Muhammad Rizwan Saleem

Towards athermal organic-inorganic guided mode resonance filters

We demonstrate guided-mode resonance filters featuring an amorphous TiO(2) layer fabricated by atomic layer deposition on a polymeric substrate. The thermal properties of such filters are studied in detail by taking into account both thermal expansion of the structure and thermo-optic coefficients of the materials. We show both theoretically and experimentally that these two effects partially compensate for each other, leading to nearly athermal devices. The wavelength shift of the resonance reflectance peak (< 1 nm) is a small fraction of the peak width (~11 nm) up to temperatures exceeding the room temperature by tens of degrees centigrade.

Broadband photon time-of-flight spectroscopy of pharmaceuticals and highly scattering plastics in the VIS and close NIR spectral ranges

We present extended spectroscopic analysis of pharmaceutical tablets in the close near infrared spectral range performed using broadband photon time-of-flight (PTOF) absorption and scattering spectra measurements. We show that the absorption spectra can be used to perform evaluation of the chemical composition of pharmaceutical tablets without need for chemo-metric calibration. The spectroscopic analysis was performed using an advanced PTOF spectrometer operating in the 650 to 1400 nm spectral range. By employing temporal stabilization of the system we achieve the high precision of 0.5% required to evaluate the concentration of tablet ingredients. In order to further illustrate the performance of the system, we present the first ever reported broadband evaluation of absorption and scattering spectra from pure and doped Spectralon®.

Replicable one-dimensional non-polarizing guided mode resonance gratings under normal incidence

Polarization-insensitive guided-mode resonance (GMR) filters have significant role in applications such as optical communication systems. Here, we report the design and fabrication of two types of simple-structured one-dimensional (1D) GMR gratings with non-polarizing resonance properties under normal incidence. A single-layer rectangular-profile TiO2 grating is fabricated by electron beam lithography and reactive ion etching, which demonstrates, for the first time in experiment, almost perfect non-polarizing filtering effect with 1D grating under normal incidence. Then, a TiO2-coated polycarbonate 1D GMR grating is fabricated by nanoimprinting and atomic layer deposition, which also exhibits good non-polarizing property and the potential of low-cost mass replication of such functional devices.

Effect of substrate overetching and heat treatment of titanium oxide waveguide gratings and thin films on their optical properties

We investigate subwavelength titanium oxide (TiO2) resonance waveguide gratings (RWGs) and TiO2 thin films of thicknesses ~200  nm fabricated by atomic layer deposition (ALD), in both amorphous and crystalline phases on fused silica substrates. The TiO2 RWGs are fabricated by electron beam lithography, reactive ion etching, and ALD. The thin films and RWGs are characterized structurally by x-ray diffraction and scanning electron microscopy. The optical characterization of RWGs and optical constants of TiO2 films are studied by an ellipsometer. RWGs are designed for TE and TM modes in such a way that an overetch effect of the fused silica substrate can be investigated. Various RWG samples are prepared by gradually increasing the overetch depth and subsequently measuring the performance of the RWGs. A close agreement between the calculated and experimentally measured resonance wavelength spectral shifts is obtained; however, the magnitudes of the measured shifts are greater than calculated ones. A parallel study related to the measurement of the refractive indices and remeasuring the optical shifts of RWGs is carried out after a heat treatment of all the samples under study. The RWGs do not reveal significant spectral changes after the heat treatment; this is primarily due to a change in the surface chemistry by the redeposition of the reaction byproducts on the grating lines.

ALD-Assisted Multiorder Dispersion Engineering of Nanophotonic Strip Waveguides

We propose a new technique for the multiorder dispersion engineering of nanophotonic strip waveguides. Unlike other techniques, the method does not require wafers with customized parameters and is fully compatible with standard wafers used in nanophotonics. The dispersion management is based on the application of nanometer-thick TiO2 layer formed by atomic layer deposition. The method is simple and reliable and allows good control of dispersion up to the fourth-order terms. The additional advantages are the reduction of propagation losses and partial compensation of fabrication tolerances.

Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition

We investigate the optical and thermo-optical properties of amorphous TiO2–Al2O3 thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO2–Al2O3 bilayers are fabricated by growing Al2O3 films of different thicknesses on the surface of TiO2 films of constant thickness (100 nm). Temperature-induced changes in the optical refractive indices of these thin-film bilayers are measured by a variable angle spectroscopic ellipsometer VASE®. The optical data and the thermo-optic coefficients of the films are retrieved and calculated by applying the Cauchy model and the linear fitting regression algorithm, in order to evaluate the surface porosity model of TiO2 films. The effects of TiO2 surface defects on the films’ thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al2O3 diffusion barrier layers. Increasing the ALD-Al2O3 thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO2. The thermo-optic coefficients of the 100 nm-thick ALD-TiO2 film and 30 nm-thick ALD-Al2O3 film in a bilayer are (0.048 ± 0.134) × 10−4 °C−1 and (0.680 ± 0.313) × 10−4 °C−1, respectively, at a temperature T = 62 °C.

Determination of thermo-optic properties of atomic layer deposited thin TiO2 films for athermal resonant waveguide gratings by spectroscopic ellipsometry

We report on variation in the refractive index of amorphous and isotropic TiO2 thin films grown by Atomic Layer Deposition (ALD) in nano optical devices. ALD-TiO2 films of thicknesses ≤ 200 nm exhibiting negative thermo-optic coefficient (TOC) due to decrease in refractive index with temperature, owing to inherent hydrophilic nature. While ALD-TiO2 films with thicknesses > 200 nm show positive TOC due to the predominance of TiO2 thickness over the very thin surface porosity region. The negative TOC of thin TiO2 films was controlled by depositing thin ALD-Al2O3 diffusion barrier films that showed impermeable behavior to block the evaporation of adsorbed water molecules on TiO2 surfaces in thermal environments. This approach turns negative sign of TOC of TiO2 thin films to positive one which is necessary to stabilize the central resonance peak of a guided mode resonance filter (GMRF). The ALD-TiO2 and ALDAl2O3 bi-layer stack was modeled by VASE analysis of spectroscopic ellipsometry using Cauchy Model to extract refractive indices at various temperatures, measured at two different angle of incidence (65° and 75°), covering a wide spectral range 380 ≤ λ ≤ 1800. The temperature dependent index and density of TiO2 films were calculated from ellipsometric measured data using Lorentz-Lorenz relation.

Effect of waveguide thickness layer on spectral resonance of a Guided Mode Resonance Filter

We demonstrated cost-effective Guided Mode Resonance Filters (GMRFs) that are diffraction gratings with spectrally narrow reflectance peaks due to resonance anomalies and couple incident light into a semi-guided mode in a corrugated waveguide. The GMRF consists of a single high index dielectric layer on a periodically corrugated transparent substrate to couple light. The GMRFs are designed by Fourier Modal Method (FMM), a numerical tool based on rigorous calculations of electromagnetic diffraction theory. In FMM the field is expanded in different regimes with the application of boundary conditions at each interface. GMRFs in polymer materials (polycarbonate) are fabricated by Nano-Imprint Lithography (NIL) from a master silicon-stamp. The master Si-stamp is fabricated by the exposure of Electron Beam Lithography (EBL) on a negative & binary tone resist, Hydrogen silsesquioxane (HSQ) to make sub-wavelength grating structures. The fabricated gratings are coated by a high refractive index TiO2 amorphous thin layer using Atomic Layer Deposition (ALD). GMRFs are demonstrated both theoretically and experimentally to shift spectral resonance towards longer wavelengths with the increase in TiO2 thickness.

Temperature-induced changes in optical properties of thin film TiO2-Al2O3 bi-layer structures grown by atomic layer deposition

We investigate the optical properties and corresponding temperature-induced changes in highly uniform thin amorphous films and their bi-layer stacks grown by Atomic Layer Deposition (ALD). The environmentally driven conditions such as temperature, humidity and pressure have a significant influence on optical properties of homogeneous and heterogeneous bi-layer stacked structures of TiO2–Al2O3 and subsequently affect the specific sensitive nature of optical signals from nano-optical devices. Owing to the super hydrophilic behavior and inhibited surface defects in the form of hydrogenated species, the thermo-optic coefficient (TOC) of ~ 100 nm thick ALD–TiO2 films vary significantly with temperature, which can be used for sensing applications. On the other hand, the TOC of ~ 100 nm thick ALD–Al2O3 amorphous films show a differing behavior with temperature. In this work, we report on reduction of surface defects in ALD–TiO2 films by depositing a number of ultra-thin ALD–Al2O3 films to act as impermeable barrier layers. The designed and fabricated heterostructures of ALD–TiO2/Al2O3 films with varying ALD–Al2O3 thicknesses are exploited to stabilize the central resonance peak of Resonant Waveguide Gratings (RWGs) in thermal environments. The temperature-dependent optical constants of ALD–TiO2/Al2O3 bi-layer films are measured by a variable angle spectroscopic ellipsometer (VASE), covering a wide spectral range 380 ≤ λ ≤ 1800 nm at a temperature range from 25 to 105 °C. The Cauchy model is used to design and retrieve refractive indices at these temperatures, measured with three angles of incidence (59°, 67°, and 75°). The optical constants of 100 nm thick ALD–TiO2 and various combinational thicknesses of ALD–Al2O3 films are used to predict TOCs using a polynomial fitting algorithm.

Impact of Atomic Layer Deposition to the fabrication of athermal guided mode resonance filters

We study athermal properties of guided mode resonance filters in polymeric substrate coated by high index, amorphous, and isotropic TiO2 thin film. The narrow spectral linewidth filters are designed by Fourier Modal Method and fabricated by Electron Beam Lithography (EBL), Nano-imprint Lithography (NIL), and Atomic Layer Deposition (ALD) and characterized by Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and variable angle ellipsometer. Thin layers of ALD coated optical materials are fabricated as waveguide layers to confine and propagate waveguide modes. The thermo-spectral properties of such filters are studied by accompanying thermal expansion coefficient (TEC) and thermo-optic coefficient (TOC) effects of the layered materials. It is demonstrated theoretically and experimentally that the effects of TEC and TOC compensate for each other to stabilize narrow spectral peak within fraction of a nanometer. The wavelength shift in spectral reflectance peak (<; 1 nm) is a small fraction of the peaks Full Width Half Maximum (FWHM ~11nm) at 100 °C temperatures.