Paolo Michetti

Polariton-mediated energy transfer between organic dyes in a strongly coupled optical microcavity

Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light-matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes.

Simulation of hydrogenated graphene field-effect transistors through a multiscale approach

In this work, we present a performance analysis of field-effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semihydrogenated graphene (i.e., graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor sp(3) tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large I-on and I-on/I-off ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.

Bound states and persistent currents in topological insulator rings

We analyze theoretically the bound state spectrum of an Aharonov Bohm (AB) ring in a two-dimensional topological insulator using the four-band model of HgTe-quantum wells as a concrete example. We calculate analytically the circular helical edge states and their spectrum as well as the bound states evolving out of the bulk spectrum as a function of the applied magnetic flux and dimension of the ring. We also analyze the spin-dependent persistent currents, which can be used to measure the spin of single electrons. We further take into account the Rashba spin-orbit interaction which mixes the spin states and derive its effect on the ring spectrum. The flux tunability of the ring states allows for coherent mixing of the edge- and the spin degrees of freedom of bound electrons which could be exploited for quantum information processing in topological insulator rings.

Analytical Model of One-Dimensional Carbon-Based Schottky-Barrier Transistors

Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, which cannot be described neither by drift diffusion nor by purely ballistic models. In carbon-based nanotransistors, source and drain contacts are often characterized by the formation of Schottky barriers (SBs), with strong influence on transport. In this paper, we present a model for 1-D field-effect transistors, taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Büttikers probe approach to dissipative transport, in which a nonballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport and, in particular, of the transition between SB- and dissipation-limited transports.

Microscopic theory of polariton lasing via vibronically assisted scattering

Polariton lasing has recently been observed in strongly coupled crystalline anthracene microcavities. A simple model is developed describing the onset of the nonlinear threshold based on a master equation including the relevant relaxation processes and employing realistic material parameters. The mechanism governing the buildup of the polariton population, namely, bosonic stimulated scattering from the exciton reservoir via a vibronically assisted process, is characterized and its efficiency calculated on the basis of a microscopic theory. The role of polariton-polariton bimolecular quenching is identified and temperature-dependent effects are discussed.

Electric Field Control of Spin Rotation in Bilayer Graphene

The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin field-effect transistors. An alternative solution can be found in magnetic doping of graphene or, as discussed in the present work, in exploiting the proximity effect between graphene and ferromagnetic oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction, that acts as an effective Zeeman field for electrons in graphene, inducing a spin precession around the magnetization axis of the FO. Here we show that in an appropriately designed double-gate field-effect transistor, with a bilayer graphene channel and FO used as a gate dielectric, spin-precession of carriers can be turned ON and OFF with the application of a differential voltage to the gates. This feature is directly probed in the spin-resolved conductance of the bilayer.

Model of tunneling transistors based on graphene on SiC

Recent experiments shown that graphene epitaxially grown on Silicon carbide (SiC) can exhibit a energy gap of 0.26 eV, making it a promising material for electronics. With an accurate model, we explore the design parameter space for a fully ballistic graphene-on-SiC tunnel field-effect transistors, and assess the dc and high frequency figures of merit. The steep subthreshold behavior can enable ION/IOFF ratios exceeding 104 even with a low supply voltage of 0.15 V, for devices with gatelength down to 30 nm. Intrinsic transistor delays smaller than 1 ps are obtained. These factors make the device an interesting candidate for low-power nanoelectronics beyond CMOS.

Exciton-phonon scattering and photoexcitation dynamics in J-aggregate microcavities

We have developed a model accounting for the photo-excitation dynamics and the photoluminescence of strongly coupled J-aggregate microcavities. Our model is based on a description of the J-aggregate film as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton-polaritons are formed, mixing superradiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a CW non resonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a paritticular set of discrete optical molecular vibrations active in relaxation processes are investigared.

Model and Performance Evaluation of Field-Effect Transistors Based on Epitaxial Graphene on SiC

In view of the appreciable semiconducting gap of 0.26 eV observed in recent experiments, epitaxial graphene on a SiC substrate seems a promising channel material for FETs. Indeed, it is 2-D-and therefore does not require prohibitive lithography-and exhibits a wider gap than other alternative options, such as bilayer graphene. Here, we propose a model and assess the achievable performance of a nanoscale FET based on epitaxial graphene on SiC, conducting an exploration of the design parameter space. We show that the current can be modulated by four orders of magnitude; for digital applications, an Ion/Ioff ratio of 50 and a subthreshold slope of 145 mV/dec can be obtained with a supply voltage of 0.25 V. This represents a significant progress toward solid-state integration of graphene electronics, but not yet sufficient for digital applications.

Analytical Model of Nanowire FETs in a Partially Ballistic or Dissipative Transport Regime

The intermediate transport regime in nanoscale transistors between the fully ballistic case and the quasi-equilibrium case, described by the drift-diffusion (DD) model, is still an open modeling issue. Analytical approaches to the problem have been proposed, based on the introduction of a backscattering coefficient, or numerical approaches consisting in the Monte Carlo solution of the Boltzmann transport equation or in the introduction of dissipation in quantum transport descriptions. In this paper, we propose a simple analytical model to seamlessly cover the whole range of transport regimes in generic quasi-1-D field-effect transistors, and apply it to silicon nanowire transistors. The model is based on describing a generic transistor as a chain of ballistic nanowire transistors in series, or as the series of a ballistic transistor and a DD transistor operating in the triode region. As an additional result, we find a relation between the mobility and the mean free path that has deep consequences on the understanding of transport in nanoscale devices.