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Three dimensional magneto-photonic crystals were realized by a low temperature sol-gel method. Self-assembled polystyrene direct opals were impregnated by CoFe2O4 nanoparticles doped silica sol-gel solution. The silica composite inverse opal structure was then created by dissolving original polystyrene photonic crystal structure using ethyl acetate. Ellipsometric and magneto-optical measurements carried-out on silica-doped monolayers evidence that the amount of nanoparticles in the matrix can reach 39%. Scanning electron microscope images of inverse opals confirm that a 3D arrangement is obtained. Transmittance measurements show a combination of photonic band gaps with the absorption of the nanoparticles. The central positions of these band gaps are in accordance with the diameter of the initial spheres through the Bragg law. Finally, non-reciprocal magneto-optical effect is demonstrated in the inverse opals through a Faraday effect hysteresis loop. It proves the ability of the method to produce magneto-photonic crystal materials.
©2013 Optical Society of America
1. Introduction
Non-reciprocal magneto-optical (MO) devices are still lacking in integrated photonic platforms, although they are essential components for optical communication systems. Indeed, in order to protect amplifiers or laser light sources from unwanted backward reflexions, isolators are required [1]. These components are mainly based on a Faraday effect and its non-reciprocal behavior. Faraday rotation (FR) appears when an incident polarized electromagnetic wave passes through a magneto-optical material under the influence of a longitudinal magnetic field. In such a medium, eigenpolarizations are circular (left and right) and the difference between the eigenmode propagation constants provides a phase shift. The Faraday rotation (θF) is proportional to the difference between the real parts of the left (𝜈L) and right (𝜈R) refractive indexes and can be written as:
where h is the thickness of the Faraday rotator material and λ is the wavelength.In order to be embedded into devices, magneto-optical materials have to possess not only high Faraday rotation, but also low absorption (α). As a consequence a merit factor (M) should be introduced as:
Current commercial isolators are based on high merit factor MO materials like YIG [2] used in bulk form (M ~100° @1550 nm). But, even if some demonstrations of integration of YIG have been realized [3–5], it is a difficult challenge to integrate these materials with classical technologies because of their high crystallisation temperature (higher than 700 °C). Thus novel integrable MO materials have to be developed.
Our group is working on the development of MO composite material using sol gel method. The main idea is to disperse crystallized magnetic nanoparticles as a MO vector in a silica matrix. This last can be spread as thin layer using classical spin or dip-coating methods. A low temperature treatment (~100 °C) is then applied to finalize the film. As a consequence, this composite material is fully compatible with classical technologies [6], and it has served to realize an integrated MO mode converter on glass substrate [7]. Cobalt ferrite (CoFe2O4) nanoparticles (NPs) were chosen to be embedded in the silica matrix, because they exhibit large FR in the 1400-1550 nm spectral range [8], which is an important area for telecommunication devices. For instance, a 320 °·cm−1 FR has been measured at 1550 nm through a thin layer doped by CoFe2O4 NPs with a volume fraction of 1.5% [6]. However, the value of the merit factor (12 °) is interesting, but is not enough for applications.
A way to increase the merit factor can be provided by a structuration of the magnetic material as a Magneto-Photonic Crystal (MPC). Due to the spatial periodicity, these media provide Photonic Band Gap (PBG). At the edge of the band-gap, due to the confinement of light and the decrease of the group velocity, dispersion curves flatten. This leads to an enhancement of the difference between the left and right propagation constants. Thus, a large MO enhancement is expected [9,10]. Such phenomenon has been observed for example in one dimensional MPC by Inoue [11] using Ta2O5/SiO2/Bi:YIG multilayers structures. In three dimensional MPC, studies have been carried out by Koerdt et al. [12] on opals impregnated with Faraday active transparent liquid. Faraday rotation, inside the PBG had been increased more than five times. Similar works was reported by Caicedo et al. [13,14] using opals doped by Ni, MnFe2O4 or γ-Fe2O3 magnetic nanoparticles. They observed that at the edge of the PBG the MO spectral behavior is modified. Based on these interesting experimental demonstrations, and on the ability of the sol-gel matrix to be arranged as a 3D inverse opal [15], we plan to build a 3D MPC using our MO composite material.
Silica inverse opal being made of 74% of air, the first objective of this work is to increase the amount of NPs in the composite to reach a MO demonstration. The second objective is the realisation of 3D MO photonic crystals, using an opal template infiltrated with this high concentrated composite. This paper is structured in two main parts. The first one is devoted to the study of the composite silica layers doped with high NPs volume fraction through the study of their MO properties. The second part is concerned by the elaboration of the 3D MPCs and their physical, optical and MO properties.
2. Magneto-optical composite material
Because of the high air content of the 3D inverse silica opal, high magnetic volume fraction should be reached in the silica matrix in order to obtain significant MO effect. Thus, in this part optical and MO properties of the magnetic composite will be studied as a function of the NPs volume fraction.
For the elaboration of the cobalt ferrite doped sol-gel solution, Tetraethyl-orthosilicate (Si(OC2H5)4 ; TEOS) was used as a precursor. Under continuous agitation, ethanol, water and HCl (0.1 N) were added into the precursor solution with the following molar ratio 2:2:0.01:1, respectively. In the following sections, this sol-gel preparation is called as “TEOS solution”.
Cobalt ferrite NPs were synthetized by coprecipitation of Fe(III) and Co(II) hydroxides in stoichiometric ratio. After 2 h of thermal treatment at 100 °C, they were subsequently transferred into aqueous acidic medium. In order to avoid the decomposition of the nanometric cobalt ferrite particles in acidic medium, it is necessary to passivate the surface by a ferric nitrate treatment [16]. The particles obtained after the step of passivation were dispersed in water in order to have an acidic magnetic fluid which pH was about 2 and behaves as a stable colloidal dispersion [17]. The NPs volume fraction of the magnetic liquid was about 7%, and the diameter of the NPs was about 10 nm (Fig. 1b).
Then, this acidic ferrofluid was added into the TEOS preparation. The TEOS solution being acidic, it insures a good stability of the nanoparticles during this doping. Then, a 0.2 µm filtration is applied to the solution. The coating of this sol-gel solution on borosilicate glass substrates was achieved with a dip-coating procedure (Fig. 1a). The dip-coating is realized using a motor helped apparatus with standard velocity (~350 µm·s−1). The composite films obtained through this process are silica-type layers doped with cobalt ferrite nanoparticles. In the following sections, they will be called “monolayers”. Finally, a 90 °C thermal treatment was applied during 1 hour to promote the condensation process of the sol-gel matrix.
For the study of the magnetic NPs volume fraction in the monolayer, four samples were made, where the quantity of the TEOS solution was increased to dilute the concentration of the magnetic NPs in the solution. 2:2, 2:3, 2:4 and 2:5 ratios of cobalt ferrite:TEOS solution monolayers were thus elaborated. As the refractive index of the bulk cobalt ferrite material is higher than 2.5 in the UV-visible spectral range [18], increasing the volume fraction of NPs should produce higher refractive index material.
Layers refractive index (n) and extinction coefficients (k), reported on Fig. 2, were measured using commercial spectroscopic ellipsometer (Jobin Yvon UVISEL). Knowing that the refractive index of the undoped TEOS matrix is about 1.44 [19,20], and that the lowest refractive index of the monolayers is about 1.6, these curves evidence that the magnetic NPs are successfully embedded in the matrix. Furthermore, this graph shows the absorption of NPs at small wavelengths, and the increase of extinction coefficient with the NPs contents.
Fig. 2 Spectral n-k graph of NPs doped monolayers (2:2, 2:3, 2:4, 2:5 are respective ratios of cobalt ferrite ferrofluid and TEOS solution).
The MO effect of the monolayers were analysed by free space polarimeter [6,21] at 820 nm wavelength in terms of FR (°). In this magneto-optical bench, samples are placed in the air gap of a bipolar electromagnet, which allows up to 8000 Oe longitudinal magnetic field. Using thicknesses, issued from ellipsometric measurements, results are given in terms of specific FR (°/cm) and reported on Fig. 3 for the four layers as a function of the magnetic field.
Fig. 3 Faraday Rotation curves of NPs doped monolayers (2:2, 2:3, 2:4, 2:5 are respective ratios of cobalt ferrite ferrofluid and TEOS solution). In the inset, normalized FR curves of the four monolayers as a function of the magnetic field. The measurements are made at a wavelength of 820 nm.
Curves show the non-reciprocity of the MO effect, and the coercitivity results from the hard magnetic dipoles behavior of cobalt ferrite NPs. The highest saturation value of the FR reaches 6.6x103 °·cm−1 in the case of 2:2 monolayer. As a comparison, Lopez-Santiago et al reported recently a specific FR of 8°/cm in cobalt ferrite nanoparticles polymer composites with a 4 wt% at 980 nm [22].
Specific FR in ferrofluids is proportional to the NPs volume fraction [23]. Thus, knowing the specific FR of the initial ferrofluid (which is 170 °·cm−1 for 1% NPs volume fraction), one can deduce the NPs volume fraction in the layers. They are reported, with the layers thickness obtained from ellipsometric measurements, in Table 1. The layer thickness difference may be explained by the influence of the ratio of cobalt ferrite ferrofluid and TEOS solution on the viscosity of the doped solution, which governs the thickness of coated layers.
Table 1. Thickness, Faraday Rotation and NP Volume Fraction of the Different Samples
Final NPs volume fraction follows roughly the increase of the ferrofluid content in the initial preparation, varying from 11% in the case of the 2:5 monolayer to 39% for the 2:2 monolayer. Due to these large values of the NPs volume fraction, one can wonder if some aggregates exist in the silica matrix, or if the NP distribution remains as a single particles random distribution. To check the quality of this NPs dispersion in the monolayers, the four FR curves were normalized, and plotted together (see inset Fig. 3). These curves are superimposed and the coercive field, which is size dependent, is the same. It proves that the NPs distribution in the matrix is a random assembly of single nanoparticles without any aggregates. Thus, the elaboration process, even for large NPs volume fraction, does not affect the magnetic behavior of the NPs.
As a conclusion of this first part, it is worthy of note that the NPs volume fraction can reach 39% in the silica matrix. This is a large increase compared to previous works [6], where the volume fraction was about 1.5%. This huge concentration is the key element of the realization of efficient 3D magneto-photonic crystals.
More generally, the elaboration process gives a composite MO material compatible with glass substrate, with a refractive index about 1.7 and associated to a large specific FR about 6000°.cm−1.
3. Magneto-photonic crystals
3.1 Elaboration
Elaboration of 3D MPCs starts with the realization of direct opal templates on glass substrate using self-assembled polystyrene (PS) spheres in aqueous dispersion. CoFe2O4 NPs FR spectrum presents two main areas of large values near 750 and 1550 nm (Fig. 4).According to the Bragg Law (Eq. (3)) [15], the PBG position (λC) is governed by the size of the periodicity of the lattice. Thus 4 different sphere diameters (), 400 and 450 nm to cover the first area, 800 and 900 nm for the second, were used in this study.
nm and nair are respectively the refractive indexes of the solid matrix and the air void. fm is the filling fraction of the solid matrix, which is respectively 0.74 and 0.26 for direct or inversed closed packed structures.Fig. 4 FR spectrum of a cobalt ferrite ferrofluid as a function of the wavelength (volume fraction = 1%).
Vertical deposition method [24] was applied to elaborate direct opals. This “bottom-up” approach provides organized face-centered-cubic (fcc) structures. First, a teflon solution holder was filled with an aqueous dispersion of PS spheres which volume fraction is 0.2% (Fig. 5a). Then, a glass substrate was dipped in this dispersion with an inclination angle of 20° with respect to the vertical (Fig. 5b). At stable 45-50 °C, the solvent evaporation induced the 3D self-arrangement of the PS spheres leading to the creation of the direct opal.
Impregnation was obtained by dipping such direct opal into the TEOS solution or cobalt ferrite doped TEOS solution during 5 minutes (Fig. 5c). The drying process was fixed for 1 hour at 90 °C (Fig. 5d), according to Nishijima et al’s work [15]. To obtain an inversed opal, PS spheres have to be dissolved. This was achieved by an immersion of the samples in ethyl acetate (Fig. 5e). After the last step, the periodic inverse opal contains air holes surrounded by a magneto-optical composite (Fig. 5f). This composite acts as high refractive index material and air voids as low refractive index material.
3.2 Optical and Magneto-optical results
Observation of 3D opals was carried out with Scanning Electron Microscopy (SEM FEI Nova nanoSEM 200). On Fig. 6 are presented SEM images of several direct and inversed opals with various diameter and magnification. These images show that a periodic arrangement can be found in all samples, either in direct or in inversed opals. Homogeneity of these arrangements covers several hundred µm2 (Figs. 6b-6d). The crack, shown in Fig. 6c, provides an inside look of the closed packed arrangement and an estimation of the thickness (roughly 5 µm). In addition, some line defects have created equilateral triangles, which can denote the good crystallization of the structure. Figures 6b, 6c and 6d confirm that after impregnation, either with doped or undoped matrix, the inverse opals maintained the closed packed arrangement of the direct opal.For further structure analysis, transmission spectroscopy was used to evidence the PBG behavior. On Fig. 7 the transmittance curve of doped inverse opals is presented. The diameter of the PS spheres used as template was 450 nm. Firstly, doped inverse opals exhibit PBG around 740 nm which is in good agreement with periodic arrangement evidenced on SEM images. Secondly, Fig. 7 shows also the transmittance curve of the 2:2 magnetic monolayer. The absorption band under 900 nm is in good agreement with extinction coefficient measurements shown in Fig. 2. It is due to the absorption of cobalt ferrite NPs. This absorption behavior is also noticeable in the doped inverse opal transmission curve. These observations indicate that magnetic NPs are embedded in the solid part of the PC, and prove the ability of our process to elaborate photonic crystals with a magnetic function.
Fig. 6 SEM image of a) 450 nm PS direct opal structure, b) non-doped 900 nm PS inverse opal, c-d) 800 nm PS cobalt ferrite NPs doped inverse opal.
Fig. 7 Transmittance curves of the 2:2 monolayer and the doped inverse opal. The diameter of the PS spheres used as template was 450 nm.
To fit the spectral FR behavior of cobalt ferrite NPs, and its two main peaks around 750 and 1550 nm, it is useful to control the PBG position. On Fig. 8, are reported transmittance curves of four doped inverse opals elaborated using 400, 450, 800 and 900 nm PS sphere diameter. This graph shows that with increasing PS spheres diameter, the PBG position of the elaborated inverse MPCs can be tuned to higher wavelengths. The insert of the Fig. 8 shows the PBG position as a function of the PS sphere diameter. The linear behavior of this curve is in good agreement with the Bragg law (Eq. (3)).
Fig. 8 PBGs of doped inverse opals elaborated using 4different sphere diameters. The graph in the inset presents the center positions of the PBG as a function of the sphere diameter.
Further transmittance analyses were done to study the PBG of doped inversed opals. Using incident angle resolved transmittance, measurements of the spectral behavior of the PBG allows the identification of some of the crystal properties: the refractive index of the lattice and the center-to-center distance in the inverse opal structure [15,25,26]. Indeed, in the case of <111> axis, when the incident angle θ is varying, the Bragg’s law is modified in order to take into account the refraction of the light at the surface of the sample and it can be expressed as:
where is the effective refractive index of the opal given by:By plotting λ2C as a function of sin2θ, a linear behavior should be obtained through this relation:Thus, the slope gives the diameter of the spheres dsph and from the y axis intercept, one can deduce.Figure 9a shows transmittance spectra of a 450nm doped inverse opal for different incident angles varying from −25 to + 25°. Curves evidence that the PBG position λC is blue shifted either for positive or negative angles, with a highest position at about 740 nm obtained at 0°. Thus, it confirms that the <111> plane is parallel to the substrate surface [26].
Fig. 9 a) Transmittance curves of a 450 nm doped inverse opal as a function of the wavelength with 0 °, ± 5 °, ± 10 °, ± 15 °, ± 20°, ± 25°angle of incidence; b) Linear plots of λ2C as a function of sin2θ.
Figure 9b shows λ2C as a function of . From the fitted parameters of the linear curve, dsph and the has been found to be equal respectively to 423 nm and 1.07. According to the certificate of calibration of the PS spheres provider, the diameter was 453 ± 9 nm. Compared to this value, a 30 nm difference appears for the void spheres of the doped inverse opals. In the case of the direct opal, this λ2C = f(sin2θ) measurement gave 454 nm for the PS sphere size. Thus, for the inverse opal, we attribute this difference of diameter between the initial PS spheres and the final void spheres to the elaboration procedure of the inverse opal. Such phenomenon has been explained as a shrinkage of the structure by Nishijima et al. [15].
From the value of the refractive index of the opal and taking into account a filling factor fm of 26%, the refractive index of the doped silica solid matrix nm was deduced: nm = 1.24. This value is lower than the 1.6 refractive index value of 2:5 magnetic monolayer (Fig. 2). This refractive index difference may be due to the porosity of the doped silica matrix in the opals, or to a deviation of the filling factor from its ideal value of 26%.
The magneto-optical behavior of an inverse opal realized with a 450 nm polystyrene spheres template was measured by free space polarimetry [6] with the same bench than the one used for the monolayers (section 2). Thus, on Fig. 10, is reported the FR of such sample measured at 820 nm as a function of the applied magnetic field. This curve has a non-reciprocal behavior with a linear slope at low field and a saturation effect at high field. The whole behaviour is identical to that obtained with classical magnetic NP doped monolayers (see Fig. 3), and is the signature of the presence of magnetic nanoparticles in the silica part of the sample. The hysteresis loop on the FR curve of Fig. 10, with a non-zero coercitive field is due to the hard dipole magnetic behavior of the cobalt ferrite NP [27]. Finally, this curve proves the magneto-optical activity of the doped inverse opals. Combined with the PBG evidenced by Fig. 7, it confirms the ability of the method to elaborate high quality 3D magneto-photonic crystals.
Fig. 10 Faraday rotation (FR) of a 450 nm doped inverse opal as a function of the magnetic field, measured at 820 nm.
Compared to other works already published on 3D MPC and based on close elaboration methods [13,28–31], the FR hysteresis loop reported on Fig. 10 seems to have a higher magnitude or a better signal to noise ratio. That is certainly due to the fact that cobalt ferrite NP have a higher MO activity compared to maghemite, nickel or magnetite [8].
These results show that our elaboration process based on the inversion of a PS direct opal through an impregnation with a doped silica sol-gel preparation is a very efficient way to obtain magneto-photonic crystals. Due to the high-quality FR curves obtained, the study of the predicted enhancement of the FR at the edges of the band-gaps is very promising. This is the subject of current works.
4. Conclusion
In the context of the development of novel magneto-optical materials candidate for integration on photonic platforms, this work was dedicated to the study of the ability of a composite silica material to behave as a magneto-photonic crystal. Firstly, compared to previous works, the volume fraction of CoFe2O4 nanoparticles in the host matrix has been increased from 1.5 to 39%. Then, using the same sol-gel approach and self-assembled polystyrene opals, doped inverse opals were elaborated. The quality of the three dimensional periodic arrangement was confirmed by SEM analysis. In the transmittance curves of the doped inverse opals, both photonic band gap and absorption band of the magnetic nanoparticles were evidenced. Finally, the magneto optical activity of the samples was demonstrated through a Faraday rotation hysteresis loop. These results show that this composite matrix obtained through a sol-gel process is a promising way to realize efficient magneto-photonic crystals. The long-term integration of such material on photonic platforms is guaranteed by the low temperature process used. Ongoing works are concerned by the study of the behavior of the FR as a function of the wavelength to check if an enhancement of this FR may be achieved in the area of the PBG.
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