GGG Crystal Research Articles

Relationship of single crystal growth and luminescence properties of Cr-doped gadolinium gallium garnet crystals for radiation dose-rate monitoring systems

For the decommissioning of Fukushima Daiichi Nuclear Plants, a novel real-time dose-rate monitor consisting of a scintillator, 100-m long optical fiber and CCD spectrometer has been proposed, and this scintillation material is required to have an emission wavelength of longer than approximately 650 nm. In this paper, as a candidate material for the system, we focused on Ce/Cr co-doped Gd3Ga5O12 (Ce/Cr:GGG) single crystals grown by the micro-pulling down method. Since the light output of this material was expected to depend on the Cr3+ concentration, we investigated the relationship between Cr segregation and luminescence properties in Ce/Cr:GGG crystal. Generally, a single photon counting method is not available to evaluate its light output accurately due to the long decay time, we used a novel technique “emission-intensity two-dimensional- mapping method” (2D-image method) to verify the light intensity at each point of the sample. This method realized estimation of the light intensity within the sample, though segregation of Cr3+ is observed. The results showed that Cr was concentrated in the center part along the radial direction in the grown crystals, while all parts had an emission wavelength of approximately 730 nm excited by X-ray. Using the 2D-image method, emission intensity was found to correspond to the Cr concentration, and its correlation coefficients between emission intensity and Cr concentration were over 0.8. We found that our material is available for the monitoring system when the Cr dense part of the crystal is cut and used.

Optical, Structural, and Mechanical Properties of Gd3Ga5O12 Single Crystals Irradiated with 84Kr+ Ions

Herein, the optical absorption, X‐ray diffraction (XRD) analysis data, and mechanical properties along the ion path are analyzed for gadolinium gallium garnet (Gd3Ga5O12 or GGG) single crystals irradiated with fast 84Kr ions to fluences of 1013–1014 ion cm−2. It is found that the optical absorption spectra of Czochralski‐grown unirradiated GGG crystal consist of relatively narrow lines in the UV spectral range associated with the 4f–4f transitions in Gd3+ ions. Transitions from the 6S7/2 ground state to the 6P, 6J, and 6D states in the Gd3+ cation are visible. An additional absorption band is also observed at 350 nm, which is associated with an uncontrolled Ca impurity. In irradiated GGG single crystals, a shift of the fundamental absorption edge by ≈30 nm to the long‐wavelength part of the spectrum is observed. The observed changes are caused by structural disturbances caused by depletion of the surface layer and an increase in the number of displaced atoms accompanied by an increase in the crystal lattice parameter. XRD analysis shows the presence of size effects and deformation of interplanar distances due to the displacement of atoms from the sites of the crystal lattice with subsequent migration. Hardness measurements show ion‐induced softening, which may be due to ion‐induced amorphization.

Effective 1.9–2.1 μm emissions in Er3+/Ho3+: GGG and Er3+/ Tm3+: GGG crystals sensitized with Er3+

In this work, we report the enhanced 2080 nm emission from Ho3+ and 1999 nm emission of Tm3+ both via Er3+ sensitization in Gd3Ga5O12 (abbr. as GGG) crystals excited with 965 nm laser diode (LD) respectively. The underlying mechanisms were analyzed based on the measurements of absorption and emission spectra as well as fluorescence decay curves. Optical parameters including absorption and emission cross-sections, as well as fluorescence decay times are evaluated and compared. Owing to the co-doped sensitizer ion Er3+, a strong absorption near 965 nm plays an important role in the enhanced 2080 nm emission by transferring pump energy to Ho3+: 5I7 level in Er, Ho: GGG crystal, similarly, transferring energy from Er3+ to Tm3+: 3F4 level is conducive to improve 1999 nm emission in Er, Tm: GGG crystal. The above results demonstrate that the introduction of Er3+ into Ho3+ or Tm3+ activated materials offers a promising approach to obtain an enhanced 1.9–2.1 μm laser, which could operate upon 965 nm LD pumping, and the efficient energy transfer plays a key role in rare-earth ions doped luminescent materials.

Self-Q-switched Nd:GGG laser

In this paper, the self-Q-switched Nd:GGG laser at 1061 nm was demonstrated. The self-Q-switched pulses were obtained by adjusting the output coupler. Under pump power of 2.1 W, the output power of 89.5 mW and pulse energy of 1.83 μJ was obtained with 1.5% output coupler, the corresponding pulse width was 4.8 μs with a repetition rate of 48.94 kHz. In addition, we have firstly reported the SQS operation in Nd:GGG crystal.

Optical absorption and Raman studies of neutron-irradiated Gd3Ga5O12 single crystals

In this work we have performed a comparative analysis of optical absorption and micro-Raman spectra for series of Gd3Ga5O12 (GGG) single crystals irradiated by fast neutrons with fluences from 1016 n/cm2 to 1018 n/cm2. It was found that the optical absorption spectra of non-irradiated Czochralski grown GGG consist of the relatively narrow lines in the UV spectral range related to the 4f–4f transitions in Gd3+. Transitions from the 6S7/2 ground state to the 6P, 6J and 6D states in a Gd3+ cation are clearly detected. For a GGG crystal containing Ca impurity ions, additional absorption band at 350 nm is observed, and it is tentatively ascribed to oxygen vacancies associated with Ca impurities. Several neutron-induced optical absorption bands, including ones related to F+ centres, were observed in fast neutron-irradiated Gd3Ga5O12 single crystals. Furthermore, clear evolution of the Raman spectra with neutron dose is also found. In particular, fine structure observed between 1350 and 1550 cm−1 is completely disappeared at high fluences, while several significant changes and the emergence of new transitions have been ascertained in spectral range of 100–850 cm−1.

Diode-pumped continuous-wave Nd:Gd3Ga5O12 lasers at 1406, 1415 and 1423 nm

We report a diode-pumped continuous-wave Nd:Gd3Ga5O12 (GGG) laser operating at 1.4 μm spectral region. A dual-wavelength laser at 1423 and 1406 nm is achieved with output power of about 2.59 W at absorbed pump power of 13.4 W. Further increasing the pump power, simultaneous tri-wavelength laser at 1423, 1415 and 1406 nm is also obtained with a maximum output power of 3.96 W at absorbed pump power of 18.9 W. Single-wavelength lasing is also realized at the three emission lines using an intracavity etalon. The laser result is believed to be the highest output power achieved in Nd:GGG crystal, at present, to the best of our knowledge.

CW and Q-switched GGG/Er:Pr:GGG/GGG composite crystal laser at 2.7 µm

We report the continuous-wave (CW) and passively Q-switched laser operations of a GGG/Er:Pr:GGG/GGG composite crystal at about 2.7 µm. Owing to the alleviation of the thermal lensing effect, the CW laser with a maximum output power of 463 mW was obtained with a slope efficiency of 15.5%. Based on the broadband saturable absorption property, a graphene saturable absorber (SA) mirror was fabricated and employed for realizing the Q-switched mid-infrared laser. Under an absorbed pump power of 2.47 W, an average output power of 186 mW was generated with a slope efficiency of 12.3%. The pulse width and the repetition rate of the laser were 360 ns and 120.5 kHz, respectively. These results indicate that the Er:Pr:GGG crystal, with the relatively lower upper-level lifetime, shows great promise for generating a short pulsed 2.7 µm mid-infrared laser using the graphene SA.

Bulk crystal growth and characterization of 2.5–5.0 µm mid-infrared laser crystals

Mid-infrared waveband lasers have demonstrated important applications in the fields of atmosphere pollution monitoring, gas flow monitoring, ocean detection, laser medicine, military confrontation and basic scientific researches etc. In this paper, we review the research development of bulk crystal growth, physicochemical, spectral and laser properties of 2.5–5.0 μm waveband mid-infrared laser crystals in recent years. A serial of gallate and aluminate crystals with low phonon energy were grown by the Czochralski method, such as Er3+ activated and rare earth ions co-doped Gd3Ga5O12(GGG), SrGdGa3O7, SrLaGa3O7, CaYAlO4 and CaGdAlO4 crystals, Ho3+ activated and rare earth ions co-doped GGG crystals, as well as Dy3+:YAlO3 crystals, and 2.5–3.4 μm waveband emissions were obtained in these crystals. High quality Dy3+:KPb2Cl5, Cr2+:CdSe and Cr2+:Fe2+:CdSe crystals were grown by the Bridgman method, and 2.5–5.0 μm waveband emissions were obtained in these crystals. The physicochemical and spectral properties of the above crystals were studied systematically, with focus on analyses of the concentration effect of activated ions Er3+, the sensitization mechanism of the co-dopant sensitized ions and the impact of the depopulated ions on the inhibition of self-termination effect. The fluorescence dynamic of the activated ions within the mid-infrared waveband, the energy transfer efficiencies between activated ions and co-dopant sensitization ions, as well as between the activated ions and co-dopant depopulation ions were calculated, and the energy transfer mechanism was discussed. Based on the results of spectroscopic investigations, the concentrations of Er3+, the types and the concentrations the co-dopant sensitization ions were optimized in different kinds of crystals. The laser properties of Er3+ activated GGG, CaYAlO4 crystals were presented. Finally, 2.7–2.8 µm multi-wavelength CW or pulsed lasers were achieved in Er:GGG, Er,Pr:GGG and Er:GGG/GGG composite crystals end-pumped by 965 nm LD or in Cr,Er:YSGG crystals by Xenon lamp, furthermore, the single longitudinal mode microchip laser was achieved in Er:GGG crystal for the first time.

Third-order nonlinearity and saturable absorbed performance of Cr^4+:Gd_3Ga_5O_12 crystal

In this work, the third-order nonlinear optical properties and saturable absorbed performance of Cr4+-doped Gd3Ga5O12 (Cr4+:GGG) crystal were investigated. By using the Z-scan technique, the third-order nonlinear properties were analyzed systematically. Compared with Cr4+:YAG crystal, Cr4+:GGG crystal has a large third-order nonlinear refractive index, ground-state absorption, and excited-state absorption cross section. It has been successfully employed as a saturable absorber for passively Q-switched and Q-switching mode-locked lasers in this paper. In the Q-switched regime, the maximum average output power of 600 mW was obtained with the shortest pulse width of 4.76 ns and the repetition rate of 41.4 kHz, corresponding to single pulse energy and pulse peak power of 14.5 µJ and 3 kW, respectively. For the Q-switching mode-locking operation, the maximum output power of 468 mW was obtained with a repetition rate of 141.2 MHz. The results indicate that Cr4+:GGG crystal has the potential to be used for passive Q-switching and mode-locking laser generation.

Single-longitudinal-mode Er:GGG microchip laser operating at 2.7 μm.

We reported on a diode-end-pumped single-longitudinal-mode microchip laser using a 600-μm-thick Er:GGG crystal at ∼2.7 μm, generating a maximum output power of 50.8 mW and the maximum pulsed energy of 0.306 mJ, with repetition rates of pumping light of 300, 200, and 100 Hz, respectively. The maximum slope efficiency of the laser was 20.1%. The laser was operated in a single-longitudinal mode centered at about 2704 nm with a FWHM of 0.42 nm. The laser had a fundamental beam profile and the beam quality parameter M(2) was measured as 1.46. These results indicate that the Er:GGG microchip laser is a potential compact mid-infrared laser source.

Diode-End-Pumped Midinfrared Multiwavelength Er:Pr:GGG Laser

We report a diode-end-pumped midinfrared multiwavelength Er:Pr:GGG laser, generating the output powers of 324 and 354 mW in CW and pulsed modes, respectively. The slope efficiencies of CW and pulsed modes are 15.18% and 16.06%, respectively. Compared with Er:GGG, the laser properties of Er:Pr:GGG are much better and the self-termination bottleneck is broken successfully. The transverse beam profile and spectra of Er:Pr:GGG laser are measured and the relationship between lasing wavelengths and operating temperature in the triwavelength emission state are also studied. The Er:Pr:GGG crystal is a potential midinfrared laser gain medium in medical and military application.

Activation effect of Ho^3+ at 284 μm MIR luminescence by Yb^3+ ions in GGG crystal

The use of Yb(3+) co-doping for the enhancement of Ho(3+):(5)I(6)→(5)I(7) mid-IR (MIR) emissions was investigated in GGG crystal for the first time. It is established that Yb(3+) highly increases Ho(3+) 2.84 μm emissions by transforming pump energy from the Yb(3+):(2)F(5/2) level to the laser upper level (5)I(6) of Ho(3+). The energy-transfer efficiency from Yb(3+):(2)F(5/2) to Ho(3+):(5)I(6) is calculated to be 96.2%. The absorption cross section, emission cross section, and fluorescence quantum efficiency are estimated and discussed. It is concluded that the Yb, Ho:GGG crystal is promising material for an LD-pumped 2.84 μm laser application.

Development of a single crystal with a high index of refraction

Time-of-flight Positron emission tomography (TOF-PET) is one of the next-generation medical imaging methods, which requires scintillators with a very short decay time. However, the shortest scintillation decay times are typically 20–30ns, and these values are not sufficient for TOF-PET. Cherenkov counters are used in high energy physics and they are expected to be applied in medical imaging due to their short decay time. Here, high-refractive index materials are necessary for Cherenkov radiators to reach a high light output. We measured refractive indices of Gd3Ga5O12 (GGG), Y3Ga5O12 (YGG) and Lu3Ga5O12 (LuGG) crystals grown by a micro-pulling-down (μ-PD) method. The GGG, YGG and LuGG crystals were found to have refractive indices of ~2.5, ~2.3 and ~2.3 at 400nm, respectively. Then we grew a 40mm diameter GGG crystal by the Czochralski method, and the emission decay times of the GGG crystals irradiated with muons and gamma rays were 10±1ns and 10±2ns, respectively, using a photomultiplier tube (Hamamatsu R6231-100). Cherenkov light of the GGG crystal could be observed for the gamma-ray irradiation.

Simultaneous dual-wavelength laser operation at 937 and 1062 nm in Nd3+:Gd3Ga5O12

Diode-end-pumped continuous-wave (cw) simultaneous dual-wavelength laser operation at 937 and 1062 nm in a single Nd3+:Gd3Ga5O12 (Nd:GGG) crystal was demonstrated. A total output power of 1.12 W at the two fundamental wavelengths was achieved at incident pump power of 17.6 W. The optical-to-optical conversion was up to 6.4% with respect to the incident pump power. To the best of our knowledge, this is first work on cw simultaneous dual-wavelength operation at 937 and 1062 nm in Nd:GGG crystal.

Study on the growth of Nd3+:Gd3Ga5O12 (Nd:GGG) crystal by the Czochralski technique under different gas flow rates and using different crucible sizes for flat interface growth

In the Czochralski growth of GGG/Nd:GGG, the percentage of oxygen flowing in the growth chamber plays an important role in eliminating or controlling a number of growth problems. In this paper we report the growth of Nd3+:Gd3Ga5O12 (Nd:GGG) crystal at different flow rates of argon and oxygen percentages (1–2%) and concluded that the optimum percentage is 1.0–1.2%. No effect on the crystal growth was observed while increasing the argon flow rate from 240LPH to 420LPH and by keeping same percentage of oxygen. The growth of Nd:GGG crystal by the Czochralski technique with a flat crystal/melt interface was also studied using different crucible sizes and under the same crystal growth conditions. In this study, it was found that the critical Reynolds number, which measures the flow driven by crystal rotation to achieve flat crystal/melt interface, is directly proportional to the crucible diameter. And the crucible diameter (D) is related with the crystal diameter (d) by equation D=Kωd2, where K is the proportionality constant and ω is the crystal rotation rate. We also found that for a given crucible, the critical Reynolds number remains the same even for different crystal diameters.

Narrow pulse width and high power passively Q-switched Nd3+:Gd3Ga5O12 laser with Cr4+:YAG saturable absorber

A compact, diode-pumped passively Q-switched Nd3+:Gd3Ga5O12 (Nd:GGG) laser with Cr4+:YAG saturable absorber has been successfully demonstrated. Stable Q-switched pulses with pulse energy of 100 μJ and high peak power of 14 kW have been obtained. The pulse width was as short as 7 ns with low repetition rate of 10 kHz. The dependence of pulse width, pulse repetition rate, pulse energy and pulse peak power on pump power have been measured respectively. Experimental results reveal that the Nd:GGG crystal with Cr4+:YAG saturable absorber is suitable for narrow pulse width and high power passively Q-switched lasers.

Ridge waveguide lasers in Nd:GGG crystals produced by swift carbon ion irradiation and femtosecond laser ablation

We report on the fabrication of ridge waveguide in Nd:GGG crystal by using swift C(5+) ion irradiation and femtosecond laser ablation. At room temperature continuous wave laser oscillation at wavelength of ~1063 nm has been realized through the optical pump at 808 nm with a slope efficiency of 41.8% and the pump threshold is 71.6 mW.

Passively mode-locked Nd:GGG laser with a semiconductor saturable absorber mirror

The mode-locking operation of the Nd:GGG crystal with a semiconductor saturable absorber mirror (SESAM) was demonstrated. Continuous wave (CW) mode-locking was obtained with the pulse duration of 11 ps and the pulse repetition rate of 40.38 MHz. At 7 W of pump power, the maximum average output power of 1.1 W was obtained with the slope efficiency of 18.1%.

Detection system for microimaging with neutrons

A new high-resolution detector setup for neutron imaging has been developed based on infinity-corrected optics with high light collection, combined with customized mounting hardware. The system can easily be installed, handled and fitted to any existing facility, avoiding the necessity of complex optical systems or further improved electronics (CCD). This is the first time optical magnification higher than 1:1 has been used with scintillator-based neutron detectors, as well as the first implementation of infinity corrected optics for neutron imaging, achieving the smallest yet reported effective pixel size of 3.375 μm. A novel transparent crystal scintillator (GGG crystal) has been implemented with neutrons for the first time to overcome limitations of traditional powder scintillators (Li6/ZnS, Gadox). The standardized procedure for resolution measurements with the Modulation Transfer Function (MTF) is summarized to facilitate comparison between instruments and facilities. Using this new detector setup, a resolution of 14.8 μm with a field of view of 6 mm × 6 mm has been achieved while maintaining reasonable count times. These advances open a wide range of new possible research applications and allow the potential for additional future developments.

Improvement of Nd:GGG crystal growth process under dynamic atmosphere composition

In this paper, the temperature dependency of the required oxygen on the quality of Nd:GGG crystal during the growth process has been investigated. Based on the thermodynamic analysis, the oxygen partial pressure in the surrounding vapor phase should be as low as possible through heating the raw material at lower temperatures to prevent the oxidation of iridium to IrO 2 ( s). In order to eliminate the volatilization of Ga 2O 3, the oxygen was charged into the gas atmosphere (N 2) near the Nd:GGG melting point. Powder X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed to characterize the crystal quality under different growth atmosphere. The experimental results confirm the validity of thermodynamic calculations. It was predicted that charging the required oxygen at about 1950 K could reduce the iridium loss rate to about 36%. Consequently, it would be expected to acquire more favorable Nd:GGG crystals with quite improved properties by applying the accurate growth conditions.