Few-cycle Surface Plasmon Polariton Generation by Rotating Wavefront Pulses

A concept for the efficient generation of surface plasmon polaritons (SPPs) with a duration of very few cycles is presented. The scheme is based on grating coupling and laser pulses with wavefront rotation (WFR), so that the resonance condition for SPP excitation is satisfied only for a time window shorter than the driving pulse. The feasibility and robustness of the technique is investigated by means of simulations with realistic parameters. In optimal conditions, we find that a $29.5$~fs pulse with $800$~nm wavelength can excite a $3.8$~fs SPP ($\sim 1.4$ laser cycles) with a peak field amplitude $2.7$ times the peak value for the laser pulse

High Field Plasmonics

Il manoscritto riguarda lo studio di effetti di plasmonica ad alti campi, ossia nel contesto dell'interazione laser-plasma ad altissima intensità (I > 10^18 W/cm^2). Si intende per "plasmonica" lo studio di plasmoni di superficie, che consistono in modi elettromagnetici all'interfaccia fra un metallo e un mezzo dielettrico. I plasmoni di superficie vengono normalmente eccitati con impulsi laser a bassa intensità.Questo regime è ben conosciuto dal punto di vista teorico e lo studio di schemi che coinvolgono effetti di plasmonica è un campo di ricerca molto vitale, motivato dalle possibili interessanti applicazioni (ad esempio per biosensori o microchip). Al contrario, non è, ad oggi, disponibile un modello teorico completo per plasmoni di superficie in regimi di altissima intensità, dove forti effetti non lineari e relativistici possono giocare un ruolo rilevante. Anche dal punto di vista sperimentale e numerico effetti di plasmonica in questo regime di interazione sono stati solo marginalmente esplorati. I risultati contenuti nella tesi sono principalmente frutto di investigazioni numeriche con codici Particle-In-Cell e risultati sperimentali relativi a effetti di plasmonica in regime di alti campi. Sono stati studiati diversi scenari, elencati di seguito. Si è studiata l'interazione di intensi impulsi laser (I~5x10^19 W/cm^2) con bersagli la cui superficie consisteva in un reticolo microstrutturato (attività sperimentale svolta presso il centro di ricerca CEA-Saclay, Gif-sur-Yvette, Francia). Si è osservata l'accelerazione di pacchetti collimati di elettroni lungo la superficie dei reticoli se irraggiati ad angoli vicini a quello atteso per l'eccitazione di un plasmone di superficie. E' stato misurato lo spettro energetico degli elettroni emessi, osservando una distribuzione piccata a 5-8 MeV e con una coda estesa fino a oltre 20 MeV. Sono state misurate cariche totali fino a oltre 100 pC. Tali caratteristiche rendono la sorgente potenzialmente di interesse per alcune applicazioni (diffrazione di elettroni ultra-veloce, produzione di fotoneutroni). Simulazioni numeriche Particle-In-Cell hanno mostrato ottimo accordo con i risultati sperimentali. Si fornisce anche un semplice modello teorico per chiarire il ruolo dei plasmoni di superficie nell'accelerazione degli elettroni Si è studiata l'interazione di impulsi laser con intensità superiori a 10^20 W/cm^2 con bersagli solidi sottili accoppiati ad una schiuma di carbonio con densità media tale da ottenere un plasma alla densità critica se completamente ionizzata (attività sperimentale svolta presso il centro di ricerca GIST, Gwangju, Corea del Sud). L'interazione laser-plasma in tali regimi consente un efficiente accoppiamento dell'impulso laser con plasmoni di volume, incrementando l'efficienza dell'assorbimento di energia da parte del bersaglio. L'attività si inquadra nel contesto dell'accelerazione di ioni con plasmi prodotti da laser (con potenziali future applicazioni in adroterapia o trattamento e diagnostica di materiali con fasci di ioni). Durante due campagne sperimentali si è osservato che la presenza di un sottile strato di schiuma consente di incrementare le energie massime degli ioni accelerati rispetto a bersagli semplici. Simulazioni numeriche con codici Particle-In-Cell hanno permesso di chiarire il ruolo giocato dalle microstrutture della schiuma. Si è studiato il ruolo di effetti plasmonici nell'instabilità di Rayleigh-Taylor “laser-driven”, che può svilupparsi in scenari di “Radiation Pressure Acceleration”, dove sottili bersagli solidi vengono accelerati direttamente dalla pressione di radiazione di un impulso laser ultraintenso. Utilizzando un semplice modello teorico si mostra che le modulazioni autoconsistenti della presione di radiazione dovute alla deformazione sinusoidale della superficie influenzano significativamente lo spettro dell'instabilità, dipendentemente dalla polarizzazione dell'impulso. L'evoluzione nonlineare è studiata con simulazioni Particle-In-Cell, che mostrano la formazione di strutture a rete. Si è studiato con simulazioni numeriche il ruolo dell'eccitazione di plasmoni di superficie nell'emissione di armoniche di ordine elevato da reticoli solidi irraggiati con impulsi laser ultra-intensi. All'eccitazione di un plasmone di superficie corrisponde un aumento dei campi elettromagnetici alla superficie del bersaglio, che incrementa l'efficienza di generazione di armoniche. Nelle simulazioni una migliore efficienza di emissione di armoiniche è stata osservata per bersagli irraggiati ad angoli vicini a quello atteso per l'eccitazione di un plasmone di superficie. I risultati presentati in quest'ultima sezione sono preliminari e saranno oggetto di studi ulteriori. The manuscript concerns the study of plasmonic effects at high fields, that is in the framework of laser-plasma interaction at ultra-high intensities (I > 10^18 W/cm^2). “Plasmonics” is the study of surface plasmons, which are electromagnetic modes at the interface between a metal and a dielectric medium. Surface plasmons are normally excited with low intensity laser pulses. This regime is well known from the theoretical point of view and the study of plasmonic schemes is a vibrant research field, motivated by the interesting possible applications (e.g. biosensors or plasmonic chips). On the other hand, a complete theoretical model is still lacking for surface plasmons in the high intensity regime, where strong non-linear and relativistic effects might play a relevant role. Also the numerical and experimental investigation of plasmonics in this regime has been limited up to now. This thesis presents mainly numerical (Particle-In-Cell simulations) and experimental results related to the study of plasmonic effects at high fields. A few scenarios were studied. They are listed hereunder. The interaction of intense laser pulses (I~5x10^19 W/cm^2) with microstructured grating targets was studied experimentally (this activity was carried out at the research center CEA-Saclay, Gif-sur-Yvette, France). We observed thee acceleration of collimated electron bunches along the surface of grating target when irradiated at angles close to that expected for the excitation of a surface plasmon. We measured the energy spectrum of the emitted electrons, observing a distribution peaked at 5-8 MeV with a tail extending up to more than 20 MeV. Total charges up to more than 100 pC were measured. These characteristics make the source interesting for some applications (like ultra-fast electron diffraction or photo-neutron generation). Particle-In-Cell numerical simulations proved to be in very good agreement with the experimental results. A theoretical model is provided to clarify the role played by surface plasmons in the electron acceleration process. The interaction of intense laser pulses (I> 10^20 W/cm^2) with solid targets coupled with a carbon foam was studied. The average density of the carbon foam was selected in order to obtain a plasma at the critical density if fully ionized (the experimental activity was carried out at the GIST research center, Gwangju, South Korea). Laser-plasma interaction in this regime allows for an efficient coupling of the laser pulse with bulk plasmons, enhanced the efficiency of the energy absorption by the target. The activity was carried out in the framework of ion acceleration with laser-produced plasmas (with potential future applications in hadron-therapy or diagnostic and treatment of materials with ion beams). During the two experimental campaigns we observed that targets coated with a thin foam allowed to obtained higher ion energies with respect to simple targets. Numerical simulations with Particle-In-Cell codes helped to clarify the role of the micro-structuring of the foam. We studied the role of plasmonic effects in laser-driven Rayleigh-Taylor instability, which may develop in Radiation Pressure Acceleration scenarios, where thin solid foils are directly accelerated by the radiation pressure of an ultra-intense laser pulse. Using a simple model it is shown that the self-consistent modulation of the radiation pressure caused by a sinusoidal rippling affects substantially the wavevector spectrum of the instability depending on the laser polarization. The nonlinear evolution is investigated by three dimensional simulations which show the formation of net-like structures. The role of surface plasmons in high order harmonic emission from solid grating targets irradiated with ultra-intense laser pulses was studied with numerical simulations. The excitation of a surface plasmon is associated with an enhancement of the electromagnetic field close to the target surface, which should increase the efficiency of harmonic generation. In the simulations, a higher emission efficiency for high-order harmonics was observed for targets irradiated at angles close to that expected for surface plasmon excitation. The results presented in this part are still preliminary. The topic will be addressed in future works

Vlasov simulation of laser-driven shock acceleration and ion turbulence

We present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear collisionless ion-acoustic shocks and solitons excited by an intense laser interacting with an overdense plasma. The use of the Vlasov code avoids problems with low particle statistics and allows a validation of particle-in-cell results. A simple original correction to the splitting method for the numerical integration of the Vlasov equation has been implemented in order to ensure the charge conservation in the relativistic regime. We show that the ion distribution is affected by the development of a turbulence driven by the relativistic "fast" electron bunches generated at the laser-plasma interaction surface. This leads to the onset of ion reflection at the shock front in an initially cold plasma where only soliton solutions without ion reflection are expected to propagate. We give a simple analytic model to describe the onset of the turbulence as a nonlinear coupling of the ion density with the fast electron currents, taking the pulsed nature of the relativistic electron bunches into account

Laser-Driven Rayleigh-Taylor Instability: Plasmonics Effects and Three-Dimensional Structures

The acceleration of dense targets driven by the radiation pressure of high-intensity lasers leads to a Rayleigh-Taylor instability (RTI) with rippling of the interaction surface. Using a simple model it is shown that the self-consistent modulation of the radiation pressure caused by a sinusoidal rippling affects substantially the wavevector spectrum of the RTI depending on the laser polarization. The plasmonic enhancement of the local field when the rippling period is close to a laser wavelength sets the dominant RTI scale. The nonlinear evolution is investigated by three dimensional simulations, which show the formation of stable structures with "wallpaper" symmetry.Comment: 5 pages, 5 figures. New version includes 2D and 3D simulations. More details in the analytical calculation are given in the previous versio

Improved analysis of the bounds from the electroweak precision tests on the 4-site model

We present a new complete analysis of the electroweak precision observables within the recently proposed 4-site Higgsless model, which is based on the SU(2)_L x SU(2)_1 x SU(2)_2 x U(1)_Y gauge symmetry and predicts six extra gauge bosons, W_{1,2} and Z_{1,2}. Within the epsilon_i (i=1,2,3,b) parametrization, we compute for the first time the EWPT bounds via a complete numerical algorithm going beyond commonly used approximations. Both epsilon_{1,3} impose strong constraints. Hence, it is mandatory to consider them jointly when extracting EWPT bounds and to fully take in to account the correlations among the electroweak precison measurements. The phenomenological consequence is that the extra gauge bosons must be heavier than 250 GeV. Their couplings to SM fermions, even if bounded, might be of the same order of magnitude than the SM ones. In contrast to other Higgsless models, the 4-site model is not fermiophobic. The new gauge bosons could thus be discovered in the favoured Drell-Yan channel already during the present run of the LHC experiment.Comment: Latex file, 35 pages, 10 figures, corrected typos, published versio

Interference effects in heavy W'-boson searches at the LHC

Interference effects are widely neglected in searches for new physics. This is the case in recent publications on searches for W'-bosons using leptonic final states. We examine the effects of interference on distributions frequently used to determine mass limits for possible W'-bosons and show that there are important qualitative effects on the behaviour of the new physics signal. There are two main consequences. Firstly, exclusion limits where interferences effects have not been considered are likely to have been overestimated. Secondly, presenting experimental results as a function of a transverse mass cut rather than in terms of the contribution of new physics to the total cross-section would be more informative.Comment: 31 pages, 8 figures. Published versio

W' production at the LHC in the 4-site Higgsless model

We study the phenomenology of the 4-site Higgsless model, based on the $SU(2)_L\times SU(2)_1\times SU(2)_2\times U(1)_Y$ gauge symmetry, at present colliders. The model predicts the existence of two neutral and four charged extra gauge bosons, Z1,Z2,W1,W2. In this paper, we focus on the charged gauge sector. We first derive limits on W1,W2-boson masses and couplings to SM fermions from direct searches at the Tevatron. We then estimate at the 7 TeV LHC the exclusion limits with the actual L=1 fb-1 and the discovery potential with the expected L=10 fb-1. In contrast to the minimal (or 3-site) Higgsless model which predicts almost fermiophobic extra gauge bosons, the next-to-minimal (or 4-site) Higgsless model recovers sizeable W1,W2-boson couplings to ordinary matter, expressing the non-fermiophobic multiresonance inner nature of extra-dimensional theories. Owing to this feature, we find that in one year from now the new heavy gauge bosons, W1 and W2, could be discovered in the final state with an electron and large missing transverse energy at the 7 TeV LHC for W1,W2-boson masses in the TeV region, depending on model parameters.Comment: 28 pages, references adde

A worksite intervention to reduce the cardiovascular risk: proposal of a study design easy to integrate within Italian organization of occupational health surveillance

BACKGROUND: Despite the substantial amount of knowledge on effectiveness of worksite health promotion (WHP) in reducing cardiovascular disease (CVD) risk, WHP programs are not systematically applied in Italy. The aim was to design an intervention easy to integrate within the Italian organization of workplace health surveillance. METHODS: We used the “pretest-posttest design”. Workers were employed in multiple occupations and resident in Veneto region, Italy. Occupational physicians (OPs) performed all examinations, including laboratory evaluation (capillary blood sampling and measure of glycaemia and cholesterolemia with portable devices), during the normal health surveillance at worksite. CVD risk was computed based on sex, age, smoking habit, diabetes, systolic pressure and cholesterol level. After excluding those with <40 years of age, missing consent, CVD diagnosis or current therapy for CVD, missing information, CVD risk <5%, out of 5,536 workers 451 underwent the intervention and 323 male workers were re-examined at 1 year. CVD risk was the most compelling argument for changing lifestyle. The counseling was based on the individual risk factors. Individuals examined at posttest were a small fraction of the whole (6% = 323/5,536). In these workers we computed the ratio pretest/posttest of proportions (such as percent of subjects with cardiovascular risk >5%) as well as the exact McNemar significance probability or the exact test of table symmetry. RESULTS: CVD risk decreased by 24% (McNemar p = 0.0000) after the intervention; in a sensitivity analysis assuming that all subjects lost to follow-up kept their pretest cardiovascular risk value, the effect (−18%) was still significant (symmetry p < 0.0000). Each prevented CVD case was expected to cost about 5,700 euro. CONCLUSIONS: The present worksite intervention promoted favorable changes of CVD risk that were reasonably priced and consistent across multiple occupations