ABSTRACT 1. INTRODUCTION

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Jemimah Morris
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1 Reflectance Fabry-Perot modulator utilizing electro-optic ZnO thin film Vikash Gulia* and Sanjeev Kumar Department of Physics and Astrophysics, University of Delhi, Delhi-117, India. * ABSTRACT Electro-optic effect in ZnO thin film prepared by rf magnetron sputtering technique is demonstrated. The effect is measured using the reflection of light polarized with the electric vector parallel to the plane of incidence of light. The reflected light intensity changes with the applied electric field and shows a modulation of about 1.5% at an applied voltage of 32V at an angle of incidence of 42.8 for Ag(Å)/ZnO(1.3µm)/Au(1Å)/Glass reflection Fabry-Perot modulator structure. The results suggest the potential of ZnO thin film for application as a thin film based reflection Fabry-Perot light intensity modulator. Keywords: electro-optic modulation, reflectance Fabry-Perot modulator, integrated optics. 1. INTRODUCTION Fabry-Perot modulator is a device incorporating an electro-optic material in the spacer layer of a Fabry-Perot device which creates a structure whose optical reflection and transmission characteristics depend on the applied voltage. Electro-optic modulators are widely used in broadband fiber-optic communication system due to their well defined transfer functions, large output optical power, wide bandwidth, high extinction ratio and low wavelength dependence. The primary role of electro-optic thin film materials in photonic devices is to modulate light waves with respect to their optical phase or amplitude. Such modulations can be achieved by means of electric field controlled indices of refraction in electro-optic thin film materials through their electro-optic effects. The characterization of the electro-optic properties of these thin film materials is of great importance. For many device applications the thickness of the electro-optic films is usually smaller or comparable to the light wavelength. Accurate and reliable detection of electro-optic effects in these thin film materials has remained a difficult and sometimes a challenging task, particularly so when films are grown on opaque substrates. Integrated optical modulators using organic as well as inorganic electro-optic media may adopt three different designs namely transmission devices, wave-guide devices and reflection devices. Transmission modulators are not compatible with the opaque substrates and their usage is thus limited. Successes were reported in the study of electro-optic effects of thin films grown on opaque substrates by using reflection mode measurement with an intensity detection approach 1-4. Incident Light Reflected Light Ag ( Ǻ) ZnO (1.3 µm) Au/Cr (1 Ǻ) Ordinary Glass Figure1: Reflection Fabry-Perot modulator structure.

2 A number of thin film electro-optic materials including lithium niobate(linbo 3 ), lead lanthanum zirconate titanate(plzt), lead zirconate titanate(pzt), potassium dihydrogen phosphate(kdp), ammonium dihydrogen phosphate(adp), barium titanate, zinc oxide(zno) and various organic polymers have exhibited attractive birefrengent electro-optic effect when deposited on suitable substrates 5-8. These thin film materials have been the focus of many studies for optical modulation. Though all these materials posses very good electro-optic properties but thin film growth of most of these materials using rf magnetron sputtering technique demands high temperature processing and hence they are difficult to grow. Even though Zinc oxide (ZnO) thin films with c-axis orientation exhibit strong piezoelectric and promising electro-optic properties 9 and can be easily grown in thin film form, a very little attention has been paid to the electro-optic characterization of these thin films. In the present work, the fabrication and performance of a ZnO thin film based Fabry-Perot electro-optic modulator operating in reflection mode at wavelength 6328 Å is described. The reflection Fabry-Perot modulator (RFPM) consists of a partially reflecting metallic layer, an electro-optic layer and a fully reflecting bottom layer. The partially reflecting layer provides impedance matching at discrete optical-path lengths which depend on the electro-optic film thickness, index of refraction and angle of incidence. The thickness of this layer depends most critically on the linear optical properties of the electro-optic film and metal and to a lesser extent on the electro-optic film thickness. 2. EXPERIMENTAL In order to fabricate the reflection Fabry-Perot modulator structure and model its performance, it is necessary to precisely control and determine the index of refraction and thickness of all the layers. A fully reflecting thin film of Cr/Au (1 Å) was thermally evaporated onto an ordinary glass substrate which has a refractive index n of 1.5 at nm. A thin film of ZnO (1.3µm) was deposited onto the Au/Cr/Glass structure by using rf magnetron sputtering technique. Finally a partially reflecting thin film of Ag ( Å) was thermally evaporated onto the ZnO/Au/Cr/Glass structure. The values of the dielectric constants of the Au and Ag layers were taken to be i and i respectively from the literature 1. Fig. 1 shows the reflection Fabry-Perot modulator structure utilizing electro-optic ZnO thin film. All the films ZnO, Cr/Au, and Ag were characterized for their thickness information using Dektak II-A surface profiler. Structural characterization of the ZnO thin films was done by X-ray diffraction method. Electro-optic ZnO thin film was grown using a rf magnetron sputtering technique. The rf power of W was applied to the 6 metallic Zn target in a reactive gas ambient of Ar and O 2. The mixed gases ratio of Ar to O 2 is :. The sputtering pressure in the chamber was maintained at 1 m Torr during deposition. p-polarized He-Ne Laser Analyzer DC Power supply Sample Photodetector Laser power meter Figure 2: Experimental setup for the measurement of reflected light intensity. Electro-optic characteristics of ZnO thin film based reflection Fabry-Perot modulator was studied using a measuring setup as shown in Fig. 2. A linearly polarized light beam with with p-polarization (electric vector parallel to plane of incidence) was incident on the RFPM surface at an angle θ. After passing through the analyzer, the component of the reflected light with the polarization parallel to the incidence plane was detected by a photo-detector. Variation in the reflected intensity was studied with angle of incidence. A sharp dip in the reflected intensity was experimentally observed at 43.6 degree which is in close agreement with the results obtained by theoretical simulations. When an electric field was applied along the direction of optic axis of ZnO film using top and bottom reflecting metallic layers as electrodes, an intensity change of the reflected light due to the change of the index of refraction of ZnO thin film was observed.

3 The light source used in the experiment was a nm He-Ne laser with the electric vector parallel to the plane of incidence. A XYθ rotation stage with rotational as well as two linear perpendicular motions and digital read out of angle procured from micro-controle (France) was used in the experiment. The least count of angle recorder was.1 degree i.e. seconds. The intensity of laser light was measured using an optical power meter (Newport 18C). 3. THEORY With the application of an external electric field across an optical medium, the distribution of electrons as well as orientation of molecules within it get disturbed. The bound electrons in the molecules of the optical medium move in a direction opposite to the applied electric field and nuclei are displaced in the direction of applied electric field. The optical medium is said to be polarized and electric dipole moment of the medium is written as p= N.e.d. Dipole moment is also written as p= N.α.E, where α= polarizability, and also p= ε. χ.e, where χ= susceptibility. N stands for the no. of molecules in the optical medium, e stands for the total charge of positive or negative particles in a molecule and d denotes separation between charges in each molecule. From the above equations it is clear that dipole moment is directly proportional to the polarizability. A change in the dipole moment changes the polarizability of the optical medium which in turn brings out a change in the susceptibility of the medium in accordance with the relation χ= N. α/ ε. A change in the susceptibility changes the dielectric constant of the medium as both are related to each other through the relation K= 1+ χ. Therefore we can say that a change in polarizabilty causes a change in the dielectric constant of the optical medium which in turn brings out a change in the refractive index of the optical medium. The reflectance of the RFPM structure is evaluated theoretically by using Fresnel s relation given as R= 1r ijklm 1 2 Where Fresnel reflection coefficient is given as r ijklm = (r ij +r jklm *exp*(2ik z1 d 1 ))/(1+r ij *r jklm *exp(2ik z1 d 1 )) In the absence of an external electric field, a light wave polarized normal to the z-direction (i.e. in x-y plane) travels as a principal wave with ordinary refractive index n o. Depending on the refractive indices and thickness of two metallic films and the electro-optic film, a phase difference is introduced among the reflected rays and the transmitted rays. When the condition for destructive interference is satisfied, dips are observed in the reflected at different values of angle of incidence. In the presence of an external electric field E z, the refractive index for a wave propagating along the optic axis of the electro-optic film and polarized along x-axis is given as, n x = n -(1/2).n 3. r 13.E z similarly, the refractive index for a wave propagating along the optic axis of the electro-optic film and polarized along y- axis is given as, n y = n -(1/2).n 3. r 13.E z where r 13 is electro-optic coefficient of the ZnO thin film used as an electro-optic media in the experiment. Changes in the refractive indices n o and n e of the electro-optic film with the applied voltage cause a shift in the angle of dip. Initially the structure is adjusted at an incident angle close to the angle of dip. A change in the reflected intensity is observed upon the application of electric field across the electro-optic film and the modulation of reflected intensity is thereby achieved. All the layers and especially the electro-optic film should be smooth and should have a parallel interface for efficient excitation of the Fabry-Perot modes. This demands highly polished and flat substrates and deposition of uniform and thin films. 4. RESULTS AND DISCUSSION For a Fabry-Perot structure having the configuration Ag/ZnO(1.3µm)/Au(1Å)/Glass, the angle of dip is found to depend on the thickness of topmost partially reflecting layer. The angle of dip showed a decrease with an increase in the thickness of topmost partially reflecting layer upto a thickness of Å and after that it remained unchanged as shown in Fig. 3. The value of reflectance (%) at dip is also found to depend on the thickness of topmost partially reflecting layer. A reflectance (%) at dip is found to first decrease with an increase in the thickness of topmost layer and after Å it starts increasing as shown in Fig. 4. Fig. 3 and Fig. 4 show theoretical and experimental variations of angle of dip and reflectance (%) at dip with the thickness of topmost layer. It may be noted that both theoretical and experimental curves are in well agreement with each other. The small deviation in the theoretical and experimental results may be attributed to the use of theoretical values of dielectric constants of all the layers in the RFPM structure. Since the dielectric constant depend upon the growth conditions of all the layers so a small deviation in the theoretical and experimental values is

4 obvious. Therefore on the basis of above studies it was concluded that the dip is sharpest at a particular thickness of Å for the topmost layer in Ag/ZnO(1.3µm)/Au(1Å)/Glass Fabry-Perot structure. Experimentally the Fabry-Perot structure having the composition Ag(Å)/ZnO(1.3µm)/Au(1Å)/Glass was fabricated Angle of dip (Degree) 45 Dip Reflectance (%) Thickness of topmost layer (Angstrom) Thickness of topmost layer (Angstrom) Figure 3: Plot of angle of dip vs. thickness of topmost layer for Ag/ZnO(1.3µm)/Au(1Å)/Glass RFPM structure. Figure 4: Plot of dip reflectance (%) vs. thickness of topmost layer for Ag/ZnO(1.3µm)/Au(1Å)/Glass RFPM structure. Variation in the reflected intensity was studied with the angle of incidence in reflection Fabry-Perot modulator structure and it was concluded that a dip is observed in the reflected intensity at a particular value of angle of incidence in the absence of external electric field. The value of angle of dip is found to depend most critically on the thickness of topmost partially reflecting layer and electro-optic layer Ez= V/micron Ez= 25V/micron Reflectance (%) Angle of incidence (Degree) Reflectance (%) Angle of incidence (Degrees) Figure 5: Plot of reflectance (%) vs. angle of incidence for Ag(Å)/ZnO(1.3µm)/Au(1Å)/Glass RFPM structure. Figure 6: Plot of experimentally observed reflectance (%) vs. angle of incidence for Ag(Å)/ZnO(1.3µm)/Au(1Å)/Glass RFPM structure.

5 The highly reflecting bottom electrode is required to be smooth and highly reflecting and its thickness is not so critical as far as the angle of dip is concerned. Angle of dip and reflectance (%) at dip remains unaffected with an increase in the thickness of bottom layer in the reflection Fabry-Perot modulator structure provided the bottom layer is highly reflecting. The performance of the fabricated device (Ag(Å)/ZnO(1.3µm)/Au(1Å)/Glass) as a light intensity modulator was examined experimentally using the measuring set-up as shown in the Fig. 2. In the absence of external electric field a sharp dip in the reflected intensity was observed at an angle of incidence of 43.6 as shown in the Fig. 5. The electrooptic modulation of a p-polarized 3 mw (632.8 nm) laser beam was studied by applying a dc voltage across the silver and gold electrodes. The applied voltage varies the refractive index of the ZnO thin film, which results in a horizontal shift in the reflectance versus θ curve. Since the angle of incidence remains fixed, the reflectance is therefore modulated. Once modulation was observed it was maximized by adjusting θ. A modulation of 1.5% was observed in the reflected intensity at an applied electric field of 25V/µm at an angle of incidence of 42.8 degree as shown in the Fig CONCLUSIONS Device fabrication and the performance of a reflectance modulator in which the intensity of a nm laser beam reflected from the topmost reflecting layer of the modulator is electrically varied, has been demonstrated. A thin film reflection Fabry-Perot modulator has been fabricated incorporating ZnO thin film as an electro-optic media. Using this device as a light intensity modulator a modulation of 1.5 % was achieved at an applied electric field of 25V/µm. The present results show the potential of ZnO electro-optic thin films for thin film electro-optic modulator applications. ACKNOWLEDGEMENTS The authors benefited greatly from the enlightening discussions and suggestions from Dr. K. Sreenivas. The authors would like to thank Dr. K. Sreenivas for providing experimental facilities to carry out the experimental work. One of the authors (VG) would like to thank University Grants Commission, India for providing a research fellowship. REFERENCES 1. Reflectance modulator utilizing an electro-optic polymer film: C. B. Rider, J. S. Schildkraut, and M. Scozzafava,, J. Appl. Phys, 7, 29(1991). 2. Electro-optic measurements of thin film materials by means of reflection differential ellipsometry: F. Wang, E. Furman, and G. H. Haertling, J. Appl. Phys., 78,9(1995). 3. Electro-optic effects of PLZT thin films M. Ishida, H. Matsunami, and T. Tanaka,, Appl. Phys. Lett., vol (1977). 4. Characterization of electro-optic polymer films by use of decal deposited reflection Fabry-Perot microcavities: P. Pretre, L. M. Wu, R. A. Hill, and A. Knoesen, J. Opt. Soc. Am. B, 15, 379(1997). 5. Comparison of electro-optic lead-lanthanum zirconate titanate films on crystalline and glass substrates: K. D. Preston and G. H. Heartling,, Appl. Phys. Lett.,, 2831(1992). 6. Integrated PLZT thin film waveguide modulators: A. Wegner et al, Ferroelectrics, 116,195(1991). 7. H. Adachi et al, Electro-optic effects of (Pb,La)(Zr,Ti)O 3 thin films prepared by rf planar magnetron sputtering, Appl. Phys. Lett., 42, 867(1983). 8. Birefrengent bistability in (Pb,La)(Zr,Ti)O 3 thin films with a ferroelectric-semiconductor interface: F. Wang and G. H. Heartling, Appl. Phys. Lett., vol. 63, 17(1993). 9. Electro-optic property of ZnO:X (X= Li, Mg) thin films: T. Nagata et al,, J. Crys. Growth, 237, 533,(2). 1. Y. C. Shen et al, A new optical method for detecting laser generated acoustic pulse in solution, Analytical Sciences, 17, 458 (1).

The 34th International Physics Olympiad

The 34th International Physics Olympiad Taipei, Taiwan Experimental Competition Wednesday, August 6, 2003 Time Available : 5 hours Please Read This First: 1. Use only the pen provided. 2. Use only the