Mark Gerrit Roelofs

Detection of ferroelectric domain reversal in KTiOPO4 waveguides

Segmented optical waveguides in KTiOPO4 are fabricated by ion exchange of Rb/Ba for K using molten nitrate salts. The waveguides have been utilized to generate efficient second‐harmonic generation (SHG) of blue light. Four techniques are described which demonstrate that the ion exchange can produce domain reversal in these waveguides, and quasi‐phase matching is utilized to obtain blue light. The experiments involve surface SHG, electrostatic toning, selective etching, and piezoelectric measurements.

Identification of Ti3+ in potassium titanyl phosphate and its possible role in laser damage

Four related Ti3+ centers in potassium titanyl phosphate (KTiOPO4 or KTP) may be created with either application of dc electric fields or with hydrogen at 800 °C. Single‐crystal electron paramagnetic resonance allows determination of the g tensors of the four centers. Hyperfine coupling is observed with one47,49 Ti and two (nearly) equivalent 31P. The most abundant center is characterized by gx=1.7709(5), gy=1.8723(5), gz=1.9462(5), principal values of ‖ATi‖ of 80, 33, and 24 MHz, and axially symmetric AP’s with principal values of 16.3 and 21.8 MHz. Optical absorption centered at 13 000–18 000 cm−1 is quite broad, relatively strong, and approximately twice as intense for light whose electric vector is polarized parallel to c as it is for a or b. A charge transfer mechanism in addition to d−d transitions may account for the absorption. Ti3+ is much more difficult to produce in hydrothermally grown KTP than in flux‐grown KTP, a difference ascribed to the presence of oxygen vacancies or interstitial potassi...

Ion exchange of Rb, Ba, and Sr in KTiOPO4

The potassium ions of potassium titanyl phosphate (KTiOPO4, or KTP) are relatively mobile and may be exchanged with other ions from molten salts. Distribution coefficients for potassium, rubidium, barium, and strontium between nitrate melts and titanyl phosphate were measured between 350 and 425 °C. At 425 °C, using melts containing K and a dopant chosen from Rb, Ba, Sr, and with dopant cation mole fractions in the melt of 0.1, Rb is incorporated in the crystal at a site fraction of 0.014, Ba at 0.007, and Sr at 0.002. Introducing 0.008 fraction of Ba increases the low‐frequency dielectric constant and loss of KTP several orders of magnitude, giving conductivities (ωe0e‘) of 4×10−5 Ω−1 cm−1 at 200 kHz. The results support a potassium vacancy mechanism for ionic conductivity in KTP. Using low concentrations of divalent ions in the melt, flux‐grown KTP has a lower uptake of divalent ions than does hydrothermally grown KTP, an effect attributed to the presence of potassium vacancies created in the high‐tempe...

Characterization of optical waveguides in KTiOPO4 by second harmonic spectroscopy

Segmented optical waveguides were fabricated in KTiOPO4 by Rb/Ba ion exchange. The waveguides were characterized by measuring, as a function of input optical wavelength, the Bragg‐reflected power, and the intensity of second harmonic (SH) radiation generated within the waveguides. Seven peaks in the SH spectra are assigned to different interactions involving the two lowest‐order spatial modes at the fundamental wavelength and the three lowest‐order spatial modes at the SH wavelength. ‘‘Combined mode’’ SH interactions are found which sum spectrally degenerate fundamental waves which are spatially nondegenerate. Analysis of the spectra is shown to be a convenient method of determining the effective indices of the propagating modes, and these then determine the refractive index depth profile of the waveguide. The relative intensities of the SH peaks are dependent on the depth of the region within the ion‐exchanged area which has a reversal of the ferroelectric polarization direction (reversed domain). In som...

Proton- and ammonium-exchanged optical waveguides in KTiOPO4

The potassium of KTiOPO4 has been exchanged for protons or ammonium ions to form optical waveguides. Proton exchange is relatively fast, does not give depth profiles expected for diffusive processes, and distinct potassium‐depleted phases are formed. The kinetics for ammonium exchange are relatively slow and diffusion limited, similar to those for Rb and Tl exchange processes. Increases in surface refractive index of up to 0.053 for proton exchange and 0.046 for ammonium exchange have been observed, with higher values for mixed NH4/H and Rb/H exchanges.

KTiOPO4 (KTP): A New Material for Optical Waveguide Applications

KTP is a unique nonlinear optical material that is widely used for second harmonic generation of Nd:YAG. It also has high electrooptic coefficients and low dielectric constants that together with its high optical damage thresholds and high thermal stability make KTP superior for integrated optical applications as well.

Ionic conductivity and damage mechanisms in KTiOPO4 crystals

The ionic conductivity and damage susceptibilities of KTP crystals are related to the defects present in the crystals, which result from the conditions of growth by the flux, high, and low temperature hydrothermal techniques. The effects of Ba impurities on the ionic conductivity and damage are also discussed.

Proton Effects in KTiOPO 4

Evidence supporting the temperature dependent defect mechanism of nonstoichiometry on the potassium and oxygen sublattices in KTP is presented. The primary compensating defects for the formation of vacant potassium sites in typical flux grown KTP are vacant oxygen sites. Protons (OH - ) are the principal defect compensating for the formation of vacant potassium sites in high temperature hydrothermal KTP. A model of the ionic conductivity in high temperature hydrothermal KTP is proposed in which specific protons participate in cooperative motion over a limited distance with the potassium vacancies migrating along the “channels” in the structure in the Z-direction. The higher activation energy measured for ionic conductivity in flux grown KTP (0.5 eV) relative to high temperature hydrothermal (0.3 eV) is suggested to be due to the energy required to dissociate from a defect complex, such as a (VO - VK). The correlation of ionic conductivity to damage susceptibility appears to be due to the levels of compensating defects for vacant potassium sites in KTP, which are related to the concentrations of Ti 3+ formed in the crystals. Further study is ongoing to understand the specific mechanisms involved in the ionic conductivity and damage in KTP grown by the flux and hydrothermal techniques.

Gray-track damage in KTiOPO4 crystals

A correlation is observed between the laser and electric-field damage susceptibilities of most KTiOPO4 (KTP) crystals. The observed gray track damage susceptibilities are found to be correlated to the concentrations of defects in the crystals which can stabilize the Ti3+ damage defect sites in KTP. These stabilizing defects are those which can have an effective positive charge, such as oxygen vacancies, protons, and Ba and F impurities. The defects present in KTP crystals depend on the technique (hydrothermal or flux) used for growth. Post-growth processing techniques have been developed which reduce the laser gray track and electric-field damage susceptibilities of both hydrothermal and flux KTP.

Integrated Manufacturing for Advanced MEAs

This program addressed a two-pronged goal for developing fuel cell components: lowering of precious metal content in membrane electrode assemblies (MEAs), thereby reducing the fuel cell cost, and creating MEAs that can operate at 120oC and 25% RH whereby the system efficiency and effectiveness is greatly improved. In completing this program, we have demonstrated a significant reduction in precious metal while at the same time increasing the power output (achieved 2005 goal of 0.6g/Kw). We have also identified a technology that allows for one step fabrication of MEAs and appears to be a feasible path toward achieving DOE’s 2010 targets for precious metal and power (approaches 0.2g/Kw). Our team partner Du Pont invented a new class of polymer electrolyte membrane that has sufficient stability and conductivity to demonstrate feasibility for operation at 120 oC and low relative humidity. Through the course of this project, the public has benefited greatly from numerous presentations and publications on the technical understanding necessary to achieve these goals.