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Abstract: This paper presents a study of the near-field optical profile of corrugated surfaces obtained from commercial digital versatile disk (DVD) patterns using a scanning near-field microscope (SNOM). The profile of the SNOM light intensity collected is related to the topography of the samples. The SNOM patterns consist of a periodic modulated light intensity along the corrugated surface with the same periodicity of the topography structure. For the virgin sample, the light profile has intensity peaks corresponding to the ridges of the DVD structure. However, when the corrugated surface is metalized with a thin gold film of 30 nm thick, the SNOM light intensity peaks are centred in the valley of the DVD structure. A Finite-Difference Time-Domain (FDTD) model has been employed to simulate the SNOM experimental data. [Copyright &y& Elsevier]

Abstract: This work shows a study of the periodic ferroelectric domains formed in LiNbO3 crystals doped with rare earths by means of scanning near field optical microscopy (SNOM) technique. It has been observed periodic structures associated with ferroelectric domains with an unexpected high value of the optical contrast working under reflectance SNOM mode. From Raman-Nath diffraction patterns, a refractive index modulation of has been calculated. These results were correlated with the ferroelectric periodic domains obtained by the SNOM technique. A light waveguide effects along the ferroelectric domains is suggested to explain the high reflectance contrast observed in SNOM experiments. [Copyright &y& Elsevier]

Abstract: This paper reports the photo-luminescence spectroscopic results of Strontium–Barium–Niobate, Sr x ,Ba1−x Nb2O5 (SBN, x =0.61 for near congruent composition) crystals doped with Cr2O, at cryogenic temperature (20K). The experimental results reveal the need of re-assignment of the Cr3+ ions defect centres in this material. For first time, a broad emission band in the near infrared region centred at ca. 950nm is reported. This emission band has micro-seconds decaytime constant and a FWHM band-width>1700cm−1 and has been ascribed to the vibronically assisted 4T2 → 4A2 transition. A much narrower emission band centred at ca. 764nm with milli-seconds decaytime constant and a FWHM band-width of ca. 170cm−1 is correlated to the 2E→ 4A2 radiative transition (R-line). [Copyright &y& Elsevier]

The emission intensity of the R-line of Cr3+ ions in near stoichiometric LiNbO3crystals codoped with MgO has been studied at a low temperature under excitation of the 4A2→4T1 transition. The R-line emission intensity associated with Cr3+ ions in the Nb5+ site, [Cr]Nb centre, was observed to decrease with the illumination time whereas the Cr3+ ions located in the Li site, [Cr]Li centres, are optically stable. This selective bleaching is explained by considering the photoionization of Cr3+ ions in Nb5+ sites and the formation of Nb4+. [Copyright &y& Elsevier]

The optical spectroscopy of Cr3+ ions in near stoichiometric LiNbO3 crystals, ‘pure’ and co-doped with MgO, has been investigated.In the near stoichiometric LiNbO3:Cr (0.2%) optical absorption and emission spectra show the usual features of broad bands and narrow lines typical of congruent samples with the an additional of a narrow emission lines centred at 738 nm and has been ascribed to the zero-phonon line of the 4T2→4A2 transition. The concentration equilibrium between the two type of centres is strongly displaced towards the [Cr]Nb centre permitting thus to obtain with accuracy the parameters of the broad bands. The R-lines associated with the [Cr]Nb centre is only observable in low-temperature emission spectrum. The presence of Fano anti-resonance lines has been observed to be more pronounced in the near stoichiometric than in congruent samples. [Copyright &y& Elsevier]

The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3+ sites; Cr3+ ion substituting Li+ and Nb5+ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3+ ions located in Nb5+ site ([Cr]Nb centres). The observation of the zero-phonon line at 13 540 cm−1 only in pure LiNbO3:Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. [Copyright &y& Elsevier]

Abstract: The optical stability of the R-lines and broad band associated with the Cr3+ ions has been studied in codoped congruent and near stoichiometric LiNbO3:Cr:Mg crystals. It has been found that the 2E→ 4A2 transition of the Cr3+ ions located in Nb5+ sites, [Cr]Nb, can be bleached under optical excitation of the high energy broad absorption band luminescence whereas the emission from the same transition but ascribed to Cr3+ ions located in Li+ sites, [Cr]Li, is optically stable. At variance, the broad emission associated with the 4T2 → 4A2 vibronic transition ascribed to these centres is optically stable. The recovery temperature for the optical bleaching has been found to be 150K, and has been correlated to Nb4+ electron traps located in Nb5+ antisite position. The different optical stability found for the R-lines and broad band emission associated with the [Cr]Nb sites confirm the existence of two [Cr]Nb centres with low and high crystal field, respectively. [Copyright &y& Elsevier]