"Nanooptics and photonics"

The role of nanoscale structural defects in optical absorption of Bi12SiO20 crystals.
Diachenko A.O.1, Panchenko T.V.1
1Physics, Electronics and Computer Systems Department, Oles Honchar Dnipro National University, Gagarin Avenue, 72, Dnipro-49010, Ukraine. 
E-mail: annadiachenko459@gmail.com

Photorefractive crystals Bi12SiO20 (BSO) of the sillenites family are used as active media in devices for recording, processing and storing optical information. Own point defects play an important role in the formation of photosensitivity spectra and impurity optical absorption of these materials. The paper presents the results of an experimental study of the influence of structural nanosized defects of nonstoichiometry on the ratio Bi:Si on stationary optical absorption and photochromic effect (PCE) in BSO crystals.
Crystals with a deficit (BSO-Bi) and an excess (BSO+Bi) of Bi from x = 10.5 and 12.5, respectively (within the homogeneity range of BSO) were grown from the BixSiO1.5x+2 melt by the Czochralski method.
It is established that the excess or deficit of Bi in the range hν = 2.2 ÷ 3.2 eV entails an increase or decrease in the stationary optical absorption of BSO, respectively. The additional absorption Δα(hν) is intracenter and is determined by the concentration of defects of nonstoichiometry. In the region of the fundamental absorption edge, where the Urbach rule is satisfied, the excess (deficiency) Bi, causes the change in the parameters of the rule with increasing (weakening) of the constant of the electron-phonon interaction.
The PCE spectra of nonstoichiometric crystals are generally similar to those for stoichiometric BSO [1]. In this case, the spectra of PCE correlate with the spectra of stationary absorption (bleaching) caused by an excess (deficit) of Bi. However, in BSO+Bi crystals an intense absorption band with a maximum at νmax = 26200 cm-1 appears. An estimate of the strength of the oscillator of the individual components of the PCE spectra gives f~10-2, which is characteristic of electrically-dipole transitions between the ground and excited states of point defects, their nature is discussed. 

1.	Panchenko T., Diachenko A. Optical absorption of Bi12SiO20:Sn crystals // Functional materials.-2015.-22(2).-P. 169-174.