Preferencje help
Widoczny [Schowaj] Abstrakt
Liczba wyników

Znaleziono wyników: 29

Liczba wyników
first rewind previous Strona / 2 next fast forward last
Wyniki wyszukiwania
help Sortuj według:

help Ogranicz wyniki do:
We have exhibited the spectroscopic properties of GdCOB:Nd 3 + in the optimum phase matching direction for frequency doubling, i.e., θ=66.3 o and φ=134.4 o , in the two n + and n - directions of polarization. This direction is suitable for self-frequency doubling of the 1060 nm laser emission in type I configuration. The laser emission cross-section is found reasonably high: 6x10 - 2 0 cm 2 . The effective nonlinear optical coefficient for SHG at 1088 nm was found 1.89 times the NGAB one, i.e., about 2.6 pm/V. From a direct comparison of self-frequency doubling under the same experimental conditions, we have demonstrated that GdCOB:Nd allowed to reach 78% of the efficiency of a NGAB sample. This is quite a good result, keeping in mind that NGAB seems to be the most efficient self-doubling crystal at the present time.
Nowadays the most successful strategy for the prediction of the tertiary structure of proteins is the homology-based modelling using known structures. A real chance to predict the general fold of a protein arises only in cases with a sufficient sequence homology (e.g. 27% over 100 residues). In this analysis we examine the phenomenon of inverse sequence similarity (ISS) in proteins and its structural meaning. In sequence data bases we found a lot of examples for ISS up to 34% identity over 204 residues and a surprisingly large number of self-inverse protein sequences. By inspection of inverse similar sequence pairs with known tertiary structures we observe that inverse sequence alignments above the threshold indicating structural similarity generally do not imply comparable folds for both. From our analysis we conclude that the straightforward employment of ISS for protein structure prediction fails even above the known threshold for ‘safe similarity’.
Pure and ytterbium-doped neodymium borotungstate (Nd 3 BWO 9 , and Nd 2 . 8 5 Yb 0 . 1 5 BWO 9 , respectively) single crystals have been grown by means of high-temperature solution growth method from 15mol% solution in PbO. The phase of both crystals was confirmed to be hexagonal with acentric space group P6 3 . Absorption and luminescence spectra as well as decay time for 4 F 3 / 2 -> 4 I 1 1 / 2 transition in neodymium borotungstate were measured and discussed. The obtained results show that Nd 3 BWO 9 is suitable as high-neodymium-content laser crystal for microchip laser applications.
Photoinduced second harmonic generation was found in BiB 3 O 6 (BiBO) glass having different degree of crystallinity. We have revealed that an increasing degree of crystallinity of BiBO glass leads to increase of the output second-order optical susceptibility. We have used a pulsed 35 MW nitrogen laser (λ=337nm; τ=50ps) as a source of photoinducing laser beam. Fundamental laser beam was generated by pulsed Nd:YAG laser (λ=1.06μm; time duration about 25 ps). Maximal value of the photoinduced second-order non-linear optical susceptibility d 2 2 2 was equal to about 5 pm/V, what is substantially higher than in case of BiBO single crystals (~3pm/V).
Temperature-dependent polarized Raman spectra of KGd(WO 4 ) 2 : (Er, Yb) single crystals have been analyzed over the 77–292K temperature range. The A g and B g spectra obtained are discussed in terms of factor group analysis. The spectra have been found to reveal the bands related to internal and external vibrations of WO 4 2− , WOW and WOOW molecular groups. Strong depolarization of the majority of the Raman bands has been observed in the whole temperature range. Some anomalies in the spectral parameters of selected Raman bands below 175K have been discussed in terms of the local distortion of WO 4 2− ions in KGd(WO 4 ) 2 : (Er, Yb) crystals.
6
100%
Monoclinic KHo(WO 4 ) 2 single crystals have been grown by means of the Top Seeded Solution Growth method from K 2 W 2 O 7 self-flux containing up to 30mol% of the crystallized compound. KHo(WO 4 ) 2 seed was selected from small crystals obtained by spontaneous crystallization and used to grow the first bulky KHo(WO 4 ) 2 crystal, from which seeds perpendicular to {111} and {110} planes, used in subsequent experiments, were cut out. Crystals grown on these seeds are designated in this paper as 111 and 110 crystals, respectively. Properly chosen thermal conditions allowed growth of crystals with one dominating crystallographic plane forming the bottom of the 111 crystals. Under conditions of very low temperature gradients, growing crystals developed bottoms consisting of several crystallographic planes, while a slightly steeper temperature gradient led to cracking of the growing crystals due to high anisotropy of thermal expansion coefficients, which is characteristic of all double tungstates. It was found that 111 KHo(WO 4 ) 2 single crystals were much bulkier than 110 ones, which had a rectangular cross-section elongated in the c direction. The bottom of 111 KHo(WO 4 ) 2 crystals was built of one (111) plane, while in 110 crystals additional planes appeared that could not be eliminated by increasing the temperature gradient because such an increase led to cracking of crystals. As-grown 111 and 110 KHo(WO 4 ) 2 single crystals were of good quality, free from such defects as bubbles, cracks, and inclusions. The morphology of KHo(WO 4 ) 2 single crystals was investigated. Miller's indices of crystallographic faces (hkl) and edge indices [uvw] were found. Positions of the crystallographic axes a, b, and c in relation to growth faces and edges were found as well. The refractive indices and optical axes have been determined. The investigations of absorption spectra of KHo(WO 4 ) 2 single crystals were performed in a wide spectral range (4000-40,000cm - 1 ) at the temperature range 1.1 up to 300K. The trichroism of KHo(WO 4 ) 2 was confirmed experimentally. This means that optical absorption is characterized by three different, independent spectra attributed to three main optical directions.
The study of KGW: (Er, Yb) was performed for the acoustic phonons [100], [001], [−101], [101], [110] and [011]. In some Brillouin spectra obtained at room temperature we observed additional modes connected with mixed polarization modes propagating as quasi-longitudinal or quasi-transverse phonons which is characteristic for low symmetry crystals. Moreover, it has been revealed, that the value of Brillouin shift and integral intensity of the longitudinal phonon [101] propagating in KGW: (Er, Yb) show the anomaly at about 175K.
The paper presents the first investigations of KGd(WO4)2 single crystals triply-doped with Pr3+/Tm3+/Yb3+, Ho3+/Tm3+/Yb3+ and Er3+/Tm3+/Yb3+ ions by micro-Raman spectroscopy. No significant influence of doping ions on the Raman bands position was observed for investigated crystals at room temperature. The highest values of wavenumbers of Raman bands were observed for Pr3+/Tm3+/Yb3+ doped KGW crystals. The low-temperature dependences of the frequency of selected modes located at about 900, 807, 766, 746, 436, 348 and 342cm−1, assigned to the stretching and bending vibrations of WO42− tetrahedra and WOW and WOOW oxygen bridges, were analyzed in the 77–293K temperature range. Some anomalies in the spectral parameters of these modes are observed at about 145K for all investigated crystals. The results were discussed in terms of the local distortion of WO42− tetrahedra and deformation of WOW and WOOW oxygen bridge bonds vibrations in low-temperature range on the basis of the mismatch of the ionic radii of Pr3+, Er3+, Ho3+, Tm3+ and Yb3+ ions in the cationic layers.
•µ-Raman investigations of NLO single crystals: Bi2ZnOB2O6:Pr were performed.•Assignment of vibrational modes was proposed.•NLO properties of Bi2ZnOB2O6:Pr are associated with the BO3 and BO4 groups and the Bi3+or Zn2+ ions.
•Investigations of KGd(WO4)2 crystals doped with Ho/Yb/Er and Ho/Tm/Yb/Er ions.•The vibrational properties were studied using micro-Raman spectroscopy.•The multicolor up-converted luminescence was investigated.
Three ordered mesoporous siliceous (OMS) materials known as MCM41s—unmodified MCM-41C16 (“C16”), and two MCM41s with different surface functionalities: MCM-41C16-SH (“C16-SH”) and MCM-41C16-NH2 (“C16-NH2”)—were synthesized and studied by inverse gas chromatography in order to determine their acceptor–donor properties. The specific retention volumes of nonpolar and polar probes that were chromatographed on these ordered mesoporous silica adsorbents were evaluated under infinite dilution conditions. Two methods were employed to calculate the standard free energy of adsorption, ΔG ads, of each chromatographed probe on the basis its specific retention volume. These ΔG ads values were then employed to estimate the van der Waals contribution and the specific contribution of the free surface energy for each MCM41. DN values (donor numbers, based on the Gutmann scale) and AN* values (acceptor numbers, based on the Riddle–Fowkes scale) were employed to determine the values of parameters that characterize the ability of the MCM41s to act as electron acceptors (parameter: K A) and donors (parameter: K D). Considering the different compositions of the probes, each of which has different acceptor–donor properties, a new chromatographic test to supplement the Grob test is suggested.
Detailed spectroscopic studies of the triply doped KGd(WO4)2:Ho3+/Yb3+/Tm3+ single crystals (which exhibit multicolor up-conversion fluorescence) are reported for the first time. The absorption spectra of crystals were measured at 10 and 300K; the room temperature luminescence spectra were excited at 980nm wavelength. The dependence of the intensity of luminescence on the excitation power for three different concentration of Ho3+, Yb3+ and Tm3+ ions was investigated. Efficient green and red up-converted luminescence of Ho3+ ions and weak blue up-conversion luminescence of Tm3+ ions were observed in spectra. The red emission of Ho3+ ions is more intensive than their green emission. Dependence of the up-conversion luminescence intensity on the excitation power and impurities concentration was also studied; the number of phonon needed for efficient up-conversion was determined for each case. All possible energy transfer processes between different pairs of the impurity ions' energy levels are also discussed.
14
88%
Single crystals of KGd(WO 4 ) 2 doped with Ho 3+ ions were grown by the top seeded solution growth method. Polarized room temperature absorption spectra were analyzed by means of the conventional Judd–Ofelt theory taking into account strong dependence of the host refractive index on the wavelength. In addition to the intensity parameters Ω 2 , Ω 4 , Ω 6 , the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region. The transitions predicted by the phenomenological model as potential transitions for laser applications are discussed. Emission spectra in the green, red, and near-infrared spectral regions were recorded for different excitation wavelengths. Comparison with spectroscopic properties of Ho 3+ ion in other crystals is discussed.
Triply-doped single crystals KGd(WO 4 ) 2 :Er 3+ /Yb 3+ /Tm 3+ , KGd(WO 4 ) 2 :Tb 3+ /Yb 3+ /Tm 3+ and KGd(WO 4 ) 2 :Pr 3+ /Yb 3+ /Tm 3+ were grown by the Top Seeded Solution Growth (TSSG) method, with an aim of getting efficient up-converted multicolored luminescence, which subsequently can be used for generation of white light. Such an aim determined the choice of the triply doped compounds: excitation of the Yb 3+ ions in the infrared spectral region is followed by red, green and blue emission from other dopants. It was shown that all these systems exhibit multicolor up-conversion fluorescence under 980nm laser irradiation. Detailed spectroscopic studies of their absorption and luminescence spectra were performed. From the analysis of the dependence of the intensity of fluorescence on the excitation power the conclusion was made about significant role played by the host’s conduction band and other possible defects of the KGd(WO 4 ) 2 crystal lattice in the up-conversion processes.
Single crystals of KGd(WO 4 ) 2 doped with Nd 3+ and Er 3+ ions were grown by the top seeded solution growth method. Detailed analysis of the room-temperature absorption spectra was performed using the conventional Judd–Ofelt theory and actual dependence of the refractive index on the wavelength. In addition to the intensity parameters Ω 2, Ω 4, Ω 6 , the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region. Those transitions which can be potentially used for laser applications have been identified for both ions.
We present a Brillouin spectroscopy study of the acoustic phonons propagating in monoclinic KY(WO 4 ) 2 single crystals. The acoustic phonons were analyzed in relation to the orientation of the optical indicatrics. The velocities of the acoustic phonons propagating in the [100], [010], [001], [101], [1 0 1¯] and [110] directions have been determined from Brillouin experiment. Moreover, the values of elastic constants C 22 , C 44 , C 66 and C 46 of KY(WO 4 ) 2 single crystals have been calculated. The results are discussed in terms of low symmetry, high monoclinic angle and strong birefringence of the KY(WO 4 ) 2 crystal investigated.
The polarized Raman spectra of KGd(WO4)2 single crystals doped with rare earth ions Nd3+, Eu3+, Ho3+ and Er3+ were carried out in the range of internal and external vibrations 20–1000 cm−1 at room temperature. No significant influence of rare earth ions doping on the band positions was observed. However, most of detected in y(xx)z and y(xy)z scattering geometry Raman bands exhibit depolarization. The highest depolarization ratio is observed for the bands at 526, 684 and 766 cm−1 assigned to the stretching vibrations of the WOOW oxygen bridge bonds. While, the bands at 348, 371 and 899 cm−1 assigned to the bending and stretching vibrations of WO2−4 tetrahedra, are almost fully polarized for all investigated crystals. The depolarization of bands is due to distortion of WO2−4 tetrahedra and WOOW bridge bonds forming the structure of KGd(WO4)2. Moreover, the lowest degree of depolarization for analyzed bands was observed for KGd(WO4)2:Nd crystals, which exhibits less affected vibrational spectra by doping ions than other investigated crystals.
Occurrence of significant elastooptical effects in PbZrO 3 single crystals under illumination by 10ns Er:glass laser at 1540nm wavelength was established. The effect is maximal for the off-diagonal piezooptical tensor component. It was found that during heating to the ferroelectric phase (at about 540K) this effect is substantially suppressed. The observed effect may be used for production of optically operated devices working at different temperatures.
20
75%
In the present work we have synthesized a novel composite materials based on praseodymium (Pr3+) doped La2CaB10O19 (LCBO: Pr3+) nanocrystallites embedded into the polymethylmethacrylate (PMMA) and polycarbonate (PC) matrices for laser operated devices. It was shown that the SHG is very sensitive to the wavelength of external light.The optimal content of the LCBO: Pr3+ nanocrystallites sized within the 30–110 nm range was equal to 5.7 wt%. It was shown that the SHG is very sensitive to the wavelength of external photoinducing illumination.
first rewind previous Strona / 2 next fast forward last
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.