Magneto-optical waveguides made of cobalt ferrite nanoparticles embedded in silica/zirconia organic-inorganic matrix

Magneto-optical waveguides made of cobalt ferrite nanoparticles embedded in silica/zirconia organic-inorganic matrix Fadi Choueikani, François Royer, Damien Jamon, Ali Siblini, Jean Jacques Rousseau, Sophie Neveu, and Jamal Charara Citation: Applied Physics Letters 94, 051113 (2009); doi: 10.1063/1.3079094 View online: http://dx.doi.org/10.1063/1.3079094 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical and magneto-optical study of nickel and cobalt ferrite epitaxial thin films and submicron structures J. Appl. Phys. 113, 084101 (2013); 10.1063/1.4792749 An X-ray absorption spectroscopy study of the inversion degree in zinc ferrite nanocrystals dispersed on a highly porous silica aerogel matrix J. Chem. Phys. 138, 054702 (2013); 10.1063/1.4789479 Magneto-optical nanoparticle-doped silica-titania planar waveguides Appl. Phys. Lett. 86, 011107 (2005); 10.1063/1.1844038 Enhanced magneto-optical Kerr effects and decreased Curie temperature in Co–Mn ferrite thin films Appl. Phys. Lett. 79, 1849 (2001); 10.1063/1.1402656 Magnetically textured - Fe 2 O 3 nanoparticles in a silica gel matrix: Optical and magneto-optical properties J. Appl. Phys. 85, 2270 (1999); 10.1063/1.369537

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APPLIED PHYSICS LETTERS 94, 051113 共2009兲

Magneto-optical waveguides made of cobalt ferrite nanoparticles embedded in silica/zirconia organic-inorganic matrix Fadi Choueikani,1 François Royer,1,a兲 Damien Jamon,1 Ali Siblini,1 Jean Jacques Rousseau,1 Sophie Neveu,2 and Jamal Charara3 1

Laboratoire Dispositifs et Instrumentation en Optoélectronique et Micro-ondes-EA 3523, Université Jean Monnet, 21 rue Paul Michelon, 42023 St Etienne Cedex 2, France 2 Laboratoire des Liquides Ioniques et Interfaces Chargées, CNRS UMR 7612, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France 3 Département de Physique, Faculté des Sciences I, Université Libanaise, C. P. 35–145 Hadath, Beadba 10152010, Lebanon

共Received 19 November 2008; accepted 16 January 2009; published online 6 February 2009兲 This paper describes a way to develop magneto-optical waveguides via sol-gel process. They are made of cobalt ferrite nanoparticles embedded in a silica/zirconia matrix. Thin films are coated on glass substrate using the dip-coating technique. Annealing and UV treatment are applied to finalize sample preparation. Therefore, planar waveguides combining magneto-optical properties with a low refractive index 共⬇1,5兲 are obtained. M-lines and free space ellipsometry measurements show a specific Faraday rotation of 250°/cm and a modal birefringence of 1 ⫻ 10−4 at 820 nm. Thus, the mode conversion efficiency can reach a maximum value around 56%. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3079094兴 Nowadays in optical telecommunication systems, the conception of integrated optic devices which allow a highspeed data transmission requires the integration of elements that have a nonreciprocal effect such as optical isolators1 and circulators.2 The nonreciprocal functionality is based on the nonreciprocal nature of the Faraday effect in the magnetooptical waveguide. Currently, the material widely used to fabricate bulk components is the ferrimagnetic garnet oxyde crystal yttrium iron garnet 共YIG兲 or bismuth substituted yttrium iron garnet 共Bi:YIG兲 deposited on a gadolinium gallium garnet substrate.3 In spite of interesting magneto-optic effects illustrated by a specific Faraday rotation of 3000 共°/cm兲 at 1300 ␮m,4 this class of material cannot be easily embedded by classical technologies5 to realize magnetooptical integrated devices. Thus, the development of new magneto-optical materials compatible with classical technologies constitutes an active area of research. For example, an interesting way is based on the use of magnetic semiconductor materials to realize nonreciprocal devices integrated on semiconductor substrates. Results obtained by Zayets et al.6 and Debnath et al.7 on a Cd1−xMnxTe layer coated on GaAs substrate show an isolation ratio around 25 dB at 750 nm. In this framework, the present letter describes the last improvements of our approach used to develop and realize magneto-optical planar waveguides integrated on glass substrate with an operating wavelength of 1550 nm. It consists of using magnetic nanoparticles as magneto-optical active element in a silica-based matrix prepared via organicinorganic process. The attractivity of such approach lies in the full compatibility of the sol-gel coating with classical integrated technologies and especially the technology on glass. Indeed, crystallized magnetic nanoparticles are dispersed in the sol-gel liquid preparation before the coating and, thus, contrarily to classical techniques not high temperature is required to obtain a magnetic behavior. Furthermore, this elaboration method is easy to implement and provides a兲

Electronic mail: [email protected].

magneto-optical thin films with a refractive index value 共⬇1,5兲 close to that of other integrated optical devices.8 The magneto-optical activity of the doping agent nanoparticles is given by the Faraday rotation which they exhibit when submitted to a longitudinal magnetic field.9 The magnitude of this effect depends on several parameters such as the nanoparticle size,10 the particle volume fraction,11 the applied magnetic field,12 and the light wavelength.9 Similar to this Faraday rotation observed in free space, a nonreciprocal effect can be achieved in planar waveguide by the TE-TM mode conversion under a longitudinal magnetic field. The efficiency of such effect is expressed as13 RM =

␪F2 , ␪F2 + 共⌬␤/2兲2

共1兲

where ␪F 共°/cm兲 is the specific Faraday rotation and ⌬␤ 共°/cm兲 is the phase mismatch between TE and TM modes: ⌬␤ = 2␲⌬Nm / ␭ and ⌬Nm is the modal birefringence. This expression proves that the TE-TM mode conversion is directly linked to the specific Faraday rotation of the material constituting the waveguide, and that it is strongly affected by the modal birefringence. Therefore, the conception of a planar waveguide that has an interesting nonreciprocal functionality needs the maximization of R M . That is the goal of our study which consists of decreasing ⌬Nm and increasing ␪F. The sol was synthesized by hydrolysis and condensation reactions. The starting materials are photosensitive precursors: methacryloxypropyltrimethoxysilane 共MAPTMS兲, tetrapropylzirconate 兵Zr关OCH共CH3兲2兴4其, and methacrylic acid 共MAA兲. MAPTMS was hydrolyzed using an aqueous solution of hydrochloric acid 共0,01 mol/L兲 as a soft catalyst for 1 h. As the zirconium precursor is very sensitive to water, it has to be complexed by MAA to avoid precipitation. It was subsequently introduced into the MAPTMS and the solution was stirred at room temperature for 1 h. A photoinitiator 共IRGACURE 651兲 was added to start the polymerization under UV-light exposure. The precursor of ZrO2 has been added in order to obtain a higher refractive index than the

0003-6951/2009/94共5兲/051113/3/$25.00 94,is 051113-1 © 2009 American InstituteDownloaded of Physics to IP: This article is copyrighted as indicated in the article. Reuse of AIP content subject to the terms at: http://scitation.aip.org/termsconditions. 195.221.0.6 On: Thu, 10 Apr 2014 08:12:20

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substrate.14 The ratio of several precursors is 10:1:1. It should be read as 10 mol MAPTMS, 1 mol zirconium alkoxyde, and 1 mol MAA. To give the magneto-optical functionality to the sol, a ferrofluid was added before being passed through a 0.2 ␮m filter. The ferrofluid is constituted of cobalt ferrite 共CoFe2O4兲 nanoparticles that have a mean size of around 9 nm. Cobalt ferrite nanoparticles were obtained by coprecipitation of Fe共III兲 and Co共II兲 hydroxide in stoichiometric ratio.15 After 2 h of thermal treatment at 100 ° C, they were subsequently transferred into acidic medium and complexed by citrate ions. Then, complexed nanoparticles were dispersed into deionized water to form the ferrofluid that has a nanoparticle volume fraction of ␾ = 7%. A previous study on the Faraday rotation behavior of these nanoparticles versus wavelength shows two resonance peaks around 750 and 1550 nm.9,16 They correspond to a large value of the Faraday rotation. For a 0.1% of volume fraction of cobalt ferrite nanoparticles, the Faraday rotation magnitude is around 11°/cm at 820 nm and 14°/cm at 1550 nm.9 Moreover, this large effect at 1550 nm is located in the optical transparency domain of the cobalt ferrite nanoparticles.9 The substrates were rinsed in flowing distilled water. Rinsing with ethanol and drying with air have completed the procedure. Using the dip-coating technique, planar waveguides are made by coating of a thin layer of doped organic-inorganic sol on Pyrex™ substrate 共ns = 1472; ␭ = 6328 nm兲. Finally, samples are annealed during 60 min at 60 ° C and UV treated 共P = 1 mW/ cm2兲, respectively, at 365 and 254 nm during 25 min for each wavelength. The annealing step homogenizes the thin layer coated on the substrate. UV exposure leads to the creation of an organic network which gives to matrix flexibility and allows the reduction in the stress.17 This fabrication process allows the realization of good quality magneto-optical thin films, confirmed by the visualization of guided mode light streaks in the films. Free space ellipsometry and M-line spectroscopy are the two techniques used to characterize doped samples. Free space ellipsometry consists of evaluating the specific Faraday rotation of the material by analyzing the polarization state of light.18 A linearly polarized light is passed through a doped thin film with a propagation direction perpendicular to its plane. Under a longitudinal magnetic field 共parallel to the light beam兲, the medium acquires a circular anisotropy. Therefore, the polarization state of the emergent light becomes elliptic.18 Two angles characterize this elliptical state: the Faraday rotation ⌰F and the Faraday ellipticity ⑀F. After being passed through a photoelastic modulator associated with a lock-in amplifier and an analyzer, the intensity of the output light is expressed by Fourier series,19,20 I=

1 2 关I0

+ IF cos共2␲ f Ft兲 + I2F cos共2␲ f 2Ft兲 + . . .兴,

25

qF (°/cm)

15 5 -8000

-4000

H (Oe)

-5 0

4000

-15 -25

FIG. 1. Specific Faraday rotation of the diluted sample of the cobalt ferrite ferrofluid used to dope the sol: the measurement is made at 820 nm and the nanoparticle volume fraction is 0.15%.

and the second one I2F are, respectively, proportional to ⑀F and ⌰F.19,20 The analysis of the emergent light intensity by the lock-in amplifier allows the determination of the rotation and the ellipticity according to the intensity of the magnetic field. M-lines consist of focusing of a laser beam on a prism placed in contact with the thin film. By application of a load to the prism, optical coupling into the film can be obtained.21 This coupling is efficient only for particular angles of incidence, which are synchronous with the propagation modes. Using the measured synchronous angle ␣m, the mode effective indices Nm and the modal birefringence ⌬Nm can be directly calculated.22 Then, refractive index n and thickness h of the film can be evaluated via numerical adjustment.23 Figure 1 presents the specific Faraday rotation of the diluted sample of the ferrofluid used as doping agent. The measurement is made at 820 nm using free space ellipsometry at room temperature. The curve has a nonreciprocal variation as a function of the magnetic field, which is the typical behavior of the Faraday effect for a ferromagnetic material.24 The saturated magnitude at 8 kOe is around 24°/cm for ␾ = 0.15%. Given the proportionality between the Faraday rotation magnitude and the nanoparticle volume fraction,11 we can estimate the specific Faraday rotation for the doping ferrofluid 共␾ = 7%兲 at about 1150°/cm. In addition, no hysteresis loop is observed. The nanoparticle dispersion has a paramagnetic behavior.24 The specific Faraday rotations of two thin films doped at two different nanoparticle volume fractions are illustrated in Fig. 2 as a function of the applied magnetic field. These room temperature measurements realized at 820 nm using free space ellipsometry show saturated Faraday rotation val250

qF (°/cm)

f=1.5 %

150

共2兲

with

f=0.65 %

50

I0 = 1 − cos共2⑀F兲sin共2⌰F兲J0共⌬m兲,

共3兲

IF = sin共2⑀F兲J1共⌬m兲,

共4兲

I2F = cos共2⑀F兲sin共2⌰F兲J2共⌬m兲,

共5兲

8000

-8000

-4000

-50 0

H (Oe) 4000

8000

-150 -250

where Jl共x兲 and ⌬m are the lth Bessel function and the moduFIG. 2. Specific Faraday rotation of SiO2 / ZrO2 matrix doped by cobalt lation amplitude of the photoelastic modulator. For low valferrite nanoparticles: measurement is made at 820 nm for two different ues of Faraday rotation andinellipticity, the first harmonic volume fractions: 0.65% and 1.5%. This article is copyrighted as indicated the article. Reuse of AIP content IisF subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 195.221.0.6 On: Thu, 10 Apr 2014 08:12:20

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TABLE I. Optical and magneto-optical features of planar waveguide at 820 and 1550 nm. ␭ n h ⌬N0 ␪F RM

820 nm 1,513 3 ␮m 1 ⫻ 10−4 250°/cm 56%

1550 nm 1,509 3 ␮m 2 , 7 ⫻ 10−4 310°/cm 22%

ues around 110 and 250°/cm, respectively, for 0.65% and 1.5% of nanoparticle volume fraction. One can easily check that this difference of magnitude respects the ratio of the nanoparticle concentration. The attractive point is that the magnitude of this Faraday effect 共250°/cm at 820 nm or 310°/cm at 1550 nm, ␾ = 1.5%兲 is close to that of YIG 共⬇200°/cm at 1550 nm兲.3 Of course, substituted YIG would give a larger effect 共⬇500°/cm at 1550 nm兲,3 but it seems interesting to note that even if we use a composite material with a small amount of magnetic particles, the same order of effect than that of classical magneto-optical material is obtained. To complete this comparison, we calculate the figure of merit of our material: F共°兲 = ␪F共deg cm−1兲 / ␣共cm−1兲. The attenuation coefficient ␣ is around 120 cm−1 at 820 nm and around 30 cm−1 at 1550 nm. Thus, the figure of merit is around 2° at 820 nm and 10° at 1550 nm. These values are less than that of YIG and substituted-YIG material 共⬇40°/cm at 1550 nm兲 共Ref. 3兲 but are located in the middle range of the figure of merit of the materials used in magneto-optical applications: between 0.1° and 150° for a wavelength in the range 共1300–1550 nm兲.3,25,26 Moreover, Faraday rotation curves illustrated in Fig. 2 present a hysteresis loop. Its coercive field is around 110 Oe and its residual Faraday effect is around 15%. These hysteresis phenomena, which do not exist for the ferrofluid, may be due to the magnetic characteristics of CoFe2O4 nanoparticles which behave as hard magnetic dipoles.27 This result is very promising to realize integrated optical components having a spontaneous Faraday rotation without any applied magnetic field. Table I summarizes the optical and magneto-optical features of the most concentrated thin film. This sample was characterized using M-line spectroscopy and free space ellipsometry at 820 and 1550 nm. Therefore, the table shows a modal birefringence around 1 ⫻ 10−4 at 820 nm and 2.7 ⫻ 10−4 at 1550 nm. This modal birefringence evolution as a function of the wavelength is due to the geometric birefringence which increases with the wavelength. This table also presents the maximum of TE-TM mode conversion calculated using expression 共1兲. The efficiency is 56% at 820 nm and 22% at 1550 nm. The difference between these two values is mainly due to the modal birefringence difference. Comparing this result with our previous works made on mineral sol-gel matrix doped by maghemite nanoparticles,8 we note a distinct improvement in the magneto-optical potentiality of such matrix. Indeed, the replacement of the mineral matrix by an organic-inorganic allows the reduction in the modal birefringence from a value of around 3 ⫻ 10−3 to 1 ⫻ 10−4. In addition, the cobalt ferrite nanoparticles embedded in the novel matrix instead of maghemite ones allow the increase in the specific Faraday rotation of the material from a value of around 25°/cm at ␾ = 2% to 250°/cm at ␾ = 1.5%.

Finally, using expression 共1兲, the combination of these two results 共high ␪F and low ⌬N兲 allows to predict an increase in the TE-TM mode conversion from 1.2⫻ 10−2% 共Ref. 8兲 to a value higher than 50%. This attractive conversion should be achieved in a waveguide whose refractive index value 共n ⬇ 1 , 5, see Table I兲 is closed to that of glass optical integrated devices. Combined with a thickness larger than that of classical magneto-optical waveguides, it should allow an efficient fiber coupling. To conclude, the composite material made of cobalt ferrite nanoparticles embedded in a silica/zirconia matrix presents a Faraday rotation larger than 300°/cm at 1550 nm 共␾ = 1.5%兲 with a 15% permanent effect at zero applied field. Combined with a low birefringence due to the use of organic-inorganic matrix, it gives magneto-optical waveguides whose potential TE-TM conversion can reach 22% at 1550 nm 共and 56% at 820 nm兲. But, in order to meet the requirements of an application in integrated optics, the figure of merit 共⬇10°兲 of this material has to be improved. The silica/zirconia surrounding matrix has a low refractive index 共⬇1,5兲 and allows the microstructuring of the waveguide using a patterned UV exposure.28 That may be interesting to realize waveguides made of magnetophotonic crystals. The short term further works consist of measuring the mode conversion of such waveguides.

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