Detailed comparative spectroscopic studies of Nd3+ doped YAG nanocrystals, transparent ceramic and single crystal have been performed. Although most of the radiative spectral properties of Nd3+ are almost in good agreement between the three hosts, the non-radiative losses are significantly high in nanocrystals, which are attributed due to the presence of large amount of hydroxyl groups on the nanocrystals surface which deteriorates the quality of the material for laser applications. In addition, wavelength dependent scattering loss for the Nd3+ doped YAG nanocrystals is found significantly high compared to those of Nd3+ doped single crystal and ceramic.
©2012 Optical Society of America
With the advancement of nanoscience and nanotechnology, transparent ceramic materials have gained much attention among researchers and photonics industry due to their several potential applications. In the fabrication of the transparent ceramics, the primary step is to synthesize the nanoparticle with proper size and shape so that the grains in the final high pressure sintered compact structure are well distributed with less pores, boundaries and other kinds of defects [1–8]. Optical transparency in polycrystalline materials is limited by the amount of light, which is scattered by their micro structural features [2,6]. The amount of light scattering therefore depends on the wavelength of the incident light. Most ceramic materials, such as Nd3+: YAG, are formed from fine powders, yielding a fine grained polycrystalline microstructure, which is filled with scattering centers comparable to the wavelength of visible light [6,9–12]. Recently different groups have reported the synthesis method to get the uniform shape nanocrystals with a desired microstructure [6,9–16]. Most of this work is concentrated towards the improvement in the microstructure of the nanocrystals, which ultimately improve the ceramic quality by reducing the scattering centers (pores and grains) on the surface of the ceramic [6,17–19]. With the improved methods and technology, optical quality of ceramics has been greatly enhanced, which has made the ceramic as a highly efficient laser media as reported in several previous publications [18,20,21].
Though the optical, mechanical and thermal properties of the bulk transparent ceramics are important in the final device fabrication, it is also important to understand these properties in the nano or micro structural grains and see how it can change while transforming from smaller size to a bulk transparent structure, especially in doped ceramic structures. Since the optical properties such as absorption, emission, optical gain, etc., can be influenced by the size, shape and distribution of the grains in the final structure. In this work, we are exploring a comparison of various optical properties of Nd doped YAG nanoparticles to those of transparent ceramic and single crystals. A detailed calculation of the inter-Stark emission cross section for the observed emission bands were done and compared with single crystal and transparent ceramic. Further, wavelength dependent scattering loss spectrum for nanocrystals has been calculated and compared with those of ceramics and single crystals. To the best of our knowledge, we are reporting for the first time the detailed comparative analyses of spectroscopic properties of Nd3+: YAG nanocrystals and its counterparts ceramic and single crystals.
2. Experimental detail
2.1 Synthesis and characterization
Nd3+ doped YAG nanocrystals were prepared by a precipitation of select rare-earth salts in a urea solution following the procedure outlined in flow chart as shown in Fig. 1 . All chemicals are 99. 9% pure and were purchased from Sigma–Aldrich. NdCl3 (0.128 mM), YCl3 (2.36 mM), and AlCl3 (3.93 mM) were dissolved on 1.0 M concentrated urea solution set with circulating bath at 90°C for 3 hrs upon constant stir. The OH- ions were generated through solubility of urea in water.The result was the formation of rare earth hydroxides. The hydroxides were then filtered from the solution and washed thoroughly with distilled water, ethanol and then dried at 80°C. The dried powder was then finally sintered at 900°C for 1 h to obtain the corresponding Nd: YAG nanoparticles.
The microstructure of the Nd3+: YAG nanocrystal was characterized by scanning transmission electron microscope (STEM). Energy dispersive X-ray (EDX) analyses were carried out using a Hitachi S-5500 at an accelerating voltage of 30 kV. High resolution TEM analyses were performed using a JEOL JEM-2010F TEM equipped with a field emission gun operated at 200 kV. The digital micrograph software GATAN was used to analyze the TEM images. The sample for XRD measurement was prepared by drying powders and dispersed on a glass substrate. The crystal structure of the sample has been analyzed using X-ray powder diffraction (XRD) at 40 KV and 30 mA in the parallel beam configuration using RIGAKU Ultima IV X-ray diffractometer with Cu Kα (λ = 1.5 A).
The sample for taking the absorption spectra has been prepared by mixing nanocrystals with transparent epoxy in the ratio of two to one and quenched in between two thin transparent glass slab putting epoxy as a reference. Since the nanocrystals were mixed with epoxy, effective index of refraction neff was used for the calculation. The absorption and transmission spectra were collected for 300–900 nm range using a UV-VIS-NIR spectrophotometer (Cary, Model 14R) in the transmission mode. The near-infrared (NIR) emission spectra were recorded under the excitation at 808 nm band of Ti-Sapphire laser (Spectra Physics, Model 3900S) pumped by an Nd: YAG laser (Spectra Physics, Model Millennia). The emission from the sample was collected with a 1.25 m single grating scanning monochromator (SPEX, Model 1250M) and detected by a liquid nitrogen cooled InGaAs detector (DSS-1GA020L, Electrooptic System Inc) for the NIR and photo multiplier tube at −950V bias (Model 1911, Horiba) for the visible. The fluorescence spectrum for NIR transitions was scanned using a 600 grooves/mm diffraction grating blazed at 1.5 µm, with a spectral resolution of 0.01 nm. The detector signal was processed in a computer coupled to the data acquisition system through a lock in amplifier (Stanford Research System, Model SR510,). The entire system was controlled though the data acquisition software Synerjy (Origin Lab), and HORIBA Jobin-Yvon). For emission lifetime studies, the 808 nm output of a pulsed (5 ns) Nd: YAG pumped MOPO SL optical parametric oscillator (Spectra-Physics, Mountain View, CA) was employed on the dry powder as the pumped source. The decay transients were averaged and recorded using a TDS 220 digital oscilloscope (Tektronix, Beaverton, OR). All measurements were carried out at room temperature. The FTIR vibrational spectrum was measured on the solid sample in the transmission mode using the FTIR spectrometer (Bruker, Model vector 22).
2.2 Phase and morphology
Figure 2(a) shows the X-ray powder diffraction patterns of Nd: YAG powders at 900 °C compared with standard JCPDS data of YAG (01-073-3184). It is clear that the patterns were basically consistent with YAG phase and no additional phases were observed. XRD results reveal that the polycrystalline YAG powder sample has a cubic phase structure with unit cell parameters calculated from the XRD patterns to be a = b = c = 12.061 Å. The values are greater than for pure YAG as expected due to partial substitution of Y3+ by Nd3+(it should be a = 12.01 A°, JCPDS No 01-073-3184). From powder X-ray diffraction patterns, the effective crystallite size (particle size) was calculated based on line broadening of XRD peaks. The nanocrystal is constituted by very fine roughly spherical nanoparticles with an average size about ~40 nm, showing a certain tendency to agglomeration.The samples were analyzed for composition using EDX technique. Figure 2(b) shows the representative EDX pattern for Nd: YAG powder annealed at 900 °C. The presence of Nd peak suggests the doping of Nd into the host. The doping of Nd3+ is also confirmed by the absorption spectra as shown in Fig. 3 . The quantitative doping concentration analysis on the final product was done based on EDX. The EDX analysis on different clusters shows that the Nd3+ concentration on the final product is 1.2 wt %, which is 0.8 wt % less than the initial NdCl2 precursor chosen in the starting composition. The dopant concentration was chosen based on the interest of the single crystal (1wt %) and the ceramic (1wt %) available to us. The morphology and the sizes of the prepared YAG nanocrystals were investigated using scanning electronic microscopy (SEM). Figure 2(c) shows the SEM micrograph taken for the samples annealed at 900°C. The high resolution lattice image for the plane (400) is shown in Fig. 2(d). It can be seen that the Nd: YAG powder is well-crystallized, which supports the XRD. The digital micrograph software was used to estimate the lattice spacing‘d’ between adjacent lattice planes in the  direction as shown in Fig. 2(d).
3. Results and data analysis
3.1. Judd-Ofelt (J-O) calculation
The room temperature UV-VIS-NIR absorption spectra of the sample are shown in Fig. 3 with their spectral band assignments. In calculating the absorption coefficient, the scattering losses were subtracted in all absorption measurements. The inset of the Fig. 4 show the images of the sample, which were analyzed for all the comparative study. It was found that all absorption bands of Nd3+ in YAG nanocrystals is similar to those of previously reported for single crystal and ceramic garnet hosts [12,23]. The spectrum is well resolved so that almost every Stark components corresponding to different manifold of Nd3+ are observed.Sixteen absorption bands identified in the room temperature absorption spectra between 300 and 1000 nm, shown in Fig. 3 and are tabulated in Table 1 . These absorption bands were chosen to determine the phenomenological Judd-Ofelt parameters [7,17,24] for polycrystalline Nd3+: YAG powder. The effective refractive index neff was calculated using the following equation as neff = xnnc + (1-x)nmed. Where nnc and nmed corresponds to the refractive index of nanocrystals and the medium respectively. The “filling factor” x describes the volume fraction occupied by the nanoparticles aggregates in the surrounding medium, which was estimated to be 0.59. The calculated wavelength dependent effective refractive indexes were reported in Tables 1 and 2 . The measured and calculated line strengths are given in Table 1 provided an RMS deviation of 0.20 x 10−20 cm2. The RMS line strength is determined to be 2.70 x 10−20 cm2. The ratio of these two values yields an RMS error of about 7.0%, which is the relative error of fit and falls at the lower end of the range of 5 to 25%, as obtained by the application of the J-O theory to other laser materials [7,24–26]. The J-O parameters for Nd: YAG nanocrystals was calculated to be, Ω2 = 0.22 x 10−20 cm2, Ω4 = 3.15 x 10−20 cm2, and Ω6 = 5.71 x 10−20 cm2, with an RMS error of 7.0% and these values are found to be similar with single crystal and ceramic with some experimental similarities.
These phenomenological J-O parameters were subsequently utilized to determine the emission line strengths corresponding to the transitions from the upper multiplet manifolds 2S + 1LJ to the corresponding lower lying multiplet manifolds 2S’ + 1LJ’ of Nd3+ in YAG nanocrystals [7,24,25], radiative decay rates from J to J’, fluorescence lifetime, and branching ratios for the manifold transitions from 4F3/2 to 4I9/2, 4I11/2, 4I13/2, and 4I15/2, as reported in Table 2. More importantly, the radiative lifetimes are calculated with and without epoxy in the sample. The radiative lifetime without epoxy was calculated to be 244 µs which is shorter (275 µs) by approximately 13% when epoxy was added to the sample. The calculated radiative lifetime of nanocrystals with epoxy was found to be greater than that of both the single crystal and ceramic. The radiative lifetime of the sample with epoxy is expected to be longer due to the high refractive index of epoxy (1.556) as s surrounding medium compared to refractive index of air (1.0). The result agrees with what was reported by Meltzer and et al .
3.2 Inter-Stark and intermanifold emission cross sections
The peak emission cross section for the inter-Stark transition is one of the most important parameters of laser design because it describes the maximum spatial amplification of emission intensity. The inter-Stark transition in Nd: YAG nanocrystals are accurately fitted by a Gaussian line shape with a respective full width at half maximum (FWHM). A Gaussian line shape is indicative of inhomogeneous broadening. This inhomogeneity is coming from changes in the local Nd3+ site symmetry, due to the substitution from Nd3+ ions on the Y3+ ion site. All the NIR emission spectra collected for nanocrystals, single crystal and ceramics under 808 nm excitation are shown in Fig. 4. The detailed characterization of the Stark energy levels of the 4I9/2, 4I11/2, 4I13/2, and 4I15/2 multiplets have been determined from the room temperature emission spectra within the manifold 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, 4F3/2 → 4I13/2, and 4F3/2 → 4I15/2 transitions of Nd3+ (4f3) in YAG nanocrystals as shown in Fig. 5 respectively. The energy-level diagrams of the Stark components for the intermanifold 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, 4F3/2 → 4I13/2, and 4F3/2 → 4I15/2 transitions are presented in Fig. 6 respectively.
The inter-Stark emission cross sections for all the Stark transitions are determined from the corresponding spectral line shapes provided they are properly resolved as shown in Fig. 5. The peak emission cross sections (σp) for all the transitions can be expressed accordingly in the following generalized equations [28–30].28–30]:
The peak emission cross sections of the inter-Stark transitions obtained for 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, 4F3/2 → 4I13/2, and 4F3/2 → 4I15/2 are presented in Table 3 , Table 4 , Table 5 , and Table 6 respectively including comparison with single crystal and ceramic. In this comparison study, we found that the inter-Stark cross section for Nd3+: YAG nanocrystal for the inter-Stark transition within the manifold 4F3/2 → 4I11/2 corresponding to 1064 nm is 4.27 x 10−20 cm2, where inter-Stark cross section for the same transition in single crystal and ceramic are found to be 3.85 x 10−20 cm2 and 3.76 x 10−20 cm2 respectively. This shows that these values are very much similar with single crystals and ceramics with some experimental uncertainties as reported in Tables 3, 4, 5, 6, and 7 .28]Table 2, and the values of λ and n are applied to Eq. (4) to determine the emission cross sections for the 4F3/2 → 4I9/2 and 4F3/2 → 4I11/2, 4F3/2 → 4I13/2 and 4F3/2 → 4I15/2 intermanifold transitions. The emission cross-sections calculated for the transitions within the manifolds 4F3/2 → 4I9/2 and 4F3/2 → 4I11/2, 4F3/2 → 4I13/2 and 4F3/2 → 4I15/2 of Nd: YAG nanocrystals are reported in Table 7. These values for the Nd: YAG nanocrystals are found to be very much similar to single crystal and ceramic with some experimental uncertainties.
3.3 Scattering loss
The scattering loss coefficients of Nd3+: YAG and pure YAG nanocrystals were calculated using Fresnel’s equation and compared with those of single crystals and ceramics. Since the dopant concentration of Nd3+ is low, the refractive indexes used were taken from the literature for YAG nanocrystals . The scattering loss coefficient, α (λ), was determined using the following equation [2,6,28];32] is one of the dominating loss within these nanocrystals mixed with the epoxy. Since the agglomeration size of the YAG nanocrystals (400 to 500 nm) approaching the optical wavelength used as shown in Fig. 2(c), the scattering loss in nanocrystals have been found more in UV range as shown in Fig. 7 . The calculation shows that maximum scattering loss is 3.42 cm−1 at 400 nm, where the scattering loss is 1.43 cm−1 at the excitation wavelength (800 nm). Furthermore, the scattering intensity was estimated using Rayleigh’s equation  and the ratio of 400 nm to 800 nm scattering loss intensity was calculated to be 14:1. This verifies that wavelength dependent scattering loss in nanocrystals is significantly high compared to those of ceramics and single crystals. The scattering loss in ceramics is due to the existence of the grain boundary but the calculation shows that single crystals also has some scattering centers which are unknown and needs more analysis.
3.4 Non-radiative processes and internal quantum yield
A quantitative way of measuring the internal quantum yield for a particular emission band is done through the fluorescence lifetime measurements. Non-radiative processes such as multi-phonon relaxation, vibrational losses by surface (such as hydroxyl) species and energy transfer (ET) interactions quench the fluorescence intensity and the efficiency. The observed lifetime of the emission can be written as .34].Fig. 8 . Multiphoton non-radiative loss (Wmp) was calculated using the following equation 35]. Using these values, the multi-phonon vibrational loss for the 1060 nm emission band was calculated to be 0.000001cm−1, which is negligible. Thus the major contribution for the non-radiative decay is coming from the water present in the sample which is confirmed by the absorption band at 3450 cm−1 in the FTIR spectrum shown in Fig. 8. The amount of water content in the samples can be obtained from the measurement of the absorption coefficient at the OH peak of the IR spectrum and is estimated to be nearly 19.5 cm−1. An OH concentration of 360 ± 100 ppm gives absorption of ~2.8 cm−1 . By comparison, the corresponding water content was estimated to be 2507 ppm (4.0 x 1020 molecules/cm3). The major sources of OH content in the Nd3+: YAG nanocrystals are from the starting chemicals and atmosphere during synthesis. A higher level of water content can be suppressed by using moisture free chemicals melted in water free environment
The internal quantum efficiency, (ηInt) can be evaluated from the ratio of the fluorescence lifetime to radiative decay time [25,32]. The fluorescence decay time obtained for the Nd3+: YAG nanocrystals corresponding to 1060 nm is 70 µs as shown in Fig. 9 , which is significantly low compared to >230 µs for single crystal and ceramics. This gives an internal quantum yield of 25.4% for the transition through 4F3/2 which is significantly less compared to > 75% for ceramic and single crystal. This shows that the major non-radiative losses are coming from the surface hydroxyl groups for present nanoparticles, which is absent in the single crystal and transparent ceramic as evidenced from the FTIR spectra in Fig. 8.
Comparison of the radiative and non radiative spectral properties such as absorption cross sections, fluorescence branching ratios, emission cross sections of the inter-Stark transitions and cross sections of the intermanifold transitions show that most of the radiative spectral properties of the three hosts are in good agreement. However the fluorescence decay time and hence the quantum efficiency of the nanocrystal was too low due to the presence of large quantity of hydroxyl groups on the nanoparticle surface. In addition, because of the large agglomeration of the particles with average size of around 400 nm, wavelength dependent optical loss calculated using Fresnel’s equation for the Nd3+ doped YAG nanocrystals is significantly high compared to those of Nd3+ doped single crystals and ceramics. Thus in practical applications of this nanocrystals for making transparent ceramics for laser or other photonics application, careful heat treatment and water free synthesis has to be adopted to achieve high fluorescence quantum yield.
This research was sponsored by the National Science Foundation (NSF) Partnerships for Research and Education in Materials (PREM), Grant Number: DMR-0934218. The authors would like to thank Dr Uwe Hommerich at Hampton University, experimental assistance in lifetime measurements on Nd3+: YAG nanocrystals.
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