Refractive Index Depth Profile Research Articles

Ellipsometry-based study of glass refractive index depth profiles obtained by applying different poling conditions.

Application of electric field and moderately elevated temperature depletes the side facing anode from alkali present in glasses. The change of composition of the treated glass results in variation of refractive index depth profile within the treated glass. Spectroscopic ellipsometry is employed for characterization of optical properties of glass treated in different conditions. The results of optical characterization are verified by secondary ion mass spectroscopy. It is found that the refractive index profile obtained from ellipsometry has a maximum value higher than the one of untreated glass. The obtained refractive index profiles are in very good agreement with concentration profiles.

TE modes of UV-laser generated waveguides in a planar polymer chip of parabolic refractive index profile

The UV-laser lithographic method is used for the preparation of Polymeric integrated-optical waveguides in a planar polymer chip. The waveguide samples are irradiated by an excimer laser of wavelength 248 nm with different doses and with the same fluencies. The refractive index depth profile for the waveguides, in the first zone is found to have a parabolic shape and Gaussian shape in the second one that can be determined by Mach–Zehnder interferometer. Both the mode field distribution and the effective mode indices for the first zone only are determined by making use of the theoretical mode and the experimental data. It is found that the model field distribution is strongly dependent on the refractive indices for each zone.

Two-dimensional interferometric characterization of laser-induced refractive index profiles in bulk Topas polymer

In this paper we precisely determine laser-induced refractive index profiles created in cyclic olefin copolymer Topas 6017 employing a sophisticated phase shifting Mach-Zehnder interferometry approach. Beyond the usual one-dimensional modification depth measurement we highlight that for straight waveguide structures also a two-dimensional refractive index distribution can be directly obtained providing full information of a waveguide's exact cross section and its gradient refractive index contrast. Deployed as direct data input in optical waveguide simulation, the evaluated 2D refractive index profiles permit a detailed calculation of the waveguides' actual mode profiles. Furthermore, conventional one-dimensional interferometric measurements for refractive index depth profiles with varying total imposed laser fluence of a 248 nm KrF excimer laser are included to investigate the effect on refractive index modification depth. Maximum surface refractive index increase turns out to attain up to 1.86·10−3 enabling laser-written optical waveguide channels. Additionally, a comprehensive optical material characterization in terms of dispersion, thermo-optic coefficient and absorption measurement of unmodified and UV-modified Topas 6017 is carried out.

Depth dependent modification of optical constants arising from H+ implantation in n-type 4H-SiC measured using coherent acoustic phonons

Silicon carbide (SiC) is a promising material for new generation electronics including high power/high temperature devices and advanced optical applications such as room temperature spintronics and quantum computing. Both types of applications require the control of defects particularly those created by ion bombardment. In this work, modification of optical constants of 4H-SiC due to hydrogen implantation at 180 keV and at fluences ranging from 1014 to 1016 cm−2 is reported. The depth dependence of the modified optical constants was extracted from coherent acoustic phonon spectra. Implanted spectra show a strong dependence of the 4H-SiC complex refractive index depth profile on H+ fluence. These studies provide basic insight into the dependence of optical properties of 4H silicon carbide on defect densities created by ion implantation, which is of relevance to the fabrication of SiC-based photonic and optoelectronic devices.

Influence of chemical polymer composition on integrated waveguide formation induced by excimer laser surface irradiation

We show that the chemical composition and the amount of residual monomers in polymethylmethacrylate significantly affect the evolution of optical waveguide formation induced by UV surface irradiation. We employ an interferometric approach in Mach-Zehnder configuration to determine the refractive index depth profile in different planar polymethylmethacrylate materials. Our results reveal a distinctive different surface and buried waveguide formation for materials having different monomer content. In particular, we find that for smaller residual monomer content buried waveguide formation is less pronounced, which is in turn preferential for a selective light guidance in planar polymer structures. Attenuation measurements confirm a difference in attenuation coefficient of 0.5dB/cm.

Underwater optical wireless communications: depth-dependent beam refraction.

Global refractive gradients in seawater cause pointing problems for optical wireless communications. A refractive index depth profile of the Pacific Ocean was calculated from measured salinity, temperature, and pressure, determining the end points of a refracted and nonrefracted 200m communication link. Numerical ray tracing was used with a point source for angles between 10° and 80° and transmission wavelengths of 500-650nm; the maximum end-point difference found was 0.23m. A 500nm laser with a 0.57° full-angle FOV was traced; the nonrefracted receiver location was outside the FOV for all links angled >15° to the vertical. However, most pointing issues underwater are unlikely to be significant with suitable FOV choice and natural scattering of the source.

Investigation of a UV-laser generated planar polymeric waveguide directly and after five years of preparation using an interferometric method

Polymeric integrated-optical waveguides were prepared in a planar chip using UV-laser lithographic method. Five waveguide samples were irradiated by an excimer laser of wavelength 248nm with different irradiation parameters (fluence and irradiation doses). Using Mach–Zehnder interferometer, the refractive index depth profiles of these samples are determined in two cases, directly and five years later of preparation. A crucial change of refractive indices profiles has been recorded after five years of preparation. In the first case we got double region waveguides fitted a Gaussian shape, while in the second one we observe a single region waveguide which has exponential shape. The photochemical reactions responsible for these changes in the two cases are demonstrated. This is quite important when such waveguides are used in the applications. Also it was interesting to investigate the aging impact on mode field distributions and the effective mode indices. The mode parameters are determined based on a theoretical model and the experimentally obtained data. The results show a notable change in the mode field distributions and the propagation coefficients as influence of aging.

Nanoscale Noncontact Subsurface Investigations of Mechanical and Optical Properties of Nanoporous Low-k Material Thin Film

Revealing defects and inhomogeneities of physical and chemical properties beneath a surface or an interface with in-depth nanometric resolution plays a pivotal role for a high degree of reliability in nanomanufacturing processes and in materials science more generally. (1, 2) Nanoscale noncontact depth profiling of mechanical and optical properties of transparent sub-micrometric low-k material film exhibiting inhomogeneities is here achieved by picosecond acoustics interferometry. On the basis of the optical detection through the time-resolved Brillouin scattering of the propagation of a picosecond acoustic pulse, depth profiles of acoustical velocity and optical refractive index are measured simultaneously with spatial resolution of tens of nanometers. Furthermore, measuring the magnitude of this Brillouin signal provides an original method for depth profiling of photoelastic moduli. This development of a new opto-acoustical nanometrology paves the way for in-depth inspection and for subsurface nanoscale imaging of inorganic- and organic-based materials.

Investigation of a UV-laser generated waveguide in a planar polymer chip using an improved interferometric method

Polymeric integrated-optical waveguides are prepared in a planar polymer chip by UV-laser lithographic methods. The waveguide samples are irradiated by an excimer laser at a wavelength Λ=248 nm with various irradiation parameters (different fluencies and irradiation doses). Mach–Zehnder interferometer is employed and the refractive index depth profiles of the waveguide samples are obtained. This profile covers two regions having exponential and Gaussian shapes. The model field distributions strongly depend on the refractive index of each region. The mode field distribution and the effective mode indices for each region have been calculated on the basis of a theoretical model and the experimentally measured data.

Refractive index depth profile and its relaxation in polydimethylsiloxane (PDMS) due to proton irradiation

In this work the refractive index depth profile, its relaxation and the absorption were investigated in 2 MeV proton irradiated PDMS. The above parameters were determined by the spectroscopic ellipsometry technique. The refractive index depth profile follows roughly the shape of the Bragg curve for the penetrating protons. It is shown that the observed difference between the refractive index of the surface ( dn = 0.01) and deeper regions ( dn = 0.08) is high enough to accomplish waveguiding that is also demonstrated in this paper. This work serves as basis for the production of integrated optical components, e.g. waveguides, gratings or other optical devices, which are planned to be produced by ion irradiation in PDMS.

Refractive index distribution in the non-linear optical layer of thermally poled oxide glasses

Large refractive index variations have been recorded by combining micro-infrared reflectance spectroscopy with simulations of interference fringes measured at the anode side of thermally poled sodium-rich boro-phosphate–niobate glasses. The method developed in this work is effective for probing the spatial distribution and depth-profile of refractive index in the non-linear optical layer. It was found that the refractive index decreases and the molar volume increases with thermal poling, leading to a net increase in molecular electronic polarizability in the non-linear optical layer of the poled glass.

Depth profiles of refractive index in thermally grown and LPCVD oxide films on silicon

Thin and thick silicon oxide films are grown by thermal oxidation of silicon in dry oxygen or by LPCVD from a mixture of monosilane with oxygen or nitrous oxide. Depth profiles of refractive index are determined in these films by monochromatic ellipsometry, with the measurements interpreted by solving an inverse problem within a multilayer model. The differences between the depth profiles are found to be consistent with well-known mechanisms of film growth under the process conditions applied.

Temperature-controlled UV-laser induced fabrication of planar polymeric waveguides

Integrated-optical waveguides may be written directly into the surface of a planar polymeric substrate by UV-excimer laser irradiation. The loss rate is relatively high due to the refractive index depth profile of waveguides produced in this way. The loss rate can be reduced significantly by use of a temperature-controlled fabrication process: during the UV-laser irradiation the polymeric substrate is heated in a controllable way by a hot plate. An explanation is given for this temperature-dependent effect.

Electrochemical preparation of a rugate filter in silicon and its deviation from the ideal structure

Porous silicon single layers are formed by anodization of silicon in a hydrofluoric acid solution at different current densities. An accurate estimation of the etch rates and refractive indices is performed. The refractive index-depth profile of a porous silicon rugate filter (PSRF) prepared by sinusoidally modulating the current density during etch is investigated. The produced PSRF is found to have a periodic pseudo-sinusoidal variation of refractive index with depth. The sinusoidal current density waveform that was employed generates a nonideal refractive index-depth profile with an asymmetric period. It is found that this deformed refractive index profile affects the quality of the characteristic resonant peak of the rugate filter. A simple modification of the current density waveform can be implemented, which allows one to achieve a true sinusoidal refractive index-depth profile characteristic of a rugate filter. The modified current density waveform allows the generation of a PSRF with a low index contrast and a sharp resonant band. The ability of these structures to act as chemical vapor sensors is tested and compared with the existing literature. The filters exhibit a red-shift in the photonic feature upon exposure to saturated ethanol vapor. The modified signal does not impose a significant change on the filter sensitivity to ethanol vapor.

Refractive index depth profile in PMMA due to proton irradiation

Proton Beam Writing has been successfully used to create buried channel waveguides in PMMA, which suggested that proton irradiation increases the refractive index. To investigate this effect, PMMA samples were irradiated by 1.7–2.1 MeV proton beam. Spectroscopic ellipsometry has been used to investigate the depth profile of the refractive index. An increase of the refractive index was observed in the order of 0.01, which is approximately one order of magnitude higher than the detection limit. The highest increase of the refractive index occurs at the end of range, i.e. we found a good correlation with the Bragg curve of the energy loss.

Characterization of porous low-k films using variable angle spectroscopic ellipsometry

Variable angle spectroscopic ellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-k films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%–4% more porosity at the interface compared with the bulk for both films. This work shows that VASE™, a nondestructive measurement tool, can be used to characterize single- and multigraded layer porous films quickly and effectively.

Nanostructured gradient index optical filter for high-speed humidity sensing

A high-speed optical humidity sensor was fabricated from a nanostructured titanium dioxide thin film. A refractive index depth profile designed to yield a narrow-bandpass optical interference filter was obtained through nanoscale porosity variations produced by a physical vapor deposition technique known as glancing angle deposition (GLAD). The GLAD process provides film morphology control on the 10 nm scale through a combination of deposition at highly oblique vapor incidence angles and controlled substrate motion. Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from water vapor adsorption/desorption. Performance analysis has shown that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270 ms and 160 ms, respectively.

Depth-resolved nanostructure and refractive index of borosilicate glass doped with Ag nanocrystals

Abstract We present an investigation of the Ag-nanocrystal depth profile as well as the corresponding refractive index depth profile of borosilicate glass that was first doped with Ag by Na+ ↔ Ag+ ion exchange and subsequently irradiated with 1 MeV Xe ions. By combining RBS, XPS, XE-AES, and transmission and reflection spectroscopy, we show unambiguously that the Ag nanocrystals are formed in a layer with a thickness (90 ± 20 nm) that is significantly smaller than the 1-MeV Xe range (340 nm). The effective refractive index for the highest Xe fluence is n = 2.1 + 0.8 i at the resonance wavelength (λ = 430 nm). The implications for the integration of surface-plasmon-resonance-based functionalities in photonic devices based on the described fabrication method are discussed.

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 Si. We use an optical depth-profiling procedure to investigate the formation of three-layer air/PLZT/STO/ITO/glass stacks, where the PLZT and STO optical properties are optimized by controlling PO2 during STO deposition, in order to form a cladding layer for potential active waveguide applications.

Active waveguides produced in lithium fluoride by He+ implantation

Planar active waveguides were produced in lithium fluoride crystals by implantation with 1.5MeV He+ ions at several doses. The colored samples have been characterized by optical absorption, photoluminescence and m-line spectroscopy. By comparing the measured guided-mode effective indices with the ones calculated by means of the Chandler–Lama approach, the depth profile of refractive index was derived, showing that there are two competitive physical mechanisms, associated with different processes of energy deposition along the ion track, responsible for positive and negative modifications of the refractive index in the irradiated volume.