Multimode Phonon Cooling via Three Wave Parametric Interactions with Optical Fields

We discuss the possible cooling of different phonon modes via three wave mixing interactions of vibrational and optical modes. Since phonon modes exhibit a variety of dispersion relations or frequency spectra with diverse spatial structures, depending on the shape and size of the sample, we formulate our theory in terms of relevant spatial mode functions for the interacting fields in any given geometry. We discuss the possibility of Dicke like collective effects in phonon cooling and present explicit results for simultaneous cooling of two phonon modes via the anti-Stokes up conversions. We show that the bimodal cooling should be observable experimentally

Some recent trends in raman spectroscopy

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Surface-enhanced Raman scattering in a two-oscillator electromagnetic model

A two-oscillator model is considered to investigate the effect of a metal substrate of dielectric function ε(ω) on the Raman scattering from a molecule absorbed on the metal surface. In the presence of the metal and an external electric field, the linear motion of the electronic and ionic oscillators in the molecule get coupled, in general. For obtaining Raman scattering at the Stokes frequency, a phenomenological nonlinear force term, which is bilinear in the oscillator amplitudes, is introduced in the equation of motion. The whole problem is considerably simplified when we use the fact that the ionic mass is much larger than the electronic mass and the ionic vibration frequency is much smaller than the electronic and optical frequencies. It is shown that because of different renormalization factors the frequency dependence of the enhancement factor F, taken to be the ratio of Raman intensity with and without the metal, is quite different from that calculated by using the familiar polarizability-derivative theory. Applying the well-known fluctuation-dissipation theorem, the new Raman line shape is also calculated to contrast it with the corresponding line shape in the absence of the metal

Surface-enhanced nonlinear-optical processes in molecules in a two-oscillator electromagnetic model

A two-oscillator electromagnetic model is used to find the effect of a solid or a metal substrate of dielectric function ε(ω) on several nonlinear optical processes in a molecule adsorbed on the surface. In particular, the case of the second-order optical mixing, the stimulated Raman scattering, the third-order polarizability for the four-wave mixing, and the case of two-photon absorption in the molecule have been considered explicitly in the approximation in which the ionic oscillator frequency is assumed to be small compared with both the electronic oscillator frequency and the optical frequencies involved. The two-oscillator model considered here, with a trilinear coupling potential function, is the same as the one recently used by us to investigate the spontaneous Raman process. From our analysis it is quite clear that the enhancement, if any, in each of the processes involves (1) the enhancement of each of the incident optical fields E→ in to E→ (0) at the molecular site, (2) the renormalization of the effective nonlinear polarizabilities at short molecular distances from the surface, and (3) the change of the outgoing radiation propagator (Green's function) from the free-space G0 to G, due to the presence of the surface. For a metal surface of a given shape, each of these factors may contain possible surface-plasmon-polariton resonances at various frequencies involved

Surface-enhanced second-harmonic generation at a metallic grating

The theory of surface-enhanced second-harmonic generation at a metallic grating is developed. Using the form of the nonlinear source polarization given by Bloembergen et al. [Phys. Rev. 174, 813 (1968)], we solve Maxwell's equations to obtain the fields at the second-harmonic frequency. The calculations are done up to second order in the surface-roughness parameter. These perturbation expressions are used to evaluate numerically the second-harmonic intensity, in various directions, produced by a plane wave incident on a metallic grating. The resonant enhancement in the second-harmonic intensity due to surface-plasmon excitation at fundamental frequency ω is discussed and the results compared with some recent experimental observations. The second-harmonic fields are also shown to get enhancement due to excitation of surface plasmons at 2ω; these, however, correspond to local-field enhancements at 2ω and are evanescent in nature

Power-constrained aware and latency-aware microarchitectural optimizations in many-core processors

As the transistor budgets outpace the power envelope (the power-wall issue), new architectural and microarchitectural techniques are needed to improve, or at least maintain, the power efficiency of next-generation processors. Run-time adaptation, including core, cache and DVFS adaptations, has recently emerged as a promising area to keep the pace for acceptable power efficiency. However, none of the adaptation techniques proposed so far is able to provide good results when we consider the stringent power budgets that will be common in the next decades, so new techniques that attack the problem from several fronts using different specialized mechanisms are necessary. The combination of different power management mechanisms, however, bring extra levels of complexity, since other factors such as workload behavior and run-time conditions must also be considered to properly allocate power among cores and threads. To address the power issue, this thesis first proposes Chrysso, an integrated and scalable model-driven power management that quickly selects the best combination of adaptation methods out of different core and uncore micro-architecture adaptations, per-core DVFS, or any combination thereof. Chrysso can quickly search the adaptation space by making performance/power projections to identify Pareto-optimal configurations, effectively pruning the search space. Chrysso achieves 1.9x better chip performance over core-level gating for multi-programmed workloads, and 1.5x higher performance for multi-threaded workloads. Most existing power management schemes use a centralized approach to regulate power dissipation. Unfortunately, the complexity and overhead of centralized power management increases significantly with core count rendering it in-viable at fine-grain time slices. The work leverages a two-tier hierarchical power manager. This solution is highly scalable with low overhead on a tiled many-core architecture with shared LLC and per-tile DVFS at fine-grain time slices. The global power is first distributed across tiles using GPM and then within a tile (in parallel across all tiles). Additionally, this work also proposes DVFS and cache-aware thread migration (DCTM) to ensure optimum per-tile co-scheduling of compatible threads at runtime over the two-tier hierarchical power manager. DCTM outperforms existing solutions by up to 12% on adaptive many-core tile processor. With the advancements in the core micro-architectural techniques and technology scaling, the performance gap between the computational component and memory component is increasing significantly (the memory-wall issue). To bridge this gap, the architecture community is pushing forward towards multi-core architecture with on-die near-memory DRAM cache memory (faster than conventional DRAM). Gigascale DRAM Caches poses a problem of how to efficiently manage the tags. The Tags-in-DRAM designs aims at efficiently co-locate tags with data, but it still suffer from high latency especially in multi-way associativity. The thesis finally proposes Tag Cache mechanism, an on-chip distributed tag caching mechanism with limited space and latency overhead to bypass the tag read operation in multi-way DRAM Caches, thereby reducing hit latency. Each Tag Cache, stored in L2, stores tag information of the most recently used DRAM Cache ways. The Tag Cache is able to exploit temporal locality of the DRAM Cache, thereby contributing to on average 46% of the DRAM Cache hits.A mesura que el consum dels transistors supera el nivell de potència desitjable es necessiten noves tècniques arquitectòniques i microarquitectòniques per millorar, o almenys mantenir, l'eficiència energètica dels processadors de les pròximes generacions. L'adaptació en temps d'execució, tant de nuclis com de les cachés, així com també adaptacions DVFS són idees que han sorgit recentment que fan preveure que sigui un àrea prometedora per mantenir un ritme d'eficiència energètica acceptable. Tanmateix, cap de les tècniques d'adaptació proposades fins ara és capaç d'oferir bons resultats si tenim en compte les restriccions estrictes de potència que seran comuns a les pròximes dècades. És convenient definir noves tècniques que ataquin el problema des de diversos fronts utilitzant diferents mecanismes especialitzats. La combinació de diferents mecanismes de gestió d'energia porta aparellada nivells addicionals de complexitat, ja que altres factors com ara el comportament de la càrrega de treball així com condicions específiques de temps d'execució també han de ser considerats per assignar adequadament la potència entre els nuclis del sistema computador. Per tractar el tema de la potència, aquesta tesi proposa en primer lloc Chrysso, una administració d'energia integrada i escalable que selecciona ràpidament la millor combinació entre diferents adaptacions microarquitectòniques. Chrysso pot buscar ràpidament l'adaptació adequada al fer projeccions òptimes de rendiment i potència basades en configuracions de Pareto, permetent així reduir de manera efectiva l'espai de cerca. Chrysso arriba a un rendiment de 1,9 sobre tècniques convencionals d'inhibició de portes amb una càrrega d'aplicacions seqüencials; i un rendiment de 1,5 quan les aplicacions corresponen a programes parla·lels. La majoria dels sistemes de gestió d'energia existents utilitzen un enfocament centralitzat per regular la dissipació d'energia. Malauradament, la complexitat i el temps d'administració s'incrementen significativament amb una gran quantitat de nuclis. En aquest treball es defineix un gestor jeràrquic de potència basat en dos nivells. Aquesta solució és altament escalable amb baix cost operatiu en una arquitectura de múltiples nuclis integrats en clústers, amb memòria caché de darrer nivell compartida a nivell de cluster, i DVFS establert en intervals de temps de gra fi a nivell de clúster. La potència global es distribueix en primer lloc a través dels clústers utilitzant GPM i després es distribueix dins un clúster (en paral·lel si es consideren tots els clústers). A més, aquest treball també proposa DVFS i migració de fils conscient de la memòria caché (DCTM) que garanteix una òptima distribució de tasques entre els nuclis. DCTM supera les solucions existents fins a un 12%. Amb els avenços en la tecnologia i les tècniques de micro-arquitectura de nuclis, la diferència de rendiment entre el component computacional i la memòria està augmentant significativament. Per omplir aquest buit, s'està avançant cap a arquitectures de múltiples nuclis amb memòries caché integrades basades en DRAM. Aquestes memòries caché DRAM a gran escala plantegen el problema de com gestionar de forma eficaç les etiquetes. Els dissenys de cachés amb dades i etiquetes juntes són un primer pas, però encara pateixen per tenir una alta latència, especialment en cachés amb un grau alt d'associativitat. En aquesta tesi es proposa l'estudi d'una tècnica anomenada Tag Cache, un mecanisme distribuït d'emmagatzematge d'etiquetes, que redueix la latència de les operacions de lectura d'etiquetes en les memòries caché DRAM. Cada Tag Cache, que resideix a L2, emmagatzema la informació de les vies que s'han accedit recentment de les memòries caché DRAM. D'aquesta manera es pot aprofitar la localitat temporal d'una caché DRAM, fet que contribueix en promig en un 46% dels encerts en les caché DRAM

Stimulated emission of x-rays from plasmas generated by short-pulse-laser-heating of solid targets

The problem of heating of a solid target to generate a nonequilibrium plasma by subnanosecond laser pulses is considered. For an appreciable absorption of energy from a Nd-glass laser, the critical density of the electrons in the plasma turns out to be 1021 cm-3. These electrons can be heated up to 107 K or more by using pulses of about 10 picosecond duration and absorbed energy flux of the order of 1021 erg cm-2 sec-1. Starting from neutral atoms in a solid with a high atomic number, e.g., Z=26, for times in the picosecond regime the relevant rate equations are solved analytically to predict densities of the atoms at different ionization levels. It is shown that during such a short time the population density of the ions isoelectronic to neon builds up to a very large amount. This in turn leads to the population inversion in the 4s → 3p soft x-ray laser transition, via the electron-impact excitation of the 4s level of the isoelectronic neon ion. For the effective pumping times of the order of 5 picoseconds, a gain of the order of 102 db cm-1 is predicted for the laser transition in Fe XVII, Co XVIII or Cu XX

Interface exciton modes and superconducting transition temperature of a metal in contact with a semiconductor

The problem of superconductivity in a metal-semiconductor system has been studied, using the dielectric formulation of superconductivity. The charge redistribution due to the quantum penetration of the metallic electrons to the semi-conductor side is approximated by a simple exponential function. The interface exciton modes are obtained within the framework of classical electrostatics, and their effect in modifying the effective electron-electron interaction near the interface is investigated. It is found that the strength of the excitonic term is small, and by itself, insufficient to lead to superconductivity. Nevertheless, it can alter the superconducting transition temperature of a metal, if it is already superconducting due to some other mechanism. This has been studied as a function of the various parameters entering in the problem

New self-consistent quasistatic approximation for screening and plasma dispersion in the electron gas

A new self-consistent quasistatic screening approach is proposed for studying the properties of an interacting electron gas. The compressibility divergence and the ferromagnetic instability found in the static unscreened Hartree-Fock approximation are nonexistent in this scheme. A better fit to the experimental data on the plasma dispersion relation than the existing calculations for free-electron metals is obtained