J. H. Choi and F. G. Shi * Department of Chemical Engineering and Materials Science University of California, Irivne, CA

Transcription

1 Dopant an concentration epenence of linear an nonlinear refractive inex an ispersion for new (Mg, BaF base fluorophosphates glass J. H. Choi an F. G. Shi * Department of Chemical Engineering an Materials Science University of California, Irivne, CA A. Margaryan an A. Margaryan AFO Research Inc, Glenale, CA USA 9109 T.G. Nieh Lawrence Livermore National Laboratory 7000 East Ave. P.O. Box 808, L-350 Livermore, CA ABSTRACT Linear an nonlinear refractive inex, Abbe number, electronic energy gap an oscillator strength are reporte for a new series of (Mg, BaF -base fluorophosphates glasses (MBBA system ope with rare earth opants (Er 3+, N 3+ in the concentration range of 6.67x x10 1 (ions/cm 3 an.5x x10 1 (ions/cm 3, respectively. The linear refractive inex is foun to increase with increasing opant concentration, while the Abbe number is foun to be remarkably concentration invariant, i.e., aroun for both opants. The average electronic ban gap is also foun to be almost opant concentration inepenent, i.e., about 4.1, while the electronic oscillator strength is foun to slightly increase with increasing opant concentration, i.e., from 6. to 6.4. The nonlinear refractive inex is foun to show a linear increase from to for the investigate opant concentration range. Those results strongly suggest the present new series of glasses can be excellent laser hosts Keywor: Refractive inex, Nonlinear refractive inex, Optical ispersion, Flurophosphates glass 1. INTRODUCTION Because of a favorable combination of low nonlinear refractive inex (ue to fluorie as a glass-forming constituent an high cross section (ue to phosphate as a glass-forming constituent, f uorophosphates glass represents one of the GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG G Q G m ˆ GnUGz GOŠ Œš Œ ŠŒPaGl ˆ a Žš gœš UŒ œbg ŒaXG`[`G_Y[G\Z]YbG ˆŸaXG`[`G_Y[GY\[XG

2 best potential laser hosts [1]. It was foun that with a fluorophosphate glass, a relatively higher egree of line broaening an smoother line shapers can be obtaine []. It was also foun that neoymium-ope fluorophosphate glasses can eliver relatively shorter pulses than pure phosphate glasses, which were attribute to the relatively higher egree of inhomogeneous line broaening in fluorophosphate glasses [3]. Other spectroscopic stuies also inicate that the fluorophosphate glass is an excellent caniate for high power lasers an broaban amplifiers in the eye-safe region aroun 1.5 µm for applications in communication, meicine, an meteorology [4-8]. It was also foun to be a goo material for the generation of ultrashort pulses [9]. G We have introuce a new family of fluorophosphates glasses which can be ope with an extremely high concentration of rare earth opants. The ope glasses exhibit low optical ispersion an high refractive inex. The objective of this paper is to report the opant type (Er 3+, N 3+ an opant concentration epenence of various optical properties incluing linear an nonlinear refractive inex for the new series of (Mg, BaF base fluorophosphates glasses.. EXPERIMENTAL ARRANGEMENT.1 Glass synthesis The reagent-grae starting materials (City Chemicals an rare earth materials such as Er O 3, an N O 3 (Spectrum Materials have above 99.99% purity. A series of batches are weighe on 0.001% accuracy an mixe thoroughly. The raw mixe materials are melte in a vitreous carbon crucible in Ar-atmosphere at C. The quenche samples are anneale at C below the transition temperature to remove an internal stress examine by the polariscope (Ruolph Instruments. Samples for optical an spectroscopic measurements are cut an optically polishe to the size of 15x10xmm 3.. Optical properties The refractive inex (n D, n F an n C is measure with a unit of Abbe refractometer (ATAGO at 0 C at the wavelength of 486, 589, an 656nm, respectively. Three ifferent measurements are carrie out to get the average value for both types of rare earth opants an at their concentration at least three samples. Abbe number is obtaine for the following expression ( nd 1 ( n n ν = (1 F C where n D, n F an n C are the refractive inices at the D-, C- an F-spectral lines. The refractive inex values are use to calculate the partial ispersion (n F -n C an reciprocal relative ispersion i.e. Abbe number (ν. The refractive inex variation with respect to photon energy can also be obtaine from the following relationship, base on the single

3 oscillator approximation suggeste by Wemple [10] E Eo n 1 = ( E E o where n is the refractive inex at a specific wavelength, E is the photon energy (=hν, E o is the average electronic energy gap for transition an E is the electronic oscillator strength. Eq.( can be written as, 1 E = n 1 E o E E E o (3 which inicates that E o an E can be etermine from the linear fit of 1/(n -1 vs. E. Nonlinear refractive inex relate to minimize self-focusing an self moulation in high power laser application can be obtaine from Eq. (4. n values are euce on the base of -line refractive inex (n an Abbe number (ν [11]. k( n 1( n n[ esu] = ν ν ( n + ( n + 1 6n (4 where the value of K is an empirical factor that might be constant; 68x10-13 [esu] for a variety of fluorie crystals an 70x10-13 [esu] for fluorie glasses. In this work, K of 70x10-13 [esu] was use to estimate the n because major compositions in our MBBA system are mostly base on (Mg, BaF. 3. RESULTS AND DISCUSSION 3.1. Linear refractive inex an Abbe number The refractive inex of a glass usually epens on iniviual ions present in the glass an their packing an polarizability, of cation. Furthermore, the refractive inex generally increases with increasing size of cation [1]. In our systems, the host materials are fixe an compose of (Mg, BaF -base fluorophosphates glass (MBBA system. Er 3+ an N 3+ ions are ope in the form of Er O 3, an N O 3. Fig. 1 shows wavelength epenence of the refractive inex of MBBA system with respect to (a N 3+ (ber 3+ ion concentration. As shown in Fig. 1, the refractive inex ecreases regarless of types of rare earth opants an their concentration as the wavelength increase. It is obvious that the refractive inex measure at D-line spectra (486nm shoul be relatively higher than those measure at F-line (589nm, C-line (656nm spectra. The relationship between refractive inex (n an photon energy (E in given by Eq. (. Lower frequency (λ o

4 results in larger E (=hc/λ o an it means that photon energy E is getting higher an total values of right-han sie in equation ( will be finally increase. Therefore, refractive inex (n will increase with ecreasing wavelength (λ o. Refractive Inex No opant 5 Wt% Er 10 Wt% Er 15 Wt% Er 0 Wt% Er Refractive Inex No opant Wt% N 5 Wt% N 10 Wt% N Wavelength (nm (a Wavelength (nm (b G Fig. 1 Wavelength epenence of the refractive inex of the MBBA system with respect to (a Er 3+, (b N 3+ ion concentration Consiering the concentration of rare earth opants (Er 3+, N 3+, the refractive inex of the MBBA system increases with the increase of the opant concentration. This is ue to the relatively ense packing of rare earth opants into host materials: Incorporation of metaphosphate compouns into fluorie leas to an extra room for rare earth ions since an incorporation of Ba(PO 3 an Al(PO 3 3, coul provie multiple sites for rare earth opants. Moreover, an incorporation of metaphosphate compouns, which results in more ense packing of rare earth opants, enhances the refractive inex because of increasing opant concentration. The expecte improvement of thermal stability an chemical urability as a result of incorporating metaphosphate compoun will be iscusse in a separate paper Refractive inex (n D Abbe number Refractive inex (n D Abbe number Refractive inex (n D Abbe number Refractive Inex (n D Abbe Number Er opant concentration (Wt% (a N opant concentration (Wt% (b G Fig. Refractive inex an Abbe number as a function of (a Er 3+ opant an (b N 3+ opant concentration

5 In general, there is an inverse relationship between the Abbe number an refractive inex in optical glasses. In other wor, the glass with a relatively high refractive inex has a relatively low Abbe number i.e. high ispersion. In case of fluorophosphates glasses, Abbe number is in the range of [1]. The reciprocal relative ispersion i.e. Abbe number (ν are calculate from refractive inex an presente with respect to two types of opants i.e. Er 3+, N 3+ ion an their concentrations. As shown in Fig., the refractive inex increases linearly with the concentration of rare earth opants (Er 3+, N 3+, but Abbe numbers (n o not noticeably ecline an are foun to be aroun The Abbe number is fairly high compare with other fluorophosphates glasses. Thus, the new MBBA system exhibits a low ispersion even at an extremely high opant concentration. 3. Average electronic energy gap (E o an the electronic oscillator strength (E The relationship between 1/(n -1 an E base on Eq. (3 is presente in Fig. 3. The overall tren is a linear ecrease with increasing rare earth opant concentration an the increase in photon energy is inepenent of types of opants. Refractive inex ispersion 1/(n -1 is relate to E o of the oscillator energy for electronic transition an E of the ispersion energy. Funamentally, the value of E, E o fitte to the linear lines in Fig. 3 epens on the neighbor cation coorination number an anion valence [13]. The MBBA system, the host materials are fixe an base on (Mg, BaF an phosphate glasses. In orer to investigate the epenence of ifferent opants (Er 3+, N 3+ an their concentration on the average electronic energy gap (E o an the electronic oscillator strength (E, the value of E o an E are calculate from the linear fitting in Fig. 3. 1/(n No opant 5Wt% Er 10Wt% Er 15Wt% Er 0Wt% Er 1/(n No opant Wt% N 5Wt% N 10Wt% N E (ev (a E (ev (b Fig. 3. Refractive inex variation on photon energy in MBBA glasses accoring to (a Er 3+ ion (b N 3+ ion concentration Figure 4 shows the epenence of ifferent types of opant (Er 3+, N 3+ an their concentration on the average electronic energy gap (E o an the electronic oscillator strength (E in the MBBA glasses. A general tenency in all glasses systems, chalcogenie, oxie an fluorie glasses, is that a larger refractive inex correspons to a smaller E o [14]. But it is note

6 that a larger refractive inex is relate to a smaller E o an it results in a larger ispersion for most glass systems. Accoring to the Wemple s equation (, refractive inex increase as the electronic ban gap (E o ecreases an the electronic oscillator strength (E increases E an E o (ev E :Electronic oscillator strength E o :Average electronic energy gap E o an E (ev E :Electronic oscillator strength E o :Average electronic energy gap Er opant concentration (Wt% (a N opant concentration (Wt% (b Fig. 4. The electronic oscillator strength (E an average electronic energy gap (E o with respect to Er 3+ an N 3+ ion concentration In the MBBA system, the average electronic ban gap (E o is almost inepenent of i.e. opant concentration. But the electronic oscillator strength (E increases with opant concentration. The electronic oscillator strength (E an average electronic ban gap (E o are not affecte by opants (Er 3+, N 3+ an their concentration. 3.3 Nonlinear refractive inex The refractive inex (n consists of linear refraction (n o an non-linear refraction (n in electromagnetic intensity (I, i.e. n=n o +n I. The nonlinear refractive inex (n can be estimate base on Eq. (4 from the ata on refractive inex an Abbe number. The knowlege of low nonlinear refractive inex (n is require for laser applications to prevent spatial intensity fluctuations in the wavefront an self-focusing [15]. Table 1. The epenence of expecte non-linear inex on Er 3+ an N 3+ ion concentrations Concentration (wt% Non-linear inex MBBA /Er [esu] MBBA /N Table 1 summarizes the nonlinear refractive inex (n for Er 3+ an N 3+ ion as well as for ifferent opant

7 concentrations. The nonlinear refractive inex is foun to be linearly epenent on the opant concentration, although the increase is insignificant. 4 CONCLUSION The opant type an concentration epenence of linear an nonlinear refractive inex an ispersion for a new (Mg, BaF base fluorophosphates glass (MBBA system have been investigate. The linear refractive inex has been foun to increase with increasing opant concentration ue to the ense packing of opant materials into host materials, while the Abbe number is foun to be remarkably concentration invariant, i.e., aroun for a wie opant concentration ranging in both opants. The average electronic ban gap has been also foun to be 4.1 almost opant concentration inepenent, i.e., about 4.1, while the electronic oscillator strength has been foun to slightly increase with increasing opant concentration, i.e., from 6. to 6.4. The non-linear refractive inex has been foun to exhibit a linear increase from to for the investigate opant concentration range. New (Mg, BaF base fluorophosphates glasses are promising materials base on results such as low nonlinear inex an low ispersion for laser application. The systematic investigation on other optical properties is also useful for material esigns. REFERENCES 1. W. Vogel, Glass Chemistry, Spring Verlag, chapter 7, 199. D. Ehrt: Proc. SPIE 1761, pp.13, D. Kopf, F.X. Kaertner, U. Keller, K.J.Weingarten: Opt. Lett. 0, 1169, S. I. Toorroki, S. Sakaguchi, M. Peeters, J. of American Ceram. Soc. 80 [] pp , Ozturk, A., Soc. Glass Technology, vol.41, (no.3, June pp.106-8, D. Warner, H. Rawson, Journal of Non-Crystalline Solis, vol.9, (no., Aug. pp.31-7, B. Kumar, R. Harris, Physics an Chemistry of Glasses, vol.5, (no.6, Dec. pp.155-8, S. Jiang, M. Myers, N. Peyghambarian: Opt. Eng. 37, pp.38, J.F. Philipps, T. T Opfer, H. Ebenorff-Heiepriem, D. Ehrt, R. Sauerbrey, Appl. Phys. B 7, pp , W. Roney, H. Halitson, T.A. King, J. Opt. Soc. Am. 48, pp.633, N.L. Boling, A.J. Glass, A. Owyoung, IEEE J. Quantum Electron. QE-14, pp. 601, Yasi, J., Fusong, J, Gan F, Chinese Physics, 3, pp.16, G. Vijaya Prakash, D. Narayana Rao, A. K. Bhatnagar, Soli state Communication 119, pp , H. Yayama, S. Fujino, K. Morinaga, H. Takebe, D.W. Hewak, D. N. Payne, J. of Non-Cryst. Solis, vol.39, (no.1-3, , 15. W. L. Smith, The hanbook of Laser Science an Technology, (Chemical Rubber Co., Boca Raton, Vol. 3, Pt. 1, pp.59, 1989