Nonlinear Optical Waveguide Multilayer Structure

The invention relates to an nonlinear optical waveguide multilayer structure.

Waveguide modulation is important in the areas of optical communication and high-speed signal processing. Switching and modulation can be accomplished in nonlinear optical waveguides by modifying the propagation constant of a guided mode through the application of an electric field via the linear electrooptic effect. The applied electric field provides a change in refractive index and can result in phase and/or intensity modulation. Voltage is applied to two electrodes placed over or alongside the waveguide. The vertical electric field is employed when one electrode is placed directly over the waveguide, while the horizontal electric field is used when the electrodes are placed on either side of the waveguide. The applied electric field is not uniform and peaks sharply near the electrode edge. See "Optical Electrode Design for Integrated Optics Modulators", by D. Marcuse, IEEE Journal of Quantum Electronics, Vol. QE-18, 393 (1982). The uniformity of the electric field can be improved by moving the two electrodes sufficiently far apart. However, this requirement necessitates a high drive voltage because the magnitude of the field is approximately equal to V/G where G is the electrode gap.

A variety of parametric devices has been developed for efficient frequency conversion. Parametric processes in nonlinear optical materials require phase matching of the interacting optical modes to achieve very efficient nonlinear optical interactions. Periodic inversion of the ferroelectric domain structure of LiNbO.sub.3 and LiTaO.sub.3 has been used for quasi-phase matching, thus permitting generation of blue light in a guided mode of the waveguide. Yamada et al formed a periodic domain structure in LiNbO.sub.3 bulk crystals by applying an external electric field. See "First-order Quasi-phase Matched LiNbO.sub.3 Waveguide Periodically Poled by Applying an External Field for Efficient Blue Second-harmonic Generation", by M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, Appl. Phys. Lett. 62, 435 (1993). The field V/T must be greater than the electric coercive force with a value of 20 kV/mm for LiNbO.sub.3, where T is the bulk crystal thickness. For instance, voltage level above 4 kV is needed for domain inversion in 200 .mu.m-thick LiNbO.sub.3 bulk crystals. For thin films grown on foreign substrates, the electric field is approximately equal to .epsilon..sub.2 V/.epsilon..sub.1 T where .epsilon..sub.1 and .epsilon..sub.2 are the respective dielectric constants of the film and substrate, and T is the total thickness of the sample. Using the known values of the dielectric constants listed, one can find that voltage with a magnitude of 10.7 kV is required for domain inversion in a c-oriented LiNbO.sub.3 film grown on a sapphire substrate with a total thickness of 200 .mu.m. Since higher voltage is required, achieving domain inversion by an electric field is more difficult in thin films than in bulk. Although one can further mill the substrate to reduce the required voltage level, a tradeoff exists between the voltage required for domain inversion and the minimum thickness required for mechanical rigidity.

FIG. 1 is a schematic diagram of an embodiment of the multilayer structure according to the invention.

FIG. 2 is a schematic diagram of another embodiment of the multilayer structure of the invention.

FIG. 3 shows a x-ray diffraction pattern of a heteroepitaxial structure of c-oriented Al.sub.2 O.sub.3 /Pd/Pt/MgO/LiTaO.sub.3.

In this invention there is provided a multilayer structure comprising in order: an oriented crystalline substrate, an epitaxial electrode on the substrate, an epitaxial buffer layer on the electrode, and an epitaxial upper layer on said buffer layer.

An advantage of a multilayer structure in accordance with this invention is that an interior electrode provides a more uniform electrical field and a high electrical field can be applied with relatively low voltage.

This invention describes a multilayer structure using an epitaxial layer as a bottom electrode to grow an optical waveguide on a single crystal substrate. When an electrode is placed on the waveguide surface, the vertical electric field is used with a magnitude of V/G, where G is in the distance between the two electrodes. The field can be sufficiently high at low voltage level because G is in the range of 1-2 .mu.m and can exhibit superior uniformity in the entire area of the waveguide. This structure can also dramatically reduce the voltage necessitated for domain inversion in ferroelectric films. For instance, about 50 V is sufficient for domain inversion. Some surface electrode geometries which utilize fringing fields can also achieve high electric field strengths with low voltages, however, these cannot produce uniform fields over large areas. Such uniform fields are required in some devices, such as prism-type electrooptic scanners.

Referring to FIG. 1 a multilayer structure 10 includes a single crystalline substrate 11, an epitaxial overlaying electrode 13, a buffer layer 15, and an epitaxial upper layer 17. The electrode consists of an adhesive layer 13a and a conducting layer 13b. Another embodiment of a multilayer structure 20 is shown in FIG. 2. The structure 20 consists of a c-oriented single crystal substrate of Al.sub.2 O.sub.3 or LiNb.sub.x Ta.sub.1-x O.sub.3 (x=0 to 1) 21, an epitaxial conducting electrode of Pd/Pt 23a & b, an epitaxial buffer layer of (111)-oriented MgO or c-oriented Mg.sub.4 (Nb.sub.x Ta.sub.1-x).sub.2 O.sub.9 25, and a c-oriented LiNb.sub.x Ta.sub.1-x O.sub.3 layer 27 grown epitaxially on the buffer layer. The term oriented means that the crystal structure is substantially aligned in one direction.

The substrates are in a preferred c-oriented orientation. Thus the nonlinear optical film may grow epitaxially in this preferred orientation. The substrate can be Al.sub.2 O.sub.3, LiNb.sub.x Ta.sub.1-x O.sub.3, Mg.sub.4 (Nb.sub.x Ta.sub.1-x).sub.2 O.sub.9 and MgO, where x is from 0 to 1. However, the same concept can be applied to "x cut" and "y cut" substrates for some applications. The materials used to fabricate the heteroepitaxial structures can be doped, lightly doped or heavily doped, as long as the doping does not significantly alter their crystallinities and optical properties.

The epitaxial electrode 13 in FIG. 1 is an essential part in this invention. This layer must adhere to substrate 11 and buffer layer 15, be stable to oxidation without forming oxides or increasing resistivities upon processing in oxygen, be oriented or preferably a single crystal film with a desired orientation to act as a seed for the growth of epitaxial buffer layer 15, and be inert with respect to substrate 11 and buffer layer 15. These minimum characteristics are mandated of the possible metal choices. Where Pt is used, the adhesion of Pt to the substrate can be improved by depositing an epitaxial adhesive layer of Pd.

The buffer layer 15 has an important contribution to this invention. Preferably, the buffer layer 15 in FIG. 1 is 1) transparent over a wide range of wavelengths, 2) an optical film with a substantially lower refractive index than the nonlinear optical film, and 3) a single crystal film structurally matching the conducting layer and the nonlinear optical film. In addition, the buffer layer must thermally match the multilayer structure so that a relatively thick film can be grown to support low loss waveguides without cracking or peeling. We have found that the oxides MgO and Mg.sub.4 (Nb.sub.x Ta.sub.1-x).sub.2 O.sub.9 (x=0 to 1) provide excellent buffer layer properties and are most suitable to form waveguides with LiNbO.sub.3 and LiTaO.sub.3.

The electrode can be grown by many conventional manners, such as e-beam evaporation, laser ablation, sputtering, or chemical vapor deposition. Minimum thickness is about 10 nm for a full coverage. There is no advantage by employing an electrode thicker than 300 nm.

The buffer layer can be grown epitaxially by many conventional manners, such as e-beam evaporation, laser ablation, sputtering, or chemical vapor deposition. It is necessary that the buffer attains a sufficient thickness, so that the strength of the evanescent tail of the mode propagating in the nonlinear optical film is negligible at the electrode-buffer layer interface to prevent guided wave loss. Useful thicknesses range from 200 to 3000 nm, preferably 400 to 800 nm.

The overlying nonlinear optical layer of LiNb.sub.x Ta.sub.1-x O.sub.3 can be grown by any conventional methods, such as rf-sputtering, laser ablation or metal organic chemical vapor deposition. Thickness ranges from 60 to 3000 nm, preferably 200 to 800 nm.

C-oriented Al.sub.2 O.sub.3 wafers were used as substrates for epitaxial growth of the multilayer structure. After a conventional cleaning in toluene, 2-propanol, and deionized water, Al.sub.2 O.sub.3 was annealed in air at 1200.degree. C. for 1 hour prior to being loaded in a chamber for deposition.

Pd, Pt and MgO were sequentially deposited directly on Al.sub.2 O.sub.3 by electron-beam evaporation. The deposition process was carried out at 3.times.10.sup.-8 Torr. The substrate was heated by a radiative heater consisting of tantalum wires. The deposition was carried out at 400.degree. C.-550.degree. C. and at 0.05-0.15 nm/s With a total thickness of 4, 40, and 500 nm for Pd, Pt, and MgO, respectively.

The targets used in laser ablation for LiTaO.sub.3 were prepared from powders using calcined Li.sub.2 CO.sub.3 and Ta.sub.2 O.sub.5 having a mole ratio of 1.10:1.00. The samples were pelletized under a pressure of 5000 psi, and sintered in air.

LiTaO.sub.3 films were grown on the structure of Al.sub.2 O.sub.3 /Pd/Pt/MgO by pulsed laser ablation. A laser pulse energy of 300-360 mJ with a 30 ns duration and a pulse rate of 4 Hz was generated by a KrF excimer laser. The beam was focused to a 5 mm.sup.2 spot onto a target of polycrystalline LiTaO.sub.3. The substrate was located 6 cm from the target and heated to 560.degree.-650.degree. C. by a resistive heater. The temperature was monitored by a thermocouple attached to an inner wall of the heater block. The deposition was carried out at a rate of 0.06 nm/pulse under an oxygen pressure of 100 mTorr. After 4000-6000 pulses the oxygen pressure was raised to 150 Torr, and the sample was cooled to room temperature.

The samples were characterized by x-ray diffraction. The distribution of c-axis orientations relative to the normal of the substrate surface was determined by x-ray rocking curve analysis, and the feature of in-plane orientation was examined by x-ray pole figure analysis. These techniques are well known and are illustrated by B. D. Cullity, Elements of X-ray Diffraction (Addison-Wesley, Reading, Mass.). The following examples are presented for a further understanding of the invention.

Thin films of 4 nm thick Pd, 40 nm thick Pt, and 500 nm thick MgO were sequentially deposited on a c-oriented Al.sub.2 O.sub.3 substrate at 400.degree. C. by e-beam evaporation, and a layer of LiTaO.sub.3 with a thickness of 250 nm was then deposited on the MgO coated substrate at 650.degree. C. by laser ablation.

The sample surface was shiny and smooth. The standard 2-theta diffraction pattern in FIG. 3 taken from the multilayer structure shows only the Al.sub.2 O.sub.3 (006), Pt(111), MgO(111) and LiTaO.sub.3 (006) diffraction peaks. The full width of the rocking curves at half maximum (FWHM) was determined to be about 0.2.degree. for Al.sub.2 O.sub.3 (006), 0.4.degree. for Pt (111), 0.8.degree. for MgO(111), and 0.8.degree. for LiTaO.sub.3 (006). The good crystal quality of the LiTaO.sub.3 film was verified by ion channeling analysis showing a minimum yield of about 0.25. Pole figure analysis indicates a good in-plane alignment of LiTaO.sub.3 and Al.sub.2 O.sub.3 although the LiTaO.sub.3 film contained high twin density. The c-oriented Al.sub.2 O.sub.3 /Pd/Pt/MgO/LiTaO.sub.3 was optically characterized by m-line spectroscopic measurements and guided optical waves were found in the LiTaO.sub.3 film.

Thin films of 4 nm thick Ti, 40 nm thick Au, and 500 nm thick MgO were sequentially deposited on a c-oriented LiTaO.sub.3 substrate at 400.degree. C. by e-beam evaporation, and a layer of LiTaO.sub.3 with a thickness of 250 nm was then deposited on the MgO coated substrate at 560.degree. C. by laser ablation. The sample surface was shiny and smooth. The standard 2-theta diffraction pattern taken from the multilayer structure shows only the Au(111), MgO(111), and LiTaO.sub.3 (006) diffraction peaks. Guided optical waves were found in the LiTaO.sub.3 film by optical measurements. The results indicate a nonlinear waveguide epitaxially grown on an electrode consisting of a Ti adhesive layer and an Au conducting layer.

The same multilayer structure was prepared as described in Example 1, except LiF was used to replace MgO as an epitaxial buffer layer. LiF and MgO posses the same NaCl structure with close lattice constants (a=4.0173 for LiF and 4.2112 for MgO), and have substantially low refractive indices with respect to LiTaO.sub.3 (n=1.392 for LiF and 1.736 for MgO). LiF was deposited on Al.sub.2 O.sub.3 /Pd/Pt at 300.degree.-400.degree. C. in vacuum by e-beam evaporation. X-ray diffraction analysis shows two peaks that can be identified as the (111) and (200) lines of LiF. The ratio of the x-ray intensity of the (200) peak to that of the (111) peak is about 6:1, indicating that LiF can not grow epitaxially in a preferred (111) orientation and is not suitable for acting as a buffer layer to grow c-oriented LiTaO.sub.3.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

10 multilayer

11 crystalline substrate

13 overlying electrode

13a adhesive layer

13b conducting layer

15 buffer layer

17 epitaxial upper layer

20 multilayer structure

21 c-oriented single crystal

23 Pd/Pt electrode

25 buffer layer

27 nonlinear optical film