1,721,288 research outputs found
Very high gain single pass two-beam coupling in 'blue' Rh:BaTiO<sub>3</sub>
No full textTwo-beam coupling has been studied at red and near-infrared wavelengths in "blue" Rh:BaTiO3. High amplification, of the order of 20,000 - 37,000, of a weak signal beam, has been measured. Rh:BaTiO3 exhibits strong intensity-dependent absorption and transmission behaviour and this effect is considered when fitting theoretical plots to the experimental data
Spectral properties and modes of surface microcavities
No full textThis thesis describes the experimental investigations into the transverse mode structure of nearly hemispherical microcavities. The nearly hemispherical microcavity structures are fabricated electrocemically through a template of self assembled latex spheres. Controlling the electrochemical parameters, such as the electrochemical solution and electrode potential. allows a wide range of nearly hemispherical microcavities to be realised.The spatial intensity profiles arid resonant frequencies of the transverse modes of nearly hemispherical microcavities are measured experimentally for a wide range of cavity lengths amid mirror curvatures. The experimental mode profiles are radially symmetric Gauss-Laguerre modes, but do not display the frequency degeneracies typical of large scale optical cavities. The nearly hemispherical microcavity samples are compared to investigate how the cavity parameters. such as cavity length and mirror curvature, affect the experimental spatial intensity profiles and resonant frequencies of the transverse modes. Higher order modes are observed despite the fact that they are forbidden due to the symmetrical coupling geometry. the symmetry breaking is shown to be produced by the surface roughness of the curved mirror.The frequency degeneracy lifting which occurs in the nearly hemispherical microcavity structures can he explained and modelled by considering non-parabolic elements in the cavity. A number of mathematical models for the cavity propagation are developed based on paraxial theory. these models are analysed and the predictions made from the models are compared with the experimental profiles and frequencies. The basic agreement between theory and experiment shows that the paraxial theory is able to model the cavity modes. However, the spectrum and the mode profiles are quite sensitive functions of the geometry of the cavity amid the surface roughness of the cavity mirrors.The nearly hemispherical microcavities are structures which offer a new fabrication technique allowing inexpensive and a uncomplicated method of fabrication. An important feature of the nearly hemispherical microcavities is the tunability, and the ease in which this can be achieved. The structures are also empty, and this will allow them, in the future, to be easily filled with functional optical materials such as liquid crystals
Light-induced removal of 180° ferroelectric domains in Rh:BaTiO<sub>3</sub>
No full textWe show that optically-induced photorefractive space-charge fields can remove 180° ferroelectric domains in rhodium-doped barium titanate. The cross section of the domains must be small (less than 100 microns) for this process to occur
Conditions for efficient build-up of power in a ring-cavity with Rh:BaTiO<sub>3</sub>
No full textNumerous interaction geometries have been developed for photorefractive materials that rely on amplification of light via the two-beam coupling effect. One of the most elegant and simple configurations is a unidirectional ring resonator, which consists of a photorefractive crystal placed in a ring cavity and pumped by an external beam. If the two-beam coupling gain is above threshold, the resonating beam will build up from the amplification of scattered light. We present results on the most crucial parameters for the effective build-up of power in the resonating beam the wavelength of the pump beam, and the type of photorefractive material. The resonating beam accumulates energy from successive amplification in a photorefractive material until saturation sets in but also loses energy from absorption and other losses such as Fresnel reflections from the crystal and imperfect mirrors. We have performed intensity-dependent modelling of the resonator's power conversion factor. The power conversion factor is defined as the ratio of the resonating beam intensity to the intensity of the external pump beam. The figure below shows the dependence of this factor on absorption coefficient for different values of coupling coefficient Gamma
Conditions for efficient build-up of power in photorefractive ring cavities
No full textWe present results on modelling of amplification of light in a photorefractive ring resonator and on the optimum parameters required to achieve a high build-up of power. We show that an efficient resonator can be realised even with moderate coupling coefficients, of the order of 5/cm, provided the value of absorption is low, namely below 0.1/cm. These two conditions can be simultaneously fulfilled in a Rh:BaTiO3 crystal, using a near-infrared (1.06µm) laser as a pump beam. The low absorption coefficient condition also relaxes the dependence on the absolute value of transmission/reflection of the out-coupling element
Application of the three-valence model of photorefraction to rhodium-doped barium titanate
No full textSmall signal and light induced absorption data taken at 633 nm and 1.06 ?m for different samples of rhodium-doped barium titanate (Rh : BaTiO3) have been analysed. These data have been used in conjunction with the three-valence model theory of photorefraction to determine ND, the total amount of rhodium in each sample. It is shown that the values of ND calculated at the two wavelengths for individual samples are inconsistent with each other, although no evidence for other impurities in the samples has been observed. This indicates that a more complex theory than the three-valence model is necessary to explain the photorefractive processes in Rh : BaTiO3, and that an additional photorefractive centre may not be sufficient to account for the observed discrepancies in ND
Dynamics and uniformity of reorientation in liquid crystal cells with PVK alignment layers
No full textWe present experimental results on dynamics of Freedericksz transition in liquid crystal cells with alignment layers made of poly(N-vinyl carbazole). DC electric field causes the build-up of, unusually strong, surface charge layers on a liquid crystal-polymer interface that screen liquid crystals from the external field. The process of surface charge build-up and discharge in illuminated areas takes place due to low dark conductivity and high photoconductivity of poly(N-vinyl carbazole), rather than due to its photorefractive nature. If the illuminating beam has a Gaussian profile, reorientation starts from a very small volume, with a clear disclination line in the reorientation spot
Electric field control of diffraction and noise in dye-doped liquid crystals
No full textWe present results on permanent gratings in highly dye-doped liquid crystal cells without polymer coatings of the cells' surfaces. The surface-mediated gratings remain in cells for months without degradation of their quality. The peak diffraction efficiency can be controlled, enhanced or decreased, by applying low voltage AC field. At low frequencies, below 1 Hz, the diffracted signal can modulated by the AC field, but the time development of the signal shows a complex response. The enhancement of diffraction can be, however, observed at all frequencies we tested (0.1 Hz – 300 kHz). The permanent gratings cannot be removed by heating above the liquid crystal phase transition temperature as on cooling the diffraction efficiency is restored
Intensity-dependent absorption and its modelling in infrared sensitive rhodium-doped BaTiO<sub>3</sub>
No full textWe investigated the intensity-dependent absorption coefficient of photorefractive Rh:BaTiO3 from the maximum (633 nm) to the near infra-red end (1.06 µm) of this crystal's sensitivity. A numerical photorefractive model, incorporating dual-wavelength illumination and a secondary photorefractive centre, gives good agreement with experiment and shows that such a two-centre model is sufficient to explain the results obtained in the visible and infrared wavelength region
- …
