Modeling of quantum transport in open systems

Whereas classical transport physics is based on the concept of a probability distribution which is defined over the phase space of the system, the concept of a phase-space distribution function in the quantum formulation of transport is difficult, since the non-commutation of the position and momentum operators (the Heisenberg uncertainty principle) precludes the precise specification of a point in phase space. However, within the formulation of quantum mechanics, various formalisms based on density matrices, Wigner functions, Feynman path integrals and Green’s functions have been developed. These embrace the quantum nature of transport; moreover, in recent years, each technique has been utilized to address key aspects of quantum transport in semiconductors. At present, there is no unifying, user-friendly approach to quantum transport in semiconductors. Density matrices, and the associated Wigner function approach, Green’s functions, and Feynman path integrals all have their application and computational strength and weakness, and all their are equivalent representations of the quantum nature of transport. In the present work the density-matrix and Wigner-function formalisms will be employed. This choice is due to the fact that the “open system problem” that here is faced, is better managed using such an approach; Indeed the density-matrix formalism is extremely useful to show the degree of quantum coherence of the system under investigations while the Wigner-function picture is the ideal instrument to describe real-space quantum devices. Such a choice will be better understood looking in more detail to the problem of open systems, i.e., systems with open spatial boundaries. The most interesting products of micro- and nanoelectronics technology are systems that operate far from equilibrium. A closer inspection of a few examples of such systems reveals that they are generally open, in the sense that they exchange matter with their environment. The present work is aimed at developing a fully microscopic theory to describe open quantum systems starting from the so-called Semiconductor Bloch equations, namely the equations that describe the coherent versus incoherent dynamics of a closed quantum system. In the context of the present work, an open system is a system that exchange locally particles with its environment. Moreover, we wish to focus upon its far-from-equilibrium behaviour, and thus the specific definition of open system will be further restricted to describe a system coupled to at least two separate particle reservoirs, so that a non-equilibrium state may be created and maintained. To specify such a system we must regard it as occupying a finite region of space, and thus the exchange of particles must consist of a current flowing through the system surface which is taken to be the boundary of the system. Our aim is to analyse in detail the problem of openness in the present sense with also the possibility to consider energy-relaxation and dephasing processes within the device active region. More specifically, our analysis will allow us to point out and overcome some basic limitations of the conventional Wigner-function formalism; this will be accomplished by introducing a Generalized Weyl-Wigner approach, able to remove such anomalies, thus recovering typical results of partially phenomenological models. In this context we shall propose a theoretical scheme where the boundary conditions are described via a source term, i.e., a term representing the particles entering the simulated region from its spatial boundaries. In particular, we shall propose and demonstrate two fully equivalent theoretical models able to describe adequately an open quantum device: the first one is characterized by a non-diagonal source term (i.e., coherent source) while the second one is characterized by a diagonal source term

Impact of structural defects on the performance of graphene plasmon-based molecular sensors

Graphene-based plasmonic devices are regarded to be suitable for a plethora of applications, ranging from mid-infrared to terahertz frequencies. In this regard, among the peculiarities associated with graphene, it is well known that plasmons are tunable and tend to show stronger confinement as well as a longer lifetime than in the noble-metal counterpart. However, due to the two-dimensional specificity of graphene, the presence of defects might induce stronger effects than in bulky noble metals. Here, we theoretically investigate the impact of structural defects hosted by graphene on selected figures of merit associated to localized plasmons, which are of key technological importance for plasmon-based molecular sensing. By considering an optimized graphene nanostructure, we provide a comparative analysis intended to shed light on the impact of the type of defect on graphene localized plasmons, that regards distinct types of defects commonly arising from fabrication procedures or exposure to radiation. This understanding will help industry and academia in better identifying the most suitable applications for graphene-based molecular sensing.Comment: 8 pages, 5 figure

Monte Carlo simulation of hot-carrier phenomena in open quantum devices: A kinetic approach

An alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries is presented. In particular, we propose replacing the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. More specifically, in this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Monte Carlo approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. The proposed simulation strategy is applyed to state-of-the-art semiconductor nanostructures

Mapping the local dielectric response at the nanoscale by means of plasmonic force spectroscopy

At the present, the local optical properties of nanostructured materials are difficult to be measured by available instrumentation. We investigated the capability of plasmonic force spectroscopy of measuring the optical response at the nanoscale. The proposed technique is based on force measurements performed by combining Atomic Force Microscopy, or optical tweezers, and adiabatic compression of surface plasmon polaritons. We show that the optical forces, caused by the plasmonic field, depend on the local response of the substrates and, in principle, allow probing both the real and the imaginary part of the local permittivity with a spatial resolution of few nanometers

Plasmomechanical Systems: Principles and Applications

AbstractExtreme confinement of electromagnetic waves and mechanical displacement fields to nanometer dimensions through plasmonic nanostructures offers unprecedented opportunities for greatly enhanced interaction strength, increased bandwidth, lower power consumption, chip‐scale fabrication, and efficient actuation of mechanical systems at the nanoscale. Conversely, coupling mechanical oscillators to plasmonic nanostructures introduces mechanical degrees of freedom to otherwise static plasmonic structures thus giving rise to the generation of extremely large resonance shifts even for minor position changes. This nanoscale marriage of plasmonics and mechanics has led to the emergence of a new field of study called plasmomechanics that explores the fundamental principles underneath the coupling between light and plasmomechanical nanoresonators. In this review, both the fundamental concepts and applications of plasmomechanics as an emerging field of study are discussed. After an overview of the basic principles of plasmomechanics, the active tuning mechanisms of plasmonic nano‐mechanical systems are extensively analyzed. Moreover, the recent developments on the practical implications of plasmomechanic systems for such applications as biosensing and infrared detection are highlighted. Finally, an outlook on the implications of the plasmomechanical nanosystems for development of point‐of‐care diagnostic devices that can help early and rapid detection of fatal diseases are forwarded

Modeling of open quantum devices within the closed-system paradigm

We present an alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries. We propose to replace the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. In this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Boltzmann-transport approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. Applications to state-of-the-art semiconductor nanostructures are discussed. Finally, the proposed approach is extended to the quantum-transport regime; to this end, we introduce an effective Liouville superoperator, able to describe the effect of the device spatial boundaries on the time evolution of the single-particle density matrix

Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters

AbstractThe weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping

Enhanced optical magnetism for reversed optical binding forces between silicon nanoparticles in the visible region.

We perform a comprehensive numerical analysis on the optical binding forces of a multiple-resonant silicon nanodimer induced by the normal illumination of a plane wave in the visible region. The silicon nanodimer provides either repulsive or attractive forces in water while providing only attractive forces in air. The enhancement of the magnetic dipole mode is attributed to the generation of repulsive forces. The sign (attractive/repulsive) and the amplitude of the optical forces are controlled by incident polarization and separation distance between the silicon nanoparticles. These optomechanical effects demonstrate a key step toward the optical sorting and assembly of silicon nanoparticles

Photoinduced Temperature Gradients in Sub-wavelength Plasmonic Structures: The Thermoplasmonics of Nanocones

Plasmonic structures are renowned for their capability to efficiently convert light into heat at the nanoscale. However, despite the possibility to generate deep sub-wavelength electromagnetic hot spots, the formation of extremely localized thermal hot spots is an open challenge of research, simply because of the diffusive spread of heat along the whole metallic nanostructure. Here we tackle this challenge by exploiting single gold nanocones. We theoretically show how these structures can indeed realize extremely high temperature gradients within the metal, leading to deep sub-wavelength thermal hot spots, owing to their capability of concentrating light at the apex under resonant conditions even under continuous wave illumination. A three-dimensional Finite Element Method model is employed to study the electromagnetic field in the structure and subsequent thermoplasmonic behaviour, in terms of the three-dimensional temperature distribution. We show how the latter is affected by nanocone size, shape, and composition of the surrounding environment. Finally, we anticipate the use of photoinduced temperature gradients in nanocones for applications in optofluidics and thermoelectrics or for thermally induced nanofabrication

Evolution of modes in a metal-coated nano-fiber.

We report on the evolution of modes in cylindrical metal/dielectric systems. The transition between surface plasmon polaritons and localized modes is documented in terms of the real and imaginary parts of the effective refractive index as a function of geometric and optical parameters. We show the evolution process of SPP and localized modes. New phenomena of coupling between SPP and core-like modes, and of mode gap and super-long surface plasmon polaritons are found and discussed. We conclude that both superluminal light and slow light can be solutions of metallically coated dielectric fibers