Ellipsometry with polarisation analysis at cryogenic temperatures inside a vacuum chamber.

In this paper we describe a new variant of null ellipsometry to determine thicknesses and optical properties of thin films on a substrate at cryogenic temperatures. In the PCSA arrangement of ellipsometry the polarizer and the compensator are placed before the substrate and the analyzer after it. Usually, in the null ellipsometry the polarizer and the analyzer are rotated to find the searched minimum in intensity. In our variant we rotate the polarizer and the compensator instead, both being placed in the incoming beam before the substrate. Therefore the polarisation analysis of the reflected beam can be realized by an analyzer at fixed orientation. We developed this method for investigations of thin cryogenic films inside a vacuum chamber where the analyzer and detector had to be placed inside the cold shield at a temperature of T ≈ 90 K close to the substrate. All other optical components were installed at the incoming beam line outside the vacuum chamber, including all components which need to be rotated during the measurements. Our null ellipsometry variant has been tested with condensed krypton films on a highly oriented pyrolytic graphite substrate (HOPG) at a temperature of T ≈ 25 K. We show that it is possible to determine the indices of refraction of condensed krypton and of the HOPG substrate as well as thickness of krypton films with reasonable accuracy.

Vibrational excitation of adsorbed molecules by photoelectrons of very low energy: acrylonitrile on Cu(100).

In this study, we report on a powerful method of primary photoelectron scattering by adsorbed species. Specifically, threshold-energy (E(kin,max) < 0.5 eV) two-photon photoelectrons (2PPE) are used to probe acrylonitrile (ACN) molecules chemisorbed onto a Cu(100) substrate, held at room temperature. This has proven to constitute a perfect tool to reveal the ACN vibrational modes in the chemisorbed state. From the dynamics of the directional (perpendicular to the copper surface) electron energy loss we conclude that only a few fundamental vibrational motions of adsorbed ACN are excited, namely the C=C, C≡N and C-H stretch modes. From the excitation probability spectra threshold energies, E(th), of these modes was extracted: E(th)(C=C) = 182(15) meV, E(th)(C≡N) = 248(16) meV--which are shifted noticeably from the equivalent gas phase values; and E(th)(C-H) ∼360-380 meV--which varies only marginally from the gas phase value. The interpretation of the excitation spectra suggests that the di-σ adsorption configuration of the terminal C- and N-atoms dominates, which agrees well with the orientation and bindings predicted in Density Functional Theory (DFT) calculations. Consistent with this is the observation that the contribution to the 2PPE excitation spectra from the C-H stretch motion is by far the largest, which are not directly affected by chemisorption bonding.

Antimatter plasmas in a multipole trap for antihydrogen.

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.

Search for laser-induced formation of antihydrogen atoms.

Antihydrogen can be synthesized by mixing antiprotons and positrons in a Penning trap environment. Here an experiment to stimulate the formation of antihydrogen in the n = 11 quantum state by the introduction of light from a CO2 continuous wave laser is described. An overall upper limit of 0.8% with 90% C.L. on the laser-induced enhancement of the recombination has been found. This result strongly suggests that radiative recombination contributes negligibly to the antihydrogen formed in the experimental conditions used by the ATHENA Collaboration.

Nearly quantum-noise-limited timing jitter from miniature Er:Yb:glass lasers.

We report on nearly quantum-limited timing-jitter performance of two passively mode-locked Er:Yb:glass lasers with a repetition rate of 10 GHz. The relative timing jitter of both lasers was measured to be 190 fs (100 Hz-1.56 MHz) root mean square. The remaining cavity-length fluctuations are below 7.5 pm in the 6 Hz-8 kHz frequency range, indicating the stability of a rugged miniature cavity setup. By actively controlling the cavity length we reduced the timing jitter to 26 fs (6 Hz-1.56 MHz). We also discuss the influence of cavity length on the practically achievable timing jitter.

Carrier-envelope offset dynamics of mode-locked lasers.

We investigate coupling mechanisms between the amplitude and the carrier-envelope offset phase in mode-locked lasers. We find that nonlinear beam steering in combination with the intracavity prism compressor is the predominant mechanism that causes amplitude-to-phase conversion in our laser. A second mechanism, induced by self-steepening, is also identified. These mechanisms are important for stabilizing the carrier-envelope offset phase and also explain the extremely low pulse-to-pulse energy fluctuations observed in some lasers with carrier-envelope lock. The coupling mechanisms described have important implications for applications of few-cycle optical pulses.

Absolute frequency measurement of the 435.5-nm (171)Yb+-clock transition with a Kerr-lens mode-locked femtosecond laser.

We have measured the frequency of the 6s(2)S(1/2)(2)-5d D(3/2)(2) electric-quadrupole transition of (171)(Yb) (+) with a relative uncertainty of 1x10(-14) , nu(Yb)=688358 979309312Hz +/-6Hz . We used a femtosecond frequency comb generator to phase-coherently link the optical frequency derived from a single trapped ion to a cesium-fountain-controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on a (171)Yb(+)-ion standard.

Clinical application of laser-induced breakdown spectroscopy to the analysis of teeth and dental materials.

OBJECTIVES: The luminous plasma generated during laser ablation of dental tissue and dental materials has been analyzed to determine qualitative and quantitative elemental composition. BACKGROUND DATA: The use of pulsed lasers for controlled material ablation now is frequently suggested as an alternative to mechanical drilling for the removal of caries and in tooth modification. Spectral analysis of the ablated plasma can be exploited to monitor precisely the laser drilling process in vivo and in real time. METHODS: Teeth samples and dental materials were ablated using pulses from a Nd:YAG laser. The line positions and intensities in the spectra, recorded in real time, were used to identify elements and to determine their relative concentrations. RESULTS: From the spectra of horizontally and vertically cut tooth slices, profiles of elemental distribution were determined; these were used in a range of monitoring applications. We showed that the transition from caries to healthy tooth material could be identified through the decrease in calcium (Ca) and phosphorus (P) concentrations, whereas nonmineralizing elements and organic materials increased in concentration. We also could relate the spatial distribution of elements to their migration or accumulation over time, for example, the migration of aluminium (Al) from dental restorative materials to the tooth matrix. CONCLUSIONS: The plasma existing during laser ablation (in vitro/in vivo) can be analyzed spectrally in real time. From the spectra, one can pinpoint high/low levels of element concentrations within the tooth matrix. Thus, this analysis could be used to monitor the ablation of material during laser dental treatment.

Amplitude-modulation-free optoelectronic frequency control of laser diodes.

A novel method is described for fast frequency modulation or frequency control of diode lasers that avoids problems associated with bias current modulation, namely, amplitude modulation and thermal phase delays. The method is based on amplitude-modulated, noninterfering control light with a wavelength near the transparency region of the laser diode, which specifically modifies the spectral gain profile to yield a constant gain but a controllable refractive index at the lasing wavelength. This permits amplitude-modulation-free frequency modulation at modulation frequencies up to the relaxation oscillation frequency. A phase lock between the emissions of two extended-cavity diode lasers that could not be achieved with bias current modulation was achieved by this method.

Diode laser frequency-noise suppression by >50dB by use of electro-optic parametric master oscillators.

We demonstrate a novel frequency-stabilization scheme for laser diodes that is capable of linewidth reduction by more than 5 orders of magnitude. In this master-slave scheme the diode laser emission is frequency-offset phase locked to the emission of an electro-optic parametric oscillator (EOPO), and the laser diode is simultaneously used as the EOPO pump source. As a result the initial frequency fluctuations of the pump are reduced to the intrinsic noise level of the EOPO, which can be extremely small. We demonstrate that subhertz linewidths of the beat notes of the signals of two of these systems can be readily achieved if acoustic perturbations are suppressed.

Intensity-induced mode shift in a femtosecond laser by a change in the nonlinear index of refraction.

The modes of the comb spectrum of a Kerr-lens mode-locked laser are frequency shifted versus the corresponding cw modes of the cavity by an intensity-induced change in the index of refraction in the Kerr medium. We demonstrate this effect and discuss novel schemes for fast frequency control of the comb spectrum.

Short-pulse properties of optical frequency comb generators.

An optical frequency comb generator, based on a simple electro-optic modulator in an optical resonator, can produce high-repetition-rate picosecond pulses. Unlike conventional picosecond lasers, the properties of these pulses are greatly affected by detuning the optical cavity and by dispersion caused by the electro-optic crystal. Picosecond pulses were studied in a physical device by numerical simulation and intensity autocorrelation measurements. The pulse width and pulse-to-pulse spacing were greatly affected by detuning the input laser frequency and the resonance of the optical resonator, and the numerical simulations showed that dispersion causes temporal ripples that are antisymmetric between pulse pairs.

Granulomatous mycosis fungoides.

We report a new observation of granulomatous mycosis fungoides. The diagnosis was able to be made only after performing multiple biopsies during the course of the disease. Initial evolution was rapidly favourable with electrontherapy. A granulomatous reaction is, except in Hodgkin's disease, a rare phenomenon in lymphoproliferative disorders, particularly in cutaneous T cell lymphoma. This variant of mycosis fungoides raises the problem of the histological differentiation from other granulomatous dermatoses, mainly sarcoidosis. Its prognostic significance is disputed and its pathogenesis remains unknown.

Generation of coherent optical radiation by electronic means: the electro-optical parametric oscillator.

We describe a novel external light control element that is capable of stabilizing both the amplitude and the frequency of an optical input field. A demanding application of this electro-optical parametric oscillator, conversion of incoherent inputs into coherent, phase-trackable outputs with small residual intensity modulation, is reported. Estimations show that commercially available light-emitting diodes have the potential to act as pump sources, although the incoherent optical input field used in these initial experiments was prepared by filtering of the coherent emission of a laser diode.

A comparison of the techniques of secondary ion mass spectrometry and resonance ionization mass spectrometry for the analysis of potentially toxic element accumulation in neural tissue.

A comparison is made of the techniques of secondary ion mass spectrometry (SIMS) and resonance ionization mass spectrometry (RIMS) for the detection of the neuro-toxic element aluminium in cortical tissue. Experiments were performed using a reflectron-type time-of-flight mass spectrometer (TOFMS) in conjunction with an Ar+ source for target sputtering and a pulsed tuneable dye laser system for resonance ionization. It is shown how isobaric interference of species such as CNH and C2H3 in the case of aluminium greatly affect the quantitative accuracy and the detection limit of aluminium in biological samples when analysed using SIMS. In contrast the use of RIMS virtually eliminates this problem, so allowing easier quantification and much lower detection limits to be achieved. Detection limits of approximately 3 ppm for aluminium in brain tissue homogenates were achieved using RIMS, with a spatial resolution of less than 100 microns.

Remote in situ analytical spectroscopy and its applications in the nuclear industry.

An experimental system based on Laser Induced Breakdown Spectroscopy (LIBS) has been used to analyze various ferrous samples. A fibre optic system has been used to transmit the incident laser pulse which produces the plasma plume at the surface of the analyte and to transmit back to a spectrometer the optical radiation emitted by the plasma. The measuring system may therefore be placed remote from the analyte which may be situated in a hostile environment such as an operating nuclear reactor. Results show that the system is capable of detecting chromium, nickel, manganese, molybdenum, silicon and vanadium at concentrations smaller than 5x10(-4) g/g