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INFRARED LIGHT TO VISIBLE LIGHT
UPCONVERSION IN HEAVY METAL FLUORIDE GLASSES
W. Sibley, D. Yeh
To cite this version:
W. Sibley, D. Yeh. INFRARED LIGHT TO VISIBLE LIGHT UPCONVERSION IN HEAVY METAL FLUORIDE GLASSES. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-391-C7-395.
�10.1051/jphyscol:1987795�. �jpa-00227100�
Colloque C7, suppl6ment au n012, Tome 48, d6cembre 1987
INFRARED LIGHT TO VISIBLE LIGHT UPCONVERSION IN HEAVY METAL FLUORIDE GLASSES
W.A. SIBLEY and D.C. YEH
Department of Physics, Oklahoma State University, Stillwater, OK 74078-0444, U.S.A.
ABSTRACT
T h e e f f i c i e n t c h a n g e of infrared light t o visible light h a s i m p o r t a n t positive implications f o r optical d e t e c t i o n a n d display devices. On the o t h e r hand, this process can, in some
instances, s u c h as t h e d e v e l o p m e n t of l a s e r sources in fibers, result in unwanted e f f e c t s . In this paper, t h e efficiency of t h e t w o photon upconversion process in heavy m e t a l fluoride glasses a n d in halide crystals is reviewed. Methods for increasing o r reducing t h e efficiency of t h e
upconversion process a s applicable t o display, o p t i c a l d e t e c t i o n devices a n d the f o r m a t i o n of e f f i c i e n t l a s e r systems in f i b e r s a r e discussed.
INTRODUCTION
Recently t h e r e has been considerable i n t e r e s t in t h e upconversion processes described by Auzel. [I] T h e conversion of infrared radiation t o s h o r t e r wavelengths by various techniques has positive implications f o r device technology. However, this s a m e process c a n have unwanted side e f f e c t s in t h e development of highly e f f i c i e n t laser s y s t e m s in fibers. Therefore, i t is important to understand the physics of t h e process a n d b e a b l e t o provide guidelines f o r b e t t e r efficiency o r f o r quenching as may b e needed. Several comprehensive review papers provide details on t h e process. [l-31 The p r a c t i c a l i n t e r e s t in frequency upconversion a r i s e s f r o m t h e possibility of makin& infrared b e a m s visible. There i s t h e opportunity t o c o n v e r t infrared light to a s p e c t r a l frequency in which e f f i c i e n t d e t e c t i o n by photomultipliers or photodiodes c a n be realized. There i s also t h e promise of using upconversion pumping f o r laser devices. Auzel [ I ] a n d o t h e r s [4-61 have p e r f o r m e d extensive investigations in this area. T h e r e c e n t work of van d e r Ziel, et a l , [ 7 l . ~ o l l a c k , et a l , [8] Baker and his collaborators, [9] ( t h e l a t t e r using a d i f f e r e n t technique of energy transfer in phospher m a t e r i a l s ) a n d others' [lo-121 h a v e resulted in considerable progress. This progress will b e reviewed with a n e y e toward understanding upconversion in various systems.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987795
C7-392 JOURNAL DE PHYSIQUE
Table I. Typical Batch Compositions (Mole %) of Heavy Metal Fluoride Glasses.
Glass BaF2 ZnF2 YbF3 ThF4 LuF3 ErF4 TmF3
HEAVY METAL FLUORIDE GLASS UPCONVERSION
Figure 1 illustrates the upconversion process in heavy m e t a l fluoride glasses doped with y b 3 + : ~ r 3 + or yb3':~m3+ whose concentrations and designations a r e given in Table I. The figure p o r t r a y s a mechanism described in detail by Auzel previously. El] In this case, incident infrared radiation of 0.97 microns is absorbed by the yb3+ ions. A portion of t h a t energy is transferred t o e i t h e r erbium o r thulium depending upon t h e glass. The r a t e equations which express transfer a r e listed below f o r both systems. [2]
Acceptor Donor Aceoptor
r'igure 1. Schematic of Upconversion Processes.
quantum efficiency of this s t a t e depends upon energy transfer and/or the multiphonon emission r a t e of t h e state. In t h e absence of energy transfer, if t h e multiphonon emission r a t e is low (less of the optical energy is used up a s heat), then a high quantum efficiency results. In order t o determine t h e multiphonon r a t e s in glass systems, Reisfeld and others b 3 - 1 9 3 have made Judd-Ofelt calculations and measured t h e temperature dependence of the lifetimes of the various ions in glasses. Some years a g o Weber [20] used this approach to compare t h e multiphonon emission rate in various types of materials. We now know t h a t the heavy m e t a l fluoride blasses have .the least multiphonon emission of any of t h e glass materials
investigated. [21,22] This results in high quantum efficiency from the emitting levels and efficient upconversion in these systems.
Aside from t h e quantum efficiency, energy transfer is very important. The t e r m s Cdi in the equations a r e energy transfer coefficients related to the probability r a t e of energy transfer
w h e r e c d is t h e radiative lifetime of the donor ion and Rda is the distance between the donor and acceptor ions. Also Q A =
. bA I
(E)dE w h e r e G ( E ) is t h e absorption cross section and fd(E) and FA(E) a r e the normalized line shape functions f o r the donor emission and the acceptor absorption. In order for e f f e c t i v e energy transfer to occur t h e relationship pdd/Cb = 1 is necessary. If'the overlap integral is large the concentration of ions can be small and hence Rda can be large. This is t h e case for ~ r ~ + : ~ b ~ + and t h e process is characterized by a single exponential t i m e constant suggesting a l l ions participate. In Tm3+:yb3+ this is not the case.The overlap integral is very small a s can be recognized from t h e positions of t h e interacting energy levels. This decreases back transfer which increases t h e efficiency but it also means t h a t only a f e w Tm3+ ions will participate in t h e energy transfer process. Only those Tm3+ ions with sufficiently small Kda (i.e. neighbors of yb3+ ions) will be effective. Therefore, in t h e l-m3+:yb3+ case two lifetimes for Tm3+ a r e observed for t h e same transition. One lifetime arises from 'I'm3+ ions too far from yb3+ to be perturbed and the s h o r t e r r from Tm3+ ions very close to yb3+ ions. Of course, t h e lifetime for upconversion mirrors t h a t of t h e l a t t e r group.
It is possible to provide suggestions on how to enhance the efficiency of upconversion through a knowledge of t h e energy levels involved and energy transfer among both donor and acceptor ions. When the interacting levels have much different energies it is important t h e donor concentration be sufficiently high for donor t o donor energy transfer so t h e energy can reach the f e w effective donor-acceptor pairs. The acceptor concentration should be low. On the other hand, for interacting energy levels of t h e same energy a lower donor concentration can be used. This has also been illustrated in research on c r y s t a l systems and is shown by t h e projected efficiencies for 10 w/cm2 of incident radiation provided in Table 11.
C7-394 JOURNAL DE PHYSIQUE
In c a s e s where reduction of upconversion light is important, t h e efficiency c a n b e reduced by high donor or acceptor concentrations or -effectively by doping the m a t e r i a l with 3d ions such as cobalt or nickel.
Table 11. Projected U conversion E f f i c i e n c i e s f o r an m c l d e n t absorbed light intensity of lo wlcrnf a t T = 303 K.
S U M M A R Y
In summary, a b e t t e r understanding of upconversion has evolved over t h e last several years. It is now possible t o use this process t o m a k e infrared light visible t o t h e e y e a t room temperature and t o pump laser sources. Even more pr0,res.s will b e made ~ l s we ~,;1y fundamental physics t o maximize t h e desired effects.
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