Laser Gain Materials Research Articles

Enhanced blue and broadband near-infrared emission of Pr3+/Yb3+ co-doped PbF2 single crystal

High quality Pr3+ single-doped PbF2 and Pr3+/Yb3+ co-doped PbF2 single crystals were grown by temperature gradient technique (TGT). By co-doping Yb3+ ion as Pr3+ ions sensitizer, the visible light emission can be regulated and the largest luminescence branching ratio can be switched from red-orange to blue-green light. Besides, when Pr3+ ions doping concentration is as low as possible, the blue light self-absorption effect can be weakened, which is beneficial to high-efficiency blue laser output. Moreover, the broad band emission around 1.3 μm from 1G4 energy level can be observed in the Pr3+/Yb3+:PbF2 crystal with the low maximum phonon energy for the first time. Absorption, emission spectra and luminescence mechanism are well analyzed by the energy transfer process and J-O theory. The laser upper energy level lifetime of Pr3+ ions has a great increase by Yb3+ ion effect. These advantageous spectroscopic characteristics show that the Pr3+/Yb3+:PbF2 crystal might be potential laser gain materials for blue-green and ∼1.3 μm near-IR laser operation.

SESAM mode-locked Tm:Y2O3 ceramic laser.

We demonstrate a widely tunable and passively mode-locked Tm:Y2O3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y2O3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y2O3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

Structure, and luminescence properties in Tm3+/Yb3+ co-doped bismuth-tellurite glass for 2 μm fiber lasers

Tm3+/Yb3+ co-doped TeO2-Bi2O3-B2O3-BaF2 glasses were synthesized by the melt-quenching method. The Raman and FT-IR spectra were deconvoluted by the Gaussian decomposition method to determine the molecule vibration glass network. And the luminescence properties at 2 μm were systematically analyzed under 980 nm laser diodes (LD) pumped according to J-O theory. Furthermore, the spectroscopic parameters, absorption, emission cross-sections, and gain coefficient of Tm3+ in TBBB3 were calculated by absorbance and fluorescence spectra. Eventually, the energy transfer process was quantitatively characterized by using a spectral overlap method. The maximum emission cross-section of Tm3+ is 9.79 × 10−21 cm2 (at 1857 nm) when the doped concentration ratio of Tm3+/Yb3+ is 0.5: 2 (mol%). Meanwhile, the backward energy transfer coefficient (CA-D) is much smaller than the forward (CD-A), and the energy transfer rate WET is 1691.5 × 10−20 cm3/s. All these results indicate that the glass presented is an ideal laser gain material at 2 μm.

Enhanced infrared luminescence of multifunctional-nanoparticle-composited transparent solids

Amorphous transparent solid glass doped with functional nanoparticles may have considerable potential in military and civilian fields, for the exploration of highly efficient laser materials. Secondary melting technology and spontaneous precipitation of precious metals were employed to prepare dual-phase nanocrystalline glass ceramics containing Ag nanoparticles and Gd2O3:Er, and the prepared ceramics exhibited high optical transmittance. The microscopic morphologies and possible structural changes of the embedded nanoparticles were confirmed by using Raman spectroscopy and transmission electron microscopy. Based on the results of the fluorescence spectra and lifetime, the source and possible causes of luminescence enhancement were explained. The crystal environment of Gd2O3 and the characteristics of Ag nanoparticles resulted in a substantial increase in infrared emission, indicating that this composite is expected to be a potentially efficient laser gain material.

High transparency Pr:Y2O3 ceramics: A promising gain medium for red emission solid-state lasers

Highly transparent 0.5 and 1.0 at% Pr-doped Y2O3 ceramics were fabricated by vacuum sintering plus hot isostatic pressing (HIP) treatment. The selection of suitable pre-sintering temperatures and right microstructures before HIP was critical to obtain high density of the final sintered bodies. The well-densified ceramics had pore-free microstructures with an average grain size of about 1 µm. It was also found that the charge states of the Pr ions could be changed through regulating the annealing atmospheres, resulting in different absorption and emission characteristics in the visible wavelength region. Annealing in reducing atmosphere (5% H2/95% Ar) favored the formation of Pr3+, resulting in stronger red emissions, while annealing in oxygen atmosphere led to the rise of lattice constant due to the concentration increase of oxygen interstitials. The H2/Ar-annealed 0.5 at% Pr:Y2O3 ceramics exhibited strong red emission at 600–675 nm, which may be a promising gain material for red solid-state lasers.

Bridgman growth and characterization of Dy:YCa4O(BO3)3 crystal

A 1-inch Dy3+:YCa4O(BO3)3 single crystal has been grown by the Bridgman method. The segregation coefficient of Dy3+ is 0.784. The structures, thermal and optical properties of Dy3+:YCOB crystal was studied. The results show that the Dy3+:YCOB crystal has high transmittance, specific heat and thermal diffusion coefficient. The Y-axis-oriented Dy3+:YCOB crystal has the largest absorption cross-section σabs(0.85×10−21cm2@444nm) and emission cross-section σem(2.54×10–21cm2@585nm). In addition, the J-O intensity parameters Ωt(t=2,4,6) of the Dy3+:YCOB crystal were calculated, where Ω2, Ω4, Ω6 are 5.01×10−20cm2, 0.33×10−20cm2, 0.52×10−20cm2 respectively. The fluorescence branching ratio β is 78.7%, and the radiative lifetime τrad is 1.588 ms. The fluorescence lifetime τf is 384.47 μs at 585 nm. The above data confirms that the Y-axis-oriented Dy3+:YCOB crystal is a good yellow laser gain material, which has potential application value in the field of yellow laser operation.

Ultra‐Low Threshold Titanium‐Doped Sapphire Whispering‐Gallery Laser

AbstractTitanium doped sapphire (Ti:sapphire) is a laser gain material with broad gain bandwidth benefiting from the material stability of sapphire. These favorable characteristics of Ti:sapphire have given rise to femtosecond lasers and optical frequency combs. Shaping a single Ti:sapphire crystal into a millimeter sized high quality (Q) whispering gallery mode resonator (Q ≈ 108) reduces the lasing threshold to 14.2 mW and increases the laser slope efficiency to 34%. The observed lasing can be both multi‐mode and single‐mode. This is the first demonstration of a Ti:sapphire whispering‐gallery laser. Furthermore, a novel method of evaluating the gain in Ti:sapphire in the near infrared region is demonstrated by introducing a probe laser with a central wavelength of 795 nm. This method results in decreasing linewidth of the modes excited with the probe laser, consequently increasing their Q. These findings open avenues for the usage of whispering gallery mode resonators as cavities for the implementation of compact Ti:sapphire lasers. Moreover, Ti:sapphire whispering‐gallery laser can also be utilized as an amplifier inside its gain bandwidth by implementing a pump–probe configuration.

Formation energy and optical excitation mechanisms of Er in GaN semi-bulk crystals

Erbium (Er) doped GaN (Er:GaN) bulk crystals are promising as an optical gain material for high energy lasers. Other than the preferred configuration of Er3+ occupying the Ga site with four nearest neighbor N atoms (ErGa), Er can also form a complex with a defect (ErGa-DX) in Er:GaN. A set of Er:GaN semi-bulk crystals were grown by hydride vapor phase epitaxy (HVPE) at different growth temperatures to allow the determination of the formation energies, EF, of Er optical centers in Er:GaN. Photoluminescence (PL) emission spectra near 1.5 μm pumped by a near band edge excitation (λexc = 375 nm) and by a resonant excitation (λexc = 980 nm) were measured, which yielded two different formation energies of EF = 2.8 eV and 3.3 eV, corresponding to the formation of ErGa-DX and ErGa in Er:GaN, respectively. As a gain medium, the formation of ErGa-DX in Er:GaN would not only reduce the pumping efficiency of Er optical centers but also increase the optical loss. Our results provide useful insights into possible strategies for enhancing the fraction of ErGa in Er:GaN and, hence, the pumping efficiency, paving the way for achieving optical gain and lasing in Er:GaN.

Diode-pumped Dy[formula omitted], Tb[formula omitted]:LuLiF[formula omitted] continuous-wave and passively Q-switched yellow lasers

We report on the power-scaling continuous-wave (cw) Dy3+, Tb3+:LuLiF 4 yellow laser, which comprises a single-emitter blue laser diode (LD) that functions as a pump source. The maximum output power of the laser is evaluated to be 220 mW at 574 nm. The passively Q-switched yellow laser is generated by using Cr4＋:Y3Al5O12 (Cr:YAG) as the saturable absorber. The shortest pulse width (full width at half maximum) is less than 582 ns with a repetition rate of 5.8 kHz. Meanwhile, the pulse energy and peak power are ∼4.5 μJ and ∼7.7 W, respectively. This work reveals that Dy3+, Tb3+:LuLiF 4 crystal is a potential laser gain material that can be used to develop compact and efficient yellow lasers, which can not only be employed in cw lasers but also in passively Q-switched pulse lasers.

Thermal conductivity and management in laser gain materials: A nano/microstructural perspective

Heat generation and thermally induced failure has been a major challenge for high-power applications in solid state lasers. Improvements in the solid state laser ceramic fabrication process offer improved mechanical toughness and comparable thermal conductivity compared to single crystal counterparts. Equally enticing is the possibility of using materials with intrinsically superior thermal/mechanical properties that are not viable in the single crystal form. Here, we review the nano/microstructural effects on optical, thermal, and mechanical properties of polycrystalline ceramics, recent developments in a variety of commonly used crystalline laser materials, and potential future directions for more robust laser gain materials for high-power applications. It is argued that the engineering microstructure with both optical and thermal performances in mind might offer breakthrough improvements in laser gain media.

Bulk-like emission in the visible spectrum of colloidal LiYF4:Pr nanocrystals downsized to 10 nm.

Though Pr3+ doped LiYF4 (LiYF4:Pr3+) bulk crystals are a well-known laser gain material with several radiative transitions, their nanocrystal counterparts have not been investigated with regards to these. Through downsizing to the nanoscale, novel applications are expected, especially in composite photonic devices. For example, nanocrystals in stable colloidal form with narrow size distribution are highly desirable to reduce scattering in such composites. Herein, we synthesized monodispersed LiYF4:Pr3+ nanocrystals having a size of 10 nm resulting in colorless clear stable colloidal dispersions and conducted an extensive optical characterization for the first time. We observed unexpected yet intense emission with excited state lifetimes comparable to bulk crystals in the visible spectrum through excitation at 444 nm and 479 nm. In macroscopic bulk crystals, this emission is only exploitable through excitation of a different, subjacent energy level. A comprehensive comparison to the bulk crystals provides deeper insight into the excitation mechanism and performance of these nanocrystals. The presented results pave the way for developing application-oriented LiYF4:Pr3+ nanocrystals as emitters with tailored properties for quantum optics or biomedical applications.

Effective sensitization of Yb3+ ions on Yb3+/Nd3+ co-doped fluoroborate glasses for NIR luminescence applications

Nd3+/Yb3+ co-doped fluoroborate glass samples were fabricated by melt quenching. The near infrared luminescence (NIR) spectral features of Nd3+/Yb3+ co-doped fluoroborate glasses were characterized by XRD, SEM, FT-IR, optical absorption, photoluminescence (PL) and PL decay spectral studies. An effective energy transfer process has been observed from Nd3+ to Yb3+ in the titled glasses under 808 nm excitation wavelength. Broad emission ranging from 850 to 1400 nm due to both Nd3+ (4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→ 4I13/2) and Yb3+ (2F5/2→2F7/2) transitions has been observed in all the glasses. The dependence of NIR emission, decay lifetime, and the energy transfer efficiency (ηETE) with the concentration of Yb3+ was studied in detailed. The PL spectra along with donor decay curves have been used to establish the energy transfer mechanism between Nd3+/Yb3+ ions. The results represent the possible sensitization of Yb3+ through 4f-4f transition of Nd3+ ions in titled glasses. The PL of the Nd3+ singly doped and Nd3+/Yb3+ co-doped titled glasses indicate their suitability as multiple pump channel sources for Yb3+ fiber laser systems. The results finally prove that the Nd3+/Yb3+ co-doped fluoroborate titled glasses are quite useful to fabricate infrared laser gain materials.

Active Q-switching operation of a Tm:SrF2 single crystal fiber laser near 2 µm

Single crystal fiber (SCF) lasers have attracted extensive attention recently. In this paper, Tm:SrF2 SCF was successfully grown by the temperature gradient method. With a compact resonator, the acousto-optically Q-switched properties of a diode-pumped Tm:SrF2 SCF laser was investigated first. At the repetition rate of 500 Hz, a highest pulse energy of 1.32 mJ and a narrowest pulse width of 51 ns were acquired with the absorbed pump power of 2.60 W, resulting in the peak power of 25.85 kW. These results indicate that Tm:SrF2 SCF is a promising laser gain material for generating ∼2 µm region pulsed lasers.

Hot-pressed Fe2+:ZnSe ceramics with powders fabricated via grinding chemical vapor deposition ZnSe polycrystalline

In this work, Fe2+:ZnSe powders were prepared by grinding a mixture consisting of high-purity ZnSe obtained via chemical vapor deposition and thermally diffused Fe2+:ZnSe. In this manner, it was possible to avoid the presence of harmful H2Se gases and achieve homogeneous distribution of Fe2+ ions in the synthesis of Fe2+:ZnSe powders. Fe2+:ZnSe transparent ceramics were fabricated by hot-pressing as-obtained powders. Phase structure and microstructural properties of the powders and ceramics were studied via X-ray diffraction and field-emission scanning electron microscopy. As-prepared Fe2+:ZnSe ceramics exhibited dense microstructure with relative density higher than 99%, but few residual micropores were found to be localized at grain boundaries. 1.0-mm-thick Fe2+:ZnSe ceramic sintered at 900 °C showed the best optical transmission, namely ∼63% at 5 µm and ∼69% at 14 µm. Additionally, hot-pressed Fe2+:ZnSe ceramics displayed an absorption peak at around 3 µm. These Fe2+:ZnSe ceramics are promising candidates as mid-infrared laser gain materials.

1.53 μm luminescent properties and energy transfer processes of Er3+/Yb3+ co-doped bismuth germanate glass laser material

In this work, Er3+/Yb3+ co-doped 40Bi2O3-30GeO2-15Ga2O3-15Na2O glass was papered by melt-quenching method. The thermal properties and spectroscopic were studied. The glass has good thermal stability and the maximum phonon energy is 750 cm−1, which is benefited for 1.53 μm emission. According to the J-O theory and absorption spectra, the J-O intensity parameters were calculated, the spectral quality factor χ = Ω4/Ω6 of BGGN4 glass is 2.236. Under 980 nm excitation, the 1.53 μm luminescence characteristics of these glasses were studied. When the Er3+/Yb3+ ions doping concentration ratio (mol%) reaches 0.5:1.0, the maximum fluorescence intensity can be obtained, and the maximum emission cross section is 6.4 × 10−21cm2. The maximum stimulated gain coefficient in the 1.53 μm band is 4.6 cm−1. Finally, the up-conversion emission and energy transition mechanism of Er3+/Yb3+ co-doped BGGN glass were studied. The results indicate the prepared Er3+/Yb3+ co-doped bismuth germanate glass have good gain performance and is an ideal 1.53 μm laser gain material.

2.5–5.5 μm mid‐infrared emission from Ni2+‐doped chalcohalide glass ceramics containing CsPbI3 perovskite nanocrystals

AbstractThe development of mid‐infrared (MIR) broadband tunable lasers urgently needs high performance laser gain materials. Transition metal (TM) ions doped glass ceramics are considered to be efficient MIR broadband laser gain media. However, it is difficult to achieve gain because of the large scattering loss and low luminescence efficiency. In this paper, GeS2–Sb2S3–CsI–PbI2 chalcohalide glass ceramics containing CsPbI3 perovskite nanocrystals are fabricated by the melt‐quenching method and subsequent heating treatment. The crystallization behavior of CsPbI3 nanophase and MIR luminescence properties of Ni2+ dopant are systematically investigated. Evidently, spherical CsPbI3 perovskite nanocrystals are precipitated and uniformly distributed in the glassy matrix, which can reduce the light scattering and make the chalcohalide glass ceramics have a high transparency. Moreover, an ultra‐broadband MIR emission in the range of 2.5–5.5 μm is observed for the first time, to our best knowledge, from Ni2+‐doped chalcohalide glass ceramics containing CsPbI3 perovskite nanocrystals. The newly developed Ni2+‐doped chalcohaldie glass ceramics could be promising gain media for MIR broadband tunable lasers.

Excellent performance of a cryogenic Nd:YAlO3 laser with low wavefront distortion based on zero thermal expansion.

High-power solid-state lasers with good beam quality are attracting great attention on account of their important applications in industry and military. However, the thermal effects generated in the laser host materials seriously limit power scaling and degrade the beam quality. Thermal lensing and thermally induced wavefront deformation are the main causes of the beam quality deterioration. Here we investigate the performance of a zero thermal expansion (ZTE) solid-state laser gain material. In a proof-of-principle experiment, an ${a}$-cut rod ${\rm Nd}\!:\!{{\rm YAlO}_3}$ (Nd:YAP) perovskite crystal is chosen to be the gain medium for ZTE around 180 K. The laser performance spanning the temperature range from 80 to 290 K is studied. The maximum output power and minimum threshold pump power were obtained at a temperature of 180 K. Moreover, the measured thermal focal power and peak-to-valley value of the wavefront distortion also reach a minimum at this temperature, an additional benefit from the crystal's ZTE coefficient. We envisage that these results will open a new route towards the development of high-power and high-beam-quality lasers through the use of ZTE gain materials.

Absorption and multi-phonon quenching in nanocrystal doped SiO2 fibers

SiO2 fibers doped with rare-earth-doped nanocrystals are promising to overcome the strong SiO2 multi-phonon quenching and could yield novel laser gain materials. So far, no attention has been paid to the question how well the nanocrystals can suppress the quenching depending on the properties of the SiO2 host glass. Here, a novel analytical model was applied to study the impact of the glass purity and composition on the quenching efficiency. Only a few experimentally accessible material and design parameters are required to compute the average quenching rate inside the nanocrystals. It is demonstrated that sufficiently low levels of quenching can only be expected for SiO2 free of impurities or dopants that increase the multi-phonon absorption. This indicates that high-purity aluminosilicate glasses, in contrast to phosphosilicate and borosilicate glasses, are ideal hosts.

Nd3+/Yb3+ co-doped mid-infrared luminescence fluorobromide glass with energy transfer and zero-thermal-quenching IR emission

Nd3+/Yb3+ co-doped fluorobromide glass samples were prepared by melt quenching. The mid-infrared (MIR) luminescence of the Nd3+/Yb3+ co-doped fluorobromide glass was investigated by Br-doping reduces the phonon state density of the matrix. The 3.9 μm MIR luminescence of the samples excited at 793 and 980 nm pump excitation was investigated in detail. There is an effective mutual energy transfer process between Nd3+ and Yb3+. It is proved under 793 nm excitation that the luminescence of Nd3+ at 3.9 μm is reduced by effective energy transfer from Nd3+:2H11/2 → Yb3+:2F5/2. At the same time, it is proved that the effective energy transfer from Yb3+:2F5/2 → Nd3+:2H11/2 under the excitation of 980 nm enhances the luminescence of Nd3+ at 3.9 μm. In addition, it is found that the samples still have good infrared (IR) luminescent properties when the temperature changes. The emission cross-sectional area and the absorption cross-sectional area are σem (3.87 × 10−20 cm2) and σabs (4.25 × 10−20 cm2). The fluorescence decay characteristics of the sample at 3.9 μm at the 2H11/2 level were investigated and the fluorescence lifetime was calculated. The gain performance of the sample was calculated and analyzed, which can reach 4.25 × 10−20 cm2. Those results prove that Nd3+/Yb3+ co-doped fluorobromide glass is the potential mid-infrared laser gain material.

Non-resonant power-efficient directional Nd:YAG ceramic laser using a scattering cavity

Non-resonant lasers exhibit the potential for stable and consistent narrowband light sources. Furthermore, non-resonant lasers do not require well-defined optics, and thus has considerably diversified the available types of laser gain materials including powders, films, and turbid ceramics. Despite these intrinsic advantages, the practical applications of non-resonant lasers have been limited so far, mainly because of their low power efficiency and omnidirectional emission. To overcome these limitations, here we propose a light trap design for non-resonant lasers based on a spherical scattering cavity with a small entrance. Using a porous Nd3+:YAG ceramic, directional laser emission could be observed with significant enhancements in the slope efficiency and linewidth (down to 32 pm). A theoretical model is also developed to describe and predict the operation characteristics of proposed non-resonant laser.