M. Gaidi

Structure and photoluminescence of ultrathin films of SnO2 nanoparticles synthesized by means of pulsed laser deposition

Tin oxide (SnO2) ultrathin films were deposited by pulsed laser deposition (PLD) onto SiO2/Si and quartz substrates, at various nominal thicknesses ranging from isolated nanoparticles (NPs) to ∼300 nm-thick films, under an oxygen background pressure of 10 mTorr. The microstructural and surface morphologies of the NP-based SnO2 films were characterized by x-ray diffraction and atomic force microscopy, as a function of their nominal film thickness. The PLD-SnO2 films were found to be composed of NPs (in the 1–6 nm range), whose size increases with the film thickness. The energy band gap, as determined from the absorption edge, was found to shift to higher values with decreasing the film thickness (i.e., decreasing the NPs size). It was found that an annealing at 700 °C under O2 ambient is a prerequisite to get a photoluminescence (PL) emission from the PLD-SnO2 films. The PL of the annealed SnO2 films was found to consist of two broad emission bands, regardless of the SnO2 film thickness. The first band is ...

Pockels response in calcium barium niobate thin films

The electro-optical response of calcium barium niobate ferroelectric thin films is characterized using a single beam setup in reflection geometry. Clear evidence of a Pockels response together with an r33 coefficient as high as 130pm∕V is found. This large value and the high Curie temperature of the material under study (>250°C) can be considered as a promising feature for the fabrication of hybrid, integrated electro-optical modulators working at a high repetition rate.

Microfabricated SrTiO3 ridge waveguides

We report the microfabrication and characterization of SrTiO3∕SiO2∕Si ridge waveguides. SrTiO3 films are very attractive for optical integrated systems as they present a high transparency in the visible and infrared wavelength range as well as a relatively high refractive index. In this work, SrTiO3 films were grown by means of a reactive pulsed-laser-deposition technique and patterned using UV photolithography and high-density plasma etching. Propagation and loss characteristics at the telecommunication wavelength of 1.55μm were investigated using top-view scattering and Fabry-Perot resonance methods. For specific ridge widths, we obtained single-mode propagation with relatively low losses (∼1.5dB∕cm), thereby demonstrating the strong potential of SrTiO3 films for guided-wave components for advanced optical integrated systems.

Transmission spectrum reconfiguration in long-period gratings electrically induced in Pockels-type media with the help of a periodical electrode structure

The perturbation to the refractive index induced by a periodic electric field from structured electrodes is analyzed for a transverse-electric-transverse-electric and transverse-magnetic-transverse-magnetic copropagating mode coupling in a planar waveguide with electrooptic (EO) core/claddings. The analysis accounts for material birefringence and propagation mode losses due to light absorption by the electrodes. The results of the mode coupling are used to calculate the EO-induced transmission spectra with account for the mode attenuation. Two types of electrode materials, gold and indium-tin oxide, are analyzed in terms of their polarization-dependent-loss contributions. It is shown that the proposed design provides possibility of fully controlling the transmission spectrum through independent electric adjustment of the bandwidth, the attenuation level, and the resonance wavelength position. However, the filter performance is highly polarization sensitive. The proposed design can provide fully reconfigurable long-period grating filter only on the basis of a polarization-maintained platform.

Hybrid integration of Ca0.28Ba0.72Nb2O6 thin film electro-optic waveguides with silica/silicon substrates

Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN-28) waveguides based on thin film technology were fabricated on SiO(2)/(100) Si substrates. By using X-ray diffraction, we confirmed the preferential c-axis orientation of the CBN structures. An effective unclamped electro-optic r33 coefficient of 12 pm/V was measured in CBN thin films by using an ellipsometric technique in reflection geometry. In addition, by means of a Fabry-Perot technique, the propagation losses of our strip loaded waveguides were estimated to be as low as 4.8 dB/cm and 6.5 dB/cm at telecommunication wavelengths for the fundamental TE and TM modes, respectively.

Optical depth profiling of strontium titanate and electro-optic lanthanum-modified lead zirconium titanate multilayer structures for active waveguide applications

Transparent polycrystalline strontium titanate (STO) and lanthanum-modified lead zirconium titanate (PLZT) thin films were deposited, respectively, on Si and on indium-doped tin oxide (ITO) coated glass by pulsed laser deposition (PLD). PLZT films are shown to exhibit electro-optic properties close to the bulk material when deposited on ITO in specific process conditions. The refractive index depth profile was determined by using a combination of variable angle spectroscopic ellipsometry and spectrophotometry, and the multisample analysis approach. PLZT films deposited at high O2 pressure, PO2, and annealed at 700°C were found to be more porous and inhomogeneous than low PO2 films. The optical properties of STO films strongly depend on PO2 as well: low PO2 depositions lead to denser film growth with homogeneous, bulk-like refractive index profile, while high PO2 depositions lead to porous and highly inhomogeneous films, exhibiting band-gap variation and formation of a 60-nm-thick interdiffusion layer on S...

Electronically reconfigurable superimposed waveguide long-period gratings

The perturbation to the refractive index induced by a periodic electric field from two systems of interdigitated electrodes with the electrode-finger period l is analyzed for a waveguide with an electro-optically (EO) active core-cladding. It is shown that the electric field induces two superimposed transmissive refractive-index gratings with different symmetries of their cross-section distributions. One of these gratings has a constant component of an EO-induced refractive index along with its variable component with periodicity l, whereas the second grating possesses only a variable component with periodicity 2l. With the proper waveguide design, the gratings provide interaction between a guided fundamental core mode and two guided cladding modes. Through the externally applied electric potential, these gratings can be independently switched ON and OFF, or they can be activated simultaneously with electronically controlled weighting factors. Coupling coefficients of both gratings are analyzed in terms of their dependence on the electrode duty ratio and dielectric permittivities of the core and cladding. The coupled-wave equations for the superimposed gratings are written and solved. The spectral characteristics are investigated by numerical simulation. It is found that the spectral characteristics are described by a dual-dip transmission spectrum with individual electronic control of the dip depths and positions. Within the concept, a new external potential application scheme is described in which the symmetry of the cross-sectional distribution of the refractive index provides coupling only between the core mode and the cladding modes, preventing interaction of the cladding modes with each another. This simple concept opens opportunities for developing a number of tunable devices for integrated optics by use of the proposed design as a building block.

Dependence of the sputter-etching characteristics of strontium–titanate–oxide thin films on their structural properties

Sputter-etching characteristics of polycrystalline strontium–titanate–oxide (STO) thin films are investigated using a high-density argon plasma. STO thin films were grown by means of a reactive pulsed-laser deposition technique in which the buffer oxygen pressure was varied to change the structural properties of the films. The sputter-etch rate of the rf-biased films is found to linearly increase with the oxygen deposition pressure. This result is shown to be related to the corresponding decrease of the film density. This dependence of the etch rate on the structural properties of the films has very important consequences on etching studies and on the optimization of etching processes.

Optical Properties Tuning of SnO2 Films by Metal Incorporation (Pt,Pd): Correlation with Microstructure Change

In this work, we report on the effect of noble metal doping (namely Pd or Pt) on the optical properties of SnO2 thin films. The optical constants (n and k) of the films, as a function of noble metal nature and content, were obtained using variable angle spectroscopic ellipsometry in the ultraviolet–visible–near infrared (UV–vis–NIR) regions. Ellipsometry analysis showed that we can tune the optical constants of SnO2 films by changing Pt or Pd doping concentration. In particular, their refractive index increases from 1.6 to ~2 while varying Pt content from 3 to 12 at. %. The origin of this optical behaviour was correlated to the microstructure change induced by metal doping. X-ray diffraction (XRD) was used to investigate the effect of doping on SnO2 lattice parameter, on crystallite size and on film preferential orientation. Atomic force microscopy (AFM) was used to estimate the surface roughness of the films. A metal concentration of ~3 at. % (for both Pt and Pd), which is known to yield the highest SnO2 gas sensing response, was found to correspond to the highest contraction of the lattice parameter of the films. Finally, the energy band gap of undoped SnO2 thin films (estimated to 4 eV) was found to shift to lower value while increasing doping concentrations.

Epitaxial CBN growth for fast electro-optic tunable devices

In this study, we report for the first time the epitaxial growth of CBN thin films on Magnesium Oxide (MgO) substrates for optical device applications. A high deposition temperature (greater than or equal to to 800 oC) is required to obtain the epitaxial growth of CBN films. A parametric study is proposed in order to elaborate CBN thin films with a crystal structure as close as possible to that of a CBN bulk single crystal and with good optical characteristics. In particular, a low oxygen pressure (1 mTorr) allows synthesizing high-quality CBN thin films with an out-off plane lattice parameter comparable to the one of CBN bulk material at low surface roughness. The optical characterization of the high-quality CBN thin films reveals a high optical transmission (greater than or equal to 85 %) and a refractive index equal to 2.22 at 1.55 μm for certain deposition conditions. These optical properties clearly indicate the potential of CBN thin films for waveguide applications. This work presents a significant first step toward the integration and the potential use of CBN films for optical device applications.