Strain-Dependent Fluorescence Spectroscopy of Nanocrystals and

Article pubs.acs.org/JPCC

Strain-Dependent Fluorescence Spectroscopy of Nanocrystals and Nanoclusters in Cr:YAG Crystalline-Core Fibers and Its Impact on Lasing Behavior Chien-Chih Lai,† Pochi Yeh,‡ Shih-Chang Wang,§ Dong-Yo Jheng,§ Cheng-Nan Tsai,∥ and Sheng-Lung Huang*,§ †

Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan Department of Electrical and Computer Engineering, University of California at Santa Barbara, Santa Barbara, California 93106, United States § Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan ∥ Institute of Electronic Engineering, Cheng Shiu University, Kaohsiung 83347, Taiwan ‡

ABSTRACT: We report the results of our experimental investigation on the strain field distribution in Cr:Y3Al5O12 (Cr:YAG) double-clad crystal fiber (DCF) via spatially resolved near-field imaging in the visible (Cr3+) and nearinfrared (Cr4+) spectral regions. The efficient and lowthreshold lasing from Cr4+:YAG DCF is well-described by a minimization of the localized strain field in the core due to the thermal expansion coefficient mismatch between YAG crystalline core and polycrystalline inner cladding (mixture of γ-Al2O3 nanocrystal and Y2O3−Al2O3−SiO2 glass). The Cr4+ fluorescence is found to be at a maximum for the DCF with a core diameter around 20 μm where the strain field in the Cr4+:YAG DCF laser is near zero. The results are presented and discussed. The strain field is known to have a crucial role in determining the optoelectronic properties of semiconductors.17−19 In terms of engineering, altering the strain while tailoring optical properties is necessary for achieving efficient fiber-optic devices. Extensive piezo-spectroscopic investigations of transition-metal ions in crystal lattices have been carried out,20−23 and the influence of strain dependence on octahedral coordinated Cr3+ in YAG in particular has been widely studied.24,25 To our knowledge, direct observation of the effects of interplay between optical and structural properties, such as size, shape, crystal orientation, and strain fields on tetrahedral Cr4+ fluorescence, has not been reported. Conventional 2D strain measurement is performed using high-resolution transmission electron microscopy (HRTEM). However, preparing good HRTEM specimens to simultaneously obtain HRTEM images and corresponding strain images is extremely difficult, especially for crystal fibers with the double-clad structure, which are nonconductive and fragile heterostructures.26−28 Alternative strain measurement techniques such as Raman spectroscopy, Xray diffraction, and ultrasound are not suitable because of insufficient spatial or spectral resolution.29−32 As a result,

1. INTRODUCTION Widespread interest in transition-metal-doped crystals has been driven for several decades by the fact that they are able to provide a broad fluorescence spectrum, which originates from their 3d electronic configurations coupled to lattice vibrations in the visible and near-infrared (NIR) spectral regions.1−7 Among such materials, ruby (Cr3+:α-Al2O3) is remarkable because of the 2E→4A2 emission of Cr3+ ions located in lattice sites with octahedral symmetry (i.e., R1−R2 lines). This transition is one of the most representative in the history of laser development.8,9 Broadly tunable laser gain media have been of particular interest since the advent of the alexandrite (Cr3+:BeAl2O4) and Ti:sapphire (Ti3+:α-Al2O3) visible range emission lasers.10,11 For emission in the NIR region, this has been demonstrated for Cr4+:Y3Al5O12 (Cr4+:YAG) double-clad crystal fiber (DCF) grown by the codrawing laser-heated pedestal growth (CDLHPG) technique. This DCF shows an exceptionally broad ASE spectrum covering the 1.2 to 1.6 μm range (3 dB bandwidth over 240 nm).12 The same fiber has also been employed in the demonstration of the first crystal fiber amplifier doped with transition-metal ions.13 More recently, the first realization of a compact, efficient, and low-threshold Cr4+:YAG DCF laser has been demonstrated, offering the potential of broad tunability in the NIR region.14−16 © 2012 American Chemical Society

Received: September 11, 2012 Revised: November 20, 2012 Published: November 21, 2012 26052

dx.doi.org/10.1021/jp309024g | J. Phys. Chem. C 2012, 116, 26052−26059

The Journal of Physical Chemistry C

Article

Figure 1. Schematic of the custom designed multipump NSOM system.

current understanding of the strain field distributions in crystalfiber-based devices at a high spatial resolution is very limited. We propose and demonstrate a spatially resolved near-field scanning optical microscope (NSOM) for the direct measurement of strain field distribution. Furthermore, we show the impact of strain fields on the fluorescence properties of Cr:YAG DCF. The difference in thermal expansion coefficient (TEC) between the YAG crystalline core and polycrystalline inner cladding (mixture of γ-Al2O3 nanocrystal (NC) and Y2O3− Al2O3−SiO2 glass) creates a significant localized strain field across the core, which can alter the Cr4+ fluorescence lifetime and emission cross section. A detailed understanding of the strain field distribution and a method of minimizing the strain make it possible to achieve low-threshold lasing. Areas with strain field were imaged using the piezo-spectroscopic effect on the R1 line, Cr3+, and Cr4+ fluorescence properties via the crystal-field ligands. This technique is applicable not only to strain-alternative Cr:YAG DCF but also to any transition-metaldoped crystals in all-fiber communications.

Spatially and spectrally resolved spectroscopy was performed to analyze the localized radial strain field in the DCF. A 532 nm laser and a 1064 nm Nd:YAG laser were used to excite the fluorescent Cr3+ and Cr4+. Further details are shown in the schematic drawing of the NSOM presented in Figure 1. The excitation laser beams were emitted from single-mode fibers and directed toward a laser-line filter and a beam expander. Then, both beams were directed by an edge filter/reflector into a filter turret to separate the laser lines before entering an inverted optical microscope (IX70, Olympus) through a 100× objective (NA = 0.95, Olympus). The tip-to-sample distance of the near-field fiber probe with 30 nm aperture size was controlled by the shear force mechanism through a feedback system. The near-field fluorescence signals were collected through an Al-coated tapered optical fiber (Nufern 630-HP for Cr3+; Nufern 980-HP for Cr4+). The near-field visible Cr3+ and NIR Cr4+ fluorescence signals were then retro-reflected toward the spectrometer and detected by a thermoelectrically cooled charge-coupled detector (BV401A-BV, ANDOR) at −50 °C and a liquid nitrogen cooled photomultiplier tube (R5509-73, Hamamatsu) at −80 °C, respectively. Corresponding near-field images were obtained by scanning the specimen with an x, y stage equipped with a closed-loop feedback system.

2. EXPERIMENTAL SECTION 2.1. Cr:YAG Crystalline-Core Fiber Growth. A 68 μm diameter Cr:YAG single-crystalline fiber was initially drawn from a 0.5 mol % Cr-doped and ⟨111⟩-oriented source rod. It was then inserted into a fused silica capillary with inner and outer diameters of 76 and 320 μm, respectively, to form the DCF structure using the CDLHPG process. The crystalline core diameter was well-controlled by the growth speed and the CO2 laser power of the CDLHPG system. In this study, drawing speeds of 3.75 to 7.5 mm/min were employed to form 11 to 25 μm diameter cores for our experimental investigations. Details of the CDLHPG technique and schematic setup are described in refs 12 and 13, respectively. 2.2. Near-Field Spectroscopic and Nanostructural Analyses. To investigate the correlation between the morphology and near-field spectra of the strained crystalline core, a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20 FEG-TEM) and a custom designed multipump near-field scanning optical microscope (NSOM, NT-MDT NTEGRA Spectra) were used.

3. RESULTS AND DISCUSSION 3.1. Nanostructure and Nanospectroscopy of HighCrystallinity Cr:YAG DCF. Figure 2a shows the end face of an 11 μm diameter-core Cr:YAG DCF with its atomic-scale image, taken along the [111] YAG (Figure 2b). No evident structural defects were observed near the core/inner-clad interface, suggesting that the slightly distorted γ-Al2O3 lattice facilitated the release of residual strain and eliminated the misfit dislocation at the interface. As shown by HRTEM observations, the core had high crystallinity and a sharp core/inner-cladding interface, exhibiting coherent planes with a preferred orientation to the γ-Al2O3 NCs in the inner cladding.26 This mechanism can be explained by a full alignment through the coherence in the 2D plan at the NC/matrix interface, analogous to the case of an Ag NC deposited onto a Cu substrate.33 In addition, coherence yet slight distortion in the 2D planes at the NC/matrix interface took place instead of the generation of 26053



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Figure 2. (a) Fine-polished end face of 11 μm diameter core Cr:YAG DCF. (b) Atomic-scale image of core/inner-clad interface of panel a. (c) Near-field Cr3+:γ-Al2O3 spectra of different individual NCs and clusters in the inner cladding, accompanied by far-field spectra for comparison. Inset: HRTEM images of the Cr3+:γ-Al2O3 NC and cluster with selected-area electron diffraction patterns. Figure 3. (a) Representative near-field fluorescence spectra for Cr3+:YAG with sharp R lines obtained at different positions, with inset showing the corresponding Cr3+ near-field fluorescence intensity mapping of the 11-μm-diameter core of the Cr:YAG DCF. (b) Zoomin image of panel a showing the strain-induced red shift of R-line spectra toward the core edge.

dislocations. This facilitates the stress accumulated by the NC/ matrix lattice difference and results in preservation of the high crystallinity of the YAG core. Figure 2c presents the near-field Cr3+:γ-Al2O3 spectra of different individual NCs and clusters, accompanied by HRTEM images for comparison. Sharp R lines (686.3 nm) associated with three phonon sidebands were clearly observed, whereas the R line of the Cr3+:γ-Al2O3 clusters had a broader line width, indicating that the latter consisted of an ensemble of Cr3+:γ-Al2O3 NCs. In fact, the coalescence of NCs may also have led to defects and disorders, such as dislocations and twins, resulting in a broadened R line.34,35 Additionally, the far-field Cr3+ spectrum was much broader and had a peak wavelength of ∼700 nm. Comparison between the far-field and near-field Cr3+ spectra revealed that the broad emission in the former comprised high-field sites (HFSs, i.e., Cr3+:γ-Al2O3 NCs) and low-field sites (LFSs, i.e., the SiO2 matrix). 3.2. Near-Field Strain Imaging of Cr:YAG DCF. In this section, a detailed characterization of the strained core of Cr:YAG DCF is presented. The performed near-field spectroscopic experiments chiefly focused on a systematic study of the strain-induced effect of Cr:YAG DCF as a function of R-line shifts, and Cr3+ fluorescence properties for different core sizes and orientations. The results clearly showed the direct impact of strain fields on the optical properties of Cr:YAG DCF and will offer useful guidelines for the design of efficient Cr:YAG DCF based active photonic devices. Figure 3a shows the representative near-field Cr3+ fluorescence spectra of the 11 μm diameter core of the Cr:YAG DCF. Typical Cr3+:YAG emission spectra were observed, containing a sharp R line (2E→4A2 transition) associated with three phonon sidebands (∼675, 705, and 725 nm), attributable to the octahedral oxygen-coordinated Cr3+. The corresponding nearfield mapping of the Cr3+ fluorescence intensity is shown in the inset of Figure 3a. Note, in the longer wavelength region for

positions 2 and 3, a weak broad band emission (750−850 nm; T2→4A2 transition) was first observed, suggesting that the Cr3+ ions occupied low LFSs near the core/inner-clad interface. This can be ascribed to the strain induced by the TEC difference between the YAG core and YAG/SiO2 mixed inner cladding. Figure 3b shows an expanded view of Figure 3a. The red-shifted R-line spectra seemingly reveal that the Cr:YAG DCF core was strained, as indicated by the measured 4T2→4A2 broad emission at the core edge shown in Figure 3a. This red-shift effect in Cr:YAG can be attributed to a compressive strain, as is the case for Nd:YAG and Nd:YVO4 analogs, whereas Yb:YAG shows an opposite behavior.36−38 The TECs of the YAG core and silica outer cladding at room temperature (RT) were 7.0 × 10−6 and 0.4 × 10−6 °C−1, respectively;39,40 whereas the inner cladding was expected to have a TEC between the two. This TEC mismatch creates a strain field in the vicinity of the core/inner-cladding during the interdiffusion process. To verify this, spectrally and spatially resolved near-field R1-line mappings were conducted. Figure 4 shows Cr3+ near-field mappings of the Cr:YAG DCF with two representative core diameters (11 and 25 μm). From Figure 4, the ⟨111⟩-hexagonal and dodecagonal shape of Cr:YAG DCF cores can be clearly distinguished. These shapes are in satisfactory agreement with the ⟨111⟩-YAG atomic structures shown in Figure 4a,d. The ⟨111⟩-hexagonal and dodecagonal atomic structures (marked with blue lines) and two crystal orientations (labeled with green arrows) were compared with those obtained by NSOM in Figure 4b,c,e,f. By filtering the R1wavelength with about 0.5 nm line width, contour maps of 4

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Figure 4. Spectrally and spatially resolved near-field Cr3+:YAG fluorescence for (a−c) 11 μm and (d−f) 25 μm diameter core with the ⟨111⟩-YAG atomic structure shown in panels a and d. Blue profiles in panels a and d represent the hexagonal and dodecagonal core shape, respectively. (b,e) R1 fluorescence mappings. (c,f) R1 center wavelength mappings.

near-field R1 fluorescence intensity and R1 center wavelength were further obtained (Figures 4b,c,e,f). As expected, the nearfield R1 center wavelength mappings (Figures 4c,f) clearly showed that the R1 line slightly shifts toward a longer wavelength in the vicinity of the core/inner-cladding interface. This indicated that the TEC-difference-induced red-shift effect occurred at the periphery of the core edge. This R-line evolution is consistent with that of ruby,22 demonstrating an increase in the compression in the vicinity of the core/innercladding interface relative to the core center. TECs have a size effect only when the average size has nanometer dimensions. Therefore, the high surface-to-volume ratio at nanosize strongly affects the physical properties as well as the structural stability, as observed in nanocrystalline γ-Al2O3 and α-Al2O3.41 In addition, the TEC mismatch of the YAG core and glass cladding is small, and the variation of temperature throughout the measurement was relatively low because of the low laser power. As a result, in this case, the size and temperature effects of the TEC mismatch were negligible. 3.3. Strain-Dependent Cr3+ Fluorescence of Crystalline Core. Furthermore, reasonable estimations for the strain variation, K, within the Cr:YAG core can be obtained from the R1-line shifts Δλ using the following equation42

Figure 5. Strain distribution in two different orientations as a function of position for various core diameters, indicating a zero strain crossover at ∼20 μm core diameter.

process (i.e., large core size) the DCF core encountered a transition from compressive strain toward tensile strain as the fused silica diffused deep into the YAG core (i.e., small core size). As a result of the transition, there was a crossover of nearly zero strain across the entire core when its diameter was ∼20 μm. Qualitatively, the presence of a zero strain can be explained by considering the TEC mismatch between core and inner cladding. According to the HRTEM results, the inner cladding was a composite of γ-Al2O3 NCs in Y2O3−Al2O3− SiO2 glass matrix due to YAG/SiO2 interdiffusion. (See Figure 2b.) The fractional content of γ-Al2O3 NCs was found to increase with inner-clad thickness, that is, decreasing core diameter from 25 to 11 μm. Additional HRTEM observations revealed that NCs beyond a certain inner-clad thickness tended to partially coalesce as microsized γ-Al2O3 clusters. It appeared that the effective TEC of the inner cladding became larger and more commensurable with that of the YAG core as the number of γ-Al2O3 NCs increased. This is because γ-Al2O3 has a higher TEC (∼8.0 × 10−6 °C−1) than that of the Y2O3−Al2O3−SiO2 matrix ((∼5 to 6) × 10−6 °C−1).43,44 This mechanism is

Δλ = λ 0{K −s exp[(1/2)D(1 − K )2 + (1/3)(D/E) (1 − K )3 ] − 1}

(1)

where λ0 is the R1 center wavelength at ambient pressure, in the present case λ0 = 687.86 nm, determined using an undoped YAG single crystal, and S, D, and E are constants of 0.3527, −4.266, and 668.7, respectively. According to eq 1, blue shift (Δλ < 0) occurs when K > 1 (tension), whereas red shift (Δλ > 0) occurs when K < 1 (compression). Using eq 1 and an analysis of the spectral shift from Figure 4c,f, the strain field distributions in two different orientations as a function of positions for various core diameters were obtained (Figure 5). The variation of the core diameters was within 5%. The results clearly revealed that in the early stage of the interdiffusion 26055



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analogous to that accounting for the increase in TEC with increasing Ca concentration in CaO−Al2O3−SiO2 glass, as inferred from the crystallization of gehlenite (CaAl2SiO7).45,46 Furthermore, the effective TEC of the inner cladding, αinner, can be reasonably estimated in terms of the TECs (αmatrix, αNC) and volume fraction (xmatrix, xNC) of the matrix and NCs as follows:47 ⎞ ⎛ x NC αinner ∝ αmatrix + (αNC − αmatrix )⎜ ⎟ ⎝ x NC + xmatrix ⎠

(2)

The above relation clearly shows that αinner is linearly proportional to xNC. This suggests that with gradually thickening of the inner cladding, larger xNC and αinner are obtained. In this regard, as core diameter varies from 25 to 11 μm, it is expected that the effective TEC of the inner cladding will eventually be larger than that of the core. On the basis of this, we conclude that zero strain will occur at a certain critical core diameter, in this case, 20 μm. In addition, on the basis of the relationship between the strain and Cr3+ fluorescence lifetime in YAG,25 one can obtain the corresponding Cr3+ fluorescence lifetime distribution across the strained core (see right vertical axis of Figure 5a,b). Clearly, the results agreed favorably as expected from the crystal-field ligands, showing that the lifetime variation is up to two times larger depending on the strength of the strain field in DCF with various core diameters (see Figure 4). The data in Figure 5 show a change (from τ0 = 1.77 ms) in Cr3+ fluorescence lifetime of ∼50%, which is linearly proportional to the applied stress.22,25 Moreover, the electric-dipole transition strength of 2 E→4A2, that is, R1-line fluorescence intensity I, was also examined in terms of lifetime distributions via the following equation, according to ref 22 ⎡⎛ ⎤2 ⎞ τη0 ⎢ ⎟⎟ − 1⎥ I( E → A 2 ) ∝ ⎜⎜ ⎢⎣⎝ τ0 − τ + τη0 ⎠ ⎥⎦ 2

Figure 6. R-line fluorescence intensity distributions for (a,b) 11 μm diameter core and (c,d) 25 μm diameter core, showing good correspondence between experimental and theoretical results.

3.4. Strain-Dependent Cr4+ Broadband Emission Cross Section of Crystalline Core. We employed crystal-field ligand theory to determine the dependence of Cr3+/Cr4+ fluorescence on strain in the Cr:YAG DCF and obtained the following possible explanations. For octahedral coordinated Cr3+ ions in YAG, compressive strain remarkably enlarges the overlapping of the 3d orbital of Cr3+ with the 2p orbital of O2−, resulting in decreased 2E energy and a red shift of the R lines; the blue shift is attributed to the tensile strain, as described by eq 1. For an active Cr4+ ion, the local atomic configuration consists of a Cr4+ ion surrounded by four oxygen ions in a tetrahedral arrangement. This tetrahedron is distorted by the tension along the S4-symmetry axis. The local symmetry thus becomes a D2d point group.48 There are three types of sites oriented along the crystallographic axes, labeled A [100], B [010], and C [001] (refer to figure 11 in ref 48). The distorted tetrahedral sites in YAG are elongated along an S4-symmetry axis, which is parallel to one of the A, B, and C axes and uniformly dispersed. Specifically, when the compressive stress is parallel to [100] (or [010], [001]), one-third of the sites are compressed along the A axis (i.e., S4 axis) (or B, C axes). In this case, reducing the stress-induced distortion of the sites along [100] makes the elongated tetrahedron closer to Td symmetry, resulting in decreasing transition probability and an increased fluorescence lifetime. Meanwhile, two-thirds of the sites are compressed perpendicularly to the S4 axis, leading to a decreased fluorescence lifetime (i.e., increasing the elongation). In this case, a net loss of the Cr4+ fluorescence lifetime is found. With the same argument as that for the case of applying tensile stress along [100] (or [010], [001]), the tetrahedral sites become more elongated, giving rise to an overall decrease of the Cr4+ fluorescence lifetime. Therefore, the above results clearly demonstrate that strain has a very different effect on the Cr3+ and Cr4+ fluorescence. This can also be seen in Figures 7a,b as well as the inset color distributions. In our case, the core edges for both core diameters suffered compressive strain relative to the core center (see Figure 4), resulting in a reduced Cr4+ fluorescence lifetime (or equivalently, Cr4+ fluorescence intensity) at the core edge. Further results exhibiting this decreasing Cr4+ fluorescence lifetime agree favorably with the

4

(3)

where η0 and τ0 are the fluorescence quantum efficiency and lifetime with zero strain, respectively, and τ is the straindependent lifetime. Figure 6a−d shows both the theoretically obtained (via eq 3, black line) and the experimentally measured (red line) fluorescence intensity as a function of radial position. The theoretical results were obtained via the measured lifetime data as well as eq 3 with η0 = 0.77 and τ0 = 1.77 ms and suggested that the estimations used to calculate the Cr3+ fluorescence lifetime and the consequent strain profile across the DCF strained core were appropriate. The discrepancy shown in Figure 6a,b, particularly at the edge, was possibly due to the simultaneous excitation of several waveguide modes in the DCF strained core. The modal interference lead to undulated and stronger Cr3+ fluorescence intensity at the core edge (Figure 6a,b). The above Cr3+ near-field results obtained by NSOM demonstrate that a strain-dependent fluorescence of Cr:YAG DCF can be achieved with systematically varied growth parameters, providing a simple yet effective way to alter the Cr3+ fluorescence lifetime as well as its intensity. As depicted above, we have shown the influence of strain dependence on octahedral Cr3+ in YAG. However, tetrahedral coordinated Cr4+ in YAG has not been studied comprehensively, particularly for fiber forms. 26056



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Table 2. Strain-Dependent Cr4+ Fluorescence Properties and Lasing Characteristics of Cr4+:YAG DCF

measured 3 dB bandwidth of the Cr4+ fluorescence spectra at RT (Table 1), as confirmed by McCumber’s theory49 ln 2/π λ 0 4 η 4πcn2 σeτf

average stress (kbar)

variation in Cr4+ lifetime (%)

variation in Cr4+ cross section (%)

threshold power Pth (mW)

slope efficiency ηs (%)

11 14 19 20 25

−8.8 −6.5 −1.5 0 +4.0

−6.4 −4.8 −1.1 0 −2.9

−20.5 −2.2 −10.4 −1.10 −24.0

96.9 89.6 83.0 78.2 97.9

7.8 31.1 21.8 33.9 4.4

fluorescence difference under 6.8 kbar,50 one can further evaluate the variation in Cr4+ fluorescence lifetime for the various core diameters. As for the variation in the Cr4+ emission cross section, eq 4 was employed before estimating the threshold power Pth and slope efficiency ηs of the Cr4+:YAG DCF laser. An important feature of this Table is that the core diameter strongly affects slope efficiency, demonstrating that the maximum slope efficiency is determined by the vanishing of the strain field. The threshold power has less dependence on the strain field. On the basis of our investigation, the maximum slope efficiency for Cr4+:YAG DCF lasers is achieved at a core diameter of ∼20 μm. To directly examine the impact of strain fields on the Cr4+:YAG DCF laser performance, we used the experimental results of the laser output power against the incident pump power for two core diameters (11 and 20 μm) for comparison. The fiber laser cavity was constructed by direct coatings onto both ends of the Cr:YAG DCF. The detail of the laser setup is depicted in ref 15. To obtain the strain-induced variation in Cr4+ fluorescence lifetime and the emission cross section of the 11 μm core fiber, we calculated the corresponding values from Table 2 using a quadratic approximation, as shown in Figure 8a. Note, as depicted by the crystal-field ligand in Section 3.4, it is sufficient to say that both applied compression and tension yield more elongated Cr4+ tetrahedrons and further decrease the Cr4+ fluorescence lifetime. This implies that maximum Cr4+ fluorescence lifetime occurs at zero strain, which corresponds to a specific core diameter as evidenced by the nonlinear (i.e., quadratic) correlation presented in Figure 8a. Assuming that τf and σe decreased quadratically with the applied stress and considering the fiber was subjected to an average stress of −8.8 kbar, we expected reduction in τf and σe of about 6.4 and 20.5%, respectively. The crystal fiber length, input, and output coupler reflectance at lasing and pump wavelength employed in Table 2 were 5.0 cm, 99.6%, 82.3%, 1.0%, and 96.0%, respectively. For the 11 μm core fiber, its τf and σe were reduced to 2.99 μs (−6.4%) and 5.10 × 10−23 m2 (−20.5%), respectively. Figure 8b shows the Cr4+:YAG DCF laser output power as a function of the incident pump power for both fibers. The equation employed to simulate the lasing behavior of the Cr:YAG DCF in Figure 8b was a quasi-four-level lumped model.14 The simulation results were in very good agreement with the obtained strain-induced reduction in τf and σe, demonstrating

Figure 7. Line-scan profiles of Cr3+ and Cr4+ near-field fluorescence intensity for (a) 11 and (b) 25 μm diameter cores, showing the opposite behavior of the fluorescence intensity at core center and edge.

Δλ =

core diameter (μm)

(4)

where Δλ is the 3 dB bandwidth of the broadband emission, λ0 is the emission peak wavelength, σe is the emission cross section, n is the refractive index of the YAG core, c is the speed of light in vacuum, τf is the Cr4+ fluorescence lifetime at RT, and η is the quantum efficiency. It is clear from eq 4 that Δλ is inversely proportional to the product of σe and τf. Note, in this case, one can measure the product of σe × τf (i.e., Cr4+ fluorescence intensity) by NSOM, as depicted in Figure 7a,b. This suggests that the measured decreasing Cr4+ fluorescence intensity induced by the compressive strain at the core edge caused an increase in 3 dB bandwidth, as depicted in Table 1. The increases in 3 dB bandwidth from the core edge to the core center were around 26.4, 7.0, 11.5, 1.1, and 26.9% for 11, 14, 19, 20, and 25 μm cores, respectively. 3.5. Impact of Strain Effect on Cr4+:YAG DCF Laser Performance. In view of the experimental results presented above, the device performance of the Cr4+:YAG DCF laser was expected to be strongly affected by the presence of a strain field, which is dependent on the core diameter in the case of doubleclad fibers. Table 2 lists strain-dependent Cr4+ fluorescence as well as lasing characteristics of the Cr4+:YAG DCF as a function of core diameter. The average stresses were obtained by measuring the R1-line shifts.42 Moreover, using the experimentally determined relation reported by Jia et al., ∼5% Cr4+

Table 1. Comparison of 3-dB Bandwidth and Emission Center Wavelength versus Core Diameters in Different Positions core diameter (μm) position Δλ (nm)

11 center 231

14 edge 314

center 265

19 edge 285

center 230 26057

20 edge 260

center 260

25 edge 263

center 250

edge 342



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REFERENCES

(1) Martyshkin, D. V.; Goldstein, J. T.; Fedorov, V. V.; Mirov, S. B. Opt. Lett. 2011, 36, 1530. (2) Zhou, S.; Feng, G.; Wu, B.; Jiang, N.; Xu, S.; Qiu, J. J. Phys. Chem. C 2007, 111, 7335. (3) Ricci, P. C.; Casu, A.; Anedda, A. J. Phys. Chem. A 2009, 113, 13901. (4) Borisov, S. M.; Gatterer, K.; Bitschnau, B.; Klimant, I. J. Phys. Chem. C 2010, 114, 9118. (5) Zeng, R.; Zhang, T.; Dai, G.; Zou, B. J. Phys. Chem. C 2011, 115, 3005. (6) Gul, S.; Cooper, J. K.; Corrado, C.; Vollbrecht, B.; Bridges, F.; Guo, J.; Zhang, J. Z. J. Phys. Chem. C 2011, 115, 20864. (7) Inamdar, D. Y.; Pathak, A. K.; Dubenko, I.; Ali, N.; Mahamuni, S. J. Phys. Chem. C 2011, 115, 23671. (8) Hu, P. Phys. Rev. Lett. 1980, 44, 417. (9) Jessop, P. E.; Szabo, A. Phys. Rev. Lett. 1980, 45, 1712. (10) Walling, J.; Peterson, O.; Jenssen, H.; Morris, R.; O’Dell, E. IEEE J. Quantum Electron. 1980, QE-16, 1302. (11) Moulton, P. F. J. Opt. Soc. Am. B 1986, 3, 125. (12) Lo, C. Y.; Huang, K. Y.; Chen, J. C.; Tu, S. Y.; Huang, S. L. Opt. Lett. 2004, 29, 439. (13) Lo, C. Y.; Huang, K. Y.; Chen, J. C.; Chuang, C. Y.; Lai, C. C.; Huang, S. L.; Lin, Y. S.; Yeh, P. S. Opt. Lett. 2005, 30, 129. (14) Lai, C. C.; Tsai, H. J.; Huang, K. Y.; Hsu, K. Y.; Lin, Z. W.; Ji, K. D.; Zhuo, W. J.; Huang, S. L. Opt. Lett. 2008, 33, 2919. (15) Lai, C. C.; Ke, C. P.; Liu, S. K.; Jheng, D. Y.; Wang, D. J.; Chen, M. Y.; Li, Y. S.; Yeh, P. S.; Huang, S. L. Opt. Lett. 2011, 36, 784. (16) Lai, C. C.; Ke, C. P.; Liu, S. K.; Lo, C. Y.; Jheng, D. Y.; Wang, S. C.; Lin, S. R.; Yeh, P. S.; Huang, S. L. Appl. Phys. Lett. 2012, 100, 261101. (17) Jain, J. R.; Hryciw, A.; Baer, T. M.; Miller, D. A. B.; Brongersma, M. L.; Howe, R. T. Nat. Photon. 2012, 6, 398. (18) Zhu, Z.; Zhang, A.; Ouyang, G.; Yang, G. J. Phys. Chem. C 2011, 115, 6462. (19) Pan, H.; Zhang, Y. W. J. Phys. Chem. C 2012, 116, 11752. (20) Sadhu, S.; Patra, A. J. Phys. Chem. C 2012, 116, 15167. (21) Xu, J. A.; Mao, H. K.; Bell, P. M. Science 1986, 232, 1404. (22) Liu, H.; Lim, K. S.; Jia, W.; Strauss, E.; Yen, W. M.; Buoncristiani, A. M.; Byvik, C. E. Opt. Lett. 1988, 13, 931. (23) Salamat, A.; Hyett, G.; Cabrera, R. Q.; McMillan, P. F.; Parkin, I. P. J. Phys. Chem. C 2010, 114, 8546. (24) Wamsley, P. R.; Bray, K. L. J. Lumin. 1994, 59, 11. (25) Shen, Y. R.; Bray, K. L. Phys. Rev. B 1997, 56, 10882. (26) Lai, C. C.; Lin, Y. S.; Huang, K. Y.; Huang, S. L. J. Appl. Phys. 2010, 108, 054308. (27) Lai, C. C.; Wang, S. C.; Lin, Y. S.; Chen, T. H.; Huang, S. L. J. Phys. Chem. C 2011, 115, 20289. (28) Lai, C. C.; Huang, K. Y.; Tsai, H. J.; Hsu, K. Y.; Liu, S. K.; Cheng, C. T.; Ji, K. D.; Ke, C. P.; Lin, S. R.; Huang, S. L. Opt. Lett. 2009, 34, 2357. (29) Vohra, Y. K.; Duclos, S. J.; Brister, K. E.; Ruoff, A. L. Phys. Rev. Lett. 1988, 61, 574. (30) Drescher-Krasicka, E.; Willis, J. R. Science 1996, 384, 52. (31) Yu, T.; Ni, Z.; Du, C.; You, Y.; Wang, Y.; Shen, Z. J. Phys. Chem. C 2008, 112, 12602. (32) Likodimos, V.; Stergiopoulos, T.; Falaras, P.; Harikisun, R.; Desilvestro, J.; Tulloch, G. J. Phys. Chem. C 2009, 113, 9412. (33) Yeadon, M.; Ghaly, M.; Yang, J. C.; Averback, R. S.; Gibson, J. M. Appl. Phys. Lett. 1998, 73, 3208. (34) Feofilov, S. P.; Kaplyanskii, A. A.; Zakharchenya, R. I. J. Lumin. 1996, 66−67, 349. (35) Bulyarskii, S. V.; Kozhevin, A. E.; Mikov, S. N.; Prikhodko, V. V. Phys. Status Solidi 2000, 180, 555. (36) Benayas, A.; Silva, W. F.; Jacinto, C.; Cantelar, E.; Lamela, J.; Jaque, F.; Vázquez de Aldana, J. R.; Torchia, G. A.; Roso, L.; Kaminskii, A. A.; Jaque, D. Opt. Lett. 2010, 35, 330. (37) Silva, W. F.; Jacinto, C.; Benayas, A.; Váazquez de Aldana, J. R.; Torchia, G. A.; Chen, F.; Tan, Y.; Jaque, D. Opt. Lett. 2010, 35, 916.

Figure 8. (a) Strain-induced Cr4+ fluorescence variations of τf and σe as a function of core diameter in Cr4+:YAG DCF. (b) Straindependent lasing behaviors of Cr4+:YAG DCF for 20 and 11 μm core diameters. The favorable agreement of the simulations with the experiments demonstrates that strain-induced variation of Cr4+ fluorescence inside the Cr4+:YAG DCF was responsible for the observed efficient and low-threshold lasing characteristics.

that the strain-dependent Cr4+ fluorescence properties of the Cr4+:YAG DCF is responsible for the observed lasing behaviors.

4. CONCLUSIONS In conclusion, we have successfully obtained near-field images of the radial strain distribution in Cr:YAG DCF using spectrally and spatially resolved NSOM. Our experimental results indicate that the strain caused by differences in TECs has a crucial impact on the optical properties of the DCF in the visible (Cr3+) and NIR (Cr4+) spectral regions. The Cr4+ fluorescence was found to be maximum for a 20 μm core fiber where the strain field is near zero, benefiting efficient and low-threshold lasing in Cr4+:YAG DCF. These results suggest that localized strain field in active crystal-fiber-based devices is an important design consideration. This new class of strain-free Cr:YAG DCF opens up new opportunities to further improve its device performance in all-optic fiber communication and biomedical applications.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mrs. L. C. Wang for conducting the HRTEM experiments using the facility at National Sun Yat-Sen University, Kaohsiung, Taiwan. This work was partially supported by the National Science Council, Taiwan under grant NSC 101-2112-M-259-005-MY3. 26058



Article

(38) Benayas, A.; Silva, W. F.; Ródenas, A.; Jacinto, C.; Vázquez de Aldana, J.; Chen, F.; Tan, Y.; Thomsom, R. R.; Psaila, N. D.; Reid, D. T.; Torchia, G. A.; Kar, A. K.; Jaque, D. Appl. Phys. A: Mater. Sci. Process. 2011, 104, 301. (39) Kikuchi, Y.; Sudo, H.; Kuzuu, N. J. Appl. Phys. 1997, 82, 4121. (40) Wynne, R.; Daneu, J. L.; Fan, T. Y. Appl. Opt. 1999, 38, 3282. (41) McHale, J. M.; Navrotsky, A.; Perrotta, A. J. J. Phys. Chem. B 1997, 101, 603. (42) Chi, Y.; Yang, H.; Liu, S.; Li, M.; Wang, L.; Zou, G. High Pressure Res. 1990, 3, 153. (43) Liu, W. R.; Li, Y. H.; Hsieh, W. F.; Hsu, C. H.; Lee, W. C.; Hong, M.; Kwo, J. J. Phys. D: Appl. Phys. 2008, 41, 065105. (44) Fahmy, A. A.; Ragai, A. N. J. Am. Ceram. Soc. 1987, 70, C-283. (45) Steimacher, A.; Astrath, N. G. C.; Novatski, A.; Pedrochi, F.; Bento, A. C.; Baesso, M. L.; Medina, A. N. J. Non-Cryst. Solids 2006, 352, 3613. (46) Martel, L.; Allix, M.; Millot, F.; Sarou-Kanian, V.; Véron, E.; Ory, S.; Massiot, D.; Deschamps, M. J. Phys. Chem. C 2011, 115, 18935. (47) Fahmy, A. A.; Ragai, A. N. J. Appl. Phys. 1970, 41, 5108. (48) Eilers, H.; Hömmerich, U.; Jacobsen, S. M.; Yen, W. M.; Hoffman, K. R.; Jia, W. Phys. Rev. B 1994, 49, 15505. (49) Kück, S.; Petermann, K.; Pohlmann, U.; Huber, G. Phys. Rev. B 1995, 51, 17323. (50) Jia, W.; Liu, H.; Wang, Y.; Hömmerich, U.; Eilers, H.; Hoffman, K.; Yen, W. M. J. Lumin. 1994, 59, 279.

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Strain-Dependent Fluorescence Spectroscopy of Nanocrystals and

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