Proton-exchanged Layer Research Articles

Improvement of photorefractive properties of proton-exchanged LiTaO 3 waveguides

are fabricated by both a proton exchange and a successive copper exchange. The influence of different fabrication steps on the refractive index profile and optical absorption is investigated. The holographic efficiency is determined together with the dark- and photo-conductivity. We show that the photorefractive properties of proton-exchanged LiTaO3 waveguides are considerably improved by the additional copper exchange. It is established that the holographic efficiency depends on the copper content, the phase of the proton-exchanged layer and the light intensity.

Relationships between structural and optical properties of proton-exchanged waveguides on Z-cut lithium niobate

By using x-ray diffraction and optical waveguide characterization, it has been shown that proton-exchanged layers on Z-cut LiNbO(3) can have as many as seven different crystallographic phases that are stable at room temperature. This study allows us to establish some correlation between fabrication parameters and the crystalline structure, the index profiles, and the propagation losses of waveguides prepared by this process.

Quantitative characterization of proton-exchanged layers in LiTaO3 optoelectronic devices by line-focus-beam acoustic microscopy

Application of line-focus-beam (LFB) acoustic microscopy is extended to quantitative characterization of proton-exchanged/annealed layers employed in LiTaO/sub 3/ optical waveguides. Several specimens of Z-cut LiTaO/sub 3/ substrates, processed under the fabrication conditions for second-harmonic generation (SHG) optoelectronic devices, were prepared for measurements of the leaky surface acoustic wave (LSAW) velocities. Remarkable decreases in LSAW velocity due to the processes of proton exchange and annealing were observed, providing very useful information on the proton concentration and depth in diffusion layer, and on the process temperature distribution. It is found that measurement sensitivity is highest in the Y-axis wave propagation direction and the resolution to the optical waveguide parameters of diffusion depth and refractive index is much greater than the conventional techniques. It is suggested that this ultrasonic method should be adopted as a new analytical technique for development and evaluation of device fabrication processes and systems destined for future mass production.

Proton-exchanged Z-cut LiNbO/sub 3/ waveguides for surface acoustic waves

Proton-exchanged (PE) waveguides in Z-cut LiNbO/sub 3/ have been fabricated using benzoic acid. Secondary-ion mass spectrometry (SIMS) measurements show that the distribution of hydrogen in the PE Z-cut LiNbO/sub 3/ samples exhibits a step-like profile with the diffusion constant D/sub 0/ and the activation energy Q of about 2.82/spl times/10/sup 8/ /spl mu/m/sup 2//h and 87.76 kJ/mol, respectively. On the other hand, the important parameters for the design of surface acoustic wave (SAW) devices are measured and discussed. The results show that the phase velocity and electromechanical coupling coefficient decrease with the increase of kd, where k is the wavenumber and d is the waveguide depth. The variation of insertion loss becomes saturated at about kd=0.068 with a maximum increase of about 4/spl sim/5 dB. The temperature coefficient of delay calculated from the frequency change of the output of SAW delay line shows an evident increase in the PE layer. Moreover, the effects of postannealing can result in a restoration of the decreased velocity and an improvement of the insertion loss.

Photo-Elastic Constants of Proton-Exchanged LiNbO 3

The acousto optic interaction on proton-exchanged 128°-rotated Y-cut LiNbO3 was investigated to evaluate the photo-elastic constants. The experimental and theoretical analysis using a surface acoustic wave of 200 MHz and a He-Ne laser beam showed that the acoustic power required for 100% diffraction depends on the proton-exchange depth and that the photo-elastic constants of the proton-exchanged layer are 76% of those in unexchanged LiNbO3. The effects of annealing on the photo-elastic constants and the diffraction efficiency were also measured.

Raman study of planar optical waveguides in LiNbO3 and LiTaO3

Proton-exchange (PE) technology has been used to form planar optical waveguides in LiNbO3 and LiTaO3 single crystals. Studying post-exchange annealed (APE) waveguides, the evolution of guide properties with decreasing the proton concentration is followed. Raman spectroscopy is used to investigate the vibrational characteristics of PE and APE layers. The spectra of PE layers show a high degree of disorder and a “paraelectric-like” form.

Proton exchanged depth determination in LiNbO3 from laser induced pyroelectric voltage measurements

We report a new nondestructive technique for the determination of depth of proton exchanged (PE) layers in LiNbO3. Relying on the difference in the magnitudes of measured pyroelectric voltages, induced by an intensity modulated laser beam, on the exchanged and unexchanged regions, the depth of the PE layer has been measured and corroborated by a conventional optical characterization method.

Surface acoustic wave distribution and acousto-optic interaction in proton exchanged LiNbO3 waveguides

The efficiency of acoustooptic (AO) interaction in YZ-cut proton exchanged (PE) LiNbO3 waveguides is theoretically analysed by determining the overlap between the optical and acoustic field distributions. The present analysis takes into account the perturbed SAW field distribution due to the presence of the PE layer on the LiNbO3 substrate determined by the rigorous layered medium approach. The overlap is found to be significant upto very high acoustic frequencies of the order of 5 GHz, whereas in the earlier analysis by vonHelmolt and Schaffer [6] for diffused waveguides, it was shown that the overlap integral rolls down to nearly zero at this high frequency range.

Analysis of hydrogen diffusion from proton-exchanged layers in LiNbO3

Hydrogen profiles for proton-exchanged layers in Z-cut LiNbO3 were obtained by secondary ion mass spectrometry measurements after annealing at 400 °C for times from 6 to 180 min. To fit the measured hydrogen profiles, it was necessary to use two separate Gaussian diffusion profiles which could be designated as a more slowly diffusing substitutional hydrogen on lithium sites and a more rapidly diffusing interstitial hydrogen. At 400 °C, analysis leads to a substitutional hydrogen diffusion coefficient Ds≊4.6×10−12 cm2/s and an interstitial hydrogen diffusion coefficient Di≊1.5×10−11 cm2/s.

Surface Acoustic Wave Properties on Proton-Exchanged 128°-Rotated Y-Cut LiNbO3

The characteristics of surface acoustic waves on proton-exchanged 128°-rotated Y-cut LiNbO3 were investigated at a frequency of 100 MHz. The changes of the phase velocity and the electromechanical coupling coefficient as a function of the proton-exchange depth were measured. From these experiments, the elastic constants and the piezoelectric constants of the proton-exchanged layer were determined. The effects of annealing on the phase velocity and the surface acoustic wave propagation loss were also measured.

Strongly reduced optical nonlinearity in lithium tantalate due to proton exchange

Reflection SHG measurements with fundamental wavelength 532 nm have been carried out on proton-exchanged LiTaO 3 waveguides. The SH signal decreased exponentially with waveguide thickness. The result is interpreted as a strongly reduced nonlinearity in the proton-exchanged layer. An increased signal was observed from annealed samples

Determination of depth of proton exchanged layer in LiNbO 3 from laser induced pyroelectric signals

A new nondestructive technique for the determination of the depth of the proton exchanged (PE) layer in LiNbO3 is reported. The techniques relies on the difference in phase of measured pyroelectric voltages, induced by a modulated laser beam incident on the exchanged and unexchanged regions. Measurements of PE layer thickness in a Z-cut LiNbO3 sample have been corroborated by prevalent, more elaborate, optical characterisation techniques.

Synchrotron radiation topographic study of Ti-diffused and proton-exchanged LiNbO3 single crystals

Titanium-diffused and proton-exchanged (PE) LiNbO3 single crystals of congruent composition were investigated by topographic methods in reflection mode using synchrotron radiation. The white beam topographs obtained for z-cut Ti:LiNbO3 samples revealed a characteristic distribution of defects with increasing concentration of Ti atoms in the near-surface region. For crystals with doses above 3.0*1020 cm-3 of indiffused atoms the density of introduced defects was fairly high resulting in rather homogeneous images. For white beam topographs of z-cut LiNbO3 samples with PE regions two images arising from the PE layer and from the substrate are distinguished, indicating the different lattice parameter of both parts of the crystal.

The effects connected with near-surface proton exchange (PE) of lithium niobate crystals

Abstract The presence of a thin near-surface PE layer on LiNbO3 wafers formed through the incidental effect of water vapor has been found out. It was established that photoinduced surface destruction (PSD) af x−, y−cut LiNbO3:Fe was caused by proton diffusion in the space charge field of photostimulated electrons. The problem changing the physical properties of LiNbO3 crystals with the PE technique is discussed.

Recovery of second-order optical nonlinearity in annealed proton-exchanged LiNbO/sub x/

The second-order nonlinear coefficients measured in proton-exchanged LiNbO/sub 3/ as a function of annealing time are discussed. Measurements of reflected second-harmonic power indicate that the second-order nonlinear coefficient d/sub 33/ is reduced to 60% of the bulk value as a result of proton exchange in pure benzoic acid. It is also shown that annealing restores the d-coefficients to almost the original value of the virgin crystal. For example, recovery to approximately 90% of the bulk value was obtained for a sample with a 0.3- mu m-bulk proton-exchanged layer, annealed for 10 h at 350 degrees C.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of the increase in the shg coefficient of proton exchanged LiNbO 3 after annealing using a grating diffraction technique

The effect of limited levels of annealing on the second harmonic generation (SHG) coefficient (d/sub 33/) of proton-exchanged LiNbO/sub 3/ has been determined by measuring nonphasematched SHG in a diffraction grating structure. The effect of annealing is to partially restore the intrinsic nonlinearity of the proton-exchanged layer. >

Characterization of sulfuric acid proton-exchanged lithium niobate

A series of bulk proton-exchanged lithium niobate waveguides has been prepared by using concentrated sulfuric acid as the proton source and varying the time and temperature of the exchange process. The optical and physical properties of the proton-exchanged lithium niobate layer have been measured and found to be essentially identical to proton-exchanged layers obtained by other workers using the benzoic acid process. Sulfuric acid offers a number of advantages over benzoic acid as a proton source, including a wider temperature liquid range and a greater ease of handling in a clean-room environment.

Measurement of reduction in SHG coefficient of LINbO 3 by proton exchanging

The SHG coefficient of a proton-exchanged layer of LiNbO3, relative to the bulk value, has been measured for the first time by non-phase-matched SHG using a grating sample. It was found that the d33 coefficient is reduced to approximately 0.5 of the bulk value by proton exchanging in pure benzoic acid.

Crystal Structure of Proton-Exchanged Layers on Single Crystals of LiNbO3

The crystal structure of proton-exchanged layers on single-crystal LiNbO3 was analyzed using the X-ray precession method. It was found that the layers were of single crystals, and that the structures varied depending on the orientation of the LiNbO3 wafers: on the (014̄) wafers, an orthorhombic phase appeared, whereas a hexagonal phase was obtained on the Z-cut (001) wafers. This suggests that the exchange reaction proceeds topotactically: protons replace Li ions with small changes in the atomic framework of oxygen and niobium.