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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 7 — Nov. 1, 2011
  • pp: 1307–1318
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Spectral linewidth narrowing and intensity enhancement of the 5G65I8 hypersensitive transition in uv-blue up-conversion from Ho3+ activated Al(NO3)3-SiO2 sol-gel glass

S. Hazarika and Reeta Rajbonshi  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 7, pp. 1307-1318 (2011)
http://dx.doi.org/10.1364/OME.1.001307


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Abstract

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 5G65I8 ﴾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

1. Introduction

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

1. G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B 27(6), 1158–1164 (2010). [CrossRef]

5

5. M. Seshadri, Y. C. Ratnakaram, D. Thirupathi Naidu, and K. Venkata Rao, “Investigation of spectroscopic properties (absorption and emission) of Ho3+ doped alkali, mixed alkali and calcium phosphate glasses,” Opt. Mater. 32(4), 535–542 (2010). [CrossRef]

].

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 5G65I8 transition in up-conversion in comparison to luminescence is studied and its influence on radiative properties are investigated.

2. Experimental

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 [6

6. S. Hazarika and S. Rai, “Structural, optical and non-linear investigation of Eu3+ ions in sol-gel silicate glass,” Opt. Mater. 27(2), 173–179 (2004). [CrossRef]

], 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 probability

Oscillator strength (fcal) of predominantly electric –dipole induced 4fn rare-earth transition ψ Jψ′ J′ can be determined using Judd-Ofelt theory [7

7. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]

,8

8. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]

] from the expression
fcal=8π2mc3hλ(2J+1)[(n2+2)2/9n]×λ=2,4,6Ωλ(|ψJUλψJ|)2,
(1)
where Ωλ, for λ = 2, 4, 6 are the Judd-Ofelt intensity parameters and are calculated from best fit Judd-Ofelt parameters(Tλ, λ = 2,4,6) and refractive index of glass(n) described by [9

9. F. Auzel, “Material for ionic solid-state lasers,” in Spectroscopy of Solid-State Laser-Type Material, B. D. Bartolo, ed. (Plenum, 1987), pp. 293.

]
Ωλ=3h8π2mc[9n/(n2+2)2](2J+1)Tλ.
(2)
In the above equations m is the mass of the electron, λ is the mean value of transition, J, the total angular momentum of the initial(ground) state of transition, [(n2 + 2)2/ 9n] is the local field correction factor through the refractive index(n) of host, Uλ is the reduced matrix element of unit tensor operator of rank λ and are independent of host matrix. The other symbols in the equation have their usual standard meanings. To determine Ωλ from 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 using
fexp=4.318×109ε(ν)dν,
(3)
where ε(ν) is the molar absorptivity at wave number ν(cm−1), with its corresponding theoretical expression given by Judd [7

7. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]

] for electric-dipole induced lanthanide transition, ψ Jψ′ J′
fed=ν(|ψJUλψJ|)2Tλ.
(4)
The theoretical oscillator strengths (fcal) are then calculated using 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 Jψ′ J′, using the Judd-Ofelt intensity parameters (Ωλ, λ = 2, 4, 6) is calculated from
A(ψJ,ψJ)=Aed+Amd,
(5a)
where Aed and Amd are electric- and magnetic- dipole induced radiative transition probabilities that are calculated using Eqs. (5b) and (5c), respectively [10

10. L. B. Shaw, B. Cole, P. A. Thielen, J. S. Sanghera, and I. D. Aggarwal, “Mid-wave IR and long-wave IR laser potential of rare-earth doped chalcogenide glass fiber,” IEEE J. Quantum Electron. 37(9), 1127–1137 (2001). [CrossRef]

]
Aed=64π4e2n(n2+2)23hλ¯3(2J+1)9λ=2,4,6Ωλ|ψJUλψJ|2
(5b)
Amd=64π4e2n33hλ¯3(2J+1)4m2c2|ψJL+2SψJ|2
(5c)
Using A(ψ J, ψ′ J′) peak emission cross-section (σem(λp)) of radiative transitions are determined with Füchtbauer-Ladenburg equation [11

11. B. F. Aull and H. P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]

],
σem(λp)=A(ψJ,ψJ)λp5I(λp)8πn2cλpI(λp)dλ,
(6)
The symbols in 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 5I85F4 + 5S2(532 nm) transition while in the later it is 5I85F5(647 nm) transition [12

12. F. Qin, Y. Zheng, Y. Yu, Z. Cheng, P. S. Tayebi, W. Cao, and Z. Zhang, “Ultraviolet and violet upconversion luminescence in Ho3+-doped Y2O3 ceramic induced by 532-nm CW laser,” J. Alloy. Comp. 509(4), 1115–1118 (2011). [CrossRef]

,13

13. M. Kowalska, G. Klocek, R. Piramidowicz, and M. Malinowski, “Ultra-violet emission in Ho: ZBLAN fiber,” J. Alloy. Comp. 380(1-2), 156–158 (2004). [CrossRef]

]. 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 5I85F5 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
Fig. 1 UV- VIS absorption spectrum of Ho3+ in (AlNO3)3-SiO2 sol-gel glass.
. The oscillator strength (fcal) of the 5I85F5 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 5I85G4 . The calculated Judd-Ofelt intensity parameters, oscillator strength of the absorption bands are tabulated along with their transition frequency in Table 1

Table 1. Absorption Transitions, Transition Frequency, Oscillator Strengths (fexp and fcal) and Judd-Ofelt Intensity Parameters (Ωλ) in Ho3+: Al (NO3)3-SiO2 Sol-Gel Glass

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. The values of λand Uλ used in calculation of fcal using Eq. (1) are taken from reported works of Carnall et. al. [14

14. W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys. 49(10), 4412–4423 (1968). [CrossRef]

].

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)
Fig. 2 (a) Up-conversion (b) Luminescence spectra of Ho3+ in (AlNO3)3-SiO2 sol-gel glass excited by(a) 641 nm and(b) 385 nm wavelength obtained with 400 W CW Xenon lamp.
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 3H65I8 (365 nm), (5G, 3G)55I8 (407nm), 5G65I8(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)55I8 (at 411 nm) and 5G65I8(at 436 nm) emission peaks, two more peaks (or features) attributed to 3K85I8 (463 nm) and 5F35I8(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 [15

15. M. J. Weber, B. H. Matsinger, V. L. Donlan, and G. T. Surratt, “Optical transition probabilities for trivalent holmium in LaF3 and YAlO3,” J. Chem. Phys. 57(1), 562–567 (1972). [CrossRef]

].

The energy transfer processes involved in this red to uv –blue up-conversion is explained in a multilevel energy scheme presented in Fig. 3
Fig. 3 Energy level scheme of Ho3+ and possible up-converted transitions routes of Ho3+ in (AlNO3)3-SiO2 sol-gel glass under 641 nm wavelength excitation.
. 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 are
dN1dt=A1,9N9W1,5N1+A6,1N6+A7,1N7+A2,1N2
(7)
dN2dt=A3,2N3W2,7N2A2,1N2
(8)
dN3dt=A4,3N4W3,8N3A3,2N3
(9)
dN4dt=A5,4N5A4,3N4W4,10N4
(10)
dN5dt=W1,5N1A5,4N5
(11)
dN6dt=A7,6N7A6,1N6
(12)
dN7dt=W2,7N2+A8,7N8A7,1N7A7,6A7
(13)
dN8dt=A9,8N9+W3,8N3A8,7N8
(14)
dN9dt=A10,9N10A9,1N9A9,8N9
(15)
dN10dt=W4,10N4A10,9N10
(16)
In the above equations Ai,j = 1/τi,j and Wj,i = Bj,i ρ are the transition probabilities of spontaneous (radiative and non- radiative, as well) emission and absorption, respectively, between levels i and j, where i or j = 1, 2 …..10 [16

16. M. Csele, Fundamentals of Light Sources and Lasers (Wiley-Interscience, 2004), Chaps. 4 and 5.

]. 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 [17

17. N. R. Giri, S. B. Rai, and A. Rai, “Intense green and red upconversion emissions from Ho3+ in presence of Yb3+ in Li:TeO2 glass,” Spectrochim. Acta [A] 74(5), 1115–1119 (2009). [CrossRef]

]
I=hνi,jAi,jNi,
(17)
where h is the Plank’s constant and νij the transition frequency between level i and j, for analysis of parameters affecting the intensities.

Thus, for 3H65I8 transition the intensity is written as
I(H36I58)=hν9,1A9,1N9
(18)
Appling the steady state condition, i.e., dNdt=0 Eq. (15) yields
N9=A10,9N10(A9,1+A9,8)
(19)
and Eq. (16), Eq. (10) and Eq. (11) give
N10=W4,10N4A10,9andN4=A5,4N5(W4,10+A4,3)=W1,5N1(W4,10+A4,3).
(20)
With the expression of N4
N10=W4,10W1,5N1A10,9(W4,10+A4,3).
(21)
Substituting Eq. (21) in Eq. (19)
N9=W4,10W1,5N1(A9,1+A9,8)(W4,10+A4,3).
(22)
I(H35I58)=hν9,1A9,1W4,10W1,5N1(A9,1+A9,8)(W4,10+A4,3).
(23)
Replacing N1 by NA , the concentration of Ho3+ ions and Wj,i by Bj,iρ for j = 4, 1 and i = 10,5 in the numerator
I(H36I58)=hν9,1A9,1B4,10B1,5ρ2NA(A9,1+A9,8)(W4,10+A4,3).
(24)
The expression may be further simplified as τ4,3 = 1/A4,3 the fast non radiative decay time from level 4→3 is a small quantity and makes 1/ τ4,3 numerically much greater than W4,10, the absorption probability which may, therefore, be ignored [16

16. M. Csele, Fundamentals of Light Sources and Lasers (Wiley-Interscience, 2004), Chaps. 4 and 5.

], resulting in the intensity expression
I(H36I58)=hν9,1A9,1B4,10B1,5ρ2NA(A9,1+A9,8)A4,3.
(25)
Similarly the intensity of the transition 5G65I8 is written as
I(G56I58)=hν6,1A6,1N6.
(26)
From Eq. (12) under steady state condition,
A6,1N6=A7,6N7
(27)
and from Eq. (13)
N7=W2,7N2(A7,6+A7,1)
(28)
As level 7 is primarily populated by ESA from level 2 contribution of fast non-radiative relaxation from level 8 is neglected in derivation of N7 from Eq. (13). Further from Eq. (8) and Eq. (9)
N2=A3,2N3(W2,7+A2,1)=A3,2A4,3N4(W2,7+A2,1)((W3,8+A3,2).
(29)
From Eq. (10) and Eq. (11) under steady state condition
N4=A5,4N5(W4,10+A4,3)=W1,5N1(W4,10+A4,3).
(30)
Using Eq. (29) and Eq. (30) N7 in terns of N1 is
N7=A3,2A4,3W2,7W1,5N1(A7,6+A7,1)(W2,7+A2,1)(W3,8+A3,2)(W4,10+A4,3).
(31)
IntensityI(G56I58)=hν6,1A7,6A3,2A4,3W2,7W1,5N1(A7,6+A7,1)(W2,7+A2,1)(W3,8+A3,2)(W4,10+A4,3).
(32)
Replacing W2,7, W1,5 and N1 by B2,,7ρ, B1,5ρ and NA, respectively, and ignoring absorption transition probabilities W2,7, W3,8 and W4,10 from the denominator of Eq. (32) for reasons cited earlier a simplified expression for intensity of the transition 5G65I8 is written as
I(G56I58)=hν6,1A7,6B2,7B1,5ρ2NA(A7,6+A7,1)A2,1.
(33)
The intensity of transition (5G,3G)55I8,
I((G5,G3)5I58)=hν7,1A7,1N7
(34)
can be written using Eq. (31) as
I((G5,G3)5I58)=hν7,1A7,1A3,2A4,3W2,7W1,5N1(A7,6+A7,1)(W2,7+A2,1)(W3,8+A3,2)(W4,10+A4,3)
(35)
Substituting W2,7 and W1,5 by B2,7ρ and B1,5ρ, N1 by NA, respectively, and ignoring W2,7, W3,8 and W4,10from the denominator as done earlier, the intensity expression simplifies to
I((G5,G3)5I58)=hν7,1A7,1B2,7B1,5ρ2NA(A7,6+A7,1)A2,1.
(36)
The three up-conversion intensity Eqs. (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 [17

17. N. R. Giri, S. B. Rai, and A. Rai, “Intense green and red upconversion emissions from Ho3+ in presence of Yb3+ in Li:TeO2 glass,” Spectrochim. Acta [A] 74(5), 1115–1119 (2009). [CrossRef]

]. The expressions further indicate the transition intensity dependence on the Ho3+ ion concentration (NA) .

4.4 5G65I8 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 [4

4. R. S. Meltzer, “Line broadening mechanisms and their measurement,” in Spectroscopic Properties of Some Rare-Earths in Optical Materials, G. Liu and B. Jacquier, eds. (Tsinghua University Press and Springer Verlag, 2005).

]. These effects are more pronounced in hypersensitive / environment sensitive 4f-4f transitions like the 5G65I8 transition in Ho3+ [5

5. M. Seshadri, Y. C. Ratnakaram, D. Thirupathi Naidu, and K. Venkata Rao, “Investigation of spectroscopic properties (absorption and emission) of Ho3+ doped alkali, mixed alkali and calcium phosphate glasses,” Opt. Mater. 32(4), 535–542 (2010). [CrossRef]

] 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+ 5G65I8 (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 5G65I8 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)55I8 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)
Fig. 4 Variation of (a) spectral quality (b) peak emission cross-section with peak emission wavelength in up-conversion and luminescence of Ho3+ transitions in (AlNO3)3-SiO2 sol-gel glas
. The relative intensity of 5G65I8 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 5G65I8 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

18. O. Svelto, Principles of Lasers (Plenum Publishing Corporation, Springer, 1998), Chap. 12.

,19

19. M. F. Joubert, B. Jacquier, and R. Moncorgé, “Exciton-exciton annihilation and saturation effect in TbF3,” Phys. Rev. B 28(7), 3725–3732 (1983). [CrossRef]

].

A graphical representation of the peak emission cross-section (σem(λp)), 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 (σem(λp)) 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

Table 2. Peak Emission Wavelength (λP), Effective Bandwidth (Δλeff), Radiative Transition Probability (A), Quality Factor (Q) and Peak Emission Cross-Section (σ(λP)) of Observed Up-Conversion and Luminescence Transitions in Ho3+: Al (NO3)3-SiO2 Sol-Gel Glass

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. It may be noted that the radiative transition probability of 3K8+5F25I8 emission only include contributions of magnetic- dipole induced radiative probability(Amd) in addition to Aed.

5. Conclusion

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 3H65I8 (365 nm), (5G, 3G)55I8 (407nm) and 5G65I8(436 nm) transitions of Ho3+. The hypersensitive 5G65I8 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 (σem(λp)) and spectral quality (Q) of transitions in up-conversion with its corresponding luminescence show reverse order variation for 5G65I8 hypersensitive and (5G, 3G)55I8 non- hypersensitive transition - increase in hypersensitive and decrease in non- hypersensitive transition in up-conversion. These variations are much pronounced for the environment sensitive 5G65I8 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 5G65I8 hypersensitive transition at 436 nm.

References and links

1.

G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B 27(6), 1158–1164 (2010). [CrossRef]

2.

M. Malinowski, M. Kaczkan, S. Stopinski, R. Piramidowicz, and A. Majchrowski, “Short-wavelength luminescence in Ho3+-doped KGd(WO4)2 crystals,” J. Lumin. 129(12), 1505–1508 (2009). [CrossRef]

3.

R. S. Quimby, M. G. Drexhage, and M. J. Suscavage, “Efficient frequency up-conversion via energy transfer in fluoride glasses,” Electron. Lett. 23(1), 32–34 (1987). [CrossRef]

4.

R. S. Meltzer, “Line broadening mechanisms and their measurement,” in Spectroscopic Properties of Some Rare-Earths in Optical Materials, G. Liu and B. Jacquier, eds. (Tsinghua University Press and Springer Verlag, 2005).

5.

M. Seshadri, Y. C. Ratnakaram, D. Thirupathi Naidu, and K. Venkata Rao, “Investigation of spectroscopic properties (absorption and emission) of Ho3+ doped alkali, mixed alkali and calcium phosphate glasses,” Opt. Mater. 32(4), 535–542 (2010). [CrossRef]

6.

S. Hazarika and S. Rai, “Structural, optical and non-linear investigation of Eu3+ ions in sol-gel silicate glass,” Opt. Mater. 27(2), 173–179 (2004). [CrossRef]

7.

B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]

8.

G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]

9.

F. Auzel, “Material for ionic solid-state lasers,” in Spectroscopy of Solid-State Laser-Type Material, B. D. Bartolo, ed. (Plenum, 1987), pp. 293.

10.

L. B. Shaw, B. Cole, P. A. Thielen, J. S. Sanghera, and I. D. Aggarwal, “Mid-wave IR and long-wave IR laser potential of rare-earth doped chalcogenide glass fiber,” IEEE J. Quantum Electron. 37(9), 1127–1137 (2001). [CrossRef]

11.

B. F. Aull and H. P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]

12.

F. Qin, Y. Zheng, Y. Yu, Z. Cheng, P. S. Tayebi, W. Cao, and Z. Zhang, “Ultraviolet and violet upconversion luminescence in Ho3+-doped Y2O3 ceramic induced by 532-nm CW laser,” J. Alloy. Comp. 509(4), 1115–1118 (2011). [CrossRef]

13.

M. Kowalska, G. Klocek, R. Piramidowicz, and M. Malinowski, “Ultra-violet emission in Ho: ZBLAN fiber,” J. Alloy. Comp. 380(1-2), 156–158 (2004). [CrossRef]

14.

W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys. 49(10), 4412–4423 (1968). [CrossRef]

15.

M. J. Weber, B. H. Matsinger, V. L. Donlan, and G. T. Surratt, “Optical transition probabilities for trivalent holmium in LaF3 and YAlO3,” J. Chem. Phys. 57(1), 562–567 (1972). [CrossRef]

16.

M. Csele, Fundamentals of Light Sources and Lasers (Wiley-Interscience, 2004), Chaps. 4 and 5.

17.

N. R. Giri, S. B. Rai, and A. Rai, “Intense green and red upconversion emissions from Ho3+ in presence of Yb3+ in Li:TeO2 glass,” Spectrochim. Acta [A] 74(5), 1115–1119 (2009). [CrossRef]

18.

O. Svelto, Principles of Lasers (Plenum Publishing Corporation, Springer, 1998), Chap. 12.

19.

M. F. Joubert, B. Jacquier, and R. Moncorgé, “Exciton-exciton annihilation and saturation effect in TbF3,” Phys. Rev. B 28(7), 3725–3732 (1983). [CrossRef]

OCIS Codes
(140.3610) Lasers and laser optics : Lasers, ultraviolet
(160.2750) Materials : Glass and other amorphous materials
(300.6540) Spectroscopy : Spectroscopy, ultraviolet
(140.3613) Lasers and laser optics : Lasers, upconversion

ToC Category:
Laser Materials

History
Original Manuscript: August 2, 2011
Revised Manuscript: October 14, 2011
Manuscript Accepted: October 15, 2011
Published: October 21, 2011

Citation
S. Hazarika and Reeta Rajbonshi, "Spectral linewidth narrowing and intensity enhancement of the 5G65I8 hypersensitive transition in uv-blue up-conversion from Ho3+ activated Al(NO3)3-SiO2 sol-gel glass," Opt. Mater. Express 1, 1307-1318 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-7-1307


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References

  1. G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B27(6), 1158–1164 (2010). [CrossRef]
  2. M. Malinowski, M. Kaczkan, S. Stopinski, R. Piramidowicz, and A. Majchrowski, “Short-wavelength luminescence in Ho3+-doped KGd(WO4)2 crystals,” J. Lumin.129(12), 1505–1508 (2009). [CrossRef]
  3. R. S. Quimby, M. G. Drexhage, and M. J. Suscavage, “Efficient frequency up-conversion via energy transfer in fluoride glasses,” Electron. Lett.23(1), 32–34 (1987). [CrossRef]
  4. R. S. Meltzer, “Line broadening mechanisms and their measurement,” in Spectroscopic Properties of Some Rare-Earths in Optical Materials, G. Liu and B. Jacquier, eds. (Tsinghua University Press and Springer Verlag, 2005).
  5. M. Seshadri, Y. C. Ratnakaram, D. Thirupathi Naidu, and K. Venkata Rao, “Investigation of spectroscopic properties (absorption and emission) of Ho3+ doped alkali, mixed alkali and calcium phosphate glasses,” Opt. Mater.32(4), 535–542 (2010). [CrossRef]
  6. S. Hazarika and S. Rai, “Structural, optical and non-linear investigation of Eu3+ ions in sol-gel silicate glass,” Opt. Mater.27(2), 173–179 (2004). [CrossRef]
  7. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev.127(3), 750–761 (1962). [CrossRef]
  8. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys.37(3), 511–520 (1962). [CrossRef]
  9. F. Auzel, “Material for ionic solid-state lasers,” in Spectroscopy of Solid-State Laser-Type Material, B. D. Bartolo, ed. (Plenum, 1987), pp. 293.
  10. L. B. Shaw, B. Cole, P. A. Thielen, J. S. Sanghera, and I. D. Aggarwal, “Mid-wave IR and long-wave IR laser potential of rare-earth doped chalcogenide glass fiber,” IEEE J. Quantum Electron.37(9), 1127–1137 (2001). [CrossRef]
  11. B. F. Aull and H. P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron.18(5), 925–930 (1982). [CrossRef]
  12. F. Qin, Y. Zheng, Y. Yu, Z. Cheng, P. S. Tayebi, W. Cao, and Z. Zhang, “Ultraviolet and violet upconversion luminescence in Ho3+-doped Y2O3 ceramic induced by 532-nm CW laser,” J. Alloy. Comp.509(4), 1115–1118 (2011). [CrossRef]
  13. M. Kowalska, G. Klocek, R. Piramidowicz, and M. Malinowski, “Ultra-violet emission in Ho: ZBLAN fiber,” J. Alloy. Comp.380(1-2), 156–158 (2004). [CrossRef]
  14. W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys.49(10), 4412–4423 (1968). [CrossRef]
  15. M. J. Weber, B. H. Matsinger, V. L. Donlan, and G. T. Surratt, “Optical transition probabilities for trivalent holmium in LaF3 and YAlO3,” J. Chem. Phys.57(1), 562–567 (1972). [CrossRef]
  16. M. Csele, Fundamentals of Light Sources and Lasers (Wiley-Interscience, 2004), Chaps. 4 and 5.
  17. N. R. Giri, S. B. Rai, and A. Rai, “Intense green and red upconversion emissions from Ho3+ in presence of Yb3+ in Li:TeO2 glass,” Spectrochim. Acta [A]74(5), 1115–1119 (2009). [CrossRef]
  18. O. Svelto, Principles of Lasers (Plenum Publishing Corporation, Springer, 1998), Chap. 12.
  19. M. F. Joubert, B. Jacquier, and R. Moncorgé, “Exciton-exciton annihilation and saturation effect in TbF3,” Phys. Rev. B28(7), 3725–3732 (1983). [CrossRef]

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