1. Introduction

In recent years, femtosecond laser pulses have been widely studied because of its ultra high power density and the corresponding multi-photon reactions. Various microstructures were observed when the femtosecond laser pulse was focused into transparent material. In particular, the space-selective modified structures can be used to fabricate three-dimensional integrated photonic devices such as optical waveguide [1], photonic crystal [2], optical memory and so on [3, 4].

Upconversion luminescence has attracted a lot of attention in the past decade. Researchers have investigated rare-earth-ion-doped transparent solid-state materials [5], in which the predominant mechanisms of the upconversion are energy transfer, excited-state absorption, cooperative upconversion, and photon avalanche. Recently, there have been some reports on upconversion of rare earth ions and transition metal ions due to multi-photon absorption under femtosecond laser irradiation [6-10]. For example, Meng et al reported intense upconversion emission of transition metal ions such as Ti⁴⁺ and Ta⁵⁺ in oxide glass by femtosecond laser irradiation. In this paper, we report for the first time the upconversion luminescence in Nb⁵⁺ ions doped silicate glass, i.e., the observation of intense visible emission under near-infrared femtosecond pulsed laser irradiation. The results indicate that transition-metal ion Nb⁵⁺ with vacant d orbitals, exhibits similar upconversion luminescence feature, thus playing a potential role in the field of optical storage, optical display etc.

2. Experiments

Traditional Na₂O-CaO-SiO₂ silicate glass system was chosen as the host for metal dopant due to its excellent glass-forming ability, higher solubility for noble elements and higher stability. The compositions (in mol %) of tested samples are 20Na₂O·10CaO·70SiO₂ (Sample 1, matrix) and 20Na₂O·10CaO·69SiO₂·1Nb₂O₅ (Sample 2), About 100 g batches were melted in a platinum crucible at temperature 1550 °C for 3 hrs. The resultant colorless and transparent glasses were cut and polished into 1 mm-thick plates.

Light source for irradiating the glass samples was a regeneratively amplified 800 nm, 120 fs, 1 kHz mode-locked Ti: Sapphire laser (Tsunami model 3960c, Spectra Physics). The samples were put on a three-dimensional XYZ stage controlled by a computer. The laser beam was focused by an optical lens with focal length of 100 mm into the glass sample, about 0.5 mm beneath the surface. Upconversion emission spectra were obtained by ZOLIX SBP 300 fluorescence spectrophotometer. Fluorescence spectra were also recorded on JASCO 6500 fluorescence spectrophotometer by using a Xe lamp as an excitation source. The emission and the excitation spectra were recorded using a JASCO FP-6500 fluorescence spectrofluorometer at room temperature. Optical absorption spectral measurements were performed with a JASCO-V570 UV-VIS-NIR spectrophotometer. All the measurements were carried out at room temperature.

3. Results and discussion

The niobium ions doped glass sample can yield a bluish green emission observable even with the naked eye under femtosecond laser irradiation. The spectrum is shown in Fig. 1. The glass sample containing niobium ions (Sample 2) produces a broad band fluorescence from 350 nm to 650 nm peaked at about 480 nm, while glass matrix (Sample 1) does not show obvious emission in this region. The emission spectrum of Sample 2 excited by 267 nm monochromatic light from a xenon lamp is also shown in Fig. 1 for comparison. Although the intensity of upconversion pumped by NIEPL is different from that by UV excitation due to the dissimilar fluorescence measurement condition, their spectral profile is almost the same, indicating that the emissions in both cases come from an identical origin.

 

Fig. 1. Emission spectra of the niobium ions doped glass sample under focused 800 nm femtosecond laser irradiation and 267 nm monochromatic light excitation, respectively.

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The log-log plots of the upconverted visible emission intensity versus pump power for NIFPL excitation are shown in Fig. 2. The upconverted emission intensity is related to the pump power through the following formula: I□Pⁿ [11], where I is the integrated intensity of fluorescence, P is pump power of the infrared laser, and n is the corresponding number of photons involved in the multiphoton absorption process. The solid points represent the experimental data; the line is linear fit with a slope equal to 3. It is obvious that the experimental data are consistent with the fit line under low-pump-power excitation. It must be a multiphoton related process when we observed visible emission of niobium ions doped glass excited at 800 nm during the femtosecond laser irradiation [12]. In the present case, we couldn’t observe any detectable structural change after irradiated by an 800 nm, 120 fs laser with the average power at about 40 mW in the glass by optical microscope. Thus we could suggest that the upconversion luminescence is not due to multiphoton ionization and avalanche ionization which are relevant to dielectric breakdown. The results indicate that the three-photon-absorption process is responsible for the appearance of the visible emission.

The PL around 480 nm found for niobium ions-doped silicate glass, as shown in Fig. 1, has not been reported until now to our knowledge. Though the emission from Sample 1 is invisible, the niobium ions-doped glass sample shows bluish green emission. Since little information is available on the emission from niobium ions in glass hosts, and there is no emission in niobium ions, we thus refer to the luminescence from closed-shell d⁰ oxo-complexes such as vanadate [13], titanium [8] and niobium - contained crystal [14]. Their optical absorption and emission are usually ascribed to charge transfer transitions between the oxygen 2p orbitals and the empty d orbitals of the transition metal ions.

 

Fig. 2. Log–log relationship between the emission intensity and the pump power of the femtosecond pulsed laser for Sample 2.

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The luminescence from closed-shell d⁰ oxo-complexes also were observed in the silicate glass as shown in Tab.1. The wavelengths of their emission and excitation behave obvious blue shift with increasing the atomic number, implying that tunable emission properties can be achieved by nd⁰ ion-doped glass materials. Such a blue shift is most likely related to the ionicity or covalence degree of ions as indicated by the electronegativity in Table 1. The electronegativity difference between M-O (M: metal element) are useful in determining if a bond is to be covalent or ionic. Generally, the smaller the electronegativity of the M atoms, the more ionic the bond will be formed between them. It is seen that the electronegativity of metallic elements decreases in the order of V, Nb and Ta, indicating the increased ionic degree of M-O bonds from a chemical viewpoint. On the other hand, the different of the electronegativity values between V and Nb elements is small; however, the ionic degree of NbO^- ₆ octahedron with high coordination is higher than that of VO₄ tetrahedron with high coordination. Since higher covalence exerts the depressing effect on transition of outer electrons of metallic ions, the corresponding energy band becomes broader, and emission or excitation shifts to the shorter wavelength.

Table 1. Luminescence from closed-shell d⁰ oxo-complexes

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Figure 3 shows the absorption spectra of Sample 1 (curve a), sample 2 (b), the difference spectrum (c) between Sample 1 and Sample 2, and the excitation spectrum of Sample 2 obtained by monitoring the emission at 480 nm (d). The difference absorption spectrum exhibits a narrow band centered at 280 nm extending from 250 to 300 nm, which may be ascribed to the charge transfer from O^2- to Nb⁵⁺. The difference of absorption intensity is high, which indicates that this absorption band contributes to bluish green emission efficiently. Indeed, strong bluish green emission excited by UV photons has been observed in the present investigation. The profile of excitation spectrum is well consistent with the difference absorption spectrum shown in Fig. 3 except for a little shift in peak position. No overlap occurs in excitation and emission spectra, which means that the excitation process of Nb–O charge transfer band can efficiently contribute to the bluish green emission.

 

Fig. 3. Absorption spectra of Sample 1 (a), sample 2 (b), the difference spectrum (c) between Sample 1 and Sample 2, and the excitation spectrum of Sample 2 obtained by monitoring the emission at 480 nm (d).

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Based on the above discussion, we propose the following mechanisms for the efficient broad band visible upconversion luminescence. That’s, Nb⁵⁺ ions is incorporated into the silicate glass network as distorted MO octahedron, substituting SiO₄ groups and giving rise to non-bridging oxygens. In the glass network, Nb⁵⁺ ions exist mainly as NbO^- ₆ octahedron which shares side with SiO₄ [16]. The transition involved is a charge transfer from oxygen to the d⁰ ion. An electron is exited from a non-bonding orbital (on the oxygen ions) to an antibonding orbital (main do on the Nb⁵⁺ ions). An electron in the 2p orbital of oxide ions is excited to the Nb⁵⁺ 4d⁰ energy level by absorbing three 800 nm photons. The excited electron thermally relaxes to the lowest vibrational level of Nb⁵⁺ 4d⁰ state; and then radiatively relaxes to the 2p orbital of oxide ions accompanied by emission of a bluish green wavelength photon.

4. Conclusions

In summary, we have experimentally demonstrated upconversion luminescence in niobium ions doped silicate glass by a focused infrared femtosecond laser irradiation. The relationship between the fluorescence intensity and the pump power density shows that the pump process is a three-photon excitation process.

The glass materials presented herein are expected to find applications in high density optical storage, colorless transparent fluorescence material and three-dimensional color displays.