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January 21, 2012

Research activities

The photorefractive effect (PRE) in LiNbO3

The photorefractive effect (PRE) was for the first time observed in LiNbO3 and LiTaO3 crystals in 1966. Since then, a great effort has been paid to get better understanding of the effect. Partial results lead to preparing new photorefractive materials - not only crystals but also different polymers and glasses, as well. Though a relatively long time has elapsed it is believed that photorefractive process in crystals was not completely under stood, yet. Mainly it is because of the complexity of the problem - interaction between light and matter. It was found in 1999-2000 that PRE in LiNbO3 crystals strongly depends on concentration and mutual interactions (relationship) between intrinsic and extrinsic defects. In our laboratory, we search for possible mechanisms responsible for photorefractivity of LiNbO3 in order to contribute to deeper understanding of the effect. There are three methods for investigation of PRE known:

    - optical polarization method
    - holographic method
    - optical records studying by means of interference imaging.

The holographic method is well known and probably the most often used. The principle of the method is studying the time-dependent diffraction of the light incident upon the record during its formation and/or decay. In our laboratory, we have developed the last method mentioned above. It is based on analysing of interference images of the photorefractive records.

Characterisation of optical fields by scanning diagnostics

The scanning optical microscopy using the tapered optical fibres has achieved high application priority in the last years especially in the optoelectronic device characterizations. Near-field scanning optical microscopy (NSOM) is a technique where a small optical probe is placed within a fraction of a wavelength of a sample and scanned over the surface. This technique allows achieving of spatial resolution of the order of 10 nm (in our conditions ~100nm), what significantly exceeds the resolution of conventional optical microscopy. High resolution of NSOM technique is due to the special optical probes prepared from tapered optical fibers with fiber tip in the range of 20-100nm. Optical fields are scanning by such optical fibers. Scanned optical field produced the evanescent optical field propagating through the fiber tip and generate in the wide-spread part of fiber propagated modes. We use the tapered optical fibre in combination with 3D micropositioning stage as a NSOM setup.

The application of near-field imaging and spectroscopy to optoelectronic devices and laser diodes provides subwavelength information on device structure, performance and output properties. Advantages of this technique could be employed for scanning of optical fields of different optical components as LEDs, lasers, photonic crystals and photonic crystal fibres (PCF). The NSOM diagnostic in combination with the laser source open possibilities to study optical properties of new materials especially formed in thin layers.