Optics Commons^™

Spectroscopic Analysis Of Tungsten Oxide Thin Films For Sensor Applications, Jose Luis Enriquez Carrejo

Open Access Theses & Dissertations

The objective of this study is targeted toward improving the quality of pure tungsten oxide (WO3) for application to the detection of poisoning gases, especially of H2S. While pure WO3 is a recognized candidate for gas sensing, its characteristics are strongly dependent on the conditions and methods used in its deposition.

Samples of WO3 thin films analyzed in this work were grown on silicon and sapphire substrates using RF magnetron sputtering at a number of different substrate temperatures and Ar:O2 pressure ratios. The properties of the samples were investigated spectroscopically with the goal of determining how variations in the above …

Two-Way Diffusion Model For Short Exposure Holographic Grating Formation In Acrylamide-Based Photopolymer, Tzvetanka Babeva, Izabela Naydenova, Dana Mackey, Suzanne Martin, Vincent Toal

Articles

A theoretical model for formation of a short exposure holographic grating is presented. The model accounts for both monomer and polymer diffusion and distinguishes between short polymer chains capable of diffusing and long polymer chains that are immobile. It is shown that the experimentally observed decrease of diffraction efficiency at higher spatial frequency can be predicted by assuming diffusion of short-chain polymers away from the bright fringes. The time evolution of the refractive index modulation after short exposure is calculated and compared with experimental results. The effects of diffusion coefficients, polymerization rates, intensity and spatial frequency of recording on the …

Optimisation Of An Acrylamide-Based Photopolymer System For Holographic Inscription Of Surface Patterns With Sub-Micron Resolution, Kevin Trainer, Kevin Wearen, Dimana Nazarova, Izabela Naydenova, Vincent Toal

Articles

We describe the optimisation of the holographic patterning of sub-micrometer surface relief structures in an acrylamide-based photopolymer. A substantial improvement in the photoinduced surface relief resolution was achieved by altering the photopolymer chemical composition and by introducing a single step post recording thermal treatment of the layers. It was observed that, by optimisation of the chemical composition of the photopolymer layers, the maximum achievable spatial frequency increases from 200 l/mm to 550 l/mm. The improvement of the surface relief amplitude by alteration of the chemical composition is limited due to the fact that both decrease of the plasticiser and increase …

Photopolymerizable Nanocomposites For Holographic Recording And Sensor Application, Elsa Leite, Izabela Naydenova, Svetlana Mintova, Louis Leclercq, Vincent Toal

Articles

Novel nanocomposites consisting of a water soluble acrylamide–based photopolymer and colloidal zeolite nanoparticles of zeolite Beta and zeolite A were prepared. The interactions between the photopolymer components and zeolite nanoparticles in the photopolymerizable nanocomposites were characterized for the first time by 13C NMR and Visible spectroscopy. It was found that the zeolite Beta nanoparticles (up to 5% wt.) behave as a non-inert additive, resulting in an effective increase in layer thickness that causes doubling of the diffraction efficiency of the nanocomposite in comparison to that of the undoped photopolymer. On the other hand, the nanocomposite containing zeolite A nanoparticles showed …

Optical Properties Of Photopolymer Layers Doped With Aluminophosphate Nanocrystals, Elsa Leite, Tzvetanka Babeva, E Ng, Vincent Toal, Svetlana Mintova, Izabela Naydenova

Articles

The optical properties of photopolymer layers consisting of an acrylamide–based matrix and microporous aluminophosphate nanocrystals of AEI- type are investigated. The compatibility of the photopolymer doped with the nanoparticles is studied. The surface and volume properties of the layers with different levels of doping with microporous nanocrystals are characterized. The effective refractive indices and absorption coefficients of the doped photopolymer layers are determined and used to calculate the refractive index and porosity of pure AEI nanoparticles used as dopants. Volume transmission gratings were recorded in the doped photopolymer layers at different spatial frequencies. By spatial monitoring of the characteristic Raman …

How Much Can Guided Modes Enhance Absorption In Thin Solar Cells?, Peter N. Saeta, Vivian E. Ferry, Domenico Pacifici, Jeremy N. Munday, Harry A. Atwater

All HMC Faculty Publications and Research

Absorption enhancement in thin metal-backed solar cells caused by dipole scatterers embedded in the absorbing layer is studied using a semi-analytical approach. The method accounts for changes in the radiation rate produced by layers above and below the dipole, and treats incoherently the subsequent scattering of light in guided modes from other dipoles. We find large absorption enhancements for strongly coupled dipoles, exceeding the ergodic limit in some configurations involving lossless dipoles. An antireflection-coated 100-nm layer of a-Si:H on Ag absorbs up to 87% of incident above-gap light. Thin layers of both strong and weak absorbers show similar strongly enhanced …

Optical Properties Of Charged Polymer With Nano-Sized Zeolite Inclusions, Temenujka Yovcheva, Izabela Naydenova, Ivanka Vlaeva, Rosen Todorov, Vincent Toal, Simeon Sainov

Conference Papers

The optical characteristics of photopolymer films doped with nanoparticles are studied. The investigated systems consist of a soft polymer matrix containing porous zeolite nanoparticles with concentrations varied in the range from 0wt.% to 7wt.%. 40μm thick layers are obtained by casting the photopolymer nanocomposite solution on glass substrates. The corona charging influence on the transmission spectra and on the surface and the effective refractive index of the dry layers is investigated.

Optical Properties Of Photopolymerisable Nanocomposites Containing Zeolite Nanoparticles, Izabela Naydenova, Tzvetanka Babeva, Elsa Leite, Nitesh Pandey, Temenujka Yovcheva, Svetlana Mintova, Vincent Toal

Conference Papers

Acrylamide-based photopolymerisable nanocomposites containing three different types of nanozeolites – Si-MFI, AlPO and BEA, were studied. The influence of the porous nanoparticles on the average refractive index, optical scattering, holographic recording properties and shrinkage were characterised.

Doubly Curved Crystal Optics For Monochromatic And Diffraction Enhanced X-Ray Imaging, Santosh Kumar Sahoo

Legacy Theses & Dissertations (2009 - 2024)

In radiographic imaging the contrast can be enhanced with the use of monochromatic radiation compared to conventional broadband source. Monochromatic beam radiation can be produced with conventional sources by diffraction off of flat monochromator crystals, but this produces a low monochromatic intensity for imaging.

Optical Properties Of Silica-Mfi Doped Acrylamide-Based Photopolymer, Tzvetanka Babeva, Rosen Todorov, Svetlana Mintova, Temenujka Yovcheva, Izabela Naydenova, Vincent Toal

Articles

The optical properties of acrylamide-based photopolymer doped with pure silica MFI-type zeolites are studied by refractometric and spectrophotometric means. Dynamic Light Scattering and Transmission Electron Microscopy are used for zeolite characterization and laser refractometry and White Light Interferometric profilometry are used for surface characterization of the composites. Refractive indices and absorption coefficients of composites are determined from their transmittance and reflectance spectra. The calculated dispersion curves are further used for deriving the zeolites refractive index and porosity and the latter compared to the values of total pore volume obtained from N2-sorption measurements. The impact of the doping level on the …

Recent And Emerging Applications Of Holographic Photopolymers And Nanocomposites, Izabela Naydenova, Pavani Kotakonda, Raghavendra Jallapuram, Tsvetanka Babeva, Denis Bade, Suzanne Martin, Vincent Toal, Svetlana Mintova

Conference Papers

Sensing applications of holograms may be based on effects such as change in the spacing of the recorded fringes in a holographic diffraction grating in the presence of an analyte so that the direction of the diffracted laser light changes, or, in the case of a white light reflection grating, the wavelength of the diffracted light changes. An example is a reflection grating which swells in the presence of atmospheric moisture to indicate relative humidity by a change is the colour of the diffracted light. These devices make use of the photopolymer’s ability to absorb moisture. In a more versatile …

Holographic Recording In Acrylamide Photopolymers: Thickness Limitations, Mohammad Mahmud, Izabela Naydenova, Nitesh Pandey, Tzvetanka Babeva, Raghavendra Jallapuram, Suzanne Martin, Vincent Toal

Articles

Holographic recording in thick photopolymer layers is important for application in holographic data storage, volume holographic filters and correlators. Here, the characteristics of acrylamidebased photopolymer layers having thickness from 250 μm to 1-mm were studied. For each layer thickness samples with three different values of absorbance were studied. By measuring both the first order diffraction efficiency growth of holographically recorded gratings and studying the diffraction patterns obtained, the influence of scattering on the diffraction efficiency of thick volume holographic gratings was analyzed. It was observed that above a particular thickness and absorbance, the first order diffraction efficiency significantly decreased because …

Fabrication Of Switchable Liquid Crystal Devices Using Surface Relief Gratings In Photopolymer, Pavani Kotakonda, Izabela Naydenova, Robert Howard, Suzanne Martin, Vincent Toal

Articles

Holographically recorded surface relief gratings in dry, self-developing acrylamide based photopolymer were used to fabricate two types of switchable liquid crystal (LC) device. One is an electrically switchable LC diffraction grating and the other is an electrically switchable polarization rotator. The electrically switchable diffraction grating was characterized by measuring the dependence of the intensity in the first diffracted order on the applied electric field. The polarization rotator was characterized by studying the influence of the applied electric field on the twist angle and the variation of intensity in the zero and the first orders of diffraction.

Nanocomposites For Novel Holographic Applications, Izabela Naydenova, Svetlana Mintova, Suzanne Martin, Vincent Toal

Articles

Nanozeolites incorporated into photopolymerizable holographic recording materials can be used for holographic sensing, data storage, and optical systems design.

Hologram-Based Humidity Indicator For Domestic And Packaging Applications, Izabela Naydenova, Raghavendra Jallapuram, Vincent Toal, Suzanne Martin

Conference Papers

A passive indicator for Relative Humidity based on a photopolymer recorded hologram has been developed. The indicator works on the principle that the wavelength reconstructed by a reflection hologram is dependent on the spacing between the fringe planes of the hologram. As the Relative Humidity changes, the hologram swells or shrinks and the apparent colour of the reconstructed image changes. The response time, operational range and reversibility of the indicator have been studied in a controlled humidity environment. Response times from less than a minute to tens of minutes depending on the holograms physical properties have been measured. Indicators with …

Holographic Recording In Nanoparticle-Doped Photopolymer, Izabela Naydenova, Hosam Sherif, Svetlana Mintova, Suzanne Martin, Vincent Toal

Conference Papers

A nanoparticle-doped acrylic photopolymer is characterised as a material for holographic recording. The influence of nanoparticles on the photopolymer dynamic range, dynamics of recording, temporal stability and mechanical stability in terms of shrinkage has been studied. The dynamics of recording and the temporal stability are investigated by real time monitoring of the build up of diffraction gratings of spatial frequencies of 200 to 2000 l/mm. The shrinkage has been characterised by recording slanted transmission gratings and observation of the change in the Bragg angle.

Characterisation Of An Acrylamide-Based Photopolymer For Data Storage Utilizing Holographic Angular Multiplexing, Hosam Sherif, Izabela Naydenova, Suzanne Martin, Colm Mcginn, Vincent Toal

Articles

An acrylamide-based photopolymer formulated in the Centre for Industrial and Engineering Optics has been investigated with a view to further optimisation for holographic optical storage. Series of 18 to 30 gratings were angularly multiplexed in a volume of photopolymer layer at a spatial frequency of 1500 lines/mm. Since the photopolymer is a saturable material, an exposure scheduling method was used to exploit the entire dynamic range of the material and allow equal strength holographic gratings to be recorded. This investigation yielded the photopolymer M/# for moderately thin layers. Photopolymer temporal stability was also studied by measuring variations of material shrinkage, …

Femtosecond Spectrotemporal Magneto-Optics, J.-Y. Bigot, L. Guidoni, E. Beaurepaire, Peter N. Saeta

All HMC Faculty Publications and Research

A new method to measure and analyze the time and spectrally resolved polarimetric response of magnetic materials is presented. It allows us to study the ultrafast magnetization dynamics of a CoPt₃ ferromagnetic film. The analysis of the pump-induced rotation and ellipticity detected by a broad spectrum probe beam shows that magneto-optical signals predominantly reflect the spin dynamics in ferromagnets.

Investigation Of The Diffusion Processes In Self-Processing Acrylamide-Based Photopolymer System, Izabela Naydenova, Raghavendra Jallapuram, Robert Howard, Suzanne Martin, Vincent Toal

Articles

Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental work on optimization of the high spatial frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive index modulation responsible for the diffraction grating build up. Monomer diffusion from dark to bright fringe areas increases the refractive index modulation. It is characterized with diffusion constant D0=1.6E-7 cm2/s. …

Investigation Of The Photoinduced Surface Relief Modulation In Acrylamide-Based Photopolymer, Izabela Naydenova, Emilia Mihaylova, Suzanne Martin, Vincent Toal

Conference Papers

A surface relief diffraction grating is inscribed in acrylamide photopolymer by holographic recording with spatial frequency below 300 l/mm. The periodic surface modulation appears in addition to a volume phase holographic grating. Due to the nature of the presented photopolymerisable material the gratings are induced by light only and no post-processing is required. An investigation of the dependance of the amplitude of the photoinduced relief modulation on spatial frequency of recording and on sample thickness has been carried out utilising white light interferometry. A model of the mechanism of surface relief formation is proposed on the basis of the measured …

Digital Holography And Application In Laser Metrology, Thian Khok Yong

Student Works (2000-2009)

A digital holography system has been developed in this study. The object is recorded and the hologram is stored electronically with a charged-couple-device (CCD) camera without any focusing optics. The reconstruction of the digitally sampled hologram is done numerically with a computer. The algorithm and software for numerical reconstruction, which consists in a numerical realization of the Fresnel-Kirchhoff diffraction integral, are designed and implemented with two­dimensional and three-dimensional simple objects. The characteristics of the system have been studied. For the CCD camera (768x494 pixels) been used with spatial resolution of about I 00 lines/mm, the angle between reference and object …

Short Terahertz Pulses From Semiconductor Surfaces: The Importance Of Bulk Difference‐Frequency Mixing, Peter N. Saeta, Benjamin I. Greene, Shun Lien Chuang

All HMC Faculty Publications and Research

The crystallographic orientation dependence of the far‐infrared (FIR) light generated at the (001) surface of a zincblende semiconductor is shown to derive principally from bulk difference‐frequency mixing. A strong modulation is observed for 1‐GW/cm² pulses on InP, which demonstrates that the radiated FIR wave produced by bulk optical rectification is comparable to that generated by the transport of photoinjected carriers. Using the bulk rectification light as a clock, we show that more than 95% of the light produced from an InP (111) crystal by 100‐fs, 100‐μJ pulses is generated in a time shorter than the excitation pulse.

Hysteresis And Anchoring Energy In Ferroelectric Liquid Crystals, Yuri Panarin

Articles

The frequency dispersion of the coercive force of Ferroelectric Liquid Crystals (FLC) cells has been detected and examined in the range of infralow (lower than 0.1 Hz) frequencies. To clarify the low-frequency dispersion, the model has been suggested, based on the arrangement of free charges and well describing the experimental curves. The method for determination of the energy of FLC anchoring at the surface, developed on the basis of the static hysteresis loop, has been proposed. The dependence of bistability and the anchoring energy upon the orientant layer thickness has experimentally been investigated.

Optical Rectification At Semiconductor Surfaces, Shun Lien Chuang, Stefan Schmitt-Rink, Benjamin I. Greene, Peter N. Saeta, Anthony F. J. Levi

All HMC Faculty Publications and Research

We show that far-infrared radiation can be generated in the depletion field near semiconductor surfaces via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This mechanism is conceptually different from those previously proposed and accounts for many recent experimental observations.