Jussi Hiltunen

Nitrogen-Doped Anatase Nanofibers Decorated with Noble Metal Nanoparticles for Photocatalytic Production of Hydrogen

We report the synthesis of N-doped TiO(2) nanofibers and high photocatalytic efficiency in generating hydrogen from ethanol-water mixtures under UV-A and UV-B irradiation. Titanate nanofibers synthesized by hydrothermal method are annealed in air and/or ammonia to achieve N-doped anatase fibers. Depending on the synthesis route, either interstitial N atoms or new N-Ti bonds appear in the lattice, resulting in slight lattice expansion as shown by XPS and HR-TEM analysis, respectively. These nanofibers were then used as support for Pd and Pt nanoparticles deposited with wet impregnation followed by calcination and reduction. In the hydrogen generation tests, the N-doped samples were clearly outperforming their undoped counterparts, showing remarkable efficiency not only under UV-B but also with UV-A illumination. When 100 mg of catalyst (N-doped TiO(2) nanofiber decorated with Pt nanoparticles) was applied to 1 L of water-ethanol mixture, the H(2) evolution rates were as high as 700 μmol/h (UV-A) and 2250 μmol/h (UV-B) corresponding to photo energy conversion percentages of ∼3.6 and ∼12.3%, respectively.

BaTiO3–SrTiO3 multilayer thin film electro-optic waveguide modulator

Mach-Zehnder electro-optic waveguide modulators were fabricated based on BaTiO3 (BTO)-SrTiO3 (STO) multilayer thin film stacks grown on single crystal MgO substrates by pulsed laser deposition. X-ray diffraction measurements confirmed the formation of a BTO-STO superlattice with periodicity of 11unit cells. Strip-loaded waveguides were formed by patterning a SixNy film deposited onto the BTO-STO stack while Al electrodes of 3mm length and 13μm separation were fabricated in the vicinity of the active waveguide arm of the Mach-Zehnder modulator. An effective electro-optic coefficient of 73pm∕V at 1550nm wavelength was determined for the deposited BTO-STO superlattice by measuring the output intensity as a function of applied electric field.

Highly sensitive biosensor based on UV-imprinted layered polymeric–inorganic composite waveguides

An evanescent field sensor utilizing layered polymeric-inorganic composite waveguide configuration was developed in this work. The composite waveguide structure consists of a UV-imprint patterned polymer inverted rib waveguide with a Ta2O5 thin film sputter-deposited on top of the low refractive index polymer layers. The results suggest that the polymer based sensor can achieve a detection limit of 3 × 10(-7) RIU for refractive index sensing and corresponding limit of about 100 fg/mm2 for molecular adsorption detection. Besides enhancing the sensitivity significantly, the inorganic coating on the polymer layer was found to block water absorption effectively into the waveguide resulting in a stabilized sensor operation. The ability to use the developed sensor in specific molecular detection was confirmed by investigating antibody - antigen binding reactions. The results of this work demonstrate that high performance sensing capability can be obtained with the developed composite waveguide sensor.

A Variable Delay Optical Buffer Based on Nonlinear Polarization Rotation in Semiconductor Optical Amplifier

In this letter, a variable delay optical buffer with three stage-cascaded buffer units based on nonlinear polarization rotation in semiconductor optical amplifier is demonstrated. The buffer is suitable for integration, fast reconfiguration, scalable, and easy control. With the proposed buffers, 1024-bit data are buffered more than 100 different buffering times at 2.5 Gb/s.

Polymeric dual-slab waveguide interferometer for biochemical sensing applications.

A polymer based dual-slab waveguide Youngs interferometer was demonstrated for biochemical sensing. Evanescent field is utilized for probing the binding events of biomolecules on the waveguide surface. Refractive index sensing in analyte and protein adsorption on the sensing surface were investigated with glucose de-ionized water solution and bovine serum albumin, immunoglobulin G solutions in phosphate buffered saline buffer. A detection limit of 10(-5) RIU and 4 pg/mm(2) was achieved for homogeneous and surface sensing, respectively. Also, the influence of water absorption inside the polymeric device on the measurement stability was evaluated. The results indicate that the waveguide polymer sensor fabricated with the spin coating technique can achieve a satisfactory sensitivity for homogeneous refractive index sensing and, as well, for monitoring molecular binding events on the surface.

X-ray diffraction and Raman investigations of thickness dependent stress effects on Pb(ZrxTi1−x)O3 thin films

Microstructure, film orientation, and optical transmission spectra of polycrystalline Nd-modified Pb(ZrxTi1−x)O3 films were studied as a function of film thickness. Pulsed laser deposition was used for the fabrication of films with thickness from 80to465nm on single-crystal MgO(100) substrates. Raman spectroscopy, x-ray diffraction, and spectrophotometry measurements were utilized in the film characterization. With the decreasing film thickness, films first oriented with c axis perpendicular to film surface, and then, after some critical thickness, changed to a-axis orientation. At the same time, compressive stress increased up to 1.3GPa and a clear blueshift of the optical absorption edge was found in transmission spectra.

Single-mode tuning of a 1540-nm diode laser using a Fabry-Pe/spl acute/rot interferometer

We realized a wavelength-tunable laser using a 1540-nm Fabry-Pe/spl acute/rot diode laser and a silicon surface micromachined Fabry-Pe/spl acute/rot interferometer device in the short external cavity configuration. This hybrid-integrated system enables the use of standard laser chips and potentially has a low cost. We obtained single-mode tuning of 13 nm, a sidemode suppression ratio of better than 25 dB, and an average single-mode fiber-coupled power of 100 /spl mu/W. The emitter can be employed in optical communication and fiber-optic sensor applications.

Atmospheric oxidation and carbon contamination of silver and its effect on surface-enhanced Raman spectroscopy (SERS).

Surface-enhanced Raman spectroscopy (SERS) is considered a highly promising technology for different analytical purposes. The applications of SERS are still quite limited due its relatively poor quantitative repeatability and the fact that SERS is very sensitive to oxidation, which is a challenge especially with silver based SERS substrates. Here, the link between these phenomena is investigated by exposing silver SERS substrates to ambient laboratory air. We show that SERS intensity decreases exponentially after the exposure, which consequently leads to an increasing standard deviation (σ) in intensity. Within a five-hour measurement window, the SERS intensity already drops by 60%, while σ triples from 7% to 21%. The SERS results are supplemented by elemental analysis, which shows that oxidation and atmospheric carbon contamination coincide with the rapid SERS intensity decrease. The results emphasize how sensitive SERS is towards atmospheric contamination and how it can also reduce the measurement repeatability – even if the substrates are exposed to air just for a very short period of time.

Low-Loss Multiple-Slot Waveguides Fabricated by Optical Lithography and Atomic Layer Deposition

We demonstrate silicon-based multiple-slot waveguides filled with dual atomic layer deposited oxide layers. Slot modes for both polarizations, transverse electric (TE) and transverse magnetic (TM), are supported in the waveguide. Propagation loss in the order of 8 dB/cm is achieved for the TE-polarization and 4 dB/cm for the TM-polarization in the waveguides with dual (Al2O3-TiO2) thin film layers. The devices are fabricated using low-temperature complementary metal-oxide-semiconductor compatible processes. To our knowledge, this is the first demonstration of dual-filled slot waveguides.

Nanoimprint Fabrication of Slot Waveguides

A nanoimprint mold for optical waveguide applications was fabricated by combining photolithography and focused ion beam (FIB) milling. The feasibility of the proposed method was demonstrated by imprinting 15-mm-long Y-branch waveguides, which had nanoscale slots embedded in one arm. Structural analysis of the FIB milled region showed surface roughness values below 2.5 nm. Characterization of the fabricated waveguides proved that 44% of the optical power was transmitted through the slot-embedded waveguide arm. Operation of slot waveguide was demonstrated at a wavelength of 1305 nm using Young interferometer devices.