Properties of Nd3+-doped and undoped tetragonal PbWO4, NaY(WO4)2, CaWO4, and undoped monoclinic ZnWO4 and CdWO4 as laser-active and stimulated raman scattering-active crystals

Signal-to-idler energy conversion from 1.9 to 2.3 µm by transient stimulated Raman chirped-pulse amplification

The combination of optical parametric and transient stimulated Raman amplification of chirped pulses demonstrates a new approach for idler energy buildup in the short-wave (SW)IR range. Optical parametric chirped-pulse amplification (OPCPA) output pulses in the wavelength range from ∼1800 nm to ∼2000 nm for the signal and from ∼2100 nm to ∼2400 nm for the idler were used as pump and Stokes seed, respectively, in a stimulated Raman amplifier based on a KGd(WO₄)₂ crystal. Both OPCPA and its supercontinuum seed were pumped with ∼1.2-ps transform-limited pulses from a Yb:YAG chirped-pulse amplifier. The transient stimulated Raman chirped-pulse amplifier provides a 33% increase in idler energy with nearly transform-limited ∼53-fs pulses after compression.

Spontaneous Raman spectra analysis and efficient nanosecond pulse Raman laser of a BaTeW2O9 crystal

In this Letter, the spontaneous Raman spectra of a novel, to the best of our knowledge, crystal α-BaTeW₂O₉ (α-BTW) are characterized and analyzed. The relative Raman gain coefficient of the α-BTW crystal is calculated to be 0.84 times that of YVO₄. With a 35-mm-long crystal, the first-order Raman laser of α-BTW operating at 1178 nm is realized. The simple external resonator setup is employed in the first-order Raman laser of α-BTW. The pump source is a lamp-pumped electric-optical Q switched Nd:YAG laser amplifier system operating at 1064 nm with a pulse width of 10 ns. The Raman laser exhibits a threshold of 14.7 MW/cm². In our experiments, a maximum pulse energy of 21.5 mJ is obtained with an optical-to-optical conversion efficiency and slope efficiency of 43.6%, 57.9%, respectively. Due to its high laser damage threshold, relative high Raman gain coefficient, and excellent thermal properties, the α-BTW crystal is a potential Raman material.

Stimulated Raman Scattering in Pb(MoO4)1−x(WO4)x with x = 0, 0.5, 0.8 and 1.0 with Combined Frequency Shifts on High- and Low-Frequency Raman Modes under Synchronous Picosecond Laser Pumping

Comparative characteristics of spontaneous and stimulated Raman scattering in Pb(MoO4)1−x(WO4)x single crystals (x = 0, 0.5, 0.8, and 1.0) including both the high (ν1) and low (ν2) frequency internal anionic group vibrations have been investigated. It was found that among these crystals, Pb(MoO4)0.2(WO4)0.8 is the most suitable for multi-wavelength Raman laser in several Raman modes simultaneously. This is caused by the optimal relative Mo/W content corresponding to the most efficient coherent combination of the (MoO4)2− and (WO4)2− vibrations enhancing the output radiation characteristics of the synchronously pumped Pb(MoO4)0.2(WO4)0.8 Raman laser. Oscillation of up to twelve, closely spaced SRS components in a range of 1128–1360 nm and the strongest pulse shortening down to 1.16 ps in comparison with not only PbMoO4 and PbWO4 but also with all the earlier investigated nominally pure scheelite-like tungstate and molybdate crystals has been achieved.

Cascaded vector vortex mode generation from a solid-state Raman laser

Cascaded vector vortex mode generation from a Ba(NO₃)₂ Raman laser cavity pumped by a vector LG_0,2 green laser was demonstrated for the first time to our knowledge. The generated Stokes outputs exhibited a second-order vector vortex beam as evidenced by linearly polarized flower-shaped spatial forms with four petals. The achieved optical conversion efficiencies of the first, second, and third Stokes outputs were 6.59%, 4.22%, and 0.11%, respectively, at a maximum pump energy of 3.6 mJ (360 mW).

Linear red/green ratiometric thermometry of Ho3+/Cr3+ co-doped red up-conversion tungstate materials

Existing optical thermometers are faced with the challenges of high sensitivity limited to a very narrow high temperature range, while also lacking low temperature sensing performance. A new linear up-conversion (UC) optical thermometer with high sensitivity over a wide temperature range was reported here. The introduction of Cr³⁺ optimized the red-green (R/G) ratio and improved the temperature sensing characteristics of Ho³⁺-doped tungstate materials. Notably, as a temperature-related parameter, the R/G emission intensity ratio of Ho³⁺/Cr³⁺ co-doped tungstate material fitted well linearly with temperature. The slope of the fitted line corresponded to the absolute sensitivity value; that is, the sensitivity was constantly 0.0217 K^-1 over the wide range of 163-663 K. This new UC temperature sensor with high sensitivity extended a new field of optical temperature measurement and demonstrated the possibility of applying this linear sensitivity effect in sensing applications. Most importantly, from an optical temperature sensing point of view, this study provided a novel and effective strategy for linear optical temperature measurement.

Multifunctional Optical Crystals for All-Solid-State Raman Lasers

In the past few decades, the multifunctional optical crystals for all-solid-state Raman lasers have been widely studied by many scholars due to their compactness, convenience and excellent performance. In this review, we briefly show two kinds of multifunctional Raman crystals: self-Raman (laser and Raman effects) crystals and self-frequency-doubled Raman (frequency-doubling and Raman effects) crystals. We firstly introduce the properties of the self-Raman laser crystals, including vanadate, tungstate, molybdate and silicate doped with rare earth ions, as well as self-frequency-doubled Raman crystals, including KTiOAsO4 (KTA) and BaTeMo2O9 (BTM). Additionally, the domestic and international progress in research on multifunctional Raman crystals is summarized in the continuous wave, passively Q-switched, actively Q-switched and mode-locked regimes. Finally, we present the bottleneck in multifunctional Raman crystals and the outlook for future development. Through this review, we contribute to a general understanding of multifunctional Raman crystals.

Upconversion luminescence and temperature sensing properties of NaGd(WO4)2:Yb3+/Er3+@SiO2 core-shell nanoparticles

Optical thermometry based on the fluorescence intensity ratio (FIR) of two thermally coupled levels in lanthanide ions has potential application in non-contact optical temperature sensing techniques. In this work, a shell of SiO2 with tunable thickness was uniformly coated on NaGd(WO4)2:Yb3+/Er3+ core upconversion nanoparticles (UCNPs). The effects of the silica shell on UC luminescence and thermal sensing properties of core-shell NaGd(WO4)2:Yb3+/Er3+@SiO2 UCNPs were investigated. Under 980 nm laser excitation, the temperature-dependent UC emission spectra of obtained samples were measured. The FIR was analyzed based on the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ in the biological temperature range of 300-350 K, in which the Boltzmann distribution is applied. The emission from the upper 2H11/2 state within Er3+ was enhanced as temperature increased due to the thermal effect. Absolute sensitivities (S A) and relative sensitivities (S R) of the core and core-shell UCNPs were calculated. It was found that after SiO2 coating, the maximum S A was enhanced by ∼2-fold (1.03% K-1 at 350 K). Especially, S A was as high as 2.14% K-1 at 350 K by analyzing the FIR of the non-thermally coupled 2H11/2 and 4F9/2 levels.

Investigation of Yb:CaWO4 as a potential new self-Raman laser crystal

High-quality Yb:CaWO₄ crystal was grown using the Czochralski method; laser action and self-Raman conversion were demonstrated.

Two-stage transient stimulated Raman chirped-pulse amplification in KGd(WO4)2 with compression to 145 fs

We report efficient amplification of chirped supercontinuum pulses in a two-stage stimulated Raman amplifier based on double tungstate [KGd(WO₄)₂] crystals, pumped with 1.2 ps transform-limited pulses at a 1030 nm wavelength. The second stage demonstrates a conversion efficiency of 55% with an output pulse energy of 0.6 mJ at a 1135 nm wavelength. The amplified Stokes bandwidth is 10 times the pump bandwidth, providing 145 fs pulses after compression.

3,5-Dihydroxy Benzoic Acid-Capped CaF2:Tb3+ Nanocrystals as Luminescent Probes for the WO4 2- Ion in Aqueous Solution

We report a facile and effective luminescence method for the determination of the WO4 2- ion in aqueous medium at initial pH = 6.3. This is achieved using 3,5-dihydroxybenzoic acid-capped CaF2:Tb3+ (5%) nanocrystals (NCs) as a luminescent probe. This is accomplished based on the energy transfer luminescence from the WO4 2- ion to the Tb3+ ion in small-size CaF2:Tb3+ NCs. Hydroxyl groups on the surface ligand helps in binding the tungstate ion to the surface of the NCs. With the gradual addition of the WO4 2- ion, the intensity of the Tb3+ excitation and emission spectra significantly increased. The linear range of the detection was from 1 to 10 μM for the WO4 2- ion (R 2 = 0.99). The calculated detection limit was 0.4 μM (by applying the 3σ/K criterion).

Enhanced photoconversion performance of NdVO4/Au nanocrystals for photothermal/photoacoustic imaging guided and near infrared light-triggered anticancer phototherapy

Although neodymium vanadate (NdVO₄) has been investigated and applied in some fields owing to its intensive ultraviolet (UV) light absorption, weak absorption in visible (Vis) and near infrared (NIR) regions constrains its environmental remediation and biomedical applications. Herein, plasmonic precious metal Au as light trapping agent is deposited onto NdVO₄ to form metal/semiconductor hybrid nanostructure for improving the Vis/NIR light absorption. NdVO₄/Au heterojunction nanocrystals (NCs) were synthesized by NdVO₄ nanorods (NRs) and plasmonic Au nanoparticles (NPs), followed by introducing polyvinylpyrrolidone (PVP) to enhance stability and biocompatibility, which exhibit elevated photocatalytic performance for organic dye degradation, photothermal conversion effect as high as 32.15% and cytotoxic reactive oxygen species (ROS) production ability. NdVO₄/Au can be internalized efficiently via endocytosis and cause apparent phototoxicity on HeLa cells. In vivo experiments further show that NdVO₄/Au can act as a high-efficiency NIR light-triggered anticancer agent with excellent tumor inhibition effect. In addition, based on outstanding light-to-heat conversion performance and thermal expansion effect under NIR irradiation, NdVO₄/Au provides photothermal (PT) and photoacoustic (PA) dual-modal imaging platform for precise cancer diagnosis and treatment. STATEMENTS OF SIGNIFICANCE: It's the first report on integrating precious metal Au and rare earth orthovanadates semiconductor into NdVO₄/Au heterojunction NCs. The as-prepared NdVO₄/Au heterojunction NCs exhibits improved absorption in Vis/NIR region and increased generation efficiency of photo-induced electron/hole pairs due to the LSPR effect, which results in enhanced photothermal conversion efficiency and the production ability of cytotoxic O₂^- and OH in comparison with pristine NdVO₄. For further clinical application, NdVO₄/Au heterojunction NCs could be served as anticancer therapeutic agent for PA/PT dual-modal imaging guided and NIR-triggered photothermal/photodynamic synergistic anticancer treatment.

Effect of pH in the Hydrothermal Preparation of Bi2WO6 Nanostructures

In this study, Bi2WO6 was prepared by the hydrothermal method. The effects of reaction temperature (150/170/200 °C) and reaction time (6/12/24 h) were investigated. The role of strongly acidic pH (1 >) and the full range between 0.3 and 13.5 were studied first. Every sample was studied by XRD and SEM; furthermore, the Bi2WO6 samples prepared at different temperatures were examined in detail by EDX and TEM, as well as FT-IR, Raman and UV-vis spectroscopies. It was found that changing the temperature and time slightly influenced the crystallinity and morphology of the products. The most crystallized product formed at 200 °C, 24 h. The pure, sheet-like Bi2WO6, prepared at 200 °C, 24 h, and 0.3 pH, gradually transformed into a mixture of Bi2WO6 and Bi3.84W0.16O6.24 with increasing pH. The nanosheets turned into a morphology of mixed shapes in the acidic range (fibers, sheets, irregular forms), and became homogenous cube- and octahedral-like shapes in the alkaline range. Their band gaps were calculated and were found to vary between 2.66 and 2.59 eV as the temperature increased. The specific surface area measurements revealed that reducing the temperature favors the formation of a larger surface area (35.8/26/21.6 m2/g belonging to 150/170/200 °C, respectively).

Stimulated Raman Scattering in Alkali-Earth Tungstate and Molybdate Crystals at Both Stretching and Bending Raman Modes under Synchronous Picosecond Pumping with Multiple Pulse Shortening Down to 1 ps

Comparative investigation of characteristics of spontaneous and stimulated Raman scattering (SRS) in different alkali-earth tungstate and molybdate crystals at both high and low frequency anionic group vibrations is presented. It has been found that, among these crystals, the SrMoO4 and SrWO4 crystals are the most perspective for SRS generation on both stretching and bending modes of internal anionic group vibrations with the strongest SRS pulse shortening under synchronous laser pumping because of not only highly intense stretching mode Raman line for efficient primary extra cavity long-shifted SRS conversion but also the widest bending mode Raman line for the strongest SRS pulse shortening down to the inverse width of the widest Raman line (~1 ps) at secondary intracavity short-shifted SRS conversion. The strongest 26-fold pump pulse shortening down to 1.4 ps at the Stokes component with the combined Raman shift in the synchronously pumped extra cavity SrMoO4 and SrWO4 Raman lasers has been demonstrated. It was found that synchronously pumped cascade SRS with combined Raman shift is more efficient in the SrWO4 crystal because the bending mode Raman line is more intense relative to the stretching mode Raman line than that in SrMoO4.

Coherent Raman Generation Controlled by Wavefront Shaping

We investigate the possibility of tailoring coherent Raman generated spectra via adaptive wavefront optimization. Our technique combines a spatial light modulator and a spectrometer providing a feedback loop. The algorithm is capable of controlling the Raman generation, producing broader spectra and an improved overall efficiency, and increasing the intensity of high-order sidebands. Moreover, by wavefront optimization we can extend the generated spectra towards the blue spectral region and increase the total power of generated sidebands. Mutual coherence and equal frequency separation of the multiple Raman sidebands are of interest for the synthesis of ultrashort light pulses with the total spectral bandwidth extending over ultraviolet, visible and near-infrared wavelengths.

Investigation on optical band gap, photoluminescence properties and concentration quenching mechanism of Pb1-x Tb3+xWO4 green-emitting phosphors

A series of monophasic Tb³⁺ (2, 5, 7, 10 and 15at%) doped PbWO₄ phosphors were successfully prepared via hydrothermal method. X-ray diffraction patterns revealed that the prepared samples possess a high crystallinity with tetragonal scheelite-type structure. FT-IR and Raman analysis exhibited a WO stretching peak of WO₄^2- group, which is also related to the scheelite structure. UV-visible diffuse reflectance spectra indicated a reduction in the optical band gap with the replacement of Pb²⁺ by Tb³⁺ ions. The presence of strong and intense emission peaks characteristic of Tb³⁺ with the dominant peak at 545nm (green, ⁵D₄→⁷F₅ transition) under UV irradiation at 320nm demonstrated an efficient energy transfer from the host to Tb³⁺ ions. Using Van Uitert's model, the concentration quenching mechanism between Tb³⁺ ions in PbWO₄:Tb³⁺ phosphor was attributed to a dipole-dipole interaction and the critical distance was determined to be ~12Å. The decay lifetimes and CIE chromaticity co-ordinates of PbWO₄:Tb³⁺ phosphors were also investigated in detail. These prepared materials might serve as a potential phosphor for LED applications.

Controllable synthesis of lanthanide Yb3+ and Er3+ co-doped AWO4 (A = Ca, Sr, Ba) micro-structured materials: phase, morphology and up-conversion luminescence enhancement

The combination of CDs can mainly maintain the morphologies of AWO₄:Yb³⁺,Er³⁺ but prominently increase their up-conversion luminescence performance.

Sub-50 ps pulses at 620 nm obtained from frequency doubled 1240 nm diamond Raman laser

We report a monolithic 1240 nm diamond Raman laser producing pulses with duration of 42-62 ps at 100 kHz repetition rate, and maximum average power of 246 mW. The Raman laser is formed by a 0.5-mm thick planar diamond, coated on both sides and pumped by ~100 ps pulses from a Q-switched 1064 nm laser. The maximum conversion efficiency from 1064 nm to 1240 nm was about 25%. The 1240 nm signal was frequency-doubled in single-pass configuration through a 10-mm long LBO crystal, enabling generation of pulses with a duration of 29-46 ps at 620 nm. The maximum average power at 620 nm was 128 mW, and the maximum conversion efficiency from 1240 nm to 620 nm was 50%. The Raman laser provides an efficient and flexible way to extend short pulse operation to wavelengths in spectral domains difficult to reach, such as 620 nm and in addition provides a simple pulse shortening mechanisms.

Self-Phase-Matched Second-Harmonic and White-Light Generation in a Biaxial Zinc Tungstate Single Crystal

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.

Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate

We report on the crystal growth, spectroscopic investigation and laser performance of Tm³⁺-doped monoclinic magnesium tungstate (Tm:MgWO₄), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 μm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

Na0.5Ce0.5MoO4 as a new light absorption material to efficiently degrade RhB under visible light irradiation

Na_0.5Ce_0.5MoO₄, with a high visible light absorption and a high conductivity, remarkably improves photocatalytic activity of Na_0.5Ce_0.5MoO₄/MoO₃ heterojunctions under visible light irradiation.

Dopant and excitation wavelength dependent color tunability in Dy3+:YVO4 and Dy3+/Eu3+:YVO4 microparticles towards white light emission

The emission color of Dy:YVO₄ and Dy,Eu:YVO₄ microstructures could be tuned towards white light and white light with a yellow undertone through varying the Ln³⁺ doping percentages as well as the excitation wavelengths.

Laser diode pumped, actively Q-switched, and mode-locked Nd:YAG/PbWO(4) Raman laser

A compact laser diode pumped, actively Q-switched, and mode-locked Nd:YAG/PbWO(4) Raman laser is demonstrated. With the incident pump power of 9.11 W and pulse repetition frequency of 15 kHz, a maximum average output power of 1.003 W at 1178 nm was obtained and the corresponding conversion efficiency was 11%. A perfect stable mode-locking operation with the modulation depth of 100% was obtained and the corresponding mode-locking pulse width was less than 207 ps. The actively Q-switched and mode-locked Raman laser was theoretically analyzed and calculations were conducted using the fluctuation rate equation model.

Thermal and spectroscopic characteristics of disordered melilite Nd:BaLaGa3O7 crystal

1 at% Nd:BaLaGa₃O₇ disordered crystal has a large emission bandwidth of 32 nm at 1060 nm.

Diode-pumped actively Q-switched Tm, Ho:GdVO4/BaWO4 intracavity Raman laser at 2533 nm

A compact diode-end-pumped actively Q-switched intracavity Raman laser with Tm,Ho:GdVO(4) laser gain medium and BaWO(4) Raman gain crystal is demonstrated for the first time. The Raman threshold is as low as 2.0 W of diode power at 802 nm. The highest average output power of 186 mW at the first Stokes wavelength of 2533 nm is obtained at a pulse repetition rate of 1 kHz, and a pump power of 2.8 W, corresponding to a diode-to-Stokes optical conversion efficiency of 6.6%. The pulsewidth and pulse energy are 7.8 ns and 0.19 mJ, respectively.

Study of relaxation oscillations in continuous-wave intracavity Raman lasers

We study the relaxation oscillations in a continuous-wave intracavity Raman laser both theoretically and experimentally. Analytic expressions for the relaxation oscillation frequency are derived from the rate-equations and are validated by experiments. We show that some important experimental parameters such as the effective Raman gain coefficient and intracavity Stokes loss can be determined simply by measuring the relaxation oscillation frequency versus pump power.

Effect of doping concentration on spectroscopic and structural properties of Nd:NaY(WO(4))(2) crystal

The optical and structural properties of NaNd(x)Y(1-x)(WO(4))(2) (NYW) single crystals grown from melt containing 1-10 at. % of Nd have been investigated. Optical absorption, photoluminescence, x-ray diffraction, and x-ray florescence studies have been carried out on grown crystals. The crystal is found to exhibit a broad absorption peak of ~16 nm width at 806 nm for 6 at. % of Nd doping. A decrease in fluorescence lifetime from 150 to 80 mus with an increase in Nd(3+) concentration is observed for 1.06 mum emission. Spectroscopic quality parameters (Omega(4)/Omega(6)) and quantum efficiency calculated from the above measurements and application of Judd-Ofelt theory are found to be 1.1 and 68%, respectively. Structural analysis through Rietveld refinement of x-ray diffraction data showed that the oxygen sublattice of NYW is not affected by changes in Nd dopant concentration, though the lattice dimensions are found to increase proportionately. The study for the first time shows that the high doping levels do not introduce any strain into the lattice. As a consequence the optical quality of the crystal does not deteriorate at high doping levels. The selection of doping concentration is therefore solely governed by the decay time requirement for a particular application.

All-solid-state parametric Raman anti-Stokes laser at 508 nm

We report a parametric anti-Stokes Raman laser using potassium gadolinium tungstate, generating output chiefly at the first anti-Stokes at 508 nm. The compact 4.5 cm long device is pumped by a Q-switched 532 nm laser and uses an off-axis Stokes resonator to provide non-collinear phase matching between the pump and the generated Stokes and anti-Stokes fields. Anti-Stokes output energies up 0.27 mJ were obtained at a conversion efficiency from the pump of 0.46%. Second- and third-order anti-Stokes lines at 486 nm and 465 nm were also observed.

Efficient diode-pumped actively Q-switched Nd:YAG/BaWO4 intracavity Raman laser

Barium tungstate (BaWO4) is employed to achieve efficient stimulated Raman scattering conversion in a compact diode-pumped actively Q-switched Nd:YAG laser. With an incident pump power of 9.2 W, 1.56 W of 1181 nm first-Stokes average output power was generated at a pulse repetition rate of 20 kHz, corresponding to an optical-to-optical conversion efficiency of 16.9%.