Showing papers by "Ludvik Martinu published in 2006"

Consecutive solvent evaporation and co-rolling techniques for polymer multilayer hollow fiber preform fabrication

Abstract: All-polymer multilayer hollow core photonic fiber preforms were fabricated using consecutive deposition from a solvent phase of two polymers with high and low refractive indices (RI). Processing techniques for two polymer pairs—polystyrene (PS)/poly(methyl methylacrylate) (PMMA) and polycarbonate (PC)/poly(vinylene difloride) (PVDF)—were established. The fabrication process involved consecutive film deposition by solvent evaporation of polymer solutions on the inside of a rotating PMMA or PC tube, used as a cladding material. By injecting right volumes of the polymer solutions into a spinning tube the thickness of each layer could be reliably controlled from 20 to 100 μm. Proper selection of solvents and processing conditions was crucial for ensuring high optical and mechanical quality of a resultant preform, as well as compatibility of different polymer films during co-deposition. Preforms of 10 layers for PMMA/PS material combination and 15 layers for PVDF/PC were demonstrated. Fabrication of preforms with higher number of layers is readily possible and is only a question of preform fabrication time. An alternative method of preform fabrication by co-rolling of polymer bilayers is also presented in this paper, drawing of PMMA/PS, PVDF/PC fibers with up to 32 layers is demonstrated.

Pulsed radio frequency plasma deposition of a-SiNx:H alloys: Film properties, growth mechanism, and applications

Abstract: In this work, we propose a fabrication process of a-SiNx:H alloys by pulsing the radio frequency (rf) signal in a low pressure plasma-enhanced chemical vapor deposition (PECVD) system. The characteristics of the films can be controlled simply by adjusting the duty cycle of the pulsed rf power, while keeping the N2∕SiH4 gas mixture constant. Spectroscopic ellipsometry analysis in the ultraviolet-visible-near infrared and far infrared ranges, atomic force microscopy, and elastic recoil detection reveal strong variations in the optical properties (1.88⩽n⩽2.75, 10−4⩽k⩽5×10−2 at 550nm), optical gap (4.01eV⩽Eg⩽1.95eV), microstructural characteristics (1.3nm⩽surfaceroughness⩽8.3nm), and chemical composition (0.47⩽x⩽1.35) of the coatings as a function of duty cycle. This behavior is interpreted in terms of radical concentration changes in the gas phase, as well as variation in the average ion bombardment energy at the film surface, leading to modifications of both chemical and physical mechanisms that sustain the film growth. Using the control of duty cycle, we fabricated two types of a-SiNx:H-based thin film devices, namely, (i) a model Fabry-Perot optical filter deposited on plastic substrate and (ii) a superlattice structure displaying a photoluminescence signal four times higher than the reference single layer. These two examples of applications point out the main advantages of this pulsed rf PECVD process, in particular, low deposition temperature, reproducibility, versatility, and ease of use.

Plasma treatment of porous SiNx:H films for the fabrication of porous-dense multilayer optical filters with tailored interfaces

Abstract: Porous and dense silicon nitride films with low (1.58) and high (1.88) refractive indices were prepared by using successively microwave and radio frequency (rf) plasma-enhanced chemical vapor deposition. Surface treatments were performed on porous layers using argon and nitrogen rf plasmas in order to densify and flatten their surface, and hence to obtain an abrupt transition between porous and dense films. The processes during deposition and interface treatment were studied by in situ real-time spectroscopic ellipsometry as well as by other characterization techniques. We show that besides the densification effect, preferential sputtering and annealing phenomena occur during plasma treatments at high bias (∣VBtreat∣>400V), leading to silicon enrichment at the film surface and chemical stabilization of the film bulk. Using atomic force microscopy, we observed a significant reduction of the thickness of the surface roughness layer after treatment for single layers (≈70% reduction) and multilayer stacks (≈6...

Interference security image structure

Abstract: An interference filter having a plurality of layers and a spectrum as a function of angle of observation is combined with a metameric element adjacent the interference filter and appearing to have a same color as the interference filter at at least one angle of observation and a contrasting color as said interference filter at at least one other angle of observation. A plurality of interference filter layers, including a metallic layer, can be provided on a support structure for the layers. The support structure is then adapted to allow the layers to be seen from a top and a bottom side, with the layers being adapted to have different spectra as a function of angle of observation in reflection and transmission.

Experimental and theoretical studies of the E + optical transition in GaAsN alloys

Abstract: We present a joint experimental and theoretical study of the optical properties of $\mathrm{GaAsN}$ alloys. The ${E}_{+}$ optical transition is identified by variable-angle ellipsometry measurements. It induces an edge characteristic of a ${M}_{0}$ critical point in the imaginary part of the dielectric function of the alloys. The experimental results are modeled by ab initio calculations of the dielectric function of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ in the nitrogen concentration range $x=0.009--0.037$. The origin of the ${E}_{+}$ transition is analyzed by calculating contributions from different conduction states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ to the dielectric function. Our study confirms that optical transitions from the valence band to electronic states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ derived from $L$ states of the conduction band of $\mathrm{GaAs}$ are at the origin of the ${E}_{+}$ edge.

Real-time in situ growth study of TiN- and TiCxNy-based superhard nanocomposite coatings using spectroscopic ellipsometry

Abstract: We investigate in situ and in real-time the growth of superhard ternary nanocomposite (nc)-TiN∕amorphous (a)‐Si3N4 and quaternary nc‐TiCxNy∕a‐SiCN films. Using nondestructive, noncontact spectroscopic ellipsometry and appropriate ellipsometric models, we determine the variation of optical constants, film resistivity, and electron scattering time and mean free path as a function of thickness and particle size. Based on the optical and electrical behavior, we propose a microstructural model of superhard nanocomposite films, postulating the interconnectivity between individual nanoparticles, presence of defects in the individual grains, and particle size varying in the initial stage of the film growth.

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

Abstract: 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...

Interface broadening due to ion mixing during thin film growth at the radio-frequency-biased electrode in a plasma-enhanced chemical vapor deposition environment

Abstract: The authors show that ion bombardment in the range of tens to a few hundreds of eV, used in ion- and plasma-assisted deposition processes, can lead to thin film growth dominated by subsurface deposition due to subplantation (shallow implantation). This can cause significant interface broadening during the initial stages of film deposition as a result of ion mixing. First, by studying the modifications of a c-Si(100) target exposed to an O2 plasma at the radio-frequency (rf)-biased electrode using in situ real-time spectroscopic ellipsometry (RTSE), the authors detect implantation, damage, and oxidation to a depth of up to ∼10nm. They validate these results using high resolution transmission electron microscopy and simulate the effects of ion-surface interactions at the rf-biased electrode by using Monte Carlo TRIDYN simulations. The simulation code, which was modified specifically to consider a broadband ion energy source, enabled the authors to reproduce depth and time relevant experimental results with ...

Engineering of waveguides and other micro-structures in dielectrics

Abstract: This invited talk presents the most recent work undertaken in our laboratory in the fields of direct laser writing of planar optical integrated circuits, laser written micro fluidics circuits, laser machining of v-grooves and laser machining in diffraction gratings.

Dynamics of ion bombardment-induced modifications of Si(001) at the radio-frequency-biased electrode in low-pressure oxygen plasmas: In situ spectroscopic ellipsometry and Monte Carlo study

Abstract: Low-pressure O2 plasma exposures were performed on c-Si(001) at a radio frequency (rf)-powered electrode in the presence of substrate self-biasing (VB) from VB=−60to−600V, in order to evaluate ion-surface interactions at the growth surface under ion bombardment conditions suitable for the fabrication of high quality optical coatings. The plasma-surface interactions were monitored in situ using real-time spectroscopic ellipsometry (RTSE), which reveals time- and ion-fluence-resolved information about depth-dependent modifications, such as damage and oxidation below the c-Si substrate surface. RTSE analysis indicates almost immediate damage formation (⪡1s) to a depth of a few nanometers below the surface after exposure to a low oxygen ion fluence (∼5×1014Ocm−2). Oxide growth is detected at intermediate fluence (∼1015–1016Ocm−2) and is attributed to O subplantation (shallow implantation); it forms near the surface of the target on top of an O-deficient interfacial damage layer (DL). Both layers experience a ...

Ion-surface interactions on c-Si(001) at the radiofrequency-powered electrode in low-pressure plasmas: Ex situ spectroscopic ellipsometry and Monte Carlo simulation study

Abstract: We use variable-angle spectroscopic ellipsometry (VASE) to investigate oxide and interface formation during plasma-oxidation of monocrystalline Si(001) at the radiofrequency (rf) powered electrode of a plasma-enhanced chemical vapor deposition reactor. HF-etched c-Si(001) wafers were exposed to an oxygen plasma under conditions similar to those used in optical coatings deposition in order to ascertain the effects of plasma-bulk interactions, and to gauge to what depth O2+ and O+ ions interact with and alter the structure and composition of the target in the presence of negative self-bias, VB. From VASE analyses, modifications are best described using a two-layer model: A top layer consisting of SiO2 and a defective interfacial layer (DL) composed of a mixture of c-Si, a-Si, and SiO2. The saturation value of the modification depth (oxide and DL thickness) increases from 3.4±0.4to9.6±0.4nm, for VB ranging from −60to−600V, respectively, and scales with Emax1∕2, where Emax is the maximum energy of ions from a...

Use of metameric filters for future interference security image structures

Abstract: In the present work, we describe innovative approaches and properties that can be added to the already popular thin film optically variable devices (OVD) used on banknotes. We show two practical examples of OVDs, namely (i) a pair of metameric filters offering a hidden image effect as a function of the angle of observation as well as a specific spectral property permitting automatic note readability, and (ii) multi-material filters offering a side-dependent color shift. We first describe the design approach of these new devices followed by their sensitivity to deposition errors especially in the case of the metameric filters where slight thickness variations have a significant effect on the obtained colors. The performance of prototype filters prepared by dual ion beam sputtering (DIBS) is shown.

Laser fabrication of microfluidics circuits

Abstract: This paper presents recent research undertaken in our laboratory in the field of direct laser writing of microfluidics circuits. Our novel technology allows the rapid fabrication of complex fluidic circuits. The fabricated channel have smooth walls and surfaces enabling the encapsulation of the circuits.

Dual laser fabrication of planar lightwave circuits

Abstract: This paper presents recent scheme based of direct dual wavelength laser writing of lightwave circuits developed in our laboratories. The system allows the fabrication of features of the order of ~1 micron by using a short wavelength laser to ablate a region preheated by a second laser.

2D refractive index measurement of CO2 laser written planar optical waveguides

Abstract: A reflectance measurement technique was used to determine the refractive index distribution of optical waveguides fabricated by a CW CO 2 direct writing technique. The uniformity of refractive index has been verified along the waveguides structure and used as input for accurate beam propagation method (BPM) calculations of waveguide devices.

CO2 laser direct writing of optical waveguides in silica thin films on silicon

Abstract: The development of low-loss, low-cost, and stable passive waveguide devices has been a major focus of research in the past few decades. Several techniques have been proposed in which direct laser writing is one of the more appealing methods since complex photolithography may no longer be required. These techniques are based on the creation of a core region by laser-induced refractive index change. Direct UV writing is one of these techniques but its application is normally restricted to highly photosensitive materials such as germania doped silica [1]. Femto-second laser writing has been used to circumvent this problem: the technique allows the creation of waveguides in a variety of bulk materials [2]. However, these methods modify the properties of the material in the exposed core region, the waveguide formation process is slow due to the writing speed of the order of 100 μm/sec [2], and are expected to cause damage in nonlinear materials. In this paper we demonstrate a simple and novel technique that allows the creation of optical waveguides in silica thin films on silicon by removing material to leave behind a core region, being defined as a region between two trenches. The proposed process is distinct from other laser writing techniques as the structure of the core region is preserved since only the cladding area is modified. Even though two trenches are required to create a waveguide the overall fabrication time is orders of magnitude shorter than with other direct laser writing techniques as the writing speed is of the order of 50 mm/sec. We believe this technique is also useful for the fabrication of waveguide devices in nonlinear materials e.g. PPLN, KTP, BBO etc. and particularly in poled glasses.