We propose and demonstrate a facile approach for ultraviolet-visible broadband generation from a sapphire crystal core–borosilicate glass cladding hybrid fiber using a laser-heated pedestal growth technique. Considerable formation of F– and F2–type color emitters is effectively facilitated by Ti4+ ions and Al3+ vacancies, retaining efficient luminescence and high crystallinity of the sapphire core. These color centers intensify the ultraviolet, blue, and green emissions at 370, 450, and 540 nm, whereas the 650-nm red emission is contributed by Cr3+ in the octahedral sites of the corundum structure. Over 1-mW white light with an optical-to-optical efficiency of up to nearly 5% and 1931 Commission International de l’Eclairage chromaticity coordinate of (0.287, 0.333) is achieved under 325-nm excitation.
© 2013 OSA
In the last decade, studies have increasingly focused on in vivo optical coherence tomography (OCT) systems and their superior capability to replace conventional invasive biopsy in various applications such as dermatology, ophthalmology, and neurology as well as cardiac catheterization [1–5]. In recent times, the rapid development of powerful OCT systems with high axial resolutions has been hampered by the lack of suitable broadband light emissions, keeping in mind the fact that the axial resolution is primarily defined by the 3-dB bandwidth and the square of the center wavelength of the light source. As a representative case, the typical axial resolution offered by commercially mature superluminescent diodes (wavelength: 800−900 nm) is 5–10 μm [6–8], which is unsuitable for three-dimensional cellular imaging. Development has also been hampered by the higher temperature sensitivity and lower reliability of these systems. Unfortunately, most visible (VIS) to near-infrared (NIR) broadband sources such as Ti3+:sapphire (Ti3+:α-Al2O3) mode-locked lasers and photonic-crystal-fiber-based supercontinuum are both expensive and extremely sophisticated [9, 10], making their use for clinical diagnostics difficult. Moreover, in many biomedical applications such as fluorescence microscopy and flow cytometry, broad ultraviolet-visible (UV-VIS) emission is of importance because most fluorochromes have large absorbances [11, 12]. In particular, wavelengths below 450 nm are difficult to cover in the aforementioned supercontinuum generations owing to the large detuning from the excitation. Therefore, there is a strong need to develop functional light sources for biophotonics with emitting wavelengths lower than those of typical Ti3+:sapphire crystals and photonic crystal fibers, especially in the ultrabroadband UV-VIS region.
Transition-metal ions are widely employed in the fields of bioimaging and telecommunication [13–15] because their 3d electronic configurations are tightly coupled to the host vibrations both in the VIS and the NIR regions. Studies have mostly focused on multiple transition-metal ions for the improvement of new broadband luminescent materials; however, this approach may lead to a deterioration in the crystal quality, and consequently, to a short lifetime and weak luminescence that cannot satisfy practical requirements . An alternative approach is to develop defect-driven broadband gain media by intentionally introducing emissive color centers such as anion/cation vacancies and interstitials into a matrix. Owing to the strong coupling between the ligand fields of the color centers and the lattice phonons, the resulting optical transition typically represents wide-range vibrational bands, as has been observed in semiconductors and metal-oxide insulators [17, 18]. Among these, thermodynamically stable sapphire in the corundum form with hexagonal close-packed (HCP) oxygen finds wide applications in the field of photonics and as an abrasive, catalyst, and insulator owing to its high mechanical strength, high electrical resistivity, and high optical transparency . The formation of corundum is of great interest in the field of cosmology owing to its occurrence in presolar stars . Several common approaches have been reported to intentionally introduce defect centers in sapphire crystals. The color centers in sapphire are mainly attributable to oxygen monovacancies (F+ and F centers) and oxygen divacancies (F2+ and F2 centers) caused by high-temperature thermochemical reactions in a reduced atmosphere [21–23] or by high-energy bombardment with high-dose particles [24–26]. However, unlike particle irradiation, normal thermochemical reduction cannot create sufficient amounts of F2–type centers, which leads to a limited emission bandwidth. Similarly, bombardment causes serious problems such as poor crystallinity and low-yield emission because heavy irradiation damages the microstructure and consequently causes concentration quenching of the luminescent centers [24–27]. Thus far, no study has successfully produced a substantial amount of F– and F2–type-based broadband emission using a facile approach while maintaining high crystallinity, which is admittedly recognized as a challenge in the fields of optical and materials science.
Herein, we propose and demonstrate a facile approach for generating UV-VIS broadband emissions from a sapphire crystal core–borosilicate glass cladding hybrid fiber by the laser-heated pedestal growth (LHPG) technique. Efficient white light with milliwatt-level output and 1931 Commission International de l’Eclairage (CIE) chromaticity coordinate of (0.287, 0.333) has been obtained by the excitation of a 325-nm laser. The broad spectrum extends from 330 nm, which is over 50 nm further into the ultraviolet region than in previously reported results. The extra bandwidth is activated via oxygen and aluminum defect centers, which were facilitated by Ti and Cr ions. In addition, the broadband luminescences can be tuned from orange-red light to greenish light and finally to white light by simply altering the defect concentrations while preserving efficient emission and high crystallinity. A possible interpretation of defect center formation is also discussed. Compared to conventional thermochemical- and bombardment-based approaches, the proposed approach requires only a different growth temperature along with a different heating environment in the LHPG system, making it more attractive for practical fiber-based broadband generation.
2.1. Device fabrications
Figure 1 shows details of the procedure by which borosilicate-glass-clad sapphire crystalline-core fibers are grown. First, a c-axis sapphire source rod with a 0.5 mm × 0.5 mm cross section was used as the starting material. The starting materials were prepared by the Czochralski method in strongly reducing conditions that favor Ti3+ over Ti4+ ions, with up to 98% Ti ions in the 3 + valence state [28, 29]. Through three diameter reduction steps by the LHPG technique, a 40-μm-diameter sapphire single-crystalline core was grown in an oxidizing atmosphere. To effectively facilitate the F– and F2–type centers and to tailor UV-VIS wideband generation, minor amounts of Ti and Cr ions (50–100 ppm) were incorporated into the 40-μm-diameter sapphire single-crystalline core. Then, this core was inserted into a borosilicate glass hollow tube with a 320-μm outer diameter and regrown using the same LHPG system to form the borosilicate-glass-clad sapphire crystalline-core fiber. Note that the glass cladding process in the second step was conducted in a ~10−3-Torr reducing environment. The schematic and end views of an as-grown 40-μm-diameter sapphire crystalline-core fiber are shown in the right-hand side of Fig. 1.
2.2. Nanostructural and spectroscopic characterizations
Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were performed using an electron probe microanalyzer (JXA-8900R, JEOL). The nanostructure of the crystalline-core glass-clad interface and the corresponding selected-area electron diffraction (SAED) were investigated by a field-emission high-resolution transmission electron microscope (HR-TEM, Tecnai G2 F20, FEI) operating at 200 kV. The HR-TEM specimen was prepared using a dual-beam focused ion beam (FIB, SMI3050, Seiko) that can cut precisely at a specific location.
Photoluminescence (PL) and Raman experiments were carried out by using a 325-nm He-Cd laser (IK3802R-G, Kimmon Koha) and analyzed by a high-spectral-resolution spectrometer (LabRAM HR 800, JOBIN-YVON) with an 1800 mm−1 grating. A 40 × objective lens with a numerical aperture of 0.50 (LMU-40X-NUV, OFR) was employed to achieve a sub-micrometer spatial resolution. To generate amplified spontaneous emission from the sapphire crystalline-core fiber, an 18-mm-long fiber was wrapped in Sn-Pb alloy at 400 °C and clamped to an Al heat sink after cooling to room temperature. Figure 2 shows the schematic of the room-temperature white light measurement. The 325-nm pump beam was first incident onto a variable attenuator, and it was coupled to the core of a sapphire crystalline-core fiber through a 40 × objective (LMU-40X-NUV, OFR). The white light output and the residual pump beam were collimated by an achromatic lens with 10-mm focal length and further filtered by a long-wavelength-pass filter (BLP01-325R-25, Semrock) before detection by a UV-enhanced Si photodetector (818-UV, Newport). The spectral sensibility of the employed photodetector is in the range of 200–1100 nm.
3. Results and discussion
3.1. Nanostructural analyses
Figures 3(a) and 3(b) show the finely polished end face of a sapphire crystalline-core fiber. The hexagon-like shape of the sapphire core can be clearly discerned in Fig. 3(b), and this is in satisfactory agreement with the c-axis HCP sapphire crystal structures shown in Fig. 3(c) . Figures 3(d) and 3(e) show the corresponding EDX mappings of the major compositions (Al and Si). Further, the EDX results also indicate that the 40-μm sapphire crystalline core is mounted quite well in the surrounding borosilicate glass cladding, reflecting effective optical wave confinement. Figure 3(f) shows the representative EDX spectrum of the sapphire crystal core, which exhibits the main Al and O peaks, whereas the Cu and Ga counts are from the supporting copper grid and the FIB ion gun, respectively. Figure 3(g) shows the low-magnification HR-TEM images taken along [11 0] at the crystal-core glass-clad interface. The atomic structure image [Fig. 3 (h)] corresponding to the red box in Fig. 3(g) reveals a sharp atomic-scale interface with no evident defects, demonstrating that the sapphire core has a nearly perfect single-crystal structure. This can be confirmed by the sharp SAED spots [inset of Fig. 3(h)].
3.2. Defect-tailored broadband tuning
Figure 4(a) shows the 325-nm-excited PL spectral evolution from the starting material to the as-grown sapphire crystalline-core fiber, as shown in Fig. 1. Several sharp peaks corresponding to the spontaneous Raman signals of the sapphire crystals are observed in the short wavelength region. A magnified view of this Raman region is shown in Fig. 4(b). According to group theory, only two A1g modes and five Eg modes are Raman active . All the observed modes are labeled in Fig. 4(b). The Raman spectra of these three samples reveal seven unambiguous characteristic c-axis sapphire peaks. Note that strongly enhancing the Raman signals of the as-grown glass-clad sapphire fiber [red lines in Fig. 4(a)] indicates the effectiveness of waveguide confinement with high crystallinity and low propagation loss of ~0.1 dB/cm. In the PL results shown in Fig. 4(a), i.e., spectrum of step 2 indicated by a red curve, three broad luminescent bands at 330–400, 400–600, and 600–650 nm are observed. These differ from those of the starting material [gray curve in Fig. 4(a)] and step 1 [blue curve in Fig. 4(a)], which show typical orange-red emissions with remarkably weak blue and UV bands because of insufficient oxygen monovacancies, as is discussed later. The corresponding chromaticity coordinates of these three samples are presented in the 1931 CIE diagram shown in Fig. 5. The CIE diagram shows that the emission colors from the starting material to the as-grown glass-clad sapphire crystalline-core fiber vary from orange-red to greenish via step 1 and then to white light via step 2. Their CIE coordinates are (0.413, 0.402), (0.260, 0.424), and (0.287, 0.333), respectively. These results clearly demonstrate the noticeable variability and different physical mechanism of color center formation during LHPG processes.
3.3. Mechanisms for color center formation
Having demonstrated that we can tune the luminescent properties, we investigate the defect formation mechanism with respect to different fiber growth environments. Referring to step 1 in Fig. 1, the PL spectrum of the 40-μm sapphire crystalline core was found to comprise seven different bands based on Gaussian deconvolution analyses, as shown in Fig. 6. These PL bands peaking at 375, 420, 458, 485, 531, 577, and 648 nm are consistent with the known F and F2 centers, Ti3+/Ti4+ ions and Ti4+-facilitated aluminum vacancies (Ti4+–), and Cr3+ ions with peaks at 380, 420, 460, 480, 510, 590, and 650 nm [23, 29, 32–34], as summarized in Table 1. The aggregated with Ti4+ ions are due to the high binding energy of such Ti4+– clusters . As listed in Table 1, the F+ luminescent band shifts from 330 nm to 352 nm because a longer excitation is used, i.e., 260 nm versus 325 nm. This is typical of classical F–type-center luminescence in phonon-assisted alkali halides [35, 36] owing to the strong coupling nature of a trapped charge to the lattice with large Huang-Rhys factors and Stokes shifts .
Among these centers, F22+, F2, and Ti4+– were found to mostly contribute to greenish emissions. However, the luminescent band at ~530 nm has also been assigned to donor centers such as interstitial aluminum ions () . At this point, a 40-μm sapphire single-crystalline core was grown in an oxidizing atmosphere, implying that the formation of Ti4+ ions is preferred, and this viewpoint persisted for long [28, 29]. This enables one to obtain a rather large Ti4+ emission band at 420 nm. The blue emission of Ti4+ is a characteristic charge-transfer transition, as observed in many wide-band-gap materials, following the scenario [29, 39, 40]
Another salient feature is that the introduction of Ti4+ causes the formation of aluminum vacancies for charge compensation. In other words, electroneutrality requires that a vacancy be formed for every three Ti4+ ions under oxidation, which is expressed as41]. Indeed, as compared to the starting material in Fig. 4, we found that the growth of the 40-μm sapphire crystalline core by heating under the present oxidation condition produces nearly 7 times enhancement at 485 nm, indicating the domination of Ti4+-facilitated at this stage. On the other hand, oxidation can cause increasing interstitial oxygen () by consuming oxygen vacancies (), resulting in no observable amount of F–type centers, as shown in Fig. 5. This can be expressed as follows:Equations (2) and (3) suggest that Schottky-type disorders exist in the as-grown 40-μm sapphire crystalline core.
It is also noteworthy that the aggregated vacant centers with Ti4+ ions are negatively charged, and therefore, the incorporation of these centers must be concomitant with the introduction of positively charged F2–type centers. This leads to a compatible concentration of F22+ centers along with a lesser amount of F2+ centers, as denoted by orange curves in Fig. 6. In this case, heat treatments were carried out in air to enhance oxygen divacancy concentrations, as conducted in Mg-doped sapphire single crystals [22, 33].
To clarify the effect of the growth environment on the broadband luminescence, we examine the vacuum-assisted heating process, referring to step 2 in Fig. 1. The glass cladding sapphire core was conducted under vacuum at ~10−3 Torr, indicating that the reduction of the sapphire core increases the concentration of Ti3+ beyond that of Ti4+. We thus see a much smaller extent of Ti4+ than that of Ti3+, i.e., 406 nm versus 446 nm in Fig. 7. The considerable amount of aggregated with Ti4+ ions centered at 471 nm can be ascribed to the isochronal introduction of negatively charged defects , because the F– and F2–type centers and Ti4+ ions are positively charged centers, attaining local charge neutrality.
In Fig. 7, for this sample in the wavelength of <400 nm, it is noteworthy that the PL intensities of F and F+ are nearly two times higher than those of the starting material and 40-μm sapphire core without glass cladding, as shown in Fig. 4(a). Furthermore, in this sample, the F22+ center shows a quietly low PL intensity. Previously, Ramírez et al. postulated the following reaction to account for the observed reversible thermal-induced interconversion between F+ and F22+ centers :Fig. 1), F+ centers aggregate more satisfactorily and energetically and form F22+ centers. Taking into consideration the fact that when a starting material is heated using a CO2 laser in the LHPG system, bulk sapphire melts at a temperature above the melting temperature of sapphire at 2,054 °C. The resulting as-grown 40-μm sapphire crystalline core contains substantial amounts of F22+ centers, exhibiting a strong emission at 577 nm, as shown in Fig. 6. Furthermore, this high-temperature heat treatment often results in large aluminum vacancy, aluminum interstitial, and oxygen interstitial concentration ratios owing to their high formation energy, as evidenced by the strong greenish light at 450–600 nm in Fig. 6. On the other hand, at temperatures below 673 K, i.e., step 2 in Fig. 1, the F+ center mobility is too low to be a cluster, yielding relatively low luminescence at ~590 nm. In fact, for step 2, the glass cladding in the LHPG process was made near the transition temperature of borosilicate glass at 525 °C, producing a pliable state, and attached to the sapphire crystalline core.
Under 325-nm laser excitation, maximum white light output power over 1 mW was achieved when the incident pump power was 24.3 mW, as shown in Fig. 8. It is noteworthy that the milliwatt-level white generation with corresponding optical-to-optical efficiency of nearly 5% is the highest among existing active waveguide schemes [42–44]. This high conversion efficiency is attributable to the large emission cross sections of the color emitters , and it is contributed to by the phonon-driven odd-parity vibrations. The emission cross sections of F– and F2–type color emitters in sapphire are one to two orders of magnitude higher than those in typical Ti3+:sapphire and Cr4+:YAG broadband gain media, namely, ~10−21–10−22 m2 versus ~10−23 m2 [28, 46–49]. In addition, from the calculated numerical aperture of 0.89 and solid angle of 2.89 in a 40-μm-diameter sapphire core–borosilicate cladding hybrid fiber, one can evaluate a luminous flux of ~0.8 lm based on the milliwatt-order white light generation, giving a luminance of ~108 cd/m2. This result is comparable to those obtained using III-V and III-nitride light-emitting diodes [50, 51].
In conclusion, a white light exhibiting a CIE chromaticity coordinate of (0.287, 0.333) and 1.16-mW output power is successfully obtained using a sapphire crystal core–borosilicate glass cladding hybrid fiber with intentionally introduced defects and dopants as color emitters by the LHPG technique. We experimentally demonstrated that a reversible thermal-induced interconversion between F+ and F22+ centers occurs simply by heating in different environments. The thus prepared glass-clad sapphire crystalline-core fiber consists of not only dopant-facilitated aluminum vacancies but also oxygen monovacancies and divacancies, which remains challenging in sapphire crystals. Our proposed facile approach can possibly enable control of broadband color tuning while attaining high crystallinity. Efficient white-light generation is suitable for the previously mentioned biomedical applications for long-range and fiber-type endoscope-compatible milliwatt-level light sources.
The authors are grateful to Prof. S. L. Huang for the insightful discussion. The authors also thank Mrs. L. C. Wang and Mr. H. D. Chiang for conducting the HR-TEM and EDX experiments at the facilities at National Sun Yat-Sen University, Kaohsiung, Taiwan. C. C. Lai acknowledges the strong funding support from the National Science Council of Taiwan via the grant NSC 101-2112-M-259-MY3 as well as the start-up funding from the National Dong Hwa University.
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