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2005 Conference on Lasers and Electro-Optics Europe UV and violet emission dynamics in Nd3+ doped fluorozirconate glasses under pulsed IR excitation R. Piramidowic_, M. Klimc_ak and M. Malinowski Institute ofMicroelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, POLAND It is well known and repeatedly proved, that the Nd3+ ion is one of the most efficient and thus most often employed activators for infra-red solid state lasers. Nevertheless, also the ultraviolet and violet emission properties of neodymium ion make it attractive for applications in diode pumped short-wavelength laser sources, based on up-conversion mechanisms of excitation. Up to now there have been a very few neodymium based up-conversion laser experiments reported [1] although the shortest wavelength up-conversion laser ever demonstrated is the neodymium doped one. Although the up-converted, short wavelength fluorescence in different neodymium doped glasses has already been reported and analysed, there are no detailed studies on the fluorescence dynamics. In this work we investigate and discuss UV and violet emission dynamics under pulsed, multi-photon infra-red excitation in Nd3+ doped ZBLAN samples of different concentrations (0.3, 1, 2 and 5 mol. %). The IR to UV and visible up-conversion is observable for all of the investigated samples and the efficiency increases rapidly with concentration, which suggests multi-ion process. The observed emission spectra, containing additional bands in green, orange and red regions of the spectrum, are considerably different from those obtained under direct excitation. The up-converted fluorescence n4 decays from the high-lying 4D3/2 and 2P3/2 states are strongly non-exponential with the characteristic rise time further suggesting multi-ion nature of the processes involved. Additionally, the cubic dependence of the luminescence intensity on the infra-red pump intensity indicates three-photon excitation scheme. On the base of obtained spectroscopic results the excitation scheme can be proposed as depicted in Fig. 1. A time dependent rate equation model has been employed to confirm the three ion excitation mechanism. The proposed model of UV fluorescence decay considers three excited energy bands of the Nd3+ ion: 4F512+4F312, 4G7/2+4G512+2G7/2 and 4D3/2÷4D 72, referred to as n2, n3 and n4. The assumed notation indicates n, as the 419,2 ground level, which is not taken into the decay n3 consideration. The rate equations for n2, n3 and n4 populations can be written as: dn,2n d ' 2= -2 X, *1?22 - 2 X2 - n2 3n +WCR n4 n2 +NWVR32 n3 -WVR21 n, +A2 72 -_ l X2 dt 2 3n t ' I`2 A 2 `2 `3 CR43 `4 -3 NR43 -4 ' IVR32 -3 ' -3 -3 dt dt4 = X) ' n3 WCR42 n4 n2 - WCR43 n4 n3 - WIVR43 n4 - T4 The main process of the model resulting in the unconverted fluorescence is a two step energy transfer depicted by coefficients XI and X2. WCR42 and WCR43 characterize cross relaxation processes, and together with Xi and X2 are the fitted parameters of the model. Ti is the radiative lifetime of the ih level, A2 and A3 represent radiative populating of the n2 band from both n3 and n4, and n3 from n4, respectively. Non-radiative processes for iXj transition are represented by WNR.j The depicted model confirms the nature of the up-conversion mechanism taking place in Nd:ZBLAN under IR excitation. Furthermore it allows estimation of both the up- conversion energy transfer coefficients as well as cross-relaxation parameters when a three ion, two step energy transfer process in taken into account. [1] W.P. Risk, T.R. Gosnell, A.V. Nurmikko, Compact blue-green lasers, Cambridge University Press 2003 4 , n,I - _- 0 0 <, 0 t CA 0 0 Co Fig.l. Simplified energy level diagram of Nd:ZBLAN, showing the proposed excitation scheme. 0-7803-8974-3/05/$20.00 ©2005 IEEE IIII I& 332

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Page 1: [IEEE CLEO/Europe. 2005 Conference on Lasers and Electro-Optics Europe, 2005. - Munich, Germany (12-17 June 2005)] CLEO/Europe. 2005 Conference on Lasers and Electro-Optics Europe,

2005 Conference on Lasers and Electro-Optics Europe

UV and violet emission dynamics in Nd3+ doped fluorozirconate glassesunder pulsed IR excitation

R. Piramidowic_, M. Klimc_ak and M. Malinowski

Institute ofMicroelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, POLAND

It is well known and repeatedly proved, that the Nd3+ ion is one of the most efficient and thus most oftenemployed activators for infra-red solid state lasers. Nevertheless, also the ultraviolet and violet emission properties ofneodymium ion make it attractive for applications in diode pumped short-wavelength laser sources, based on

up-conversion mechanisms of excitation. Up to now there have been a very few neodymium based up-conversion laserexperiments reported [1] although the shortest wavelength up-conversion laser ever demonstrated is the neodymiumdoped one.

Although the up-converted, short wavelength fluorescence in different neodymium doped glasses has alreadybeen reported and analysed, there are no detailed studies on the fluorescence dynamics. In this work we investigate anddiscuss UV and violet emission dynamics under pulsed, multi-photon infra-red excitation in Nd3+ doped ZBLANsamples of different concentrations (0.3, 1, 2 and 5 mol. %).

The IR to UV and visible up-conversion is observable for all of the investigated samples and the efficiencyincreases rapidly with concentration, which suggests multi-ion process. The observed emission spectra, containingadditional bands in green, orange and red regions of the spectrum, are considerablydifferent from those obtained under direct excitation. The up-converted fluorescence n4decays from the high-lying 4D3/2 and 2P3/2 states are strongly non-exponential with thecharacteristic rise time further suggesting multi-ion nature of the processes involved.Additionally, the cubic dependence of the luminescence intensity on the infra-red pumpintensity indicates three-photon excitation scheme. On the base of obtained spectroscopicresults the excitation scheme can be proposed as depicted in Fig. 1. A time dependent rateequation model has been employed to confirm the three ion excitation mechanism. Theproposed model of UV fluorescence decay considers three excited energy bands of theNd3+ ion: 4F512+4F312, 4G7/2+4G512+2G7/2 and 4D3/2÷4D 72, referred to as n2, n3 and n4. The

assumed notation indicates n, as the 419,2 ground level, which is not taken into the decay n3consideration. The rate equations for n2, n3 and n4 populations can be written as:

dn,2nd'2=-2 X, *1?22 - 2 X2 - n2 3n +WCR n4 n2 +NWVR32 n3 -WVR21 n, +A2 72 -_ l X2dt 2 3n

t 'I`2 A 2 `2 `3 CR43 `4 -3 NR43 -4 ' IVR32 -3 ' -3 -3dt

dt4 = X) ' n3 WCR42 n4 n2 - WCR43 n4 n3 - WIVR43 n4 - T4

The main process of the model resulting in the unconverted fluorescence is a twostep energy transfer depicted by coefficients XI and X2. WCR42 and WCR43 characterizecross relaxation processes, and together with Xi and X2 are the fitted parameters of themodel. Ti is the radiative lifetime of the ih level, A2 and A3 represent radiative populatingof the n2 band from both n3 and n4, and n3 from n4, respectively. Non-radiative processesfor iXj transition are represented by WNR.j

The depicted model confirms the nature of the up-conversion mechanism takingplace in Nd:ZBLAN under IR excitation. Furthermore it allows estimation of both the up-conversion energy transfer coefficients as well as cross-relaxation parameters when athree ion, two step energy transfer process in taken into account.

[1] W.P. Risk, T.R. Gosnell, A.V. Nurmikko, Compact blue-green lasers, CambridgeUniversity Press 2003

4 ,

n,I-

_- 0

0 <,0t

CA

00Co

Fig.l. Simplified energylevel diagram ofNd:ZBLAN, showing theproposed excitationscheme.

0-7803-8974-3/05/$20.00 ©2005 IEEE

IIII

I&

332