Role of Yb(3+) ions on enhanced ~2.9 μm emission from Ho(3+) ions in low phonon oxide glass system.

The foremost limitation of an oxide based crystal or glass host to demonstrate mid- infrared emissions is its high phonon energy. It is very difficult to obtain radiative mid-infrared emissions from these hosts which normally relax non-radiatively between closely spaced energy levels of dopant rare earth ions. In this study, an intense mid-infrared emission around 2.9 μm has been perceived from Ho(3+) ions in Yb(3+)/Ho(3+) co-doped oxide based tellurite glass system. This emission intensity has increased many folds upon Yb(3+): 985 nm excitation compared to direct Ho(3+) excitations due to efficient excited state resonant energy transfer through Yb(3+): (2)F5/2 → Ho(3+): (5)I5 levels. The effective bandwidth (FWHM) and cross-section (σem) of measured emission at 2.9 μm are assessed to be 180 nm and 9.1 × 10(-21) cm(2) respectively which are comparable to other crystal/glass hosts and even better than ZBLAN fluoride glass host. Hence, this Ho(3+)/Yb(3+) co-doped oxide glass system has immense potential for the development of solid state mid-infrared laser sources operating at 2.9 μm region.

Laser materials operating in the 2–5 μm region are gaining much interest in recent years because of their wide applications in automotive, pharmaceutical and medical industries12. This wavelength region being known as “finger print region” for many molecules, lasers operating at this region can potentially act as chemical sensors too3. Especially, Mid Infrared (MIR) photonics has gained momentum ever since the invention of quantum cascade lasers4 which are currently being served as laser sources in MIR region. However, because of their miniature size, heat dissipation has become a main threat where, around 70% of input pump power has been obsessive only for heat generation5. On the other hand optical parametric oscillator (OPO) which also being served as MIR laser sources are expensive and require highly coherent pump sources6. In recent years, significant thrust has been driven on rare earth doped low phonon fluoride and chalcogenide167 based glasses because of their flexible geometry and easy fiberization than that of crystals. Although fluoride glasses are thermally more stable than chalcogenide glasses, chemical durability and mechanical strength of these glasses are inferior to that of oxide glasses. Chalcogenide glasses on the other hand, having high transparency far beyond fluoride glasses, and can potentially allow many MIR emission transition from rare earth ions. Conversely, high refractive index of these materials may restrict high peak powers of MIR emissions. Further, preparation process for fluoride and chalcogenide based glasses are onerous compared to oxide glasses. Hence, rare earth doped oxide based glasses with low phonon energy and extended infrared transmission are attaining high surge to serve as MIR laser sources provided the glasses should have zero/low OH− content. In this contest, tellurite glasses (transparency upto ~6 μm) have been under strenuous study because of their advantages over fluoride and chalcogenide based glasses. There are few reports on ~2.7 μm emission from Er3+ ions8910, ~2.8 μm, ~4 μm from Ho3+ ions11, 12, 13, 14 and ~2.9 μm, ~3.3 μm from Dy3+ ions1516 when doped in tellurite glasses. However, the reported ~4 μm emission from Ho3+ ions is subject to revision since harmonic peak of Ho3+: ~2 μm emission arises exactly at ~4 μm and it is very difficult to eliminate this harmonic peak unless one suppresses the ~2 μm emission by using a proper high pass IR cut-on filter >2 μm. Also, there is no mention of filters used while recording the ~4 μm MIR emission in their report1314.

In preview of Ho3+ ion energy level structure, it can facilitate numerous NIR and MIR emission transitions when doped into a suitable low phonon host material6. However, one of the major shortcomings of Ho3+ is the lack of ground state absorption (GSA) transitions17 that overlap with convenient high-power pump sources. Thus, sensitized excitation with suitable rare earth ion co-doping having strong absorption at pump powers seems to be a better choice. Earlier, we have demonstrated18 energy transfer (Yb3+ → Ho3+) based enhanced and efficient ~2 μm emission from Ho3+/Yb3+ ions co-doped Tellurium-Barium-Lanthanum oxide glass system having low phonon energy. Also, visible up-conversion emissions from Ho3+ ions on account of bi-photonic absorption by Yb3+ ions under 980 nm excitation in the same host has been reported19. The present work mainly aims to investigate the MIR emission transitions from Ho3+ ions by considering the extended IR transparency (upto ~6 μm) and low phonon energy (Fig. 1 of Supplementary data) of this oxide based glass system. The dependence of emission transition strengths on pump wavelengths, role of Yb3+ co-doping on the enhancement of Ho3+ mid-infrared emissions and the possible energy transfer mechanisms were discussed in detail and reported here.

Figure 1

NIR emission spectra of Yb3+/Ho3+ co-doped glass under different excitation wavelengths (colour codes were given for different excitation wavelengths).

Results and Discussion

The emission spectra in NIR (1.0–2.5 μm) and MIR (2.5 to 4.5 μm) regions were recorded at room temperature under similar experimental conditions using suitable combinations of low pass and high pass filters (Fig. 2 of Supplementary data) at excitation and emission channels respectively. Figure 1 presents the NIR emission spectra of co-doped sample recorded by exciting Yb3+ (985 nm) and Ho3+ (464 nm, 653 nm, 1183 nm and 1836 nm) ions corresponding to their intense absorption transitions. The spectra have revealed characteristic emission bands from Yb3+ and Ho3+ ions which are designated to appropriate transitions depending on their energy positions. Further, there is a clear dependence of excitation wavelength on emission intensity where an ~8 fold enhancement in ~2 μm emission has been demonstrated under Yb3+ (985 nm)excitation compared to direct Ho3+ ion excitations (except 1836 nm excitation, where only ~4 fold enhancement was observed). This has been attributed to an effective energy transfer from Yb3+ to Ho3+ ions with estimated transfer efficiency of 86% (ref. 18).

Figure 2

MIR emission spectra of Yb3+/Ho3+ co-doped glass under different excitation wavelengths.

The insets (a & b) of Fig. 1 show the magnified view of emission transitions in respective wavelength regions along with partial energy level diagram projecting those emission transitions. The spectra shown in inset (a) of Fig. 1 (1300–1700 nm region) appears to be greatly dependent on the excitation wavelength. Under Ho3+: 464 nm excitation, two emission bands observed at 1400 nm and 1530 nm are assigned to the transitions 5S2 → 5I5 and 5F5 → 5I6 respectively. But, the Ho3+: 653 nm and 1183 nm excitations show only 5F5 → 5I6 emission transition slightly at higher energy side ~1490 nm. In case of 1183 nm excitation, this emission is normally not anticipated due to huge energy level mismatch. However, the experimental emission data suggests, this emission transition is only possible with second photon absorption from 5I7 to 5F5 state through Excited State Absorption (ESA) process. The calculated excited state lifetime of 5I7 state is in the order of ~5 ms (radiative lifetime calculated through J-O analysis, Table 1 of Supplementary data)18 clearly indicates the probability of ESA from 5I7 → 5F5 is at higher side. Interestingly, under Yb3+: 985 nm excitation an enhancement in intensity of emission at ~1550 nm is observed with slight red shift in its peak. Based on the energy level position of Ho3+ ions, this emission peak is assigned to 5I5 → 5I7 transition20. As discussed above, the Ho3+: ~1550 nm emission can originate from 5F5, 5I5 levels, to ascertain the originating state probability, the radiative transition rate calculated from J-O analysis have been considered as an effective tool. The emission probability (from J-O analysis) for ~1550 nm originating from Ho3+: 5I5 level is ~65% (Table 1 of Supplementary data), whereas the ~1490/~1530 nm emission originating from Ho3+: 5F5 level is only 4% clearly indicates the prominence of 5I5 → 5I7 transition over 5F5 → 5I6. Further, upon Yb3+: 985 nm excitation the 5I5 level is effectively populated through resonant energy transfer process which enhances the transition probability of Ho3+: 5I5 → 5I7 (~1550 nm) over Ho3+: 5F5 → 5I6 transition.

Table 1

MIR emission properties comparison with other host materials.

Host	~2.9 μm ΔλFWHM(nm)	(cm2) 10−20	Reference
LLFC	~85	1.9	[21]
PbF2C	~120	1.4	[22]
GGGC	~130	0.3	[23]
Yb-Ho-YAGC	~50	1.2	[24]
TZNF60G	80	1.51	[11]
FluoroaluminateG	~60	1.91	[25]
ZBLANG	5	0.5	[26]
TBLG	180	0.91	Present Work

G: Glass; C: Crystal.

The MIR emission spectra of Ho3+ ions recorded under different excitation wavelengths are shown in Fig. 2(a–f). The spectra revealed an intense emission band peaking at around 2886 nm corresponding to Ho3+: 5I6 → 5I7 intra-manifold transition under all considered excitation wavelengths. The intensity of the band greatly depends on the excitation wavelength as shown in Fig. 2(f). Interestingly, compared to direct excitations of Ho3+ ions, Yb3+ sensitized excitation (985 nm) yielded a remarkable enhancement in the emission peak intensity. Since, the high absorption cross section (~1.9 × 10–20 cm2) of Yb3+ ions at 985 nm practically enhances the 2F5/2 excited state population density which favours an efficient energy transfer from Yb3+ to Ho3+ ions in the present host18. The MIR emission intensity has increased to ~15 folds, ~8 folds, ~4 folds and ~5 folds compared to direct Ho3+: 653, 464, 1183 and 1836 nm excitations respectively. The excitation spectrum as shown in Fig. 3 recorded for 2886 nm emission depicts several well resolved absorption bands of Ho3+ ions having lower intensity than that of Yb3+ ions specifies the significance of Yb3+ sensitization in the present host. The emission cross-section of 5I6 → 5I7 transition (at ~2.9 μm) calculated using Fuchtbauer-Ladenburg equation is found to be 9.068 × 10–21 cm2 (Fig. 3 of Supplementary data). Additionally, the full width at half maximum (FWHM) of this emission transition is 180 nm which is quite high compare to other tellurite glass hosts reported in the literature1112. The high band width is very much useful for ultra-broadband tunable laser sources. Table 1 compares the MIR emission properties of this glass with different crystal/glass hosts reported in the literature11212223242526. The emission cross-section value is comparable to other hosts and even better than ZBLAN fluoride glass.

Figure 3

Excitation spectrum of Yb3+/Ho3+ co-doped glass monitoring 2886 nm emission of Ho3+ ions.

Energy transfer mechanism

The probable energy transfer mechanism contributing to this intense MIR emission at ~2.9 μm may be through resonant energy transfer from Yb3+: 2F5/2(highest Stark component) → Ho3+: 5I5 alongside the phonon assisted Yb3+: 2F5/2 → Ho3+: 5I6 energy transfer process. Since the strong absorption of Yb3+ ions suppresses the weak absorption transition, 5I8 → 5I5 of Ho3+ ions in the region 850–930 nm, and in order to elucidate the resonant energy transfer mechanism, absorption spectra of both Yb3+ singly doped and Yb3+/Ho3+ co-doped samples were measured with slow scan rate as shown in Fig. 4.

Figure 4

Slow scanned absorption spectra of Yb3+ doped and Yb3+/Ho3+co-doped glass.

Inset (a): difference absorption spectrum; Inset (b): magnified view of Yb3+ absorption band.

The absorption spectrum of co-doped glass in the higher energy side (850–930 nm) indicates the superposition of Ho3+: 5I8 → 5I5 absorption band which could be clearly visualized in the difference spectrum shown in the inset (a) of Fig. 4. The broadness of absorption band of Yb3+ ions is shown in magnified view starting from ~820 to 1100 nm in the inset (b) of Fig. 4 emphasising the greater overlap of Yb3+: 2F5/2 and Ho3+: 5I5 energy levels. Further, the mechanism of resonant energy transfer from Yb3+: 2F5/2 → Ho3+: 5I5 also be supported by the experimentally detected NIR emission spectrum where the enhanced emission at ~1550 nm (Ho3+: 5I5 → 5I7) has been observed in case of Yb/Ho co-doped sample which is originating from 5I5 level under 985 nm excitation.

To understand the dynamics of the MIR emission originating state i.e., Ho3+: 5I6, the decay rates were studied under different excitation wavelengths by monitoring the intense emission from 5I6 level i.e., 1.2 μm. Figure 5 presents the decay curves of the NIR emission at 1.2 μm corresponding to Ho3+: 5I6 → 5I8 transition under different excitation wavelengths. The decay curves were well fitted to single exponential function and the lifetime values are presented in Fig. 5 (as measured decay curves of 5I6 level under different excitations are shown in Fig. 4 of Supplementary data). Considering the fact that, the lifetime of the ions in the excited state has less influence on the different emission transitions originating particularly from the same excited state. If we see the radiative transition probability rate (calculated from J-O analysis; Table 1 of Supplementary date) of the different emission transitions originating from 5I6 level, 80.5 % of Ho3+ ions were contributing for NIR 1.2 μm emission and 10.5% for MIR ~2.9 μm emission. Interestingly upon Yb3+ excitation, the decay rate of 5I6 level has been enhanced to ~1.7 times (compared to Ho3+: 464 and 653 nm excitation) and ~2.3 times (compared to Ho3+: 1183 nm excitation). The slow feeding of excited ions from Yb3+: 2F5/2 (lifetime of Yb3+ ions in Yb3+ singly doped glass is ~0.63 ms where as in Yb3+/Ho3+ co-doped sample it is 0.09 ms (ref. 18)) to Ho3+: 5I5 state through resonant energy transfer mechanism is contributing for the excessive population at Ho3+: 5I6 state which inturn enhanced its decay rate upon Yb3+ excitation.

Figure 5

Decay dynamics of Ho3+: 5I6 excited state. Inset: Ho3+: 5I6 → 5I8 emission intensity variation with different excitation wavelengths.

The partial energy level diagram illustrated in Fig. 6 presents the experimentally observed NIR and MIR emission transitions with possible Yb3+ → Ho3+ energy transfer mechanisms in the present host. In presence of Yb3+ ions, the energy transfer (Yb3+ → Ho3+) may take place mainly in three routes ET1, ET2 and ET3 as indicated in Fig. 6. From the observed emission intensities and their calculated emission transition strengths, probability of energy transfer mechanism may follow the trend ET1 > ET2 > ET3. The energy transfer micro parameter (CDA) has been calculated using the spectral overlap method. For ET1 process, the acceptor, Ho3+ ions absorption band 5I8 → 5I5 (peaking at ~900 nm) has been considered. For ET2 process, Ho3+ ions absorption band 5I8 → 5I6 (peaking at ~1200 nm) has been considered. In both the cases, donor Yb3+ ions emission cross-section calculated from Reciprocity Method (RM) has been considered19 for calculating the CDA values. The obtained values are 6.05623 × 10−33 cm6 s−1 for ET1 process and 1.38 × 10−40 cm6 s−1 for ET2 process. The CDA values strongly suggest that ET1 is more prominent than ET2. ET1 is resonant energy transfer process where as ET2 is phonon assisted energy transfers process. ET3 is mainly based on Excited State Absorption (ESA) process which depends on rare earth ion concentration, excited state (Ho3+: 5I6, 5I7) lifetime and pump power density. Under direct Ho3+ excitations at 464 nm and 653 nm, it follows multiple relaxations from different excited states to yield a less intense MIR emission transitions. Ho3+: 1183 and 1836 nm excitations show a considerable intense MIR emission transitions, again these transitions are dependent on the population densities at 5I7 level which is always a less probable route. Though a strong resonant energy transfer ET1 process (Yb3+: 2F5/2 → Ho3+: 5I5) observed in the present low phonon oxide glass system, emission at ~4 μm could not be perceived due to the low radiative transition probability (4%) or strong interaction with OH−ions (Fig. 5 of Supplementary data) in the glass matrix.

Figure 6

Partial Energy level diagrams of Yb3+ and Ho3+ ions depicting experimentally observed NIR and MIR emission transitions along with possible energy transfer mechanisms in the present glass system.

Conclusions

An intense and broad (FWHM = 180 nm) MIR emission peaking at around 2.9 μm (5I6 → 5I7) from Ho3+ ions with a cross-section () of 9.1 × 10−21 cm2 has been reported in a Yb3+/Ho3+ co-doped low phonon tellurium oxide based glass system. MIR emission intensity increased to many folds upon Yb3+ excitation at 985 nm compared to direct Ho3+ ion excitations which has been attributed to high absorption cross-section at pump wavelength followed by resonant energy transfer from Yb3+ → Ho3+ ions. The decay dynamics indicate Yb3+ sensitized excitation effectively enhance the Ho3+: 5I6 level decay time by increasing the population density through Yb3+: 2F5/2 → Ho3+: 5I5 resonant energy transfer process. The hassle free fabrication of these oxide glasses along with ease of fiberization make them potential candidates for the development of solid state MIR laser sources compatible to the commercially available high energy pump sources.

Materials and Methods

The Ho3+/Yb3+ co-doped tellurite glass of composition (mol %) 80TeO2-15(BaF2 + BaO)-3La2O3-1Ho2O3-1Yb2O3 and only Yb3+ doped glass (mol %) 80TeO2-15(BaF2 + BaO)-4La2O3-1Yb2O3 were prepared by melt quenching technique. The raw chemicals used such as TeO2, BaF2, BaCO3, La2O3, Ho2O3 and Yb2O3 were of GR grade as procured from Sigma-Aldrich, USA with ≥99.99% purity. Each batch to yield approximately of 10 g glass was melted for about an hour in a pure platinum crucible at 700−750 oC in an electrical furnace. The molten batch was stirred intermediately using a thin platinum rod to attain bubble free and homogeneous melt. The cast glasses were annealed for about 2 hours at the temperature close to glass transition range to avoid thermal stress in the glass. For optical measurements, the samples were cut and polished to a plate-shape.

The absorption spectra of the samples were recorded using UV-VIS-NIR absorption spectrophotometer (Model: Lamda950, Perkin Elmer, USA). The emission and excitation spectra of the sample were recorded at room temperature on spectrofluorimeter (Model: Quantum Master enhanced NIR from PTI, USA) fitted with double monochromators on both excitation and emission channels. The NIR (1–2.5 μm) and MIR (2.5–5 μm) emission spectra of the sample were recorded through Peltier cooled InGaS solid state detector (Model: J23TE2-66C-R02M-2.4, Judson technologies, USA) and LN2 cooled InSb detector (Model: P7751-02, Hamamatsu, Japan) respectively. The emission channels were equipped with 2000 nm and 4000 nm blazed gratings for InGaS and InSb detectors respectively. Appropriate low-pass and high-pass filters from Edmund Optics, Inc, USA were used at excitation and emission channels to avoid excitation and emission wavelength’s higher order harmonics in the recorded emission spectrum. The transmission spectra of the used filters were given in the Supplementary file for ready reference. The decay kinetics of Ho3+: 5I6 excited state were measured under different excitation wavelengths (464, 653, 985 & 1183 nm) by monitoring 1.2 μm emission corresponding to 5I6 → 5I8 transition using 60 W Xenon flash lamp as source and LN2 cooled NIR PMT as detector in gated mode (Model: NIR PMT 1.7 from Hamamatsu, Japan) on same spectrofluorimeter.

Additional Information

How to cite this article: Balaji, S. et al. Role of Yb3+ ions on enhanced ~2.9 µm emission from Ho3+ ions in low phonon oxide glass system. Sci. Rep. 6, 29203; doi: 10.1038/srep29203 (2016).