Femtosecond nonlinear optical properties of lead-germanium oxide amorphous films Diego Rativa, Renato E. de Araujo, Cid B. de Araújo, Anderson S. L. Gomes, and Luciana R. P. Kassab Citation: Applied Physics Letters 90, 231906 (2007); doi: 10.1063/1.2747174 View online: http://dx.doi.org/10.1063/1.2747174 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/23?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 12:17:55 http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/2079205716/x01/AIP-PT/APL_ArticleDL_1213/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?x http://scitation.aip.org/search?value1=Diego+Rativa&option1=author http://scitation.aip.org/search?value1=Renato+E.+de+Araujo&option1=author http://scitation.aip.org/search?value1=Cid+B.+de+Ara�jo&option1=author http://scitation.aip.org/search?value1=Anderson+S.+L.+Gomes&option1=author http://scitation.aip.org/search?value1=Luciana+R.+P.+Kassab&option1=author http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://dx.doi.org/10.1063/1.2747174 http://scitation.aip.org/content/aip/journal/apl/90/23?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov Femtosecond nonlinear optical properties of lead-germanium oxide amorphous films Diego Rativa and Renato E. de Araujo Departamento de Engenharia Eletrônica e Sistemas, Universidade Federal de Pernambuco, 50740-530 Recife, Pernambuco, Brazil Cid B. de Araújoa� and Anderson S. L. Gomes Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil Luciana R. P. Kassab Laboratório de Vidros e Datação, Faculdade de Tecnologia de São Paulo, CEETEPS/UNESP, 01124-060 São Paulo, Sã Paulo, Brazil �Received 21 March 2007; accepted 14 May 2007; published online 6 June 2007� The nonlinear �NL� response of lead-germanium oxide amorphous films was investigated using a Ti:saphire laser delivering pulses of �150 fs at 800 nm. The Kerr shutter technique was employed to reveal the time response of the nonlinearity that is smaller than 150 fs. The sign and magnitude of the nonlinearity were obtained using a novel technique called thermally managed eclipse Z scan which allows the simultaneous characterization of cumulative and noncumulative NL effects. The NL refractive index of electronic origin, n2�2�10−17 m2/W, and the NL absorption coefficient, �2�3�103 cm/GW, were determined. © 2007 American Institute of Physics. �DOI: 10.1063/1.2747174� The search of new materials with large nonlinearity is motivated by the development of devices for optical process- ing, all-optical switching and optical limiting, among other applications. Several families of heavy-metal oxide �HMO� glasses that are promising candidates for such photonic de- vices have been reported.1–10 In particular, compositions of HMO glasses containing lead and/or bismuth are strong can- didates and have been studied in the past few years.6–10 Re- cent measurements with bismuth oxide based glasses from the femtosecond to the nanosecond regime6,7 demonstrated that the presence of heavy-metal atoms is very important to enhance the nonlinearity. Previous studies using pulses of 35 and 100 fs at wavelengths in the range of 600–1250 nm also revealed increasing of the HMO glasses nonlinearity with the increase of heavy-metal content.8 Measurements of fifth- and seventh-order nonlinearities of some HMO glasses were per- formed at 790 nm with pulses of 100 fs.9 Among the HMO families, the lead-germanate glasses deserve a lot of attention because they are simple to prepare, have high refractive indices ��2�, present large transmission in the visible and in the near infrared, are very stable and resistant to moisture, and have small cutoff phonon energy ��700 cm−1�. For instance, experiments performed with la- ser pulses of 15 ps at 1064 nm have shown that HMO glasses based on PbO–GeO2 present large nonlinear �NL� refractive index, n2�10−18 m2/W, negligible NL absorption coefficient �2, and good figure of merit for all-optical switch- ing applications.10 More recently large efficiency of second harmonic generation in Er3+ doped PbO–GeO2 induced by two-color optical poling was obtained.11 Although films containing heavy-metal constituents also attract a great deal of interest1,12–14 the femtosecond NL properties of amorphous films of lead-germanium oxides have not been investigated yet. Recently, the nonlinearity of lead-germanium based films �LGFs� was studied using a 15 ps neodymium doped yttrum aluminum garnet laser at 1064 nm and its second harmonic at 532 nm.15 NL refractive indices of �10−16 m2/W and NL absorption coefficient vary- ing from �102 cm/GW at 1064 nm to �103 cm/GW at 532 nm were measured. In this letter, we report on the LGF nonlinearity in the femtosecond regime as well as present measurements of n2 and �2 that show large nonlinearity for excitation at 800 nm. The experiments were made using the Kerr shutter technique16 and the thermally managed eclipse Z-scan �TM-EZ scan� technique.17 Large values of n2�2�10−17 m2/W and �2�3�103 cm/GW were mea- sured. Carbon disulfide �CS2� was used as a reference mate- rial to confirm our data. Films with a thickness of 1.5 �m were fabricated on quartz substrates using the rf sputtering method �50 W, 14 MHz�. Pure argon plasma was used at a constant pressure of 5.5 mTorr. The glassy targets were obtained by melting the starting materials in an alumina crucible at 1050 °C for 1 h, quenched in air, in a heated graphite mold, annealed for 1 h at 420 °C, and then cooled to room temperature in- side a furnace. The films obtained exhibit good optical qual- ity, high mechanical strength, and large adherence to quartz substrate. For the NL experiments we used a Ti-sapphire laser �800 nm, 150 fs, 76 MHz�. The Kerr shutter setup is well known.16 The laser beam is split into two beams with differ- ent intensities. The electric field of the strong �pump� beam is set at 45° with respect to the electric field of the weak �probe� input beam. When the pulses of both beams overlap spatially and temporally at the sample position, the probe beam polarization rotates due to the birefringence induced in the sample by the pump beam. Then, a fraction of the probe beam passes through a polarizer crossed to the input probe beam polarization. A slow detector is used to record the a�Author to whom correspondence should be addressed; electronic mail: cid@df.ufpe.br APPLIED PHYSICS LETTERS 90, 231906 �2007� 0003-6951/2007/90�23�/231906/3/$23.00 © 2007 American Institute of Physics90, 231906-1 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 12:17:55 http://dx.doi.org/10.1063/1.2747174 http://dx.doi.org/10.1063/1.2747174 http://dx.doi.org/10.1063/1.2747174 probe signal as a function of the delay between the pump and the probe pulses. The TM-EZ scan technique is a combination of the eclipse Z scan18 with the thermally managed Z scan.19 The new technique was introduced recently for studies of liquids, solids, and biomaterials.7,17 TM-EZ scan technique presents the sensitivity of eclipse Z scan and gives the simultaneous measurements of the nonthermal and thermal nonlinearities of the material under study. The use of lasers with high rep- etition rate allows measurements with large sensitivity and better signal-to-noise figure. The experimental setup for TM-EZ scan, shown in Fig. 1, employs a disk in front of the detector such that the eclipsed beam collected by a lens is directed towards the detector. For large disk and small NL phase shift ���0�0.2� the relationship between �Tpv and ��0 can be written as �Tpv=0.68 �1−Sd�−0.44 ���0�, where Sd is the fraction of the beam blocked by the disk, given by Sd= �1−exp�−2rd 2 /wd 2��, with rd being the disk radius and wd the beam radius at the disk position. The NL phase shift is given by ��0=kn2I0Leff where I0 is the excitation peak intensity within the sample, k=2� /�, Leff = �1−exp�−�oL�� /�o, and L is the sample length. The chop- per is the new element responsible for the thermal or other cumulative effects management, introduced to modify the conventional EZ-scan setup. In short, the TM-EZ scan method consists in acquiring the time evolution of the EZ- scan signal, for the sample placed in the pre- and postfocal positions of its focal plane with respect to lens L3. The time resolution of the system is determined by the chopper open- ing time � o=10 �s in our setup�, which depends on the finite size of the beam waist on the chopper wheel. By ex- trapolating the time evolution curves for t� o, noncumula- tive signals at both the pre- and postfocal positions are ob- tained. The photodetector information is sent to a digital scope and then processed. From these measurements, using the formalism described in Ref. 20, the EZ-scan curves can be constructed and the contribution of cumulative effects �such as thermal effects� and electronic nonlinearities can be inferred, provided no other mechanism besides the electronic nonlinearity are present in the relatively short time of the chopper opening rise time. The technique is sensitive to cu- mulative effects such as thermal effect and contributions due to absorption of excited carriers from states of long depopu- lation time. Figure 2 shows the Kerr shutter results obtained for CS2 and for the LGF sample. As is well known, CS2 has two decay times, a fast one ��50 fs� and a slower one ��2 ps�. On the other hand, the signal due to the LGF is symmetric. The inset in Fig. 2 shows the result in detail to illustrate the fast behavior of the signal, demonstrating that the sample response is limited by the pulse duration �150 fs�. The power dependence of the Kerr signal intensity versus pump power indicated a dependence of the signal with the square of pump beam intensity �Isignal IprobeIpump 2 �, which arises due to the phase shift imposed on the probe beam by the pump beam. Figures 3 and 4 show the results of the TM-EZ scan experiments. The solid lines are the best-fit curves obtained using the procedure described in Ref. 20. For the sake of comparison, as well as intensity calibration, we first per- formed measurements for liquid CS2 contained in a cell of 2 mm. For measurements of the signal temporal evolution the cell is placed in the peak and valley transmittance posi- FIG. 1. Experimental setup: L1–L5 are biconvex lenses. BS is a beam splitter. Pd1 and Pd2 are photodiodes. Ch is a chopper. FIG. 2. Normalized Kerr shutter signals for CS2 �solid squares� and the lead-germanium film �solid circles�. The inset shows the Kerr shutter signal for the film in an expanded scale. FIG. 3. Time evolution of the TM EZ-scan signal at pre- and postfocal positions. �a� Liquid CS2. �b� Lead-germanium film. 231906-2 Rativa et al. Appl. Phys. Lett. 90, 231906 �2007� 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 12:17:55 tion and the results are recorded for different delay times. The crossing of the two temporal evolution curves obtained at the pre- and postfocal positions indicates the presence of both cumulative and noncumulative nonlinearities. By extrapolating the time evolution curves of CS2 for t=0, the normalized peak-valley transmittance, for Sd=0.99, is �Tpv=0.25, corresponding to 1.6 GW/cm2 at the focus. The curves cross at c�150 �s, indicating the dominance of the thermal nonlinearity on CS2 after the crossing. Figure 3�b� shows the result for the LGF sample. Lower light intensity �0.5 GW/cm2� was used to prevent damage of the sample. In this case the temporal evolution curves cross at c�30 �s. By extrapolating the curves to t=0 the total change in the normalized transmittance is �0.005. From the measurements of Fig. 3�b� we obtain n2=2±1�10−17 m2/W. This result represents the average of eight measurements at different sample regions. A typical signal profile obtained for t� cross is shown in Fig. 4�a� where the solid line represents the theoretical fitting. NL absorption was also measured and Fig. 4�b� shows a typical result corresponding to �2= �3±1��103 cm/GW. To analyze the present measurements we first recall the previous results obtained using a 15 ps laser at 1064 nm and its second harmonic at 532 nm.15 In that experiment n2 and �2 did not change when the laser intensity was increased by almost one order of magnitude, indicating that high-order nonlinearities were not present. In the present experiment n2 is smaller than that for 532 and 1064 nm by about one order of magnitude; �2 at 800 nm has the same order of magnitude than that at 532 nm and is one order of magnitude larger than that at 1064 nm. The increase of �2 is attributed to energy states located inside the energy gap due to possible micro- scopic defects in the film. The localized states may present long relaxation time1 and originate a tail in the absorption spectrum of the LGF such that the linear absorption coeffi- cient at 1064 nm is 6.7�102 cm−1 and its value at 800 nm is 7.2�102 cm−1. It is probable that the value of �2 is en- hanced through resonance with intermediate states. Of course, the localized states may also originate cumulative effects that would increase the value of n2, but the value measured using the TM-EZ scan technique is of pure elec- tronic origin and it is not affected by cumulative effects. The large values obtained for n2 and �2 indicate that the LGF herein studied can be used as optical limiter for laser pulses of 150 fs. Financial support by the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico �CNPq� and Fundação de Amparo a Ciência e Tecnologia do Estado de Pernambuco �FACEPE� is acknowledged. This work was performed under the Millenium Institute on Non- linear Optics, Photonics and Bio-Photonics Project and the Nanophotonics Network Program. The Instituto Tecnológico da Aeronáutica is also acknowledged for the sputtering equipment used for the film production. 1See, for instance, M. Yamane and Y. Asahara, Glasses for Photonics �Cambridge University Press, Cambridge, UK, 2000�. 2K. Tanaka, J. Mater. Sci.: Mater. Electron. 16, 633 �2005�. 3T. Hashimoto, T. Yamamoto, T. Kato, H. Nasu, and K. Kamiya, J. Appl. Phys. 90, 533 �2001�. 4E. L. Falcão-Filho, C. B. de Araújo, C. A. C. Bosco, G. S. Maciel, L. H. Acioli, M. Nalin, and Y. Messaddeq, J. Appl. Phys. 97, 013505 �2005�. 5L. A. Gomez, C. B. de Araújo, D. N. Messias, L. Misoguti, S. C. Zílio, M. Nalin, and Y. 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Lett. 90, 231906 �2007� 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 12:17:55