R-3 Repository :: Browsing by Author "Tittel, Frank K."

Browsing by Author "Tittel, Frank K."

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A compact mid-infrared dual-gas CH4/C2H6 sensor using a single interband cascade laser and custom electronics
(SPIE, 2017) Ye, Weilin; Zheng, Chuantao; Tittel, Frank K.; Sanchez, Nancy P.; Gluszek, Aleksander K.; Hudzikowski, Arkadiusz J.; Lou, Minhan; Dong, Lei; Griffin, Robert J.
A compact mid-infrared (MIR) dual-gas sensor system was demonstrated for simultaneous detection of methane (CH4) and ethane (C2H6) using a single continuous-wave (CW) interband cascade laser (ICL) based on tunable laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS). Ultracompact custom electronics were developed, including a laser current driver, a temperature controller and a lock-in amplifier. These custom electronics reduce the size and weight of the sensor system as compared with a previous version based on commercial electronics. A multipass gas cell with an effective optical length of 54.6 m was employed to enhance the absorption signal. A 3337 nm ICL was capable of targeting a C2H6 absorption line at 2996.88 cm-1 and a CH4 line at 2999.06 cm-1. Dual-gas detection was realized by scanning both the CH4 and C2H6 absorption lines. Based on an Allan deviation analysis, the 1 σ minimum detection limit (MDL) was 17.4 ppbv for CH4 and 2.4 ppbv for C2H6 with an integration time of 4.3 s. TDLAS based sensor measurements for both indoor and outdoor mixing ratios of CH4 and C2H6 were conducted. The reported single ICL based dual-gas sensor system has the advantages of reduced size and cost without influencing the midinfrared sensor detection sensitivity, selectivity and reliability.
A computer controlled, narrow linewidth, broadly tunable, continuous wave laser
(1979) Ketkar, Suhas Narayan; Dunning, F. Barry; Tittel, Frank K.; Estle, Thomas L.
A continuous wave, tunable, single frequency dye ring laser has been developed. This laser is capable of giving output powers, in a single longitudinal mode, in excess of 7 mV, using different dyes and Second Harmonic Generation or Sum Frequency Mixing schemes, a wavelength coverage from 26 nm to 8 nm can be achieved. Using a PDP 11/3 minicomputer and a CAMAC dataway, control of a linear as well as a ring laser, oscillating in a single longitudinal mode, has been accomplished. The various optical elements in the dye laser cavity and the nonlinear mixing device are synchronously tracked. The laser can be operated both as a low resolution (4 GHz bandwidth) and a high resolution (.1 GHz bandwidth) visible and ultraviolet radiation source. Using Boyd and Kleinman theory for Second Harmonic Generation of focussed Gaussian beams,theoretical estimates for the wavelength and angle bandwidths are made. Some of the results have been experimentally verified.
A Miniaturized QEPAS Trace Gas Sensor with a 3D-Printed Acoustic Detection Module
(MDPI, 2017) Yang, Xiaotao; Xiao, Youhong; Ma, Yufei; He, Ying; Tittel, Frank K.
A 3D printing technique was introduced to a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and is reported for the first time. The acoustic detection module (ADM) was designed and fabricated using the 3D printing technique and the ADM volume was compressed significantly. Furthermore, a small grin lens was used for laser focusing and facilitated the beam adjustment in the 3D-printed ADM. A quartz tuning fork (QTF) with a low resonance frequency of 30.72 kHz was used as the acoustic wave transducer and acetylene (C2H2) was chosen as the analyte. The reported miniaturized QEPAS trace gas sensor is useful in actual sensor applications.
A NDIR Mid-Infrared Methane Sensor with a Compact Pentahedron Gas-Cell
(MDPI, 2020) Ye, Weilin; Tu, Zihan; Xiao, Xupeng; Simeone, Alessandro; Yan, Jingwen; Wu, Tao; Wu, Fupei; Zheng, Chuantao; Tittel, Frank K.
In order to improve the performance of the large divergence angle mid-infrared source in gas sensing, this paper aims at developing a methane (CH4) sensor with non-dispersive infrared (NDIR) technology using a compact pentahedron gas-cell. A paraboloid concentrator, two biconvex lenses and five planar mirrors were used to set up the pentahedron structure. The gas cell is endowed with a 170 mm optical path length with a volume of 19.8 mL. The mathematical model of the cross-section and the three-dimension spiral structure of the pentahedron gas-cell were established. The gas-cell was integrated with a mid-infrared light source and a detector as the optical part of the sensor. Concerning the electrical part, a STM32F429 was employed as a microcontroller to generate the driving signal for the IR source, and the signal from the detector was sampled by an analog-to-digital converter. A static volumetric method was employed for the experimental setup, and 20 different concentration CH4 samples were prepared to study the sensor’s evaluation, which revealed a 1σ detection limit of 2.96 parts-per-million (ppm) with a 43 s averaging time.
A near-infrared gas sensor system based on tunable laser absorption spectroscopy and its application to CH4/C2H2 detection
(SPIE, 2017) He, Qixin; Zheng, Chuantao; Liu, Huifang; Wang, Yiding; Tittel, Frank K.
A near-infrared (NIR) dual-channel differential gas sensor system was experimentally demonstrated based on tunable laser absorption spectroscopy (TLAS) and wavelength modulation spectroscopy (WMS). The sensor consists of four modules, including distributed feedback (DFB) lasers for the detection of targeted gases, a custom portable DFB driver compatible for butterfly-packaged DFB lasers, a 20cm-long open-reflective gas-sensing probe and a custom cost-effective lock-in amplifier for harmonic signal extraction. The optical and electrical modules were integrated into a standalone sensor system, which possesses advantages of user-friendly operation, good stability, small volume and low cost. With different DFB lasers, the sensor system can be used to detect different gases. Two DFB diode lasers with emission wavelengths of 1.65 μm and 1.53 μm were used to detect CH4 and C2H2, respectively. Standard CH4 and C2H2 samples were prepared and experiments were carried out to evaluate the performance of the two-gas TLAS sensor system. The relation between the second harmonic amplitudes (2f) and gas concentrations was obtained for the two gases by means of calibration. Both the detection error and the limit of detection (LoD) were determined experimentally. The sensor system will be useful in industrial trace gas monitoring due to its use of a low-loss optical fiber and an open-reflective gas-sensing probe.
A Sensitive Carbon Dioxide Sensor Based on Photoacoustic Spectroscopy with a Fixed Wavelength Quantum Cascade Laser
(MDPI, 2019) Qiao, Shunda; Qu, Yanchen; Ma, Yufei; He, Ying; Wang, Yao; Hu, Yinqiu; Yu, Xin; Zhang, Zhonghua; Tittel, Frank K.
A photoacoustic spectroscopy (PAS) based carbon dioxide (CO2) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 μm in the mid-infrared spectral region matched a fundamental CO2 absorption line. Amplitude modulation of the laser intensity was used to match the resonant photoacoustic (PA) cell. The noise from the background was reduced with the correlation demodulation technique. The experimental results showed that the sensor had excellent signal stability and a concentration linear response. When the integration time was 1 s, a 1σ minimum detection limit (MDL) of 2.84 parts per million (ppm) for CO2 detection was achieved. The long-term stability of the sensor was evaluated by means of an Allan deviation analysis. With an integration time of ~100 s, the MDL was improved to 1 ppm. This sensor was also used to measure the CO2 concentration from some common emission sources, such as cigarette smoking, automobile exhaust, and the combustion of some carbon-containing materials, which confirmed the stability and robustness of the reported FW-QCL based CO2-PAS sensor system.
A Sensitive Carbon Monoxide Sensor Based on Photoacoustic Spectroscopy with a 2.3 μm Mid-Infrared High-Power Laser and Enhanced Gas Absorption
(MDPI, 2019) Qiao, Shunda; Ma, Yufei; He, Ying; Yu, Xin; Zhang, Zhonghua; Tittel, Frank K.
A photoacoustic spectroscopy (PAS)-based carbon monoxide (CO) gas sensor with a high-power laser and an enhanced gas absorption was demonstrated. The light source was a distributed feedback (DFB), continuous wave (CW) diode laser with a high output power of ~8 mW to give a strong excitation. The target gas received optical absorption enhanced two times by using a right-angle prism reflecting the laser beam. In order to reduce the noise from the background, wavelength modulation spectroscopy (WMS) and second-harmonic detection techniques were used. The modulation frequency and modulation depth were optimized theoretically and experimentally. Water vapor was added in the PAS sensor system to increase the vibrational–translational (V–T) relaxation rate of the CO molecule, which resulted in an ~8 times signal enhancement compared with the using of a dry CO/N2 gas mixture. The amplitude of the 2f signal had a 1.52-fold improvement compared to the one with only one time absorption. The experimental results showed that such a sensor had an excellent linear response to the optical power and gas concentration. At 1 s integration time, a minimum detection limit (MDL) for CO detection of 9.8 ppm was achieved. The long-term stability of the sensor system was evaluated with an Allan deviation analysis. When the integration time was 1100 s, the MDL improved to be 530 ppb. The detection performance of such a PAS-based CO sensor can be further improved when a laser with a higher output power and increasing optical absorption times is used.
A tunable laser-based mid-infrared source for use in trace gas detection
(1997) Wang, Shunxi; Tittel, Frank K.
A new widely tunable mid-infrared (IR) source based on difference-frequency generation (DFG) in Gallium Selenide (GaSe) for use in high-resolution spectroscopy and trace gas detection is described. The characteristics of type-I phasematching in GaSe are studied. Preliminary results on the improvement of the dispersion relations for GaSe are presented. Spectroscopic feasibility experiments are reported that involve the spectroscopy of ethylene $\rm (C\sb2H\sb4)$ at low pressures near 950 $\rm cm\sp{-1}.$ Two near-IR Ti:Sapphire lasers served as convenient pump sources, which can be eventually replaced by two high power near-IR diode lasers. Design issues (including optimum elliptical focusing conditions), potential improvements and new directions are also discussed.
Acoustic Detection Module Design of a Quartz-Enhanced Photoacoustic Sensor
(MDPI, 2019) Wei, Tingting; Wu, Hongpeng; Dong, Lei; Tittel, Frank K.
This review aims to discuss the latest advancements of an acoustic detection module (ADM) based on quartz-enhanced photoacoustic spectroscopy (QEPAS). Starting from guidelines for the design of an ADM, the ADM design philosophy is described. This is followed by a review of the earliest standard quartz tuning fork (QTF)-based ADM for laboratory applications. Subsequently, the design of industrial fiber-coupled and free-space ADMs based on a standard QTF for near-infrared and mid-infrared laser sources respectively are described. Furthermore, an overview of the latest development of a QEPAS ADM employing a custom QTF is reported. Numerous application examples of four QEPAS ADMs are described in order to demonstrate their reliability and robustness.
An electron-beam controlled discharge laser o the C-->A transition of xenon fluorite
(1985) Stiegler, Harvey J.; Wilson, William L.; Tittel, Frank K.; Sauerbrey, R. A.
A XeF(C->A) laser utilizing an electron-beam controlled electric discharge for excitation was investigated. Studies were made of He, Ne, and Ar as buffer gases with various combinations of NF, F2, and Xe. The partial pressure of the various gas components was varied to find a mixture which would achieve optimum performance. Energy deposition by the discharge was confirmed by observing the intensity of fluorescence. Positive optical gain was achieved for a duration of 3 ns to 4 ns and a peak gain of > 1% cm was measured. Spectral narrowing and gain sufficient to overcome cavity losses were evidence of laser action. Output observed from the laser was on the order of 1 uJ/liter of active volume. Performance was limited by inability to rapidly deposit sufficient energy into the discharge. The possibility of a longterm absorption phenomenon related to the electric discharge may also have limited performance.
Analysis of overtone flexural modes operation in quartz-enhanced photoacoustic spectroscopy
(The Optical Society, 2016) Tittel, Frank K.; Sampaolo, Angelo; Patimisco, Pietro; Dong, Lei; Geras, Antonina; Starecki, Tomasz; Spagnolo, Vincenzo
A detailed investigation of a set of custom quartz tuning forks (QTFs), operating in the fundamental and first overtone flexural modes is reported. Support losses are the dominant energy dissipation processes when the QTFs vibrate at the first overtone mode. These losses can be decreased by increasing the ratio between the prong length and its thickness. The QTFs were implemented in a quartz enhanced photoacoustic spectroscopy (QEPAS) based sensor operating in the near-IR spectral range and water vapor was selected as the gas target. QTF flexural modes having the highest quality factor exhibit the largest QEPAS signal, demonstrating that, by optimizing the QTF prongs sizes, overtone modes can provide a higher QEPAS sensor performance with respect to using the fundamental mode.
Application of diode-laser-pumped difference-frequency generation to spectroscopic trace gas detection in the atmosphere
(1995) Petrov, Konstantin Petrovich; Tittel, Frank K.
The application of diode-laser-pumped tunable IR sources to trace gas detection in air is discussed. The design and technology issues of CW tunable mid-infrared difference-frequency generation are addressed. Feasibility experiments are reported in which the detection of CH$\sb4$, and CO was performed at low pressure and in air using two laser sources near 3.2 $\mu$m, and 4.7 $\mu$m, respectively. They were based on difference-frequency mixing in AgGaS$\sb2$ at room temperature. Pump sources used were a diode-pumped Nd:YAG laser at 1064 nm, a laser diode at 693 nm, and a tunable fiber-coupled Ti:Sapphire laser near 795 nm, and 813 nm. Detection limits of 2.4 ppb*m/$\rm\sqrt{Hz}$ for CO, and 9.0 ppb*m/$\rm\sqrt{Hz}$ for CH$\sb4$ in air are estimated for 0.1 $\mu$W of IR probe power based upon the performance of the sources and detector.
Application of Micro Quartz Tuning Fork in Trace Gas Sensing by Use of Quartz-Enhanced Photoacoustic Spectroscopy
(MDPI, 2019) Lin, Haoyang; Huang, Zhao; Kan, Ruifeng; Zheng, Huadan; Liu, Yihua; Liu, Bin; Dong, Linpeng; Zhu, Wenguo; Tang, Jieyuan; Yu, Jianhui; Chen, Zhe; Tittel, Frank K.
A novel quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a micro quartz tuning fork (QTF) is reported. As a photoacoustic transducer, a novel micro QTF was 3.7 times smaller than the usually used standard QTF, resulting in a gas sampling volume of ~0.1 mm3. As a proof of concept, water vapor in the air was detected by using 1.39 μm distributed feedback (DFB) laser. A detailed analysis of the performance of a QEPAS sensor based on the micro QTF was performed by detecting atmosphere H2O. The laser focus position and the laser modulation depth were optimized to improve the QEPAS excitation efficiency. A pair of acoustic micro resonators (AmRs) was assembled with the micro QTF in an on-beam configuration to enhance the photoacoustic signal. The AmRs geometry was optimized to amplify the acoustic resonance. With a 1 s integration time, a normalized noise equivalent absorption coefficient (NNEA) of 1.97 × 10−8 W·cm−1·Hz−1/2 was achieved when detecting H2O at less than 1 atm.
Atmospheric ammonia measurements and implications for particulate matter formation in urban and suburban areas of Texas
(2013-09-16) Gong, Longwen; Griffin, Robert J.; Cohan, Daniel S.; Tittel, Frank K.
In order to improve the current understanding of the dynamics of ammonia (NH3) in the Greater Houston and Dallas-Fort Worth (DFW) areas and to examine the effects of NH3 on local and regional air quality with respect to particulate matter formation, intensive field investigations were made. A 10.4-μm external cavity quantum cascade laser based-sensor employing conventional photo-acoustic spectroscopy was used to conduct real-time and continuous measurements of atmospheric NH3 in this work. Results from the Houston campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 8.7 ppb with a mean of 2.4±1.2 (1σ) ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1±2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH3 mixing ratios existed between weekdays and weekends. During the simultaneous high-time-resolution measurements of gaseous and aerosol species in summer, elevated NH3 levels occurred around mid-day when NH4+ (0.5 ± 1.0 μg m-3) and SO42- (4.5 ± 4.3 μg m-3) also increased considerably, indicating that NH3 likely influenced aerosol particle mass. NH4+ mainly existed in the form of (NH4)2SO4 and NH4HSO4; by contrast, the formation of NH4NO3 and NH4Cl was suppressed. Power plant plumes were found to be potential contributors to the enhancements in NH3 at the urban sampling site under favorable meteorological conditions. Increased particle number concentrations were predicted by the SAM-TOMAS model downwind of a large coal-fired power plant when NH3 emissions (based on these measurements) were included, highlighting the potential importance of NH3 with respect to particle number concentration. Measurements also show the role of NH3 in new particle formation in Houston under low-sulfur conditions. Results from the DFW campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 10.1 ppb, with a mean of 2.7 ± 1.7 ppb. The diurnal profile of NH3 exhibited a daytime increase, likely due to increasing temperatures affecting temperature-dependent sources in the study region. Automobiles might be potential sources of NH3 on Sundays based on the Pearson’s correlation coefficient between NH3 and carbon monoxide, but the relationship did not exist on weekdays and Saturdays, probably due to decreased traffic volume and different traffic composition. According to the results from the EPA PMF 3.0 model, biogenic (primarily vegetation and soil) emissions were major contributors to gas-phase NH3 levels measured at the suburban site during the campaign. In addition, agriculture (especially livestock-related activities) also was expected to be a potentially significant source of NH3 based on the nature of the region. Inorganic aerosol components of submicron particles (PM1) (4.41 ± 2.13 μg m-3) were dominated by SO42- (1.25 ± 0.66 μg m-3), followed by NH4+ (0.44 ± 0.24 μg m-3) and NO3- (0.12 ± 0.11 μg m-3). Pearson’s correlation coefficients between NH4+, SO42-, and NO3- imply that particulate NH4+ mainly existed as (NH4)2SO4 and that NH4NO3 was not formed during most of the study period, likely due to high temperatures (30.15 ± 4.12 oC) over the entire campaign. Ambient aerosols tended to be nearly neutral. Theoretical calculations of thermodynamic equilibrium were performed to consider the formation of NH4NO3 and NH4Cl. When relative humidity (RH) was lower than deliquescence relative humidity (DRH), the partial pressure products of PNH3PHNO3 and PNH3PHCl were smaller than the associated equilibrium constants, indicating the lack of NH4NO3 and NH4Cl formation. When RH was above DRH, higher levels of NO3- often were observed. A strong relationship between NO3- and SO42- at higher RH suggests that NH4NO3 might be formed on the moist surface of pre-existing sulfate aerosols. In the particle mixture, (NH4)2SO4 reduces the equilibrium constant, making the aqueous system a more favorable medium for NH4NO3 formation. In addition, measured particle number size distributions showed that an aerosol nucleation and growth event was coincident with humid periods characterized by substantially increased concentrations of particulate NH4+, NO3-, and SO42-. Excess NH4+ also was found to be correlated closely with NO3- during this episode when elevated PM1 levels imply aqueous NH4NO3 formation.
Automated Evaluation of Critical Features in VLSI Layouts Based on Photolithographic Simulations
(SME Press, 1996-01-01) Cavallaro, Joseph R.; Sengupta, Chaitali; Tittel, Frank K.; Wilson, William L. Jr.; Center for Multimedia Communication
This paper describes a CAD tool (An Integrated CAD Framework) which links VLSI layout editors to lithographic simulators and provides information on the simulated resolution of a feature to the circuit designer. The designer can modify the original layout based upon this analysis to create compact circuits with better yield capabilities. The objective of this project is to improve the manufacturability of high density VLSI integrated circuits.
Automated Evaluation of Critical Features in VLSI Layouts Based on Photolithographic Simulations
(IEEE, 1997-11-01) Sengupta, Chaitali; Cavallaro, Joseph R.; Wilson, William L.; Tittel, Frank K.; Center for Multimedia Communication
In this paper, we address the problem of identifying and evaluating “critical features” in an integrated circuit (IC) layout. The “critical features” (e.g., nested elbows and open ends) are areas in the layout that are more prone to defects during photolithography. As feature sizes become smaller (sub-micron range) and as the chip area becomes larger, new process techniques (such as, using phase shifted masks for photolithography), are being used. Under these conditions, the only means to design compact circuits with good yield capabilities is to bring the design and process phases of IC manufacturing closer. This can be accomplished by integrating photolithography simulators with layout editors. However, evaluation of a large layout using a photolithography simulator is time consuming and often unnecessary. A much faster and efficient method would be to have a means of automatically identifying “critical features” in a layout and then evaluate the “critical features” using a photolithography simulator. Our technique has potential for use either to evaluate the limits of any new and nonconventional process technique in an early process definition phase or in a mask house, as a postprocessor to improve the printing capability of a given mask. This paper presents a CAD tool (an Integrated CAD Framework) which is built upon the layout editor, Magic, and the process simulator, Depict 3.0, that automatically identifies and evaluates “critical features”
Beat frequency quartz-enhanced photoacoustic spectroscopy for fast and calibration-free continuous trace-gas monitoring
(Springer Nature, 2017) Wu, Hongpeng; Dong, Lei; Zheng, Huadan; Yu, Yajun; Ma, Weiguang; Zhang, Lei; Yin, Wangbao; Xiao, Liantuan; Jia, Suotang; Tittel, Frank K.
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a sensitive gas detection technique which requires frequent calibration and has a long response time. Here we report beat frequency (BF) QEPAS that can be used for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications. The resonance frequency and Q-factor of the quartz tuning fork (QTF) as well as the trace-gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the QTF is demodulated at its non-resonance frequency. Hence, BF-QEPAS avoids a calibration process and permits continuous monitoring of a targeted trace gas. Three semiconductor lasers were selected as the excitation source to verify the performance of the BF-QEPAS technique. The BF-QEPAS method is capable of measuring lower trace-gas concentration levels with shorter averaging times as compared to conventional PAS and QEPAS techniques and determines the electrical QTF parameters precisely.
Cavity-enhanced photoacoustic sensor based on a whispering-gallery-mode diode laser
(Copernicus Publications, 2019) Pan, Yufeng; Dong, Lei; Wu, Hongpeng; Ma, Weiguang; Zhang, Lei; Yin, Wangbao; Xiao, Liantuan; Jia, Suotang; Tittel, Frank K.
A cavity-enhanced photoacoustic (CEPA) sensor was developed based on an ultra-narrow linewidth whispering-gallery-mode (WGM) diode laser. A cavity-enhanced photoacoustic module (CEPAM) was designed to match the output beam from the WGM-diode laser, resulting in an increase in the excitation light power, which, in turn, significantly enhanced the photoacoustic signal amplitude. The results show that a signal gain factor of 166 was achieved, which is in excellent agreement with the power enhancement factor of 175 after considering the power transmissivity. The performance of the sensor was evaluated in terms of the detection sensitivity and linearity. A 1σ detection limit of 0.45 ppmV for C2H2 detection was obtained at atmospheric pressure with a 1 s averaging time.
Characterization of Iron Doped Lithium Niobate for Holographic Storage Applications
(1976-06-20) Shah, Rajiv R.; Kim, Dae M.; Rabson, Thomas A.; Tittel, Frank K.
A detailed study of eight systematically chosen Fe:LiNb0₃ crystals is presented. Correlation between the photorefractive sensitivity and various chemical properties of Fe:LiNb0₃ is investigated in order to ascertain optimum performance of Fe:LiNb0₃ crystals in holographic storage and display applications. Concentrations of Fe²⁺ and Fe³⁺ ions have been determined from optical and EPR spectra while impurities have been detected from x-ray emission and infrared spectra. Particular emphasis is placed on investigating the dependence on Fe³⁺ ion concentrations of the photorefractive sensitivity. The photorefractive sensitivity is shown to depend primarily on the concentration of Fe²⁺ ions in Fe:LiNb0₃. This fact seems to suggest that Fe²⁺ ions are the impurity centers responsible for the photorefractive effect in Fe:LiNb0₃. Spectral dependence of the photorefractive sensitivity and its modification due to oxygen annealing are also reported. Our results indicate that an unannealed Fe:LiNb0₃ crystal containing 0.05 mole % Fe with 20-25% of the ions in the Fe²⁺ state and the remainder in the Fe³⁺ state possesses the most favorable photorefractive sensitivity.
Characterizing Engineered Nanomaterials: From Environmental, Health and Safety Research to the Development of Shaped Nanosphere Lithography for Metamaterials
(2012-09-05) Lewicka, Zuzanna; Colvin, Vicki L.; Tittel, Frank K.; Wong, Michael S.
In this thesis two issues in nanotechnology have been addressed. The first is the comprehensive characterization of engineered nanomaterials prior to their examination in toxicology and environmental studies. The second is the development of a method to produce nanostructure arrays over large areas and for low cost. A major challenge when assessing nanomaterial’s risks is the robust characterization of their physicochemical properties, particularly in commercial products. Such data allows the critical features for biological outcomes to be determined. This work focused on the inorganic oxides that were studied in powdered and dispersed forms as well as directly in consumer sunscreen products. The most important finding was that the commercial sunscreens that listed titania or zinc oxide as ingredients contained nanoscale materials. Cell free photochemical tests revealed that ZnO particles without any surface coating were more active at generating ROS than surface coated TiO2 nanoparticles. These studies make clear the importance of exposure studies that examine the native form of nanomaterials directly in commercial products. The second part of this thesis presents the development of a new method to fabricate gold nanoring and nanocrescent arrays over large areas; such materials have unique optical properties consonant with those described as metamaterials. A new shaped nanosphere lithography approach was used to manipulate the form of silica nanospheres packed onto a surface; the resulting array of mushroom structures provided a mask that after gold evaporation and etching left either golden rings or crescents over the surface. The structures had tunable features, with outer diameters ranging from 200 to 350 nm for rings and crescent gap angles of ten to more than a hundred degrees. The use of a double mask method ensured the uniform coverage of these structured over 1 cm2 areas. Experimental and theoretical investigations of the optical properties of the arrays revealed the optical resonances in the infrared region. Finally, in the course of developing the nanorings, etch conditions were developed to deposit large area arrays of polystyrene nanodoughnuts with diameters from 128 to 242 nm. These non-conductive structures provide an ideal template for further attachment of magnetic of optically emissive nanoparticles.

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