1. Introduction

With the rapid progress of thin-film waveguide technology, highly integrated chip-scale photonic devices are desirable for widespread applications in optical amplification, laser, sensing, etc [1,2]. Traditionally, optical waveguide amplifier based on thin-film doped with trivalent rare-earth (RE) ions, specifically Er³⁺ has received much attention after the success of Er-doped fiber amplifiers at 1.5 μm [3,4]. Such a type of devices can compensate for optical coupling and waveguide losses [4]. But, the near-infrared (NIR) emission band of RE-doped films is narrow, severely limiting further development of optical communication. Fortunately, Bi-doped materials with broadband NIR luminescence covering the whole optical communication region are emerging [5–8]. Recently, various types of Bi-doped fiber lasers and amplifiers have been demonstrated in 1150~1550 nm region [9]. Considering these advantages, the integration of Bi-doped thin-films with dielectric and semiconductor wafers will open up broad prospects for the development of integrated optics and optoelectronics. However, further fundamental investigation and applications of the materials are hindered due to the lack of high-quality films with ultra-broadband NIR emission. At present, NIR luminescence has only been exploited in very few Bi-doped silica and silicate films with complicated configuration prepared by co-sputtering at high temperature [10–13]. Yet, without the sensitization of Si nanocrystals, no NIR luminescence was observed. Moreover, it should be helpful to understand the physical mechanism by investigating NIR luminescence in a new type of material systems, i.e. Bi-doped films.

Pulsed laser deposition (PLD) has proven to be a powerful technique of preparing complex oxide thin-films, which can realize stoichiometric transfer of compositions from the target [13]. Meanwhile, germanate glasses have high refractive index, low phonon energy, excellent transparency in the infrared region, and Bi-doped germanate glasses own high NIR luminescence efficiency with large full-width at half maximum (FWHM) [14,15]. Here, we report on the PLD deposition of Bi-doped germanate glass films grown on various substrates, including LaAlO₃, GaAs, Si, and silica. Facile strategies are followed to realize ultra-broadband luminescence from NIR to mid-infrared (MIR) in Bi-doped films. Our results will deepen the understanding of the properties of infrared luminescence in Bi-doped materials.

2. Experimental

Glass target with a nominal composition (in mol%) of 20MgO-5Al₂O₃-75GeO₂-1Bi₂O₃ was prepared by the method described elsewhere [15]. Films were deposited by PLD using KrF excimer laser (λ = 248 nm, pulse length ~20 ns). Photoluminescence (PL) and excitation (PLE) spectra, and the fluorescence decay curves were measured on FLS920 fluorescence spectrophotometer (Edinburgh Instrument Ltd., U.K.). NIR and MIR PL spectra were measured by cooled InGaAs and InSb detectors, respectively. A 450 W Xe lamp, 200 mW laser diodes at the wavelength of 808 and 980 nm were used as the exciting sources. X-ray diffraction (XRD) measurements were carried out using a D/MAX-2550pc diffractometer with Cu Kα as the incident radiation source. Raman spectra were measured on a Horiba Jobin Yvon HR800 Raman spectrometer with a 488 nm laser as the excitation source. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1(a) shows the XRD pattern of the film deposited on silica substrate. There is only a broad diffraction hump at ~21 degree, indicating the amorphous state of the film. As shown in the inset of Fig. 1(a), in contrast to the colorless silica substrate, the film looks orange-yellow with the naked-eye. Figure 1(b) presents PL spectra of films deposited on LaAlO₃, GaAs, Si and silica substrates. All films show emissions peaked at around 1500 nm with FWHM over 450 nm, covering all the O + E + S + C + L (1260~1625 nm) bands of optical communication. PL spectra excited by other wavelengths from ultra-violet (UV) to NIR show similar PL profile. Hence, the observed NIR PL arises from the grown films and the substrates seem to have weak effect on the luminescence. The emission intensity droping at 1380 and 1500~1650 nm is due to the drop of spectral response of InGaAs detector.

 

Fig. 1 (a) XRD pattern of the film deposited at 450 °C with oxygen pressure (P_O2) of 1.0 Pa on silica substrate. The inset shows the digital photographs of the silica substrate and film. (b) PL spectra of films deposited at 450 °C with P_O2 of 1.0 Pa on different substrates. The inset illustrates standard optical communication bands.

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As silica substrate is transparent in 0.2~2.5 μm region [16], the following films are deposited on silica substrate. PL spectra of films deposited in various conditions are shown in Figs. 2(a)-2(c). In Fig. 2(a), as the deposition temperature increases, the emission intensity increases till the temperature is up to 450 °C, and then decreases as the temperature rises further. In Fig. 2(b), as annealing time extends, the emission intensity decreases all the way. Similar luminescence behaviors occur when the films are excited by other wavelengths. At low deposition temperature, low atoms mobility and large amounts of defects may influence the quality of the films, leading to the low emission intensity [13]. Due to the high vapor pressure of both Bi and Bi₂O₃, high temperature or long hours of annealing leads to the decrease of the amount of Bi species in the films resulting in the decrease of emission intensity [17]. In our study, the optimal process condition was found to be 450 °C of deposition temperature without annealing, producing the most intense luminescence. In Fig. 2(c) and the inset, as P_O2 rises, the emission peak continues blue-shifting from 1500 to 1200 nm, and the PL diminishes at 14.0 Pa. Similar phenomenon was observed when the films were excited by the light of other wavelengths. In contrast, no luminescence was observed in the films free of Bi. And no visible luminescence arising from Bi³⁺ and Bi²⁺ was evident in all these films. Thus, it is suggested that the observed NIR PL should result from Bi species of low valence states in the films. Figure 2(d) shows PL spectra of the glass target. The glass target presents an emission peak at 1140 nm under an excitation of 350 nm which is much shorter than that for films. The inset shows the dependence of emission peak position on the excitation wavelength. As the excitation wavelength is changed from 300 to 980 nm, the emission peak position has a large shift in the range of 1080~1330 nm. The observation is possibly due to the co-existence of several different Bi infrared active centers (BIA) [14].

 

Fig. 2 (a)-(c) PL spectra of the films. (a) P_O2: 1.0 Pa, deposition temperatures (T): 300~600 °C. (b) P_O2: 1.0 Pa, T: 450 °C, in situ annealed at 450 °C for 0~2.0 h. (c) T: 450 °C, P_O2: 1.0~14.0 Pa, the inset is the emission peak position as a function of P_O2. (d) PL spectra of the glass target. The inset is the emission peak position as a function of the excitation wavelength.

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PL and PLE spectra of the film deposited at 450 °C under 1.0 Pa are shown in Figs. 3(a)-3(b). The peak positions of both PL and PLE spectra remain unchanged. This phenomenon was also observed in the films prepared under other P_O2, which was different from that found in the glass target. Figure 3(c) presents PLE spectra of films deposited at 450 °C under different P_O2. Films deposited under 1.0~9.0 Pa show an excitation band at around 360~400 nm and two shoulders at around 450~500 and 550~600 nm. While the film deposited under 12.0 Pa shows three bands at around 276, 312 and 533 nm. All these PLE spectra are distinct from those of as-made glass [14]. Figure 3(d) compares the fluorescent decay curves of the as-made glass and films deposited under different P_O2. All measured curves show second-order exponential decay. The mean lifetime of the glass target is 181.3 μs, while that of films deposited under 1.0~12.0 Pa is 22.6, 29.4, 23.5, 15.2, 17.9 and 62.3 μs, respectively. The lifetime of the films is much shorter than that of the glass target, which indicates that different NIR active centers may exist in films and the glass target.

 

Fig. 3 (a)-(b) PL and PLE spectra of the film deposited at 450 °C under 1.0 Pa. (c) PLE spectra of films deposited at 450 °C under different P_O2. (d) Fluorescence decay curves of the glass target and films deposited under different P_O2 excited by 350 nm. The monitored emission wavelength of films in Figs. 3(c)-3(d) is as the peak positions in the inset of Fig. 2(c). The monitored emission wavelength for the glass target is 1200nm.

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Raman spectra were measured as shown in Fig. 4. There is no Bi related Raman mode. For the glass target, the measured bands at 230 and 330 cm⁻¹ are ascribed to Raman active bending mode of Q² and Q¹ tetrahedra [18], respectively. Band at 464 cm⁻¹ is attributed to symmetric stretching vibration of Ge-O-Ge bands associated with three-membered rings of [GeO₄] [19], while the band at 535 cm⁻¹ corresponds to symmetric bending mode of Ge-O-Ge band in [GeO₄] [20]. The bands at 810 and 890 cm⁻¹ are due to TO and LO split antisymmetric stretching of Ge-O-Ge bands [19], respectively. It should be pointed out a lack of comparability between Raman mode intensities of glass and films, keeping in mind that the thickness of films deposited under oxygen pressure of 1.0 and 6.5 Pa are about 800 and 650 nm by a step profiler, while that of glass is 4 mm. Moreover, the luminescence excited by 488 nm leads to different rise of the background of the signals. The main difference of the films from glass is the up-shift of the bands at 464 and 535 cm⁻¹ to 495 and 560 cm⁻¹, indicating the shortened length of Ge-O-Ge bond in [GeO₄]. The band at 890 cm⁻¹ diminishes demonstrating the loss of LO split antisymmetric stretching of Ge-O-Ge bands. These changes imply the increase of inhomogeneities and decrease of connectivity of the network in films in contrast to the as-made glass. However, there is nearly no shift of Raman bands in different film samples, suggesting the weak effect of the film structure on its luminescence properties.

 

Fig. 4 Raman spectra of the glass target and films deposited at 450 °C.

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To provide a deep understanding of the effects of various defects (oxygen vacancy, etc.) on the luminescence properties of films, we compare the PL of as-grown films without any post-annealing and the films annealed under 0.5 atm and 1.0 Pa. PL spectra and fluorescence decay curves of these films are shown in Figs. 5(a)-5(b). Compared to that of the as-grown film, the position of emission peak from both annealed films is slightly changed. The emission intensity of the sample annealed under 0.5 atm is a little stronger than that under 1.0 Pa. Both decay curves show a second-order exponential decay. The mean lifetimes for the films annealed under 1.0 Pa and 0.5 atm are 27.6 and 46.8 μs, respectively, which are longer than that of the film without annealing. Thus, the defects have little influence on the emission peak position of the films deposited under different oxygen pressure, but they are responsible for the decrease in the emission intensity and decay time. Figure 5(c) shows PL spectra of the as-grown films without any post-annealing. The emission ranges from NIR to MIR with a peak at 1650 nm. The drop in the region shorter than 1400 nm is caused by the low spectral response of InSb detector at specific wavelength region.

 

Fig. 5 (a) PL spectra and (b) fluorescence decay curves of films. The films are deposited at 450 °C under 1.0 Pa without annealing or in situ annealed at 450 °C under 0.5 atm and 1.0 Pa for 1 h, respectively. (c) PL spectra of the film deposited at 450 °C under 1.0 Pa detected by InGaAs and InSb detector.

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Finally, the different luminescence properties of glasses and films could be understood. For glasses, some factors like the redox of Bi species during melting, the different oxidization environment in different micro-areas during casting, may play important role in the formation of Bi species, resulting in the co-existence of several kinds of BIA with different characteristics [14]. By contrast, for the film prepared under certain P_O2 in this work, all Bi species undergo quite similar oxidization environment, thus BIA in the specific film has similar feature. Therefore, the peak positions of both PL and PLE spectra remain unchanged, which is clearly evident in Figs. 3(a)-3(b).

As shown in Eq. (1), Bi^{x + n} and Bi^x are Bi species in oxidation and reduction states, respectively; n refers to the number of transferred electrons [9]. As P_O2 decreases, Bi species will be reduced. Moreover, the utilization of high power laser in PLD could induce the reduction of doping active ions [21]. Earlier studies suggested that BIA of low valence state could be generated by laser irradiation [22]. Here, laser ablation of the glass target is capable of inducing the reduction of Bi species, including the generation of Bi clusters of low valence state. Thus, NIR luminescence in films comes from Bi species of low valence states, and the valence state of most BIA in films is lower than that in as-made glass. Earlier studies showed that the lifetime of Bi⁺ and Bi⁰ (τ> 100 μs) is much longer than that of Bi clusters, such as Bi₅³⁺ and Bi₈²⁺ (several microseconds) [6,14,23–26]. Moreover, Bi clusters, such as Bi₅³⁺ and Bi₈²⁺, correspond to NIR and MIR luminescence, respectively [23–25]. Thus, most BIA in films may be consisted by Bi clusters. In addition, compared to as-made glass, the amorphous films in this work possess inhomogeneities and low connectivity of the network, which could be more suitable for the occurrence of Bi clusters. Though the valence state of BIA is not specified here, we can infer that the emission at longer wavelength comes from Bi species of lower valence states compared to that at shorter wavelength.

4. Conclusion

In conclusion, Bi-doped germanate films with ultra-broadband infrared luminescence have been prepared by PLD. The emission peak has a large blue-shift as oxygen pressure increases during deposition. The emission intensity can be greatly affected by the thin-film processing. When the excitation wavelength is changed, the emission peak positions of Bi-doped film prepared under certain oxygen partial pressure remain unchanged. Our work demonstrates broad excitation band covering from UV to NIR region. By considering the availability of the obtained Bi-doped films integrated with various conventional dielectric and semiconductor wafers, the presented results make the developed Bi-doped thin-film photonic materials promising for widespread applications in optical and optoelectronic integrated devices.