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Spontaneous and stimulated Raman scattering in ZnWO4 crystals
Abstract
Spontaneous and stimulated Raman scattering (SRS) are studied in ZnWO4 crystals with a wolframite structure. The polarised Raman scattering spectra corresponding to all the six independent Raman tensor components are measured. The frequencies of the complete set of vibrational modes are identified. The threshold pump energies for SRS in ZnWO4 and KGd(WO4)2 crystals are measured upon excitation by picosecond 1047-nm pulses of a Nd:YLF laser. The SRS gains for ZnWO4 crystals are determined based on the measured thresholds and spectroscopic parameters of the crystals.
... where all even vibrations (g) are Raman active and all odd vibrations (u) are IR active [26][27][28][29]. Of the 18 possible active Raman vibration modes, at least 16 distinct vibration bands were identified in the spectra shown in Figure 2a. ...
Iron niobates, pure and substituted with copper (Fe1−xCuxNbO4 with x = 0-0.15), were prepared by the solid-state method and characterized by X-ray diffraction, Raman spectroscopy, and magnetic measurements. The results of the structural characterizations revealed the high solubility of Cu ions in the structure and better structural stability compared to the pure sample. The analysis of the magnetic properties showed that the antiferromagnetic-ferromagnetic transition was caused by the insertion of Cu 2+ ions into the FeNbO4 structure. The pure FeNbO4 structure presented an antiferromagnetic ordering state, with a Néel temperature of approximately 36.81K. The increase in substitution promoted a change in the magnetic ordering, with the state passing to a weak ferromagnetic order with a transition temperature (Tc) higher than the ambient temperature. The origin of the ferromagnetic ordering could be attributed to the increase in super-exchange interactions between Fe/Cu ions in the Cu 2+-O-Fe 3+ chains and the formation of bound magnetic polarons in the oxygen vacancies.
... Insert in (a) is the SEM image with a red circle where the EDS spectrum is taken from. The data in black and red color in (b) correspond to our experiment and reference[31,32], respectively. ...
Monolayer tungsten disulfide (2D WS2) films have attracted tremendous interest due to their unique electronic and optoelectronic properties. However, the controlled growth of monolayer WS2 is still challenging. In this paper, we report a novel method to grow WS2 through chemical vapor deposition (CVD) with ZnO crystalline whisker as a growth promoter, where partially evaporated WS2 reacts with ZnO to form ZnWO4 by-product. As a result, a depletion region of W atoms and S-rich region is formed which is favorable for subsequent monolayer growth of WS2, selectively positioned on the silicon oxide substrate after the CVD growth.
... Other higher-order nonlinear effects that contribute to white-light generation partly have their origin in the sum-frequency generation governed by Raman scattering. In particular, ZnWO 4 shows efficient (> 50%) picosecond multiple Stokes and anti-Stokes generation when it is used as a Raman-active crystal in solid-state lasers, based on stimulated Raman scattering (SRS) 33 . In the latter case, the strong SRS-active Raman mode at 907 cm −1 is the internal stretching W-O Ag-mode in the − WO 6 6 octahedra 17 . ...
Second-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities.
... The bands about 300 cm À1 were assigned to the modes of terminal WO 2 group. The W-O bonds of intermediate length are characteristic of bridging W-O bonds and are assigned to Raman mode stretching wavenumbers in the 700-1000 cm À1 [2,[23][24][25]. ...
... The terms '' g and u '' subscripts indicate the parity under inversion in centrosymmetric b-ZnMoO 4 crystals. Therefore, only 18 active vibrational modes are expected in Raman spectra of b-ZnMoO 4 crystals as represented by Eq. (3) below [59]: ...
In this paper, a combined theoretical and experimental study on the electronic structure and photoluminescence (PL) properties of beta zinc molybdate ( b-ZnMoO4) microcrystals synthesized by the hydrothermal method has been employed. These crystals were structurally characterized by X-ray diffraction
(XRD), Rietveld refinement, Fourier transform Raman (FT-Raman) and Fourier transform infrared (FTIR) spectroscopies. Their optical properties were investigated by ultraviolet–visible (UV–Vis) absorption
spectroscopy and PL measurements. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level have been carried out. XRD patterns, Rietveld refinement, FTRaman and FT-IR spectra showed that these crystals have a wolframite-type monoclinic structure. The
Raman and IR frequencies experimental results are in reasonable agreement with theoretically calculated
results. UV–Vis absorption measurements shows an optical band gap value of 3.17 eV, while the calculated band structure has a value of 3.22 eV. The density of states indicate that the main orbitals involved
in the electronic structure of b-ZnMoO4 crystals are (O 2p-valence band and Mo 4d-conduction band).
Finally, PL properties of b-ZnMoO4 crystals are explained by means of distortions effects in octahedral
[ZnO6] and [MoO6] clusters and inhomogeneous electronic distribution into the lattice with the electron
density map.
... The terms '' g and u '' subscripts indicate the parity under inversion in centrosymmetric b-ZnMoO 4 crystals. Therefore, only 18 active vibrational modes are expected in Raman spectra of b-ZnMoO 4 crystals as represented by Eq. (3) below [59]: ...
We report the third order nonlinear optical properties of Er3+-doped BaMoO4 nanostructures, and its dependence on Er-dopant concentration. BaMoO4 nanostructures with different concentration of Er were synthesized by chemical precipitation method and were characterized by UV–Vis absorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and fluorescence measurements. The incorporation of Er ions shifted the absorption band of BaMoO4 towards higher wavelength and enhanced the light absorption in the visible region. XRD patterns showed that the powders crystallize in scheelite-type tetragonal structure. The nonlinear optical behavior of the nanostructures was investigated by a Z-scan technique at 532 nm using continuous wave Nd:YAG laser. Experimental results suggested that the addition of Er can considerably enhance the nonlinear absorption coefficient and refractive index of BaMoO4 which could be used as a potential for nonlinear optical device applications.
Tetragonal tungstate CsLa(WO4)2:Nd3+ was found to be an attractive multifunctional simultaneously Nd3+-laser and stimulated Raman scattering (SRS)-active crystal. A multitude of Stokes and anti-Stokes components is generated in the visible and near-infrared spectral region producing frequency combs with widths of ~8600 cm−1 and ~9560 cm−1 under pumping at 0.532 07 µm and 1.064 15 µm wavelengths, respectively. All registered nonlinear emission lines are identified and attributed to a single SRS-promoting vibration ν s(WO4) mode with energy of ω SRS ~956 cm−1. Moreover, a passively Q-switched LD-pumped nanosecond self-Raman CsLa(WO4)2:Nd3+ laser is reported. An overview of Ln3+-doped tungstate self-Raman lasers is given as well.
The present joint experimental and theoretical work provides in-depth understanding on the morphology, structural, electronic, and optical properties of ZnWO4 nanocrystals. Monoclinic ZnWO4 nanocrystals were prepared at three different temperatures (140, 150, and 160 °C) by a microwave hydrothermal method. Then, the samples were investigated by X-ray diffraction with Rietveld refinement analysis, field-emission scanning electron microscopy, transmission electronic microscopy, micro-Raman and Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, and photoluminescence measurements. First-principles theoretical calculations within the framework of the density functional theory were employed to provide information at the atomic level. The band structure diagram, density of states, Raman and infrared spectra were calculated to understand the effect of structural order–disorder on the properties of ZnWO4. The effects of the synthesis temperature on the above properties were rationalized. The band structure revealed direct allowed transitions between the VB and CB and the experimental results in the ultraviolet-visible were consistent with the theoretical results. Moreover, the surface calculations allowed the association of the surface energy stabilization with the temperature used in the synthesis of the ZnWO4 nanocrystals. The photoluminescence properties of the ZnWO4 nanocrystals prepared at 140, 150, and 160 °C were attributed to oxygen vacancies in the [WO6] and [ZnO6] clusters, causing a red shift of the spectra. The ZnWO4 nanocrystals obtained at 160 °C exhibited excellent photodegradation of Rhodamine under ultraviolet light irradiation, which was found to be related to the surface energy and the types of clusters formed on the surface of the catalyst.
A short-range force constant model was applied to investigate the phonons in wolframite ZnWO4 crystals in their monoclinic phase having space group P2/c and symmetry C2h . The normal symmetry coordinates for the monoclinic wolframite were computed. The zone center phonons were calculated by using 8 stretching and 6 bending force constants. The calculated results are in very good agreement with the observed ones. The infrared frequencies were assigned for the first time. The potential energy distribution was also investigated for determining the significance of the contribution from each force constant toward the Raman and infrared wavenumbers.
In the search for stable and efficient photocatalysts beyond TiO2 , the tungsten-based oxide semiconductors silver tungstate (Ag2 WO4 ), copper tungstate (CuWO4 ), and zinc tungstate (ZnWO4 ) were prepared using solution combustion synthesis (SCS). The tungsten precursor's influence on the product was of particular relevance to this study, and the most significant effects are highlighted. Each sample's photocatalytic activity towards methyl orange degradation was studied and benchmarked against their respective commercial oxide sample obtained by solid-state ceramic synthesis. Based on the results herein, we conclude that SCS is a time- and energy-efficient method to synthesize crystalline binary tungstate nanomaterials even without additional excessive heat treatment. As many of these photocatalysts possess excellent photocatalytic activity, the discussed synthetic strategy may open sustainable materials chemistry avenues to solar energy conversion and environmental remediation.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Output charactaristic of external resonator PbWO 4 Raman laser excited by nanosecond pulse is studied. An actively Q-switched Nd:YAG laser is used as the pumping source, with pulse width of 31.4 ns and the maximum output energy of 200 mJ at 1064 nm. The Raman laser cavity is a flat-concave one. The relation between the pulse width of output Raman pulses and pumping energy is measured. The optical spectra and the pulse waveforms of Raman pulses are analyzed. The relation between the energy of output Raman pulses and pumping energy is measured, and the relation between the conversion efficiency and pumping energy is calculated. When the pumping energy is 42 mJ, the obtained maximum output energy and conversion efficiency of first-Stokes pulse are 10 mJ and 24%, respectively. The wavelength of the first-Stokes pulse of the Raman laser is 1177.6 nm. The typical pulse width the of the first-Stokes pulses is 20 ns.
Barium praseodymium tungstate (Ba1-xPr2x/3)WO4 crystals with (x = 0, 0.01, and 0.02) were prepared by the coprecipitation method. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier-transform Raman (FT-Raman) and Fourier-transform infrared (FT-IR) spectroscopies. The shape and size of these crystals were observed by field emission scanning electron microcopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. Moreover, we have studied the photocatalytic (PC) activity of crystals for degradation of rhodamine B (RhB) dye. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all crystals exhibit a tetragonal structure without deleterious phases. FT-Raman spectra exhibited 13 Raman-active modes in a range from 50 to 1000 cm−1, while FT-IR spectra have 8 infrared active modes in a range from 200 to 1050 cm−1. FE-SEM images showed different shapes (bonbon-, spindle-, rice- and flake-like) as well as a reduction in the crystal size with an increase in Pr3+ ions. A possible growth process was proposed for these crystals. UV-vis absorption measurements revealed a decrease in optical band gap values with an increase of Pr3+ into the matrix. An intense green PL emission was noted for (Ba1-xPr2x/3)WO4 crystals (x = 0), while crystals with (x = 0.01 and 0.02) produced a reduction in the wide band PL emission and the narrow band PL emission which is related to f-f transitions from Pr3+ ions. High photocatalytic efficiency was verified for the bonbon-like BaWO4 crystals as a catalyst in the degradation of the RhB dye after 25 min under UV-light. Finally, we discuss possible mechanisms for PL and PC properties of these crystals.
Two-photon absorption (TPA) coefficients were measured in tungstate and molybdate crystals – BaWO4, KGW, CaMoO4, BaMoO4, CaWO4, PbWO4 and ZnWO4 upon different orientations of excitation polarization with respect to the crystallographic axes. Trains of 25 ps pulses with variable radiation intensities of third (349 nm) harmonics of passively mode-locked 1047 nm Nd:YLF laser were used for interband two-photon excitation of the crystals.
It was suggested that in the case, when 349 nm radiation pumping energy exceeds the bandgap width (hν>Eg), the nonlinear excitation process can be considered as two-step absorption.
The interband two-photon absorption in all the studied crystals induces the following one-photon absorption from the exited states, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the excitation intensity. This fact was taken into account under analysis of the experimental dependences of the reciprocal transmission on the excitation intensity. Laser excitation in the transparency region of the crystals caused stimulated Raman scattering (SRS) not for all the crystals studied. The measured nonlinear coefficients allowed us to explain the suppression of SRS in crystals as a result of competition between the SRS and TPA.
The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm{sup 2}. The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO{sub 4} and ZnWO{sub 4} crystals reaches 50-60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients {beta} for the crystals vary over a wide range, namely, from {beta} = 2.4 cm/GW for PbMoO{sub 4} to {beta} = 0.14 cm/GW for CaMoO{sub 4}, and the values of {beta} can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.
A high-pressure Raman scattering study of wolframite-type Mn(0.97)Fe(0.03)WO(4) is presented up to 10.4 GPa. The phonon wavenumbers vary linearly with pressure. The mode Grüneisen parameters are larger for many bending and lattice modes when compared to the stretching modes due to the larger compressibility of Mn(Fe)O(6) octahedra when compared to WO(6) octahedra. Combining the pressure-dependent Raman data of this work with the temperature-dependent Raman data on this crystal previously reported by us has allowed estimation of the temperature-dependent pure lattice and intrinsic anharmonic contributions to the observed total Raman shifts as a function of temperature. It has been found that the observed unusual hardening of the 884, 698 and 674 cm(-1) stretching modes upon heating from 4 to about 150-200 K followed by the usual softening above 150-200 K is a result of a positive intrinsic anharmonic contribution and a negative pure lattice contribution; i.e., up to about 150-200 K the anharmonic contribution surpasses the lattice contribution and the total Raman shift is slightly positive whereas above 150-200 K the lattice contribution becomes dominant and the Raman bands exhibit the usual softening with increasing temperature.
A comparison of the spectroscopic parameters of Raman-active vibronic modes in various crystalline materials with a view to the use of these crystals for stimulated Raman scattering (SRS) is presented. It includes data on the Raman frequency shift, linewidth, integral, and peak Raman scattering cross sections. For steady-state SRS the highest Raman gain coefficient has been proved to be in barium nitrate and sodium nitrate crystals; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium tungstate and strontium tungstate are proposed as new highly efficient Raman materials for both SRS cases.
Spectroscopic, laser, and χ⁽³⁾ nonlinear optical properties of tetragonal PbWO4, NaY(WO4)2, CaWO4, and monoclinic CdWO4 and ZnWO4 were investigated. Particular attention was paid to Nd³⁺-doped and undoped PbWO4 and NaY(WO4)2 crystals. Their absorption and luminescence intensity characteristics, including the peak cross sections of induced transitions, were determined. Pulsed and continuous-wave lasing in the two ⁴ F 3/2→ ⁴ I 11/2 and ⁴ F 3/2 →⁴ I 13/2 channels was excited. For these five tungstates, highly efficient (greater than 50%) multiple Stokes generation and anti-Stokes picosecond generation were achieved. All the observed scattered laser components were identified. These results were analyzed and compared with spectroscopic data from spontaneous Raman scattering. A new crystalline Raman laser based on PbWO4 was developed for the χ⁽³⁾ conversion frequency of 1-µm pump radiation to the first Stokes emission with efficiency up to 40%. We classify all the tungstates as promising media for lasers and neodymium-doped crystals for self-stimulated Raman scattering lasers.
The vibrational (IR absorption and Raman) spectra of the tungstates of the composition ZnWO4, CdWO4, β-LiYb(WO4)2, LiFe(WO4)2 and LiIn(WO4)2 crystallizing in the wolframite structural type, including those of lithium isotope 6Li7Li, iron isotope 54Fe58Fe and zinc isotope 64Zn68Zn substituted samples have been studied. Assignments and analysis of the vibrations of the crystal cationic sublattices are presented. The character of the vibrational spectrum has been shown to depend on the way in which the cation alternation occurs.
The theory of transient stimulated Raman scattering has been extended to include an arbitrary shape of the laser pump pulse. It is shown that the maximum Stokes gain depends on the total energy content per unit area of the pump pulse, and not on the instanteous intensity for an exciting pulse of short duration. The Stokes pulse has a leading edge which rises sharply to a maximum, where the maximum occurs with some delay with respect to the maximum of the pump pulse. The trailing edge follows the decay of the pump. In a nondispersive medium, the gain is not reduced by frequency broadening of the laser output, while in a dispersive medium, considerable gain reduction is expected. Numerical results for various laser-pulse shapes and spectral distributions are presented.
The temporal characteristics of stimulated Raman scattering (SRS) under 22-ps laser excitation were studied in eight oxide
crystals with a T
2 optical-phonon dephasing time variable by up to two orders of magnitude. The measured SRS pulse width was shortened from
13.8 ps for Ba(NO3)2 (T
2 = 26.5 ps) to 4 ps for the LiNbO3 (T
2 = 0.38 ps) crystal. The dependence of SRS pulse width on the dephasing time was analyzed in the framework of the known SRS
theory.
This chapter describes the state of the art of crystalline and fiber Raman lasers based on the simulated Raman scattering
(SRS) effect in crystals and silica-based fibers. It includes historical and theoretical background, analysis of properties
of known and newly developed high-efficient SRS crystals, such as LiIO3, Ba(NO3)2, NaNO3, PbNO3, CaCO3, KGW, BaWO4, SrWO4,BaMoO4,SrMoO4, PbWO4, and germanosilicate and phosphosilicate fibers. A large set of data on IR Raman shifters and lasers operating in the CW,
nanosecond, and picosecond regimes with low and high repetition rates is given. Some applications of Raman lasers in medicine,
ecology, fiber optics, and communications are discussed.
Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material.OCIS: 300.6450; 290.5910; 190.2640
Spontaneous Raman spectra of tungstate (MeWO4) and molybdate (MeMoO4) crystals with sheelite structure were investigated (Me=Ca, Sr, Ba, Pb). The energy shift and line broadening of internal vibrational modes were measured in the temperature range from 77 K to the melting point by means of a high temperature Raman spectroscopic technique. The phase transition from sheelite to cubic structure was discovered at a temperature of 1600 K in a BaMoO4 crystal. Vibrational level diagrams for “free” [WO4]2− and [MoO4]2− tetrahedrons were built on the basis of spontaneous Raman spectra of tungstate and molybdate crystals in the molten state. The effect of mass and electronegativity of Me2+ cations on the position and linewidth of internal vibrational modes was investigated. It was shown that the variation of vibration level diagrams in sheelite series could change the probability of the dephasing and splitting relaxation mechanisms. As a result, the consequent line narrowing of totally symmetric Raman vibrations of [WO4] and [MoO4] tetrahedron complexes was registered in the Raman spectra in a series of sheelite crystals moving from calcium to strontium and barium. Due to the narrowest linewidth and the highest peak cross-section of the Raman line in barium tungstate and molybdate crystals, they were proposed as the most efficient sheelite crystals for Raman laser development.