Characterization and application of a channel–planar composite waveguide

The paper presents complex waveguide structures which have been fabricated on a basis of the gradient waveguide on the top of which a uniform waveguide film of high refractive index is deposited. The gradient waveguides were fabricated using the ion exchange method, whereas the uniform film with the application of the sol-gel method. The application of such a waveguide structure in the system of difference interferometer makes it possible to obtain high sensitivities of the effective refractive index against the changes of sensor layer parameters, in particular when the interference of the basic mode TM₀ with the first order mode TM₁ is applied. The paper presents the theoretical analysis of complex waveguide structures, their fabrication technology and the results of experimental studies.

This year, the European Conference on Optical Chemical Sensors and Biosensors (Europt(r)ode) celebrated its 10th anniversary. The Europt(r)ode series was initiated by Otto S. Wolfbeis in 1992 and over the last two decades has realized his vision of providing researchers working on various types of optical chemical and biological sensors with a specialized conference to foster development in the field and to promote international collaboration. After the success of Europt(r)ode I in Graz in 1992, Europt(r)ode has continued as a biennial conference in Florence (chaired by A. M. Scheggi), Zurich (chaired by U. Spichiger-Keller), Munster (chaired by K. Camman), Lyon (chaired by P. R. Coulet), Manchester (chaired by R. Narayanaswamy), Madrid (chaired by G. Orellana), Tuebingen (chaired by G. Gauglitz) and Dublin (chaired by B. D. MacCraith and C. McDonagh). In 2010, Europt(r)ode X was held in Prague, 28–31 March, and attracted more than 250 participants from 27 countries. During the almost two decades of its existence, Europt(r)ode has become a major scientific meeting on optical chemical sensors and biosensors attracting scientists not only from Europe, but from across the globe. Because of the multidisciplinary nature of the field, Europt(r)ode has always brought together a diverse audience including scientists and engineers in physics, chemistry and biology. Their engaging discussions have stimulated interdisciplinary and international collaborations and helped to shape scientific schools from which several generations of PhD students have graduated. The scientific program of Europt(r)ode X comprised a total of 230 scientific presentations covering various aspects of research and development of optical chemical sensors and biosensors. These presentations included 5 plenary lectures, 10 invited lectures, 34 contributed oral presentations and 181 poster presentations. The plenary lectures were given by Maria C. Moreno Bondi (University Complutense, Spain), Andrew deMello (Imperial College, UK), Teri W. Odom (Northwestern University, USA), Michael J. Sailor (University of California, USA) and Otto S. Wolfbeis (University of Regensburg, Germany). The plenary and invited lectures were selected to cover traditional topics as well as emerging areas, such as optofluidics for chemical analysis, nanoparticles and nanostructures for optical biosensing, silicon photonics biosensors, photonic fibres for label-free biosensing, artificial receptors and polymers for label-free biosensing in complex media. The oral and poster presentations fell into six main thematic areas. The largest number of presentations (nearly 25%) were concerned with the topic Optical platforms and methods for sensing. This high percentage documents the pivotal role that optics plays today in optical chemical sensing and biosensing and a high number of new original developments realized in the last few years. Moreover, new optical solutions to sensing were also presented under the thematic area Multisensors and sensor arrays, which accounted for 8% of the presentations. Advances in the development of new receptors were less represented at Europt(r)ode X with less than 5% of presentations submitted for the thematic area Biological and synthetic receptors. Europt(r)ode X testified to the growing importance of nanophotonics and nanomaterials in optical sensing with nearly 17% of the presentations submitted to Published in the special issue Optical biochemical and chemical sensors (Europtrode X) with Guest Editor Jiři Homola.

The mode structure of a four-layer optical waveguide consisting of an ion-exchanged channel waveguide in glass coated with a highly refractive nanoscale dielectric film has been studied. It has been found that varying the geometric dimensions of the film changes the polarization states of the second and third waveguide modes: the TM0 mode is transformed into the TE1 mode, and the TE1 mode becomes the TM0 mode. Based on this, a new method of designing a waveguide polarization converter has been proposed and experimentally confirmed.

Titanium dioxide (TiO2) thin films were deposited on Si wafer and glass substrates by direct current (dc) magnetron sputtering, by which the photocatalytic phase, anatase, was obtained under the optimum deposition conditions of O2/(O2+Ar) flow ratios and substrate biases. TiO2 thin films doped with Sn or Nb were prepared on glass substrates by dc/rf (radio frequency) magnetron co-sputtering, in which dc and rf were utilized for Ti and Sn or Nb targets, respectively, and the samples were post-annealed at temperatures ranging from 473 to 773 K for 1 h in ambient air. Microstructure, surface morphology, chemistry, and optical properties of the un-doped, Sn-doped, and Nb-doped TiO2 films were characterized in details by glancing incidence X-ray diffraction (GIXD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS), differential thermal analysis/thermogravimetry (DTA/TG), atomic force microscopy (AFM), UV/visable spectrophotometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). In addition, the photocatalytic activity was evaluated by measurement of the degeneration of methylene blue under UV/visible irradiation. The analytical results of GIXD show that the anatase phase is the dominant phase at an O2/(O2+Ar) flow ratio of over 40 %, whereas the films exhibit the rutile phase dispersed in amorphous matrix at an O2/(O2+Ar) flow ratio of 20 %. Microstructure and optical properties of the TiO2 films are dependent closely upon the rf power applied to the substrates as a bias. The anatase phase with columnar structure was obtained for the films prepared at rf powers of 0 and 5 W; whereas the films exhibit a rutile phase with equi-axial crystals at rf powers of 15 and 25 W. The rf-bias-assisted TiO2 films have a smoother surface than that without rf bias, and the absorbance edges shift to long wavelengths. The TiO2 films with a rutile phase have a higher refractive index, whereas better photocatalytical properties were observed for the films having anatase phase. TiO2 films with various Sn or Nb doping contents were investigated on the microstructure, optical properties, and photocatalytic activities. All the as-deposited TiO2 films on glass substrate were amorphous. The concentration of Sn in rf powers of 100 and 200 W and Nb in rf powers of 200, 300, and 400 W were 6.65 and 16.4 at.% for Sn and 1.98, 3.56, and 4.9 at.% for Nb, respectively, by X-ray photoelectron spectroscopy. GIXD revealed a polycrystalline phase for the Nb-doped films post-annealed at temperature 523 K, in contrast to the un-doped one that has to be annealed at temperature 723 K, indicating that Nb dopant can enhance the crystallization of amorphous TiO2. Furthermore, the Nb-doped film post-annealed at 673 K was found to have an anatase-dominanted phase with a fine grain microstructure, while the Sn-doped film post-annealed at 673 K was found to have a coexistence of anatase/rutile duplex and amorphous structure, as observed by using TEM. Heat treatment also induces a change in the surface morphology of the TiO2 films examined by FE-SEM. The optical properties of the TiO2 films were characterized by UV/visible spectrophotometry. The average transmittance of the Sn-doped and Nb-doped films is higher than 85%. Absorption zone in visible regime and optical band gap of the Sn-doped or Nb-doped TiO2 films increase with the doping contents. Under visible light irradiation, all the Nb-doped TiO2 films exhibit better photocatalytic activity than that of un-doped and Sn-doped ones. Among them, the Nb-doped TiO2 film with rf power of 400 W post-annealed at temperature 673 K shows the best photocatalytic performance.

Integrated optical components based on ion-exchanged glass waveguides are attractive for several reasons. Significant progress has occurred in the past decade in the understanding of the ion-exchange process and its relationship to the dopant profile. It is now possible to model the thermal as well as electric field enhanced ion exchange with reasonable accuracy and to design ion-exchanged waveguides and devices with prespecified characteristics. This paper first describes the theory of the ionexchange process and its application to planar and 2-D (channel) waveguides. Following this, we review the latest results for several passive devices based on single-mode channel waveguides. These include couplers, power dividers, MUX/DEMUX, and polarization splitters. Finally, we point out the latest trends and novel emerging applications of ionexchanged waveguides, such as waveguide lasers and amplifiers, all optical switching, and integration of passive waveguides on glass with optoelectronic devices. We conclude by presenting some of the issues to be addressed for the technology to make an impact on the market.

In this simulation work, the wave vector and reflectance properties of a 1DPC made of dielectric/metamaterial multilayer structure employing TMM, for TE and TM modes are studied. For TE mode, it is observed that the obtained wide bandgap increases with increase in the angle of incidence, whereas the narrow bandgap also shows a minor increase with increase in the incident angle. It is noted that bandgap increases with increase in the incident angle in TE mode, while bears opposite behavior in TM mode and decreases with increase in the incident angle. If the DPS thickness is increased, the number of bands increases in both TE and TM modes, while the wide bandgaps for TE and TM decreases in width. In both cases of the increased DPS width, it is found that the narrow bandgap obtained in TM mode is showing enhancement with an increase in incident angle, and exhibits ultra‐tunable behavior. Some other researchers have reported earlier that a complementary split‐ring resonator based metamaterial with effective medium ratio has influence on the extraction of effective permittivity, permeability, and refractive index, where DNG regions exist in the ranges 6.34–7.39 and 8.20–9.98 GHz. Comparing with the proposed DNG, the study finds that the aforesaid values correspond to frequency regime taken in our case, 1–10 GHz, particularly the middle of the range 1–10 GHz, where broadband is obtained, which is satisfactory. The insights included in the article can be employed in designing microwave broadband reflectors, tunable mirrors, and can be proved to be useful in photovoltaic cells. Such PC can also be employed in designing super lenses as well as optical cloaking, and the miniaturization of antennas.

Dispersion relations of refractive indexes and extinction coefficients of constituent materials were obtained by comparing the experimental and simulated transmission spectra of single-layer Er2O3 and TiO2 films. We designed multilayer thin films of [Er2O3/TiO2]6/[Er2 O3]2/[TiO2/Er2O3]6 such that they can act as photonic crystals exhibiting a resonance wavelength of 514 nm. We prepared single-layer TiO2 and Er2O3 thin films on quartz glass substrates by employing a magnetron sputtering deposition method. Uniform films exhibiting very high transmittance values were obtained. Deposition rates of the TiO2 and Er2O3 films were 0.44 and 0.82 nm/s, respectively. [Er2O3/TiO2]6/[Er2 O3]2/[TiO2/Er2O3]6 multilayer films, to be used for optical bandpass filter applications, were also prepared using a multitarget sputtering deposition method. Uniform and transparent films were obtained; however, the wavelength corresponding to the highest transmittance was observed around 490 nm through ultraviolet-visible near-infrared spectroscopic measurements. The shift in the wavelength can be attributed to the low crystallinity and variations in the thicknesses of Er2O3 and TiO2 films.

Superhydrophilic TiO2 thin film by nanometer scale surface roughness and dangling bonds

We describe the waveguide properties of the asymmetric dielectric slab formed by a very-high-index film on a low-index substrate. The analysis is appropriate for Pb-salt films grown on fluorite structure substrates. The mode reflectivities, surface scattering losses, and gain enhancements for the low-order TE and TM modes are considered. The stronger confinement of the TM modes leads to larger gain enhancements than for the TE modes. For the film thicknesses of interest, 2–4 μm, the TE and TM mode reflectivities are comparable. Using the theory of Tien, the TE modes show substantially lower surface scattering losses than TM modes of the same order. The magnitude of this loss, however, depends on an unknown parameter σ, the variance of the surface height. Experimental results are presented for thin-film diode lasers made with Pb Schottky barriers on p-type PbTe (index 6.4) grown epitaxially on BaF2 substrates (index 1.42). The spontaneous emission at 77°K is shown to agree with the band-band recombination model, assuming k conservation. Laser emission at 6.5 μm, both pulsed and cw, is observed at 10–15°K. Well above threshold the spectra generally show two dominant modes spaced by ≃ 1.4 meV. This splitting is attributed to a strain-induced shift of the energy bands. Using the deformation potentials for PbTe the observed splitting leads to an estimate −2 × 10−4 of the strain component in the [111] direction normal to the film surface. All observed laser lines are TE polarized. An analysis is made of the gain and loss parameters at threshold. The optical gain is estimated from the measured current and quantum efficiency, while the free carrier and reflection losses are calculated from the device parameters. The remaining losses can then be estimated from the threshold condition. If these losses are attributed to surface scattering, the analysis can be used to give an upper limit on σ of 0.03–0.06 μm. In every case the surface scattering loss is dominant in establishing the laser threshold, which is consistent with the strong preference shown for TE modes.

PurposeThis paper aims to describe the techniques used in industrial optical chemical sensors and to consider future prospects.Design/methodology/approachThis paper discusses the techniques and technologies used in today's optical chemical sensors. It highlights their limitations and considers briefly certain new technological developments.FindingsThis paper shows that techniques such as wet reagent‐aided photometry, UV absorption, spectroscopy and UV fluorescence satisfy a range of industrial chemical sensing applications and that optode technology is making limited commercial inroads. It identifies the need for inexpensive, wet reagent‐free chemical sensors and suggests that both solid‐state electrodes and lab‐on‐a‐chip devices may ultimately resolve this issue.Originality/valueThis paper provides a technical insight into the state of optical chemical sensing and illustrates that generic families of inexpensive chemical sensors are yet to be developed.

Non-UV activated superhydrophilicity of patterned Fe-doped TiO2 film for anti-fogging and photocatalysis

The aim of this work is to deposit nanocrystalline TiO2 films by DC reactive facing-target sputtering (FTS) technique. The TiO2 films have been deposited in Fluorine doped tin-oxide (FTO) glass substrate by FTS with sputtering pressure of 2.0 Pa, and Ar/O2 gas ratio of 7∶3. The main attraction of this paper is to correlate the structural, optical and morphological properties of these prepared TiO2 film with variation of deposition time (like 2, 4 and 12 hrs). The thickness of TiO2 film increases with the increase of deposition time. The correlation between the thickness variance and the structural, optical and morphological properties of TiO2 thin films has been investigated in details. From the results, it is cleared that the crystallinity, grain size and surface roughness enhance but cluster size reduces with the increase of deposition time. These prepared films have chanced to apply directly for dye-sensitized solar cell, photoelectrochemical cell and water splitting applications.

NiO/TiO2 films with various NiO film thicknesses ranging from 10 to 320 nm were deposited on silicon and glass substrates by e-beam evaporation at 200 °C, and then annealed in H2 atmosphere at 500 °C for 1 h in order to reduce the NiO film to Ni grains on the TiO2 film. The structures of titanium oxide, NiO, and Ni/TiO2 were determined by X-ray diffraction (XRD) analysis, and the morphology of the Ni/TiO2 films was observed by scanning probe microscopy. The ultraviolet–visible (UV–vis) transmittance and vis absorption of the Ni/TiO2 films were measured by UV–vis spectrophotometry. The results showed that an amorphous titanium oxide was obtained as deposited at 200 °C and that the structure change to the anatase phase after 500 °C annealing. As deposited, crystalline NiO films with XRD patterns similar to that of powder were obtained; however, diffraction peaks of (111) and (200) Ni appeared after annealing in H2 atmosphere. Ni nanograins, coarsened grains, and films were obtained on the TiO2 films when the NiO film with a thickness from 10 to 320 nm was reduced in H2 atmosphere at 500 °C. The transmittance of the Ni/TiO2 films decreased with an increase in Ni particle size. The vis absorption measurement showed that the peak shifted toward a shorter wavelength with a decrease in Ni particle size.

TiO2 film was prepared onto glass substrates with ultrasonic spray pyrolysis technique by using the less reported Ti (IV) chloride precursor, and then a highly anatase TiO2 film was successfully produced by annealing at 550 °C for 3 h in air. The structural, optical, and surface properties were characterized in detail by means of X-ray diffraction (XRD) patterns, UV–vis spectroscopy, spectroscopic ellipsometry, photoluminescence spectrometry, atomic force microcopy (AFM), and energy dispersive X-ray (EDX) spectroscopy. Furthermore, to investigate the application potential as a catalyst, the photocatalytic property of TiO2 film was tested by the degradation of methylene blue dye at various time intervals under UV light. It was determined that photocatalytic degradation of methylene blue using TiO2 film has the highest correlation with first-order velocity law, and the degradation efficiency was successfully achieved at 66.1% after 1 h. Consequently, an alternative, inexpensive, easily applicable, and highly efficient material was developed for the removal of organic dyes in wastewater.

Titanium dioxide (TiO2) thin films were grown on glass substrates at room temperature using RF magnetron sputtering technique. Effect of the post-annealing for 1 h at 400–600 °C on the structural, morphological, optical and waveguide properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible spectrophotometry and m-lines spectroscopy (MLS). XRD studies show that as-grown and post-annealed TiO2 films exhibit (101) XRD peak corresponding to the anatase phase of TiO2. Higher annealing temperatures result in a significant increase in crystallinity. The grain size values were calculated and found to be about 15–37 nm. From the analyses made on the SEM micrographs and AFM images, it was revealed that the morphology and surface roughness of the thin films were influenced by the heat treatment temperature. The UV–visible spectroscopy analyses show that as-grown TiO2 films were transparent in the visible region with an average transmittance of more than 75 % and the transmittance decreases slightly with an increase in annealing temperature. Annealed TiO2 films also exhibit an increase in the values of direct optical band gap. MLS measurements at 633-nm wavelength put into evidence that TiO2 planar waveguides demonstrate a well-guided fundamental mode for both transverse electric and transverse magnetic polarized light. Moreover, the refractive index was found to increase with temperature and to approach to the anatase TiO2 single-crystal value for the TiO2 film annealed at 600 °C.