Down conversion and efficient NIR to visible up-conversion emission analysis in Ho3+/Yb3+ co-doped tellurite glasses

Abstract

A series of glasses with chemical composition (50−x−y) TeO₂–30ZnO–10YF₃–10NaF–xHo₂O₃–yYb₂O₃ (x = 0.5 and y = 0.5, 1.0, 3.0, 5.0 mol%) were prepared by melt-quenching procedure. The absorption spectra, excitation, down conversion emission spectra, up-conversion (UC) emission spectra and decay time measurements were analyzed. In down conversion, the visible emission transition intensity associated with ⁵F₄ → ⁵I₈ (547 nm), ⁵F₅ → ⁵I₈ (657 nm), and ⁵F₄ → ⁵I₇ (755 nm) of Ho³⁺ ions decreased with Yb³⁺ concentration due to the energy transfer (ET) process from Ho³⁺ to Yb³⁺ ions. In up-conversion, on exciting with 980 nm diode laser beam, we observed a strong green (543 nm) and red (657 nm) UC emissions, that refers to the energy level transitions; ⁵F₄ (⁵S₂) → ⁵I₈ and ⁵F₅ → ⁵I₈ of Ho³⁺. The influence of excitation power on UC intensities studies revealed that the population at ⁵F₄ (⁵S₂) and ⁵F₅ levels of Ho³⁺ ion occurs due to two-photon absorption process associated energy transfer from Yb³⁺ to Ho³⁺. The influence of Yb³⁺ doped concentration on UC was studied, and it is observed that both the green and red UC intensities improved significantly on increasing Yb³⁺ ions concentration.

Introduction

The lasers operating in the visible region of electromagnetic spectrum have been employed for a number of applications like optical recording, display, medical diagnosis, etc. [1]. Energy-up conversion (UC) property of rare earth (RE) ions are an efficient way to obtain visible lase under IR pumping. Besides the visible laser, material exhibiting UC luminescence have also getting special consideration due to their potential use for IR sensing, in photovoltaics as solar NIR concentrators, and biological classification [2,3]. It is essential to understand the exact mechanism, in order to exploit a competent up-converting material and to enhance their efficiency.

A number of studies were reported on tellurite glasses during the last few years owing to its attractive attributes. It is transparent for a wide range of wavelengths (0.35–5 μm) in comparison to silicate glasses (0.2–3 μm), and has large index of refraction [4,5]. It also has excellent stability, and lower phonon energy. Among the number of RE ions available, Ho³⁺ has exceptional UC luminescence due to its apparent transitions which spans in the visible region [6,7]. Even though, Ho³⁺ is an excellent candidate, it has some draw backs, like one cannot directly excite it with commercially available diode laser (LD), 980 nm because of the absence of appropriate absorption levels in it. Fortunately, co-doping with Yb³⁺ enables sensitization of Ho³⁺ to realize visible UC transition due to the accessibility of its large absorption cross-section about 980 nm [8,9] in addition to its efficient energy transfer ability to Ho³⁺[9,10]. Ho³⁺ can be employed into matrixes as activators to initiate interesting luminescent mechanisms. Due to their rich energy level occurrence, the emissions in red, green and many other wavelengths can occur under IR laser excitations for ex. 980 nm wavelength. With Ho³⁺, Yb³⁺ addition as a sensitizer and a co-dopant can bring improved up-conversion emissions [11,12]. Balaji et al. have analyzed phonon assisted energy transfer process in Ho³⁺/Yb³⁺ co-doped telluride glass, and found that in order to achieve a substantial energy transfer the difference in energy between ²F_5/2 (Yb³⁺⁾ and ⁵I₆ (Ho³⁺⁾ levels can be crossed with the aid of host phonons [13]. It can be noted that because of the non-resonant transitions the Yb³⁺ emission and Ho³⁺ absorption has a very poor spectral overlap of about 1500 cm⁻¹ energy gap. Since the telluride glasses have a ∼750 cm⁻¹ of phonon energy, nearly two or three phonons are needed to cross the transition gap in energy. On the contrary, the host systems that have high phonon energy like phosphate, silicate, and borate can achieve this energy gap bridging with a smaller number of phonons, but non radiative transition from ⁵I₇ level of Ho³⁺ restricts the 2 μm emission probabilities significantly. Their study also estimated the energy transfer microparameters to be reasonably good magnitude, which substantiates that the ET process benefits from host phonons. Thus, the telluride glass is promising host for Ho³⁺/Yb³⁺ co-doping for efficient NIR emissions and up-conversions. For the last few years, numerous research works were conducted on Yb³⁺/Ho³⁺ doped tellurite glasses for the advance of solid-state UC green emitting lasers, mid-infrared optical fiber amplifiers and laser devices [11–16]. The studies reported by Wang et al., have shown enhanced luminescent properties via tailoring the phonon energy and crystal fields’ environment in Ho³⁺/Yb³⁺ based silicate system [17]. Le et al. reported Ho³⁺/Yb³⁺ in tellurite glasses with enhanced up-conversion NIR emissions [18]. Suresh et al. [19] studied Ho^3+/Yb³⁺ co-doped zinc tellurite glasses that exhibit blue-green photon into NIR photons transitions and it is found useful for solar conversion and optical amplifier applications. Zhu et al. [20] presented the emission improvement in the 2.0 μm band and effective energy transfer process in Ho³⁺/Yb³⁺/Er³⁺ tri-doped tellurite glasses for solid state laser applications. Guangning et al. [21] depicted the mid-infrared 2.0 μm and 4.1 μm wideband emission in Ho³⁺/Yb³⁺ doped tellurite–germanate glasses that are suitable for mid-infrared fiber and laser materials applications. Azam et al. [22] explained the photo luminescence studies with Judd–Ofelt analysis in Ho³⁺/Ho³⁺–Yb³⁺ doped/co-doped lead tellurite glasses for use in optical multi-functional devices. Chao et al. [23] discussed enhancement in UC and 2 μm mid-infrared emission in Ho³⁺/Yb³⁺ co-dopped tellurite glass for UC and mid-infrared luminescent material applications. Dogan et al. [24] focused on the UC luminescence in Ho^3+/Yb³⁺ co-doped tellurite glasses and their temperature sensing properties which are useful for lighting and optical temperature sensing applications. Although there are a lot of studies reported on the up-conversion luminescence and NIR emission on Ho³⁺/Yb³⁺ co-doped material, the research on Ho³⁺/Yb³⁺ co-doped glass is still expected to bring interesting luminescent properties in varying crystal environment, heat treatment, phonon energies, dopant concentrations, excitation wavelengths, pump powers, etc.

Based on the above facts, in this work, we have synthesized a new set of Ho³⁺/Yb³⁺ co-doped tellurite glasses, comprising the composition TeO₂–ZnO–YF₃–NaF. The visible and NIR down conversion emission and intense UC emissions observed in the green and red of Ho³⁺ in the synthesized tellurite glasses with varied amounts of Yb³⁺ are analyzed. In the interim the energy transfer mechanism between Yb³⁺ to Ho³⁺ ions were analyzed.

Experimental details

Glass preparation

A series of glasses with chemical composition (50−x−y) TeO₂–30ZnO–10YF₃–10NaF–xHo₂O₃–yYb₂O₃ (x = 0.5 and y = 0.5, 1.0, 3.0 and 5.0 mol%) were prepared by melt-quenching technique. In an agate mortar, rigorously mixed high purity TeO₂ (99.9%), ZnO (99.9%), YF₃ (99.9%), NaF (99.99%), Ho₂O₃ (99.99%) and Yb₂O₃ (99.99%) raw materials. An electrical furnace at air atmosphere is maintained at 1050 °C and 10 g batches of the sample were melted for 30 min. Then the melts were quenched via pouring them into a preheated brass mould which is maintained at 200 °C. For the purpose of avoiding the internal thermal stress-initiated cracking, the glasses were annealed at 300 °C for 4 h, and then brought back to room temperature via natural cooling.

Sample measurements

The density of glass samples was measured using distilled water as an immersion liquid which works based on Archimedes principle. The absorption spectra of the samples were recorded using Perkin Elmer spectrophotometer (Lambda 1050) which operates over the wavelength ranges 300–2000 nm with a spectral resolution of 1 nm. The down conversion emission, excitation, and decay curve analysis were done using Horiba Fluorolog-spectrofluorimeter, that consists of CW and pulsed Xe lamps. The output signals were recorded using a visible photodiode detector (l PPD-850) and by an infrared Hamamatsu photomultiplier. The up-conversion was obtained by Horiba Fluorolog-spectrofluorimeter with laser diode used as excitation source operating at 980 nm. All the studies were conducted at room temperature.

Results and discussion

Physical properties

The density of the glass matrix influences the bulk properties of the glass samples, such as linear refractive index, etc. [25]. The variation of density (g/cm³) and molar volume (cm³/mol) with respect to Yb³⁺ ion concentration in tellurite glasses is studied and is shown in Fig. 1. Both density and molar volume increase with increasing Yb³⁺ concentration; this could be due to the generation of excess non-bridging oxygen (NBO) bonds, that dictates the structure of the respective glass networks, and the replacement of TeO₂ with heavier Yb₂O₃ in the base glass composition [26].

Fig. 1.

The variation of density (g/cm³) and molar volume (cm⁻³) as a function of Yb³⁺ ion concentration in oxyfluoro tellurite glasses.

Optical absorption spectra

The optical absorption spectra of Ho³⁺/Yb³⁺ co-doped tellurite glasses are illustrated in Fig. 2. Absorption peaks at 420, 450, 474, 489 541, 645, 896, 1160 and 1955 nm are referred to the ground level of Ho³⁺ ions (⁵I₈) to highly excited levels (⁵G₅, ⁵G₆, ⁵F₂, ⁵F₃, ⁵F₄, ⁵F₅, ⁵I₅, ⁵I₆ and ⁵I₇, respectively) transitions. The Yb³⁺ ions ground level (²F_7/2) to the excited level ²F_5/2 transitions, are referred to the strong absorption peak observed at 978 nm. Therefore, the doping of sensitizer Yb³⁺ ion provided a channel for mid-infrared emission of Ho³⁺.

Fig. 2.

Optical absorption spectra of oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions.

Excitation and down-conversion emission

Fig. 3 exhibits the excitation spectra of Ho³⁺-doped oxyfluoro tellurite glasses. As shown in Fig. 3, the excitation profile for 980 and 1216 nm emission closely resembles its absorption spectrum from 350 to 600 nm. Hence, some of the excitation bands can be readily assigned to ⁵I₈ → ³H₆ (362 nm), ⁵I₈ → ⁵G₄ (388 nm), ⁵I₈ → ⁵G₅ (420 nm), ⁵I₈ → ⁵G₆, ⁵F₁ (455 nm), ⁵I₈ → ³F₂ (475 nm), ⁵I₈ → ³F₃ (487 nm), and ⁵I₈ → ³F₄ (539 nm) transitions of Ho³⁺ ions, respectively.

Fig. 3.

Excitation spectra (a) λ_emi = 980 nm and (b) λ_emi = 1216 nm of oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions.

The intensity of excitation bands depends on the monitored emission wavelength. For instance, when the spectra were measured at the emission wavelength of 980 nm (Yb³⁺ ion), see Fig. 3(a), the intensity of the bands increased up to 1.0 mol% Yb³⁺ ion contraction. Whereas the intensity of the same bands decreased when monitored at 1216 nm (Ho³⁺ ion), see Fig. 3(b). The energy transfer (ET) from Ho³⁺ to Yb³⁺ ions is responsible for this variation in bands intensity. When the Yb³⁺ ion concentration exceeds 1.0 mol%, the opposite effect is observed. This could be attributed to energy back transfer (EBT) from Yb³⁺ to Ho³⁺ ions via the cross relaxation channels [28], which will discuss below. The ET between Ho³⁺ and Yb³⁺ confirmed by the analysis of down conversion emission spectra, as discussed below.

From Fig. 3, the maximum excitation wavelength is located at 455 nm. Excitation into the strongest ⁵I₈ → ⁵G₆, ⁵F₁ transition of Ho³⁺ at 455 nm yields the emission spectrum, as illustrated in Fig. 4(a) in visible (VIS) and Fig. 4(b) in NIR region. The visible spectrum (Fig. 4(a)) consists of the emission bands associated with ⁵F₄ → ⁵I₈ (547 nm), ⁵F₅ → ⁵I₈ (657 nm), and ⁵F₄ → ⁵I₇ (755 nm) of Ho³⁺ ion's transitions. The intensity of these bands decreases with Yb³⁺ concentration due to ET process from Ho³⁺ to Yb³⁺ ions takes place through cross relaxation channels (⁵S₂, ⁵F₄): Ho³⁺, ²F_7/2: Yb³⁺ → ⁵I₆: Ho³⁺, ²F_5/2: Yb³⁺ (in Fig. 5, marked as CR₁) and ⁵F₅: Ho³⁺, ²F_7/2: Yb³⁺ → ⁵I₇: Ho³⁺, ²F_5/2: Yb³⁺ (in Fig. 5, marked as CR₂) [28]. Whereas NIR spectra (Fig. 4(b)) consists of emission bands at 980 nm, 1021 nm and 1216 nm, which are referred to the Yb³⁺: ²F_5/2 → ²F_7/2, Ho³⁺: ⁵F₄, ⁵S₂ → ⁵I₆, and Ho³⁺: ⁵I₆ → ⁵I₈ energy transitions. In contrast to visible emission spectra, although Yb³⁺ ion lack an absorption band in the visible region, the intensity of the 980 nm band increases with Yb³⁺ concentration up to 1.0 mol%, which confirms further that ET takes place from Ho³⁺ to Yb³⁺ ions. However, beyond 1.0 mol% of Yb³⁺ concentration, the intensity drops for 980 nm band and raises for 1216 nm band have been observed. The change in intensity may be due to EBT from Yb³⁺ to Ho³⁺ ions. Further, despite of change in intensity of 980 nm emission band, change in spectral profile has also been observed with the increase in Yb³⁺ ion concentration. This could be due the fact that luminescence reabsorption by the Yb³⁺ neighboring ions [29]. Fig. 4(c) shows the infrared fluorescence spectra under 980 nm excitation. The emission bands can be readily assigned to ⁵I₆ → ⁵I₈ (1201 nm) transition of Ho³⁺ ions.

Fig. 4.

Emission spectra in (a) 500–800 nm (λ_exc = 455 nm), (b) 900–1250 nm (λ_exc = 455 nm), and (c) 1100–1250 nm (λ_exc = 980 nm) oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions.

Fig. 5.

Down conversion energy level diagram of oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions.

Up-conversion emissions and energy transfer process

In order to demonstrate the up-conversion emission fluorescence, sensitive fluorescence spectrophotometer adopting at 980 nm pumping laser was used and the up-conversion spectra obtained for the investigated glasses are shown in Fig. 6. The emission bands centering around 546, 661 and 775 nm wavelengths are assigned to the transitions from the Ho³⁺ ion's (⁵F₄, ⁵S₂) to ⁵I₈, ⁵F₅ to ⁵I₈, and (⁵F₄, ⁵S₂) to ⁵I₇ levels, respectively. The intensity of these UC emissions gradually raised against Yb³⁺ content up to 1.0 mol% and the emission dropped dramatically with further increase in Yb³⁺ ion's concentration. The non-radiative energy transfer process overrides and the resulting concentration quenching effect plays a dominant role in the reduction of UC intensities [27]. The earlier studies reported that the upconversion emission intensity of Ho³⁺/Yb³⁺ glass centered at 754 nm was very weak compared to the emissions bands centered at 546 nm and 669 nm [22,30–36] and the ratio of peak intensity between 546 nm and 659 nm UC emissions is less than 1, while it is more than 1 in some other studies [37–39]. The value of UC intensity bands centered at 546 nm and 661 nm are maximized when the concentration of Yb³⁺ is 1.0 mol%.

Fig. 6.

Up-conversion emission spectra with different Yb³⁺ concentration in oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions.

In order to explain the energy transfers from Yb³⁺ to Ho³⁺ ions, the energy diagram given in Fig. 7 is utilized. The studies on power dependency of UC emissions revealed that the intense emissions in green and red is two-photon absorption enabled. The Yb³⁺ ion to Ho³⁺ ion energy transfer populates the Ho³⁺ ion's ⁵S₂ and ⁵F₄ levels which leads to the emission in green and red wavelengths. When excited with near infrared (NIR) photons (980 nm), at first the ground ²F_7/2 level Yb³⁺ ion gets excited to the ²F_5/2 level because of its large absorption cross-section. From the excited level of Yb³⁺ ion, the energy transfer takes place to the Ho³⁺ ions which are initially at the ground level, which further gets excited to a ⁵I₆ metastable level through energy transfer (ET). Then, via the excited state absorption (ESA) or the ET, the Ho³⁺ ions existing at the ⁵I₆ excited level reabsorb the incoming NIR wavelength photons to populate the ⁵S₂ (⁵F₄) energy levels. From there, they return to the ground ⁵I₈ level with the emission of intense radiation in the green region around 546 nm, owing to the energy gap. Also, originated from some non-radiative relaxations, the ⁵S₂ (⁵F₄) levels of Ho³⁺ ions get transferred to the ⁵F₅ level quickly. Moreover, some possible excited state absorption (ESA) or the ET enables the ⁵F₅ level to get populated, which ultimately lead to an intense red-light emission at about 661 nm wavelength [40]. Meantime, the ⁵F₄ (⁵S₂) level Ho³⁺ ions deplete to the ⁵I₇ metastable state thereby emitting a weak NIR emission at about 754 nm wavelength [41,42].

Fig. 7.

Up conversion energy level diagram of Ho³⁺/Yb³⁺ co-doped oxyfluoro tellurite glasses.

The UC involves nonlinear optical processes, and hence the UC emissions should be excitation laser intensity dependent. In order to analyze that, the UC emission spectra of the sample under varying pump power scheme is studied, and the obtained graph is given in Fig. 8(a). It shows the UC emission intensities’ variation with pump power for all observed transitions. With the increase of pump power, all UC emission intensities shoot up. For these three luminescence peaks, the dependence of integrated up-conversion emission intensities, obtained from Fig. 8(a), is presented in Fig. 8(b). The number of photons (n) take part into the UC process is revealed by linear fit on the intensity-power variation plotted as double logarithmic curve. The slope obtained can give the n value, as it assumed to follow the relation (I ∼ Pⁿ) [43]. For 0.5 mol% Ho³⁺ and 1.0 mol% Yb³⁺ ions co-doped tellurite glass, the n values of green, red and NIR emissions are 2.07, 2.54 and 2.06, respectively; confirming that three UC emission transitions are populated predominantly by two-photons absorption process. The high n value of red transition (⁵F₅–⁵I₇) is indicative of multiphoton cooperative process, in addition to the two-photon absorption process.

Fig. 8.

(a) Different pump power oxyfluoro tellurite glasses doped with Ho³⁺/Yb³⁺ ions; (b) dependence integrated up-conversion emission intensities on excitation power in oxyfluoro tellurite glass co-doped with 0.5 mol% Ho³⁺ and 1.0 mol% Yb³⁺ ions.

Fluorescence decay analysis

In order to unravel the ET process the lifetime measurement was carried out. The time resolved emission spectra (decay curves) of 546 nm (⁵F₄,⁵S₂) emission of Ho³⁺ ions by exciting at 455 nm in tellurite glasses are shown in Fig. 9. All decay curves are well fitted with single exponential function. The obtained decay times found to be decreased from 20.23 to 10.44 μs. The reduction in lifetimes with the increase of Yb³⁺ ions concentration warrants the occurrence of strong energy transfer process from Ho³⁺ to Yb³⁺ ions, via cross relaxation channels (⁵S₂, ⁵F₄): Ho³⁺, ²F_7/2: Yb³⁺ → ⁵I₆: Ho³⁺, ²F_5/2: Yb³⁺ and ⁵F₅: Ho³⁺, ²F_7/2: Yb³⁺ → ⁵I₇: Ho³⁺, ²F_5/2: Yb³⁺ (see in Fig. 6) [14].

Fig. 9.

Decay curve of the 546 nm emission (Ho³⁺: ⁵F₄ level) for oxyfluoro tellurite glass co-doped with Ho³⁺/Yb³⁺ ions.

Conclusion

In this report, a series of glasses with chemical composition (50−x−y) TeO₂–30ZnO–10YF₃–10NaF–xHo₂O₃–yYb₂O₃ (x = 0.5 and y = 0.5, 1.0, 3.0 and 5.0 mol%) is prepared by melt-quenching technique. The absorption spectra, excitation, visible emission, up-conversion emission spectra and decay time measurements were characterized. Under the 980 nm laser excitation, the sample exhibited intense green (546 nm), red (661 nm), and NIR emissions (754 nm). The UC emission intensity variation study with respect to excitation pump power indicated that the two-photon absorption mechanism based on the energy transfer from Yb³⁺ to Ho³⁺ was the source of the Ho³⁺ population at ⁵F₄ (⁵S₂) and ⁵F₅ levels. With the increase of Yb³⁺ ions concentration, the green and red up-conversion emission intensities show significant enhancement. Hence, the present results indicate that the newly synthesized Ho³⁺/Yb³⁺-co-doped TeO₂–ZnO–YF₃–NaF glass with intense up-conversion emissions in the green and red wavelengths could be a promising material that will find applications in the visible-band fiber lasers and photonics.

Funding

This research was funded by Kuwait Foundation for the Advancement of Sciences, Kuwait, grant number PR18-14SP-11.

Acknowledgments

S.K. acknowledges Kuwait Foundation for the Advancement of Sciences (KFAS) for the research fund (PR18-14SP-11). This work is a part of the activities of the FunGlass project. R.D. acknowledges the funding from the European Union Horizon 2020 research and innovation program under grant agreement No. 739566 and S.E. acknowledges SERB, India for NPDF No. PDF/2020/002494.