Fengnian Xia de la Universidad de Yale). Finalmente, en el Capítulo 6, mostramos a través de simulaciones mecánico-cuánticas (introducidas en el Capítulo 1), que tanto la respuesta óptica lineal como la no lineal de las nanoestructuras de grafeno pueden ser dramáticamente alteradas por la presencia de una sola molécula vecina que transporte o bien una carga elemental o un dipolo permanente. En base a estos resultados, afirmamos que las estructuras de grafeno nanoscópicas podrían ser una plataforma eficiente para detectar moléculas portadoras de carga o dipolos.Postprint (published version

Time-modulated near-field radiative heat transfer

We explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can results in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale heat flow

Manipulating the Interaction between Localized and Delocalized Surface Plasmon Polaritons in Graphene

The excitation of localized or delocalized surface plasmon polaritons in nanostructured or extended graphene has attracted a steadily increasing attention due to their promising applications in sensors, switches, and filters. These single resonances may couple and intriguing spectral signatures can be achieved by exploiting the entailing hybridization. Whereas thus far only the coupling between localized or delocalized surface plasmon polaritons has been studied in graphene nanostructures, we consider here the interaction between a localized and a delocalized surface plasmon polariton. This interaction can be achieved by two different schemes that reside on either evanescent near- field coupling or far-field interference. All observable phenomena are corroborated by analytical considerations, providing insight into the physics and paving the way for compact and tunable optical components at infrared and terahertz frequencies.Comment: 6 pages, 4 figure

Nonlinear Plasmonic Sensing with Nanographene

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene.Peer ReviewedPostprint (published version

Moving Media as Photonic Heat Engine and Pump

A system consisting of two slabs with different temperatures can exhibit a non-equilibrium lateral Casimir force on either one of the slabs when Lorentz reciprocity is broken in at least one of the slabs. This system constitutes a photonic heat engine that converts radiative heat into work done by the non-equilibrium lateral Casimir force. Reversely, by sliding two slabs at a sufficiently high relative velocity, heat is pumped from the slab at a lower temperature to the other one at a higher temperature. Hence the system operates as a photonic heat pump. In this work, we study the thermodynamic performance of such a photonic heat engine and pump via the fluctuational electrodynamics formalism. The propulsion force due to the non-reciprocity and the drag force due to the Doppler effect was revealed as the physical mechanism behind the heat engine. We also show that in the case of the heat pump, the use of nonreciprocal materials can help reduce the required velocity. We present an ideal material dispersion to reach the Carnot efficiency limit. Furthermore, we derive a relativistic version of the thermodynamic efficiency for our heat engine and prove that it is bounded by the Carnot efficiency that is independent of the frame of reference. Our work serves as a conceptual guide for the realization of photonic heat engines based on fluctuating electromagnetic fields and relativistic thermodynamics and shows the important role of electromagnetic non-reciprocity in operating them.Comment: 26 pages, 7 figures, and supplementary material

Toward next-generation nanophotonic devices

In this thesis, we aim to explore several novel designs of nanostructures based on graphene to realize various functionalities. We briefly introduce the fundamental concepts and theoretical models used in this thesis in Chapter 1. Following the macroscopic analytical method outlined in the first chapter, in Chapter 2 we show that simple simulation methods allow us to accurately describe the optical response of plasmonic nanoparticles, including retardation effects, without the requirement of large computational resources. We then move to our proposed first type of device: optical modulators. We explore graphene sheets coupled to different kinds of optical resonators to enhance the light intensity at the graphene plane, and so also its absorption, which can be switched on/off and modulated through varying the level of doping, as explored in Chapter 3. Unity-order changes in the transmission and absorption of incident light are predicted upon electrical doping of graphene. Heat deposition via light absorption can severely degrade the performance and limit the lifetime of nano-devices (e.g., aforementioned optical modulators), which makes the manipulation of nanoscale heat sources/flows become crucial. In Chapter 4, we exploit the extraordinary optical and thermal properties of graphene to show that ultrafast radiative heat transfer can take place between neighboring nanostructures facilitated by graphene plasmons, where photothermally induced effects on graphene plasmons are taken into account. Our findings reveal a new regime for the nanoscale thermal management, in which non-contact heat transfer becomes a leading mechanism of heat dissipation. Apart from the damage caused by heat deposition, generated thermal energy can be in fact used as a tool for photodetection (e.g., silicon bolometers for infrared photodetection). In Chapter 5, we show that the excitation of a single plasmon in a graphene nanojunction produces profound modifications in its electrical properties through optical heating, which we then use to demonstrate an efficient mid-infrared photodetector working at room temperature based on theoretical predictions that are corroborated in an experimental collaboration with the group of Prof. Fengnian Xia in Yale University. Finally, in Chapter 6, we show through microscopic quantum-mechanical simulations, introduced in the first chapter, that both the linear and nonlinear optical responses of graphene nanostructures can be dramatically altered by the presence of a single neighboring molecule that carries either an elementary charge or a small permanent dipole. Based on these results, we claim that nanographenes can serve as an efficient platform for detecting charge- or dipole-carrying molecules.En esta tesis, pretendemos explorar varios diseños novedosos de nanoestructuras basadas en grafeno, con diversas funcionalidades. Tras presentar brevemente los conceptos fundamentales y los modelos teóricos utilizados en esta tesis en el Capítulo 1, en el Capítulo 2 mostramos la posibilidad de describir la respuesta de nanopartículas plasmónicas (incluyendo efectos de retardo) mediante métodos de simulación semi-analíticos sencillos y sin la necesidad de emplear grandes recursos computacionales. Posteriormente, empleamos estos modelos en el desarrollo de un primer tipo de dispositivo: moduladores ópticos. Añadiendo láminas de grafeno acopladas a diferentes tipos de resonadores ópticos, podemos mejorar la intensidad de la luz en el plano del grafeno, y por lo tanto también su nivel de absorción, la cual puede ser modulada a voluntad mediante el nivel de dopado electrostático del grafeno, como se explora en el Capítulo 3. Los modelos empleados predicen cambios en la transmisión del orden de la unidad, produciendo así la absorción total por parte del dispositivo de la luz incidente. En esta clase de dispositivos, así como en todos los dispositivos nanofotónicos, la producción de calor mediante la absorción de la luz puede degradar severamente su rendimiento, así como limitar su vida útil, lo que hace que la manipulación de la fuente y el flujo de calor en la nanoescala sea una componente crucial del desarrollo. En el Capítulo 4, empleamos las extraordinarias propiedades ópticas y térmicas del grafeno para mostrar que puede tener lugar una transferencia ultrarrápida de calor radiativo entre nanoestructuras vecinas, facilitada por los plasmones del grafeno, los cuales a su vez experimentan efectos fototérmicos asociados con este proceso de disipación. Nuestros hallazgos revelan un nuevo régimen para la energía térmica a nanoescala, en la que la transferencia de calor radiativa se convierte en el mecanismo principal de disipación de calor. Además de los daños causados por la deposición de calor, la energía térmica generada puede ser de hecho usada como herramienta para la fotodetección: tal es el caso, por ejemplo, de los bolómetros de silicona, empleados para la fotodetección por infrarrojos. En el Capítulo 5, mostramos que la excitación de un solo plasmón en una unión de grafeno altera radicalmente sus propiedades eléctricas debido al calentamiento óptico. Este hecho puede ser empleado para demostrar el funcionamiento eficaz de un fotodetector en la región media de los infrarrojos a temperatura ambiente, tanto a través de predicciones teóricas como su corroboración experimental (en colaboración con el grupo del Prof. Fengnian Xia de la Universidad de Yale). Finalmente, en el Capítulo 6, mostramos a través de simulaciones mecánico-cuánticas (introducidas en el Capítulo 1), que tanto la respuesta óptica lineal como la no lineal de las nanoestructuras de grafeno pueden ser dramáticamente alteradas por la presencia de una sola molécula vecina que transporte o bien una carga elemental o un dipolo permanente. En base a estos resultados, afirmamos que las estructuras de grafeno nanoscópicas podrían ser una plataforma eficiente para detectar moléculas portadoras de carga o dipolos

Continuous-Wave Multiphoton Photoemission from Plasmonic Nanostars

Highly nonlinear optical processes, such as multiphoton photoemission, require high intensities, typically achieved with ultrashort laser pulses and, hence, were first observed with the advent of picosecond laser technology. An alternative approach for reaching the required field intensities is offered by localized optical resonances such as plasmons. Here, we demonstrate localized multiphoton photoemission from plasmonic nanostructures under continuous-wave illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp tips with structural features smaller than 5 nm, leading to near-field-intensity enhancements exceeding 1000. This large enhancement facilitates 3-photon photoemission driven by a simple continuous-wave laser diode. We characterize the intensity and polarization dependencies of the photoemission yield from both individual nanostars and ensembles. Numerical simulations of the plasmonic enhancement, the near-field distributions, and the photoemission intensities are in good agreement with experiment. Our results open a new avenue for the design of nanoscale electron sources