Visible upconversion emissions in Pr³⁺-doped TeO₂–ZnO glass

Abstract

The Pr³⁺-doped tellurite zinc oxide (TZO) glasses by conventional melt and quenching technique have been prepared. The absorption spectra of samples doped with different concentrations of triply ionized praseodymium ions have been analyzed. Several upconversion emission bands of the Pr³⁺ ions doped in tellurite zinc oxide glasses under 980 nm excitation have been observed. The possible excitation mechanisms responsible for upconversion emissions spanning from blue to near infrared region have been discussed in detail.

1 Introduction

In recent years, there has been a growing research interest in the investigation of optical properties of rare earth elements introduced as doping because of their potential applications in optical devices, upconverters, optical fibers, amplifiers and sensors [1–3]. Among the rare earth ions, triply ionized praseodymium has been intensively studied due to its wide emission spectral range extended from blue to the near infrared (NIR) region of the electromagnetic spectrum [4–15].

Triply ionized praseodymium in crystals and glasses under NIR excitation has been observed to emit radiations in the visible region [7–9]. The infrared to visible upconversion in Pr³⁺-doped KPb₂Cl₅ crystal has been studied by Balda et al. [7]. A strong orange and weak blue, green and red upconversion emission bands have been observed under the direct excitation into the ¹G₄ level. Rai et al. [9] have studied frequency upconversion in the Pr³⁺-doped tellurite glass upon excitation at ~816 nm and the most probable channel for the upconversion was excited state absorption (ESA) process. A frequency doubling in Pr³⁺ ions doped gallium-lanthanide-sulfide glass by the excitation of 1.319 and 1.064 μm laser pulses has been observed at ~660 and ~532 nm, respectively, with the conversion efficiency of 0.02 % [10]. Choi and co-workers [11] have studied the effect of addition of the Er³⁺ in the Pr³⁺-doped selenide glass upon excitation at ~1.48 μm and marked the enhancement in the emission intensity of the Pr³⁺ ions caused by the codoping of Er³⁺ ions. The optical properties of heavy metal oxide glassy materials codoped with Pr₆O₁₁ and silver nanoparticles have been examined on the basis of optical absorbance and luminescence spectra due to the Surface Plasmon interactions [12, 13]. Frequency upconversion in the Pr³⁺-doped TeO₂-Li₂O binary glass by the decay curve analysis under 890 nm continuous radiation has been reported [14]. The mechanism responsible for the frequency upconversion emission was observed to be an excited state absorption assisted by the emission of phonons. The photoluminescence studies of Pr³⁺ ions doped in lead germanate glass upon direct excitation into the ¹D₂ state using an excitation line of 594 nm from a dye laser pumped by a second harmonic of Q-switched Nd: YAG laser of nanosecond pulses have been reported [15]. The energy transfer process involving the Pr³⁺ ions in pairs due to dipole–dipole interaction in the ¹D₂ state has been found to be dominant process for the upconversion emission.

The search of suitable host materials for the doping of Pr³⁺ ions in terms of better optical properties is still in progress. The oxide hosts such as tellurite lead oxide (TPO), lead germanate (PGO), tellurite zinc oxide (TZO), etc. are often described as heavy metallic oxide glasses and these are efficient hosts for the modern world of photonics as they have a broad range of spectral properties such as low loss and long transmission windows. Tellurite glasses have been extensively investigated due to its low cut-off phonon energy, high refractive index, large dielectric constant, high mechanical strength, high chemical durability, non-hygroscopic in nature, high optical damage threshold, small absorption coefficient within the wavelength range of interest and transparent over the visible and NIR regions [16–20]. Tellurite-based glasses with 60–90 mol% TeO₂ possess a combination of properties, which makes them attractive materials for various photonic applications. They used to combine a wide transmission window (350 nm–5 μm) with good thermal stability, corrosion resistance and materials with losses of 20 dB/km have been fabricated [21, 22]. Among the heavy metal oxide glasses, tellurite zinc oxide glass is a promising oxide glass host for the photoluminescence applications because of the large band gap (3.37 eV) and an exciton binding energy (60 meV) of zinc oxide [23].

Although the rare earth doped tellurite based glassy materials have been investigated, but to the best of our knowledge there is no such information available about the upconversion properties in TZO glass especially with Pr³⁺ ions. In view of this, the present paper deals with the study of frequency upconversion in the Pr³⁺-doped TZO glass under non-resonant excitation corresponding to the ³H₄ → ¹G₄ transition and mechanism involved in the upconversion emissions have been discussed.

2 Experimental

To prepare the Pr³⁺-doped TZO glasses, the used chemical composition of the ingredients viz. TeO₂, ZnO and Pr₆O₁₁ are the following:

$$ \left( {80 - x} \right){\text{ TeO}}_{2} + \, 20{\text{ZnO}} + \, x{ \Pr }_{6} {\text{O}}_{11} , $$

where x = 0.3, 0.5, 0.75 and 1.0  mol%.

Various compounds have been thoroughly crushed in an agate mortar to get a homogeneous mixture. The homogenous mixture was then heated in an alumina crucible using an electric furnace at 735 °C for half an hour. The liquid melt is then air quenched by putting it on a brass plate placed at 400 °C. The melt was then cooled to room temperature to get properly annealed glasses of desired shape and size. Optical absorption spectra of all the doped samples were recorded with a double beam spectrophotometer in the wavelength range of 400–700 nm. For the non-resonant excitation corresponding to the ³H₄ → ¹G₄ transition of the Pr³⁺ ions, a CW diode laser lasing at 980 nm was used. The laser beam was focussed onto the sample with a 10 cm focal length lens. The radiation emitted from the Pr³⁺ ions was collected in a direction perpendicular to the incident beam and the signal was analyzed by a monochromator equipped with a photomultiplier tube. All the measurements were done at room temperature.

3 Results and discussion

3.1 Absorption studies

Figure 1 shows the absorption spectra due to 4f-4f transitions of trivalent praseodymium ions doped in tellurite Zinc oxide glass samples in the spectral range of 400–700 nm. The valence electronic configuration of Pr³⁺ ion is [5s²5p⁶4f²], which designates ³H₄ as the ground state. The analysis of the observed absorption spectra is summarized as follows:

Fig. 1

Absorption spectra of 0.3, 0.5 and 1.0 mol% Pr³⁺ ions doped in TZO glass from 400 to 700 nm wavelength range

• 1. The intensity of absorption peaks increases with increase in the rare earth ion concentrations. For 1.0 mol% Pr³⁺-doped in tellurite zinc oxide glass, the peak intensity is observed to be maximum among all the prepared samples due to the large absorption coefficient.

• 2. There appear four absorption bands in the 400–700 nm region peaking at ~450, ~473, ~488, ~591 nm corresponding to the ³H₄ → ³P_2, ³H₄ → ³P₁, ³H₄ → ³P₀ and ³H₄ → ¹D₂ transitions, respectively, assignments are in agreement with the values reported in the other host materials [7, 9, 15, 24]. The absorption transitions are inhomogeneously broadened because of the site to site variations in the strength of crystal field.

3.2 Upconversion studies

The upconversion spectra of Pr³⁺-doped in tellurite zinc oxide glasses under non-resonant excitation into the ¹G₄ state using the laser excitation of 980 nm in the 350–1,000 nm region are shown in Fig. 2. There appears several upconversion emission bands peaking at ~450, ~528, ~540, ~598, ~620, ~627, ~674, ~690, ~707 and ~866 nm and attributed to the ³P_2,1,0 → ³H₄, ³P₁ → ³H₅, ³P₀ → ³H₅, ¹D₂ → ³H₄, ³P₀ → ³H₆, ³P₀ → ³F₂, ¹D₂ → ³H₅, ³P₀ → ³F₃, ³P₀ → ³F₄ and ¹D₂ → ³F₂ transitions, respectively. The broad and intense upconversion emission band in the blue region is due to the contribution of ³P₂ → ³H₄, ³P₁ → ³H₄ and ³P₀ → ³H₄ transitions. The upconversion intensity of several emission bands is observed to increase with increase in the concentration of Pr³⁺ ions.

Fig. 2

Upconversion emission spectra of 0.3, 0.5 and 1.0 mol% Pr³⁺-doped TZO glass from 350 to 1,000 nm wavelength range

The log–log plot of the upconversion intensity versus concentration of Pr³⁺ ions shows a linear variation (Fig. 3), thereby showing that only one Pr³⁺ ion is participating in the upconversion process. To get the information about the number of photons involved in the upconversion emission process, log–log plot of the upconversion intensity versus NIR laser pump power has been made and observed to provide the slope ~2.09 ± 0.05 and ~2.65 ± 0.1 (Fig. 4) for the emissions in the orange and blue regions peaking at ~598 and ~450 nm, respectively. This confirms the participation of two and three NIR laser photons, which are responsible for the orange and blue upconversion emissions, respectively, and hence for the upconversion emission throughout the visible region to occur.

Fig. 3

Log–log plot of blue upconversion intensity and Pr³⁺ ion concentration

Fig. 4

Log–log plot of the upconversion intensities and laser intensity of a 598 nm and b 450 nm emission

To know the whole mechanism involved in the upconversion emission of Pr³⁺-doped in tellurite zinc oxide glass, we assume the schematic energy level diagram (shown in Fig. 5).

Fig. 5

Schematic energy level diagram of Pr³⁺ ion. The virtual level is shown in dotted straight line and denoted as V

The ground state Pr³⁺ ions are excited to the ¹G₄ state through ground state absorption followed by multiphonon emission. The excited Pr³⁺ ions in the ¹G₄ state (~9,800 cm⁻¹) are again pumped through the excited state absorption (ESA) process to a virtual excitation level V at ~20,004 cm⁻¹ (Fig. 5), which lies above the ¹D₂ state (~16,920 cm⁻¹). Since there is no suitable matching of any energy level of excited Pr³⁺ ion, excited Pr³⁺ ions relax non-radiatively via the emission of four numbers of phonons associated with the lattice vibrations in the host matrix and ultimately the ¹D₂ state is populated. As, the phonon energy of tellurite glass is ~750 cm⁻¹ [25]. The population in the ¹D₂ state may also be achieved by the following processes.

• 1. Through energy transfer (ET) between the Pr ³⁺ ions in the ¹ G ₄ state: In this, one ion (acceptor) after receiving the energy from the other ion (donor) in the ¹G₄ state is excited to the higher energy state at ~19,600 cm⁻¹ and then the other ion (donor) after transferring its excitation energy to the acceptor relaxes to the ³H₅ state. Thereafter, the acceptor ion returns back to the ¹D₂ state by the multiphonon emission. For this, there should be participation of two Pr³⁺ ions, but the log–log plot of upconversion intensity versus rare earth ion concentration provided the slope ~0.85 ± 0.04 (Fig. 3). Thus, there is contribution of only one Pr³⁺ ion. Therefore, the energy transfer process in present case is not a possible source of excitation.

• 2. Excited state absorption (ESA) involving Pr ³⁺ ions in the ³ F ₄ state: As the ³F₄ level may be sufficiently populated via the nonradiative relaxation from the ¹G₄ state and also through the (¹G₄, ³H₄ → ³H₅, ³F₄) cross-relaxation energy transfer process, which is again less probable.

Moreover, after non-resonant excitation into the ¹G₄ state, the upconversion emissions from the ³P₂, ³P₁ and ³P₀ levels are also observed. The energy used to excite the Pr³⁺ ions to ³ P states by a consecutive two photon absorption process is not sufficient, because the ³P₂, ³P₁ and ³P₀ levels are at ~2,218, ~1,136 and 487 cm⁻¹ just above the virtual level V at ~ 20,004 cm⁻¹ attained by the first ESA through the ¹G₄ state (Fig. 5). Also, the ³P₂, ³P₁ and ³P₀ levels are lying above at the energy gap of ~5,300, 4,220 and 3,570 cm⁻¹ from the ¹D₂ state. The pump power dependence of the blue upconversion emission around ~450 nm, which is due to superposition of the ³P₂ → ³H₄, ³P₁ → ³H₄ and ³P₀ → ³H₄ transitions, shows cubic behavior with the laser power (Fig. 4). This shows that the upconversion emissions from these ³P₂, ³P₁ and ³P₀ levels are due to the stepwise absorption of three photons followed by the ESA process.

4 Conclusion

The Pr³⁺ ions doped in TeO₂–ZnO glasses have been successfully synthesized by the melt and quenching technique. The NIR to visible upconversion emissions in the Pr³⁺-doped TeO₂–ZnO glasses have been observed upon non-resonant excitation into the ¹G₄ level. The ESA process is observed to be the dominant one for the emissions throughout the visible region. The broad and intense blue upconversion band centred at ~450 nm is the superposition of the ³P₂ → ³H₄, ³P₁ → ³H₄ and ³P₀ → ³H₄ transitions. The present host materials may be suitable one in making the NIR to visible upconverter.