Calcium Lithium Niobium Gallium Garnet Research Articles

Spectroscopy and 2.8 µm laser operation of disordered Er:CLNGG crystals.

We report on the first, to the best of our knowledge, laser operation on the 4I11/2 → 4I13/2 transition of erbium-doped disordered calcium lithium niobium gallium garnet (CLNGG) crystals with broadband mid-infrared emission properties. A 41.4 at.% Er:CLNGG continuous-wave laser generated 292 mW at 2.80 µm with 23.3% slope efficiency and a laser threshold of 209 mW. Er3+ ions in CLNGG feature inhomogeneously broadened spectral bands (σSE = 1.79 × 10-21 cm2 at 2.79 µm; emission bandwidth, 27.5 nm), a large luminescence branching ratio for the 4I11/2 → 4I13/2 transition of 17.9%, and a favorable ratio of the 4I11/2 and 4I13/2 lifetimes, exhibiting values of 0.34 ms and 1.17 ms (for 41.4 at.% Er3+), respectively.

Kerr-lens mode-locking of an Yb:CLNGG laser.

We report on a Kerr-lens mode-locked laser based on an Yb3+-doped disordered calcium lithium niobium gallium garnet (Yb:CLNGG) crystal. Pumping by a spatially single-mode Yb fiber laser at 976 nm, the Yb:CLNGG laser delivers soliton pulses as short as 31 fs at 1056.8 nm with an average output power of 66 mW and a pulse repetition rate of ∼77.6 MHz via soft-aperture Kerr-lens mode-locking. The maximum output power of the Kerr-lens mode-locked laser amounted to 203 mW for slightly longer pulses of 37 fs at an absorbed pump power of 0.74 W, which corresponds to a peak power of 62.2 kW and an optical efficiency of 20.3%.

SWCNT-SA mode-locked Tm,Ho:LCLNGG laser.

Sub-100 fs pulse generation from a passively mode-locked Tm,Ho-codoped cubic multicomponent disordered garnet laser at ∼2 µm is demonstrated. A single-walled carbon nanotube saturable absorber is implemented to initiate and stabilize the soliton mode-locking. The Tm,Ho:LCLNGG (lanthanum calcium lithium niobium gallium garnet) laser generated pulses as short as 63 fs at a central wavelength of 2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ∼102.5 MHz. Higher average output power of 121 mW was obtained at the expense of longer pulse duration (96 fs) at 2067.6 nm using higher output coupling. To the best of our knowledge, this is the first report on mode-locked operation of the Tm,Ho:LCLNGG crystal.

Evaluation of Growth, Thermal, and Spectroscopic Properties of Er3+-Doped CLNGG Crystals for Use in 2.7 μm Laser

A series of optical-quality Er3+-doped calcium lithium niobium gallium garnet (CLNGG) single crystals with different Er3+ ion concentration (10, 15 and 30 at.%) has been grown by the Czochralski method. A comparative study of their structure, thermal, and spectroscopic properties is performed. Crystal structure was analyzed with X-ray powder diffraction (XRPD) and refined by the Rietveld method, results showing that the Er:CLNGG crystal possesses a cubic structure with space group Ia3¯d, and the lattice constants decrease linearly as the Er3+ concentration increase. The complete set of thermal properties were systematically studied for the first time. It has been found that all the thermal conductivities increase with temperature, indicating a glass-like behavior. Effect of Er3+ concentration on spectroscopic properties of Er:CLNGG crystals was studied. Results show that with the Er3+ concentration increase, the NIR fluorescence around 1600 nm weakens, while the Mid-IR fluorescence intensity around 2700 nm strengthens. Fluorescence lifetime of 4I13/2 decreased faster than that of 4I11/2 with the Er3+ concentration increase, which is beneficial for surmounting the “bottleneck” effect to achieve 2.7 μm laser. All the results show that CLNGG crystal with high Er3+ concentration is a potential candidate for the 2.7 μm laser.

Optical properties of Sm3+ doped Ca3(Nb,Ga)5O12 and Ca3(Li,Nb,Ga)5O12 single crystals

Sm3+ doped partially disordered calcium-niobium-gallium-garnet (CNGG) and calcium-lithium-niobium-gallium-garnet (CLNGG) single crystals were grown by the Czochralski method. Spectroscopic characteristics of Sm:CNGG and Sm:CLNGG single crystals were obtained by high-resolution absorption and emission spectra and emission dynamics measurements. Partial energy levels of Sm3+ doped CNGG and CLNGG were extracted from the measured spectra. The Judd-Ofelt theory has been applied to evaluate the Ωt (t=2, 4, 6) intensity parameters, radiative transition rates Ar, branching ratios β, and radiative lifetime τr of the fluorescent Sm3+ 4G5/2 level. The emission cross-sections for the 4G5/2→6HJ (J=5/2, 7/2, 9/2) transitions of special interest for visible laser application were obtained by the Fuchtbauer - Ladenburg equation. The interaction between Sm3+ ions is dipole–dipole type and the CDA microparameter for both crystals was calculated based on Inokuti-Hirayama model.

Passive Q-switching and Q-switched mode-locking operations of 2 μm Tm:CLNGG laser with MoS_2 saturable absorber mirror

With MoS2 as saturable absorber, passive Q-switching and Q-switched mode-locking operations of a Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) laser were experimentally demonstrated. The Q-switched laser emitted a maximum average output power of 62 mW and highest pulse energy of 0.72 μJ. Q-switched mode locking was also obtained in the experiment. The research results will open up applications of MoS2 at the mid-infrared wavelength.

Excited-state absorption in erbium-doped calcium lithium niobium gallium garnet

Excited-state absorption (ESA) spectra of Er3+ in the partially disordered calcium lithium niobium gallium garnet (CLNGG), recorded in the spectral range 400–900nm, are presented and discussed. The use of a pump wavelength of 973nm enables a complete separation of the ESA spectra from ground-state absorption (GSA) and stimulated emission. For the identification of various ESA transitions, the Er3+ energy levels above S3/24 in CLNGG are determined using GSA spectra at low temperature. Excitation spectra for the population of (S3/24,H211/22) levels by two-step sequential absorption with I13/24 as an intermediate level are generated using the recorded ESA and GSA spectra.

Judd–Ofelt analysis of Er3+ ions in calcium lithium niobium gallium garnet

The Judd–Ofelt parameters of Er3+ in the partially disordered calcium lithium niobium gallium garnet are calculated using absorption spectra. The results (Ω2=5.58×10−20cm2; Ω4=1.21×10−20cm2; Ω6=0.92×10−20cm2) are used to calculate the radiative lifetimes of the Er3+ energy levels up to 4G11/2 and the branching ratios of the radiative transitions originating on them. The luminescence lifetimes of Er3+ excited levels 4I13/2, 4I11/2, 4F9/2, (4S3/2, 2H211/2) and 2H29/2 are also presented.

Crystal field disorder effects in the optical spectra of Nd3+ and Yb3+-doped calcium lithium niobium gallium garnets laser crystals and ceramics

The optical spectroscopic properties of RE3+ (Nd, 1 at. % or Yb, 1 to 10 at. %)-doped calcium-lithium-niobium-gallium garnet (CLNGG) single crystals and ceramics in the 10 K–300 K range are analyzed. In these compositionally disordered materials, RE3+ substitute Ca2+ in dodecahedral sites and the charge compensation is accomplished by adjusting the proportion of Li+, Nb5+, and Ga3+ to the doping concentration. The crystals and ceramics show similar optical spectra, with broad and structured (especially at low temperatures) bands whose shape depends on temperature and doping concentration. At 10 K, the Nd3+ 4I9/2 → 4F3/2, 5/2 and Yb3+ 2F7/2 → 2F5/2 absorption bands, which show prospect for diode laser pumping, can be decomposed in several lines that can be attributed to centers with large differences in the crystal field. The positions of these components are the same, but the relative intensity depends on the doping concentration and two main centers dominate the spectra. Non-selective excitation evidences broad emission bands, of prospect for short-pulse laser emission, whereas the selective excitation reveals the particular emission spectra of the various centers. The modeling reveals that the nonequivalent centers correspond to RE3+ ions with different cationic combinations in the nearest octahedral and tetrahedral coordination spheres, and the most abundant two centers have 4Nb and, respectively, 3Nb1Li in the nearest octahedral sphere. At 300 K, the spectral resolution is lost. It is then inferred that the observed optical bands are envelopes of the spectra of various structural centers, whose resolution is determined by the relative contribution of the temperature-dependent homogeneous broadening and the effects of crystal field disordering (multicenter structure, inhomogeneous broadening). The relevance of spectroscopic properties for selection of pumping conditions and of laser design that would enable utilization of the broad optical bands for efficient laser emission and reduced heat generation is discussed.

Diode-pumped mode-locked femtosecond Tm:CLNGG disordered crystal laser

A diode-end-pumped passively mode-locked femtosecond Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) disordered crystal laser was demonstrated for the first time to our knowledge. With a 790 nm laser diode pumping, stable CW mode-locking operation was obtained by using a semiconductor saturable absorber mirror. The disordered crystal laser generated mode-locked pulses as short as 479 fs, with an average output power of 288 mW, and repetition rate of 99 MHz in 2 μm spectral region.

Synthesis and Characterization of Erbium Lithium Niobium Gallium Garnet

The synthesis of a new garnet, Er3Li0.275Nb0.275Ga4.45O12 (ErLNGG), by solid-state reaction, is reported. A comparison of the fluorescence spectra of Er3+ in ErLNGG, Er-doped (0.1 at.%) calcium lithium niobium gallium garnet (CLNGG), and Er(50 at.%):CLNGG is presented. The narrowing of luminescence lines suggests an ordering of the partially disordered CLNGG with the increase of erbium concentration.

Growth and characterization of Nd:CLNGG crystal

The disordered laser crystal neodymium-doped calcium lithium niobium gallium garnet (Nd:CLNGG) was successfully grown by the Czochralski method. Its thermal properties, including the average linear thermal expansion coefficient, thermal diffusion coefficient, specific heat, and thermal conductivity were measured, and continuous-wave (CW) laser performance at 1.06 μm was demonstrated. The maximum power of 1.48 W was achieved with corresponding optical conversion efficiency of 12.4% and slope efficiency of 16.2%.

Vertical Bridgman growth of calcium lithium niobium gallium garnet crystals

The growth of calcium lithium niobium gallium garnet (CLNGG) crystal has been carried out using platinum crucibles in a vertical Bridgman (VB) furnace with three resistance-heating zones. Transparent CLNGG crystals grown from the congruent melts with and without weight loss compensation are different in color and are 25 mm in diameter and about 70 mm in length. The phase identification of the sintered raw materials, grown crystals and white compound formed on the side surface of the grown crystal has been done using the powder X-ray diffraction method. The formation of the white compound is related to the {1 0 0} facet growth near the side surface. The naturally selected growth direction of the CLNGG crystal grown without a seed is near 〈1 1 1〉 , which is in good agreement with the morphological importance analysis according to the BFDH law. The VB-grown CLNGG also shows a cleavable feature parallel to {1 1 0} face and no absorption peaks in the wavelength range of 1100–1600 nm. The linear thermal-expansion coefficient of the CLNGG crystal along 〈1 1 1〉 direction is also reported and compared with that of CNGG, GGG and platinum crucible.

Growth, RBS-ERDA characterisations and modelling in Nd 3+-doped calcium–lithium–niobium–gallium garnet (CLNGG:Nd) crystal

Laser crystals with disordered structure present a series of advantages for diode pumping or short pulse generator as compared with ordered crystals. Such a system proved to be the calcium–lithium–niobium–gallium garnet (CLNGG) doped with Nd 3+. This paper presents the growth, RBS and ERDA analysis of specially doped CLNGG. Based on structural and spectral data, a structure modelling is proposed.

The nature of nonequivalent Nd3+ centers in CNGG and CLNGG

Abstract Results on the spectroscopic investigations and modeling of Nd3+ nonequivalent center structure in calcium niobium gallium garnet (CNGG) and calcium lithium niobium gallium garnet (CLNGG) are presented. The low temperature absorption, emission under selective excitation and lifetimes on a large variety of samples with Nd3+ varying between 0.1 and 5 at.% are analyzed. At least five different types of centers have been separated. Based on spectral data along with previous reports on the composition of the crystals a model of Nd3+ five-center structure in CNGG and CLNGG disordered crystals is proposed. The relative intensities for all the centers can be explained consistently by the structural possibilities of occupation of the first cationic octahedral sphere around the Nd3+ site. Starting from an unique average composition of this sphere our modeling suggests a correlated placement of the cations in the first octahedral sphere around Nd3+: the larger charge of Nd3+ as compared to the substituted Ca2+ leads to a local depletion of Nb5+ ions, accompanied by a corresponding enhancement of the lower-charge cations ( Ga 3+ , Li + ) or vacancies as compared with the undoped crystals.

Picosecond laser with active mode locking and calcium lithium niobium gallium disordered Nd3+-activated garnet

A laser was constructed in which the active medium was a crystal of calcium lithium niobium gallium garnet activated with Nd3+ ions. An acoustooptic modulator made of an LiNbO3 crystal and a passive LiF switch with F2 colour centres were used in this laser. The combination of active mode locking and Q switching made it possible to generate laser pulses of 10–15 ps duration at the wavelength λ = 1061.2 nm with a peak power of about 15 MW.

Calcium lithium niobium gallium garnet crystals activated with Cr3+, Tm3+, and Ho3+ions as a new active medium for λ ≈ 2 μm lasers

An investigation was made of the luminescence and lasing spectra of a calcium lithium niobium gallium garnet crystal activated with chromium, thulium, and holmium ions. Kinetics of decay of the Cr3+ luminescence was investigated and it was concluded that energy transfer from the Cr3+ to the Tm3+ ions was efficient. An investigation was made of the lasing characteristics of this crystal (λl = 2.09 μm) under free-running conditions at room temperature.

Calcium lithium niobium gallium garnet crystals activated with Er3and Cr3+as a new active medium for 3-μ lasers

An investigation was made of the luminescence spectra and lasing properties of a calcium lithium niobium gallium garnet crystal activated with chromium and erbium. The lifetimes of the upper (4I11/2) and lower (4I13/2) active levels of the Er3+ ion were determined. A study was made of the kinetics of decay of the Cr3+ luminescence and it was concluded that energy was transferred from the Cr3+ to the Er3+ ions. Lasing properties of the crystal (λl = 2.707 μm) were studied under free-running conditions at room temperature.