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.
We demonstrated the design, fabrication and characterization of three Resonant Waveguide Gratings (RWGs) with
different polymer substrate materials [polycarbonate (PC), cyclic-olefin-copolymer (COC) and Ormocomps). The RWGs
are designed by Fourier Modal Method and fabricated by Electron Beam Lithography, Nanoimprinting and Atomic
Layer Deposition. RWGs are investigated for athermal filtering device operation over a wide range of temperatures.
Spectral shifts of RWGs are described in terms of thermal expansion and thermo-optic coefficients of the selected
substrate and waveguide materials. Furthermore, the spectral shifts are explained on the basis of shrinkage strains,
frozen-in stresses and the molecular chain orientation in polymeric materials. The thermal spectral stability of these
filters was compared by theoretical calculations and experimental measurements. For PC gratings, there is a good
agreement between calculated and measured results with a net spectral shift of 0.8 nm over 75 °C wide temperature
range. Optical spectral characterization of COC and Ormocomp gratings showed larger red spectral shifts than predicted
by theoretical calculations. The deviation (0–1.5 nm) for the COC grating may result in by high modulus and inherent
stresses which were relaxed during heating and accompanied with the predominance of the thermal expansion
coefficient. The Ormocomps gratings were subjected to UV-irradiation, causing the generation of compressive
(shrinkage) strains, which were relieved on heating with a net result of expansion of material, demonstrated by thermal
spectral shifts towards longer wavelengths (0–2.5 nm). The spectral shifts might also be caused partially by the
reorientation and reconfiguration of the polymer chains.
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.
We investigate substrate overetch effect on resonance properties of sub-wavelength titanium oxide (TiO2) Guided Mode Resonance Filters (TiO2-GMRFs). The TiO2-GMRF is designed and fabricated to possess a non-polarizing behavior, which is strongly dependent on substrate (fused silica) overetch depth. For non-polarizing gratings at resonance, TE- and TM-modes have the same propagation constants. However, an overetch substrate effect results in splitting of the degenerate modes, which is studied theoretically and experimentally. The TiO2-SiO2 GMRFs are designed by Fourier Modal method (FMM) based on the rigorous calculation of electromagnetic diffraction theory at a designed wavelength of 850 nm. The TiO2-SiO2 gratings are fabricated by Atomic Layer Deposition (ALD), Electron Beam Lithography (EBL), and Reactive Ion Etching (RIE), and they are subsequently characterized structurally by Scanning Electron Microscopy (SEM) and optically by a spectroscopic ellipsometer. Several grating samples are fabricated by gradually increasing the overetch depth into fused silica and measuring the extent of TE- and TM-mode-splitting. A close agreement between the calculated and experimentally measured resonance wavelength spectral shift is found to describe the mode splitting of non-polarizing gratings.
Guided mode resonance (GMRF) phenomena occurs when the evanescent orders of a diffraction grating are coupled to the waveguide modes and propagate out at given optical parameters such as wavelength, angle, and state of polarization of incident light. The outcoupling field from a waveguide is, in general, polarization sensitive. Polarization insensitive 1D subwavelength grating structures with high diffraction efficiency at normal and oblique incidence are required, for example, in optical communications where output light may possess any polarization state. This means that an s- or p-polarized input optical field, which generally couples TE- or TM-modes in the waveguide under different resonance conditions, can be tuned at one resonance by selecting suitable grating parameters, regardless of the input polarization state. All of the polarization insensitive devices fabricated to date either employing a method which is not cost-effective or simple enough to some extent. In this work, we report the design and fabrication of two types of non-polarizing binary-structured onedimensional (1D) GMRF at normal incidence. A single layer binary-profile TiO2 resonant grating (grating-I) is fabricated by Atomic layer deposition (ALD), electron beam lithography (EBL) and reactive ion etching (RIE), which demonstrates almost perfect non-polarizing filtering effect with 1D grating under normal incidence. A double layer rectangular-profile polycarbonate-TiO2 1D GMR grating (grating-II) is fabricated by nanoimprint lithography (NIL) and ALD which also shows good non-polarizing property and the potential of cost-effective mass fabrication of such functional devices.
We investigated high efficiency organic-inorganic hybrid sub-wavelength binary diffraction gratings as partially
athermalized waveguides. The performance of the grating is evaluated in terms of low spectral shifts in heating
environment. The efficiency was determined to be least effective in temperature environment around room temperature.
The spectral characteristics of waveguide remain thermally stable by selecting optical grade polymer materials with high
thermal expansion coefficients, subsequently deposited by high index, amorphous TiO2 thin films by atomic layer
deposition (ALD) process. The spectral shifts towards longer and shorter wavelengths were investigated in terms of two
main parameters, thermal expansion coefficient (TEC) and thermo-optic coefficient (TOC) respectively. Realization of
partially athermalized waveguides are described by complete agreement in theoretically calculated and experimentally
measured results in the temperature range of 100 °C.
We investigate an affordable, accurate and large-scale production method to fabricate subwavelength grating structures by hot embossing replication in polycarbonate substrates. We use inorganic hydrogen silsesquioxane (HSQ), a high resolution, binary, negative electron beam resist, on silicon substrate to make a stamp for replication. The stamp is fabricated without any etching processes and with simple process steps. The process starts by spin coating an HSQ-resist layer on a silicon substrate. The desired film thickness is achieved by adjusting the spinning speed and time. The resist material is then subjected to e-beam writing and development followed by a heat treatment to enhance the hardness and to obtain hot embossing stamp material properties comparable with solid SiO2. A comparison with and without the silicon etching is also performed. We demonstrate that a high quality stamp for thermal nano-imprint lithography for optical gratings can be fabricated using an inexpensive process without an etching step. The process results in a uniform imprinting density over the entire grating surface and high imprint fidelity. The reflectance spectra of replicated grating structures are also shown to be in agreement with theoretical calculations.
We investigated an affordable, accurate and large scale production method to fabricate sub-wavelength grating structures
by replication in polycarbonate substrates by hot embossing. We used hydrogen silsesquioxane (HSQ) a high resolution,
binary, inorganic, negative electron beam resist, on silicon substrate to make a stamp for replication. We fabricated the
stamp on silicon by using HSQ-resist without any etching process with simple process steps. The process starts by
depositing an HSQ-resist layer on a silicon substrate and by a measurement of the desired film thickness by adjusting the
spinning speed and time. The resist material is then subjected to e-beam writing followed by a heat treatment to enhance
the hardness and to reveal properties analogous to solid SiO2 as a hot embossing stamp material. A comparison study is
made with and without the etching process with different etching rates. We demonstrate that an effective and inexpensive
stamp for thermal nano-imprint lithography (NIL) for optical gratings is provided without an etching process, which
gives a uniform imprinting density over the entire grating surface and high imprint fidelity. The reflectance spectra of
replicated grating structures are also shown to be in agreement with theoretical calculations.
We investigate the design of binary grating structures, e.g. resonance waveguide filters (RWFs), with
subwavelength feature sizes, taking the temperature dependence of different material parameters into account. Our final
goal is to demonstrate devices with athermal operation. We design the binary grating structures to be made in polymer
substrates, such as polycarbonate (PC), due to their potential for low cost, mass fabrication. The high thermal expansion
coefficient (TEC) of polymers, compared to inorganic optical materials, enhances the thermal sensitivity of the grating
structures. The gratings are designed using Fourier Model Method (FMM) by considering both thermal expansion and
thermo-optic effects on the resonance wavelength shift. The fabrication of RWF structures is proposed by e-beam
lithography, creating a master stamp and copying the structures into a polymer substrate by some replication techniques,
followed by an ALD deposition of TiO2. When the resonance waveguide grating RWG is designed for nearly room
temperature operation at a peak wavelength of 633 nm with a full width half maximum FWHM of 3 nm (TM mode
reflectance), the peak wavelength shifts 0.2 nm /50C when only the TEC is taken into account. However, taking into
account also the thermo-optic coefficients TOCs of PC and TiO2, the peak position shifts to 0.4 nm/ 50C on the opposite
side of spectral central wavelength. Thus the overall shift reduces to 0.2 nm /5 0C, illustrating partial athermalization. It
was also observed that thermo-optic coefficient TOC contributed more significantly than TEC effect. The wavelengths
shift was almost linear with respect to temperature for both effects and showed slopes of 0.0673, 0.0422 and 0.02352 for
TOC, TEC and combined effects, respectively.
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