Laser-assisted atom probe tomography of Ti/TiN films deposited on Si.

Laser-assisted atom probe tomography (L-APT) was used to examine superconducting TiN/Ti/TiN trilayer films with nominal respective thicknesses of 5/5/5 (nm). Such materials are of interest for applications that require large arrays of microwave kinetic inductance detectors. The trilayers were deposited on Si substrates by reactive sputtering. Electron energy loss microscopy performed in a scanning transmission electron microscope (STEM/EELS) was used to corroborate the L-APT results and establish the overall thicknesses of the trilayers. Three separate batches were studied where the first (bottom) TiN layer was deposited at 500°C (for all batches) and the subsequent TiN/Ti bilayer was deposited at ambient temperature, 250°C, and 500°C, respectively. L-APT rendered an approximately planar TiN/Si interface by making use of plausible mass-spectral assignments to N31+, SiN1+, and SiO1+. This was necessary since ambiguities associated with the likely simultaneous occurrence of Si1+ and N21+ prevented their use in rendering the TiN/Si interface upon reconstruction. The non-superconducting Ti2N phase was also revealed by L-APT. Neither L-APT nor STEM/EELS rendered sharp Ti/TiN interfaces and the contrast between these layers diminished with increased film deposition temperature. L-APT also revealed that hydrogen was present in varying degrees in all samples including control samples that were composed of single layers of Ti or TiN.

A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics.

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured across a sample over a frequency range of 1 GHz - 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S(11) minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se(2) (CIGS) sample. The S(11) and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires.

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (µ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.

Ion-exchanged waveguide lasers in Er3+/Yb3+ codoped silicate glass.

We investigated an Er(3+)/Yb(3+) codoped silicate glass as a host material for waveguide lasers operating near 1.5 microm. Spectroscopic properties of the glass are reported. Waveguide lasers were fabricated by K(+)-ion exchange from a nitrate melt. The waveguides support a single transverse mode at 1.5 microm. An investigation of the laser performance as a function of the Yb:Er ratio was performed, indicating an optimal ratio of approximately 5:1. Slope efficiencies of as great as 6.5% and output powers as high as 19.6 mW at 1.54 microm were realized. The experimental results are compared with a waveguide laser model that is used to extract the Er(3+) upconversion coefficients and the Yb(3+)-Er(3+) cross-relaxation coefficients. The results indicate the possibility of obtaining high-performance waveguide lasers from a durable silicate host glass.

Nd:LiTaO(3) waveguide laser.

Waveguide lasers operating near 1092 and 1076 nm were fabricated in Z-cut Nd-Ti codiffused LiTaO(3). The Nd diffusion was at 14000 degrees C for 120 h. Samples from two wafers were examined. The Nd film starting thickness was 7 nm in wafer 1 and 15 nm in wafer 2. Ti stripes, 8-15 microm wide, were diffused at 1500 degrees C for 4 h for wafer 1 (130-nm stripe thickness) and 2 h forwafer 2 (100-nm stripe thickness). Pumping was at 750 nm. Threshold occurred at 330 mW of absorbedpump power for the best waveguides from wafer 1 and100 mW for the best waveguides from wafer 2. The slope efficiency of the latter was 0.07%.

Extended-cavity operation of rare-earth-doped glass waveguide lasers.

Channel waveguides fabricated in Nd-doped glass were used as gain elements for extended-cavity lasers. End-fire pumping was performed with a Ti:sapphire laser operating at 807 nm. The 4-nm FWHM output spectrum was centered near 1057 nm. Slope efficiencies were typically 4-11%, with thresholds near 20 mW. Active mode locking and Q switching were separately performed; mode-locked pulse widths were roughly 80 ps FWHM. Q-switched peak power was 1.2 W. The cw output narrowed to 7 GHz and tuned over a range of 24 nm when a grating provided feedback; single-frequency operation resulted when a high-reflectivity étalon was added.

Y-branch waveguide glass laser and amplifier.

A Y-branch channel waveguide laser operating near 1057 nm was fabricated by electric-field-assisted ion exchange in Nd-doped silicate glass. The overall length was 24 mm. Optical pumping was performed with a cw Ti:sapphire laser. Mirrors were bonded to the polished waveguide facets. The slope efficiency was 5.1% when a 4%-transmitting output coupler was used. Threshold was reached at 26-mW absorbed pump power. The device exhibited a single-pass small-signal gain of 0.034 dB/mW when operated as an amplifier. The 3-dB splitting loss of the Y-branch structure was overcome when the absorbed pump power was approximately 85 mW.

Integrated-optic laser fabricated by field-assisted ion exchange in neodymium-doped soda-lime-silicate glass.

A continuous-wave integrated-optic channel waveguide laser operating at 1.057 microm has been fabricated in neodymium-doped soda-lime-silicate laser glass. The device was end-fire pumped with the 0.528-microm line of an argon-ion laser. Threshold for laser action occurs for an absorbed pump power of 31 mW. The slope efficiency for the integrated-optic laser is estimated to be 0.5%. Field-assisted ion exchange in a eutectic melt of CaNO(3) and KNO(3) was used to form the waveguide.

Direct measurement of effective indices of guided modes in LiNbO(3) waveguides using the Cerenkov second harmonic.

Second-harmonic generation in the form of Cerenkov radiation has been used to make direct, contact-free measurements of the effective indices of guided modes in LiNbO(3) channel waveguides. X-cut substrates were used, and channels oriented along the y axis were formed by either proton exchange or titanium indiffusion. For end-fire TE excitation, the Cerenkov harmonic radiates forward at a shallow angle into the substrate, leaves the output facet of the substrate, and appears as bright bands in the far field. These bands are seen to form a one-to-one correspondence with the m lines that are due to guided modes when a prism coupler is simultaneously clamped to the sample surface. The effective indices of the guided modes can be obtained by a simple calculation that involves only the substrate index and the angle at which the Cerenkov harmonic is radiated into the substrate. This technique permits effective index measurements of cladded waveguides where prism coupler measurements cannot be made. Examples of the technique in which index oil is used as a guide overlayer are given.

Direct-current bias stable Ti:LiNbO(3) TE-TM mode converters produced by magnesium postdiffusion.

Depression of the surface index of Ti:LiNbO(3) channel waveguides by secondary diffusion of MgO forms a buffer layer that optically isolates the guided modes from attenuation caused by electrodes placed over the guides. For x-cut, z-propagating mode-conversion devices, we find high stability to optical drift when a dc voltage as high as 60 V is applied to electrodes (typical gap of 5 microm) to establish bias operating points. Photoconductivity is found to limit the long-term stability. Devices operating at 0.633-microm wavelength generally suffer significant drift in their mode-conversion efficiency when 100 microW or more of throughput power is present and a bias of 14 V is used. 10-microW throughput at the same wavelength is found to be stable over an 8-h period. At 0.8-microm wavelength, bias voltages as high as 60 V may be used for stable operation even at 100-microW optical throughput.

Secondary-ion mass spectroscopy characterization of proton-exchanged LiNbO(3) waveguides.

Channel waveguides fabricated in LiNbO(3) substrates by proton exchange in melts of benzoic acid have been analyzed by secondary-ion mass spectroscopy. The depletion of lithium from the waveguide regions, owing to proton exchange, was found to depend to a strong degree on guide width as well as diffusion time for common (249 degrees C) ion-exchange temperatures. In addition to a reduced depth, a guide formed by diffusing through a 6-microm-wide metallic aperture for 10 min suffered 60% less lithium depletion near its surface than one formed by diffusing through a 12-microm aperture for the same time. Optical measurements were performed at 1060 and 1320 nm in guides of width ranging from 5 to 10 microm.

Optical waveguides in LiTaO(3) formed by proton exchange.

We describe the fabrication of optical waveguides in LiTaO(3) using proton exchange in hot benzoic acid. All specimens analyzed were X cut. An increase in surface extraordinary index of ~0.14 has been observed for samples exchanged for 3 h at a melt temperature of 249 degrees C. No change was observed in the ordinary index. Waveguide fabrication was facilitated by further baking the samples for an extended period of time at a temperature hotter than the acid melt. The latter technique also served to minimize the surface damage that is characteristic of acid baths exceeding roughly 4 h. The depth of proton exchange in X-cut LiTaO(3) appears to be considerably less than would be the case for X-cut LiNbO(3) under the same conditions.