Dynamics of phase transformations in Si and Ge upon strong excitation with UV femtosecond laser pulses

Femtosecond laser processing of semiconductors has evolved into a mature, high-precision fabrication technique, enabling a wide range of applications. While initially most studies have employed pulses at near infrared wavelengths, the interest in using UV laser pulses is constantly increasing due to the different excitation conditions as a consequence of the much shorter optical penetration depth, leading to an improved resolution. In this context, fundamental studies on the temporal dynamics of phase transformations triggered by such pulses are necessary in order to comprehend and eventually control the complex phase transformation pathways. Here, we report a detailed time-resolved study on the phase transformation dynamics of crystalline silicon and germanium upon irradiation with single 400 nm, 100 fs laser pulses in the moderate and high excitation regime. To this end, we have employed fs-resolved optical microscopy with a probe wavelength of 800 nm to study the reflectivity evolution of the irradiated surface over a temporal window ranging from 100 fs up to 20 ns. At moderate excitation fluence, the data reveals the entire sequence of laser-induced processes, starting from the generation of a free-electron plasma, non-thermal melting, ablation onset and expansion of a semi-transparent ablation layer with sharp interfaces. At excitation with peak fluences more than 30 times the ablation threshold, an anomalous transient high-reflectivity state is observed, which might be indicative of a recoil pressure-induced liquid–liquid phase transition. Moreover, 70 nm-thick amorphous surface layers are formed in both materials after irradiation at moderate fluences. Overall, our results provide relevant information on both, transformation dynamics and final state of both materials for fs-pulse excitation in the near-UV wavelength range.This work is part of the projects TEC2008-001183, PID2020-112770RB-C21 and TED2021-130894B-C22 and has been financed by MCIN/AEI/10.13039/501100011033 (Ministry of Science and Innovation/State Agency of Investigation) and European Union NextGenerationEU/PRTR, as well as by the “Generalitat Valenciana” (project PROMETEO/2021/006)

Slow interfacial reamorphization of Ge films melted by ps laser pulses

Melting and rapid solidification is induced in 50-nm-thick amorphous Ge films on glass substrates by single laser pulses at 583 nm with a duration of 10 ps. The solidification process is followed by means of reflectivity measurements with ns time resolution both at the air/film (front) and the substrate/film (back) interfaces. Due to interference effects between the light reflected at the film-substrate and film-liquid interfaces, the back side reflectivity measurements turn out to be very sensitive to the melt depth induced by the laser pulse and their comparison to optical simulations enables the determination of the solidification dynamics. For low fluences, only a thin layer of the film is melted and solidification occurs interfacially leading to reamorphization of the molten material. The results provide a critical interface velocity for amorphization of ∼4 m/s, much slower than the one that has widely been reported for elementary semiconductors. For high fluences, the molten layer depth approaches the film thickness and the results are consistent with a bulk solidification process. In this case, recalescence effects upon solid phase nucleation become important and lead to the formation of crystallites distributed throughout the whole resolidified volume. © 1998 American Institute of Physics.Jan Siegel acknowledges the funding of the European Community through a grant (ERB40001GT954352) within the Training and Mobility of Researchers Program. This work has been partially supported by CICYT (Spain) under project TIC93-0125.Peer Reviewe

Gso Projection, BRST Cohomology and Picture-Changing in N=2 String Theory

We investigate in detail the critical $N{=}2$ fermionic string with and without a global ${\bf Z}_2$ twist. An analysis of BRST cohomology shows that twisted sectors contain massless `spacetime' fermions which are {\it non-local\/} with respect to the standard massless boson. However, two distinct GSO projections exist, one (untwisted) retaining merely the usual boson and its spectral-flow partner, the other (twisted) yielding two fermions and one boson, on the massless level. The corresponding chiral BRST-invariant vertex operators are constructed in certain pictures, and their fusion and picture-changing are investigated, including the construction of inverse picture-changing operators. The $N{=}2$ `spacetime supersymmetry' generators are {\it null\/} operators, since the twisted massless states fail to interact. The untwisted three- and four-point functions are recalculated at tree-level.Comment: 42 pages, LaTeX, no figures, 115 kb, ITP-UH-05/94, DESY 94-104 (extensive revision and expansion of earlier version

Mathematic Modelling of Reverse Osmosis System Design for Detection of Estrogens in Water

Estrogenní sloučeniny jsou podskupinou environmentálních polutantů s názvem endokrinní disruptory. Jedná se o širokou skupinu látek, schopných narušovat hormonální rovnováhu organismu a ovlivňovat jeho reprodukční, vývojové a behaviorální funkce. V životním prostředí se vyskytují ve velmi malých koncentracích (ng/l). To činí jejich detekci a odstranění značně problematické. Experimentální zařízení vyvíjené v Centru pro výzkum toxických látek v prostředí (RECETOX) využívá principu reverzní osmózy (RO) pro zakoncentrování stopových množství mikropolutantů (např. estrogenů) s cílem snížení detekčního limitu analytických metod. RO se zdá být nadějnou alternativou k doposud běžně používaným metodám, které jsou časově i finančně náročné. Cílem diplomové práce bylo vytvořit matematický model RO, který by popisoval průběh zakoncentrování estrogenů a mohl tak přispět k optimalizaci experimentálního zařízení. Matematický model byl vytvořen v prostředí MATLAB-simulink a jeho funkčnost byla ověřena srovnáním s třinácti experimentálními výsledky. Testovacími vzorky byly pitná voda, roztoky NaCl o koncentracích 0,002–2 g/l a vodný roztok 17-ethinylestradiolu o koncentraci 25 ng/l. Při srovnání výsledků zakoncentrování roztoků NaCl na deskové RO-membráně byla odchylka matematického modelu a experimentálch dat 0,9–4,9 %. Analogické srovnání modelovaného a experimentálního zakoncentrování estrogenu ukázalo odchylku 3,6 %. Srovnání modelovaného a experimentálního zakoncentrováním roztoků NaCl na vinuté RO-membráně ukázalo odchylku 5,9–8,7 %. Doposud byla při výpočtu výtěžnosti zakoncentrování používána jako teoretická (referenční) hodnota poměr objemu retentátu na začátku a na konci procesu. Z výsledků diskutovaných v diplomové práci se však tento postup jeví jako nepřesný, a přesnějšího výpočtu výtěžnosti by bylo dosaženo za použití teoretické hodnoty zakoncentrování získané navrženým matematickým modelem. Pro zvýšení spolehlivosti výsledků však bude třeba provést experimentání zakoncentrování dalších estrogenů, a také zjistit příčiny odchylek modelovaných a experimentálních hodnot objemů retentátu na konci zakoncetrování.Estrogen compounds are a subgroup of the environmental pollutants named Endocrine Disrupting Chemicals. It is a large group of chemicals which are capable of causing hormonal imbalance of an organism and affecting its reproductive, developmental or behavioral functions. In the environment, they are found in very low concentrations (below ng/l). This makes their detection and elimination highly problematic. The Research Centre for Toxic Compounds in the Environment (RECETOX) has been developing an experimental device which employes reverse osmosis (RO) to concentrate the trace amounts of micropollutants (e.g. estrogens). The aim is to lower the detection limit of subsequent analytical methods. RO seems to be a promising alternative to commonly used methods which are more time consuming and costly. The aim of this thesis is to create a mathematical model of RO, which would describe the process of estrogen concentration and therefore could contribute to the optimization of experimental device. The mathematical model was created in MATLAB-simulink and verified by comparison to thirteen experimental results. The solutions used to test the model were NaCl at 0.002–2 g/l and drinking water with 17-ethinylestradiol of 25 ng/l. In order to concentrate the retentate on a flat-sheet RO membrane the model achieves a mean relative error in the range of 0.9–4.9 % for NaCl and drinking water. For the concentration of estrogens the deflection is 3.6 %. Comparing the model and experimental concentration of NaCl on a spiral wound RO-membrane, the deviation is in the range of 5.9–8.7 %. So far, ratio of retentate volume from the beginning to the end of the process has been used to determine the process recovery. However, due to the results obtained in the thesis, this approach appears inaccurate. A more accurate recovery would likely be achieved by using the theoretical concentration value obtained by the proposed mathematical model. However, to increase reliability, the further measurements with estrogens are needed, and also to determine the causes of deflection of model and experimental retentate volume values at the end of the concentration

Self-Dual Supergravity from N=2 Strings

A new heterotic N=2 string with manifest target space supersymmetry is constructed by combining a conventional N=2 string in the right-moving sector and a Green-Schwarz-Berkovits type string in the left-moving sector. The corresponding sigma model is then obtained by turning on background fields for the massless excitations. We compute the beta functions and we partially check the OPE's of the superconformal algebra perturbatively in $\alpha'$, all in superspace. The resulting field equations describe N=1 self-dual supergravity.Comment: 32 pages, Latex, discussion in pages 10, 11 revised so that it is compatible with the complex structure chosen in Appendix A. Appendix A slightly expanded. Final versio

Pressure-induced transient structural change of liquid germanium induced by high-energy picosecond laser pulses

The temporal evolution of the reflectivity of germanium at 514 nm upon irradiation with single high-energy picosecond laser pulses has been measured using a streak camera. It is found that, for a well-defined high fluence range, the reflectivity of the laser-induced molten phase attains a value of 0.85, considerably above the value reported for liquid Ge in thermal equilibrium (0.75). This behavior is consistent with a strong densification of the liquid phase remaining after the explosive vaporization of a thin surface layer. Within the specified fluence interval, this anomalously high reflectivity state is independent of the fluence and lasts tens of nanoseconds. Both characteristics point to the presence of a pressure-induced transient structural change in liquid germanium. © 2005 American Institute of Physics.Peer Reviewe

Covariant Computation of the Low Energy Effective Action of the Heterotic Superstring

We derive the low energy effective action of the heterotic superstring in superspace. This is achieved by coupling the covariantly quantized Green-Schwarz superstring of Berkovits to a curved background and requiring that the sigma model has superconformal invariance at tree level and at one loop in \a'. Tree level superconformal invariance yields the complete supergravity algebra, and one-loop superconformal invariance the equations of motion of the low energy theory. The resulting low energy theory is old-minimal supergravity coupled to a tensor multiplet. The dilaton is part of the compensator multiplet.Comment: 59 pages, LaTeX, with two figures (needs epsfig

Model Checking Race-freedom When "Sequential Consistency for Data-race-free Programs" is Guaranteed

Many parallel programming models guarantee that if all sequentially consistent (SC) executions of a program are free of data races, then all executions of the program will appear to be sequentially consistent. This greatly simplifies reasoning about the program, but leaves open the question of how to verify that all SC executions are race-free. In this paper, we show that with a few simple modifications, model checking can be an effective tool for verifying race-freedom. We explore this technique on a suite of C programs parallelized with OpenMP