YAG Host Research Articles

Novel lanthanide-doped Y3-xNaxAl5-yVyO12 garnets: Synthesis, structural and optical properties

In this study the novel garnet-type Y2.47Na0.5Ln0.03Al5O12, Y2.97Ln0.03Al4.95V0.05O12, Y2.92Na0.05Ln0.03Al4.9833V0.0167O12 (Ln = Ce3+, Tb3+, Eu3+) phosphors were successfully synthesized by the sol-gel method for the first time. The structural, morphological and optical properties were characterized by using multiple characterization techniques. The XRD and FTIR results confirmed that all obtained phosphors are monophasic yttrium aluminium garnet compounds, i.e. the doping of Ce3+, Tb3+, Eu3+, Na+, V5+ ions does not induce any impurity phases, indicating the successful incorporation of these dopants into the YAG host. The formation of novel garnets was probed using 27Al, 51V, 23Na MAS NMR techniques. SEM micrographs revealed that almost in all cases, the surface of the obtained luminophores is porous and consists of homogeneously distributed irregular sphere-like shape particles, which tend to form larger agglomerates. The optical properties of obtained compounds were also investigated by recording their excitation and emission spectra and calculating the colour coordinates in the CIE 1931 colour space.

Nd3+→ Cr4+ energy transfer in co-doped Nd,Cr:Y3Al5O12 garnet transparent ceramics

Herein, we quantitatively analyze the Nd3+ → Cr4+ energy transfer efficiency in the co-doped transparent ceramic YAG host and determine that it can be maintained at less than a 20 % loss of Nd3+ excited states when the Cr4+ absorption coefficient at 1064 nm is less than 1.0 cm−1. Furthermore, we conclude that the energy transfer mechanism can be fully accounted for on the basis of classic Förster-Dexter theory, as the transfer efficiency deduced from the emission kinetics is consistent with that independently determined from the spectroscopic properties of Cr4+ absorption and Nd3+ emission. Systematic studies of trends for Cr3+ → Cr4+ conversion are reported as a function of the oxygen annealing temperature, initial Cr3+ concentration, and incorporation of divalent (Mg2+ and Ca2+) charge compensators. Lastly, no effect of energy migration among the Nd3+ ions was observed.

Growth of Cr,Yb:YAG single crystals for self-Q-switched monolithic solid-state lasers

The saturable absorbers play key role in passively Q-switched lasers, and Cr4+-doped crystals present attractive advantages for Yb:YAG lasers. At present the major problem is the low doping concentration of Cr4+ and the effective way to promote Cr3+→Cr4+ conversion. From this view, we introduce divalent additives Ca2+ and Mg2+ as charge compensators to ensure the charge state of Cr4+ in the tervalence environment of Yb:YAG. The results reveal that this method can promote Cr3+→Cr4+ conversion effectively and an optimal CaO/MgO ratio is 1:4. The corresponding Cr0.01at.%,Yb5at.%:YAG single crystals present enhanced absorption coefficients and fluorescence intensity at 941 nm and 1030 nm, and decreased fluorescence lifetimes and decay curves than Yb:YAG. The thermal expansion coefficient and laser damage threshold of Cr,Yb:YAG single crystal are approaching the YAG host material. The results show that Cr,Yb:YAG crystal has good potential for self-Q-switched monolithic solid-state lasers.

Ultra-Small YPO4 -YAG:Ce Composite Nanophosphors with a Photoluminescence Quantum Yield Exceeding 50.

Synthesis of high quality colloidal Cerium(III) doped yttrium aluminum garnet (Y3 Al5 O12 :Ce3+ , "YAG:Ce") nanoparticles (NPs) meeting simultaneously both ultra-small size and high photoluminescence (PL) performance is challenging, as generally a particle size/PL trade-off has been observed for this type of nanomaterials. The glycothermal route is capable to yield ultra-fine crystalline colloidal YAG:Ce nanoparticles with a particle size as small as 10nm but with quantum yield (QY) no more than 20%. In this paper, the first ultra-small YPO4 -YAG:Ce nanocomposite phosphor particles having an exceptional QY-to-size performance with an QY up to 53% while maintaining the particle size ≈10nm is reported. The NPs are produced via a phosphoric acid- and extra yttrium acetate-assisted glycothermal synthesis route. Localization of phosphate and extra yttrium entities with respect to cerium centers in the YAG host has been determined by fine structural analysis techniques such as X-ray diffration (XRD), solid state nuclear magnetic resonance (NMR), and high resolution scanning transmission electron microscopy (HR-STEM), and shows distinct YPO4 and YAG phases. Finally, a correlation between the additive-induced physico-chemical environment change around cerium centers and the increasing PL performance has been suggested based on electron paramagnetic resonance (EPR), X-ray photoelectronspectrometry (XPS) data, and crystallographic simulation studies.

Ionic selective occupation, lattice modulation and photoluminescence properties of garnet typed orange-yellow phosphors for white-light-emitting diodes

Orange-yellow luminescent Y2-xSr1-yAl4SiO12: xCe3+, yMn2+ (x = 0.04–0.06, y = 0.1–0.6, abbreviated as YSAS: xCe3+, yMn2+) phosphors with garnet structure were prepared by the high temperature solid-phase method. The matrix energy band is calculated by DFT, where YAG is 4.535 eV and YSAS is 4.874 eV. The Rietveld refinement of the XRD data indicates that Ce3+ mainly replaces the Y3+ sites, while Mn2+ selectively occupies. The Gaussian peaks of the fluorescence spectrum prove that Mn2+ occupies two luminescent centers, which are further determined by Van Uitert's empirical formula. The doping of Ce3+/Mn2+ units can effectively enhance the structural rigidity of the YAG host and promote the red-shift of the emission peak position of Mn2+. The fluorescence decay kinetics test proves that Ce3+ can transfer energy to Mn2+, and the transfer efficiency reaches 72.08% at y = 0.5. The thermal stability test of the phosphor shows the luminous intensity at 473 K still maintains 86% of the room temperature. The phosphor prepared in this experiment was combined with a blue light chip to make a w-LED device, and the performance showed Ra = 86.1, CCT = 5168 K. The above results show that the emission color of the garnet structure can be tuned by energy transfer, which is attractive for future applications.

Peculiarities and the red shift of Eu2+ luminescence in Gd3+-admixed YAG phosphors

Set of Eu2+-doped (Gd; Y)3Al5O12 (GYAG) powder samples with different Gd/Y ratio was synthesized by inorganic reaction in the solid state from high purity Gd2O3, Y2O3, Al2O3 and EuS. Their structural, scintillation and optical properties were investigated thoroughly by means of X-ray powder diffraction, time-resolved luminescence spectroscopy, photothermal deflection spectroscopy and electron paramagnetic resonance in a broad temperature range to assess the influence of Gd admixture into the YAG host. A phenomenological model was fitted to the measured temperature dependences of the photoluminescence emission spectra and photoluminescence decay times to better understand the dynamics of the 5d excited state of the Eu2+ center. Thermoluminescence and delayed recombination measurements were performed to study the trap distribution and thermally induced ionization of the activator's excited state. Application potential of studied phosphors either in the field of scintillators or solid state white light emitting diode (LED) phosphors is discussed.

Color tunability and synergistic effect of PiG materials based on YAG:Ce3+ phosphor in SCS:Eu3+ glass

Efficient white-emitting phosphor-in-glass (PiG) was successfully synthesized by synergistic complexation of Ce3+-doped YAG phosphor and Eu3+-doped sodium calcium silicate glass. The crystalline phosphor was obtained via sol-gel route using polyvinyl alcohol (PVA), whereas glasses were prepared by melting-quenching method. Rare-earth ions were successfully incorporated into the YAG and glass host lattices without changing their original structures. Through adjusting the Eu3+-doped glass and Ce3+:YAG phosphorus contents in the PiG, it was possible to optimize the chromaticity emission from yellow-green to orange-red in CIE 1931 diagram. Combination of a violet LED and samples emissions, allowed to achieve the chromaticity parameters closer to an ideal white light source [CIE-(0.33, 0.33); CCT 5000K]. Moreover, adjusting the luminescence intensity ratio between the LED and the PiG emission provides a tuning route for smart white light devices.

Elastic and thermodynamic properties of cerium‐doped yttrium aluminum garnets

AbstractCerium‐doped yttrium aluminum garnets (Y3‐xCexAl5O12, Ce:YAGs) are promising yellow light‐emitting components of solid‐state white light‐emitting diodes. Although there have been numerous studies examining the effects of Ce concentrations on the luminescent properties of Y3‐xCexAl5O12, the impacts of Ce dopant on the elastic and thermodynamic properties are not well understood. In this work, we used resonant ultrasound spectroscopy (RUS) to determine the effects of Ce doping (0.025, 0.1, 1 at. %) on the elastic and thermodynamic properties of Y3‐xCexAl5O12. The elastic moduli calculated via the Voigt–Reuss–Hill (VRH) method demonstrated that low Ce dopant concentrations (≤0.1 at. %) induced negligible effects on the elasticity of the YAG host matrix, while a high Ce concentration (1 at. %) yielded significant softening. RUS spectral analysis and SEM images suggested that the elastic softening originated from microstructural differences induced at higher Ce dopant concentrations. In addition, we demonstrated an increase in elastic anisotropy at higher Ce concentrations, which further elucidated the correlations between structure and elasticity of Y3‐xCexAl5O12. Debye temperatures (θD), heat capacities (Cp), and thermal conductivities (κ) were calculated for Ce:YAGs through the relations of RUS‐derived parameters (sound velocities, elastic moduli) and previously determined thermal expansion coefficients. Ce:YAG was found to have a significant reduction in θD, Cp, and κ at Ce concentrations ≥1 at. %. Lastly, extrapolation of Cp and κ to higher temperatures allowed the modeling of thermal stress experienced by Y3‐xCexAl5O12 disks up to 1073.15 K.

Ce-doped YAG single-crystals prepared by continuous wave (CW)–CO2 laser combustion technique with attractive characteristics and moderate white LED performance

In this work, the influence of continuous wave –CO2 laser-induced solution combustion as a new and effective technique to prepare YAG:Ce phosphor was investigated. The as-prepared ceramic displayed a direct pure YAG phase within the laser exposure time of 1.5 min with an attractive characteristics and moderate white light-emitting diode (WLED) performance, suggesting that laser synthesis has a role to play in accelerating the diffusion of the cations and an increase in reaction rate. However, the thermal treatment step plays a key role to adjust efficient luminescence processes. In addition, the luminescence properties were strongly dependent on the sintering strategies due to the expulsion of defects (e.g. CeO2 and other impurity phases) from matrix. We showed experimentally that increasing the luminescence intensity has an effect observed on the CIE chromaticity parameters and the theoretically calculated luminous efficiency of radiation (LER). This work presents the effects of thermal treatment properties on the YAG host lattice and explains the mechanism of YAG formation under CO2 laser beam.

Effect of γ-irradiation on structural, morphological and luminescence properties of cerium-doped Y3Al5O12 nano-material

ABSTRACTCerium-doped yttrium aluminum garnet (YAG: Ce3+) nanopowder phosphors have been elaborated by sol–gel process and annealed at 900°C for 2 h. The prepared phosphors were exposed to gamma radiation, using 60Co source, at different doses ranging from 5 to 100 kGy. The influence of γ-irradiation on the structural, morphological and luminescence properties of YAG: Ce3+ phosphors were investigated in detail by X-ray diffraction, field emission scanning electron microscopy (FESEM), Fourier transforms infrared spectroscopy (FTIR) and photoluminescence measurements. The XRD analysis confirmed the presence of single cubic phase for all samples of YAG: Ce3+ nanophosphors independent of γ-rays dose. FESEM micrograph results revealed that the particles present flate-like shapes and high density of dislocation for sample irradiated at 100 kGy of γ-ray. The YAG: Ce3+ nanophosphors showed broad green–yellow emission band in the range of 450–700 nm with maximum intensity at 538 nm assigned to the 5d → 4f transitions of Ce3+ ion. The emission intensity of YAG: Ce3+ phosphors vary with the γ-ray irradiation and reach the maximum for sample irradiated to a dose of 25 kGy. The variation of luminescence intensity is related to the crystallite size and Ce4+ ions content in YAG host nanomaterial.

Preparation and characterization of bright high quality YAG: Eu3+ thin films grown by sol–gel dip-coating technique

In the present study, highly transparent thin films of europium trivalent (Eu3+) -doped yttrium aluminum garnet (YAG: Eu3+), deposited on quartz substrates with good optical properties were successfully prepared by sol–gel dip-coating technique. Thin film thicknesses were incremented by increasing the number of coated layers. The relationship between the film thickness and its structural and optical properties were highlighted. Prepared thin film samples were characterized by Rutherford Backscattering Spectroscopy (RBS), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence spectroscopy (PL) and UV–Visible spectroscopy. RBS analysis shows that the film thicknesses range from 74 nm to 803 nm. According to XRD results, the single cubic phase of YAG was formed and no impurity phases appeared. Also, the crystallite size increased with the increase of film thickness. FE-SEM images reveal spherical shape particles with the formation of cracks. All samples present a dominate and intense orange emission centered at 594 nm characteristic of magnetic dipole 5D0→7F1 transition of Eu3+ ion in the YAG host material. Also, it was found that the emission intensity increases with increasing film thickness and correlated with its density. The UV–Vis spectroscopy revealed that the films were transparent and the degree of transparency depends on the film thickness.

Concentration dependence of energy transfer Ce3+→Er3+ in YAG host

In the doubly doped YAG:Ce,Er single crystal we show the effect of the nonradiative energy transfer from the donor (Ce3+) to the acceptor (Er3+) in terms of the Förster-Dexter model. Such an energy transfer process is enabled by the overlap of the donor and acceptor emission and absorption spectra, respectively, and makes the donor to de-excite faster. This phenomenon can be used to tailor the luminescence decay of a phosphor in order to increase the rate at which it converts incoming radiation, if the decrease of its quantum efficiency can be tolerated. Quantitative comparison of experimental data obtained at the sample set with large concentration interval of Er3+ acceptor (up to the concentration Y2.835Al5O12: Ce0.006Er0.159) with the mentioned theoretical model allows the determination of the microscopic parameters of the energy transfer process, such as the critical concentration of the Er3+ ions. The Er concentration higher than 1% has proven to shorten the 1/e decay time from 62 ns below 30 ns. The concentration dependence of the absorption lines of the Er3+ is also discussed.

Third order nonlinearity and optical limiting behaviors of Yb:YAG nanoparticles by Z-scan technique

The present report explores third order nonlinear optical behavior of phase pure Yb3+:YAG nanoparticles for the first time by Z-scan technique. The measurement was carried out using diode pumped continuous wave (CW) Nd:YAG laser at 532 nm. The Yb:YAG nanoparticles exhibit characteristic near-infrared (NIR) emission at 1030 nm under 940 nm excitation. The nanoparticles exhibit high nonlinear refractive index (n2 = 8.649 × 10−8 cm2/W) and low nonlinear absorption coefficient (β = 0.109 × 10−4 cm/W) giving an appreciable figure of merit (FOM) of ∼74.50. The excitation power (8.2–10.5 W cm−2) dependent emission spectra were recorded to study exchange energy interaction of Yb3+ ions with YAG host lattice. By utilizing the nonlinear refractive index ‘n2’ from Z-scan measurement, thermo-optic coefficient (dn/dt) was calculated to demonstrate Yb3+:YAG nanomaterial for high power compact solid state laser gain amplifier systems.

Dielectric and microstructural properties of YAG:Dy3+ ceramics

Yttrium aluminium garnet (Y3Al5O12:YAG) singly doped with Dy3+ at different concentrations was prepared by solid state reactions using repeated heating cycles over the temperature range of 1300–1600 °C. X-ray powder diffraction analysis confirms the presence of a well-crystallized YAG perovskite phase with cubic structure (by Rietveld refinement). The rare earth dopant is successfully integrated into the YAG host lattice without any major changes in the original structure. The temperature dependence, up to 250 °C, of the conductivity, dielectric constant, dielectric loss, and loss tangent, at various frequencies of up to 5.0 MHz for undoped and doped crystals is compared to understand the electrical and structural characteristics. The experimental results reveal that Dy3+ dopants in YAG crystal significantly influence the conductivity, dielectric constant, and lossy mechanisms, which is probably due to the 3d-Al ions and 4f-Dy ions incorporated at different positions of both tetrahedral and octahedral symmetries in YAG:xDy3+ ceramics.

Transition of Emission Colours as a Consequence of Heat-Treatment of Carbon Coated Ce3+-Doped YAG Phosphors.

To modify the luminescence properties of Ce3+-doped Y3Al5O12 (YAG) phosphors, they have been coated with a carbon layer by chemical vapor deposition and subsequently heat-treated at high temperature under N2 atmosphere. Luminescence of the carbon coated YAG:Ce3+ phosphors has been investigated as a function of heat-treatment at 1500 and 1650 °C. The 540 nm emission intensity of C@YAG:Ce3+ is the highest when heated at 1650 °C, while a blue emission at 400–420 nm is observed when heated at 1500 °C but not at 1650 °C. It is verified by X-ray diffraction (XRD) that the intriguing luminescence changes are induced by the formation of new phases in C@YAG:Ce3+-1500 °C, which disappear in C@YAG:Ce3+-1650 °C. In order to understand the mechanisms responsible for the enhancement of YAG:Ce3+ emission and the presence of the blue emission observed for C@YAG:Ce3+-1500 °C, the samples have been investigated by a combination of several electron microscopy techniques, such as HRTEM, SEM-CL, and SEM-EDS. This local and cross-sectional analysis clearly reveals a gradual transformation of phase and morphology in heated C@YAG:Ce3+ phosphors, which is related to a reaction between C and YAG:Ce3+ in N2 atmosphere. Through reaction between the carbon layer and YAG host materials, the emission colour of the phosphors can be modified from yellow, white, and then back to yellow under UV excitation as a function of heat-treatment in N2 atmosphere.

Giant Enhancement of Nonlinear Optical Response in Nd:YAG Single Crystals by Embedded Silver Nanoparticles.

We report on the enhancement and modulation of nonlinear optical response in an Nd:Y3Al5O12 (Nd:YAG) laser crystal through embedded silver nanoparticles (NPs) fabricated by Ag+ ion implantation. The linear absorption spectrum of the sample clearly reveals a localized surface plasmon resonance (SPR) band from 350 to 700 nm correlated to the Ag NPs. By using the Z-scan technique with femtosecond pulses at a wavelength of 515 nm, which is considered as an optical excitation within the SPR band, the nonlinear refraction index reaches values as high as ∼10–12 cm2/W, enhanced by ∼4 orders of magnitude in comparison to that of unimplanted Nd:YAG (without Ag NPs). In addition, it has been shown that embedded Ag NPs in the Nd:YAG host reveal saturable absorption signifying the nonlinear responses. We have also observed that the nonlinear absorption coefficients depend significantly on the excitation energy and can be modulated by varying the fluence of Ag+ ions.

Enhanced visible to near infrared downshifting in Ce3+/Yb3+-co-doped yttrium aluminum garnet through Pr3+ doping

Novel YAG:Ce3+, Pr3+, Yb3+ phosphors were synthesized using a high temperature solid state reaction method. The XRD patterns revealed that the obtained samples belong to the single phase YAG. The optical spectra demonstrated that Pr3+ could be a bridge to facilitate the energy transfer from Ce3+ to Yb3+ in the YAG host. Blue light excited Ce3+ and a yellow emission from Ce3+ together with a near infrared emission of Yb3+ was obtained. The effect of Pr3+ doping on the energy transfer efficiency was explored. When the doping concentration of Pr3+ increased, the yellow emission intensities of Ce3+ decreased and the near infrared emission intensity of Yb3+ increased monotonously. Hence, the efficiency of energy transfer from Ce3+ to Yb3+ is enhanced by the addition of Pr3+, implying that the YAG:Ce3+, Pr3+, Yb3+ phosphors are promising downshifting materials and have the potential to be used in Si-based solar cells.

Assessment of mechanisms for enhanced performance of TiO2/YAG:Yb+3,Er+3 composite photocatalysts for organic degradation

Wastewater remediation is a key component to a sustainable water management program and the use of photo-activated catalysis has emerged as a promising advanced oxidative process for the decomposition of organic contaminants to innocuous products. Although the process is feasible using UV photons, only ∼5% of solar insolation is comprised of UV photons and few advances have been made to improve upon commercial TiO2 (i.e., P25). Here, composite catalysts containing both an upconversion phase (Yb+3/Er+3/Y-Al garnet or YAG host) and a photocatalyst (TiO2) are assessed for enhancing the photocatalytic activity of TiO2 for organic degradation using simulated insolation. Samples of the upconverting phosphor (YAG host) were prepared utilizing the Pechini method with varying molar concentration of Yb+3 ions [0%, 10%, 15%, or 20%] and a constant concentration of Er+3 ions [2%]. The composite materials were then obtained via calcination of the various YAG:Yb+3,Er+3 samples with titania at molar concentrations of [10%, 15%, or 20%]. Structures and properties of the upconverting phosphors and composite materials were verified with characterization analyses including X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, and photoluminescence spectroscopy. For rose bengal degradation in a batch slurry reaction using a simulated ‘daylight’ spectrum as the irradiation source, results indicated that a mixed phased, Y-Al oxide doped with 2mol% Er+3 and 15mol% Yb+3 with 10mol% doped YAG/balance TiO2 composite yielded a 42% higher rate constant and apparent quantum yield than TiO2 alone. Additional kinetic studies with portions of the solar spectrum (i.e., UV bulbs or UV or IR LEDs) were conducted to further probe the effect of photon energies and suggested that rate and apparent quantum yield enhancements were largely attributed to increased amounts of organic adsorption and close proximity between phases, which coincided with the presence of the monoclinic phase (not cubic) of the Y-Al oxide structure. Upconversion and decreased charge carrier recombination rates were deemed to provide minimal contribution to these enhancements.

Optical spectroscopy and decoherence studies ofYb3+:YAG at 968 nm

The $^{2}F_{7/2}\ensuremath{\leftrightarrow}^{2}F_{5/2}$ optical transitions of $\mathrm{Y}{\mathrm{b}}^{3+}$ doped into ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5}{\mathrm{O}}_{12}$ (YAG) were studied for potential quantum information and photonic signal processing applications. Absorption and fluorescence spectroscopy located the energy levels of the ground $^{2}F_{7/2}$ and excited $^{2}F_{5/2}$ manifolds, allowing inconsistencies between previous assignments of crystal field splittings in the literature to be resolved. These measurements reveal an unusually large splitting between the first and second levels in both the ground and excited multiplets, potentially providing for reduced sensitivity to thermally induced decoherence and spin-lattice relaxation. Spectral hole burning through two-level saturation was observed, determining the excited state lifetime to be 860 \ensuremath{\mu}s and resolving ambiguities in previous fluorescence measurements that were caused by the large radiation trapping effects in this material. Optical decoherence measurements using two-pulse photon echoes gave a homogeneous linewidth of 18 kHz for an applied magnetic field of 1 T, narrowing to 5 kHz at 2.5 T. The observed decoherence was described by spectral diffusion attributed to $\mathrm{Y}{\mathrm{b}}^{3+}\mathrm{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{3+}$ magnetic dipole interactions. Laser absorption determined an inhomogeneous linewidth of 3.6 GHz for this transition in this 0.05%-doped crystal, which is narrower than for any other rare-earth-ion transition previously studied in the YAG host. The temperature dependence of the transition energy and linewidth of the lowest $^{2}F_{7/2}$ to lowest $^{2}F_{5/2}$ transition centered at 968.571 nm measured from 4 K to 300 K was well described by phonon scattering at higher temperatures, with an additional anomalous linear temperature-dependent broadening at temperatures below 80 K. Two magnetically inequivalent subgroups of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions were identified when a magnetic field was applied along the $\ensuremath{\langle}111\ensuremath{\rangle}$ axis, as expected for the ${D}_{2}$ sites in the cubic symmetry crystal, with ground and excited state effective $g$-values of ${g}_{\mathrm{g}}=3.40$ (3.34) and ${g}_{\mathrm{e}}=1.04$ (2.01), respectively. Together with the convenient diode laser wavelength of this transition, our study suggests that $\mathrm{Y}{\mathrm{b}}^{3+}$:YAG is a promising material system for spectral hole burning and quantum information applications.

A possible new origin of long absorption tail in Nd-doped yttrium aluminum garnet induced by 15 MeV gold-ion irradiation and heat treatment

When ion irradiation introduces point-defects in semiconductors/insulators, discrete energy levels can be introduced in the bandgap, and then optical transitions whose energies are lower than the bandgap become possible. The electronic transitions between the discrete level and the continuous host band are observed as a continuous tail starting from the fundamental edge. This is the well-known mechanism of the absorption tail close to the band-edge observed in many semiconductors/insulators. In this paper, we propose another mechanism for the absorption tail, which is probably active in Nd-doped yttrium aluminum garnet (Nd:YAG) after ion irradiation and annealing. A Nd:YAG bulk crystal was irradiated with 15 MeV Au5+ ions to a fluence of 8 × 1014 ions/cm2. The irradiation generates an amorphous layer of ∼3 μm thick with refractive index reduction of Δn = −0.03. Thermal annealing at 1000 °C induces recrystallization to randomly aligned small crystalline grains. Simultaneously, an extraordinarily long absorption tail appeared in the optical spectrum covering from 0.24 to ∼2 μm without fringes. The origin of the tail is discussed based on two models: (i) conventional electronic transitions between defect levels and YAG host band and (ii) enhanced light scattering by randomly aligned small grains.