Quasi-phase-matching of high-order-harmonic generation using multimode polarization beating

The generalization of quasi-phase-matching using polarization beating and of multimode quasi-phase-matching (MMQPM) for the generation of high-order harmonics is explored, and a method for achieving polarization beating is proposed. If two (and in principle more) modes of a waveguide are excited, modulation of the intensity, phase, and/or polarization of the guided radiation will be achieved. By appropriately matching the period of this modulation to the coherence length, quasi-phase-matching of high-order-harmonic radiation generated by the guided wave can occur. We show that it is possible to achieve efficiencies with multimode quasi-phase-matching greater than the ideal square wave modulation. We present a Fourier treatment of QPM and use this to show that phase modulation, rather than amplitude modulation, plays the dominant role in the case of MMQPM. The experimental parameters and optimal conditions for this scheme are explored

One Hundred Years of the Cosmological Constant: from 'Superfluous Stunt' to Dark Energy

We present a centennial review of the history of the term known as the cosmological constant. First introduced to the general theory of relativity by Einstein in 1917 in order to describe a universe that was assumed to be static, the term fell from favour in the wake of the discovery of the expanding universe, only to make a dramatic return in recent times. We consider historical and philosophical aspects of the cosmological constant over four main epochs: (i) the use of the term in static cosmologies (both Newtonian and relativistic); (ii) the marginalization of the term following the discovery of cosmic expansion; (iii) the use of the term to address specific cosmic puzzles such as the timespan of expansion, the formation of galaxies and the redshifts of the quasars; (iv) the re-emergence of the term in today's Lamda-CDM cosmology. We find that the cosmological constant was never truly banished from theoretical models of the universe, but was sidelined by astronomers for reasons of convenience. We also find that the return of the term to the forefront of modern cosmology did not occur as an abrupt paradigm shift due to one particular set of observations, but as the result of a number of empirical advances such as the measurement of present cosmic expansion using the Hubble Space Telescope, the measurement of past expansion using type SN 1a supernovae as standard candles, and the measurement of perturbations in the cosmic microwave background by balloon and satellite. We give a brief overview of contemporary interpretations of the physics underlying the cosmic constant and conclude with a synopsis of the famous cosmological constant problem.Comment: 60 pages, 6 figures. Some corrections, additions and extra references. Accepted for publication the European Physical Journal (H

Complete spatial characterization of an optical wavefront using a variable-separation pinhole pair

We present a technique for measuring the transverse spatial properties of an optical wavefront. Intensity and phase profiles are recovered by analysis of a series of interference patterns produced by the combination of a scanning X-shaped slit and a static horizontal slit; the spatial coherence may be found from the same data. We demonstrate the technique by characterizing high harmonic radiation generated in a gas cell, however the method could be extended to a wide variety of light sources.Comment: 4 pages, 3 figures, 1 tabl

Einstein's cosmology review of 1933: a new perspective on the Einstein-de Sitter model of the cosmos

We present a first English translation and analysis of a little-known review of relativistic cosmology written by Albert Einstein in late 1932. The article, which was published in 1933 in a book of Einstein papers translated into French, contains a substantial review of static and dynamic relativistic models of the cosmos, culminating in a discussion of the Einstein-de Sitter model. The article offers a valuable contemporaneous insight into Einstein's cosmology in the 1930s and confirms that his interest lay in the development of the simplest model of the cosmos that could account for observation, rather than an exploration of all possible cosmic models. The article also confirms that Einstein did not believe that simplistic relativistic models could give an accurate description of the early universe.Comment: Accepted for publication in the European Physical Journal (H). Includes an English translation of a little-known review of cosmology written by Albert Einstein in 1933. 20 pages, 4 figure

Historical and philosophical reflections on the Einstein-de Sitter model

We present some historical and philosophical reflections on the paper "On the Relation Between the Expansion and the Mean Density of the Universe", published by Albert Einstein and Willem de Sitter in 1932. In this famous work, Einstein and de Sitter considered a relativistic model of the expanding universe with both the cosmological constant and the curvature of space set to zero. Although the Einstein-deSitter model went on to serve as a standard model in 'big bang' cosmology for many years, we note that the authors do not explicitly consider the evolution of the cosmos in the paper. Indeed, the mathematics of the article are quite puzzling to modern eyes. We consider claims that the paper was neither original nor important; we find that, by providing the first specific analysis of the case of a dynamic cosmology without a cosmological constant or spatial curvature, the authors delivered a unique, simple model with a straightforward relation between cosmic expansion and the mean density of matter that set an important benchmark for both theorists and observers. We consider some philosophical aspects of the model and provide a brief review of its use as a prototype 'big bang' model over much of the 20th century.Comment: Some revisions and corrections to original MS. Accepted for publication in the European Physical Journal (H

A Novel Smart Device Student Response System For Supporting High Quality Active Learning In The Engineering And Science Disciplines

This paper proposes and presents a unique smart device student response system (SDSRS) that allows for a more flexible input than existing classroom response systems, such as clickers. This, in turn, allows students to respond to the lecturer with higher quality and more relevant information and, thus, improves their active learning. This is of particular relevance in the science and engineering disciplines where methodology is as important, if not more so, than the final answer. The SDSRS allows the lecturer to quickly obtain this pertinent information in real-time within the classroom environment. The proposed system consists of three main elements, namely a student sketch application, a lecturer view-and-edit application and a central cloud-based service to co-ordinate the exchange of information between the two applications. The system was evaluated within two different engineering classrooms, with very positive feedback obtained from both the lecturers and students involved. Details of the evaluation process, and the feedback obtained, are presented within

A Novel Smart Device Student Response System For Supporting High Quality Active Learning In The Engineering And Science Disciplines

This paper proposes and presents a unique smart device student response system (SDSRS) that allows for a more flexible input than existing classroom response systems, such as clickers. This, in turn, allows students to respond to the lecturer with higher quality and more relevant information and, thus, improves their active learning. This is of particular relevance in the science and engineering disciplines where methodology is as important, if not more so, than the final answer. The SDSRS allows the lecturer to quickly obtain this pertinent information in real-time within the classroom environment. The proposed system consists of three main elements, namely a student sketch application, a lecturer view-and-edit application and a central cloud-based service to co-ordinate the exchange of information between the two applications. The system was evaluated within two different engineering classrooms, with very positive feedback obtained from both the lecturers and students involved. Details of the evaluation process, and the feedback obtained, are presented within

Combined visible and near-infrared OPA for wavelength scaling experiments in strong-field physics

We report the operation of an optical parametric amplifier (OPA) capable of producing gigawatt peak-power laser pulses with tunable wavelength in either the visible or near-infrared spectrum. The OPA has two distinct operation modes (i) generation of >350 uJ, sub 100 fs pulses, tunable between 1250 - 1550 nm; (ii) generation of >190 uJ, sub 150 fs pulses tunable between 490 - 530 nm. We have recorded high-order harmonic spectra over a wide range of driving wavelengths. This flexible source of femtosecond pulses presents a useful tool for exploring the wavelength-dependence of strong-field phenomena, in both the multi-photon and tunnel ionization regimes.Comment: 14 pages, 9 figures, This paper was published in Proceedings of SPIE 10088, Nonlinear Frequency Generation and Conversion: Materials and Devices XVI, doi 10.1117/12.225077

Quasi-phase-matching of high-order-harmonic generation using polarization beating in optical waveguides

A scheme for quasi-phase-matching high-harmonic generation is proposed in which polarization beating within a hollow core birefringent waveguide modulates the generation of harmonics. The evolution of the polarization of a laser pulse propagating in a birefringent waveguide is calculated and is shown to periodically modulate the harmonic generation process. The optimum conditions for achieving quasi-phase-matching using this scheme are explored and the growth of the harmonic intensity as a function of experimental parameters is investigated