Neutron radiography with ultracold neutrons

The neutron transmission factor of very thin films may be low if the neutron energy is comparable to the pseudo-potential of the film material. Surprisingly, perhaps, it is relatively easy to obtain neutrons with such low energies in sufficient numbers to produce neutron radiographs.     

Measuring the phase of spatially coherent polychromatic fields

Realistic optical fields are never completely monochromatic and the interpretation of the concept of phase is not straightforward. It has been pointed out by Wolf that, if a strict condition of spatial coherence is imposed, the statistically averaged behavior of a polychromatic field can be described by an associated monochromatic wave. We show that, for spatially coherent polychromatic optical fields, the measurement of the phase of the second-order correlations determines the phase of this associated, spatially coherent field. We verify this prediction using a novel interferometric technique for measuring the cross-spectral density of a steady-state optical field.     

Interferential polychromatic filters

The reflection properties of one-dimensional generalized Cantor-like multilayer (GCLM) are investigated numerically in the visible range. Strong correlation between the stack geometry and the properties of the optical reflection spectra is found, namely spectral scalability and sequential splitting. The construction of multilayer systems according to the definite Cantor distribution brings improvements to the reflection properties. In particular, the widening of the band gap and the thin peak appearance in the reflection spectra whose number increases with the division number in the (GCLM). Optical properties of (GCLM) inserted between two periodic stacks are numerically investigated. We chose SiO₂(L) and TiO₂ (H) as two elementary layers. The study configuration is H(LH)⁵[GCLM]^PH(LH)⁵ which forms an effective interferential filter in the visible spectral range. We show that the number of resonator peaks is dependent on the repetition of the number P of the (GCLM). The best performances are obtained in particular for the symmetrical configurations of the (GCLM) and especially for P an odd number.Received: 10 July 2003, Published online: 30 January 2004PACS: 61.44.Br Quasicrystals - 42.70.Qs Photonic bandgap materials - 42.79.Ci Filters, zone plates, and polarizers     

Quantitative optical phase microscopy

We present a new method for the extraction of quantitative phase data from microscopic phase samples by use of partially coherent illumination and an ordinary transmission microscope. The technique produces quantitative images of the phase profile of the sample without phase unwrapping. The technique is able to recover phase even in the presence of amplitude modulation, making it significantly more powerful than existing methods of phase microscopy. We demonstrate the technique by providing quantitatively correct phase images of well-characterized test samples and show that the results obtained for more-complex samples correlate with structures observed with Nomarski differential interference contrast techniques.     

Imaging with polarized neutrons

While polarized neutrons have long proved to be an outstanding tool for the investigation of magnetic structures by scattering, we report on their potential for real space investigations of magnetic fields on a macroscopic scale by neutron imaging. Due to the ability of neutrons to penetrate thick layers of matter and their high sensitivity to magnetic fields owed to their magnetic moment, neutron imaging enables the investigation of magnetic fields even in bulk samples of condensed matter. We demonstrate how neutrons provide images of magnetic fields trapped in or expelled by superconductors or even reveal the path of electric currents due to the corresponding magnetic fields.     

Quantitative phase imaging with a scanning transmission x-ray microscope

We obtain quantitative phase reconstructions from differential phase contrast images obtained with a scanning transmission x-ray microscope and 2.5 keV x rays. The theoretical basis of the technique is presented along with measurements and their interpretation.     

Instrumentation with polarized neutrons

Neutron scattering with polarization analysis is an indispensable tool for the investigation of novel materials exhibiting electronic, magnetic, and orbital degrees of freedom. In addition, polarized neutrons are necessary for neutron spin precession techniques that path the way to obtain extremely high resolution in space and time. Last but not least, polarized neutrons are being used for fundamental studies as well as very recently for neutron imaging. Many years ago, neutron beam lines were simply adapted for polarized beam applications by adding polarizing elements leading usually to unacceptable losses in neutron intensity. Recently, an increasing number of beam lines are designed such that an optimum use of polarized neutrons is facilitated. In addition, marked progress has been obtained in the technology of ³He polarizers and the reflectivity of large-m supermirrors. Therefore, if properly designed, only factors of approximately 2–3 in neutron intensity are lost. It is shown that S-benders provide neutron beams with an almost wavelength independent polarization. Using twin cavities, polarized beams with a homogeneous phase space and P>0.99 can be produced without significantly sacrificing intensity. It is argued that elliptic guides, which are coated with large m polarizing supermirrors, provide the highest flux.     

Interaction of neutrons with nanoparticles

Two hypotheses concerning the interaction of neutrons with nanoparticles and having applications in the physics of ultracold neutrons (UCN) are considered. In 1997, it was found that, upon reflection from the sample surface or spectrometer walls, UCN change their energy by about 10^?7 eV with a probability of 10^?7–10^?5 per collision. The nature of this phenomenon is not clear at present. Probably, it is due to the inelastic coherent scattering of UCN on nanoparticles or nanostructures weakly attached at the surface, in a state of Brownian thermal motion. An analysis of experimental data on the basis of this model allows one to estimate the mass of such nanoparticles and nanostructures at 10⁷ a.u. The proposed hypothesis indicates a method for studying the dynamics of nanoparticles and nanostructures and, accordingly, their interactions with the surface or with one another, this method being selective in their sizes. In all experiments with UCN, the trap-wall temperature was much higher than a temperature of about 1 mK, which corresponds to the UCN energy. Therefore, UCN increased their energy. The surface density of weakly attached nanoparticles was low. If, however, the nanoparticle temperature is lower than the neutron temperature and if the nanoparticle density is high, the problem of interaction of neutrons with nanoparticles is inverted. In this case, the neutrons of initial velocity below 10² m/s can cool down, under certain conditions, owing to their scattering on ultracold heavy-water, deuterium, and oxygen nanoparticles to their temperature of about 1 mK, with the result that the UCN density increases by many orders of magnitude.     

Observation of polychromatic vortex solitons

We demonstrate experimentally the formation of polychromatic single- and double-charge optical vortex solitons by employing a lithium niobate crystal as a nonlinear medium with defocusing nonlinearity. We study the wavelength dependence of the vortex core localization and observe self-trapping of polychromatic vortices with a bandwidth spanning over more than 70 nm for single-charge and 180 nm for double-charge vortex solitons.     

New gravitational experiment with ultracold neutrons

The results of a new neutron gravitation experiment are reported. The change in the energy of a neutron falling to a known height in the Earth’s gravitational field is compensated by an energy quantum ?Θ transferred to the neutron as a result of the phase modulation of the neutron wave. A phase diffraction grating moving across the direction of the propagation of the neutron wave is used as a modulator. The experiment has been carried out with ultracold neutrons Interference filters, neutron analogues of Fabry-Perot interferometers, are used for the spectrometry of ultracold neutrons. The force m _g g _n acting on the neutron in the Earth’s gravitational field has been measured with an accuracy of about 0.2%.     

Narrowing effect of focused polychromatic laser with small Fresnel number

Starting from Huygens-Fresnel diffraction integral, the field representation of focused polychromatic laser beams is derived. Intensity distributions of the focused polychromatic laser are investigated with different Fresnel number and bandwidth at focal plane. It is found that the width of transverse intensity distribution of the focused polychromatic laser decreases with increasing the bandwidth at the focal plane and zero local time, and the transverse intensity presents more obvious narrowing effects with smaller Fresnel number. A physical explanation for the narrowing effects is given based on a simplified expression.     

Quantitative coherence analysis of dual phase grating x-ray interferometry with source grating

Dual phase grating x-ray interferometry is compatible with common imaging detectors,and abandons the use of an absorption analyzer grating to reduce the radiation dose.When using x-ray tubes,an absorbing source grating must be introduced into the dual phase grating interferometer.In order to attain a high fringe visibility,in this work we conduct a quantitative coherence analysis of dual phase grating interferometry to find how the source grating affects the fringe visibility.Theoretical analysis shows that with the generalized Lau condition satisfied,the fringe visibility is influenced by the duty cycle of the source grating and the transmission through the grating bar.And the influence of the source grating profile on the fringe visibility is independent of the phase grating type.Numerical results illustrate that the maximum achievable fringe visibility decreases significantly with increasing transmission in the grating bar.Under a given transmission,one can always find an optimal duty cycle to maximize the fringe visibility.These results can be used as general guidelines for designing and optimizing dual phase grating x-ray interferometers for potential applications.     

Shape invariant propagation of polychromatic fields

When a polychromatic field propagates the transverse distribution of the power spectrum generally changes from one plane to another. Conditions are found under which the spectrum distribution maintains the same shape in any cross-section. Examples of fields satisfying these conditions are given.     

Nonlinear directional coupler for polychromatic light

We demonstrate that a nonlinear directional coupler with special bending of waveguide axes can be used for all-optical switching of polychromatic light with a very broad spectrum covering all of the visible region. The bandwidth of the suggested device is enhanced five times compared with conventional couplers. Our results suggest novel opportunities for the creation of all-optical logical gates and switches for polychromatic light with white-light and supercontinuum spectra.     

光谱开关与多色光场的奇点光学效应

推导出多色高斯光束和高斯-谢尔模型光束通过杨氏实验双缝传输的谱强度公式.对完全空间相干光和部分空间相干光照明杨氏实验装置出现的光谱开关作了详细研究,并判断其是否属于奇点光学效应. 结果表明：多色场奇点光学效应的判据应当是光谱开关出现时的谱强度极小值S_min=0,而不是总光强极小值I_min=0. 当用多色高斯-谢尔模型光束照明杨氏实验装置时,在近场和远场产生的光谱开关都不属于奇点光学效应. 当用多色高斯光束照明杨氏实验装置时,只有远场产 关键词： 光谱开关 奇点光学效应 完全相干和部分相干 多色光场     

Analysis of the phase tunneling time for cold neutrons through a neutron interference filter

Explicit expressions are obtained for the energy dependence of the particle transmission coefficient and phase tunneling time through two rectangular barriers near resonance. The resonance half-width and the phase tunneling time for neutrons in resonance are calculated.