Uv-blue up-conversion at 365, 407 and 436 nm is reported in Ho3+ activated Al (NO3)3-SiO2 sol-gel glass under 641nm excitation. Large intensity enhancement and spectral linewidth narrowing is observed for the 5G6→5I8 ﴾436 nm﴿ hypersensitive transition in up-conversion compared to its luminescence and is attributed to amplified spontaneous emission (ASE).The influence of linewidth narrowing on emission properties of the transition is quantitatively analyzed. Using theoretical rate equations in steady state the proposed energy transfer routes for populating higher emitting states in the up-conversion are verified.
© 2011 OSA
Trivalent Holmium ion (Ho3+) has several high lying metastable states capable of emitting in the UV-VIS region. However, investigations on such short wavelength emissions are comparatively less and Ho3+ ions are primarily investigated for its mid infra-red (IR) emissions doped in different matrices for use in fibre communication system. The short wavelength emissions in rare-earth activated matrices are successfully described by laser generation multilevel schemes and mechanisms, such as sensibilisation, cascade generation, up-conversion and technologically can be more important because of its better efficiency in storage, transmission of data etc [1–5].
In this communication uv-blue up-conversion in Ho3+ ions doped sol-gel derived silica glass in presence of Aluminum Nitrate nonahydrate (Al(NO3)3,9H2O) is reported. Further, spectral linewidth and intensity of the hypersensitive 5G6→5I8 transition in up-conversion in comparison to luminescence is studied and its influence on radiative properties are investigated.
2.1 Sample synthesis
Sample for the study was prepared by sol-gel technique where polymerization of a metal alkoxide –tetraethylorthosilicate (TEOS)) solution in methanol – is catalyzed using doubly distilled water and dilute nitric acid. 0.01M(0.047 g) of Ho2O3 and 0.03M(0.14 g) of Al (NO3)3, 9H2O, with molar concentration (M) considered for 12.5 ml of solvent, were dissolved in 10.5 ml mixture of methanol(8.75 ml), distilled water(1.25 ml) and dilute nitric acid(0.5 ml) taken in proportion of 70 (Methanol): 10 (H2O): 4 (HNO3) parts by stirring continuously for 10 min in a magnetic stirrer. To this solution, 2ml of TEOS, that amounts to the remaining 16 parts in a mixture of 12.5ml, was added and further stirred for 1h till formation of gel starts. At this juncture the entire mass of gel was poured into plastic mould and left to dry and solidify at temperature of 25-27°C(room temperature). With progress of hydrolysis the gel solidified to form a stiff hard mass (Xerogel / sol-gel glass) in (2-3 days) 48-72 h time. It may be mentioned that dissolution of salts in solvent mixture is subject to salt solubility. Holmium, taken as Ho2O3 in this study, partially dissolves in the solvent mixture used for preparation of sol. The undissolved particles in the liquid phase diffused uniformly into the pores of the host network formed by removal of solvent from the gel (via hydrolysis) as glass formation progressed. This is reflected by the colour uniformity observed in the host (sol-gel glass),which the diffused rare–earth dopants impart. The sample was heat treated at 500°C for 5h after it solidified for reduction in hydroxyl content and densification prior to spectroscopic investigation. On densification a pale orange, transparent material of refractive index (n) 1.497, density (d) 1.453 g/cc and thickness (t) 0.2cm was obtained.
XRD and IR analysis of the Al(NO3)3-SiO2 sol-gel glass reported in author’s earlier work , indicates of an amorphous structure with Si-O and Si-O-Si bonds forming the glass network and signs of Al3+ entering the network as AlO4 tetrahedra when treated beyond 300°C.
The mol% concentration of the constituents of the prepared sample worked out as 3Al(NO3)3 – 96SiO2 – 1Ho2O3. In these calculation the M.W. % of the fraction of Ho2O3 and Al (NO3)3 in the total weight of the sample was considered.
The salts and reagents used in sample preparation were of spectroscopic grade with 99.9% purity. Holmium, used as oxide was supplied by Aldrich (Germany) and the tetraethylorthosilicate (metal alkoxide) was of Merck (Germany).
2.2 Refractive index, density and spectra
The refractive index (n) of the glass sample was measured with a Abbe refractometer (Almicro, India, AB 24) using sodium source (λ = 589.3 nm) and monobromonapthelene as the contact liquid. The density of the sample was measured in accordance to Archimedes principle with xylene as the immersion liquid.The UV- VIS absorption and up-conversion & luminescence spectra were recorded in commercially available spectrophotometer (Analytic Jena, Specord 200) and Spectrofluorimeter (Perkin Elmer, MPF 44B), respectively. The wavelengths used for excitation of up-conversion and luminescence were the 641 and 385 nm, respectively, of a xenon lamp source (400 W). All measurements and recordings were done at temperature 27-29 0 C.
3. Theoretical Considerations
3.1 Judd-Ofelt intensity parameters and the radiative transition probability9]Eq. (2) Tλ, the Judd-Ofelt parameters needs to be evaluated by least square fit analysis of equations formed by correlating experimentally determined oscillator strength (fexp) from absorption spectra, derived using7] for electric-dipole induced lanthanide transition, ψ J → ψ′ J′Eq. (1) and values of Ωλ derived from Eq. (2)..
The radiative transition probability A(ψ J, ψ′ J′) of a rare-earth transition ψ J → ψ′ J′, using the Judd-Ofelt intensity parameters (Ωλ, λ = 2, 4, 6) is calculated fromEqs. (5b) and (5c), respectively 11],Eqs. (5a) and (5c) are same as in Eq. (1). L and S represent orbital and spin angular momentum of the rare-earth ions. In Eq. (6) I(λp) is the measured intensity of the transition with peak at λp, n the refractive index of host.
4. Results and discussion
4.1 Absorption spectra and pump band oscillator strength
Up-conversion transitions show variations in their emission wavelength with pump band transition oscillator strength and wavelength apart from factors like nature of hosts, power of excitation etc. Uv-violet and uv-blue-green Ho3+ up-conversion reported in Y2O3 ceramic and fluoro- zirconate (ZBLAN) fibre, respectively, bears evidence to such variations. In the former case, where transitions are in uv-violet, excitation is by the 5I8 →5F4 + 5S2(532 nm) transition while in the later it is 5I8 →5F5(647 nm) transition [12,13]. Change in excitation wavelength affects the energy transfer amongst higher emitting levels causing shift in the wavelength range and number of observed transitions in up-conversion. Thus, for an efficient energy transfer in up-conversion choice of a pump band capable of storing large pump energy –the measure of which can be had from the oscillator strength of absorption transition terminating at the level ─ and suitably placed with respect to emitting energy levels is to be made from absorption spectra. In the present case the transition at 641nm in the UV-VIS absorption spectrum of Ho3+ doped Al(NO3)3-SiO2 sol-gel glass, assigned to 5I8 →5F5 Ho3+absorption transition is responsible for up-conversion excitation in the glass. The UV-VIS absorption spectrum of Ho3+ doped Al(NO3)3-SiO2 sol-gel glass with twelve assigned bands is presented in Fig. 1 . The oscillator strength (fcal) of the 5I8 →5F5 transition used for up-conversion excitation in the present glass is 2.064 × 10−6 . The excitation transition responsible for the luminescence in the present case is 5I8 →5G4 . The calculated Judd-Ofelt intensity parameters, oscillator strength of the absorption bands are tabulated along with their transition frequency in Table 1 . The values of and used in calculation of fcal using Eq. (1) are taken from reported works of Carnall et. al. .
4.2. Up- conversion and mechanism of energy transfer
The uv-blue up-conversion spectrum of Ho3+ in Al (NO3)3-SiO2 sol-gel glass in the range of 350 −500 nm is shown in Fig. 2(a) alongside its luminescence spectrum in Fig. 2(b). Sharp, well defined up-converted emission peaks observed on excitation by 641 nm wavelength are attributed to 3H6→5I8 (365 nm), (5G, 3G)5→5I8 (407nm), 5G6→5I8(436 nm) 4f-4f radiative transitions of Ho3+. Emission peaks (or bands) beyond 450 nm in up-conversion appear diffused in a strong background and hence not considered. In addition to (5G, 3G)5→5I8 (at 411 nm) and 5G6→5I8(at 436 nm) emission peaks, two more peaks (or features) attributed to 3K8 → 5I8 (463 nm) and 5F3→ 5I8(498 nm) transitions are observed in the luminescence spectrum under 385 nm excitation. Assignments of Ho3+ transitions here are verified with those reported in LaF3 crystal .
The energy transfer processes involved in this red to uv –blue up-conversion is explained in a multilevel energy scheme presented in Fig. 3 . Sequential absorption of excitation/ pump energy via excited state absorption (ESA) by the ground state manifolds of Ho3+ followed by multi-phonon relaxations are considered responsible for populating the upper energy levels 3H6, (5G,3G)5, 5G6 of the up-converted emissions achieved in Ho3+:Al (NO3)3-SiO2 sol-gel glass.
In the first step ESA from ground manifolds 5I4.5,6 excites Ho3+ ions to higher energy states 3L9, 5G4 and (5G,3G)5, respectively, by absorption of a second pump photon, the first of which is absorbed in ground state absorption(GSA) at 5I8 that precedes ESA . A close look into the oscillator strengths of the absorption transitions terminating at these higher energy levels (Table 1) indicates that except for (5G,3G)5 level, the oscillator strength- an indicator of the storage capacity- of the other levels is comparatively small, thereby reducing the possibility of a substantial population build up at these levels necessary for an intense radiative transition. But, successive fast non-radiative decays to closely separated lower levels viz. 3L9,→ 3H6→5G4→(5G,3G)5→5G6 (separation ~1800 cm −1) following ESA help to populate 3H6 and 5G6 levels responsible for radiative transitions at 365 nm and 436 nm, respectively. The (5G,3G)5 state is primarily populated by ESA and contribution by non-radiative relaxation from 5G4 level is negligible as a very small population accumulates via ESA in the level owing to its small absorbance (see absorption spectra).
Population in the 5I4, 5, 6, Ho3+ ground manifolds, the initial levels of the considered ESA, results from non-radiative de-excitation to lower levels in steps after excitation of Ho3+ ions to 5F5 level by ground state absorption (GSA).
The excitation processes by ESA discussed above in the energy level scheme can be represented as: 1. 5I4 (Ho3+) + pump photon →3L9(Ho3+). 2. 5I5 (Ho3+) + pump photon →5G4. 3.5I6 (Ho3+) + pump photon → (5G, 3G)5, (Ho3+) . In addition to ESA, energy transfer (ET) mechanism that can co–exist with ESA may also be considered. In ET process an excited ion transfers energy to another excited ion along resonant channels that may be represented in this present case as: 1 5I4 (Ho3+) + 5F5(Ho3+) →3L9 (Ho3+) + 5I8(Ho3+). 2. 5I5 (Ho3+) + 5F5(Ho3+)→5G4(Ho3+) + 5I8(Ho3+). 3. 5I6 (Ho3+) + 5F5(Ho3+)→(5G,3G)5 (Ho3+) + 5I8(Ho3+).
The concentration of Ho3+ ions (in ions/cm3 = Avagado’s number (NA) x mol% of RE ions x density of glass / M.W. of the glass) in the Al(NO3)3-SiO2 glass where energy transfer processes in up-conversion are described is 1.89 x 1022 ions/cm3 of the sample.
4.3 Rate equation analysis
Considering only up-conversion via ESA, rate equations required to describe the processes involved in up-conversion mechanism explained in the energy level scheme in Fig. 3 are16]. Bj,i, τi,j and ρ are Einstein’s B coefficient, spontaneous decay time and pump energy density, respectively. Ni = 1…10 represents population of Ho3+ ions in levels marked 1…..10 in Fig. 3. In writing these rate equations approximations like consideration of stimulated emissions from levels, possible non- radiative relaxations among closely spaced higher energy levels like… 3K8,3F2,3 etc. are neglected.
In steady state conditions, where the time derivatives of populations in levels are set equal to zero, the rate equations are solved simultaneously to derive expressions for emission intensity in terms of state population using 
Thus, for 3H6→5I8 transition the intensity is written asEq. (15) yieldsEq. (16), Eq. (10) and Eq. (11) giveEq. (21) in Eq. (19)16], resulting in the intensity expressionEq. (12) under steady state condition,Eq. (13) Eq. (13). Further from Eq. (8) and Eq. (9)Eq. (10) and Eq. (11) under steady state conditionEq. (29) and Eq. (30) N7 in terns of N1 isEq. (32) for reasons cited earlier a simplified expression for intensity of the transition 5G6→5I8 is written asEq. (31) asEqs. (25), (33) and (36) show quadratic dependence of transition intensity on the pump energy density (ρ). It confirms the involvement of two photon absorption in the emission processes - a characteristic of up-conversion transitions . The expressions further indicate the transition intensity dependence on the Ho3+ ion concentration (NA) .
4.4 5G6→5I8 transition: in up-conversion and luminescence
In disordered or amorphous materials linewidths of transitions are inhomogeneous broadened due to the different local environment experienced by all colour centres . These effects are more pronounced in hypersensitive / environment sensitive 4f-4f transitions like the 5G6→5I8 transition in Ho3+  and can have a large influence on spectral properties of transition. Effective linewidths (Δλeff) of inhomogeneously broadened transitions as in glass, ceramics etc. are measured as the fullwidth at half maximum(FWHM) intensity .Apart from serving as an excellent probe to characterize disorders in materials, measurement of such linewidths gives an insight into the spectral quality/purity (Q) of transitions. Q = λp/ Δλeff, λp is the peak emission wavelength.
The spectral quality of the hypersensitive Ho3+ 5G6→5I8 (436 nm-blue) in this silica sol-gel glass in presence Al (NO3)3 is found to be larger in the up-conversion compared to luminescence due to narrowing of linewidth (Δλeff) in up-conversion. Its is observed that with a ~1nm shift towards blue the 5G6→5I8 transition at 436 nm is 2.13 times narrower and 1.4 times more intense in the up-conversion compared to the luminescence. However, opposite is the case of the non-hypersensitive (5G, 3G)5→5I8 transition. Its linewidth is broader by ~3nm with a λp shift of ~4nm towards the blue in the up-conversion. A plot of spectral quality /purity (Q) vs. λp of transitions in up-conversion and luminescence is presented in Fig. 4(a) . The relative intensity of 5G6→5I8 transition in up-conversion and luminescence is derived by comparing normalized intensity ratio of I(5G6):I((5G, 3G)5) calculated from the up-conversion and luminescence spectra of Ho3+ in Fig. 2.
The enhancement of 5G6→5I8 transition intensity in up-conversion over luminescence is a situation of intensity variation with pump energy density (ρ), considering linear and quadratic dependence of intensities on pump energy density (ρ) in luminescence and up-conversion, respectively. Narrowing of transition linewidth and the variation in peak to peak intensity ratio of I(5G6) to I((5G, 3G)5)transitions under the circumstances described, i.e. with increased pump energy density, are spectral signature of amplified spontaneous emission (ASE) [18,19].
A graphical representation of the peak emission cross-section (), calculated using Eq. (6), with peak emission wavelength(λp ) for up-conversion and luminescence transitions is shown in Fig. 4(b). Computed values of peak emission wavelength (λP), effective bandwidth(Δλeff,) transition probability(A), quality factor(Q) and peak emission cross-section () of observed up-conversion and luminescence transitions in Ho3+: Al (NO3)3-.
SiO2 sol-gel glass using the equations described in Sec. 3 are tabulated in Table 2 . It may be noted that the radiative transition probability of 3K8+5F2→5I8 emission only include contributions of magnetic- dipole induced radiative probability(Amd) in addition to Aed.
The quadratic character of intensities resulting from steady state equation analysis of the up-converted emissions in Ho3+: Al(NO3)3-SiO2 sol-gel glass under 641 nm excitation in a multilevel scheme is an indication of two photon involvement in the process and establishes the experimentally observed uv-blue up-conversion emissions in to be 3H6→5I8 (365 nm), (5G, 3G)5→5I8 (407nm) and 5G6→5I8(436 nm) transitions of Ho3+. The hypersensitive 5G6→5I8 transition showed significant narrowing of effective linewidth (Δλeff) and intensity enhancement I(λp) in up-conversion, which may be attributed to amplified spontaneous emission(ASE) from the 5G6. However, for conclusive evidence variation of intensity and linewitdh of the hypersensitive transition in up-conversion at different excitation power needs to be studied. Comparison drawn from graphical representation of properties like emission cross-section () and spectral quality (Q) of transitions in up-conversion with its corresponding luminescence show reverse order variation for 5G6→5I8 hypersensitive and (5G, 3G)5→5I8 non- hypersensitive transition - increase in hypersensitive and decrease in non- hypersensitive transition in up-conversion. These variations are much pronounced for the environment sensitive 5G6→5I8 transition. The radiative properties data of transitions complied in Table 2 are indicative of the glass in study to be a potential optical material for the up-converted 5G6→5I8 hypersensitive transition at 436 nm.
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