1. Introduction

Scintillating optical fiber sensors have been used to detect high energy particles, such as gamma ray, X-ray, or thermal neutrons in many applications including environmental monitoring around nuclear waste sites, geophysical exploration, brachytherapy, and diagnostic radiology [1–9]. Base on the mechanisms of detection, these sensors can be classified as: intrinsic sensors and extrinsic sensors. In extrinsic sensors, the optical fiber is used only to transmit an optical signal to the detector while coupled to an external scintillator. These types of sensors are based on thermoluminescence [1], optically stimulated luminescence [10]. However, they normally require extra components to the measurement facility which increase the complexity of the system for radiation dosimetry. In intrinsic sensors, the scintillating optical fiber with rare-earth ions doping is able to act as both the sensing medium and the signal transmission medium. Mechanisms such as the radiation induced luminescence and radiation induced absorption are used to detect ionizing radiations [11, 12]. Such sensors have clear advantage of structural and refractive index properties similar to commercial optical fibers, making its very useful in low loss remote dosimetry where direct access is denied [13]. Moreover, simultaneous remote and real-time measurement can be implemented.

In particular, Ce³⁺ ion doped YAG and YAP single crystal fibers have been fabricated using laser heated pedestal growth [14], micro-pulling down [15] due to its important scintillation properties of the crystal materials, such as high light yield, fast decay time, and good energy resolution. There are several oxygen vacancy and anti-site defects (AD) of Y3 + Al (Y³⁺ cations in a-sites of Al³⁺ cations) to relate luminescence [16, 17]. However, most of these fibers have been unclad leading to increases of the scattering loss of scintillation signal. In the mean time, the smaller core diameter and the uniformity of the fiber diameter provide obstacles to meet the requirements of remote fiber optic dosimeter systems. Although Ce³⁺ doped SiO₂ optical fibers have been investigated by sol-gel and powder in tube method [18], the concentration of activator ions is difficult to improve due to ease of volatilization and oxidation under high temperature. To improve the characteristic of the single crystal fibers, SiO₂ glass-cladding crystal fibers have been fabricated by employing a tower-drawing with the rod-in-tube (RIT) technique [19]. It is worth noting that the spectral characteristics of the rare-earth ion are strongly host-dependent [20]. A number of point defects can be created in various lattice sites by the high temperature of fiber fabrication due to the presence of both complex cation sublattices and cations of equal charge state [17]. Meanwhile, SiO₂ glass material provides a lot of defects as recombination centers because of the short-range order and the absence of long-range order with an extended three-dimensional network which lacks symmetry and periodicity in silica [20]. One needs to notice that the dependency of scintillating and optical properties on single crystal and silica glass host is still not well understood.

In this paper, the scintillating and optical properties of the SiO₂ glass-cladding YAP:Ce crystal fibers (SYCF) fabricated by a modified RIT (m-RIT) method [21] are studied. We report the photoluminescence (PL) and decay properties of SYCF with YAP:Ce crystal and pure SiO₂ glass based fiber fabrication materials. A recombination competition mechanism of Ce³⁺ ion between YAP and SiO₂ substrate utilizing a microstructure model is used to explain the results. All measurements are directly made using the SYCF samples at room temperature. The origins of recombination center defects, created during the fiber fabrication, are discussed.

2. Fabrication of SYCF optical fiber

The preform of SYCF was fabricated using the m-RIT technique. For a more detailed description of the fabrication procedure see ref [21]. The preform core material was a commercial YAP:Ce crystal rod. A pure SiO₂ glass tube was prepared as the preform cladding. Due to the different thermal expansion coefficients between YAP crystal and silica glass, the preform went through the graphite furnace gradually to guarantee efficient melting. The slow drawing speed could also help prevent crack during the fiber fabrication process. The preform was drawn into the SYCF using a drawing tower at an slow pulling rate of 0.4 m/min and the lower temperature of the graphite furnace with approximately 2000°C than conventional RIT [19, 22].

The compositions of the SYCF sample in core area are analyzed by Scanning Electron Microscope (JSM-6700F cold field emission SEM, Japan) combining with Energy Dispersive Spectrometer (EDS) (SEM-EDS, MX80-EDS, OXFORD, England) as shown in Fig. 1. The concentration of Ce ion is approximately 0.21 at% and slightly lower than 0.3 at% of Ce ion in YAP crystal source rod, and the concentration of Si ion is about 29.50 at% and lower than 45.63 at% in SiO₂ glass tube. The components of Y and Al belong to YAP host is also obtained, as listed in Fig. 1 inset table. The discrepancy in Ce and Si concentration can be attributed to the effective inter-diffusion of Ce ion between YAP:Ce based core and SiO₂ based cladding.

 

Fig. 1 The EDS spectrum and concentration of compositions in the fabricated SYCF core. The inset is the magnified spectrum of Ce component in the red dotted line enclosed frame.

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To further analyze the characteristic of the effective inter-diffusion via m-RIT method at different fabrication temperature, the refractive index difference (RID) between the SYCF core and the cladding is measured by a commercial refractive index profiler (S14, Photon Kinetics Inc.) and reported in Fig. 2. The fiber sample (denoted as SYCF #1) was fabricated using the above condition mentioned at section 2. For comparison, another fiber sample (denoted as SYCF #2) was fabricated at the heating temperature of about 2100°C in air via RIT method. RID of the SYCF #1 is Δn = 0.0756. The numerical aperture is calculated to be 0.85 at 633 nm. The core and cladding diameters were approximately 50 and 120 μm, respectively. Moreover, the RID of SYCF #2 is only 0.0439 which is still one order of magnitude higher than that of commercial single mode fiber (SMF). Due to the intensified inter-diffusion at high temperature, the boundary between the core and the cladding is not steep but gradually change. Combining with EDS analysis, the variation of RID is much clearer indicating the inter-diffusion can be controlled by the fabrication procedure. The flat refractive index distribution of SYCF #1 fiber core demonstrates that optimized temperature can improve the structure of the crystal fiber and SiO₂ cladding during the fiber fabrication procedure.

 

Fig. 2 RID of the SYCF and SMF samples.

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3. Experimental results and discussion

3.1 PL properties of the SYCF materials

PL spectral measurement was recorded by a fluorescence spectrometer (FS980, Edinburgh Instruments) with xenon lamp excitation. The emission range was set from 300 to 600 nm. The excitation range from 230 to 370 nm was set up to meet the emission. In order to increase the intensity of luminescence, all of the SYCF samples with a typical length of about 30 mm were cleaved, polished and placed horizontally on the sample holder. It is worth noting that the fiber materials were under the influence of high temperature and inter-diffusion during the fiber fabrication. The optical transmission spectrum of such type of fiber has been investigated in our previous work [21]. The measured PL spectra of SYCF, crystal source rod and SiO₂ glass-tube, are shown in Figs. 3(a)-3(c), respectively. The emission spectrum of SYCF covers the 320-600 nm range with a primary peak at ~370 nm under 306 nm excitation, as shown in Fig. 3(a). Three decomposed Gaussian components are laid at 348, 381 and 434 nm and denoted as region I, II, III, respectively. The R-square value, namely the coefficient of determination, of the fitting line was 0.9994. It is clearly higher than 0.9963 for two Gaussian fittings. We assume that the three regions are tentatively related to Ce³⁺ ions in YAP and SiO₂ host. The emissions of region I and II originates with a 5d-4f¹ transition of the Ce³⁺ ion in SYCF and YAP [23]. The separation width with approximately 2489 cm⁻¹ is in agreement with the faint energy difference from 2000 to 2500 cm⁻¹ in different host lattices by the spin-orbit coupling interaction [24–26]. On the other hand, the emission peaks at 210 nm (5.9 eV) and 295 nm (4.2 eV) in undoped YAP crystal were reported and belonged to self-trapped excitons and a recombination center, respectively (see Ref [27]. and references there in). Therefore, the impurity Ce³⁺ ions usually acted as “crystal-modifying” element to occupy YAP lattice. The shoulder peak in region III of the SYCF is cooperating with SiO₂ rather than the YAP lattice. This is because there does not exist a peak longer than 430 nm in YAP:Ce crystal [28]. Moreover, the excitation spectra of the SYCF and YAP:Ce rod appears a broad band peaking at ~300 nm attributing to 4f→5d orbital transition of Ce³⁺ ion, which belongs to Ce³⁺ ions substitute for Y³⁺ in sites with C_1h symmetry in YAP:Ce crystal [23, 29]. The 5d sublevels of Ce³⁺ ion in YAP:Ce crystal are experimentally laid at 244, 270, 292, 303 nm as shown in Fig. 3(b). The 244 nm band is ascribed to charge transfer (CT), analogous to those reported for the perovskite host [30]. The peak below 250 nm in SYCF disappeared when compare with YAP:Ce. This is because the AD increase can be aggravated as an inevitable consequence of the high temperature during fiber fabrication [17].

 

Fig. 3 Optical properties of SYCF (a), YAP:Ce crystal rod (b) and pure SiO₂ glass (c). (a) Emission spectrum of SYC fiber by the 306 nm excitation and excitation spectrum of SYC fiber monitored at 370 nm; (b) emission spectrum by the 303 nm excitation and excitation spectrum monitored at 377 nm in YAP:Ce crystal rod; (c) emission spectrum of SiO₂ glass by the 240 nm excitation and excitation spectrum monitored at 397 nm. Regions I and II relate to Ce³⁺ 5d→²F_5/2and 5d→²F_7/2, respectively, and region III relates to Ce³⁺ in SiO₂ host.

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Specially, the shoulder of the emission in SYCF extended to 600 nm is a new recombination center rather than composition from YAP and SiO₂ host. The energy level diagram from the measured PL spectrum fitted by Gaussian function is presented in Fig. 4. This result suggests that the emission spectrum of the fiber is not directly overlapped. The β luminescence band at 400 nm [31, 32] and the 470 nm band [33] have been reported and is ascribed to oxygen vacancies defect in pure bulk SiO₂ and on the nano-sized silica surface. The formation mechanism to which whether the emission belongs Ce³⁺ doped or pure SiO₂ is still questionable. We shall discuss the possible origin of these mechanisms in the following section.

 

Fig. 4 Energy-level diagram of SYC fiber.

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3.2 Decay kinetics

To take further insight into the luminescence mechanism of the SYCF, YAP:Ce crystal and pure SiO₂, the decay kinetics of the three samples were investigated as shown in Fig. 5. Due to the difference in decay time and excitation wavelength, two measurement setups in the ns time and μs time range were employed. In the ns range, the decay spectra of the 370 and 486 nm emission were performed by a microscopic fluorescence lifetime measurement system (MFLMS) (QM/TM/NIR, Photon Technology International) under excitation of 320 nm laser and FS980 under excitation with 405 nm laser, respectively. The decay life of the μs time range was measured by employing FS980 with xenon excitation. The decay time curves were fitted using the exponential equation $I(t)=A\exp (-t/\tau )+B$.

 

Fig. 5 Measured PL decay time of SYCF materials, including YAP:Ce crystal, pure SiO₂ and SYCF. (a) Nanosecond order decay time of SYCF and YAP:Ce; (b) microsecond order decay time of pure SiO₂.

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In Fig. 5(a), the main components of decay times of SYCF and YAP:Ce samples are all about 30 ns around 370 nm that is related to the 5d-4f transition of Ce³⁺ ion in YAP host. This is in good agreement with previous reports [28, 34, 35], and merely a few ns difference is probably due to the ion-pair resonance of energy transfer in rare earth crystal [36]. Meanwhile, in order to distinguish the mechanism of region III, the decay curve of SYCF at 486 nm is obtained by an excitation laser at 405 nm. The 61 ns decay time is clearly longer than that of the emission at 370nm. The presence of this result is an inevitable consequence of Ce³⁺ diffusion from YAP to SiO₂ host because of two reasons: first, the decay of emission peak at 486 nm does not originate from the d-f radiative transition of Ce³⁺ ion in YAP but similar to SiO₂, which is much close to the results reported in bulk SiO₂:Ce by Vedda and Chiodini [18, 37]. The slow decay of Ce³⁺ ion might be obtained at time scale longer than several hundreds ns [37], but the corresponding luminescence intensity of the SYCF are very weak therefore cannot be detected by the microscopic fluorescence lifetime measurement system. In fact, the decay time of SiO₂ material without Ce³⁺ doping varies from 1 μs to 10 ms at different wavelength from 288 to 826 nm [38]. It is worth noting that the scintillation decay shows a noticeable action of fast components originating from radiative recombination of Ce³⁺ centers [17]. In particular, the rare earth ions play an important role as activators of luminescence doped in different materials. As a comparison, the decay component of a pure undoped SiO₂ sample with emission peak at 397 nm is τ = 5.8 μs as shown in Fig. 5(b). The results above illustrate that incorporation of Ce³⁺ ions in SYCF are the result of effective inter-diffusion of Ce³⁺ from YAP to SiO₂ lattice. It is well known that the decay kinetics response is mainly determined by the radiative transition. Hence, the PL decay time of SYCF from 370 to 486 nm validates the above hypothesis of Ce³⁺ doped in YAP and SiO₂ host.

3.3 Competition of Ce³⁺ ion between YAP and SiO₂ microstructure

The result obtained at the study of PL and decay component allowed us to consider possible mechanisms of luminescence center between YAP and SiO₂ host. The optical properties are affected by the total crystal field splitting under different lattice. Crystal field splitting is confirmed by the size and shape of the first anion coordination polyhedron structure around Ce³⁺ [29]. In this case, SiO₂ material as fiber glass tube has no long-range order so that a short-range order atomic arrangement in glass is characterized by an extended three-dimensional network which lacks of symmetry and periodicity [20]. The unique amorphous network of SiO₂ is able to create ample space allowing the inter-diffusion of Ce³⁺ from YAP to SiO₂. Meanwhile, the competition mechanism of Ce³⁺ between YAP and SiO₂ is the perturbing defect form by the high temperature in the preform and fiber fabrication. Growth of stable single crystal YAP:Ce is difficult due to decreased stability of the perovskite phase during the single crystal growth process [39]. Pauling’s rules [40], i.e., the minimum free energy principle, in crystal chemistry for the ionic structure can be utilized to evaluate the coordination of the rare-earth ions in crystal. According the rules, the number of coordination anions around the rare-earth cations can be determine by the rare-earth ion to ligand ionic radius ratio. The radius ratio range of 0.414-0.732 should occupy a six-coordinated site to form a octahedron structure [20]. The Y³⁺ cation (0.97 Å) [41] with radius ratio (R_Y³⁺/R_O^2- = 0.643<0.732) should be six-coordinated by O^2- ion. In YAP structure, the Al³⁺ cations are distributed over the octahedron a-position formed by six oxygen anions O^2-, whereas Y³⁺ cations are located in the spaces between the octahedrons created by twelve nearest O^2- anions [16]. The Ce³⁺ ion radius (1.01Å) is similar to the Y³⁺ ion radius so that it easily replaces Y³⁺ site to form Y3 + Al AD in the YAP lattice. To obtain a more precise idea how the mechanism of inter-diffusion of Ce³⁺ are related to the hosts, we compare SiO₂ and YAP:Ce octahedron arrangement, as the models of SiO₂ and YAP:Ce based on the minimum free energy principle are schematically shown in Fig. 6. Anion and cation vacancies and the AD with the impurity can be realized. The perturbation of structure will aggravate with the Ce³⁺ into YAP. On the other hand, Ce³⁺ ion from YAP:Ce crystal as fiber core material to SiO₂ as fiber cladding is increase with the high temperature of fabrication about 2000°C. This is the reason for the PL of SYCF extends to 600 nm that indicates Ce³⁺ ion has been successfully deposited on SiO₂ networks.

 

Fig. 6 Idealized fragment of SiO₂ and YAP:Ce crystal structure.

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The Si-O distribution functions have revealed that the network system is randomly scattered but homogeneously distributed in the matrix [42]. The network structure can be characterized in terms of rings using the shortest path analysis (see Ref [43], and references there in), which closed paths containing nSi-O segments are referred to as n-membered rings (MRs). In recent years, we reported [42, 44, 45] that the 3-MRs and 4-MRs microstructure models are found to be the most common types in the silica optical fiber materials. In our previous works, the 3-MR segment model of the amorphous silica network has been investigated to calculated the electronic transitions, oscillator strengths and configurations using TDDFT//b3lyp//RECPs [42, 46]. Based on the relationship between the fluorescence characteristics and the oscillator strength, the decay time τ for spontaneous emission is given by the Einstein transition probabilities according to the following formula (in au) [47]

4. Conclusion

In conclusion, we investigated the optical properties of the Ce³⁺-doped SiO₂ glass-cladding fiber fabricated by m-RIT method. EDS and RID analysis indicates that the concentration of Ce³⁺ is decrease due to the successful inter-diffuse effect of Ce³⁺ ion from YAP:Ce crystal core to SiO₂ glass-cladding in the SYCF. The PL properties, including three excitation peaks at 270, 292, 303 nm and a wide emission peaks at 370 nm, are related to the Ce³⁺ ion d-f transition. Meanwhile, a long shoulder of emission extends to 600 nm is observed that is significantly distinguished to the luminescence feature in YAP:Ce crystal. Moreover, using decay kinetics method, the decay times of emission bands at 370 and 486 nm in SYCF sample are 29 and 61 ns, respectively. The results illustrate an impossibility of luminescence lifetime (>μs) in pure silica material. According to previous works and the minimum free energy principle, we calculate the decay time (peak at 486 nm) of Ce-3MR for the SYCF, which is in good agreement with our experiment results. Therefore, the band extends to 600 nm is still Ce³⁺ scintillation center. This study provides a microstructural understanding for the SiO₂ cladding YAP:Ce optical fiber. We believe that such fiber with fast radiative life, super wide luminescence band, better optical guiding capability, high temperature resistances can provide practical applications in the remote radiative environment monitoring and in radiotherapy.