Negative nonlinear absorption in Er 3+ -doped fluoroindate glass W. Lozano B., Cid B. de Araújo, L. H. Acioli, and Y. Messaddeq Citation: Journal of Applied Physics 84, 2263 (1998); doi: 10.1063/1.368292 View online: http://dx.doi.org/10.1063/1.368292 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/84/4?ver=pdfcov Published by the AIP Publishing [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.225 On: Tue, 14 Jan 2014 13:13:48 http://scitation.aip.org/content/aip/journal/jap?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1744363738/x01/AIP-PT/JAP_CoverPg_101613/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?x http://scitation.aip.org/search?value1=W.+Lozano+B.&option1=author http://scitation.aip.org/search?value1=Cid+B.+de+Ara�jo&option1=author http://scitation.aip.org/search?value1=L.+H.+Acioli&option1=author http://scitation.aip.org/search?value1=Y.+Messaddeq&option1=author http://scitation.aip.org/content/aip/journal/jap?ver=pdfcov http://dx.doi.org/10.1063/1.368292 http://scitation.aip.org/content/aip/journal/jap/84/4?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 4 15 AUGUST 1998 [This a Negative nonlinear absorption in Er 31-doped fluoroindate glass W. Lozano B., Cid B. de Araújo, and L. H. Acioli Departamento de Fı´sica, Universidade Federal de Pernambuco, 50670-901 Recife, PE Brazil Y. Messaddeq Instituto de Quı´mica, UNESP, 14800-900 Araraquara, SP Brazil ~Received 18 July 1997; accepted for publication 15 May 1998! We report the observation of negative nonlinear absorption in fluoroindate glasses doped with erbium ions. The pumping wavelength is 800 nm which is doubly resonant with Er31 ions transitions. A large nonlinear intensity dependence of the optical transmittance and strong upconverted fluorescence are obtained. The dependence of the upconverted fluorescence intensity with the laser power is described by a system of coupled-rate equations for the energy levels’ populations. ©1998 American Institute of Physics.@S0021-8979~98!07816-5# on o sio at g l ie th ba se ns p- m f iv ve ns a o in su th i d as t dure m- any o- ob- ults r- and he es- ci- eter, it- nals pe eam he ul- ing ich is in- the Fig. fi- I. INTRODUCTION Presently there is great interest in the study of the n linear absorption behavior of new materials because of p sible photonic applications such as frequency upconver lasers,1,2 all-optical switches,3,4 and bistable devices.5 Among the new materials available to date, fluoroind glasses doped with rare earth~RE! ions appear as promisin candidates to be used in photonic devices because of the multiphonon emission rates and high fluorescence effic cies for RE ions, as compared when they are doping o glasses. Various spectroscopic studies of RE doped InF3-based glasses have appeared in recent years. For instance, studies on the optical properties of fluoroindate glas doped with Eu31, Gd31, Pr31, Nd31, Tm31, and Dy31 were reported.6–11Frequency upconversion and energy tra fer processes involving Pr31-, Er31-, and Nd31-doped flu- oroindate glasses were also presented12–16 illustrating the large potentiality of this glass family as efficient optical u converters. Moreover, applications such as flashla pumped lasers,17 temperature sensor,18 and processing o waveguides for integrated optics19 are already known. In this paper we report the first observation of negat nonlinear absorption~NNA! in Er31-doped fluoroindate glass, at room temperature. This effect, previously obser in highly doped erbium–yttrium aluminum garnets20 and in semiconductors,21 is characterized by a decrease of the tra mission with increasing intensity. It is usually due to double resonance of the incident radiation with a pair electronic transitions. It can be studied directly by observ the transmission through the sample or indirectly by mea ing the resulting upconverted radiation as a function of intensity of the incident laser. This upconverted radiation only present when the highest excited state has a large ra tive branching ratio to the lowest lying states, as is the c presented here. From the applied point of view this effec also important for optical limiting applications22 and for op- eration of bistable devices.23 2260021-8979/98/84(4)/2263/5/$15.00 rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue - s- n e ow n- er sic s - p e d - f g r- e s ia- e is II. EXPERIMENT The samples used, prepared according to the proce described in Refs. 6, 7, 12, and 14 have the following co positions in mol %: (392x) InF3–20ZnF2–20SrF2–16 BaF2–2GdF3–2NaF–1GaF32xErF3 ~x51 and 3!. The samples present good optical quality and do not show crystallization under examination with the optical micr scope. Optical absorption spectra in the 200–800 nm were tained with a diode array spectrophotometer and the res are similar to the ones already presented in Ref. 14. The excitation of NNA was performed using a comme cial continuous-wave diode laser operating at 800 nm delivering ;5 mW. The laser beam was focused on t sample using a lens with 10 cm of focal length. The fluor cence, collected perpendicularly to the direction of the in dent beam and dispersed by a 0.5 m grating spectrom was detected with a GaAs photomultiplier. For the transm tance measurements Si photodiodes were used. The sig were processed with a lock-in or a digital oscillosco coupled to a personal computer for data processing. III. RESULTS AND DISCUSSION Figure 1 shows the dependence of the transmitted b intensity as a function of the incident laser intensity. T laser wavelength was chosen at 800 nm in order to be sim taneously resonant with one erbium transition originat from the ground state,4I 15/2→4I 9/2, and also with transitions associated to excited states4I 11/2→(4F5/2,4F3/2) and 4I 13/2 →2H11/2. The experiments were performed with the two Er31 con- centrations available and both samples exhibit NNA wh causes a decreasing transmission as the laser intensity creasing. The obtained results are shown in Fig. 1, where points represent the experimental data. The solid lines in 1 were obtained using the expressionT5(12R)2e2a0d@1 1a2 /a0I 0(12R)(12e2a0d)#2 i for the transmittance, where R50.04 is the air–glass interface reflectivity,I 0 is the laser intensity on the front surface of the sample,d represents the sample length,a0 is the measured linear absorption coef 3 © 1998 American Institute of Physics ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:13:48 e I. n th tr re of 3 - up . I a a lu ro nt ia w first s r- s ls rgy l 1 than em- the op- as a si w 2264 J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Lozano B. et al. [This a cient, anda2 is the nonlinear absorption coefficient. Th parameters used in these fittings are presented in Table The fluorescence spectrum observed in the 500–700 range for the sample withx53 is shown in Fig. 2. The spectral bands correspond to transitions2H11/2→4I 15/2 ~;520 nm!, 4S3/2→4I 15/2 ~;550 nm!, and4F9/2→4I 15/2 ~;670 nm!. The spectrum for the other sample is similar except for band intensities which decreases with the erbium concen tion. The frequency upconversion process was monito through the fluorescence at;550 nm and the dependence the emitted signal with the laser intensity is shown in Fig. We note that the sample withx53 presents a strong nonlin ear dependence~slope 5 in the log–log plot! while an almost quadratic dependence is observed for the other sample. We have also studied the dynamics of the frequency conversion process as a function of the pump intensity these temporal studies the excitation beam was chopped frequency of 8 Hz while the upconverted fluorescence w monitored with a digital oscilloscope. The temporal reso tion of the detection system is 0.1 ms. Figure 4 shows the dependence of the fluorescence f the x53 sample for four different intensities. It is importa to note that the signal rise time is;10 ms, although the lifetime of the fluorescing level is 0.6ms.14 This behavior indicates that long-lived states participate as intermed stages in the upconversion process. In order to understand the upconversion dynamics first consider the energy level scheme of Fig. 5~a! which TABLE I. Parameters used to fit the theoretical expression of the inten dependent transmittance. d(cm) a0(cm21) a2(cm21/W) x51 0.29 0.317 0.001 x53 0.28 1.028 0.631 FIG. 1. Transmittance at 800 nm as a function of the incident laser po for the samples withx51 andx53. rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue m e a- d . - n t a s - m te e indicates possible steps in the excitation process. The step is a resonant transition4I 15/2→4I 9/2 followed by a fast nonradiative relaxation to level4I 11/2 followed by a slow nonradiative decay to level4I 13/2. Excited state absorption ~ESA! to highly excited states,4I 11/2→(4F3/2,4F5/2), and 4I 13/2→2H11/2 occur and originate the frequency upconve sion emissions. It is important to note that cross relaxation (2H11/2,4I 15/2)→(4I 9/2,4I 13/2) and (4F7/2,4I 15/2)→(4I 11/2, 4I 11/2) may contribute to increase the population of leve 4I 11/2 and 4I 13/2 and thus the emission at;550 nm is en- hanced. The red emission at;670 nm (4F9/2→4I 15/2) occurs after nonradiative relaxation4S3/2→4F9/2. In Fig. 5 we present a simplified scheme for the ene levels including all the essential excitation steps. Leve corresponds to the ground state4I 15/2, level 2 corresponds to either4I 11/2 or 4I 13/2, and level 3 represents levels4S3/2 and all other levels of higher energies. State4F9/2 is not consid- ered in the model because the red emission is weaker the green fluorescence. A four level system was also ployed but no significant improvements compared to FIG. 2. Upconverted fluorescence spectrum excited with a diode laser erating at 800 nm. FIG. 3. Dependence of the upconverted fluorescence emitted at 550 nm function of the incident laser intensity. ty er ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:13:48 2265J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Lozano B. et al. [This a FIG. 4. Fluorescence intensity at 550 nm vs time during laser illumination with an excitation intensity of 56.6 W/cm2 ~a!; 24 W/cm2 ~b!; and 7.5 W/cm2 ~c!. Sample withx53. b hic in - in f nted re- b- ues 6. r- ent pare on. he on from three level model are obtained. The observed temporal havior is reproduced using a rate-equation approach w takes into account the contributions due to the two pump ratesR1(1→2) andR2(2→3). The equations for the popu lation densities assume the form: ṅ152R1n11W2n21W31n32Sn1n3 , ~1! ṅ25R1n12~R21W2!n21W32n312Sn1n3 , ~2! ṅ35R2n22W3n32Sn1n3 , ~3! where n1 , n2 , and n3 denote the population densities levels 1, 2, and 3, respectively, andn11n21n351. The pa- rametersW2 andW311W325W3 are the relaxation rates o levels 2 and 3, respectively.S is the effective cross- relaxation parameter associated to processes@4S3/2( 2H11/2) →4I 9/2#1@4I 15/2→4I 13/2# and @4S3/2( 2H11/2)→4I 13/2# rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue e- h g 1@4I 15/21 4I 9/2#. The software packageMATHEMATICA was used to solve numerically the system of equations prese above. The agreement of the model with the experimental sults can be verified in Fig. 4, where the solid line was o tained using the parameters indicated in Table II. The val of W31, W32, andW2 were obtained from Refs. 14 and 1 The ratio between the pumping ratesR1 and R2 , and the cross-relaxation rateS, were determined from the upconve sion intensity measurements as a function of the incid laser intensity. From the steady state solution of Eqs.~1!–~3! we can calculate the upconverted fluorescence and com the results with the observed behavior under cw excitati In Fig. 6 we show the results of the comparison with t experimental results from Fig. 3. It is important to menti that the parameters used here are the same as obtained the fitting of the transient experiments. ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:13:48 ra e t i as ss he rve on - all he sh- not that eri- the an ex- ser, tion A ong the fec- lari- , we va- g ur- ar rte rate The the 2266 J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Lozano B. et al. [This a We note that the dependence of the cross-relaxation with the erbium concentration~as shown in Table II! is a consequence of the change in the distances between th tive ions. This dependence explains why the NNA effec less pronounced in the sample withx51. The nonlinear be- havior of transmittance and upconverted fluorescence function of laser intensity is mainly due to the large cro FIG. 5. Simplified energy levels scheme.~a! Relevant levels of Er31; and ~b! three-level model used in the calculations. TABLE II. Parameters used to fit the temporal behavior of the upconve fluorescence. W31(s 21) W32(s 21) W2(s21) S(s21) R1 /R2 x51 638 262 100 1750 0.012 x53 781 321 109 1300 0.012 rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue te ac- s a - relaxation which enables the excitation of two ions in t (4I 11/2,2I 13/2) states when pumping transition4I 15/2→4I 9/2. A closely related effect which is also possible to obse with our samples is the phenomenon of phot avalanche.16,24–26 The main difference is that in the ava lanche effect the sample’s absorption is initially very sm andconstant, with the laser tuned far from resonance in t first step. Under these conditions there is an intensity thre old where the transmission starts to change, which is what happens in the present measurements. We can say the negative nonlinear absorption observed for the exp mental situation discussed here is a limiting case where avalanche effect is absent~even for very low powers the transmission depends nonlinearly on the intensity!, because the ratio between the pumping ratios,R1 /R2 , is too large. V. CONCLUSION We have reported negative nonlinear absorption in Er31-doped fluoroindate glass pumped at 800 nm. The perimental results, obtained using a low power diode la depend on the erbium concentration because the interac among ion pairs which contribute significantly to the NN process is strongly dependent on the relative distance am Er31 ions. The presented theoretical model agrees with experimental results and provides an estimate for the ef tive cross-relaxation rate among erbium ions. From the results presented here and due to the simi ties between fluoroindate glass and fluorozirconate glass conclude that other nonlinear effects, such as photon a lanche, may also be observed in InF3-based glasses usin inexpensive diode lasers with appropriate wavelengths. F ther experiments along this line will be performed in the ne future. d FIG. 6. Upconverted fluorescence intensity vs the pumping rate (R2). The circles are experimental results and the solid line was obtained using the equation approach to calculate the population of the fluorescing level. values for the parameters are those indicated in Table II obtained fitting theoretical expressions to the results of Fig. 4. ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:13:48 es to l p s. g, A , J A . ett. s. . l. ett. l. 2267J. Appl. Phys., Vol. 84, No. 4, 15 August 1998 Lozano B. et al. [This a ACKNOWLEDGMENTS This work was supported by the Brazilian Agenci Conselho Nacional de Desenvolvimento Cientı´fico e Tecno- lógico ~CNPq!, Financiadora Nacional de Estudos e Proje ~FINEP!, Fundac¸ão Coordenac¸ão de Pessoal de Nı´vel Supe- rior ~CAPES!, and Fundac¸ão de Amparo a` Ciência e Tecno- logia ~FACEPE!. Stimulating discussions with G. S. Macie and N. Rakov are also acknowledged. 1T. Herbert, R. Wannermacher, R. M. Macfarlane, and W. Length, Ap Phys. Lett.60, 2592~1992!. 2S. C. 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