Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-μm wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet–visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of 10 − 5 to 10 − 2 cm − 1 and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of 10 − 4 to 10 − 2 and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods.
Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. However, potential material candidates such as spinel and sapphire have scattering and absorption levels near the 1 μm wavelength region that are undesirable for future HEL applications. Indeed, absorption measurements in these materials indicate that residual loss in this region can be associated with a weak absorption tail. This tail has been experimentally characterized for amorphous semiconductor materials, long-wave infrared glasses, and certain optical fiber materials, but no comprehensive characterization has been done on crystalline window materials. Measurements of both weak scattering and absorption are essential for understanding loss mechanisms in these low loss materials. In this work, scattering measurements on spinel and sapphire samples obtained from different vendors are reported for wavelengths of 532 nm, 633 nm, 1064 nm, and 1550 nm. These measurements were obtained using a bidirectional scatterance probability distribution function instrument. The surface and bulk absorptions of these materials at wavelengths of 355 nm, 532 nm, and 1064 nm were also measured using photothermal common path interferometry. Additionally, a photoacoustic technique was used to obtain the absorption measurements at 1064 nm. In addition, ultraviolet-visible spectroscopy measurements were made to detect any impurity bands. These measurements contribute to a better understanding of the low-loss phenomenon near the 1 μm wavelength in both spinel and sapphire.
This work focuses on the scalable synthesis and processing of nanostructures in polymer matrix nanocomposites (PMNCs) for applications that require photochemical functionality of these nanostructures. An in situ vapor deposition process using various metal and metal oxide precursors has been used to create a range of nanocomposites that display photochromic and photocatalytic behaviors. Under specific processing conditions, these composites consist of discrete nanoparticles distributed uniformly throughout the bulk of an optically transparent polymer matrix. Incorporating other chemical species as supplementary deposition agents in the synthesis process can modify these particles and produce complicated nanostructures with enhanced properties. In particular, work has been carried out to structure nanoparticles using laser irradiation. Starting with metallic or metal oxide nanoparticles in the polymer matrix, localized chemical vapor deposition in the near-particle environment has been carried out using laser irradiation to decompose chemical precursors leading to the formation of secondary structures surrounding the seed nanoparticles. Control of the spatial and temporal characteristics of the excitation source allows for synthesis of nanocomposites with a high degree of control over the location, composition and size of nanoparticles in the matrix and presents the opportunity to produce patterned materials with spatially varying properties.
Laser ultrasonic methods have been used to characterize the elastic behaviors of commercially-available and legacy nuclear graphites. Since ultrasonic techniques are sensitive to various aspects of graphite microstructure including preferred grain orientation, microcrack orientation and porosity, laser ultrasonics is a candidate technique for monitoring graphite degradation and structural integrity in environments expected in high-temperature, gas-cooled nuclear reactors. Aspects of materials texture can be assessed by studying ultrasonic wavespeeds as a function of propagation direction and polarization. Shear wave birefringence measurements, in particular, can be used to evaluate elastic anisotropy. In this work, laser ultrasonic measurements of graphite moduli have been made to provide insight into the relationship between the microstructures and the macroscopic stiffnesses of these materials. In particular, laser ultrasonic measurements have been made using laser line sources to produce shear waves with specific polarizations. By varying the line orientation relative to the sample, shear wave birefringence measurements have been recorded. Results from shear wave birefringence measurements show that an isostatically molded graphite, such as PCIB, behaves isotropically, while an extruded graphite, such as H-451, displays significant ultrasonic texture. Graphites have complicated microstructures that depend on the manufacturing processes used, and ultrasonic texture in these materials could originate from grain orientation and preferred microcrack alignment. Effects on material isotropy due to service related microstructural changes are possible and the ultimate aim of this work is to determine the degree to which these changes can be assessed nondestructively using laser ultrasonics measurements.
Laser ultrasonic line sources have been used to study the ultrasonic properties of nuclear graphites. These materials
exhibit varying degrees of porosity and texture that relate directly to the conditions imposed during material processing-extruded materials display significant texture while the anisotropy of molded materials is significantly lower. Both the
texture (related to grain orientation) and porosity impact the long term performance of graphite under service conditions
and methods are needed to assess the microstructural states of these materials during service. Laser ultrasonic
measurements can be used to assess aspects of material microstructure by measuring longitudinal and shear wavespeeds
as a function of propagation direction and polarization. While porosity-related effects are independent of propagation
direction for materials with spherical pores, material texture (related to preferred grain orientation) produces anisotropic
wave propagation effects. In particular, propagation perpendicular to extrusion directions can produce shear wave
birefringence effects that can be used to assess texture. Ultrasonic measurements in this work were made using laser
ultrasonic methods that yield waveforms that can be interpreted using elastodynamic models for wave propagation in
anisotropic materials. In particular, models for laser ultrasonic line sources in transversely isotropic materials have been
used to simulate laser sources in nuclear graphites. The effects of optical penetration (related to material porosity) have
been incorporated to produce synthetic waveforms that can be used to extract modulus information from experimental
measurements. Current results hold open the opportunity for porosity and texture assessment using limited sets of
ultrasonic measurements.
In addition to visible and near-IR emission, recent investigations have shown that electromagnetic pulses (EMP) in the
microwave and RF regions of the spectrum are generated during femtosecond laser-matter interactions if the laser source
is sufficiently intense to ablate and ionize an illuminated solid target material. Although the mechanisms for the laserinduced
EMP pulse are not fully characterized, it is reported that this phenomenon arises from two mechanisms
associated with terawatt to petawatt level laser interactions with matter: (1) ionization via propagation in air, and (2)
plasma generation associated with the laser-excited solid material. Over the past year, our group has examined the
microwave emission profiles from a variety of femtosecond laser ablated materials, including metals, semiconductors,
and dielectrics. We have directed our measurements towards the characterization of microwave emission from ablated
surfaces in air using laser peak powers in excess of 1012 Watts (energy/pulse ~50 mJ, pulse width ~30 fs, laser diameter
at target ~200 microns). We have characterized the temporal profile of the microwave emission and determined the
emission from all samples is omni-directional. We have also observed a difference in the minimum fluence required to
generate emission from conducting and insulating materials although the peak amplitudes from these materials were
quite similar at the upper laser energy levels of our system (~50 mJ).
The localization of brachytherapy seeds in relation to the prostate is a key step in intraoperative treatment planning (ITP)
for improving outcomes in prostate cancer patients treated with low dose rate prostate brachytherapy. Transrectal
ultrasound (TRUS) has traditionally been the modality of choice to guide the prostate brachytherapy procedure due to its
relatively low cost and apparent ease of use. However, TRUS is unable to visualize seeds well, precluding ITP and
producing suboptimal results. While other modalities such as X-ray and magnetic resonance imaging have been
investigated to localize seeds in relation to the prostate, photoacoustic imaging has become an emerging and promising
modality to solve this challenge. Moreover, photoacoustic imaging may be more practical in the clinical setting
compared to other methods since it adds little additional equipment to the ultrasound system already adopted in
procedure today, reducing cost and simplifying engineering steps. In this paper, we demonstrate the latest efforts of
localizing prostate brachytherapy seeds using photoacoustic imaging, including visualization of multiple seeds in actual
prostate tissue. Although there are still several challenges to be met before photoacoustic imaging can be used in the
operating room, we are pleased to present the current progress in this effort.
Fatigue damage sensing and measurement in aluminum alloys is critical to estimating the residual useful lifetime of a
range of aircraft structural components. In this work, we present electrical impedance and ultrasonic measurements in
aluminum alloy 2024 that has been fatigued under high cycle conditions. While ultrasonic measurements can indicate
fatigue-induced damage through changes in stiffness, the primary indicator is ultrasonic attenuation. We have used laser
ultrasonic methods to investigate changes in ultrasonic attenuation since simultaneous measurement of longitudinal and
shear properties provides opportunities to develop classification algorithms that can estimate the degree of damage.
Electrical impedance measurements are sensitive to changes in the conductivity and permittivity of materials - both are
affected by the microstructural damage processes related to fatigue. By employing spectral analysis of impedance over a
range of frequencies, resonance peaks can be identified that directly reflect the damage state in the material. In order to
compare the impedance and ultrasonic measurements for samples subjected to tension testing, we use processing and
classification tools that are matched to the time-varying spectral nature of the measurements. Specifically, we process
the measurements to extract time-frequency features and estimate stochastic variation properties to be used in robust
classification algorithms. Results are presented for fatigue damage identification in aluminum lug joint specimens.
Femtosecond laser induced breakdown spectroscopy (LIBS) has been shown to be sensitive to a variety of ERC's
(explosive-related compounds) deposited on substrates. In LIBS, surface material is excited by a high-powered laser
pulse forming a plasma. The optical emission from this plasma is collected and spectrally analyzed to determine the
surface species entrained in the excitation event. The detection of explosive related compounds in the field presents
many challenges, one of these being the wide variety of materials surfaces that might be covered with ERC's. Results
from femtosecond and nanosecond LIBS of ERC's of metal, glass, and polymer substrates show that the optical
properties of the substrate play a large role in the observed emission. Results indicate that nanosecond LIBS of ERC's on
metal surfaces yield strong atomic emission while nanosecond LIBS of ERC's on glass results in some molecular
emission. Molecular emission is also present in femtosecond LIBS spectra of ERC's on all surfaces but is particularly
strong for metal substrates. In particular emission from the CN molecular fragment could provide a means to understand
the effect of the substrate on the excitation event in nitroaromatic compounds since it is present in both nanosecond LIBS
spectra of the TNT/glass system and femtosecond LIBS spectra of the TNT/Al system. The origins of this CN molecular
fragment are currently being studied since fragmentation and reaction processes in LIBS events are not fully understood
at this time.
We investigate the use of low frequency (10-70 MHz) laser ultrasound for the detection of fatigue damage.
While high frequency ultrasonics have been utilized in earlier work, unlike contacting transducers, laser-based
techniques allow for simultaneous interrogation of the longitudinal and shear moduli of the fatigued material. The
differential attenuation changes with the degree of damage, indicating the presence of plasticity. In this paper, we
describe a structural damage identification approach based on ultrasonic sensing and time-frequency techniques.
A parsimonious representation is first constructed for the ultrasonic signals using the modified matching pursuit
decomposition (MMPD) method. This decomposition is then employed to compute projections onto the various
damage classes, and classification is performed based on the magnitude of these projections. Results are presented
for the detection of fatigue damage in Al-6061 and Al-2024 plates tested under 3-point bending.
We use femtosecond laser mass spectrometry (FLMS) to study the fragmentation patterns of solid phase explosive materials subjected to femtosecond laser pulse irradiation. In condensed phase FLMS a compound deposited on a solid substrate is desorbed into vacuum by femtosecond irradiation forming a plume of ionized and neutral species. Positive or negative ions are accelerated by an electric potential, allowed to drift in the field-free region of a time-of-flight (TOF) mass spectrometry instrument, and flight-times are recorded by a micro-channel plate detector and a digital oscilloscope. From the value of the accelerating field and the ion flight time, the mass-to-charge ratio of each ion is obtained. In this paper we report femtosecond laser mass spectra for the positive and negative ions formed by desorbing TNT and RDX with 150 fs pulses centered at 800 nm. The fragmentation pathways for the formation of the observed ions are described and are used to interpret femtosecond laser induced breakdown spectroscopy results.
We use femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of TNT and RDX. A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical emission from the plasma is spectrally analyzed to determine the material composition. Femtosecond LIBS results for TNT and RDX on aluminum substrates and glass slides are reported. Results are examined in terms of the optical properties of the substrate and the strong linear absorption for aluminum is contrasted with the weaker multiphoton absorption for glass. Optical microscope images of the ablated explosives are shown for femtosecond and nanosecond laser excitation. Fragmentation studies by femtosecond laser mass spectrometry are used to interpret LIBS results.
In this work, we describe femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of explosive-related compounds (ERCs). A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical radiation from the plasma is spectrally analyzed to determine the material composition. LIBS is minimally destructive because only a minute amount of material is consumed in the process. LIBS also enables remote analysis because only optical access to the material is needed. Femtosecond LIBS results for TNT on brass and molybdenum substrates, and RDX on molybdenum substrates are reported. We will also show the effect of detection gate delay and gate width on the enhancement of spectral information provided by LIBS.
Diffuse reflectance spectrum (DRS) technique is extensively used in UV/visible and middle/near infrared for characterizing/analyzing powders and samples with rough surface. In this paper, we report on the DRS investigation of explosives & related compounds in the THz region, which is more difficult because of the limitations of far infrared sources, beam splitters and detectors. We also discussed the penetration depth and the sensitivity for this technique in the THz range.
THz diffuse reflectance spectra (50-680 cm-1) were taken on a Bruker 66V/S FTIR spectrometer with a Specac diffuse reflectance accessory. A number of explosive and related compounds were investigated in both transmission and diffuse reflectance modes, and a good agreement between transmission and diffuse reflection spectra was demonstrated. Our experimental results show that DRS technique has advantages over transmission spectrum technique, such as better sensitivity and easier sample preparation. Therefore, the THz DRS has the potential for the standoff detection of explosives and related compounds in the real world applications.
The detection of buried anti-personnel mines has presented problems for current mine detection methods, such as ground penetrating radar. Terahertz (THz) radiation provides the ability to obtain higher resolution images as well as the ability to obtain spectroscopic information on the explosives in and around the buried mines. Propagation of THz in granular media is studied using techniques adapted from material science for foams and ceramics and adapted to sand and soil. We then give early results of 2-D reflection imaging of objects buried in sand. A terahertz time domain reflection system with a GaAs photoconductive emitter, a ZnTe electro-optic detector, and a rapid delay scanning mechanism was used to produce and collect the transmitted THz signal.
Far infrared spectra of 14 commonly used explosive samples have been measured by using Fourier Transform Infrared Spectroscopy (FTIR) and THz Time-Domain Spectroscopy (THz TDS). New absorption resonances between 20 cm-1 and 650 cm-1 are reported. Below 20 cm-1, no clear absorption resonances are observed in all the explosives. There is a good consistency of far-IR spectrum measured by Far-FTIR and by THz TDS in explosives 3,5-DNA and 2,4-DNT. Observed far-IR spectrum of TNT is compared with a previously reported theoretical calculation.
The research center established by Army Research Office under the Multidisciplinary University Research Initiative program pursues a multidisciplinary approach to investigate and advance the use of complementary analytical techniques for sensing of explosives and/or explosive-related compounds as they occur in the environment. The techniques being investigated include Terahertz (THz) imaging and spectroscopy, Laser-Induced Breakdown Spectroscopy (LIBS), Cavity Ring Down Spectroscopy (CRDS) and Resonance Enhanced Multiphoton Ionization (REMPI). This suite of techniques encompasses a diversity of sensing approaches that can be applied to detection of explosives in condensed phases such as adsorbed species in soil or can be used for vapor phase detection above the source. Some techniques allow for remote detection while others have highly specific and sensitive analysis capabilities. This program is addressing a range of fundamental, technical issues associated with trace detection of explosive related compounds using these techniques. For example, while both LIBS and THz can be used to carry-out remote analysis of condensed phase analyte from a distance in excess several meters, the sensitivities of these techniques to surface adsorbed explosive-related compounds are not currently known. In current implementations, both CRDS and REMPI require sample collection techniques that have not been optimized for environmental applications. Early program elements will pursue the fundamental advances required for these techniques including signature identification for explosive-related compounds/interferents and trace analyte extraction. Later program tasks will explore simultaneous application of two or more techniques to assess the benefits of sensor fusion.
KEYWORDS: Ultrasonics, Near field, Ultrasonography, Near field optics, Transducers, Near field scanning optical microscopy, Spatial resolution, Copper, Wave propagation, Microscopes
The ability to characterize the sub-surface mechanical properties of a bulk or thin film material at the sub-micron level has applications in the microelectronics and thin film industries. In the microelectronics industry, with the decrease of line widths and the increase of component densities, sub-surface voids have become increasingly detrimental. Any voids along an integrated circuit (IC) line can lead to improper electrical connections between components and can cause failure of the device. In the thin film industry, the detection of impurities is also important. Any impurities can detract from the film's desired optical, electrical, or mechanical properties. Just as important as the detection of voids and impurities, is the measurement of the elastic properties of a material on the nanometer scale. These elastic measurements provide insight into the microstructural properties of the material. We have been investigating a technique that couples the high-resolution surface imaging capabilities of the apertureless near-field scanning optical microscope (ANSOM) with the sub-surface characterization strengths of high-frequency ultrasound. As an ultrasonic wave propagates, the amplitude decreases due to geometrical spreading, attenuation from absorption, and scattering from discontinuities. Measurement of wave speeds and attenuation provides the information needed to quantify the bulk or surface properties of a material. The arrival of an ultrasonic wave at or along the surface of a material is accompanied with a small surface displacement. Conventional methods for the ultrasound detection rely on either a contact transducer or optical technique (interferometric, beam deflection, etc.). However, each of these methods is limited by the spatial resolution dictated by the detection footprint. As the footprint size increases, variations across the ultrasonic wavefront are effectively averaged, masking the presence of any nanometer-scale sub-surface or surface mechanical property variations. The use of an ANSOM for sensing ultrasonic wave arrivals reduces the detection footprint allowing any nanometer scale variations in the microstructure of a material to be detected. In an ANSOM, the ultrasonic displacement is manifested as perturbations on the near-field signal due to the small variations in the tip-sample caused by the wave arrival. Due to the linear dependence of the near-field signal on tip-sample separation, these perturbations can be interpreted using methods identical to those for conventional ultrasonic techniques. In this paper, we report results using both contact transducer (5 MHz) and laser-generated ultrasound.
This paper describes current efforts to apply spatially and temporally localized microwave processing techniques to ensure uniformity of material properties in polymer composite materials. In large polymer composite structures, high temperatures caused by exothermic resin cure can degrade the mechanical properties of the composite. In this work, resin cure temperature data was obtained during microwave processing from a series of thermocouples embedded at various lateral locations relative to the microwave source and uniformly through the thickness of the composite structure. Using this temperature information, the potential for localized microwave-accelerated cure to reduce the occurrence of material degradation from resin over-temperature was evaluated.
KEYWORDS: Ultrasonics, Signal detection, Signal to noise ratio, Interferometers, Pulsed laser operation, Ultrasonography, Inspection, Modulation, Laser systems engineering, Process control
Laser ultrasonic techniques for in-process materials characterization show significant potential for applications where rapid, remote sensing is a requirement. While the potential is great, relatively few on-line industrial systems currently exist owing to difficulty associated with designing and implementing robust laser ultrasonic systems. Although laser-based ultrasound is becoming widely used as a laboratory tool for materials characterization, transitioning this technology into a robust industrial process control system poses several problems. Chief among these are suitable lasers for ultrasound generation, ultrasound detection, interferometer design, required signal processing, and overall system performance. This manuscript addresses each of these issues in turn, and gibes examples of industrial process control implementations where appropriate. Finally, recent advances in increasing laser based ultrasonic sensitivity are discussed.
Controlled heating of a test specimen with a laser source provides several advantages for flaw detection using shearographic detection. This stressing method is non-contacting, can be localized, and allows defect information to be obtained while heating. In addition, the beam profile can be tailored to aid in the detection of different defect types. This paper presents results of simultaneous observations of material response to an applied thermal load using both TRIR and shearographic detection methods. Of particular importance is the demonstration that the depth of a defect can be determined by measuring the time-dependence of the shearographic fringe development during heating.
KEYWORDS: Signal to noise ratio, Acoustics, Pulsed laser operation, Signal detection, Ultrasonics, Laser systems engineering, Modulation, Signal generators, Optical testing, Mirrors
It is shown that improvement in the detection sensitivity of laser-ultrasonic systems may be obtained by generating narrowband acoustic signals using both temporal and spatial modulation of the generating laser. A laser-generated acoustic tone burst waveform will have lower peak amplitudes than a single acoustic pulse providing the same system SNR. Consequently, lower power density laser pulses may be used to avoid surface damage.
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