Photon-electron correlations in double Compton scattering

The exact matrix element for the production of two circularly polarised photons in a double Compton scattering when the initial photon is also circularly polarised is calculated. The differential cross-sections for the various photon-electron polarisation correlations are obtained for high energy photons (i.e.) taking only near forward emission of photons. It is found that irrespective of the spin of the initial electron, the cross-section for either right or left circular polarisation of all three photons is zero under this approximation.     

Polarisation-sensitive optical elements in azobenzene polyesters and peptides

In this article, we describe fabrication of polarisation holographic optical elements in azobenzene polyesters. Both liquid crystalline and amorphous side-chain polyesters have been utilised. Diffractive optical elements such as lenses and gratings that are sensitive to the polarisation of the incident light have been fabricated with polarisation holography. Computer-generated optical elements and patterns have also been written with a single polarised laser beam. Recording of polarisation defects enabling easy visualisation is also shown to be feasible in azobenzene polyesters.     

The Goos-Hänchen shift on a layer

A beam of linearly polarised light suffering total internal reflection is shifted longitudinally. Two different shifts are found, belonging to the eigenstates of TE and TM polarisation. A much smaller transverse shift occurs when left and right circular polarisation are considered as eigenstates. The measured shifts agree well with theoretical predictions and are referred to as Goos-Hänchen effect. In this contribution shifts are investigated for the case of frustrated total internal reflection on a thin optical barrier. As well known, there appears, in addition to the reflected beam, a transmitted beam for which hitherto the shifts were not reported.     

A surprising method for polarising antiprotons

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to v/c ≈ 0.002 the cross-section for spin-flip is as large as about 2 ^. 10¹³ barn as shown by new QED calculations of the triple spin cross-sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately 1.5 ^. 10¹⁰ /(mm² s) appears to be feasible by means of a radioactive ¹¹C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100MeV) as well as at high (1GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 10¹⁰ antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure of merit by a factor of about ten.     

On the dependence of the fluorescence intensity of a single impurity molecule on the polarization of the incident light in birefringent crystals (pentacene in p-terphenyl)

A theoretical consideration of the propagation of a polarised light in a birefringent crystal is given. The dependence of the fluorescence intensity on the polarisation of the incident laser beam, on the distance of the molecule from the entrance crystal surface and on the orientation of the transition dipole moment is found. The values of parameters are given for p-terphenyl as the host.     

Laser with simultaneous Gaussian and conical refraction outputs

We report a double-sided two-mirror neodymium-doped KGd(WO₄)₂/Nd:KGW conical refraction laser which produces a linearly polarised output from the flat output coupler and a cone-refracted output from the curved output coupler. The linearly polarised output had a polarisation extinction ratio of 20:1 with a Gaussian beam profile and a measured M ²=1. The unpolarised output had the intensity and polarisation distribution corresponding to a cone-refracted Gaussian beam with extinction ratio of 2.7:1.     

A measurement of the beam asymmetry parameter Σ for neutral pion photoproduction in the energy range 1.2–2.8 GeV

An experiment has been completed at the Daresbury synchrotron to measure the asymmetry in the photoproduction cross section of neutral pions on hydrogen, for photons polarised normal to and in the production plane. The source of polarised photons was coherent bremsstrahlung of electrons traversing the lattice structure of diamond and the polarisation P of the γ beam was calculated from the measured intensity of the coherent spike. The asymmetry parameter Σ, defined as Σ = (σ⊥ ? σ|)/(σ⊥ + σ|) where σ⊥(σ|) are the cross sections for photons polarised perpendicular (parallel) to the production plane, has been measured over a range of photon energies from 1.2 to 2.8 GeV and over a range of ?t (the square of the four-momentum transfer) from 0.13 (GeV/c)² to 1.4 (GeV/c)². A marked energy variation in the value of Σ is found over the energy region 1.6–1.8 GeV.     

Propagation of a polarised polychromatic beam through a birefringent plate

The effect of propagation of a polarised polychromatic beam having a truncated gaussian spectral profile has been studied. The degree of polarisation of the output beam is seen to be less than of the of the standard deviation of the profile on the degree of polarisation of the output has been considered. Finally, the special case of a flat-top spectral profile has been discussed.     

Carrier transport, ultrafast spin dynamics and polarisation switching in InGaAsP multiple quantum wells

Excitonic optical nonlinearities are exploited in room temperature InGaAsP multiple quantum wells to determine the in-well carrier mobilities and spin relaxation processes. The picosecond time scale of the spin relaxation leads to the creation of a fast all-optical polarisation switch by using a circular polarised excite and a linear polarised probe pulse. It is demonstrated that the operation of the switch is dependent on the sense of circular polarisation of the probe. An optically induced birefringence is identified.     

Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.     

High intensity Compton scattering in a strong plane-wave field of general form

Photon emission by an electron embedded in a strong external field of general form is studied theoretically. The external field considered is a plane-wave electromagnetic field of any number of components, period and polarisation. Exact, Volkov solutions of the Dirac equation with the 4-potential of the general external field are obtained. The photon emission is considered in the usual perturbation theory using the Volkov solutions to represent the electron. An expression for the transition probability of this process is obtained after the usual spin and polarisation sums, trace calculation and phase space integration. The final transition probability in the general case contains a single sum over contributions from external field photons, an integration over one of the phase space components and the Fourier transforms of the Volkov phases. The validity of the general expression is established by considering specific external fields. Known specific analytic forms of the transition probability are obtained after substitution of the 4-potential for a circularly polarised and constant crossed external field. As an example usage of the general result for the transition probability, the case of two circularly polarised external fields separated by a phase difference is studied both analytically and numerically.     

Study on low-energy positron polarimetry

A polarised positron source has been proposed for the design of the international linear collider (ILC). In order to optimise the positron beam, a measurement of its degree of polarisation close to the positron creation point is desired. In this contribution, methods for determining the positron polarisation at low energies are reviewed. A newly developed polarisation extension to GEANT4 will provide the basis for further polarimeter investigations.      

Linearly polarised three-colour lasing emission from an evanescent wave pumped and gain coupled fibre laser

A novel Whispering--Gallery--Mode (WGM) fibre laser, emitting linearly polarised three-colour light, is demonstrated by pumping and gain coupling with evanescent waves. The pump light is longitudinally coupled into a bare optical fibre immersed in a dye solution of lower refractive index. The dye molecules around the bare fibre are excited by the evanescent waves of pump light when they propagate along the fibre in a total internal reflection. When the pump beam within the fibre is a meridian beam, the WGM lasing emission from the fibre laser is a linearly polarised transverse electric wave, while it is a mixed wave of the linearly polarised transverse electric and magnetic waves if the pump beam is a skew beam within the fibre. Because the excited molecules are located within the evanescent field of WGM, a good spatial overlap between the dye gain and the evanescent field leads to a high pumping efficiency and a longer gain distance along the fibre. Once the bare fibre is inserted into three glass capillaries filled with Rhodamine 6G, 610 and 640 dye solutions, respectively, WGM laser oscillations at the wavelengths of 567--575, 605--614 and 656--666~nm occur simultaneously, and a linearly polarised three-colour lasing emission is achieved in a single optical fibre.     

Influence of spin–orbit interaction,Zeeman effect and light polarisation on the electronic and optical properties of pseudo-elliptic quantum rings under magnetic field

ABSTRACT

We theoretically investigated the influences of the magnetic field and light polarisation on the electronic and optical properties of a GaAs/GaAlAs pseudo-elliptic quantum ring, modelled by an outer ellipsis and an inner circle, in the presence of the Rashba and Dresselhaus spin–orbit interactions and Zeeman effect. We show that Aharonov-Bohm oscillations of the energy spectrum are not affected by the presence of the Zeeman effect alone but, in the presence of Rashba and Dresselhaus spin–orbit couplings, the periodicity of certain levels becomes hardly definite. The Zeeman effect generally enhances/diminishes the separation levels produced by Rashba/Dresselhaus interactions (SOI) and when both types of SOI are considered, the effect depends on their relative strength. The magnetic field can trigger spin-flip for each type of spin–orbit interaction and Zeeman effect or their combination through anticrossings in the energy spectra. Our results reveal that the absorption spectra are very sensitive to the magnetic field and light polarisation. For all polarisations considered, the magnetic field increment leads to the redshift or blueshift of some particular peaks (an effect of this ring geometry) and a better separation of the peaks. The x-polarised light determines spectra with many small, but separated peaks while the circular polarised light leads to spectra with large peaks of high amplitude.     

Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Σ , was measured at ELSA in the reaction p → ηp using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range E _γ = 800 to 1400MeV with the maximum polarisation of P _γ = 49 % obtained at 1305MeV. Both dominant decay modes of the η into two photons and 3π⁰ were used to extract the beam asymmetry from the azimuthal modulation of the cross-section. The measurements cover the angular range Θ_cm ≃ 50 -150 degrees. Large asymmetries up to 80% are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.     

Geometrical and wave optics of paraxial beams

Most calculational techniques used to evaluate beam propagation are geared towards either fully coherent or fully incoherent beams. The intermediate partial-coherence regime, while in principle known for a long time, has received comparably little attention so far. The resulting shortage of adequate calculational techniques is currently being felt in the realm of x-ray optics where, with the advent of third generation synchrotron light sources, partially coherent beams become increasingly common. The purpose of this paper is to present a calculational approach which, utilizing a "variance matrix" representation of paraxial beams, allows for a straightforward evaluation of wave propagation through an optical system. Being capable of dealing with an arbitrary degree of coherence, this approach covers the whole range from wave to ray optics, in a seamless fashion.     

Some polarisation effects in photoemission from a cubic crystal

The energy distribution spectra of photoelectrons emitted normal to the (100) face of tungsten were investigated with respect to their polarisation dependence. The results are compatible with the assumption that surface photoemission vanishes if the exciting light is polarised parallel to the emitting surface. This is demonstrated for the examples of emission from a high density of states at the surface, a surface resonance, and an adsorbate induced level. The measurements also show that emission following direct optical excitation in the bulk is polarisation sensitive, supporting the idea of an anisotropic excitation along a star of k-vectors in the bulk. Some indication is also given of emission in ‘secondary cones’.     

Polarisation parameters in positive pion photoproduction

Measurements have been made of the polarisation parameters Σ, T and P for the process γp → π⁺n in the photon energy range 520–2250 MeV at c.m. angles between 30° and 120°. These data were obtained in a double polarisation experiment, using the polarised photon beam from the Daresbury electron synchrotron incident on a polarised proton target. The data are compared with predictions from current theoretical analyses.     

部分相干椭球光笼

 从理论上研究部分相干光束经透镜聚焦的三维光强分布。研究结果表明,在几何焦点附近的光强分布不仅依赖于入射部分相干光束的光强分布,而且还依赖于入射光束的空间相干度。基于这一点,提出了一种控制部分相干光束的空间相干度,在几何焦点附近获得了类似椭球光笼形状的光强分布的新方法。通过控制入射部分相干光的空间相干度,来改变聚焦在几何焦点附近的三维光强分布,从而获得希望得到的光强分布。