Thin-film Optical Interference Research Articles

Influence of Al-doped ZnO transparent electrodes on thin-film interference in Cu2ZnSn(S,Se)4 thin-film solar cells prepared via a sputtering method

Transparent electrodes are often regarded as a trade-off between transmittance and sheet resistance, although the effect of thin-film interference on the performance of an optoelectronic device is significant. In this paper, we report the effect of optical thin-film interference of an Al-doped ZnO (AZO) transparent electrode on the performance of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. First, we distinguished the transmittance in the overall wavelength region (450–800 nm) of the AZO transparent electrode from the transmittance in the maximum absorption region for CZTSSe solar cells (550–700 nm). From the results, a thicker AZO transparent electrode showed higher transmittance (87.1%) than that of a thinner AZO transparent electrode (82.9%) in the maximum absorption region. These electrodes were applied in CZTSSe solar cells, which showed a difference of up to 26% in power conversion efficiency. In addition, the colors generated by the optical interference between the AZO transparent electrodes. We anticipate that our results will be useful for improving the performance and/or color tuning of thin-film solar cells for application in building-integrated photovoltaics.

The effect of gap on the quality of glass-to-glass welding using a picosecond laser

In order to study the influence of air gap and focus position on the quality of ps-laser glass-to-glass welding we implemented a method of detecting the air gap between glass surfaces based on optical thin film interference. The method allows real-time optimization of the welding process. We measure the air gap between glass surfaces to be bonded and the laser focal position, which are set as the constraint conditions affecting the nonlinear self-focusing of the laser beam. The processing parameters of picosecond laser welding are determined and the welding strength and the morphology of bonded zone are detected and observed. Laser welding with good performance can be achieved with an air gap of 2 μm. SEM microscopy shows that the bonding region between two glass surfaces is very smooth in the vicinity of the tear-drop shaped cross-sectional structure of the welded glass. A tensile strength inspection fixture is designed, and we measured a tensile fracture strength of over 30 MPa for the welded glass. Through confocal microscope analysis, the width and depth of fracture morphology of welded samples are shown to change with air gap, which is consistent with the theoretical analysis.

Spatially resolved electrochemistry enabled by thin-film optical interference.

Herein, we report an approach with sub-micrometer spatial resolution for studying local electrochemistry based on thin-film optical interference. The spatial resolution is achieved by successive interferometric imaging of a nanochannel membrane coated electrode during an electrochemical (EC) reaction. The EC reaction of redox molecules on the electrode induces variation of the refractive index of the nanochannel membrane, leading to changes of the intensity of interferometric light. Local EC reactions thus can be studied from the optical signal. The refractometry-based approach is versatile and label-free, and has promising application in nanosensing and nanocatalysis.

Self-Aligned IGZO TFTs with Boron Implanted Source/Drain Regions

Self-aligned channel regions in thin-film transistors (TFTs) have advantages in reduced parasitic capacitance and stage delay, and a reduction in overhead real estate. A common method used to fabricate self-aligned a-Si:H TFTs is to utilize a through-glass exposure of photoresist which is blocked by the opaque metal bottom-gate electrode [1,2]. This process does not require an additional photomask or lithographic alignment, and thus supports low production cost. Sputtered IGZO has been introduced into flat panel display product manufacturing, exhibiting a channel mobility of approximately an order of magnitude higher than a-Si:H. The working source/drain electrodes in IGZO TFTs can be direct metal contact regions to the IGZO, without the need for additional processes such as doping to render the IGZO conductive. Proper metallurgy and annealing processes can provide ohmic behavior with minimal series resistance [3], however this usually requires several microns of gate-to-source/drain overlap in order to ensure such behavior. Various self-aligned channel strategies have been demonstrated that either utilize through-glass exposure or a top-gate structure [4,5]; however such methods must address associated process integration challenges. The focus of this study is a novel technique to realize bottom-gate (BG) Indium-Gallium-Zinc Oxide (IGZO) TFTs that is not design-rule dependent or dependent on lithographic alignment. This work presents an alternative method to attain a self-aligned IGZO channel on the traditional staggered BG TFT structure which utilizes top-side exposure and optical thin-film interference. Reflection from the underlying bottom-gate electrode can result in a lower effective exposure of positive-working photoresist, creating a mirror image of the gate electrode in photoresist above the IGZO channel region (see Figure 1). One-dimensional verification of this technique was demonstrated using an underlying molybdenum gate electrode, 100 nm SiO2 gate dielectric, 50 nm IGZO, and AZ MIR 701 positive photoresist imaging layer. Top-side illumination at g-line (λ = 436 nm) with slight underexposure resulted in “reflection gates” that were aligned to the underlying bottom metal electrodes (see Figure 2). This pattern definition can then be used to protect the channel region during subsequent source/drain “activation” processes under investigation which include ion implantation, UV irradiation and plasma exposure [5-7]. Details of the lithographic process, selective source/drain activation, and resulting TFT operation will be presented. [1] K. Asama, T. Kodama, S. Kawai, Y. Nasu, and S.Yanagisawa, “Self-Alignment Processed A-Si TFT Matrix Circuit for LCD Panels”, SID Intl. Symp. Digest of Technical Papers, pp. 144-145, 1983. [2] Y. Kuo, “A New Process Using Two Photo-Masks to Prepare Trilayer Thin Film Transistors”, J. Electrochem. Soc., pp. 138, 1991. [3] T. Mudgal, N. Walsh, R.G. Manley, and K.D. Hirschman, “Impact of Annealing on Contact Formation and Stability of IGZO TFTs,” ECS J. Solid State Sci. Technol. 2014 volume 3, issue 9, Q3032-Q3034 (2014) / DOI: 10.1149/2.006409jss [4] Z. Xia, L. Lu, J. Li, H. S. Kwok, and M. Wong, “A Bottom-Gate Metal-Oxide Thin-Film Transistor With Self-Aligned Source/Drain Regions”, IEEE Trans. Electron Devices, vol. 65, no. 7, pp. 2820–2826, 2018. [5] R. Chen, W. Zhou, M. Zhang, M. Wong, and H. S. Kwok, “Self-Aligned Indium-Gallium-Zinc Oxide Thin-Film Transistor With Source/Drain Regions Doped by Implanted Arsenic”, IEEE Electron Device Lett., vol. 34, no. 1, pp. 60–62, 2013. [6] M.-H. Kim, S.-Y. Choi, S.-H. Jeon, J.-H. Lim, and D.-K. Choi, “Stability Behavior of Self-Aligned Coplanar a-IGZO Thin Film Transistors Fabricated by Deep Ultraviolet Irradiation”, ECS J. Solid State Sci. Technol., vol. 7, no. 4, pp. Q60–Q65, 2018. [7] X. He, L. Wang, X. Xiao, W. Deng, L. Zhang, M. Chan, and S. Zhang, “Implementation of Fully Self-Aligned Homojunction Double-Gate a-IGZO TFTs”, vol. 35, no. 9, pp. 927–929, 2014. Figure 1

(Invited) Photo-Assisted Etching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

Silicon can be made porous by electrochemical (anodization) etching in hydrofluoric acid (HF). This is the most controlled and used technique for the preparation of porous silicon (PSi). The porous structure depends on several factors, such as silicon resistivity and type, as well as HF concentration and anodization current. In the case of lightly-doped p-type silicon, uniform sponge-like nano-structures can be formed. This type of PSi layer is characterized in particular by its porosity and thickness. PSi is interesting for a variety of properties, some related to quantum confinement in its nanostructure, such as luminescence and porosity-dependent refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing. The post-anodization behavior of PSi in various environments is of interest for stability and degradation evaluation. The evolution of PSi is sometimes made intentional in order to further tailor the PSi properties after anodization. Typically, the effects of PSi oxidation (mainly for stabilization purposes), etching (to increase the porosity) or degradation in various environments (typically for biotechnology) are sometimes studied. Recently, we have made use of a particular behavior of the PSi photoconduction in HF: when PSi is illuminated in HF, only the charge carriers photogenerated in the Si substrate contribute to the measured photocurrent, and thus, the photocurrent is a measure of the optical transmission of PSi. This behavior allowed us to measure the PSi optical constants for various optical wavelengths and PSi porosities in situ and thus for the most pristine form of PSi and avoiding all the problems related to the drying and handling of high-porosity PSi free-standing layers1. We have also used the photoconduction to study the chemical etching of PSi in HF. We could monitor the etching as it proceeded and developed a model that allowed us to derive the HF-dependent etching rates and the absorption coefficient of PSi for a large range of porosities not explored so far, from as-prepared to close to 100% porosity (PSi fully dissolved)2. A small degree of tuning of the photoluminescence (PL) band of PSi may be achieved by adjusting anodization conditions, but fine tuning remains difficult. A few attempts aiming at controlling the PL or understanding PSi PL in HF have been attempted by using post-anodization photo-assisted etching in HF. The photoetching process itself was not studied in much detail, and were focusing on as-formed luminescent PSi. A better understanding of the photoetching process would be welcome to get better control and understanding of the effects. In this paper, we show how the photoetching of PSi (initially luminescent or not) can be monitored and its rate evaluated using the photoconduction technique that we have used previously in other studies1 , 2. The photocurrent was continuously measured during the illumination of PSi by monochromatic light, which was inducing photoetching. As PSi was etched, the PSi absorption was decreasing, more light was transmitted to the substrate and, as a result, the photocurrent was increasing. Figure 1 shows results for an initial porosity of 62%, an illumination wavelength of 450 nm, and different PSi thicknesses. A saturation state is reached when the PSi layer does not absorb anymore at the illumination wavelength. The waves observed for 1 μm and 2 μm-thick layers were attributed to thin film optical interference. The curve for 16 μm-thick PSi was affected by reproducibility problems because of instabilities in the diffusion of species and hydrogen bubbles inside the pores. For clean photoetching study, 4-μm thick PSi layers were used. Figure 2 shows the time evolution of the photocurrent for 4 μm-thick PSi layers, with different illumination powers at 450 nm. As expected, the photocurrent at saturation is proportional to the incident illumination power. Solid lines are fits obtained using a model that will be presented. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the photoetching reaction. The maximum photoetch rate was derived. The evolution of the in depth porosity profile can be obtained from the model as well. The effects of a range of parameters will be presented. 1 B. Gelloz, H. Fuwa, and L. H. Jin, Ecs Journal of Solid State Science and Technology 5 (3), P190 (2016). 2 B. Gelloz, K. Ichimura, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 6 (1), R1 (2017). Figure 1

Reducing N2O induced cross-talk in a NDIR CO2 gas sensor for breath analysis using multilayer thin film optical interference coatings

Carbon dioxide (CO2) gas sensing is an important aspect in the biomedical field of capnography, where cheap, fast and accurate measurement of exhaled CO2 vs. time is crucial in the evaluation of lung and tracheal function during surgical anaesthesia and is an under-used bio-marker for underlying respiratory conditions. Current detection methods do not adequately meet these requirements and suffer from considerable cross-talk associated with the commonly used anaesthetic gas nitrous oxide (N2O). In this work, we report how cross-talk can be reduced in a commercially available, low power (35mW) non-dispersive infrared (NDIR) CO2 gas sensor using thin film multilayer optical filters. Current sensor spectral response, spans 2500nm–5000nm via use of a pentanary alloy LED/photodiode optopair grown by molecular beam epitaxy (MBE), resulting in sensor sensitivity to gases with absorption bands in this region, including N2O. To reduce the effective spectral response of the sensor, capturing only CO2, a multilayer thin film optical interference bandpass filter has been designed and deposited directly onto the diode epi-structures using microwave plasma assisted DC magnetron sputtering. Three different coating configurations have been explored; LED-only coated, photodiode only coated and both coated. Gas sensor response to N2O for each coating configuration has been explored. It was found that application of an optical bandpass filter onto both the sensor LED and photodiode only was the most effective method of reducing sensor response to N2O, however no signal was observed in one of the two “LED and PD coated”, therefore optimal coating configuration for cross-talk reduction is subject to further investigation.

Porous Silicon-Based Biosensors: Towards Real-Time Optical Detection of Target Bacteria in the Food Industry.

Rapid detection of target bacteria is crucial to provide a safe food supply and to prevent foodborne diseases. Herein, we present an optical biosensor for identification and quantification of Escherichia coli (E. coli, used as a model indicator bacteria species) in complex food industry process water. The biosensor is based on a nanostructured, oxidized porous silicon (PSi) thin film which is functionalized with specific antibodies against E. coli. The biosensors were exposed to water samples collected directly from process lines of fresh-cut produce and their reflectivity spectra were collected in real time. Process water were characterized by complex natural micro-flora (microbial load of >107 cell/mL), in addition to soil particles and plant cell debris. We show that process water spiked with culture-grown E. coli, induces robust and predictable changes in the thin-film optical interference spectrum of the biosensor. The latter is ascribed to highly specific capture of the target cells onto the biosensor surface, as confirmed by real-time polymerase chain reaction (PCR). The biosensors were capable of selectively identifying and quantifying the target cells, while the target cell concentration is orders of magnitude lower than that of other bacterial species, without any pre-enrichment or prior processing steps.

Numerical techniques for high-throughput reflectance interference biosensing

We have developed a robust and rapid computational method for processing the raw spectral data collected from thin film optical interference biosensors. We have applied this method to Interference Reflectance Imaging Sensor (IRIS) measurements and observed a 10,000 fold improvement in processing time, unlocking a variety of clinical and scientific applications. Interference biosensors have advantages over similar technologies in certain applications, for example highly multiplexed measurements of molecular kinetics. However, processing raw IRIS data into useful measurements has been prohibitively time consuming for high-throughput studies. Here we describe the implementation of a lookup table (LUT) technique that provides accurate results in far less time than naive methods. We also discuss an additional benefit that the LUT method can be used with a wider range of interference layer thickness and experimental configurations that are incompatible with methods that require fitting the spectral response.

Design of optical fiber linear filter basing on optical film interference

A novel tunable F–P linear filter used in optical fiber communication is reported in this paper. The filter consists of two fiber planes relative to the incident fiber and the outgoing optical fibers, and two fiber ends are, respectively, coated with reflective optical thin film. The F–P interferometric cavity improves the transmittance response characteristics of traditional F–P cavity and helps to improve the linearity and linear range. The optical thin film interference matrix analysis method on the transmission of the filter response is calculated and analyzed. The analyzed results show that the new linear filter has the advantages of good linearity, wide linear range and linear interval adjustable.

Optical Detection of <em>E. coli</em> Bacteria by Mesoporous Silicon Biosensors

A label-free optical biosensor based on a nanostructured porous Si is designed for rapid capture and detection of Escherichia coli K12 bacteria, as a model microorganism. The biosensor relies on direct binding of the target bacteria cells onto its surface, while no pretreatment (e.g.by cell lysis) of the studied sample is required. A mesoporous Si thin film is used as the optical transducer element of the biosensor. Under white light illumination, the porous layer displays well-resolved Fabry-Pérot fringe patterns in its reflectivity spectrum. Applying a fast Fourier transform (FFT) to reflectivity data results in a single peak. Changes in the intensity of the FFT peak are monitored. Thus, target bacteria capture onto the biosensor surface, through antibody-antigen interactions, induces measurable changes in the intensity of the FFT peaks, allowing for a 'real time' observation of bacteria attachment. The mesoporous Si film, fabricated by an electrochemical anodization process, is conjugated with monoclonal antibodies, specific to the target bacteria. The immobilization, immunoactivity and specificity of the antibodies are confirmed by fluorescent labeling experiments. Once the biosensor is exposed to the target bacteria, the cells are directly captured onto the antibody-modified porous Si surface. These specific capturing events result in intensity changes in the thin-film optical interference spectrum of the biosensor. We demonstrate that these biosensors can detect relatively low bacteria concentrations (detection limit of 10(4) cells/ml) in less than an hour.

Optical Detection of <em>E. coli</em> Bacteria by Mesoporous Silicon Biosensors

A label-free optical biosensor based on a nanostructured porous Si is designed for rapid capture and detection of Escherichia coli K12 bacteria, as a model microorganism. The biosensor relies on direct binding of the target bacteria cells onto its surface, while no pretreatment (e.g. by cell lysis) of the studied sample is required. A mesoporous Si thin film is used as the optical transducer element of the biosensor. Under white light illumination, the porous layer displays well-resolved Fabry-Pérot fringe patterns in its reflectivity spectrum. Applying a fast Fourier transform (FFT) to reflectivity data results in a single peak. Changes in the intensity of the FFT peak are monitored. Thus, target bacteria capture onto the biosensor surface, through antibody-antigen interactions, induces measurable changes in the intensity of the FFT peaks, allowing for a 'real time' observation of bacteria attachment. The mesoporous Si film, fabricated by an electrochemical anodization process, is conjugated with monoclonal antibodies, specific to the target bacteria. The immobilization, immunoactivity and specificity of the antibodies are confirmed by fluorescent labeling experiments. Once the biosensor is exposed to the target bacteria, the cells are directly captured onto the antibody-modified porous Si surface. These specific capturing events result in intensity changes in the thin-film optical interference spectrum of the biosensor. We demonstrate that these biosensors can detect relatively low bacteria concentrations (detection limit of 104 cells/ml) in less than an hour.

Can interference patterns in the reflectance spectra of GaN epilayers give important information of carrier concentration?

Low-temperature reflectance spectra of a series of Si-doped GaN epilayers with different doping concentrations grown on sapphire by metal-organic chemical vapour deposition were measured. In addition to the excitonic polariton resonance structures at the band edge, interference oscillating patterns were observed in the energy region well below the band gap. The amplitudes of these oscillation patterns show a distinct dependence on the doping concentrations of the samples. From the thin-film optical interference principle, an approach connecting the amplitude of the interference oscillations and the impurity scattering was established. Good agreement between experiment and theory is achieved.

Self-Cleaning Organic Vapor Sensor Based on a Nanoporous TiO2 Interferometer

Porous thin films of TiO(2) are prepared and their use as chemical sensors for organic vapor analytes is investigated. Thin-film optical interference (Fabry-Perot) fringes in the reflectance spectrum are monitored using Reflectometric Interference Fourier Transform Spectroscopy (RIFTS). Three analytes are employed to probe the sensitivity of the porous TiO(2)-based sensors as a function of analyte vapor pressure: dodecane, isopropyl alcohol (IPA), and pentane. Measured lower limits of detection (3, 30, and 11, 000 ppmv for dodecane, IPA, and pentane, respectively) track the saturation vapor pressures (P(sat)) of the analytes (0.134, 45, and 513 Torr at 25°C for dodecane, IPA, and pentane, respectively); the analyte with the lowest value of P(sat) shows the lowest LLOD. Recovery of the sensor after a saturation dose of analyte is also dependent on P(sat): the sensor displays good recovery from pentane and IPA, and sluggish and incomplete recovery from dodecane. However, irradiation of the porous TiO(2) sensor with UV light in the presence of air accelerates recovery, and this process is attributed to photo-catalyzed oxidation of the analyte at the TiO(2) surface.

Engineering Nanostructured Porous SiO2 Surfaces for Bacteria Detection via “Direct Cell Capture”

An optical label-free biosensing platform for bacteria detection ( Escherichia coli K12 as a model system) based on nanostructured oxidized porous silicon (PSiO(2)) is introduced. The biosensor is designed to directly capture the target bacteria cells on its surface with no prior sample processing (such as cell lysis). The optical reflectivity spectrum of the PSiO(2) nanostructure displays Fabry-Pérot fringes characteristic of thin-film interference, enabling direct, real-time observation of bacteria attachment within minutes. The PSiO(2) optical nanostructure is synthesized and used as the optical transducer element. The porous surface is conjugated with specific monoclonal antibodies (immunoglobulin G's) to provide the active component of the biosensor. The immobilization of the antibodies onto the biosensor system is confirmed by attenuated total reflectance Fourier transform infrared spectroscopy, fluorescent labeling experiments, and refractive interferometric Fourier transform spectroscopy. We show that the immobilized antibodies maintain their immunoactivity and specificity when attached to the sensor surface. Exposure of these nanostructures to the target bacteria results in "direct cell capture" onto the biosensor surface. These specific binding events induce predictable changes in the thin-film optical interference spectrum of the biosensor. Our preliminary studies demonstrate the applicability of these biosensors for the detection of low bacterial concentrations. The current detection limit of E. coli K12 bacteria is 10(4) cells/mL within several minutes.

Preparation and characterization of a pH- and thermally responsive poly(N-isopropylacrylamide-co-acrylic acid)/porous SiO(2) hybrid.

A multifunctional nanohybrid composed of a pH- and thermoresponsive hydrogel, poly(N-isopropylacrylamide-co-acrylic acid), poly(NIPAM-co-AAc) is synthesized in-situ within the mesopores of an oxidized porous Si template. The hybrid is characterized by electron microscopy and by thin film optical interference spectroscopy. The optical reflectivity spectrum of the hybrid displays Fabry-Pérot fringes characteristic of thin film optical interference, enabling direct, real-time observation of the pH- induced swelling and volume phase transitions associated with the confined poly(NIPAM-co-AAc) hydrogel. The optical response correlates to the percentage of AAc contained within the hydrogel, with a maximum change observed for samples containing 20% AAc. The swelling kinetics of the hydrogel are significantly altered due to the nanoscale confinement; displaying a more rapid response to pH or heating stimuli relative to bulk polymer films. The inclusion of AAc dramatically alters the thermoresponsiveness of the hybrid at pH 7, effectively eliminating the lower critical solution temperature (LCST). The observed changes in the optical reflectivity spectrum are interpreted in terms of changes in the dielectric composition and morphology of the hybrids.

THE ORIGIN OF COLOR IN "FIRE" OBSIDIAN

A variety of obsidian from Glass Buttes, Oregon, known as “fire” obsidian and named for thin layers showing various colors, was investigated with field-emission scanning electron microscopy, X-ray energy-dispersive spectroscopy, electron back-scatter diffraction, and optical spectroscopy methods. Our study reveals that the thin layers mainly consist of concentrated nanometric crystals of magnetite. The thin layers, which have a thickness of 300 to 700 nm, give rise to brilliant colors in reflection. The color is caused by thin-film optical interference, in which the thin layers have a higher calculated index of refraction (1.496 < n < 1.519) than that of the host glass (n = 1.481).

Oxidation of evaporated porous silicon rugate filters

Rugate filters are thin-film optical interference coatings with sinusoidal variation of the refractive index. Several of these filters were fabricated with glancing angle deposition, which exploits atomic competition during growth to create nanoporous materials with controllable effective refractive index. This method enables the fabrication of devices with almost arbitrary refractive index profiles varying between the ambient, 1.0, and the index of the film material, in this case silicon with an index of 4.0 (at 600 nm). As these filters are inherently porous, oxidation of the silicon can occur throughout the device layer, and here we study the intentional oxidation of silicon filters by high-temperature reaction with gaseous oxygen. We find that a significant portion of the silicon filter oxidizes in approximately 10 min when heated to 600 degrees C-650 degrees C in an oxygen environment; oxidation then continues slowly over several hours. The presence of water vapor has little apparent effect on the oxidation reaction, and attempts to oxidize with ozone at room temperature were unsuccessful. As silicon filters oxidize to become silica, spectral blueshifts and increased short-wavelength transmittance are observed. Measured and calculated transmittance spectra generally agree, although the lack of absorption and dispersion in the theoretical model limits detailed comparison.

Recent progress in improving low-temperature stability of infrared thin-film interference filters

The degeneration of performance of an optical thin-film interference filter associated with the change of temperature is not acceptable. In this letter, we report a new progress in improving low-temperature performance of infrared narrow-band filters by using Pb(1-x)Ge(x)Te initial bulk alloy with appropriate Ge concentration x. It can be found that there exists a critical temperature for the investigated narrow-band filter, at which the temperature coefficient of filter is exactly zero. Therefore, by means of controlling the composition in (Pb(1-x)Ge(x))(1-y)Te(y) layers, the temperature coefficient of filter can be tunable at the designated low-temperature. In our present investigation, when temperature varies from 300 to 85 K, a shift of peak wavelength of 0.05935 nm.K-1 has been achieved.

Gradient-index narrow-bandpass filter fabricated with glancing-angle deposition

Glancing-angle deposition (GLAD) is a fabrication method capable of producing thin films with engineered nanoscale porosity variations. GLAD can be used to create optical thin-film interference filters from a single source material by modification of the film refractive index through control of film porosity. We present the effects of introducing a layer of constant low density into the center of a rugate thin-film filter fabricated with the GLAD technique. A rugate filter is characterized by a sinusoidal refractive-index profile. Embedding a layer of constant refractive index, with a thickness equal to one period of the rugate index variation, causes a narrow bandpass to appear within the filter's larger stop band. Transmittance measurements of such a gradient-index narrow-bandpass filter, formed with titanium dioxide, revealed an 83% transmittance peak at a vacuum wavelength of 522 nm, near the center of the stop band, with a FWHM bandwidth of 15 nm.

Shock Induced Reaction Observed via Ultrafast Infrared Absorption in Poly(vinyl nitrate) Films

Ultrafast laser shock generation methods and broad-band infrared reflection absorption spectroscopy have provided evidence that shock-induced chemistry in a condensed-phase energetic material occurs on a time scale of tens of picoseconds and involves the nitro group as a primary reactant. Femtosecond broad-band infrared reflection absorption spectroscopy was used to monitor films of the energetic polymer poly(vinyl nitrate) during shock loading and rarefaction. At ∼18 GPa, poly(vinyl nitrate) films exhibited loss of absorption in the nitro group stretch modes that did not recover upon rarefaction, providing an indication of initial chemical reaction. At pressures ≤9 GPa, the observed spectral changes are ascribed to thin film optical interference effects, without chemical reaction. The loss of infrared absorption required an induction time of tens of picoseconds after shock passage, supporting reaction mechanisms that require vibrational energy transfer rather than prompt reaction.