Observation of stimulated emission from a single Fe-doped AlN triangular fiber at room temperature

Aluminum nitride (AlN) is a well known wide-band gap semiconductor that has been widely used in fabricating various ultraviolet photo-electronic devices. Herein, we demonstrate that a fiber laser can be achieved in Fe-doped AlN fiber where Fe is the active ion and AlN fiber is used as the gain medium. Fe-doped single crystal AlN fibers with a diameter of 20–50 μm and a length of 0.5–1 mm were preparated successfully. Stimulated emission (peak at about 607 nm and FWHM ~0.2 nm) and a long luminescence lifetime (2.5 ms) were observed in the fibers by a 532nm laser excitation at room temperature. The high quality long AlN fibers are also found to be good optical waveguides. This kind of fiber lasers may possess potential advantages over traditional fiber lasers in enhancing power output and extending laser wavelengths from infrared to visible regime.

garnets, YVO 4 and silica. A suitable span between energy levels might be built up, which can be utilized to realize the spontaneous or stimulated photon emissions. In fact, the photoluminescence (PL) in Cu, Mn, Cr, Ti, or Ni-doped AlN [21][22][23][24] has already been observed. Another aspect deserves being mentioned is that AlN is transparent to a wide wavelength of light from ultraviolet to infrared and exhibits very high refractive index ~2. 15 25 . So, in the context of fiber laser, AlN is an attractive material for the gain medium, which may help further increase the power output. However, AlN as a fiber used for fiber lasers has not been reported to date.
The Fe:AlN fibers were grown by vapor-solid process in an induction heating furnace 24 . The detailed description of growth and characterization of Fe:AlN fibers can be found in the supporting information.
The powder X-ray diffraction patterns in Fig. 1a show characteristic peaks with single phase as wurtzite AlN for 0.13 at.% and 0.28 at.% Fe-doped AlN fibers. No other impure peaks were detected within the instrumental resolution, confirming the Fe-doping did not destroy the hexagonal structure of AlN. Figure 1b is the side-view image of Fe:AlN fibers with the average length of about 0.5-1 mm. From the SEM image of Fe:AlN fibers as shown in the inset of Fig. 1b, it can be seen that the Fe:AlN fibers exhibit triangular profiles with diameters ranging from 20 to 50 μ m. Figure 1c shows the typical HRTEM image of the Fe:AlN fibers. The spacing of 2.49 Å between adjacent lattice planes corresponds to (002) spacing, indicating [0001] is the growth direction for the Fe:AlN fibers. This [0001] growth direction is also confirmed by the result of selected area electron diffraction (SAED) as shown in the inset of Fig. 1c. These results establish the quality monocrystalline nature of the Fe:AlN fibers. The room temperature excitation and emission spectra of Fe:AlN fibers are shown in Fig. 1d. The excitation spectra exhibit a broad band centered at 495 nm which indicates that there is a strong absorption at this wavelength. The emission spectrum of Fe:AlN fibers excited at 495 nm shows a broad band centered at about 600 nm, showing that Fe:AlN has potential candidate gain medium for fiber lasers considering the difference in emission and excitation wavelengths is not wide since the difference, i.e. the quantum defect, determines the surplus heat in lasing action. Figure 2a is the far-field image of a representative single Fe:AlN fiber. The far-field image Fig. 2b,c show optically pumped (325 nm, He-Cd laser, 7 mW) emission from a single Fe:AlN fiber (0.13 at.% Fe and 0.28 at.% Fe, respectively). The area I in Fig. 2b,c is the in-situ PL under laser excitation, most of which was guided along the fibers and emitted at the fiber end. The orange emission as shown in Fig. 2b,c is very consistent with the emission wavelength measured by Xe lamp as shown in Fig. 1d. The localization of bright emission at the end of the fibers (area II) with relatively weak emission in other regions suggests that strong waveguiding behavior 8,26 . Further investigations reveal that all the obtained Fe:AlN fibers have a similar waveguide effect, suggesting that the obtained Fe:AlN fibers can act as good optical waveguide media. These observations demonstrate that they are suitable to realize lasing action.  Figure 3 shows the intensity-dependent PL spectra of a representative 0.28 at.% Fe-doped AlN fiber excited by a 532 nm laser (Nd:YAG) at room temperature. At low excitation intensities, a broad weak emission band appears at about 607 nm which is a little shift compared with the emission at about 600 nm excited by the Xe lamp as shown in Fig. 1d. However, with increasing excitation power density to exceed the threshold (2 mW/μ m 2 ), a super narrow emission of a single lasing mode (peak at about 607 nm and FWHM ~0.2 nm) occurs. Above the threshold, the integrated emission intensity increases rapidly with the pump power density as shown in Fig. 3 right inset. Apparently, the increment of emission intensity with the excitation power density demonstrates that 0.13 at.% Fe-doped AlN fibers can also realize stimulated emission under a higher intensity excitation, with a threshold power density of around 4.5 mW/μ m 2 . However, For 0.13 at.% Fe-doped AlN fibers, the threshold (4.5 mW/μ m 2 ) is larger than 0.28 at.% Fe-doped, but the lasing intensity is lower as shown in Fig. 3 right inset. The different threshold of Fe:AlN fibers may mainly be attributed to two aspects. Firstly, the fibers themselves are natural Farby-Perot (F-P) cavity to realize stimulated emission or lasing. The threshold of a F-P cavity is inversely proportional to the cavity length (L) by G th ~(2L) −1 ln(R 1 R 2 ) −1 , where R 1 and R 2 are the end facet reflections 27 . According to this relationship, different length and end facet roughness of Fe-doped AlN fibers may cause different thresholds. Secondly, different Fe contents in two samples are also responsible for the variation of the threshold [28][29][30] . However, we did not observe stimulated emission in undoped AlN fibers even the pump power density reached 20 mW/μ m 2 . This indicates that Fe ions play an important role in achieving stimulated emission in AlN. However, in some cases, different PL features were observed in fibers doped with similar Fe contents, as shown in Figure S1(b,c). The reason is not clear at the moment, but we expect that the different PL feature can be attributed to inhomogeneous dopings in AlN fibers. Comparison of the emission intensity at 607 nm of 0.0, 0.13 & 0.28 at.% Fe-doped AlN fibers under different pump fluence was summarized in Table 1.  In addition, for high-quality fibers, single-, and multimode lasing may be achieved along the crystallographic c axis with the two end facets acting as end mirrors forming a F-P cavity. The mode-space Δ λ could be calculated by using the expression 31 Δ λ = λ 2 /2 nL, where L is the laser cavity length, n is the refractive index (2.15), and λ is the resonant wavelength (607 nm). For a Fe:AlN fiber with length of 0.5-1 mm, the mode-space between the closest longitudinal modes is expected to be 0.09-0.18 nm. The full width at half maximum of the lasing peak is 0.2 nm as shown in Fig. 3. Therefore, it should exist 1-2 F-P modes for the observed lasing at around 607 nm. This calculated value of longitudinal modes is in good agreement with the experimental results as shown in Fig. 3. However, there should exist multiple transverse modes considering the diameter of the fiber is ~50 μ m, though they are not observed in the present study. Figure 4 shows the luminescence decay curves of the energy level transition from the Fe:AlN fibers at the wavelength of 607 nm. As can be seen, the decay can be well characterized by an exponential function and the luminescence lifetimes of about 2.6 ms (0.13 at.%) and 3.1 ms (0.28 at.%) could be obtained by fitting the decay curve at room temperature. The long lifetimes manifest the high crystal quality of the fibers. It is worth noting that the lifetime in Fe:AlN is much larger than most current laser materials 32 . For example, the lifetime of Mn 3+ doped GSGG, YSGG, GGG, YGG, YAG is < 0.5 μ s, 4.7 μ s, 2.7 μ s, 104.4 μ s, 1.11 ms, respectively. Although the mechanism of long lifetime remains unclear at this stage, the long luminescence lifetime provide further evidence that Fe:AlN may be a promising system for high power fiber lasers.
To our best knowledge, the detailed energy levels of Fe 3+ in hexagonal AlN remain unreported. However, the detailed energy levels of Fe 3+ in GaN may be taken as a reference for analysis because of the similar lattice constant and crystal field between GaN and AlN 25 . When Al site of AlN was occupied by Fe 3+ , the impact of the N ligand field in the form of a Stark effect cause the d 5 configuration of Fe 3+ on Al site to split into the ground state 6 A 1 (S) and the excited states 4 T 1 (G), 4 T 2 (G) and 4 E(G) as shown in Fig. 3 left inset [33][34][35] . In GaN, the energy between 4 T 2 (G) and 6 A 1 (S) is about 2.009 eV (617 nm), which is smaller than the stimulated emission at 2.043 eV (607 nm). This may be the result of the smaller lattice constant and stronger crystal field of AlN. Therefore, the stimulated emission at 607 nm (2.043 eV) may be attributed to the 4 T 2 (G)-6 A 1 (S) transition of Fe 3+ (left inset in Fig. 3) even though a detailed investigation of energy levels of Fe ion in AlN still needs to be done. In this regard, stimulated emission may also be realized in AlN crystal which doped with other transition metals such as Mn, Co and so on.
In summary, the stimulated emission of photons in Fe:AlN fibers was demonstrated. Stimulated emission (peak at about 607 nm and FWHM ~0.2 nm) and very long luminescence lifetime (2.5 ms) were first observed in the fibers under light excitation at room temperature. The high quality AlN fibers also are good optical waveguides. These results suggest that Fe:AlN have potential applications in high power fiber lasers.