Laser Crystal Research Articles

Property optimization of Er/Yb-codoped LiNbO3 crystals for LNOI lasers and amplifiers

Lithium-niobate-on-insulator (LNOI) is regarded as an ideal platform for integrated photonics. As an essential part, active devices based on rare-earth (RE) doped LNOI have made significant progress, with erbium/ytterbium-codoped lithium niobate (Er/Yb-codoped LN) showing great potential. In this paper, a series of Er/Yb-codoped LN crystals were successfully grown by the Czochralski method, featuring a fixed Er3+ concentration with various Yb3+ doping levels and a fixed RE3+ (RE3+ = Er3+ + Yb3+) concentration. The absorption and emission spectra were measured, and the transfer efficiencies along with relative conversion efficiencies were also displayed. It was concluded that the optimal composition of codoped LN is ∼1.0 mol. % Er3+ and 1.0 mol. % Yb3+, which can enhance active devices by providing lower threshold power, reduced transmission loss, higher transfer efficiency, and improved conversion efficiency. These results will advance the development of high performance LNOI lasers and amplifiers.

High-performance 320-nm continuous-wave solid-state laser

For the first time, to our knowledge, we realized a high-performance 320-nm continuous-wave (CW) laser with a fiber-coupled blue laser diode as the pump source. A V-shaped folded cavity is constituted by three mirrors, with a Pr3+:LiYF4 (Pr:YLF) as the laser crystal and a LiB3O5 (LBO) as the frequency doubling crystal, respectively. Under an absorbed pump power of 19.5 W, the maximum 320-nm output power reaches 4.26 W, corresponding to an optical conversion efficiency of 21.8% and a root-mean-square (RMS) power fluctuation of 0.61%, which give the best results of a 320-nm CW laser up to now. The excellent performance will make this ultraviolet laser source very suitable for practical applications, including semiconductor detection, spectral analysis, microscopic imaging, and biotechnology.

Application of confocal laser scanning microscopy to investigation of micro crystals in transparent amorphous media: Photoluminescence tomography and spectroscopy of CdZnSSe crystallites in historical silicate glass

Recently, photoluminescence tomography based on the confocal laser scanning microscopy with two-photon excitation has been developed to study the distribution of point and extended defects in the bulk of ZnSe laser crystals. This article presents the use of the tomography to investigate luminescent micro inclusions in transparent amorphous media such as silicate glass. Studies of CdZnSSe crystals synthesized in a silicate glass melt have been carried out using the tomography with both two-photon and single-photon excitation of luminescence. Zn-rich glass manufactured in the 19th century has been found to contain micron-sized crystals of CdξZn1−ξSζSe1−ζ (ξ≈0.5, ζ≈0.5) of hexagonal crystal system exhibiting intense photoluminescence. The photoluminescence band of these crystals has been found to peak at about 2.1 eV (∼590nm) at 300 K. Minor shifts in the maximum of bands and changes in their shape in individual crystals or at tomogram points within the crystal are associated with variations in their composition. Changes in the photoluminescence band shape and maximum due to the excitation of luminescence in CdZnSSe crystallites by laser radiation of different energies have also been observed in this study.

Growth and spectral properties of Er3+/Yb3+/Ce3+:YPO4 crystal

Er3+/Yb3+/Ce3+ tir-doped YPO4 crystals were successfully synthesized by high temperature solution method with Pb2P2O7 as flux. The growth, structures and spectral characterizations, including polarization absorption spectra, polarization fluorescence spectra and lifetime, were systematically discussed. It has broad absorption band around 973 nm with a full width at half maximum of 20.76 nm and 14.71 nm for π- and σ-polarizations, respectively, which is suitable for efficient pumping of InGaAs laser diodes. Based on J-O theory, spectral parameters such as oscillator strength, line strength, J-O intensity, fluorescence branching ratio, spontaneous transition rate and radiation lifetime were calculated systematically. In the polarization fluorescence spectra, four strong emission peaks were observed at 1505, 1537, 1561 and 1586 nm. The results show that Er3+/Yb3+/Ce3+:YPO4 crystal may be a promising laser crystal suitable for 1.5–1.6 μm.

Structure–luminescence property relationships of Yb3+-Doped Y2Mg3-xCax(SiO4)3 single crystals for 0.95–1.2 μm broadband emissions

This study successfully grew Yb3+-doped Y2Mg3-xCax(SiO4)3 (x = 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75) (Yb: YCMS) novel single crystals with multiple disordered structures. To the best of our knowledge, this is the first systematic report on the relationship between the luminescence properties (including emission intensity, emission peak position, full width half maximum, and fluorescence lifetime) and the local structure of the luminescent ions based on first-principles calculations. The wide half-height width of emission (103.10 nm@1047 nm) have been realized on the first-grown Yb: YCMS crystals. This study provides a novel solution for designing laser crystals with tunable optical properties. The comprehensive properties of Yb3+-doped YCMS crystals, such as ultra-wide emission bandwidth and high fluorescence lifetime, are promising gain materials for ultrafast laser applications.

Incorporating Er:YAP Microcrystals Into Tellurite Fiber Using Volumetric Interface Doping

AbstractIn this semi‐quantitative study, laser crystal microparticles which are potentially capable of generating mid‐infrared (MIR) light are incorporated into MIR transmitting optical fibers using a volumetric interface doping technique that combines interface doping with volumetric doping. Using confocal microscopy with a refractive index matching fluid, the Er:YAP microcrystals (MCs) inside the tellurite glass fiber are observed based on the detection of the green upconversion fluorescence emission of Er3+ ions, produced under excitation at 976 nm and localized at the central region within the fiber. The key outcome from this proof‐of‐concept study is that MC particles that are fused with the glass during the fiber drawing process survived heat treatment because the MC particles are exposed for a short time to a glass fluid with high viscosity of ≈105 Pa.s, which prevented the glass from exerting a dissolution effect. The survival of the MCs demonstrated the viability of the doping technique for fabricating fibers with exotic crystal‐glass combinations for applications including good refractive index matching across the pump and lasing bands of rare earth ions. This latter parameter provides significant particle size flexibility whilst minimizing additional loss from scattering especially at MIR wavelengths where the MC diameter‐to‐wavelength ratio becomes smaller.

Growth of YAG:Nd laser crystals by Horizontal Directional Crystallization in Protective Carbon‐Containing Atmosphere

AbstractLaser‐quality yttrium‐aluminum garnet single crystals doped with neodymium (YAG:Nd) of a concentration up to 1 at. % is grown by the method of horizontal directional crystallization from a molybdenum crucible in the protective reducing atmosphere based on argon, СО, and hydrogen. It is found that the content of carbon impurity in the grown crystals does not exceed 5·10−3 wt %, the content of molybdenum being on the level of 1.5·10−3 wt %. The optical quality of the crystals depends on the composition of the growth atmosphere and annealing. It is shown that, besides the bands of neodymium ion absorption, the crystals are characterized by the intense absorption in the UV edge of the spectrum at 370 nm wavelength, and by the wide absorption band with a maximum at 580 nm caused by formation of F and F+‐centers. The absorption at 370 and 580 nm can be eliminated by annealing. The structure perfection of the crystals is characterized by the rocking curve half‐width (β) which value varies within the limits of 10–14 arc. sec for (001) plane. Laser testing demonstrates the parameters comparable with those of YAG:Nd crystals grown by the Czochralski method from iridium crucible.

Modern developments in lasing with liquid crystals

A review of the recent developments in the field of lasing with liquid crystals (LCs) is presented. After an introduction into the principle of lasing the different relevant liquid crystal phases to the field are introduced, namely, the nematic and chiral nematic phase, Blue Phases, twist grain boundary and ferroelectric liquid crystals. The classic examples of liquid crystal lasing are shortly discussed, together with a variety of possibilities for tuning the lasing wavelength, before the modern trends in LC lasing are discussed in detail. These are particularly random lasers, where the effects of nanoparticles, quantum dots and solitons are highlighted, as well as localized surface plasmon resonance. Other modern laser systems that have attracted recent interest, white lasers, whispering gallery mode lasers and those with biological materials, for example, cellulose nanocrystals, are also introduced and the latest developments outlined.

Directional growth and composition design of the highly disordered Yb3+ doped Ca(Gd1-xLux)AlO4 crystals for ultrafast lasers

The unique disordered structure of the CaGdAlO4 crystal, combined with its exceptional thermal and spectral properties, has contributed to its growing popularity in the field of ultrafast lasers. In this work, high quality Yb3+ doped CaGdAlO4 crystals were directionally grown by the rapid and cost-effective micro-pulling down method for the first time. The anisotropic analysis determined [100]-orientation as the optimal growth direction due to its superior spectral, thermal and continuous-wave laser properties compared to that of [001]-orientation. Based on the optimized [100]-orientation, two novel crystals, Yb3+:CaGd1-xLuxAlO4 (x = 0.05 and 0.1), were designed by incorporating Lu3+ into Gd3+ site. Benefiting from the enhanced structure disorder induced by Lu3+ co-doping, the Yb3+:CaGd1-xLuxAlO4 crystals demonstrated superior spectral and pulsed laser performance compared to that of Yb3+:CaGdAlO4, featuring a broader emission spectrum, narrower pulse duration and higher peak power. The utilization of anisotropy and composition design strategies effectively enhanced the spectral and pulsed laser properties, providing a feasible way for the optimization of compact and high-performance laser devices.

Sol-Gel Pt-VO2 Films as Selective Chemoresistive and Optical H2 Gas Sensors.

In this work, VO2 (M1/R) thin films were exploited as H2 gas sensors. A flat film morphology, obtained by furnace annealing, was compared with a laser-induced nanostructured one. The combination of the environmentally friendly sol-gel approach with the ultrafast laser crystallization allows for significant reductions in energy consumption and related emissions during the fabrication of VO2 sensors. By decorating the sensors' surface with Pt nanoparticles (NPs), the sensor response was enhanced exploiting the hydrogen spillover effect. The Pt/VO2 sensors, tested at operating temperatures between 20 and 200 °C and for concentration of H2 from few ppm to 50000 ppm, offered a dual chemoresistive and optical sensing mode. Low operating temperatures of 150 °C were achieved, along with a detection limit as low as 2 ppm and a perfect baseline recovery. Both sensors guaranteed specific selectivity toward H2, without response to NO2 or humidity, and long-term stability over 500 h. The H2 sensing mechanism, for both the monoclinic and rutile VO2 phases, was investigated through in operando X-ray Diffraction and in situ X-ray Photoelectron Spectroscopy tests. The interaction was found to be based on the reversible formation of HxVO2 bronze, along with the reversible variations in the oxidation state of V.

Astigmatic thermal lensing due to surface bulging in Yb:KYW laser crystals

One of the sources of thermal lensing in crystals is surface bulging that results from an inhomogeneous temperature distribution. We investigate a thermal lens caused by surface bulging in an end-pumped Yb:KYW crystal, which serves as a gain medium in lasers and optical amplifiers. The surface profile of the pumped crystal is measured using a Fizeau-type interferometer and compared with a numerical simulation using a finite element method. The study reveals that due to anisotropic thermal expansion, the surface shape of the Yb:KYW crystal is anisotropic and the profile of the expansion is transversely displaced with respect to the pump beam profile which generates the temperature distribution. The observed surface bulging gives rise to aberrations and deflection of the transmitted beam. It was found that the surface bulging introduces astigmatism that is significantly larger than previously estimated [Appl. Opt. 56, 3857 (2017)10.1364/AO.56.003857]. Our results allow the evaluation of the bulging contribution with improved accuracy. We show that these effects can be significant in certain designs of amplifiers and lasers.

Crystal growth, spectra and laser properties of Tm3+ and Ho3+ co-doped GdScO3 crystal

Tm,Ho:GdScO3 single crystal was grown by the Czochralski method. Its quality and effective segregation coefficient of crystal were characterized, and spectral properties of crystal (100) plane were studied. The absorption cross-section of Tm,Ho:GdScO3 at 793 nm is 1.61 × 10−20 cm2 and emission cross-section at 2020 nm is 1.39 × 10−20 cm2. The laser performance of the crystal by 793 nm LD end-pumped was investigated. The maximum output power of 240 mW was obtained with a slope efficiency of 9.1 % in CW(continuous wave) pump mode. In pulse mode, the maximum average output power of 507 mW was achieved. The maximum slope efficiency of 13.7 % was obtained with 100 Hz and 400 μs. The laser wavelength was 2096 nm. The beam quality factors Mx2/My2 were fitted to be 1.81/1.80. These results indicate that Tm,Ho:GdScO3 crystal is a promising candidate medium for near-infrared laser application.

Multi-stage and multi-colour liquid crystal reflections using a chiral triptycene photoswitchable dopant.

The photomodulation of the helical pitch of cholesteric liquid crystals results in dynamic and coloured canvases that can potentially be used in applications ranging from energy-efficient displays to colour filters, anti-counterfeiting tags and liquid crystal (LC) lasers. Here we report on the analysis of a series of photoswitchable chiral dopants that combine the large geometrical change and bistability of hydrazone switches with the efficient helical pitch induction of the chiral motif, triptycene. We elucidate the effects that conformational flexibility, dispersion forces and π-π interactions have on the chirality transfer ability of the dopant. We then use the irradiation time with visible light (442 nm) combined with a simple digital light processing microscope projection set-up to draw numerous stable multi-coloured images on an LC canvas, showcasing the fine control this dopant yields over the LC assembly.

Growth, first-principles calculations and optical properties of a novel near-infrared Ho: NLGW laser crystal

A rigid structure was constructed using an isotopic doping strategy, and a novel near-infrared 1 at% Ho3+: NaLa0.93Gd0.06(WO4)2 (Ho: NLGW) laser crystal with a size of Φ 18 mm × 30 mm was successfully grown by the Czochralski method. X-ray rocking curves and Rietveld refinement indicate that the grown crystal possesses high crystalline quality and crystallizes in the I41/a space group with lattice parameters: a = b = 5.3518 Å, c = 11.6438 Å and V = 333.5051 Å3. The Ho: NLGW crystal exhibits a low coefficient of thermal expansion (8.472 × 10−6 K−1), and the c-axis thermal conductivity increases from 1.235 W/(m × K) to 1.404 W/(m × K). The thermal properties of the Ho: NLGW crystal surpass those of other Ho3+-doped tungstate series and mainstream near-infrared laser crystals, suggesting that it is easier to obtain excellent laser gain. For revealing the excellent thermal properties, the elastic moduli of the NLW and NLGW crystals have been calculated by first principles. With the introduction of Gd3+, the structural stiffness of the NLW as a whole has been increased to optimize the mechanical properties in each axial direction and to obtain the reason for the larger Debye temperature. Finally, the transmission, absorption and near-infrared emission spectra of the Ho: NLGW crystal were investigated. The crystal exhibits a high transmittance of 88.5 % and an emission cross-section of 0.84 × 10−20 cm2 calculated according to the J-O theory. Experimental results indicate that the grown Ho: NLGW crystal can achieve large laser output more easily and are expected to be an excellent medium for 2 μm solid-state lasers.

Dynamic Transition from Branched Flow of Light to Beam Steering in Disordered Nematic Liquid Crystal (Laser Photonics Rev. 18(10)/2024)

Dynamic Transition from Branched Flow of Light to Beam Steering in Disordered Nematic Liquid Crystal (Laser Photonics Rev. 18(10)/2024)

Enhanced cone-shaped lasing from cholesteric liquid crystals.

Strong conical lasing in the violet-blue range of the spectrum from dye doped cholesteric liquid crystals is obtained. In contrast to the previously reported cases, the cone laser emission is sharply increased at the local maximums of the luminescence peaks of the dyes. Besides that, for obtaining the cone lasing strong focusing of the pumping beam is not required. Extremely low pump density is achieved for the cholesteric liquid crystal lasers. It is important that the cone lasing can be observed without simultaneous manifestation of any other laser emissions.

Growth, optical and thermal properties of YbxSm1-xCa4O(BO3)3 crystals

YbxSm1-xCOB (x = 0.1, 0.2) crystals were grown by the Bridgman method for the first time. The purpose of this paper is to evaluate the application prospect of Yb:SmCOB crystal in quasi-parametric chirped pulse amplification (QPCPA) and frequency-doubling laser. The phase structure, thermal properties and optical properties of Yb:SmCOB were studied, and the density of states of SmCOB crystal was calculated by first-principles. The effective segregation coefficient Keff of Yb0.1Sm0.9COB and Yb0.2Sm0.8COB crystals are 0.89 and 0.88, respectively. The thermal diffusivity and thermal conductivity of Yb:SmCOB decrease with increasing temperature. The specific heat increases with the increase of temperature and eventually tends to be constant. The specific heat of Yb:SmCOB crystal is greater than 0.70 J/(g·K) at room temperature. The transmittance of Yb:SmCOB crystal reaches 87 % in the range of 500 ∼ 900 nm. With the increase of Yb3+ doping concentration, the UV absorption cut-off edge is red shifted. The frequency doubling emission peak of Yb:SmCOB crystal at 490 nm was measured by 980 nm laser. Yb:SmCOB has the characteristics of high transmittance and high specific heat, and has application potential in laser frequency doubling and QPCPA.

Development and Validation of a One‐Dimensional Finite Difference Simulation Scheme for Polymer Laser Powder Bed Fusion with Application to the Effect of the Inter Layer Time

The division of the polymer laser powder bed fusion process polymers (PBF‐LB/P) into temporal regimes originating from laser motion, thermal diffusion, viscous flow, crystallization kinetics, and powder application is exploited by considering only the building direction. The reduction of dimensionality enables fast simulations which are used to investigate the influence of the inter layer time on the final part density. Numerical simulation results are validated by experiments using polyamide 12 (PA 12) with good agreement in terms of part density. It is shown that an inter layer time of 90 s leads to nearly dense PA 12 parts while a time of 45 s leads to less dense parts. The cooling effect of the applied powder is identified as a cause for insufficient densification of the previous layer. The simulation tool is quantitatively validated against experimental results for the surface temperature of PA 12 as function of scan speed and hatch distance, for the melt pool depth of polyamide 6 as function of scan speed and for the melt pool depth of polyetherketoneketone as function of laser power. The presented simulation method enables rapid process parameter adjustment for new polymer materials in the PBF‐LB/P process.

Characterization and modeling of laser transmission welded polyetherketoneketone (PEKK) joints: Influence of process parameters and annealing on weld properties

Welding high-performance thermoplastics has gained popularity across various industries such as automotive, aerospace, and medical. Laser transmission welding (LTW) has emerged as an effective method for joining thermoplastic parts due to its precise control and high joint quality. PAEK (polyaryletherketone) are wide spreading over various industrial applications as a substitute to metals and thermosets when high durability and performance are required. Polyetherketoneketone (PEKK) is one of these PAEK and it has received less attention than PEEK until now. PEKK, being a semi-crystalline thermoplastic, requires additional care during processing due to its propensity to crystallize. This study presents both experimental and numerical investigations into LTW of PEKK molded parts, aiming to understand the influence of welding parameters and crystallinity on weld joint morphology and mechanical properties. PEKK plates, prepared in amorphous and semi-crystalline states, are subjected to LTW using a 975 nm diode laser. Material characterization confirms differences in crystallinity between the samples, which affect their thermal and optical properties, which are crucial for welding. Welding tests are conducted with varying laser power (between 75 and 95 W) and semi-transparent part thickness (2 and 4 mm). The morphology of joints is analysed. Assemblies undergo post-weld annealing treatment to examine its influence on weld crystallinity and consequent mechanical properties. Results reveal an anisotropic distribution of crystallinity within the heat-affected zone (HAZ). The depths of the molten layer (ML) and semi-crystalline layer (scL) vary with laser power and assembly type. A notable decrease in weld strength with laser power is highlighted, while annealing leads to enhanced crystallinity and improved weld strength. Despite variations, high weld strengths are achieved with annealing. Computational modelling elucidates the complex interplay between laser irradiation, temperature distribution, and crystallization kinetics observed experimentally. Overall, this comprehensive investigation provides valuable insights into optimizing LTW parameters for PEKK parts.

Fabrication of Circular Defects in 2-Dimensional Photonic Crystal Lasers with Convex Edge Structure

We have developed circular defects in 2-dimensional photonic crystal lasers that allow current injection for interconnected optical communications. However, when cleaving the sample to measure the output light, the output light intensity changes due to the cleaving position. In a previous study, we proposed a new end face structure called a convex edge structure. In this paper, we design the electron beam lithography patterns to fabricate this structure. With this design, it is possible to eliminate the effect of different cleaving positions and ensure that the cleavage tolerance is larger than the cleavage position error. We also develop the fabrication technology for this structure, fabricate samples, and measure the output light experimentally. The optical properties of the fabricated sample are similar to well-fabricated samples with normal cleavage edge faces. We are assured that these results contribute to future work such as accurate manufacturing and improving the end face configuration to obtain higher outputs.