1 Introduction

Recently, metal nanoparticles (MNPs) and semiconductor nanoparticles (SNPs)-doped glasses have attracted much attention due to their unique linear and nonlinear optical (NLO) properties which are different from those of bulk metals [1–7]. They have a large third-order NLO susceptibility because of a large local-field enhancement factor and strong resonant absorption around the surface plasma peak of MNPs and large enhancement from the quantum confinement effect of SNPs [1, 2, 5]. The MNPs- and SNPs-doped glasses are usually fabricated by various processes such as melt-quenching, femtosecond laser irradiation and sol–gel methods [8–12]. Among these techniques, melt-quenching method remains the most convenient method as it offers facile access to a variety of glass systems doped with different crystalline phases with a simple and controllable process. By this process, noble metals such as Au and Ag as well as chalcogenide semiconductor quantum dots (QDs) can be precipitated in glasses of diverse chemical compositions, thus allowing for the investigation of optical properties and the quantum confinement effect [13, 14]. In comparison, the precipitation of non-noble MNPs and SNPs such as Si, Ga, Bi, Ge, Al and ZnTe has been reported only in a limited number of oxide glasses recently [5, 6, 15]. Lin et al. [5] reported that the metal and semiconductor NPs (Si, Cu, Ge, and Bi)-doped glasses prepared by the conventional melt-quenching method exhibit large third-order optical nonlinearities (χ (3), about 10−8 esu) and ultrafast response time (within picoseconds). As they usually form strong covalent bond in the glass network, the precipitation of these NPs has been difficult and apparently quite selective for parent glass. However, their unique optical properties combined with the difficulty in the stabilization of their NPs in the colloidal solution phase have stimulated growing interest in the search of suitable glass systems for doping with these NPs, which have been expected to find applications in photonic fields, such as ultrafast all-optical switching, optical communication and optical storage.

In this investigation, we report the precipitate Te NPs in an oxide glass of GeO2–MgO–B2O3–Al2O3–TeO2 by the conventional melt-quenching method. The precipitation of Te NPs in the glasses was confirmed by X-ray photoelectron spectroscopy, Raman spectroscopy and transmission microscopy. The NLO properties of the Te NPs-doped glasses were investigated by using the Z-scan technique. The results reveal that the Te NPs-doped glasses exhibit excellent NLO properties, which are attractive for photonics applications.

2 Experimental

The germanate glass samples with the molar compositions of (60 − x)GeO2–xTeO2–20MgO–15B2O3–5Al2O3 (simplified as GTx, x = 0, 0.5, 1 and 3) were prepared by the conventional melt-quenching method. The raw materials are analytical grade MgO, H3BO3, Al2O3, 3 N TeO2 and 5 N GeO2. For each batch of 20 g raw materials was mixed thoroughly and melted in alumina crucibles at 1500 °C for 30 min under air atmosphere. The melts were quenched on a stainless steel plate where the as-cast glasses were cooled down to room temperature. The glass samples were cut into small plates and then optically polished before optical characterizations.

The linear absorption spectra were recorded between 300 nm and 1800 nm using a UV-3600 spectrophotometer (Shimadzu Instruments). Raman spectra were recorded in the range of 100–1000 cm−1 using a LabRAM HR800 Raman spectrometer (Horiba Jobin Yvon Instruments) with 514 nm Ar+-ion laser as excitation source. The oxidation state of Te in glass was characterized using an ESCALAB 250XI photoelectron spectrometer (Thermo Instruments) with Al-Kα X-ray as the excitation source, and the C 1s (284.8 eV) peak was used as the energy reference. The microstructure of the samples was observed by a Tecnai G2 F30 S-Twin transmission electron microscope (Philips-FEI Instruments), operating at 300 kV. The NLO properties of these glasses were measured by using Z-scan setup equipped with a Ti/sapphire laser with a wavelength of 800 nm, pulse width of 130 fs and repeating frequency of 1 kHz. The refractive index of the sample was measured by a Model 2010/M prism coupler (Metricon Instruments). All the measurements were taken at room temperature.

3 Results and discussion

The glass without TeO2 is colorless and highly transparent in the visible and near infrared (NIR) region. With increasing in TeO2 concentration, the as-cast glass changes its color from brown to black, as shown in the inset of Fig. 1. The linear optical absorption spectra of the glasses are also shown in Fig. 1. As shown clearly, with increasing in TeO2 concentration, the optical absorption coefficient increases rapidly from 300 nm to 1200 nm. The change of optical absorption coefficient can be attributed to the presence of Te-related centers, presumable in the form of elemental Te.

Fig. 1
figure 1

Linear optical absorption spectra of the glasses. The inset is the photographs of the glasses

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To confirm the precipitation of elemental Te in the glasses, we recorded XPS and Raman spectra for the glasses. Figure 2a, b show the high-resolution XPS spectra in the Te 3d5/2 region for the glasses GT1 and GT3. The observed spectra can be divided into a band located at 573 eV and another band located at 576 eV, corresponding to the bonding energies of Te in the elemental Te and TeO2, respectively [16–18]. The Raman of the glasses GT0, GT0.5, GT1 and GT3 are shown in Fig. 2c. The spectrum shows two sharp bands located at around 120 and 140 cm−1 in the glasses GT1 and GT3, which are characteristic Raman signatures of elemental Te [19–21]. With increasing in TeO2 concentration, the intensity of two characteristic bands increases, implying the growth of Te concentration. Due to the low TeO2 concentration, the glass GT0.5 does not show two characteristic bands of elemental Te. From the above results of the XPS and Raman spectra analyses, we can conclude that part of TeO2 is reduced to elemental Te by a decomposition reaction in the glass during high-temperature melting process.

Fig. 2
figure 2

High-resolution XPS spectra in the Te 3d5/2 region of the glasses: a GT1; b GT3. c Raman spectra of the glasses GT0, GT0.5, GT1 and GT3

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To further confirm the precipitation of Te NPs in the glasses, the crushed glass powders of GT1 were observed with high-resolution transmission electron microscope (HRTEM). The HRTEM images reveal a large quantity of NPs in the glass GT1, as shown in Fig. 3. Form a closer observation of lattice fringes, the spacing of 0.235 and 0.314 nm is found, which correspond to the inter-planar distances of the (104) and (012) plane of Te (JCPSD No. 23-1000, d 104 = 0.231 nm, d 012 = 0.311 nm), respectively. In addition, the inter-planar spacing of 0.244, 0.274 and 0.300 nm match the (005) plane, (103) plane and (004) plane, which is absent from JCPSD No. 23-1000 as it is not observable by X-ray diffraction. All these results clearly indicate the presence of Te NPs in the glass.

Fig. 3
figure 3

Typical HRTEM images of the glass GT1

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We then examine the nonlinear optical response of the Te NPs-doped glasses by using the Z-scan technique. Typical normalized Z-scan transmittance curves for open-aperture measurement of glasses GT0 and GT1 are shown in Fig. 4. The results show that the germanate glass with 1 % TeO2 exhibits clear NLO response at the excitation laser power of 95 μW, in sharp contrast to the germanate glass without TeO2. The NLO response of the glasses changes dramatically with increasing of laser power.

Fig. 4
figure 4

Typical Z-scan normalized transmittance curves for open-aperture measurement of glass samples. a At laser power at 95 μW. b At different excitation powers for the glass sample GT1

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According to NLO theory [22–25], the nonlinear absorption coefficient (β) can be determined by fitting the open-aperture Z-scan normalized transmittance curves using Eq. (1):

$$T(z) = 1 - \frac{{\beta I_{0} L_{\text{eff}} }}{{2\sqrt 2 [1 + (z/z_{0} )^{2} ]}}$$
(1)

where \(I_{0}\) is the peak laser intensity at the focus, \(z_{0} = \frac{{\pi \omega_{0}^{2} }}{\lambda }\) is the diffraction length of the beam, \(\omega_{0}\) is the beam waist radius at the focus (~45 μm in our case) and T(z) is the normalized transmission. L eff is the effective thickness of the glass, determined by \(L_{\text{eff}} = (1 - e^{ - \alpha L} )/\alpha\). L is the physical thickness of the glass measured to be 0.0809 cm, and α is the linear absorption coefficient estimated to be 10.816 cm−1 at 800 nm. Consequently, L eff of the glass is calculated to be 0.0539 cm. The calculated nonlinear absorption coefficient (β) at different laser power is revealed in Table 1, which shows that β is dependent on the laser power. In particular, with laser power increased from 95 to 185 μW, the change of β becomes much slow. The imaginary part of the third-order NLO susceptibility (Imχ (3)) can be calculated by [24–26]

$$\text{Im} {\kern 1pt} \chi^{\left( 3 \right)} ({\text{esu}}) = \frac{{10^{ - 7} c\lambda n^{2} }}{{96\pi^{2} }}\beta$$
(2)

where λ is the wavelength of the laser, c is the light speed in vacuum and n = 1.61 is the refractive index. Here, the Imχ (3) value is presented in Gaussian units (esu). The calculated Imχ (3) at different laser power is also shown in Table 1. With laser power increased from 95 to 185 μW, the imaginary part of the third-order NLO susceptibility (Imχ (3)) is weakly dependent on the laser power.

Table 1 β and Imχ (3) at different laser powers
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The dependence of β and Imχ (3) on laser power has also been reported for a SiO2 glass, SiO2–Na2O–CaO glass and SiO2–Na2O–B2O3 glass [27, 28]. The presence of a peak β value can be ascribed to the change of nonlinear absorption processes. There are many processes of nonlinear absorption, such as saturable absorption (SA) process and two-photon absorption (TPA) process. The TPA process is dominated at low laser power. With increasing of laser power, the TPA process becomes saturated and changes to SA process. This change leads to the β decreasing with increasing of laser power at high laser power [29, 30].

Tellurite glasses usually show large third-order NLO susceptibility up to 1.4 × 10−12 esu (TeO2 glass) [31], (1–2) × 10−12 esu(TeO2–ZnO–Li2O–Bi2O3 glass) [32], and 2 × 10−12 esu (TeO2–ZnO–Nb2O5–MoO3 glass) [33], which is about 50 times larger than silica glass (~2.8 × 10−14 esu) [31, 32] and 10 times larger than Bi2O3–GeO2–TiO2 glass (~1×10−13 esu) [34]. Such a large third-order NLO susceptibility of tellurite glass may stem from the unique network structure. The tellurite glass consists of TeO4 trigonal bipyramid unit, which has two equatorial and two axial oxygen atoms. The hybridization of Te atomic orbital in the axial plane is d 2z p z, while that in the equatorial plane is sp2. The empty 5d orbital and the 5s and 5p orbital of Te atom participate in the formation of the sp 3 d hybrid molecular orbital of TeO4 trigonal bipyramid. Previous report suggested that the large third-order NLO susceptibility of tellurite glass is ascribed to the empty 5d orbital of Te atom [31]. However, comparing to TeO2 concentration (in mol%) in Ref. 31 (100 %), Ref. 32 (78 %), and Ref. 33 (70–78 %), TeO2 concentration in this investigation is only 1 %. Therefore, the contribution of TeO4 to the NLO properties is small, and large third-order NLO susceptibility in our investigation is mostly ascribed to Te NPs, not TeO4 trigonal bipyramid. Our results indicated that the precipitation of Te nanoparticles leads to the observation of NLO property.

4 Conclusion

Glasses doped with Te NPs were prepared by the conventional melt-quenching method. The coloration and optical properties of glasses have been investigated. The origin of coloration in the glasses is due to the precipitation of Te NPs, which was revealed by XPS spectra, Raman spectra and TEM observation. The NLO properties were obtained by Z-scan technique. The values of β and Imχ (3) were estimated to be 1.34–4.13 cm/GW and (0.880–2.712) × 10−12 esu with laser power form 15.5 to 185 μW. The results show that the germanate glass doped with Te NPs exhibits excellent NLO properties, which may find potential applications in photonics and related fields.