Christoph Buchal

Optical waveguiding in magnetron-sputtered Na0.5K0.5NbO3 thin films on sapphire substrates

Preferentially oriented perovskite-structured Na0.5K0.5NbO3 (NKN) thin films have been deposited on hexagonal Al2O3(011_2) substrates using rf magnetron sputtering of a stoichiometric, high-density, ceramic target. Structural and film surface properties were measured using x-ray diffraction and atomic force microscopy, respectively. Optical and waveguiding properties were characterized using a prism-coupling technique. We observed sharp and distinguishable TM and TE propagation modes and measured the refractive index of NKN thin films of different thicknesses. The ordinary and extraordinary refractive indices were calculated to be no=2.247±0.002 and ne=2.216±0.002 for a 2.0-μm-thick film at 632.8 nm. This implies a birefringence Δn=ne−no=−0.031±0.002 in the film. These first results show the potential use of rf-sputtered NKN films as an electro-optical active material.

Photorefractive effect in proton-implanted Fe-doped KNbO 3 waveguides at telecommunication wavelengths

We report on photorefractive two-beam coupling in proton-implanted Fe-doped KNbO3 waveguides at wavelengths from 632.8 to 1550 nm. Exponential gain coefficients of 11 and 0.9 cm-1 were measured at wavelengths of 632.8 nm and 1550 nm, respectively. Photorefractive response times as low as 120 µs are reported at 632.8 nm for a pump power of 4 mW. It is shown that proton irradiation increases the effective trap density, decreases the response time, and extends the photorefractive sensitivity of Fe-doped KNbO3 crystals toward near-infrared wavelengths.

Cerenkov-type second-harmonic generation in KNbO/sub 3/ channel waveguides

Cerenkov-type second-harmonic generation using KNbO/sub 3/ channel waveguides produced by MeV He/sup +/-ion implantation is presented from the viewpoint of device design. We derive the Cerenkov phase-matching condition for multimode waveguides and utilize Cerenkov-angle analysis as a tool for contact-free measurement of the effective indexes of guided modes of ion-implanted KNbO/sub 3/ channel waveguides at a wavelength of 860 nm. The measured mode indexes are in full agreement with calculations based on the effective-index method and the refractive index-depth profiles of ion-implanted KNbO/sub 3/ waveguides. The efficiency of Cerenkov-type second-harmonic generation is modeled using analytical approximations of the field distributions of the fundamental and the Cerenkov-radiation modes in embedded-channel waveguides. The acceptance width for Cerenkov-type frequency doubling in these ion-implanted waveguides is about one order of magnitude wider than for noncritical phase-matched second-harmonic generation in bulk KNbO/sub 3/ crystals. Based on the theoretical simulations, guidelines for optimum device design are given, and the possibility to increase the ultimate conversion efficiency to about 30% W/sup -1/ cm/sup -1/ through lateral-resonance enhancement of the second-harmonic field in KNbO/sub 3/ channel waveguides is demonstrated.

Mode propagation losses in He + ion-implanted KNbO 3 waveguides

The losses of ion-implanted potassium niobate (KNbO3) waveguides are evaluated theoretically and experimentally in dependence on wavelength, irradiation dose, waveguide thickness, and waveguide width. Irradiation-induced absorption and tunneling are identified as the main sources of loss. The contributions from surface scattering and intrinsic material absorption are shown to be small. The attenuation due to irradiation-induced absorption and tunneling is calculated from the experimentally determined complex refractive-index profile. The optical loss is minimum in the red part of the spectrum and increases toward the blue because of absorption and toward the infrared because of tunneling. A minimum loss of less than 0.2 cm-1(1 dB cm-1) was measured in an ion-implanted KNbO3 waveguide at a wavelength of 0.633 μm. On the basis of a theoretical model we give guidelines for the formation of optimized waveguides for specific applications, e.g., second-harmonic generation.

VERTICAL SILICON METAL-SEMICONDUCTOR-METAL PHOTODETECTORS WITH BURIED COSI2 CONTACT

We report on design, fabrication, and characterization of an ultrafast vertical metal–semiconductor–metal photodetector. A CoSi2 layer in silicon acts both as a bottom Schottky contact and a buried light reflector. A semi‐transparent metallization on top of a photosensitive silicon mesa serves as top Schottky contact. Time‐domain studies of the pulse response are performed by electro‐optic sampling measurements on photodetectors integrated monolithically into microstrip transmission lines. At room temperature, carrier sweep‐out is dominated by hopping transport involving shallow traps. At low temperatures, hopping transport is strongly suppressed. As a consequence, the speed of the diode is considerably enhanced, reaching a pulse response with a full width at half maximum of 6.5 ps.

Low‐temperature annealing of ion‐implanted KNbO3 waveguides for second‐harmonic generation

Results on annealing experiments of He+ ion‐implanted KNbO3 planar and channel waveguides are reported. Annealing at 150 °C for several hours leads to a reduction of the waveguide attenuation constant by more than 5 dB cm−1 at a wavelength of 457 nm without significant change of the profile of the mode confining barrier. A minimum waveguide attenuation constant of 1.3 dB cm−1 in planar and 2.2 dB cm−1 in channel waveguides at 515 nm was achieved. Second‐harmonic generation measurements in the waveguides showed that the conversion efficiency can be improved by more than a factor of 2 by the annealing process.

Ferroelectric thin films for optical applications

Abstract Presently the installation of optical fibers for high bandwidth communication services experiences an explosive growth on a world wide scale. As a consequence, a growing demand for more complex integrated optical devices is forseeable. At present, the technology for elec-trooptic integrated devices and components has been mostly satisfied by bulk LiNbO3, which is by far the most important optical ferroelectric. We will review the state of the art of “near-surface-modified”-ferroelectric devices, which have been patterned on bulk substrates and their potential counterparts to be fabricated completely in thin film technology on different substrates. Special emphasis will be laid upon theepitaxy of LiNbO3, BaTiO3 and (Pb, La) (Zr, Ti)O3.

Birefringence phase-matched blue light second-harmonic generation in a KNbO3 ridge waveguide

Ridged channel waveguides in KNbO3 were fabricated by a technique using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. A continuous-wave second-harmonic output power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW in a 0.73 cm long waveguide. Phase matching was provided by material birefringence without need of periodic poling.

Ion implantation for photorefractive devices and optical emitters

Abstract For the development of optical devices both defect engineering and dopant introduction are the most important current uses of ion implantation. Defect engineering is a well-established concept for changing the optical index. Especially in well-ordered crystalline materials, as the optical ferroelectrics, the nuclear recoils from ion implantation generate disorder, which lowers the polarizability and the optical index. Since MeV implantation of light ions provides the largest damage at the end of the ion range, numerous optical waveguides have been formed by the low index barrier in the depth. In addition, it could be demonstrated recently, that the implantation generated Frenkel defects within the oxides are electrically active. They can be used to improve the photorefractive (PR) properties significantly, as has been shown for single crystals of KNbO3. In the second part we will briefly mention the latest results of different groups on the luminescence of some rare earth ions, as Tb or Er, implanted into oxides or into silicon in the form of an erbium–oxygen complex. These ions from luminescent centers, which can be activated by the impact of electrons or the recombination of electron–hole pairs. The experiments are motivated by the search for optical electroluminescent emitters, being compatible with silicon-based microelectronics.

Linear and nonlinear optical properties of KNbO3 ridge waveguides

Ridged channel waveguides in KNbO3 were produced using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. We investigated the linear and nonlinear optical characteristics of the waveguides. The effective mode indices are derived from the refractive index profiles using the effective index method. The losses are investigated as a function of wavelength and of the geometrical parameters channel width and ridge height. A minimum loss of 2 dB cm−1 is measured at a wavelength of 0.633 μm. We investigated the power handling capabilities at visible and near-infrared wavelengths. Second-harmonic generation in these waveguides is studied both theoretically and experimentally with regard to its dependence on the guide fabrication parameters. Phase-matching configurations for blue light second-harmonic generation are evaluated on the basis of the dispersion of the effective mode indices. Overlap integrals are calculated on the basis of the field distributions derived from the refract...