A 3+m periodic domain-inverted pattern on the C-face of LiNbQs has been written using direct electron-beam writing for first-order gratings used in second harmonic generation. The domain formation mechanism has been investigated, especially the shape of the domain and need for a conductor on the C+ face during the electron-beam writing process.The quasiphase-matching (QPM) technique,' where phase matching is based on the periodic modulation of the nonlinear coefficient, has been shown to give highly efficient second harmonic generation.' Using this technique, by selecting appropriate period of modulation, it is possible to phase match an arbitrary wavelength of light. The periodic modulation of the nonlinear coefficient is achieved in LiNb03 by reversing the domain polarization using Ti:indiffusion, (Ref. 3) Li ion outdiffusion,@ and by electronbeam bombardment.7*8 The efficiency of light frequency conversion depends upon the period of the domain inversion grating. Recent results reported on frequency conversion using the QPM technique'*25 use a third-order grating because of fabrication difficulties at the smaller grating period. It is, however, desirable to have a first-order grating as the frequency conversion efficiency can be increased by a factor of 9 compared to that obtained with a third-order grating. In this letter we report the fabrication of a firstorder grating using a direct electron-beam writing technique. Also, the domain reversal mechanism inherent in the formation of such gratings is examined, particularly the shape of the domain and need for some form of electrical ground on or near the C+ face of the LiNbO,.Direct electron-beam writing was achieved using a Phillips 525M scanning electron microscope (SEM) converted for this purpose. Beam currents used were in the range of 3-7 nA and the beam voltage ranged between 20 and 30 kV. The electron-beam spot size was 0.5 ,um. Patterns were written with saturated filament current at beam voltages of 20, 25, and 30 kV. The best grating resolution was obtained at 30 kV. Although surface cracking was observed at high voltages (30 kV) and at slower scan velocities (235 ym/s) with a beam current of 7 nA, surface cracking was avoided by reducing the beam current while keeping the beam voltage high. Samples used in this study were 500~pm-thick Z-cut LiNbO, obtained from Crystal Technology. The domain inversion process is controlled by the electric field created by electron bombardment. Hence a 30-nm film of Ta metal was sputtered on the C+ face, which acted as a ground electrode. Samples were scanned on the C-face where the electron beam deposited a -ve charge on the surface. The scan velocities were between 200 and 800 pm/s. Typical sheet resistance of the metal film was 200 fi/cm2. Samples were mounted in the SEM by grounding the metal film with graphite paste or were isoa)Present address: lated from ground using double-sided scotch tape between the sample and the metal holder. Domain inversion was revealed by etching the LiNbOs sample in a solution of two p...

This paper reports the results of structural analysis of proton-exchanged lithium niobate optical waveguides fabricated in Z-, X-, and Y-cut substrates immersed in pure benzoic acid. Rutherford backscattering spectrometry, nuclear reactions, secondary ion mass spectrometry, scanning electron microscopy, and x-ray diffraction were used to measure atomic composition profiles and the marked lattice distortion induced by the proton exchange process in the waveguiding layer. H and Li concentration measurements indicate an exchange of about 70% of the Li atoms are present in the virgin LiNbO3 crystal.

When Y-cut LiNbO3 substrates are proton exchanged in pure benzoic acid to fabricate optical waveguides, they suffer surface damage, and a consequent degradation in optical properties. This effect is mainly produced by a remarkably large strain in the exchanged layer in a direction normal to the surface. This strain leads to a large number of cracks and to the peeling off of the exchanged layer itself. This paper presents a probable explanation of the mechanism involved.

We report the results of a systematic study on planar optical waveguides fabricated in z-cut LiNbO3 by proton exchange in benzoic acid. It was found that the refractive index varied with depth and could be accurately modeled by a step index profile with Δn=0.126. Diffusion coefficients have been calculated from mode effective refractive index measurements, assuming a step index profile, and hence a value for the activation energy for the proton exchange process has been deduced. The lowest measured optical propagation loss in single-mode waveguide at a 633-nm wavelength was 2.4 dB/cm.

Domain inversion in c-cut KTiOPO4 was produced by scanning an electron beam on the −c face. The domain reversal occurred through the 1 mm thickness of the sample. Second-harmonic conversion efficiency of 7×10−5 was measured for a fifth-order grating by focusing the beam to a 7 μm spot in a 500-μm-long domain-inverted KTP crystal. This efficiency is close to the theoretical value of 9×10−5. The measured phase matching bandwidth was 1.9 nm, which is in agreement with the theoretical value of 1.5 nm indicating that the domain-inverted grating is reasonably uniform over its entire length.