A compact streak camera for 150 fs time resolved measurement of bright pulses in ultrafast electron diffraction

We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction UED experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations

Size Doesn't Matter: Towards a More Inclusive Philosophy of Biology

notes: As the primary author, O’Malley drafted the paper, and gathered and analysed data (scientific papers and talks). Conceptual analysis was conducted by both authors.publication-status: Publishedtypes: ArticlePhilosophers of biology, along with everyone else, generally perceive life to fall into two broad categories, the microbes and macrobes, and then pay most of their attention to the latter. ‘Macrobe’ is the word we propose for larger life forms, and we use it as part of an argument for microbial equality. We suggest that taking more notice of microbes – the dominant life form on the planet, both now and throughout evolutionary history – will transform some of the philosophy of biology’s standard ideas on ontology, evolution, taxonomy and biodiversity. We set out a number of recent developments in microbiology – including biofilm formation, chemotaxis, quorum sensing and gene transfer – that highlight microbial capacities for cooperation and communication and break down conventional thinking that microbes are solely or primarily single-celled organisms. These insights also bring new perspectives to the levels of selection debate, as well as to discussions of the evolution and nature of multicellularity, and to neo-Darwinian understandings of evolutionary mechanisms. We show how these revisions lead to further complications for microbial classification and the philosophies of systematics and biodiversity. Incorporating microbial insights into the philosophy of biology will challenge many of its assumptions, but also give greater scope and depth to its investigations

Comparison of infrared laser beam shaping by diffractive and refractive methods

Infra-red laser beam shaping has the inherent difficulty that simple ray tracing methods often yield anomalous results, due primarily to the propagation effects at longer wavelengths. Techniques based on diffraction theory have been developed to overcome this, with associated parameters to determine when one approach is needed versus another. In this paper, infra-red (IR) beam shaping by diffractive methods is investigated and compared to refractive methods. Theoretical results on the beam shapers are calculated through a combination of analytical and numerical techniques, and using both ideal and non-ideal inputs. We show that the diffractive optical element (DOE) is remarkably resilient to input errors of wavelength and beam quality, while the refractive shaper is found to be difficult to model. Optical elements based on the two approaches were designed, and then fabricated from ZnSe. A comparison between the fabricated elements and the designed elements is presented, and some of the findings on practical problems in having such elements fabricated are highlighted.Conference Pape

Infrared, visible and ultraviolet broadband coherent supercontinuum generation in all-normal dispersion fibers

We numerically investigate supercontinuum (SC) generation in fibers with all-normal group velocity dispersion (GVD) under femtosecond pumping, including photonic crystal fibers (PCF), optical nanofibers and suspended core PCF. It is shown that all-normal dispersion (ANDi) fibers are ideally suited to generate extremely flat and more than octave spanning SC spectra which are highly coherent over the entire bandwidth. Due to the suppression of soliton fission in the normal GVD regime, the SC spectra are mainly generated by self-phase modulation and optical wave breaking dynamics, resulting in smooth spectral profiles without significant fine structure. A single pulse is maintained in the time domain, which can be externally compressed to the few-cycle regime. We present specific design examples of ANDi PCF for pumping at 1080 nm and extend the concept to optical nanofibers for deep ultraviolet (UV) SC generation at 400 nm pump wavelength as well as tapered suspended core PCF for visible and near UV SC generation at 465 nm and 530 nm pump wavelength. First "proof of principle" experiments confirm the basic findings of the numerical simulations and show the feasibility of the proposed SC generation scheme. © 2010 SPIE.Conference Pape

Templated polar order of a guest in a quasiracemic organic host

A quasiracemic mixture of Dianin's compound and its thiol derivative enforces additional anisotropy of the guest-accessible space, thus facilitating a net polar arrangement of guest molecules; guest alignment is rationalized in terms of van der Waals volume considerations. © 2010 The Royal Society of Chemistry.Articl

Development and use of a turbidity analyzer for studying the solution crystallization of polyolefins

Cooling a solution of a crystalline polyolefin from 140°C to room temperature causes the dissolved polymer to crystallize. If a laser beam passes through this solution, the crystallization will cause the beam to scatter, which thereby decreases the intensity of the beam. With this principle, it is possible to follow the crystallization of polyolefins under controlled cooling. An instrument capable of doing these analyses was manufactured, and several different polyolefins were analyzed. The effect of the experimental parameters are illustrated for both cooling and reheating experiments. In addition, an interesting dependence on molecular weight was also observed for a series of metallocene polypropylenes. © 2008 Wiley Periodicals, Inc.Articl

Second harmonic generation as a technique to probe buried interfaces

Since the advances of femtosecond laser technology during the last decade, optical second harmonic generation (SHG) has proven itself a powerful tool to investigate the electronic and structural properties of semiconductor materials. Its advantage lies in the fact that it is a contact-less, non-intrusive method that can be used in situ. It is sensitive to systems with broken symmetry, in particular interfaces and surfaces. The Si/SiO2 system is technologically important since it forms a component of most modern electronic equipment. Furthermore, it has been shown that it is possible to induce an electric field across this interface by means of laser irradiation as a result of defect formation and defect population. This electric field can be measured since it determines the SHG signal. The anisotropy of the SHG signal from the Si/SiO2 interface was measured and showed four-fold symmetry, illustrating that the SHG technique was able to characterise the electrical properties of the interface below the 5 nm thick oxide layer

Second harmonic generation as a technique to probe buried interfaces

Since the advances of femtosecond laser technology during the last decade, optical second harmonic generation (SHG) has proven itself a powerful tool to investigate the electronic and structural properties of semiconductor materials. Its advantage lies in the fact that it is a contact-less, non-intrusive method that can be used in situ. It is sensitive to systems with broken symmetry, in particular interfaces and surfaces. The Si/SiO2 system is technologically important since it forms a component of most modern electronic equipment. Furthermore, it has been shown that it is possible to induce an electric field across this interface by means of laser irradiation as a result of defect formation and defect population. This electric field can be measured since it determines the SHG signal. The anisotropy of the SHG signal from the Si/SiO2 interface was measured and showed four-fold symmetry, illustrating that the SHG technique was able to characterise the electrical properties of the interface below the 5 nm thick oxide layer

Optimal control of the population dynamics of the ground vibrational state of a polyatomic molecule

Simulating coherent control with femtosecond pulses on a polyatomic molecule with anharmonic splitting was demonstrated. The simulation mimicked pulse shaping of a Spatial Light Modulator (SLM) and the interaction was described with the Von Neumann equation. A transform limited pulse with a fluence of 600 J/m2 produced 18% of the population in an arbitrarily chosen upper vibrational state, n =2. Phase only and amplitude only shaped pulse produced optimum values of 60% and 40% respectively, of the population in the vibrational state, n=2, after interaction with the ultra short pulse. The combination of phase and amplitude shaping produced the best results, 80% of the population was in the targeted vibrational state, n=2, after interaction. These simulations were carried out with all the population initially in the ground vibrational level. It was found that even at room temperatures (300 Kelvin) that the population in the selected level is comparable with the case where all population is initially in the ground vibrational state. With a 10% noise added to the amplitude and phase masks, selective excitation of the targeted vibrational state is still possible. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).Conference Pape