Photoinduced effects in thin films of Te20As30Se50 glass with nonlinear characterization

Photoinduced effects in thin films of Te 20 As 30 Se 50 glass with nonlinear characterization

K. Fedus, G. Boudebs, Cid B. de Araújo, M. Cathelinaud, F. Charpentier, and V. Nazabal

Citation: Applied Physics Letters 94, 061122 (2009); doi: 10.1063/1.3082083 View online: http://dx.doi.org/10.1063/1.3082083

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/6?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in

Effect of Zn incorporation on the optical properties of thin films of Se 85 Te 15 glassy alloy AIP Conf. Proc. 1512, 552 (2013); 10.1063/1.4791156

Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge 16 As 29 Se 55 thin films

Appl. Phys. Lett. 98, 201111 (2011); 10.1063/1.3591978

Third order nonlinearities in Ge - As - Se -based glasses for telecommunications applications J. Appl. Phys. 96, 6931 (2004); 10.1063/1.1805182

Influence of ultraviolet-light exposure on electron-beam written gratings in amorphous As–Se thin films coated with different metals

Appl. Phys. Lett. 79, 2004 (2001); 10.1063/1.1405810

Photoinduced transformations in amorphous Se 75 Ge 25 thin film by XeCl excimer-laser exposure J. Appl. Phys. 83, 5381 (1998); 10.1063/1.367367

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 193.52.40.1 On: Tue, 03 May 2016 14:56:27

Photoinduced effects in thin films of Te

₂₀

As

₃₀

Se

₅₀

glass with nonlinear characterization

K. Fedus,¹G. Boudebs,^1,a兲Cid B. de Araújo,²M. Cathelinaud,^3,4F. Charpentier,⁵and V. Nazabal⁵

1Laboratoire des Propriétés Optiques des Matériaux et Applications, FRE CNRS 2988, Université d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France

2Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, Brazil

3Institut FRESNEL, UMR CNRS 6133, Université Paul Cézanne, Ecole Centrale Marseille, Université de Provence, 13397 Marseille, France

4Missions des Ressources et Compétences Technologiques, UPS CNRS 2274, 92135 Meudon, France

5Sciences Chimiques de Rennes, UMR6226, Université Rennes 1, 35042 Rennes, France

共Received 3 November 2008; accepted 23 January 2009; published online 13 February 2009兲

We discuss the influence of photoinduced effects

共PIEs兲

on the measurements of nonlinear refractive indices and nonlinear absorption coefficients. A chalcogenide glass film Te₂₀As₃₀Se₅₀ was studied using picosecond laser pulses at 1064 nm. The nonlinear imaging technique with phase object

共NIT-PO兲

and the Z-scan technique were applied and their results are compared. The NIT-PO technique reveals clearly the influence of PIE on the samples’ response, while by using theZ-scan technique we measured the deepness of ablated regions

共holes兲

produced during the measurements. ©2009 American Institute of Physics.

关DOI:

10.1063/1.3082083兴

The current interest in photonic devices is driving in- tense research to characterize new nonlinear

共NL兲

materials.

Usually to evaluate the materials performance for future ap- plications, the materials have to be investigated as thin films deposited on transparent substrates. Therefore, large number of papers are being published with basis on the use of four wave-mixing

共FWM兲 共Ref.

1兲andZ-scan²techniques to de- termine the materials’ NL parameters.^3–9 In particular the Z-scan technique has been much employed because it re- quires a simple experimental setup and the interpretation of the results seems to be straightforward. Among the NL films studied using FWM and Z-scan, the ones based on chalco- genide glasses deserve special attention because they may present large NL index of refractionn₂and large NL absorp- tion coefficient ␤^.

Chalcogenide glasses are known to be photosensitive but this has been ignored in most of the publications with infra- red lasers. Since FWM and Z-scan are techniques that re- quire long exposure time of the samples to high intensity lasers, it is difficult to observe changes produced by each successive laser shot. Indeed photoinduced effects

共PIEs兲

may produce permanent microscopic modifications in the samples^10,11that change the refractive index. Another prob- lem with the measurements reported is the possibility of laser ablation of the films that leads to giant changes in the samples’ transmission.

In this letter we report NL experiments using a 4.8 ␮^m thick Te₂₀As₃₀Se₅₀ chalcogenide glass film. The NL image technique with phase object

共

NIT-PO

兲 共

Ref. 12

兲

and the Z-scan technique² were applied. The NIT-PO technique is based on a 4f-coherent imaging system with a ␭/4 phase object

共PO兲

placed at the entry of the imaging system in such way that it allows observation of the NL image time evolu- tion for each laser shot.¹³ On the other hand, the Z-scan method masks completely the PIE contribution and normal

Z-scan profiles are obtained, which would be attributed to pure instantaneous electronic effect even when the glass film suffers changes at the beginning of the measurement. In the present work we evaluated the performance of the two NL techniques. Numerical simulation is also presented to sup- port the interpretation of the results.

The film deposition procedure was described in Ref.14.

The sample has a relatively large energy gap Eg= 1.56 eV

共⬇

800 nm

兲

and its linear optical absorption at 1064 nm is smaller than the sensitivity of the measurement appar- atus.^14,15 The NL experiments were performed using a Nd doped yttrium aluminium garnet laser

共1064 nm, 17 ps; 10

Hz兲 and the experimental setups employed for both tech- niques

共

Z-scan and NIT-PO

兲

were presented in Refs.12,13, and16.

Figures 1共a兲and 1共b兲show an example of NIT-PO ex- perimental image together with its profile. The phase contrast

共defined as a difference between the light intensity passing

through the PO and the intensity passing outside兲^12,13should be constant in time for a given NL phase shift

共

PS

兲

. However the curves presented in Fig.1共c兲show variations in the phase contrast as a function of time for four different peak intensi- ties inside the sample: 1.2, 2.0, 2.8, and 3.3 GW/cm². There are 50 laser shots between each successive data point. The temporal variations in the phase contrast for intensities above 1.2 GW/cm² reveals the presence of cumulative PIE and indicate ablation threshold for the sample between 1.2 and 2.0 GW/cm². The oscillatory behavior observed for 2.8 and 3.3 GW/cm² is related to high PS values and has been pre- dicted and explained theoretically in Ref. 17. Each oscilla- tion period shown in Fig.1共c兲corresponds to a PS change of 2␲. The PS should be attributed to the third order nonlinear- ity and to ablation of holes occurring for each laser shot. In the competition between the two contributions the ablation process becomes dominant very rapidly. For I₀= 3.3 GW/

cm² we observed contrast reduction after 1500 laser shots that can be explained by the total ablation of the thin film in the impact region, confirmed by scanning the sample surface

a兲Author to whom correspondence should be addressed. Electronic mail:

georges.boudebs@univ-angers.fr.

APPLIED PHYSICS LETTERS94, 061122

共

2009

兲

0003-6951/2009/94共6兲/061122/3/$25.00 94, 061122-1 © 2009 American Institute of Physics

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 193.52.40.1 On: Tue, 03 May 2016 14:56:27

using an atomic force microscope

共

AFM

兲

. According to Ref.

17, for high PS it is difficult to know from the image contrast when the induced dephasing is positive or negative. A corre- spondence between negative contrast and negative dephasing is only valid for PS smaller than ␲

共see Fig. 7 in Ref.

17兲.

SequentialZ-scan traces were made in three regimes of intensity: low, high, and low. The first experiment with low intensity, not shown here, was made in order to be sure that the fresh laser impact region has no holes or defects. Flat transmittance curves were obtained in the absence of NL contributions. Afterwards NL acquisitions with large inten- sity were performed. The insets of Figs.2共a兲and2共b兲show the experimental Z-scan results

共

filled circles

兲

using the closed- and open-aperture schemes, respectively. The laser intensity was 3.3 GW/cm²

共pulse energy of 4

␮J兲and the Z-scan profiles obtained indicate positive values for n₂

=

共9.0⫾

1.4兲⫻10⁻¹⁶ m²/W and ␤= 760⫾80 cm/GW. The transmittance of the numerical aperture in the closed- aperture Z-scan experiment was equal to 0.4. The profile obtained in the open-aperture scheme is narrower than the profile expected when only a two-photon absorption

共TPA兲

process occurs. However, the data can be fitted considering the contribution of TPA together with the absorption of ex- cited carriers.¹⁸ After the NL regime experiment, a Z-scan trace was obtained with very low intensity to prevent rel- evant NL contribution. The laser beam was incident on the same impact point as the strong laser in the NL experiments.

The same experimental results are shown in Fig.2

共

a

兲 共

filled circles兲 and in the inset of Fig. 2共a兲

共empty circles兲. These

results indicate that the sample changes due to the laser pulses and it behaves as a divergent lens for low intensity acting as a material having negativen₂. The closed-aperture Z-scan profile at low intensity can be explained considering the presence of a hole,¹⁹ while the decrease in the transmit- tance around the focus

共z

= 0兲, shown in Fig. 2共b兲

共filled

circles兲 and in the inset of Fig.2共b兲

共empty circles兲, are at-

tributed to the photodarkening effect. To support our inter- pretation, we performed numerical simulation of low inten- sity acquisitions using the field propagation optical transfer

function

共see Chap. 3 in Ref.

20兲inside a 4f system, with the specimen moving in the focal region as in Ref.21. We con- sidered that the sample contributes for a constant linear PS due to a hole having the same spatial Gaussian shape than the focused beam. The permanent change in the linear PS, created by PIE during the NL regime, is assumed to be ␸L

= 2␲

共n

0−j␬

兲ᐉ

0exp共−j␳²/␻2f

兲

/␭, wheren₀= 2.9 is the linear refractive index of the film at␭= 1064 nm,^14,15ᐉ0is the peak depth inside the hole,␳ is the radial coordinate in the focal plane, and ␻f is the beam waist of the focused beam. The imaginary part of the linear refractive index ␬ describes an absorption effect due to photodarkening. Then, the linear transmittance of the sample in the impact region is described byT_L= exp

共

−j␸L

兲

.

In Figs. 2共a兲 and 2共b兲 we present experimental

共filled

circles兲and numerical simulation

共lines兲

results for low in- tensity. The best agreement was obtained for␬^{= 1.1 m−1and} ᐉ0= 10 nm. The value of ᐉ0 is in good agreement with the hole deepness measured by AFM

共⬃

15 nm

兲 关

Figs.3

共

a

兲

and 3

共

b

兲兴

. However, the experimental closed-apertureZ-scan pro- file is narrower than the theoretical curve. This may be due to the threshold intensity to be reached in order to produce ab- lation in the film. Indeed, the pattern used in the numerical calculation is Gaussian with beam waist of 25 ␮m, while the AFM image shows a steep profile with a hole diameter of

0 50 100 150

0 50 150 200

x y

(a) ^{0 100 200 300 400}

0 500 1000 1500 2000 2500

I(arb.units)

(b) x

Normalizedphasecontrast

0 50 100 150 200 250

0 0.2 0.4 0.6 0.8 1

(c) time (s)

FIG. 1.共a兲Experimental image of the PO obtained by a CCD camera using the NIT-PO technique. 共b兲 Profile corresponding to 共a兲. 共c兲 Normalized phase contrast vs time: empty circles, empty squares, stars, and filled circles are for incident intensity of 1.2, 2.0, 2.8, and 3.3 GW/cm², respectively.

-300 -20 -10 0 10 20 30

0.2 0.4 0.6 0.8 1

z (mm)

Normalizedtransmittance

(b)

z (mm) T

-30 -20 -10 0 10 20 30

0.92 0.94 0.96 0.98 1 1.02 1.04 1.06

z (mm)

Normalizedtransmittance

(a)

z (mm) T

FIG. 2. Experimental data 共filled dots兲and numerical results 共lines兲.共a兲 Closed-aperture and共b兲open-apertureZ-scan transmittanceTin low inten- sity regime共with negligible nonlinearity兲showing the presence of ablated region and photodarkening effect, respectively. The inset of 共a兲shows the closed aperture Z-scan trace in the NL regime 共filled circles兲 at I₀

= 3.3 GW/cm², and in the linear regime共empty circles兲. The inset of共b兲 shows the open-aperture Z-scan trace in the NL regime at 3.3 GW/cm² 共filled circles兲and in the linear regime共empty circles兲.

061122-2 Feduset al. Appl. Phys. Lett.94, 061122共2009兲

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 193.52.40.1 On: Tue, 03 May 2016 14:56:27

⬇15

␮m. It should be added that the inspection of the ab- lated film under the optical microscope did not reveal any damage.

In summary, the n₂ response of the Te₂₀As₃₀Se₅₀ film studied is attributed not only to the optical nonlinearity but also to nanoablation of the sample. The good agreement be- tween experiment and numerical simulation proves that holes as well as photodarkening can appear during the Z-scan and/or NIT-PO measurements in the picosecond regime masking the NL response of the sample. Another important result is the use of theZ-scan technique that allows measure- ment of very small holes with⬇10 nm deepness showing the high sensitivity of this method to estimate not only n₂ but also the film thickness and optical absorption coefficients due to the photodarkening phenomenon. Finally, the present results suggest that PIE contributions may occur for other transparent thin films and many published results would re- quire a careful revision.

We acknowledge the financial support of CAPES/

COFECUB and PEPS programs. One of us

共C.B.A.兲

ac- knowledges the support of the Brazilian agencies CNPq and

FACEPE. We are indebted to R. Mallet and R. Filmon of Laboratoire S.C.I.A.M, in Angers, for providing us with AFM pictures and profiles.

1R. L. Sutherland, Handbook of Nonlinear Optics共Dekker, New York, 1996兲.

2M. Sheik-Bahae, A. A. Said, T. H. Wei, D. Hagan, and E. W. Stryland, IEEE J. Quantum Electron. 26, 760共1990兲.

3A. Zakery and S. R. Elliott, J. Non-Cryst. Solids 330, 1 共2003兲, and references therein.

4J. T. Bloking, S. Krishnaswami, H. Jain, M. Vlcek, and R. P. Vinci, Opt.

Mater. 17, 453共2001兲.

5M. Yamane and Y. Asahara,Glasses for Photonics共Cambridge University Press, Cambridge, 2000兲.

6A. Zakery and M. Hatami, J. Opt. Soc. Am. B 22, 591共2005兲.

7W. F. Wang, Z. W. Wang, J. G. Yu, C. L. Liu, X. J. Zhao, and Q. H. Gong, Chem. Phys. Lett. 399, 230共2004兲.

8K. Ogusu, J. Yamasaki, S. Maeda, M. Kitao, and M. Minakata,Opt. Lett.

29, 265共2004兲.

9C. B. de Araújo, A. Humeau, G. Boudebs, V. D. Del Cacho, and L. R. P.

Kassab,J. Appl. Phys. 101, 066103共2007兲.

10K. Tanaka,J. Non-Cryst. Solids 352, 2580共2006兲, and references therein.

11G. Chen, H. Jain, M. Vlcek, and A. Ganjoo,Phys. Rev. B 74, 174203 共2006兲.

12G. Boudebs and S. Cherukulappurath,Phys. Rev. A 69, 053813共2004兲.

13G. Boudebs and C. B. de Araújo,Appl. Phys. Lett. 85, 3740共2004兲.

14V. Nazabal, M. Cathelinaud, W. Shen, P. Nemec, F. Charpentier, H. Lher- mite, M.-L. Anne, J. Capoulade, F. Grasset, A. Moreac, S. Inoue, M.

Frumar, J.-L. Adam, M. Lequime, and C. Amra, Appl. Opt. 47, C114 共2008兲.

15W. Shen, M. Cathelinaud, M. Lequime, V. Nazabal, and X. Liu, Opt.

Commun. 281, 3726共2008兲.

16G. Boudebs, and S. Cherukulappurath,Opt. Commun. 250, 416共2005兲.

17J.-L. Godet, H. Derbal, S. Cherukulappurath, and G. Boudebs,Eur. Phys.

J. D 39, 307共2006兲.

18A. A. Said, M. Sheik-Bahae, D. J. Hagan, T. H. We, J. Wang, J. Young, and E. W. van Stryland,J. Opt. Soc. Am. B 9, 405共1992兲.

19B. M. Patterson, W. R. White, T. A. Robbins, and R. J. Knize, Appl. Opt.

37, 1854共1998兲.

20J. W. Goodman,Introduction to Fourier Optics, 2nd ed. 共McGraw–Hill, New York, 1996兲.

21G. Boudebs, K. Fedus, C. Cassagne, and H. Leblond, Appl. Phys. Lett.

93, 021118共2008兲. (a)

z(nm)

x (m) y (m)

0

50 50

x (m)

z(nm)

(b)

FIG. 3. 共Color online兲AFM image 共a兲and its profile 共b兲of the ablated region created duringZ-scan measurements共⬃50 laser shots兲with a maxi- mum of 3.3 GW/cm²in the focus.

061122-3 Feduset al. Appl. Phys. Lett.94, 061122共2009兲

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 193.52.40.1 On: Tue, 03 May 2016 14:56:27