Potassium Gadolinium Tungstate Research Articles

Highly efficient continuous-wave solid-state Raman crystal lasers at 555 and 559 nm

High-power efficient continuous-wave Nd:YVO4/KGW Raman lasers at 555 and 559 nm are achieved by using a double-sided dichroic coating output coupler to improve the resonance quality factor. The Np-cut potassium gadolinium tungstate (KGW) is used to generate the Stokes waves at 1159 and 1177 nm by placing the polarization of the1064 nm fundamental wave parallel to the Ng and Nm axes, respectively. The lithium triborate (LBO) crystal with the cut angle in the XY plane for the type-I phase matching is used to perform the intracavity sum frequency generation for yielding the green light at 555 nm and the lime light at 559 nm at the optimal phase matching temperature. Experimental results were systematically accomplished to comprehend the optimal cavity length for the conversion efficiency. Under the optimal cavity length, the output powers can reach 6.6 and 6.3 W at a pump power of 22 W for the wavelengths of 555 and 559 nm, respectively. The conversion efficiencies can be up to 30% and 28.6% for 555 and 559 nm, respectively.

Effect of Femtosecond Ultraviolet and Infrared Laser Wavelength on Plasma Characteristics of Metals, Ceramics and Glass Samples Using Femtosecond Laser-Induced Breakdown Spectroscopy.

In this work, we present studies on the effect of laser wavelengths on the laser-induced plasma characterization using a femtosecond (fs) ytterbium-doped potassium-gadolinium tungstate (Yb:KGW) laser. Plasma plumes of copper, steel, ceramics, and glass samples were induced using a multiple shot of 200 fs laser pulses with 1030 nm and 343 nm wavelengths at fixed laser fluence () and analyzed using the laser-induced breakdown spectroscopy (LIBS) technique. Time-resolved fs-LIBS measurements were performed on the same set of samples and under the same experimental conditions. For the calculation of plasma parameters, the set of spectral lines of Cu(I) (for copper sample), Fe(I) (for steel sample), and Ca(I), K(I) (for glass and ceramics samples) were observed. The plasma electron temperature and density were evaluated from the Boltzmann plots and Stark-broadening profiles of the plasma spectral lines, assuming the local thermodynamic equilibrium condition. Time-resolved plasma temperature and electron density for fs-LIBS using ultraviolet (UV) and infrared (IR) laser wavelengths were analyzed and no significant dependence on fs laser wavelength was observed for any of the samples. However, for all samples the signal-to-noise ratio (SNR) significantly increased using UV laser radiation: copper (), steel (), glass (), and ceramics (). Therefore, by using a fs UV laser wavelength for laser-induced breakdown spectroscopy experiments, for certain materials the SNR and at the same time the limit of detection can be significantly enhanced.

Features of cluster ion treatment of the surface of KGd(WO<sub>4</sub>)<sub>2</sub>:Nd single crystal

The features of the surface treatment of single crystals of potassium gadolinium tungstate doped with neodymium ions with low- and high-energy cluster argon ions are considered. Two radically different treatment modes were used: low-energy for more efficient surface smoothing and high-energy for more efficient target etching. Using atomic force microscopy, the topography of the target surface was analyzed before and after cluster ion treatment. Treatment in a low-energy mode was shown to smooth out irregularities on the target surface formed by chemical-mechanical polishing at an etching depth of less than 100 nm. The root-mean-square roughness and maximum height difference of the initial and treated surfaces of potassium gadolinium tungstate doped with neodymium ions were compared. Survey X-ray photoelectron spectra of the initial surface of a KGd(WO4)2:Nd single crystal and after the cluster ion treatment in different modes are presented. The intensities of the potassium and gadolinium peaks were shown to decrease after cluster ion treatment in both modes. A significant decrease in the concentration of potassium atoms in the subsurface layer of the target is explained by the predominant sputtering of potassium as a lighter chemical element. The mutual decrease in the concentrations of gadolinium and potassium atoms can be explained by the weak bonds of these atoms in the lattice of the KGd(WO4)2:Nd single crystal.

Impact of Laser Cavity Configurations and Pump Radiation Parameters on the Characteristics of High-Energy Yellow Raman Lasers Based on Potassium Gadolinium Tungstate Crystals

In this paper, the influence of pump radiation parameters and laser resonator configurations on the characteristics of high-energy 589 nm KGW Raman lasers is investigated and explained. The best energy efficiency is obtained in lasers with a flat resonator, while the best angular characteristics are obtained in lasers with an unstable resonator. The use of stable spherical resonators results in the worst energy and angular characteristics. The angular divergence in lasers with flat cavities increases with pump intensity because of wing growth, whereas the core width remains unchanged. The structure of the wings is recorded and analyzed.

Sub-Nanosecond Passively Q-Switched Yellow and Orange Raman Lasers

Sub-nanosecond high-peak-power passively Q-switched yellow and orange lasers are compactly developed using a near-concentric resonator with intracavity-stimulated Raman scattering (SRS) as well as second harmonic generation (SHG). The fundamental wave is generated from an a-cut Nd:YVO4 crystal. The Stokes wave for an SHG yellow (579 nm) or orange (589 nm) laser is generated from a Np-cut potassium gadolinium tungstate (KGW) with the Ng or Nm axis parallel to the c-axis of Nd:YVO4 crystal. The optimal cavity length is systematically explored in a near-concentric configuration to achieve sub-nanosecond high-peak-power pulses. The shortest pulse widths for the yellow and orange output pulses are down to 0.67 ns and 0.70 ns, respectively. The highest peak powers for the yellow and orange output pulses are up to 176 and 138 kW, respectively.

Two-photon absorption of radiation in potassium-gadolinium tungstate (KGW) crystals and its effect on the operation of high-energy yellow Raman lasers

Two-photon absorption of radiation in potassium-gadolinium tungstate (KGW) crystals and its effect on the operation of high-energy yellow Raman lasers

Low-threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate.

We report on high average power, low threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate with amplified as well as unamplified pulses from a Yb:KGW oscillator. An octave-spanning supercontinuum was produced in undoped potassium gadolinium tungstate (KGW), which demonstrated robust, damage-free long-term performance with a total average pump power of 6.4 W. The supercontinuum generation was unambiguously attested by the distinctive features of the phenomenon: beam filamentation visualized via filament-induced luminescence; conical emission and its characteristic angular distribution captured by angle-resolved spectral measurements; and pulse splitting that produced the sub-pulses with well-behaved phases, as retrieved from the measurements employing a second harmonic frequency-resolved optical gating technique.

KGW and YVO4: two excellent nonlinear materials for high repetition rate infrared supercontinuum generation.

We present an experimental investigation of supercontinuum generation in potassium gadolinium tungstate (KGW) and yttrium vanadate (YVO4) crystals pumped with 210 fs, 1030 nm pulses from an amplified Yb:KGW laser operating at 2 MHz repetition rate. We demonstrate that compared to commonly used sapphire and YAG, these materials possess considerably lower supercontinuum generation thresholds, produce remarkable red-shifted spectral broadenings (up to 1700 nm in YVO4 and up to 1900 nm in KGW) and exhibit less bulk heating due to energy deposition during filamentation process. Moreover, durable damage-free performance was observed without any translation of the sample, suggesting that KGW and YVO4 are excellent nonlinear materials for high repetition rate supercontinuum generation in the near and short-wave infrared spectral range.

Synthesis of Adjacent Stokes Spectra in a Two-Stage Transient Stimulated Raman Chirped-Pulse Amplifier

The synthesis of adjacent overlapped Stokes spectra in two stages of a transient stimulated Raman chirped-pulse amplifier, tuned respectively to the vibrational modes at 901 and 767 cm−1 of a potassium gadolinium tungstate [KGd(WO4)2] crystal, is demonstrated. The contribution of the spatio-temporal overlap of seed and pump pulses, as well as the self-phase modulation, was investigated. The noncollinear configuration allows the composite bandwidth at the central wavelength of 1120 nm to be increased by a factor of 23 compared to the pump pulse bandwidth of 1.6 nm. After reaching a conversion efficiency of 35% in the second stage, the compressibility of a chirped Stokes pulse with a tailored spectrum was also investigated.

Noncritical acousto-optic Bragg phase matching: analysis of orthorhombic and monoclinic crystal systems.

Anisotropic acousto-optic diffraction in crystals is the fundamental phenomenon that is used to design acousto-optic tunable filters. Noncritical and quasicollinear phase-matching geometries of Bragg acousto-optic diffraction are compared for acoustic symmetry planes in four crystal systems (tetragonal, trigonal, orthorhombic, and monoclinic). The results for uniaxial crystals are reviewed and generalized for biaxial crystals. It is shown that cubic frequency dependence on the Bragg angle exists in two symmetry planes of orthorhombic crystals and conditionally exists in the symmetry plane of monoclinic crystals. It is also shown that there are two points in the symmetry plane of monoclinic crystals where noncritical phase matching takes place in quasicollinear diffraction geometry that can be used to design high-resolution tunable filters. Phase-matching configurations in α-iodic acid and potassium gadolinium tungstate crystals are analyzed.

Raman wavelength conversion in a multipass cell.

Positively chirped femtosecond pulses at 1030 nm are wavelength-converted using spontaneous and stimulated Raman scattering in a potassium gadolinium tungstate crystal inserted inside a multipass cell. Recirculation in the cell and the Raman material allows both a high conversion efficiency and good spatial beam quality for the generated Stokes beams. The converted pulses can be compressed to sub-picosecond duration. Multipass cells could be an appealing alternative to other Raman shifter implementations in terms of thermal effects, control of the Raman cascade, and overall output beam quality.

High-order stimulated Raman scattering in KGd(WO4)2 employing aperture

An approach that employs the aperture in front of the Raman crystal for the generation of high-order stimulated Raman scattering (SRS) was proposed. Furthermore, first- to fifth-order anti-Stokes components (508, 486, 465, 447, and 430 nm) and first- to fifth-order Stokes components (559, 588, 621, 658, and 700 nm) were generated by a potassium gadolinium tungstate (KGW) Raman generator, which were the highest-order SRS generated in KGW. The effect of the aperture was discussed, and the angular dependence of Stokes was calculated theoretically and experimentally.

Investigation of high-energy extracavity Raman laser oscillator and single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal

Herein, a high-energy all-solid-state extracavity Raman oscillator and a high-energy single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal are theoretically and experimentally investigated. First, a high-energy 1064 nm nanosecond laser system with a maximum output energy of 7.03 J at a repetition rate of 1 Hz is built as the pumping source for stimulated Raman scattering (SRS). Then, the output spectra and energy of multiorder Stokes lasers from 1.16 to 1.49 μm are gradually detected as the 1064 nm pumping laser energy increases for both Raman laser structures. For the extracavity Raman oscillator, under a pumping energy of 1.8 J, three-order Stokes lasers with a maximum energy of 448 mJ are obtained when the electric field direction of the pumping laser is parallel to the Ng optical axis of the KGW crystal (E∥Ng). Moreover, to obtain a higher output of the Raman laser without damaging the cavity mirrors, a single-pass Raman generator is studied for comparison purposes. As the 1064 nm pumping laser energy of 2.8 J is injected, the measured maximum output energy of a two-order Stokes laser is ~ 676 mJ for the E∥Ng condition; this is the highest Stokes energy output of the nanosecond solid-state Raman lasers to the best of our knowledge.

Extreme acoustic anisotropy in crystals visualized by diffraction tensor

Anisotropic acoustic wave propagation in single crystals is a fundamental phenomenon enabling operation of various acoustic, acousto-electronic and acousto-optic devices. It provides a variety of device performances and application fields, but its role in pre-estimation of achievable device characteristics and location of crystal orientations with desired properties is often underestimated. A geometrical image of acoustic anisotropy can be an important tool in design of devices based on wave propagation in single crystals or combinations of anisotropic materials. We propose a fast and robust method for survey and visualization of acoustic anisotropy based on calculation of the eigenvalues of bulk acoustic wave (BAW) diffraction tensor (curvature of the slowness surface). The stereographic projection of these eigenvalues clearly reveals singular directions of BAW propagation (conical acoustic axes) in anisotropic media and areas with large and small BAW beam divergence. The method is illustrated by application to three crystals of different symmetry used in different types of acoustic devices: paratellurite, lithium niobate and potassium gadolinium tungstate. The specific features of acoustic anisotropy are discussed for each crystal in terms of their potential application in devices. In addition, we demonstrate that visualization of acoustic anisotropy of lithium niobate helps to find orientations supporting propagation of high-velocity surface acoustic waves.

High-power diode-pumped Nd:GdVO4/KGW Raman laser at 578 nm.

A diode-pumped neodymium-doped gadolinium vanadate (Nd:GdVO4) laser is developed as a compact efficient yellow light at 578 nm by means of intracavity stimulated Raman scattering (SRS) in a potassium gadolinium tungstate (KGW) crystal and the second-harmonic generation in a lithium triborate crystal. The SRS process with a shift of 768cm-1 is achieved by setting the polarization of the fundamental wave along the Ng axis of the KGW crystal. The self-Raman effect arising from the Nd:GdVO4 crystal is systematically explored by employing two kinds of coating specification for the output coupler. With a specific coating on the output coupler to suppress the self-Raman effect, the maximum output power at 578 nm can reach 3.1 W at a pump power of 32 W. Moreover, two different lengths for the Nd:GdVO4 crystal are individually used to verify the influence of the self-Raman effect on the lasing efficiency.

A theoretical insight into the microstructure and electronic properties of Ho3+-doped potassium gadolinium tungstate

We have systematically studied the structural evolutions of trivalent holmium ion (Ho3+) doped potassium gadolinium tungstate (KGd(WO4)2) crystals by means of the CALYPSO structure search method coupled with first-principles calculations. The unbiased global minimum structure search is successful in reproducing the standardized monoclinic structure with C2/c phase for KGd(WO4)2. For the first time, the lowest energy structure of Ho3+-doped KGd(WO4)2 system is identified, which possesses a significantly different monoclinic configuration with P2 space group. It is found that the impurity Ho3+ ions can substitute the sites of Gd3+ ions, forming the [HoO8]13− local units with C2 site symmetry. Our electronic calculations show that the energy band gap of Ho3+-doped KGd(WO4)2 is equal to 3.03 eV. The impurity of Ho3+ ion remains the insulating character of Ho:KGW system. These results could provide important information for understanding the structural evolution and electronic properties of other rare-earth-doped materials.

Hybrid femtosecond laser system based on a Yb:KGW regenerative amplifier for NP polarization

We report a hybrid femtosecond laser system based on a femtosecond Yb-doped fiber laser and a Yb-doped potassium gadolinium tungstate (Yb:KGW) regenerative amplifier. To match the central wavelength of the seed source, a Yb:KGW crystal is used in the regenerative amplifier for Np polarization. We study and optimize the dynamics of nonlinear amplification to alleviate the gain narrowing effect. With optimization, the system can output 270 fs pulses with 21 μJ pulse energy at a 60 kHz repetition rate.

Potassium gadolinium tungstate nanocrystals doped with holmium ions as candidates for optical imaging

Holmium-doped monoclinic KGd(WO4)2 nanoparticles were synthesized by the mechano-chemical method. The structure and morphology of KGW:Ho 10 at.% nanopowders were investigated with the use of X-ray Powder Diffraction, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Emission and excitation spectra of KGW nanocrystals were also measured using a fluorometer with a xenon lamp and OPO laser source. The influence of nanoparticles of KGW doped with 10 at.% of holmium on the viability of J774.1 mouse BALB/c monocyte macrophage cells was tested by the WST8 assay. Preliminary immunity cell studies demonstrated a high viability of KGW:Ho 10 at%. Hence, these nanoparticles are promising for imaging applications. To characterize and quantify the physical stability of the nanoparticles of KGW:Ho 10 at.% suitable for biomedical applications, nanoparticles of KGW:Ho in the range of 80–200 nm were prepared.

Theoretical improvement and experimental research on Raman coupled equations

The classic Raman coupled equations, which describe the principles of the interaction between the incident and generated laser, have important applications in numerous domains. However, the classic Raman equations ignored the off-axis generated laser energy, which cannot be ignored in certain circumstances. We improved the classic equations by considering the off-axis laser energy of the stimulated Raman scattering. A single-pass multiwavelength potassium gadolinium tungstate crystal Raman generator pumped by a picosecond Nd:YAG laser was realized. On the basis of theoretical calculation and experimental verification, the improved equations produced numerical results for the output spectrum and output laser conversion efficiency that have a better agreement with the experimental results than the classic equations.

Three-color vibrational CARS thermometry of fuel-rich ethylene/air flames using a potassium gadolinium tungstate Raman-active crystal as a source of narrowband probe radiation.

Three-color broadband vibrational coherent anti-Stokes Raman scattering (CARS) temperature measurements were carried out in laminar fuel-rich sooting ethylene/air flames. Stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman-active potassium gadolinium tungstate [KGd(WO4)2] crystal was employed as a source of narrowband probe radiation. In the three-color CARS experiment, this wavelength-shifted radiation enables N2-based vibrational CARS temperature measurements in sooting flames free of the signal interference with the absorption/emission bands of the flame intermediate radicals C2. Spatial temperature profiles for different fuel-rich atmospheric pressure ethylene/air flames are presented in comparison with the results of two-color broadband vibrational and dual-broadband pure rotational CARS temperature measurements. The comparison shows the suitability of the three-color CARS measurement technique employing the KGd(WO4)2 crystal for accurate, C2 interference-free, temperature measurements in sooting flames.