ABSTRACT
Holograms are recorded with focused beams in an iron-doped lithium niobate crystal. The diffraction efficiency shows a maximum after several seconds of recording, unlike in the case of writing with two homogeneous plane waves in the same crystal. This behavior can be attributed to a compensation field caused by incomplete illumination of the crystal. The field finally stops the bulk photovoltaic effect, which is the main driving force of the process. Based on this assumptions, we derive an analytical expression for the evolution of the diffraction efficiency which correctly fits the experimental data.
ABSTRACT
Congruently melting undoped lithium niobate crystals are irradiated with 20 MeV (3)He ions which penetrate the entire crystal volume. Radiation damage effects are directly visualized by transmission electron microscopy (TEM) where damage zones with diameters of 4 nm give rise to circular Fresnel fringe contrasts. These regions of modified material, appearing circular in cross-section, are interpreted as damage cascades inflicted by fast Nb and O atoms displaced in knock-on collisions with primary (3)He ions. This two-step displacement process results in local density changes manifested by the contrast behaviour of Fresnel fringes observed in TEM images.
ABSTRACT
Nb K-edge x-ray absorption fine structure spectra from a single lithium niobate (LN) crystal irradiated with high-energy (3)He ions of 41 MeV and from unexposed crystal as the reference material are compared. The differences in the x-ray absorption near edge structure (XANES) spectra are interpreted by simulating Nb K-edge XANES spectra with the FEFF8.2 code. It is found that displacements of Nb and Li atoms, as well as Li and O vacancies, are likely to cause structural disorder leading to change in the refractive index of LN and to diminished birefringence. This finding is in agreement with previous results obtained from SRIM-2003 simulations and transmission electron microscopy measurements.
ABSTRACT
A method is presented to acquire the absorption cross sections of dopants in photorefractive lithium niobate crystals utilizing doubly doped samples. The absorption cross section of one dopant must be well known. By illumination with ultraviolet light, electrons are transferred from one centre to the other. From the changes of the absorption spectra, the absorption cross section of the centre under investigation is deduced. For a wavelength of λ = 577 nm the absorption cross section of Mn(3+) is determined by this method to be σ(Mn(3+),577 nm)(o) = (9.2 ± 1.3) × 10(-19) cm(2) for ordinarily polarized light. The described method can be adapted to other dopants and host materials.
ABSTRACT
In atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and non-periodic patterns at nanometer scales.
ABSTRACT
Angular-multiplexed hologram recording in iron-doped lithium niobate crystals was carried out with near-infrared light. An incremental recording schedule with active phase locking of the light pattern onto the hologram was used. Continuous and reproducible recording of holograms of equal efficiency was achieved, and a hologram multiplexing number, M/#=2 , for a 5-mm-thick crystal was obtained at a 760-nm wavelength of light.
