Order-dependent structure of High Harmonic Wavefronts

The physics of high harmonics has led to the generation of attosecond pulses and to trains of attosecond pulses. Measurements that confirm the pulse duration are all performed in the far field. All pulse duration measurements tacitly assume that both the beam's wavefront and intensity profile are independent of frequency. However, if one or both are frequency dependent, then the retrieved pulse duration depends on the location where the measurement is made. We measure that each harmonic is very close to a Gaussian, but we also find that both the intensity profile and the beam wavefront depend significantly on the harmonic order. Thus, our findings mean that the pulse duration will depend on where the pulse is observed. Measurement of spectrally resolved wavefronts along with temporal characterization at one single point in the beam would enable complete space-time reconstruction of attosecond pulses. Future attosecond science experiments need not be restricted to spatially averaged observables

Gaussian-Schell analysis of the transverse spatial properties of high-harmonic beams

High harmonic generation (HHG) is a compact source of coherent, ultrafast soft x-ray radiation. HHG is increasingly being used as a source to image biological and physical systems. However, many imaging techniques such as coherent diffractive imaging, and ptychography require coherent illumination. Characterization the spatial coherence of HHG sources is vital if these sources are to kind widespread applications. Here a new method for characterizing coherent radiation is used to investigate the near- and far- field spatial properties of high harmonic radiation generated in a gas cell. The intensity distribution, wavefront curvature, and complex coherence factor are measured for a range of harmonic orders, and the Gaussian-Schell model is used to determine the properties of the harmonic beam in the plane of generation. Our results show the measured spatial properties of the harmonic beam are consistent with the finite spatial coherence of the driving laser beam as well as variations of the atomic dipole phase. These findings are used to suggest new approaches for controlling and optimizing the spatial properties of light for imaging applications

Oriented rotational wave-packet dynamics studies via high harmonic generation

We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wavepacket is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation

Scaling and root planing with and without periodontal flap surgery

. Complete removal of calculus is a primary part of achieving a “biologically acceptable” tooth surface in the treatment of periodontitis. Rabbani et al. reported that a single episode of scaling did not completely remove subgingival calculus and that the deeper the periodontal pocket, the less complete the calculus removal. The purpose of the present study was to evaluate the effectiveness of scaling relative to calculus removal following reflection of a periodontal flap. Each of 21 patients who required multiple extractions had 2 teeth scaled, 2 teeth scaled following the reflection of a periodontal flap, and 2 teeth serve as controls. Local anesthesia was used. Following extraction, the % of subgingival tooth surfaces free of calculus was determined using the method described by Rabbani with a stereomicroscope. Results showed that while scaling only (SO) and scaling with a flap (SF) increased the % of root surface without calculus, scaling following the reflection of a flap aided calculus removal in pockets 4 mm and deeper. Comparison of SO versus SF at various pocket depths for % of tooth surfaces completely free of calculus showed 1 to 3 mm pockets to be 86% versus 86%, 4 to 6 mm pockets to be 43% versus 76% and >6 mm pockets to be 32% versus 50%. The extent of residual calculus was directly related to pocket depth, was greater following scaling only, and was greatest at the CEJ or in association with grooves, fossae or furcations. No differences were noted between anterior and posterior teeth or between different tooth surfaces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73823/1/j.1600-051X.1986.tb01461.x.pd

Updatable phase filters for correlation with incoherent light

The possibility for exploiting an electronically addressed liquid crystal spatial light modulator as a phase filter, for optical correlators using incoherent light, is investigated. In such correlators, the phase filter must be specially designed with an iterative algorithm which takes into account certain degradations of the modulators. The degradations were considered in some detail. For the experiments, an electronically addressed liquid crystal spatial light modulator, derived from a commercially available video projector was adapted for use as the phase filter. The phase and amplitude modulation properties of this modulator were experimentally measured and full – 2π dynamic range for phase modulation was obtained, although accompanied by some parasitic amplitude modulation. This liquid crystal spatial light modulator was incorporated into an incoherent on-axis light correlator, to demonstrate successful updatable, real-time correlations

Versatile approach for frequency resolved wavefront characterization

Spatial characterization of high harmonics (HH) and XUV coherent radiation is of paramount importance, along with its temporal characterization. For many applications it will be necessary to accurately measure the beam properties, just as it is important to know the beam characteristics for many laser experiments. For example, high harmonics and attosecond pulses are being proposed as a front-end for the next generation X-ray free electron lasers. This oscillator-amplifier-like arrangement will require well characterized high harmonic sources. On the other hand, the electromagnetic radiation carries the combined signature of underlying quantum physical processes at the molecular level and of the cooperative phase matching. For example, accurate reconstruction of the high harmonic spatial wavefront, along with its temporal profile, gives us a complete range of tools to apply to the fundamental quantum properties and dynamics associated with high harmonic generation. We present a new concept of frequency resolved wavefront characterization that is particularly suitable for characterizing XUV radiation. In keeping with tradition in the area we give it an acronym - SWORD (Spectral Wavefront Optical Reconstruction by Diffraction). Our approach is based on an analysis of the diffraction pattern of a slit situated in front of a flat-field spectrometer. As the slit is scanned, the spectrally resolved diffraction pattern is recorded. Analyzing the measured diffractogram, we can reconstruct the wavefront. The technique can be easily extended beyond the XUV spectral region. When combined with temporal characterization techniques all information about the beam can be measured. \ua9 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).Peer reviewed: YesNRC publication: Ye

Creating High-Harmonic Beams with Controlled Orbital Angular Momentum

A beam with an angular-dependant phase Φ ¼ lϕ about the beam axis carries an orbital angular momentum of lℏ per photon. Such beams are exploited to provide superresolution in microscopy. Creating extreme ultraviolet or soft-x-ray beams with controllable orbital angular momentum is a critical step towards extending superresolution to much higher spatial resolution. We show that orbital angular momentum is conserved during high-harmonic generation. Experimentally, we use a fundamental beam with jlj ¼ 1 and interferometrically determine that the harmonics each have orbital angular momentum equal to their harmonic number. Theoretically, we show how any small value of orbital angular momentum can be coupled to any harmonic in a controlled manner. Our results open a route to microscopy on the molecular, or even submolecular, scale.192931sciescopu