Microfabricated magnetic waveguides for neutral atoms

We describe how tightly confining magnetic waveguides for atoms can be created with microfabricated or nanofabricated wires. Rubidium atoms guided in the devices we have fabricated would have a transverse mode energy spacing of K. We discuss the creation of a single-mode waveguide for atom interferometry whose depth is comparable to magneto-optical trap (MOT) temperatures. We also discuss the application of microfabricated waveguides to low-dimensional systems of quantum degenerate gases, and show that confinement can be strong enough to observe fermionization in a strongly interacting bosonic ensemble. Received 1st December 1998 and Received in final form 23 February 1999     

Manipulation of cold atoms by an adaptable magnetic reflector

Adaptive optics for cold atoms has been experimentally realized by applying a bias magnetic field to a static magnetic mirror. The mirror consist of a 12-mm-diameter piece of commercial videotape, having a sine wave of wavelength 25.4 μm recorded in a single track across its width, curved to form a concave reflector with radius of curvature R=54 mm. We have studied the performance of the mirror by monitoring the evolution of a 24 μK cloud of ⁸⁵Rb atoms bouncing on it. A uniform static external magnetic field was added to the mirror field causing a corrugated potential from which the atoms bounce with increased angular spread. The characteristic angular distribution of the surface normal is mapped at the peak of the bounce for atoms dropped from a height of R/2 and at the peak of the second bounce for a drop height of R/4. In a second experiment a time-dependent magnetic field was applied and the angular distribution of the cloud was measured as a function of field frequency. In this scheme we demonstrate a corrugated potential whose time-dependent magnitude behaves like a diffraction grating of variable depth. Finally a rotating field was added to generate a corrugated potential that moves with a velocity given by the product of the external field rotation frequency and the videotape wavelength. This travelling grating provides a new method of manipulation as cold atoms are transported across the surface by surfing along the moving wave. Two theoretical methods have been developed to predict the behaviour of atoms reflecting from these stationary, variable magnitude and moving corrugated potentials. A simple analytic theory provides excellent agreement for reflection from a stationary corrugated potential and gives good agreement when extended to the case of a travelling grating. A Monte Carlo simulation was also performed by brute force numeric integration of the equations of motion for atoms reflecting from all three corrugated potential cases. Received: 1 December 1999 / Revised version: 3 February 2000 / Published online: 5 April 2000     

Analysis of the local conformation of proteins with two-dimensional fluorescence techniques

Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides. Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids. In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca²⁺ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone. Received: 10 February 2000 / Published online: 13 September 2000     

Cold atomic beam from a rubidium funnel

We report an experimental demonstration of a continuous, slow and cold beam of rubidium atoms from a two-dimensional magneto-optic trap or atomic funnel. Typically 7.3(7)×10⁸ atoms/s are ejected from the funnel with a variable velocity in the range 2–8 m/s and a temperature of 45–55 μK in the moving frame. This represents the first demonstration of sub-Doppler laser cooling in an atomic beam and temperatures as low as ≈25 μK have been observed. Received: 30 September 1999 / Published online: 5 April 2000     

Optical manipulation of group III atoms

We present details for the laser manipulation of group III atoms, specifically aluminum, gallium, and indium. The practical considerations of accomplishing this manipulation are discussed and alternative schemes are presented for each species. The possibility of using such an optical technique for composition modulation during semiconductor growth for the fabrication of quantum wire and quantum dot structures is also discussed. Received: 29 September 1999 / Published online: 5 April 2000     

GaN blue diode lasers: a spectroscopist’s view

We have characterized the spectroscopic properties of one of the first samples of blue-emitting diode lasers based on GaN. With such a laser diode operated inside a standard extended cavity arrangement we find a mode-hop free tuning range of more than 20 GHz and a linewidth of 10 MHz. Doppler-free spectroscopy on an indium atomic beam reveals the isotope shift between the two major indium isotopes as well as efficient optical pumping. Received: 8 November 1999 / Revised version: 31 January 2000 / Published online: 8 March 2000     

狭缝式高灵敏裂变中子探测系统

研制了狭缝-外延式高灵敏大面积PIN裂变中子探测系统. 其对14MeV，2.5MeV中子灵敏度可达10^-16C·cm²，比原有典型的脉冲裂变中子探测系统高4个量级. 采用外延式铅狭缝准直结构，研制灵敏区尺寸为60mm，厚度为200μm—300μm的大面积PIN半导体探测器、以Be膜为衬底，有效直径为60的²³⁵U裂变靶，解决了该探测器研制中的高灵敏度和n/γ分辨难题. 该系统已在实践中获得成功应用. 关键词： 裂变中子探测系统 大面积PIN探测器 大面积裂变靶 高灵敏探测系统     

Effects of Annealing on Microstructure and Optical Properties of TiO2 Sculptured Thin Films

The evolution of microstructure and optical properties of TiO2 sculptured thin films under thermal annealing is reported. XRD, field emission SEM, UV-Vis-NIR spectra are employed to characterize the microstructural and optical properties. It is found that the optimum annealing temperature for linear birefringence is 500℃. The maximum of transmission difference for linear birefringence is up to 18%, which is more than twice of that in as-deposited thin films. In addition, the sample annealed at 500℃ has a minimum of column angle about 12℃. The competitive process between the microstructural and optical properties is discussed in detail. Post-annealing is a useful method to improve the linear birefringence in sculptured thin films for practical applications.     

Room-Temperature Ferromagnetism in Semiconducting TiO2-δ Nanoparticles

TiO2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that anatase is the dominant phase with small amounts of the futile phase of TiO2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit unprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6 × 10^-3 emu/g for the sample annealed in H2/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under different conditions indicates that the amounts of Ti^3＋ or Ti^2＋ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.     

Pulsed holmium laser ablation of tissue phantoms: correlation between bubble formation and acoustic transients

5 Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 μm, duration: 180 μs FWHM), were delivered through 400 and 600 μm diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated using simultaneous flash micro-videography and pressure recording for radiant exposures of 20–382 J/cm². Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of up to 4.2 mm in length were observed in gels with a Young’s modulus of 2.9×10⁵ Pa at a pulse energy of 650 mJ. An increase of Young’s modulus (reduction in water content) led to a monotonic reduction of bubble size. In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased from 100±14 bars to 17±6 bars with increasing Young’s modulus over the studied range. Theoretical analysis suggested a major influence on bubble dynamics of the mass and energy transfer through the bubble boundary. Received: 26 August 1996/Revised version: 10 February 1997