Broad-band distributed Bragg reflectors with photorefractive Ti : Cu : LiNbO/sub 3/ waveguide gratings

We have realized broad-band distributed Bragg reflectors with photorefractive gratings recorded at 441.6 nm in channel Ti : Cu : LiNbO/sub 3/ waveguides. Proton-assisted copper exchange is used to enable a high level of copper doping and, thereby, achieve an extremely large modulation of refractive index (/spl ges/ 5/sup */10/sup -4/) within a photorefractive grating. Experimental structures demonstrate reflectivities up to 17% with full-width at half-maximum bandwidths in excess of 1.2 nm at center wavelengths around 1.55 /spl mu/m.     

Double ion exchange for high-index waveguides in LiTaO/sub 3/

The authors describe the fabrication and properties of optical waveguides formed in Y-cut LiTaO/sub 3/ by simultaneous processing of zinc-lithium, proton-lithium and zinc-proton exchange in the melts K/sub 2/SO/sub 4/-Na/sub 2/SO/sub 4/-ZnSO/sub 4/-LiSO/sub 4/-KHSO/sub 4/ at 540 degrees C. A surface increase of the refractive index of 0.14 was observed for the extraordinary polarisation and 0.11 for the ordinary polarisation. The dynamics of the process is discussed.<>     

Characterization of alpha-phase soft proton-exchanged LiNbO3 optical waveguides

Waveguides in LiNbO3 are realized by a soft proton exchange (SPE) process with use of a melt of stearic acid highly diluted by lithium stearate. No phase transitions are formed when alpha-phase waveguides are obtained by SPE. The alpha-phase presents the same crystalline structure as that of pure LiNbO3 crystal, and it maintains the excellent nonlinear and electro-optical properties of the bulk material. The kinetics of the SPE method is studied by the use of secondary-ion mass spectrometry and prism-coupling techniques. The hydrogen effective diffusion coefficient as well as the self-diffusion coefficients of H+ and Li+ ions are determined as a function of the proton-exchange temperature for X-cut LiNbO3.     

Simulation of the real efficiencies of high-efficiency silicon solar cells

The temperature dependences of the efficiency η of high-efficiency solar cells based on silicon are calculated. It is shown that the temperature coefficient of decreasing η with increasing temperature decreases as the surface recombination rate decreases. The photoconversion efficiency of high-efficiency silicon-based solar cells operating under natural (field) conditions is simulated. Their operating temperature is determined self-consistently by simultaneously solving the photocurrent, photovoltage, and energy-balance equations. Radiative and convective cooling mechanisms are taken into account. It is shown that the operating temperature of solar cells is higher than the ambient temperature even at very high convection coefficients (~300 W/m² K). Accordingly, the photoconversion efficiency in this case is lower than when the temperature of the solar cells is equal to the ambient temperature. The calculated dependences for the open-circuit voltage and the photoconversion efficiency of high-quality silicon solar cells under concentrated illumination are discussed taking into account the actual temperature of the solar cells.     

The temperature dependence of the characteristics of crystalline-silicon-based heterojunction solar cells

Technical Physics Letters - Temperature dependences of the photovoltaic characteristics of (p)a-Si/(i)a-Si:H/(n)c-Si singlecrystalline- silicon based heterojunction-with-intrinsic-thin-layer (HIT)...     

Structure and properties of optical waveguides in stoichiometric LiNbO<Subscript>3</Subscript> crystals

The structures of proton-exchanged and implanted waveguides are studied by X diffraction analysis and vibration spectroscopy. On the basis of the absorption’s spectroscopy data in the visible region and the data on the shift of the fundamental absorption edge in the nearultraviolet region, a comparative analysis of technologically conditioned variations in the nonlinear optical properties of proton-exchanged and implanted waveguides in stoichiometric LiNbO₃ crystals is conducted.     

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.     

LiNbO3 optical waveguide fabrication by high-temperatureproton exchange

An efficient method, the high-temperature proton exchange (HTPE), to fabricate high-quality LiNbO₃ optical waveguides is studied. The new proton exchange source, the stearic acid diluted by lithium stearate, is proposed for HTPE process. The known soft proton exchange (SPE) process can be realized by HTPE. There are no phase transitions when the α-phase waveguides are fabricated by SPE. This phase presents the same crystalline structure as that of LiNbO₃ and, as expected, maintains the excellent nonlinear and electrooptical properties of the bulk material. The kinetics of HTPE is studied     

Structural phase diagram of HxLi1-xNbO3 waveguides: The correlation between optical and structuralproperties

We show that proton exchanged H_xLi_1-xNbO₃ single-crystalline solid solutions exhibit very complex structural chemistry, which is different from those known for powders. Seven crystallographic phases have been identified in H_xLi_1-xNbO₃ layers. Correlation between the crystal structure and the ordinary and extraordinary refractive indices has been experimentally determined which allows us to explain some of the observed optical phenomena and to predict the characteristics of the great variety of proton exchanged waveguides