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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 6 — Jun. 1, 2012
  • pp: 864–871
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Tuning the magneto-optic response of maghemite doped poly(phenylmethylvinyl siloxane) through electric field based nanoparticle orientation

Ganapathy Kumar, Satish M. Mahajan, Holly A. Stretz, and Sanjay K. Apte  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 6, pp. 864-871 (2012)
http://dx.doi.org/10.1364/OME.2.000864


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Abstract

Maghemite nanoparticles were doped in optical poly phenyl methyl vinyl siloxane and oriented by externally applied electric fields before curing. Consequent change in the morphology of the nanocomposite was observed and characterized using small angle x-ray scattering (SAXS). After curing, Faraday rotation measurements were carried out at 632.8 nm. Electric field based alignment in addition to presence of the nanodopants enhanced the magneto-optic sensitivity by 14.3-48.6% for the polymer nanocomposites. A linear trend was observed between orienting electric fields and measured Faraday rotation angles. Limits on applicable electric fields for dopant concentrations were also ascertained from the magneto-optic response.

© 2012 OSA

1. Introduction

Nanocomposite optical materials exhibiting very high Faraday rotation capabilities are employed in a multitude of applications ranging from current sensors, fiber optic current transformers, magneto-optic isolators, high powered lasers, data storage devices and magneto-optic imagery [1

1. P. R. Watekar, S. Ju, S. A. Kim, S. Jeong, Y. Kim, and W. T. Han, “Development of a highly sensitive compact sized optical fiber current sensor,” Opt. Express 18(16), 17096–17105 (2010). [CrossRef] [PubMed]

4

4. D. Golubchik, E. Polturak, G. Koren, and S. G. Lipson, “A high resolution magneto-optical system for imaging of individual magnetic flux quanta,” Opt. Express 17(18), 16160–16165 (2009). [CrossRef] [PubMed]

]. Investigations of glass and polymer based nanocomposites for the express purpose of developing highly magneto-optic Faraday rotators have been widely discussed with pertinence to aforementioned applications [5

5. L. Sun, S. Jiang, and J. R. Marciante, “All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber,” Opt. Express 18(6), 5407–5412 (2010). [CrossRef] [PubMed]

7

7. M.-C. Oh, J.-K. Seo, K.-J. Kim, H. Kim, J.-W. Kim, and W.-S. Chu, “Optical Current Sensors Consisting of Polymeric Waveguide Components,” J. Lightwave Technol. 28(12), 1851–1857 (2010). [CrossRef]

]. Those efforts have reported and substantiated the presence of Faraday effect leading to high magneto-optic sensitivities (Verdet constants) and have encouraged further study on such materials.

2. Experimental procedure

Ferric nitrate, glycine and ammonium nitrate were combusted at 650°C and yielded hematite nanocrystals (α-Fe2O3). The hematite thus obtained was then mixed with polyethylene glycol in a combustion reactor at 400°C to obtain the maghemite nanocrystals (γ-Fe2O3) [10

10. S. K. Apte, S. D. Naik, R. S. Sonawane, B. B. Kale, and J. O. Baeg, “Synthesis of Nanosize-Necked Structure α- and γ-Fe2O3 and its Photocatalytic Activity,” J. Am. Ceram. Soc. 90(2), 412–414 (2007). [CrossRef]

]. X-ray diffraction confirmed the material phase. The polymer nanocomposite samples were prepared by mixing phenyl methyl vinyl siloxane with dicumyl peroxide (99:1) and doping the mixture with maghemite at 0.03-wt % and 0.06-wt%. The dopant concentrations were chosen so as to observe possibilities of physical orientation even at extremely low concentrations. Following a 12 hour sonication process and filtering using 0.45 micron filters, the mixture was subjected to DC electric fields from 0.0 kV/cm to 7.5 kV/cm in a custom built electric field chamber capable of generating 10 kV/cm. Thermal cure was then carried out for 12 hours to get the nanocomposite (henceforth known as γ-PMVS). Characterization of the γ-PMVS was performed using a Rigaku Ultima IV x-ray diffractometer with the small angle x-ray scattering (SAXS) setup (CuKα-1.54Å) at an operating voltage of 40 kV/44 mA. The SAXS method was used as a diagnostic tool to observe occurrence of nanoparticle orientation and to get nanoparticle size distribution. The nanoparticles were also observed using a field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM).

Measurement of Faraday rotation (FR) angle was carried out using a phase sensitive detection setup (Fig. 1
Fig. 1 Experimental setup for determination of Faraday rotation angle and Verdet constant.
) comprised of a 632.8nm Helium Neon Laser, a 1000:1 linear polarizer, a Wollaston prism, a Helmholtz coil and an SRS 830 lock in amplifier with two silicon photodetectors (0.3 A/W responsivity at 632.8nm). The Helmholtz coil was capable of generating magnetic fields upto 35 mT-rms at a frequency of 60 Hz. The sample was kept in the system such that the orientation of nanoparticles was orthogonal to the external magnetic field generated by the Helmholtz coil as well as to the direction of propagation of the laser light. Differential phase measurements and relative change in photodetector intensities measured with a lock-in amplifier were used to calculate the Faraday rotation angle with respect to change in magnetic field strength. Data acquisition was performed using a LabVIEW based virtual instrument (VI) to record and analyze data [1

1. P. R. Watekar, S. Ju, S. A. Kim, S. Jeong, Y. Kim, and W. T. Han, “Development of a highly sensitive compact sized optical fiber current sensor,” Opt. Express 18(16), 17096–17105 (2010). [CrossRef] [PubMed]

]. The results were averaged over thousand data points acquired for every magnetic field and had an accuracy of < 1%. This was also established earlier and confirmed during the course of this work [1

1. P. R. Watekar, S. Ju, S. A. Kim, S. Jeong, Y. Kim, and W. T. Han, “Development of a highly sensitive compact sized optical fiber current sensor,” Opt. Express 18(16), 17096–17105 (2010). [CrossRef] [PubMed]

].

3. Faraday effect and theory for nanoparticle orientation

The Faraday effect describes the angle through which linearly polarized light rotates while propagating through a material under a parallel magnetic field. This angle is termed the Faraday rotation angle, θf (deg) and is directly proportional to the length of the material L (cm) and the applied magnetic field B = µH (T) .The proportionality constant is represented by V, called the Verdet constant (deg/T-cm) and is the magneto-optic sensitivity of the material. The FR angle is given by Eq. (1)

θf=μVHL
(1)

The maghemite nanoparticles are confocal ellipsoids with an aspect ratio of 3:1 (as shown in Figs. 2(a)
Fig. 2 (a) TEM image of a maghemite nanoparticle. (b) FESEM of the dopant maghemite particles. (c) Nanoparticle axes with respect to applied electric field.
and 2(b)). Application of external electric fields would result in induced dipole moments, causing a torque to orient the nanoparticles. For an electric field E0 applied in the ‘x’ direction (Fig. 2(c)), orientation occurs at an angle α, and the electric field at the nanoparticle-polymer interface, Enp, which is responsible for the orienting torque, can be written as follows [11

11. A. E. DePrince and R. J. Hinde, “Accurate Computation of Electric Field Enhancement Factors for Metallic Nanoparticles Using the Discrete Dipole Approximation,” Nanoscale Res. Lett. 5(3), 592–596 (2010). [CrossRef] [PubMed]

]:
Enp=E0[1(εnεp1)(ab22)01(a2+u)3/2(b2+u)du]
(2)
where [(εn/ εp)-1] is a material permittivity compensation factor with εn, εp being the relative permittivities of the nanoparticle and the polymer respectively and 0.5ab201(a2+u)3/2(b2+u)du is a dimensionless integral dependent on the physical dimensions of the nanoparticle. It also represents a size based enhancement factor for the electric potential at the interface. For a particle aligned along the x’y’z’ axes with an alignment/orientation angle ‘α’ to an electric field applied along the x-direction as shown in Fig. 2(c), the torque acting on the nanoparticle is given by ‘τ’. This torque was determined earlier as [12

12. J. Park and W. Lu, “Orientation of core-shell nanoparticles in an electric field,” Appl. Phys. Lett. 91(5), 053113 (2007). [CrossRef]

]

τ=νεpEnp2sin2α2
(3)

θf=VBL=V(μH+ΛEnp)L
(5)

This equation shows that with increase in the orienting electric field strength, the Faraday rotation angle changes its value while maintaining the same Verdet constant. This implies an improvement in the signal to noise ratio of the material, thus suggesting an improvement in sensitivity for applications in magneto-optic sensing.

4. Results

4.1 SAXS characterization and electron microscopy

Small angle x-ray scattering was employed as a primary means of evaluating a change in orientation based on a change in the scattering intensities due to incident diffraction angles. The samples that were subjected to electric field orientations were characterized at incident angles of 2Theta from 0.1 degrees to 4 degrees. The resulting scattering intensity vs 2theta curves are shown in Fig. 3(a)
Fig. 3 (a) SAXS scattering intensities for electric field oriented polymer nanocomposites. (b) Nanoparticle size distribution obtained via SAXS. (c) XRD info for maghemite with diffraction peaks.
. The measured scattering intensities increased for samples with orienting e-fields upto 5 kV/cm and showed a decrease in intensity for the sample oriented at 6.25 kV/cm. Based on the shift in scattering intensities and considering the cubic crystal structure of maghemite (diffraction peaks shown in Fig. 3(c)), the electric fields applied for orientation may have re-oriented the nanocrystal to its original state of anisotropy or a symmetrical position.

The nanoparticle size distribution for the electric field oriented γ-PMVS samples was determined via SAXS (Fig. 3(b)) and showed presence of nanoparticles ranging from sizes of 50 nm to 150 nm. It was observed that the distribution peaks shifted for the samples with increased orienting electric fields. This may be attributed to the electric fields possibly resulting in aggregation of the nanoparticles in addition to alignment during the orientation process. The sizes of the nanoparticles were also observed using transmission electron microscopy (accelerating voltage 120 kV). The TEM micrographs of sectioned γ-PMVS samples showing the anisotropic maghemite nanoparticles in PMVS and oriented maghemite nanoparticles in PMVS are furnished in Figs. 4(a)
Fig. 4 (a) TEM image of anisotropic maghemite nanoparticle in PMVS. (b) Oriented maghemite nanoparticles in PMVS. E0 is the electric field applied and τ is the torque causing the orientation.
and 4(b). The sizes were found to be averaged at ~100 nm and were in agreement with nanoparticle size distribution obtained via SAXS.

4.2 Verdet constant measurements

Figures 5(a)
Fig. 5 Measured Faraday rotation angles for (a) 0.03wt% γ-PMVS and (b) 0.06wt% γ-PMVS.
and 5(b) show the plots of the measured Faraday rotation angle (in degrees) versus the applied AC magnetic fields (in mT-rms) for the 0.03 wt% and 0.06 wt% γ-PMVS samples. The corresponding Verdet constants were determined from the slopes of Figs. 4(a) and 4(b). Their values ranged from (3.5-4 °/T-cm) for 0.03 wt% doping and (2.1- 3.12 °/T-cm) for 0.06 wt% doping concentrations. The enhancement of Verdet constants due to the maghemite doping and e-field alignment was found to be about 14.3% and 48.6% for 0.03 wt% and 0.06 wt% doping respectively. The blank undoped polymer gave a Verdet constant of 1.419 °/T-cm. From the plots, a substantial increase in FR angle can be clearly observed for samples oriented at different electric fields. This increase in the numerical value of FR angle was found to be linear for the γ-PMVS. This variation of FR angle at an applied magnetic field of 34.1 mT-rms is shown in Fig. 6(a)
Fig. 6 (a) Variation of measured Faraday rotation angle with respect to orienting electric field. The readings correspond to a magnetic field of 34.1 mT rms. (b) Non-linear magneto-optic response as a function of electric field duration.
. Such a response, from an application perspective, could signify an improved SNR in sensing.

Application of electric fields to the composite, causes an off-center charge displacement inside the maghemite nanoparticles. Consequently, the wavefunction overlap decreases between any weakly confined ions and so, the nanoparticles, under the influence of the dipole electric moment, alter their state of anisotropy from the resultant torque. To account for the decreased magneto-optic behavior for the 0.03 wt-% and the 0.06 wt-% γ-PMVS samples, application of high electric fields decreases the ionic exchange interaction in maghemite with increase in dopant concentration but magnetically induced Zeeman transition energies are still prevalent [15

15. X. J. Li and K. Chang, “Electric-field tuning s-d exchange interaction in quantum dots,” Appl. Phys. Lett. 92(7), 071116 (2008). [CrossRef]

]. This explains the observation of lower values of FR angle for the samples with increased maghemite concentration and gives a method for preferential tuning of the magneto-optics for this particular polymer-semiconductor nanocomposite via orienting electric fields.

Magneto-optic response of the material was also measured for orienting electric field durations of 1, 2, 5, 10 and 20 minutes. An e-field of 5 kV/cm appears to cause the optimal orientation based on observations from the SAXS plots. Therefore, the variation of FR angle for an e-field of 5 kV/cm at aforementioned durations is plotted in Fig. 6(b). This showed a non-linear change in FR angle as the orienting e-field duration increased. Interpolation of the data points for longer durations revealed that the FR angle may saturate beyond applied durations of 20 minutes. In general, physical orientation of the nanoparticles due to the applied e-fields also tunes the optical transmittance of the sample by affecting the birefringence of the sample in accordance with the Kerr effect. Since the Faraday rotation angle is also dependent on the birefringence of the sample to an extent, it is given by the equation
θf=(nLnR)πL/λ
(6)
where nL and nR are the left and right circularly polarized light components respectively. Owing to the large applied e-field strengths it is also possible that the nanoparticles may have been subjected to quantum confined Stark effect resulting in a change in optical clarity of the sample due to a change in the optical absorption bandgap of the nanoparticles.

5. Conclusions

Electric fields were applied to orient maghemite nanoparticles doped in poly-phenyl methyl vinyl siloxane. Small angle x-ray scattering was employed to observe the orientation and change in morphology of the nanocomposite. After curing, Faraday rotation measurements of these e-field oriented polymer nanocomposites revealed a change in the numerical value of FR angles which could result in improving the signal to noise ratio for sensing applications. These observations could facilitate development of potential materials for applications in optical current sensing, magneto-optic isolators and modulators.

Acknowledgments

The authors wish to thank Dr. Jibao He of Tulane University for help with the transmission electron microscopy and the Center for Manufacturing Research, Tennessee Tech University for the use of SAXS instrumentation acquired through NSF grant DMR-0923042.

References and links

1.

P. R. Watekar, S. Ju, S. A. Kim, S. Jeong, Y. Kim, and W. T. Han, “Development of a highly sensitive compact sized optical fiber current sensor,” Opt. Express 18(16), 17096–17105 (2010). [CrossRef] [PubMed]

2.

H. Yoshida, K. Tsubakimoto, Y. Fujimoto, K. Mikami, H. Fujita, N. Miyanaga, H. Nozawa, H. Yagi, T. Yanagitani, Y. Nagata, and H. Kinoshita, “Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator,” Opt. Express 19(16), 15181–15187 (2011). [CrossRef] [PubMed]

3.

A. I. Savchuk, V. I. Fediv, S. A. Savchuk, and V. V. Makoviy, “Optical and magneto-optical diagnostics of polymer/nanoparticles colloidal solutions and composites,” Proc. SPIE 7388, 73880Z (2009). [CrossRef]

4.

D. Golubchik, E. Polturak, G. Koren, and S. G. Lipson, “A high resolution magneto-optical system for imaging of individual magnetic flux quanta,” Opt. Express 17(18), 16160–16165 (2009). [CrossRef] [PubMed]

5.

L. Sun, S. Jiang, and J. R. Marciante, “All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber,” Opt. Express 18(6), 5407–5412 (2010). [CrossRef] [PubMed]

6.

A. Lopez-Santiago, P. Gangopadhyay, J. Thomas, R. A. Norwood, A. Persoons, and N. Peyghambarian, “Faraday rotation in magnetite-polymethylmethacrylate core-shell nanocomposites with high optical quality,” Appl. Phys. Lett. 95(14), 143302 (2009). [CrossRef]

7.

M.-C. Oh, J.-K. Seo, K.-J. Kim, H. Kim, J.-W. Kim, and W.-S. Chu, “Optical Current Sensors Consisting of Polymeric Waveguide Components,” J. Lightwave Technol. 28(12), 1851–1857 (2010). [CrossRef]

8.

H. Koerner, J. Jacobs, D. Tomlin, J. Busbee, and R. Vaia, “Tuning Polymer Nanocomposite Morphology: AC Electric Field Manipulation of Epoxy–Montmorillonite (Clay) Suspensions,” Adv. Mater. 16(4), 297–302 (2004). [CrossRef]

9.

M. Mittal, P. P. Lele, E. W. Kaler, and E. M. Furst, “Polarization and interactions of colloidal particles in ac electric fields,” J. Chem. Phys. 129(6), 064513 (2008). [CrossRef] [PubMed]

10.

S. K. Apte, S. D. Naik, R. S. Sonawane, B. B. Kale, and J. O. Baeg, “Synthesis of Nanosize-Necked Structure α- and γ-Fe2O3 and its Photocatalytic Activity,” J. Am. Ceram. Soc. 90(2), 412–414 (2007). [CrossRef]

11.

A. E. DePrince and R. J. Hinde, “Accurate Computation of Electric Field Enhancement Factors for Metallic Nanoparticles Using the Discrete Dipole Approximation,” Nanoscale Res. Lett. 5(3), 592–596 (2010). [CrossRef] [PubMed]

12.

J. Park and W. Lu, “Orientation of core-shell nanoparticles in an electric field,” Appl. Phys. Lett. 91(5), 053113 (2007). [CrossRef]

13.

A. Yariv and P. Yeh, Optics of Waves in Crystals (Wiley, 2003).

14.

Yu. S. Dadoenkova, I. L. Lyubchanskii, Y. P. Lee, and Th. Rasing, “Electric field controlled Faraday rotation in an electro-optic/magneto-optic bilayer,” Appl. Phys. Lett. 97(1), 011901 (2010). [CrossRef]

15.

X. J. Li and K. Chang, “Electric-field tuning s-d exchange interaction in quantum dots,” Appl. Phys. Lett. 92(7), 071116 (2008). [CrossRef]

OCIS Codes
(160.3820) Materials : Magneto-optical materials
(230.3810) Optical devices : Magneto-optic systems

ToC Category:
Magneto-Optical Materials

History
Original Manuscript: April 30, 2012
Revised Manuscript: May 20, 2012
Manuscript Accepted: May 21, 2012
Published: May 24, 2012

Citation
Ganapathy Kumar, Satish M. Mahajan, Holly A. Stretz, and Sanjay K. Apte, "Tuning the magneto-optic response of maghemite doped poly(phenylmethylvinyl siloxane) through electric field based nanoparticle orientation," Opt. Mater. Express 2, 864-871 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-6-864


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References

  1. P. R. Watekar, S. Ju, S. A. Kim, S. Jeong, Y. Kim, and W. T. Han, “Development of a highly sensitive compact sized optical fiber current sensor,” Opt. Express18(16), 17096–17105 (2010). [CrossRef] [PubMed]
  2. H. Yoshida, K. Tsubakimoto, Y. Fujimoto, K. Mikami, H. Fujita, N. Miyanaga, H. Nozawa, H. Yagi, T. Yanagitani, Y. Nagata, and H. Kinoshita, “Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator,” Opt. Express19(16), 15181–15187 (2011). [CrossRef] [PubMed]
  3. A. I. Savchuk, V. I. Fediv, S. A. Savchuk, and V. V. Makoviy, “Optical and magneto-optical diagnostics of polymer/nanoparticles colloidal solutions and composites,” Proc. SPIE7388, 73880Z (2009). [CrossRef]
  4. D. Golubchik, E. Polturak, G. Koren, and S. G. Lipson, “A high resolution magneto-optical system for imaging of individual magnetic flux quanta,” Opt. Express17(18), 16160–16165 (2009). [CrossRef] [PubMed]
  5. L. Sun, S. Jiang, and J. R. Marciante, “All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber,” Opt. Express18(6), 5407–5412 (2010). [CrossRef] [PubMed]
  6. A. Lopez-Santiago, P. Gangopadhyay, J. Thomas, R. A. Norwood, A. Persoons, and N. Peyghambarian, “Faraday rotation in magnetite-polymethylmethacrylate core-shell nanocomposites with high optical quality,” Appl. Phys. Lett.95(14), 143302 (2009). [CrossRef]
  7. M.-C. Oh, J.-K. Seo, K.-J. Kim, H. Kim, J.-W. Kim, and W.-S. Chu, “Optical Current Sensors Consisting of Polymeric Waveguide Components,” J. Lightwave Technol.28(12), 1851–1857 (2010). [CrossRef]
  8. H. Koerner, J. Jacobs, D. Tomlin, J. Busbee, and R. Vaia, “Tuning Polymer Nanocomposite Morphology: AC Electric Field Manipulation of Epoxy–Montmorillonite (Clay) Suspensions,” Adv. Mater.16(4), 297–302 (2004). [CrossRef]
  9. M. Mittal, P. P. Lele, E. W. Kaler, and E. M. Furst, “Polarization and interactions of colloidal particles in ac electric fields,” J. Chem. Phys.129(6), 064513 (2008). [CrossRef] [PubMed]
  10. S. K. Apte, S. D. Naik, R. S. Sonawane, B. B. Kale, and J. O. Baeg, “Synthesis of Nanosize-Necked Structure α- and γ-Fe2O3 and its Photocatalytic Activity,” J. Am. Ceram. Soc.90(2), 412–414 (2007). [CrossRef]
  11. A. E. DePrince and R. J. Hinde, “Accurate Computation of Electric Field Enhancement Factors for Metallic Nanoparticles Using the Discrete Dipole Approximation,” Nanoscale Res. Lett.5(3), 592–596 (2010). [CrossRef] [PubMed]
  12. J. Park and W. Lu, “Orientation of core-shell nanoparticles in an electric field,” Appl. Phys. Lett.91(5), 053113 (2007). [CrossRef]
  13. A. Yariv and P. Yeh, Optics of Waves in Crystals (Wiley, 2003).
  14. Yu. S. Dadoenkova, I. L. Lyubchanskii, Y. P. Lee, and Th. Rasing, “Electric field controlled Faraday rotation in an electro-optic/magneto-optic bilayer,” Appl. Phys. Lett.97(1), 011901 (2010). [CrossRef]
  15. X. J. Li and K. Chang, “Electric-field tuning s-d exchange interaction in quantum dots,” Appl. Phys. Lett.92(7), 071116 (2008). [CrossRef]

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