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spring 1998 logo1998 MRS Spring Meeting & Exhibit

April 13 - 17, 1998 | San Francisco
Meeting Chairs: John A. Emerson, Ronald Gibala, Caroline A. Ross, Leo J. Schowalter

Symposium M—Integrated Magneto-Optics - Materials and Devices


Miguel Levy 
Dept of Applied Physics 
Columbia Univ 
New York, NY 10027 

Joseph Lorenzo
Opto-electronic Components Branch
USAF Research Laboratory
Hanscom AFB, MA 01731-2909

Masud Mansuripur 
Univ of Arizona 
Optical Sciences Center 
Tucson, AZ 85721 

Yasuyuki Okamura
Osaka/Wakayama Univ
Osaka, 560 JAPAN

Bethanie Stadler 
Dept of Electrical Engr 
Univ of Minnesota 
4-174EE/CSCI Bldg 
Minneapolis, MN 55455 

Raymond Wolfe
Bell Labs, Lucent Technologies
Rm 7D212
Murray Hill, NJ 07974

Symposium Support 
*Air Force Office of Scientific Research 
*Army Research Office 

Proceedings published as Volume 517 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chair: Raymond Wolfe 
Monday Morning, April 13, 1998 
Pacific C
9:00 AM *M1.1 

Metalorganic Chemical Vapor Deposition (MOCVD) process was developed for deposition of magnetic-optic RE-YIG films at a low deposition temperature. The magnetic properties of the films were systematically studied as a function of the composition. Stoichiometric YIG films with high Ce concentration in substitution of Y were obtained with a large Faraday rotation. Ce-YIG thin films can be epitaxially grown on lattice matched GGG substrates at a temperature as low as 600C. They have excellent optical and magnetic properties along with high Faraday rotations that were comparable to that of the films deposited by high temperature liquid phase epitaxy (LPE). The films deposited on single crystal (100) MgO substrates are polycrystalline and have good magnetic properties. Sputter deposited MgO buffer layer was demonstrated for preventing the decomposition and chemical reaction of GaAs and InP substrates resulting in successful deposition of YIG films on GaAs, InP, and Si substrates at a substrate temperature of 550. The films grown on MgO buffered GaAs substrates possessed good magnetic properties. 

9:30 AM M1.2 
OPTIMIZED NONRECIPROCAL RIB WAVEGUIDES FOR INTEGRATED MAGNETO-OPTIC ISOLATORS. Michael Wallenhorst, Norbert Bahlmann, Manfred Lohmeyer, Peter Hertel, Horst Dösch, University of Osnabrück, Dept of Physics, Osnabrück, GERMANY. 

Garnet films of composition (Lu/Tm,Bi)3(Fe,Ga/Al)5O12 are grown by liquid-phase epitaxy on [111]-oriented substrates of gadolinium gallium garnet. Ferrimagnetic films with positive or negative Faraday rotation as well as paramagnetic films without Faraday rotation can be produced by variations of the rare earth ion substitutions. The Faraday rotation can be calculated with a molecular field model. Optical rib waveguides in single and double layer garnet films with different Faraday rotation are realized. The nonreciprocal phase shift of the TM0-Mode is studied both theoretically and experimentally at a wavelength of 1.3 m. Results show that the maximum nonreciprocal effect of double layer films with opposite Faraday rotation is 1.6 times as large as that of comparable single layer waveguides. But, because of the large temperature dependence of the Faraday rotation of the positive rotating films, these waveguides show a large temperature dependence of the nonreciprocal phase shift. This problem can be avoided if the positive rotating layer is replaced by a paramagnetic layer. Agreement between calculations and measurements is excellent. 

9:45 AM M1.3 

A design of the waveguide magneto-optic isolator for the integrated optics requires the adjoining areas with in-plane magnetization (Faraday area) and out-of-plane magnetization (Cotton-Mouton area) in a single iron garnet film[1]. Such waveguide isolator has been fabricated by using the laser-annealing technique[2]. Localized annealing with a focused laser beam strongly modified the growth-induced magnetic anisotropy, and changed the direction of magnetization in the annealed area. The annealing temperature should be higher than 1100 ºC. An improper annealing condition caused the surface damages[3]. In this report, we studied the surface morphology of the laser annealed (YBi)3(FeGa)5O12 and (GdLuBi)3(FeGa)5O12 garnet films by using Atomic Force Microscope (AFM). The magnetic domain structures in the successfully laser-annealed area were investigated by using Magnetic Force Microscope(MFM), and Near-field Scanning Optical Microscope(NSOM), as well as by the ordinary polarization microscope. AFM images showed the atomically flat surface of the laser-annealed region in which the growth-induced magnetic anisotropy was strongly reduced. In a damaged area, the re-growth of garnets and orthferrites were observed. MFM and NSOM images showed sharp changes in the direction of magnetization at the boundary between the laser-annealed region and the non-laser-annealed region. 

10:30 AM M1.4 
INTEGRATED OPTICAL ISOLATOR EMPLOYING NONRECIPROCAL PHASE SHIFT BY WAFER DIRECT BONDING. Hideki Yokoi, Tetsuya Mizumoto, Tokyo Institute of Technology, Dept of Physical Electronics, Tokyo, JAPAN. 

A novel configuration of an integrated optical isolator employing nonreciprocal phase shift is proposed. The nonreciprocal phase shift is experienced by TM modes traveling in a magnetooptic waveguide where magnetization is aligned transversely to light propagation direction in film plane. The magnetooptic waveguide in the isolator has a structure of magnetic garnet / GaInAsP / InP. This can be fabricated by wafer direct bonding technique which makes two wafers bond with each other without any adhesives. The waveguide has a semiconductor guiding layer so that low optical propagation loss is expected. Moreover, this device has high compatibility with other semiconductor optical devices. In the integrated isolator, an optical interferometer is composed of two tapered couplers, nonreciprocal phase shifters and a reciprocal one. The interferometer is designed so that the phase difference between the light waves propagating into the two arms becomes 0 and 180 for forward and backward traveling waves, respectively. This can be accomplished by 90 nonreciprocal phase shift and the same amount of reciprocal one. Two types of the nonreciprocal phase shifters can be considered. One is nonreciprocal phase shifters in two arms and the other is a nonreciprocal phase shifter in one of the arm. The former utilizes push-pull type phase shift, which reduces the device length. In the latter case, the nonreciprocal phase shift originates only in one arm so that the isolator needs simple magnetization control compared with the other waveguide optical isolators. The required length for the nonreciprocal phase shifter can be reduced to less than 300 m by employing an optimum structure. 

10:45 AM M1.5 
CRYSTAL ION SLICING OF MAGNETIC AND FERROELECTRIC OXIDE FILMS. M. Levy, R.M. Osgood, Jr., Department of Applied Physics, Columbia University, New York, NY; A. Kumar, H. Bakhru, Department of Physics, State University of New York at Albany, Albany, NY. 

Yttrium iron garnet (YIG), bismuth-substituted YIG (Bi-YIG) and lithium niobate constitute particularly important materials for integrated-photonic device fabrication, because of their strong magneto-optic (Bi-YIG) and electro-optic (LiNbO3) response characteristics. Magnetic garnets (Bi-YIG) are needed for on chip thin film optical isolators, while lithium niobate is used to fabricate highly efficient electro-optic modulators. However, it has been heretofore impossible to integrate devices of these oxide systems on semiconductor platforms because of the complex high-temperature chemistry, as well as the usual problems of lattice matching, inherent in the growth of mixed oxides on single-crystal semiconductor surfaces. We report here on a novel technique to detach thin oxide films from their growth substrates and bond them onto semiconductor platforms. Deep ion-implantation is used to create a buried sacrificial layer in single-crystal YIG, Bi-YIG and LiNbO3 epitaxial layers. Helium ions at 3.8 MeV of energy are implanted several microns below the top surface with little residual damage to the near-surface region. The damage generated by the implantation induces a large etch selectivity between the sacrificial layer and the growth substrate. Eight to ten-m-thick LiNbO3 and YIG films have been lifted off from their original substrates by wet etching. The detached films have been bonded to silicon and gallium arsenide substrates using van der Waals forces. Study of the waveguiding characteristics in 3m-thick epitaxial Bi-YIG layers after ion implantation shows negligible excess loss after rapid thermal annealing. No changes are detected in magnetic coercivity or in the size or shape of the magnetic domains in the Bi-YIG epilayers, showing that the ion-implantation does not affect the uniaxial anisotropy in the garnet. This work was supported under MURI/DARPA contract #F49620-96-1-0111, and DARPA/AFOSR contract # F49620-95-1-0403. 

11:00 AM M1.6 
INTEGRATION OF YTTRIUM IRON GARNET FILMS VIA RF SPUTTERING. Bethanie J.H. Stadler, and Anand Gopinath, Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN. 

For truly integrated magneto-optical (MO) devices, it will be important to have the ability to grow MO materials monolithically onto specialized platforms. We have deposited undoped and Ce-doped yttrium iron garnets via rf sputtering onto a variety of substrates, including gadolinium gallium garnet, quartz, and buffered semiconductor substrates. Two buffer layers were researched for the semiconductor substrates: SiO2 and MgO. The films were deposited using separate Y/Ce and Fe targets. The power to the respective guns and substrate rotation were used to control composition. Reactive rf sputtering was employed and the effect of the oxygen content in the sputtering gas on the film properties was determined. We have measured the optical and magneto-optical properties of the films along with their structure and composition. Ce has been seen to substantially increase Faraday rotation in YIG films and also to increase the refractive index from 2.05 to 2.2 in the near infrared. This will be important in waveguide applications. 

11:15 AM M1.7 
SELECTED-AREA SPUTTER EPITAXY OF A MAGNETO-OPTIC GARNET PATTERNED FILM ON A SUBSTRATE WITH ION-BEAM BOMBARDED MICROSTRUCTURE PATTERN. Yasuyuki Okamura and Sadahiko Yamamoto, Osaka University, Graduate School of Engineering Science, Division of Advanced Electronics and Optical Science, Toyonaka, Osaka, JAPAN; also with Wakayama University, Faculty of Systems Engineering, Department of Optomechatronics, Wakayama, JAPAN. 

Patterned microstructure in a magneto-optic garnet film plays an important role in materializing nonreciprocal integrated optical devices, display devices, and magnetic recording media. So far a channel waveguide buried in an iron garnet film and a two-dimensional electrically addressed magneto-optic light modulator have been demonstrated. Here we experimentally investigate selected-area sputter epitaxy of a rare-earth iron garnet strip film on a substrate, whose surface is partially bombarded with ion-beam etching, for its application to an embedded optical waveguide and a two-dimensional spatial light modulator. Recently we found that controlled ion-beam etching into a nonmagnetic garnet substrate is suitable for selected-area sputter epitaxy of a cerium substituted yttrium iron garnet (Ce:YIG) film, which is one of promising magneto-optic materials, and successfully grew such a film on a gadolinium gallium garnet (GGG) substrate whose surface was bombarded with ion-beam etching. In this work we fabricate a patterned strip selected-area film with microstructure. Firstly, a mask pattern with line wide of 8m in 16m period was used to check the possibility of patterning. The lines were directed along 110 or 112 lying perpendicular to the (111) oriented GGG substrate. At the substrate temperature of 750, a periodical structure was built in a Ce:YIG film. Scanning electron microscopic observation revealed that an amorphous strip was embedded in a crystalline film, whose cross section was a trapezoid. Strip mask patterns oriented along 112 and 110 direction yielded symmetric and asymmetric sidewalls of the embedded stripe, respectively. The result is explained in terms of the anisotropic facet growth of a garnet crystal investigated for a sphere. Furthermore use of a narrow line of 3m width led us to an amorphous strip with a triangle shape of cross section perfectly surrounded in a crystalline film. Thereby we can fabricated a buried channel optical waveguide with only one process. 

11:30 AM M1.8 
MAGNETO-OPTICAL PROPERTIES OF Co DOPED Bi3Fe5O12 GARNET FILM. T. Okuda, T. Mima, K. Ando, N. Ohmori, N. Adachi, and H. Ohsato, Nagoya Institute of Technology, Nagoya, JAPAN. 

Bi3 Fe5 O12 garnet (BIG) is all artificial material which shows giant Faraday effect. Since Co2+ ions on tetrahedral (d) site and octahedral [a] sites contribute to increase of Faraday rotation angle and magnetic anisotropy, respectively, we investigated the effects of Co doping on the magnetic and magneto-optical properties of BIG. The films were prepared by means of reactive ion beam sputtering. As the Co source targets, CoO, CoFe2O4, Co metal and Bi3 Fe5-2xCoxGexO12(BFCG) ceramic were employed. (100) oriented Gd3 (ScGa)5 O12 garnet wafer used as the substrate was held at 500ºC during deposition. When the CoO, CoFe2O4 or Co metal targets were used, by XRD measurement, formation of BIG was cofirmed, however, spinel phase was scarecely found. Prominent increase of the saturation magnetization, the coercive force, and the Faraday rotation angle (F) with negative sign at around 1.5 m were observed. The coercive force obtained from Faraday hysteresis loop measured at 633nm was about 100 Oe and that at 1.5 m was larger than 1kOe. From these results, it was deduced that Co ions may exist in the BIG film as the fine particles of Co spinel ferrite. No evidence which suggests the presence of Co ion in garnet structure was obtained. When the BFCG target was employed, garnet phase was formed up to X=1. Remarkable increase of F with positive sign at around 1.5 m, which corresponds to 1.1deg/m/atom was observed. The saturation magnetization steeply decreased from 1.6kG (BIG) to 0.9kG. The coercive force increased from 100 Oe (BIG) to 1.7kOe and direction of the magnetic easy axis changed from in-plane to plane normal. From these results, we concluded that considerable amount of Co2+ ions were substituted for Fe3+ ions at the (d) and [a] sites of BIG. 

Chair: Miguel Levy 
Monday Afternoon, April 13, 1998 
Pacific C
1:30 PM *M2.1 

The development of integrated optic components for optical communication systems has intensified the need for integrated nonreciprocal devices such as isolators and circulators. The advantages of these nonreciprocal devices include compatibility with waveguide optics, low magnetic field requirements, and low cost. Although several waveguide nonreciprocal devices have been proposed and demonstrated, no practical device has yet been achieved. This is mainly because they require complicated structures. This talk illustrates our recently developed integrated isolators. Their structure is simple, in that it consists solely of a single-mode channel magneto-optic waveguide. The operating principle is that a backward fundamental TM mode wave is converted to higher-order or radiation TE modes while a forward fundamental TM mode wave propagates with no mode conversion. Operation is achieved by employing a combination of nonreciprocal phase shift and TM-TE mode conversion. A very large Faraday rotation of Ce-substituted yttrium iron garnet magneto-optic film enables us to realize these integrated isolators. The principle, fabrication and optical characteristics will be addressed in detail. Further, our recent study on a hybrid-integrated polarization-independent optical circulator based on a nonreciprocal Mach-Zehnder interferometer will be reviewed. 

2:00 PM M2.2 
A COMPARISON OF AN IMPROVED DESIGN FOR TWO INTEGRATED OPTICAL ISOLATORS BASED ON NONRECIPROCAL MACH-ZEHNDER INTERFEROMETRY. Norbert Bahlmann, Manfred Lohmeyer, Michael Wallenhorst, Horst D{otsch, Peter Hertel, University of Osnabr{uck, Dept of Physics, Osnabr{uck, GERMANY. 

Nonreciprocal rib waveguide structures can be used to realize integrated optical isolators. The nonreciprocal phase shift is the difference between the forward and backward propagation constants of TM modes in magneto-optic waveguides. It can be optimized with respect to absolute value and temperature dependence if double layer waveguides with different magnetic and nonmagnetic layers are prepared. In this paper we propose an improved design for two different Mach-Zehnder interferometer isolators the nonreciprocal parts of which are formed by such double layer waveguides. One concept utilizes a nonreciprocal and a reciprocal arm. In the other case both arms are nonreciprocal but with opposite sign of the nonreciprocal phase shift. A particular property of both concepts is that the lengths of the nonreciprocal arms are well defined. The rest of the interferometer is made by reciprocal rib waveguides. Therefore, the nonreciprocal phase shift is well known. The concepts are compared with regard to isolation ratio, forward losses and fabrication tolerances. Moreover, we simulate the entire isolators by a finite difference beam propagation calculation. 

2:15 PM M2.3 
RADIATIVELY COUPLED WAVEGUIDE CONCEPT FOR AN INTEGRATED MAGNETOOPTIC CIRCULATOR. Manfred Lohmeyer, Michael Shamonin, Norbert Bahlmann, Peter Hertel, Horst Dötsch, Dept of Physics, University of Osnabrück, GERMANY. 

Three-guide couplers with multimode central waveguides allow for remote coupling between optical channels. A simple three mode approximation turns out to be sufficient for the description of the main features of the power transfer behavior. The specific form of the relevant modes suggests the design of integrated optical isolators and circulators based on magnetic garnet materials. These novel devices are superior to the conventional nonreciprocal coupler with respect to total length and admissible fabrication tolerances. We characterize the isolation performance and the transmission loss for the proposed devices by propagating mode simulations and estimate the influence of geometry parameter deviations. 

3:00 PM *M2.4 
MAGNETOSTATIC WAVES-BASED INTEGRATED MAGNETOOPTIC DEVICES IN YIG-GGG WAVEGUIDES*Chen S. Tsai, Department of Electrical and Computer Engineering and Institute for Surface and Interface Sciences, University of California, Irvine, CA. 

Magnetostatic Waves-based integrated magnetooptics is concerned with magnetooptic (MO) interactions between guided-optical waves and magnetostatic waves (MSW), and the resulting devices in yttrium iron garnet-gadolinium gallium garnet (YIG-GGG) waveguides. MSWs can be readily and efficiently generated by applying a microwave signal to a microstrip line transducer deposited directly on the YIG layer or brought over it. The carrier frequency of the MSW can be tuned, typically from 0.5 to around 30 GHz, by simply varying an external bias magnetic field. MO interactions result from the moving optical gratings induced by the MSW through the Faraday and Cotton-Mouton effects. Specifically, non-collinear coplanar MO Bragg interactions between the guided-optical waves and the MSWs in the YIG-GGG waveguides have resulted in guided-wave MO Bragg cell modulators. Such MO modulators have shown desirable features similar to that of acoustooptic (AO) Bragg cell modulators, but with potentially superior performance characteristics. These MO modulators have thus paved the way for realization of a variety of MSW-based integrated optic devices such as high-speed optical scanners and space switches, electronically tunable carrier frequency band RF spectrum analyzers, and optical frequency shifters and modulators with application to RF signal processing and communications. In this presentation a selective review of the materials, physics and technology involved as well as the resulting devices will be given. 

3:30 PM M2.5 
MAGNETOSTATIC WAVE OSCILLATOR-BASED INTEGRATED MAGNETOOPTIC BRAGG CELL MODULATOR*Jun Su and Chen S. Tsai, Department of Electrical and Computer Engineering and Institute for Surface and Interface Sciences, University of California, Irvine, CA. 

Magnetooptic (MO) Bragg interactions between guided-optical waves and magnetostatic waves in YIG-GGG waveguides have demonstrated considerable potentials toward realization of integrated MO devices for wideband real-time RF signal processing and optical communications.1 In this paper, we report a novel MO Bragg cell modulator which utilizes a magnetostatic forward volume wave (MSFVW)-based oscillator. The MSFVW oscillator consists of a 9 m-thick and 6 mm-wide Bi-substituted YIG film, a pair of microstrip line transducers at a separation of 5.0 mm, and a feedback loop. A solid state amplifier is connected to the input transducer via a dual directional coupler. The output transducer of the delay line is fed into the amplifier through a sampling coupler. The fed power is controlled using a variable attenuator. The transmission and reflection characteristics of the delay line as well as the oscillating frequencies are measured at the output ports of the directional coupler. By tuning the bias magnetic field from 2350 to 2900 Oe, the oscillating frequency is found to increase linearly from 2.112 to 3.274 GHz. An optical beam of 1.313 m in wavelength is used to measured the Bragg diffraction efficiency. In order to compare such MSFVW oscillator-based MO Bragg cell modulator with the conventional MO Bragg cell modulator, namely, the one involving just the MSFVW delay line, we also measure the MO Bragg diffraction efficiencies by disconnecting the feedback loop. The amplifier input is then connected to a sweep generator which supplies the signals in the same frequencies as the measured oscillating frequencies under the corresponding magnetic biasing conditions. The experimental data have shown that the MSFVW oscillator-based MO Bragg cell modulator is considerably more efficient than the one based on the delay line alone. For example, when the biasing magnetic field is set at 2450 Oe, the MSFVW oscillator produces an output frequency of 2.435 GHz and the corresponding MO Bragg diffraction efficiency is measured at 33.3%. In contrast, the measured diffraction efficiency for the conventional modulator is 17.4% at the same frequency and input power. Both theoretical calculation and experimental data show that enhancement of MO Bragg diffraction efficiency in the oscillator-based modulator can be attributed to reduction in the insertion loss of the MSFVW delay line. 

3:45 PM *M2.6 

Although discovery of the magneto-optic effect by Michael Faraday occurred 150 years ago, its practical applications have become possibly only in recent years, with the advent of high Verdet constant materials, and very high strength rare-earth magnetic materials. 
The application in isolators and circulators of the Faraday effect depends upon the non-reciprocity nature of the effect, which causes light to behave differently in the reverse direction over the same path. Because the Faraday effect affects the state of polarized light, the optical isolator has been conceived to minimize optical ``feedback''. In the circulator, the principle enables a change in direction of light traveling a reverse path. 
A review of the many kinds of magneto-optic materials used in such devices today, and performance characteristics of these devices, will be presented. 

4:45 PM M2.7 
A LENS-FREE AND MIRROR-FREE FIBER INTEGRATED OPTICAL MAGNETIC FIELD SENSOR USING Bi-SUBSTITUTED GARNET. Hisashi Minemoto, Daisuke Ishiko, Nobuki Itoh and Satoshi Ishizuka, Matsushita Electric Industrial Co., Ltd., Osaka, JAPAN. 

We have developed a novel lens-free and mirror-free fiber integrated optical magnetic field (or current) sensor for a power distribution system. A Bi-substituted rare earth iron garnet ((BiGdLaY)3(FeGa)5O12) [1] was used as a Faraday rotator for good temperature characteristics. We adopted a bent optical fiber to realize a small-sized sensor in the magnetic field direction. We also used direct coupling of the optical path without any lens or alignment of optical components. The sensor was fabricated on a glass substrate having three grooves formed by a precise rotating blade saw. An optical fiber was bent into an inverted U shape by heat treatment. This bent fiber was bonded in the grooves with epoxy resin. Then, other grooves were formed and the fiber was simultaneously cut to insert such optical components as a polarizer, an analyzer and a Faraday rotator. We used a (BiGdLaY)3(FeGa)5O12 crystal and two glass polarizing plates, all with dimensions of 1x1x0.5 mm3. These optical chip components were inserted in the grooves and bonded with epoxy resin. Use of a bent optical fiber and optical chip components allows easy assembly with no need for alignment. The temperature dependence and linearity error of this sensor's sensitivity were less than +2% and 1%, respectively.