The plasma mirror - A subpicosecond optical switch for ultrahigh power lasers

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers ~TW–PW!. The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csDt,lLaser must be met (cs : sound speed, Dt: time from plasma formation to the peak of the pulse!. The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.

Wavelet analysis of bender element signals

Accurate determination of shear wave arrival time using bender elements may be severely affected by a combination of near-field effects and reflected waves. These may mask the first arrival of the shear wave, making its detection difficult in the time domain. This paper describes an approach for measuring the shear wave arrival time through analysis of the output signal in the time-scale domain using a multi-scale wavelet transform. The local maxima lines of the wavelet transform modulus are observed at different scales, and all singularities are mathematically characterised, allowing subsequent detection of the singularity relating to the first arrival. Examples of the use of this approach on typical synthetic and experimental bender element signals are also supplied, and these results are compared with those from other time and frequency domain approaches. The wavelet approach is not affected by near-field effects, but instead is characterised by a relatively consistent singularity related to the shear wave arrival time, across a range of frequencies and noise levels.

Efficient excitation of dielectric-loaded surface plasmon-polariton waveguide modes at telecommunication wavelengths

The excitation of surface plasmon-polariton (SPP) waveguide modes in subwavelength dielectric ridges deposited on a thin gold film has been characterized and optimized at telecommunication wavelengths. The experimental data on the electromagnetic mode structure obtained using scanning near-field optical microscopy have been directly compared to full vectorial three-dimensional finite element method simulations. Two excitation geometries have been investigated where SPPs are excited outside or inside the dielectric tapered region adjoint to the waveguide. The dependence of the efficiency of the SPP guided mode excitation on the taper opening angle has been measured and modeled. Single-mode guiding and strong lateral mode confinement of dielectric-loaded SPP waveguide modes have been characterized with the near-field measurements and compared to the effective-index method model.

Precise Subnanometer Plasmonic Junctions for SERS within Gold Nanoparticle Assemblies Using Cucurbit[n]uril "Glue"

Cucurbit[n]urils (CB[n]) are macrocyclic host molecules with subnanometer dimensions capable of binding to gold surfaces. Aggregation of gold nanoparticles with CB[n] produces a repeatable, fixed, and rigid interparticle separation of 0.9 nm, and thus such assemblies possess distinct and exquisitely sensitive plasmonics. Understanding the plasmonic evolution is key to their use as powerful SERS substrates. Furthermore, this unique spatial control permits fast nanoscale probing of the plasmonics of the aggregates "glued" together by CBs within different kinetic regimes using simultaneous extinction and SERS measurements. The kinetic rates determine the topology of the aggregates including the constituent structural motifs and allow the identification of discrete plasmon modes which are attributed to disordered chains of increasing lengths by theoretical simulations. The CBs directly report the near-field strength of the nanojunctions they create via their own SERS, allowing calibration of the enhancement. Owing to the unique barrel-shaped geometry of CB[n] and their ability to bind "guest" molecules, the aggregates afford a new type of in situ self-calibrated and reliable SERS substrate where molecules can be selectively trapped by the CB[n] and exposed to the nanojunction plasmonic field. Using this concept, a powerful molecular-recognition-based SERS assay is demonstrated by selective cucurbit[n]uril host-guest complexation.

Tip-enhanced fluorescence imaging of quantum dots

We have imaged the fluorescence from a single quantum dot cluster using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought within a few nanometers from the sample surface, the resulting enhancement in quantum dot fluorescence in the vicinity of the tip leads to a resolution of about 60 nm. We determine this enhancement of the fluorescence to be about fourfold in magnitude, which is consistent with the value expected as a result of competition between fluorescence quenching and electromagnetic field enhancement. (C) 2005 American Institute of Physics.

Tip-enhanced Raman microscopy: practicalities and limitations

The feasibility of apertureless scanning near-field Raman microscopy, exploiting the local enhancement in Raman scattering in the vicinity of a silver or gold tip, was investigated. Using the finite difference time domain method we calculated the enhancement of electric field strength, and hence Raman scattering, achieved through the resonant excitation of local modes in the tip. By modelling the frequency-dependent dielectric response of the metal tip we were able to highlight the resonant nature of the tip-enhancement and determine the excitation wavelength required for the strongest electric field enhancement, and hence Raman scattering intensity, which occurs for the excitation of modes localized at the tip apex. It is demonstrated that a peak Raman enhancement of 10(7)-fold should be achievable with <5 nm spatial resolution. We show that surface-enhanced Raman scattering from carbon contamination on a silver or gold tip can be significant. However, we find for a tip of radius of curvature 20 nm that the Raman enhancement should decay totally within 20 nm from the tip. Hence withdrawal of the tip by this distance should lead to the disappearance of the tip-enhanced signal, leaving only that from carbon contamination on the tip itself and the intrinsic signal from the sample. Copyright (C) 2003 John Wiley Sons, Ltd.

Spectral impairment of enclosed higher order ambisonics

Ambisonics is spatial audio technique that attempts to recreate a physical sound field over as large an area as possible. Higher Order Ambisonic systems modelled with near field loudspeakers in free field as well as in a simulated room are investigated. The influence of reflections on the image quality is analysed objectively for both a studio-sized and large reproduction environment using the relative intensity of the reproduced sound field. The results of a simulated enclosed HOA system in the studio-sized room are compared to sound field measurements in the reproduced area.

BROAD-BAND EXCITATION AND EMISSION OF SURFACE-PLASMONS

The well known advantages of using surface plasmons, in particular the high sensitivity to surface adsorbates, are nearly always compromised in practice by the use of monochromatic excitation and the consequent lack of proper spectroscopic information. This limitation arises from the angle/wavelength selective nature of the surface plasmon resonance. The work described here uses an elegant broadband excitation/decay scheme in a substrate(silica)-grating profiled photoresist-Ag film geometry. Laser radiation of wavelength 488 nm, incident through the silica substrate, excites by near-field coupling a broad band of surface plasmons at the photoresist-Ag interface within the spectral range of the photoresist fluorescence. With a judicious choice of grating period this mode can cross-couple to the mode supported at the Ag-air interface. This latter mode can, in turn, couple out to light by virtue of the same grating profile. The spectral distribution of the light emitted due to this three-step process has been studied as a function of the angle of emission and depth of the grating profiled surface for each polarization. It is found that the optimum emission efficiency occurs with a groove depth in the region of 65 nm. This is considerably greater than the optimum depth of 40 nm required for surface plasmon-photon coupling at a Ag-air interface or, in other words, for the last step of the process in isolation.

Polarization Distortion as a Means for Securing Wireless Communication

This paper presents a simple polarization encoding strategy that operates using only single element dual port transmit and receive antennas in such a way that selective spatial scrambling of QPSK data can be effected. The key transmitter and receiver relationships needed for this operation to occur are derived. The system is validated using a cross dipole antenna arrangement. Unlike all previously reported physical layer wireless solutions the approach developed in this paper transfers the security property to the receive side resulting in very simple transmit and receive side architectures thus avoiding the need for near field modulated array technology. In addition the scheme permits, for the first time, multiple spatially separated secured receive sites to operate in parallel.

A Local Superlens

Superlenses enable near-field imaging beyond the diffraction limit. However, their widespread implementation in optical imaging technology so far has been limited by large-scale fabrication, fixed lens position and specific object materials. Here, we demonstrate that a dielectric lamella of sub-wavelength size in all three spatial dimensions behaves as a compact superlens that operates at infrared wavelengths and can be positioned to image any local microscopic area of interest on the sample. In particular, the lamella superlens may be placed in contact with any type of object and therefore enables examination of hard-to-scan samples e.g. with high topography or in liquids, without altering the specimen design. This lamella-based local superlens design is directly applicable to sub-wavelength light-based technology such as integrated optics.

Manipulating plasmon modes in tip-enhanced Raman microscopy

The influence of polarization on the plasmon modes excited in tip-enhanced near-field optical microscopy have been investigated using the Finite Difference Time Domain Method. Analysis of the calculated results have laid particular emphasis on the ability to align local field enhancements with the orientation of molecules in order to optimize Raman signals, with particular relevance to recent experimental work on carbon nanotubes.

Optically controlled dense current structures driven by relativistic plasma aperture-induced diffraction

The collective response of charged particles to intense fields is intrinsic to plasma accelerators and radiation sources, relativistic optics and many astrophysical phenomena. Here we show that a relativistic plasma aperture is generated in thin foils by intense laser light, resulting in the fundamental optical process of diffraction. The plasma electrons collectively respond to the resulting laser near-field diffraction pattern, producing a beam of energetic electrons with a spatial structure that can be controlled by variation of the laser pulse parameters. It is shown that static electron-beam and induced-magnetic-field structures can be made to rotate at fixed or variable angular frequencies depending on the degree of ellipticity in the laser polarization. The concept is demonstrated numerically and verified experimentally, and is an important step towards optical control of charged particle dynamics in laser-driven dense plasma sources.

Photon nanojet lens: Design, fabrication and characterization

In this paper, a novel nanolens with super resolution, based on the photon nanojet effect through dielectric nanostructures in visible wavelengths, is proposed. The nanolens is made from plastic SU-8, consisting of parallel semi-cylinders in an array. This paper focuses on the lens designed by numerical simulation with the finite-difference time domain method and nanofabrication of the lens by grayscale electron beam lithography combined with a casting/bonding/lift-off transfer process. Monte Carlo simulation for injected charge distribution and development modeling was applied to define the resultant 3D profile in PMMA as the template for the lens shape. After the casting/bonding/lift-off process, the fabricated nanolens in SU-8 has the desired lens shape, very close to that of PMMA, indicating that the pattern transfer process developed in this work can be reliably applied not only for the fabrication of the lens but also for other 3D nanopatterns in general. The light distribution through the lens near its surface was initially characterized by a scanning near-field optical microscope, showing a well defined focusing image of designed grating lines. Such focusing function supports the great prospects of developing a novel nanolithography based on the photon nanojet effect.

A Review of Numerical Modelling of Wave Energy Converter Arrays

Large-scale commercial exploitation of wave energy is certain to require the deployment of wave energy converters (WECs) in arrays, creating ‘WEC farms’. An understanding of the hydrodynamic interactions in such arrays is essential for determining optimum layouts of WECs, as well as calculating the area of ocean that the farms will require. It is equally important to consider the potential impact of wave farms on the local and distal wave climates and coastal processes; a poor understanding of the resulting environmental impact may hamper progress, as it would make planning consents more difficult to obtain. It is therefore clear that an understanding the interactions between WECs within a farm is vital for the continued development of the wave energy industry.To support WEC farm design, a range of different numerical models have been developed, with both wave phase-resolving and wave phase-averaging models now available. Phase-resolving methods are primarily based on potential flow models and include semi-analytical techniques, boundary element methods and methods involving the mild-slope equations. Phase-averaging methods are all based around spectral wave models, with supra-grid and sub-grid wave farm models available as alternative implementations.The aims, underlying principles, strengths, weaknesses and obtained results of the main numerical methods currently used for modelling wave energy converter arrays are described in this paper, using a common framework. This allows a qualitative comparative analysis of the different methods to be performed at the end of the paper. This includes consideration of the conditions under which the models may be applied, the output of the models and the relationship between array size and computational effort. Guidance for developers is also presented on the most suitable numerical method to use for given aspects of WEC farm design. For instance, certain models are more suitable for studying near-field effects, whilst others are preferable for investigating far-field effects of the WEC farms. Furthermore, the analysis presented in this paper identifies areas in which the numerical modelling of WEC arrays is relatively weak and thus highlights those in which future developments are required.

Lanthanide oxide nanotubes and nanorods: synthesis, processing, luminescence and catalytic properties

Esta tese relata estudos de síntese, caracterização da estrutura e das propriedades de fotoluminescência e aplicações de nanotubos e nanobastonetes de óxidos de lantanídeos em pontas para microscopia de força atómica, catálise heterogénea e compósitos de base polimérica. Há um interesse crescente em compreender como o confinamento quântico decorrente da redução do tamanho de partícula pode influenciar a eficiência da luminescência, a dinâmica dos estados excitados, a transferência de energia e os efeitos de termalização de nanoluminóforos. Em nanocristais dopados com lantanídeos (Ln3+), e apesar da localização dos estados 4f, ocorrem efeitos de confinamento quântico via interacção com os modos vibracionais da rede. Em particular, a termalização anómala, descrita para uma variedade de nanocristais dopados com Ln3+, tem sido atribuída à ausência de modos vibracionais de menor frequência. Este nanoconfinamento pode ter impacto na dinâmica da luminescência, bem como na transferência de energia mediada por modos vibracionais e processos de upconversion. Nesta tese, relata-se o estudo deste efeito em nanotubos de Gd2O3:Eu3+. A influência de parâmetros como a concentração de európio e as condições de calcinação também foi investigada. Algumas aplicações destes óxidos de lantanídeos também foram exploradas, nomeadamente a modificação de pontas usadas em microscopia de força atómica com nanobastonetes de Gd2O3:Eu3+, lograda através de dielectroforese, técnica que não degrada a emissão de luz (rendimento quântico 0.47). As pontas modificadas são estáveis sob condições de trabalho, podendo ser aplicadas, por exemplo, em microscopia óptica de varrimento de campo próximo (SNOM). A oxidação em fase líquida do etilbenzendo foi investigada usando como catalisador nanotubos de CeO2, em presença dos oxidantes hidroperóxido de t-butilo e H2O2, e do solvente acetonitrilo, e temperaturas entre 55 e 105 ºC. Nanobastonetes de Gd2O3:Eu3+ recobertos com sílica foram preparados pelo método sol-gel. Esta cobertura resultou num aumento, quer do rendimento quântico de emissão, de 0.51 para 0.86 (excitação a 255 nm), quer dos tempos de vida,de 1.43 para 1.80 ms (excitação a 394.4 nm). A superfície dos nanotubos cobertos com sílica foi modificada com o agente de acoplamento metacrilato de 3-(trimetoxissilil)propilo que permitiu a preparação de compósitos através da subsequente polimerização in-situ do estireno por técnicas de miniemulsão e solução. ABSTRACT: This thesis reports on the synthesis, characterisation of the structure and photoluminescence properties, and applications of nanotubes and nanorods of lanthanides oxides in atomic force microscopy tips, heterogeneous catalysis and polymer-base composites. There is a growing interest in understanding how size-dependent quantum confinement affects the photoluminescence efficiency, excited-state dynamics, energy-transfer and thermalisation phenomena in nanophosphors. For lanthanide (Ln3+)-doped nanocrystals, and despite the localisation of the 4f states, confinement effects are induced mostly via electron-phonon interactions. In particular, the anomalous thermalisation reported for a handful of Ln3+-doped nanocrystals has been rationalised by the absence of lowfrequency phonon modes. This nanoconfinement may further impact on the Ln3+ luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, this effect is investigated in Gd2O3:Eu3+ nanotubes. The influence of parameters such as europium concentration and calcination procedure is also studied. Some applications of these lanthanides oxides have been explored, for instance the modification of atomic force microscopy tips with photoluminescent Gd2O3:Eu3+ nanorods, using dielectrophoresis, a technique which preserves the red emission of the nanorods (quantum yield 0.47). The modified tips are stable under working conditions and may find applications in scanning near-field optical microscopy. The liquid-phase oxidation of ethylbenzene over CeO2 nanotubes has been investigated, using tert-butyl-hydroperoxide and H2O2 as the oxidising agents, and acetonitrile as the solvent, in the range 55-105 ºC. Gd2O3:Eu3+ nanorods have been coated with silica via a sol-gel approach. The silica coating increases both, the Eu3+ absolute emission quantum yields from 0.51 to 0.86 (255 nm excitation), and decay times from 1.43 to 1.80 ms (394.4 nm excitation). The silica coating was modified with 3- (trimethoxysilyl) propyl methacrylate and, subsequently, composites have been prepared by in-situ radical polymerisation of styrene via miniemulsion and solution routes.