The stability of silver nanoparticles on indium tin oxide coated glass substrates under atmospheric condition was
investigated. These nanoparticles were fabricated using electron beam lithography. Energy dispersive spectroscopy
analysis revealed a high concentration of sulfur in the silver nanoparticles exposed to laboratory air for 12 weeks at room
temperature. Morphological changes in the silver nanoparticles were also observed for nanoparticles stored under the
same conditions. In contrast, silver nanoparticles kept in vacuum did not show chemical or morphological changes after
12 weeks. The present work clearly shows the need to consider ambient exposure when using Ag nanoparticles for
Multicharged ion beams (MCI) are promising tools to probe or modify the surface of materials with applications in
microelectronics and nanotechnology. Ion beam lines are parts of the MCI systems connecting the ion source with the
processing chamber and they perform the function of extracting, accelerating, decelerating, focusing and scanning the
ion beam on the surface of the target. In our work we present results of modeling of an MCI beam line using the
SIMION code to simulate the flight of ions, with the purpose of optimizing the yield of the line and avoiding spurious
effects due to interaction of the ions with the metallic elements of the line, such as heating, outgassing and excessive Xray
emission. We show that a two stage ion extractor could significantly reduce ion beam losses.
A compact remote Raman spectroscopy system was developed at NASA Langley Research center and was
previously demonstrated for its ability to identify chemical composition of various rocks and minerals. In
this study, the Raman sensor was utilized to perform time-resolved Raman studies of various samples such
as minerals and rocks, Azalea leaves, and a few fossil samples. The Raman sensor utilizes a pulsed 532 nm
Nd:YAG laser as excitation source, a 4-inch telescope to collect the Raman-scattered signal from a sample
several meters away, a spectrograph equipped with a holographic grating, and a gated intensified CCD
(ICCD) camera system. Time resolved Raman measurements were carried out by varying the gate delay
with fixed short gate width of the ICCD camera, allowing measurement of both Raman signals and
fluorescence signals. Rocks and mineral samples were characterized, including marble, which contains
CaCO3. Analysis of the results reveals the short (~10-13 s) lifetime of the Raman process and shows that the
Raman spectra of some mineral samples contain fluorescence emission due to organic impurities. Also
analyzed were a green (pristine) and a yellow (decayed) sample of Gardenia leaves. It was observed that
the fluorescence signals from the green and yellow leaf samples showed stronger signals compared to the
Raman lines. It was also observed that the fluorescence of the green leaf was more intense and had a
shorter lifetime than that of the yellow leaf. For the fossil samples, Raman shifted lines could not be
observed due to the presence of very strong short-lived fluorescence.
Self-absorption is used in laser induced breakdown spectroscopy to obtain quantitative analytical information. In this
approach two plasmas are generated with a laser pulse that is split into two beams separated by a few millimeters and
incident on the target material. One of the beams generates plasma that acts as the light source analogous to that used in
standard atomic absorption spectroscopy, while the other generates plasma that is used as the analyte. The lines emitted
from the light source plasma are absorbed while passing through the analyte plasma. This technique was applied to Cu-
Zn samples with different Cu/Zn concentrations. The results show that the strongly self absorbed Cu 324 nm and 327 nm
lines can be effectively used to probe the Cu concentration, while the Cu 330 nm line does not show strong selfabsorption.
A compact remote Raman sensor system was developed at NASA Langley Research Center. This sensor is an
improvement over the previously reported system, which consisted of a 532 nm pulsed laser, a 4-inch telescope, a
spectrograph, and an intensified CCD camera. One of the attractive features of the previous system was its portability,
thereby making it suitable for applications such as planetary surface explorations, homeland security and defense
applications where a compact portable instrument is important. The new system was made more compact by replacing
bulky components with smaller and lighter components. The new compact system uses a smaller spectrograph
measuring 9 x 4 x 4 in. and a smaller intensified CCD camera measuring 5 in. long and 2 in. in diameter. The previous
system was used to obtain the Raman spectra of several materials that are important to defense and security applications.
Furthermore, the new compact Raman sensor system is used to obtain the Raman spectra of a diverse set of materials to
demonstrate the sensor system's potential use in the identification of unknown materials.
Recent and future explorations of Mars and lunar surfaces through rovers and landers have spawned great interest in
developing an instrument that can perform in-situ analysis of minerals on planetary surfaces. Several research groups
have anticipated that for such analysis, Raman spectroscopy is the best suited technique because it can unambiguously
provide the composition and structure of a material. A remote pulsed Raman spectroscopy system for analyzing
minerals was demonstrated at NASA Langley Research Center in collaboration with the University of Hawaii. This
system utilizes a 532 nm pulsed laser as an excitation wavelength, and a telescope with a 4-inch aperture for collecting
backscattered radiation. A spectrograph equipped with a super notch filter for attenuating Rayleigh scattering is used to
analyze the scattered signal. To form the Raman spectrum, the spectrograph utilizes a holographic transmission grating
that simultaneously disperses two spectral tracks on the detector for increased spectral range. The spectrum is recorded
on an intensified charge-coupled device (ICCD) camera system, which provides high gain to allow detection of
inherently weak Stokes lines. To evaluate the performance of the system, Raman standards such as calcite and
naphthalene are analyzed. Several sets of rock and mineral samples obtained from Ward's Natural Science are tested
using the Raman spectroscopy system. In addition, Raman spectra of combustible substances such acetone and isopropanol are also obtained.
For exploration of planetary surfaces, detection of water and ice is of great interest in supporting existence of life on other planets. Therefore, a remote Raman spectroscopy system was demonstrated at NASA Langley Research Center in collaboration with the University of Hawaii for detecting ice-water and hydrous minerals on planetary surfaces. In this study, a 532 nm pulsed laser is utilized as an excitation source to allow detection in high background radiation conditions. The Raman scattered signal is collected by a 4-inch telescope positioned in front of a spectrograph. The Raman spectrum is analyzed using a spectrograph equipped with a holographic super notch filter to eliminate Rayleigh scattering, and a holographic transmission grating that simultaneously disperses two spectral tracks onto the detector for higher spectral range. To view the spectrum, the spectrograph is coupled to an intensified charge-coupled device (ICCD), which allows detection of very weak Stokes line. The ICCD is operated in gated mode to further suppress effects from background radiation and long-lived fluorescence. The sample is placed at 5.6 m from the telescope, and the laser is mounted on the telescope in a coaxial geometry to achieve maximum performance. The system was calibrated using the spectral lines of a Neon lamp source. To evaluate the system, Raman standard samples such as calcite, naphthalene, acetone, and isopropyl alcohol were analyzed. The Raman evaluation technique was used to analyze water, ice and other hydrous minerals and results from these species are presented.
A model of the spectral responsivity of In1–GaSb p-n junction infrared photodetectors is developed. This model is based on calculations of the photogenerated and diffusion currents in the device. Expressions for the carrier mobilities, absorption coefficient, and normal-incidence reflectivity as a function of temperature are derived from extensions made to Adachi and Caughey-Thomas models. Contributions from the Auger recombination mechanism, which increase with a rise in temperature, are also considered. The responsivity is evaluated for different doping levels, diffusion depths, operating temperatures, and photon energies. Parameters calculated from the model are compared with available experimental data, and good agreement is obtained. These theoretical calculations help us to better understand the electro-optical behavior of In1–GaSb photodetectors, and can be utilized for performance enhancement through optimization of the device structure.
An Indium Gallium Arsenide linear photodiode array in the 1.1-2.5 μm spectral range was characterized. The array has 1024X1 pixels with a 25 μm pitch and was manufactured by Sensors Unlimited, Inc. Characterization and analysis of the electrical and optical properties of a camera system were carried out at room temperature to obtain detector performance parameters. The signal and noise were measured while the array was uniformly illuminated at varying exposure levels. A photon transfer curve was generated by plotting noise as a function of average signal to obtain the camera gain constant. The spectral responsivity was also measured, and the quantum efficiency, read noise and full-well capacity were determined. This paper describes the characterization procedure, analyzes the experimental results, and discusses the applications of the InGaAs linear array to future earth and planetary remote sensing mission.
A multilayered infrared Ge/Si quantum-dot photodetector is fabricated by pulsed laser deposition. Forty successive Ge quantum dot layers, each covered with a thin Si layer, are deposited. Deposition is monitored by in situ reflection high-energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. Current-voltage measurements reveal typical diode characteristics, while responsivity measurements show an absorption peak around a 2-μm wavelength
With the availability of terawatt laser systems with subpicosecond pulses, laser damage to optical components has become the limiting factor for further increases in the output peak power. Evaluation of different material structures in accordance to their suitability for high-power laser systems is essential. Multi-shot damage experiments, using 110 fs laser pulses at 800 nm, on polycrystalline single layer gold films and multi-layer (gold-vanadium, and gold-titanium) films were conducted. The laser incident fluence was varied, in both cases, from 0.1 to 0.6 J/cm2. No evidence of surface damage was apparent in the gold sample up to a fluence of 0.3 J/cm2. The multilayer sample experienced the onset of surface damage at the lowest fluence value used of 0.1 J/cm2. Damage results are in contrast with the time resolved ultrafast thermoreflectivity measurements that revealed a reduction of the thermoreflectivity signal for the multilayer films. This decrease in the thermoreflectivity signal signifies a reduction in the surface electron temperature that should translate in a lower lattice temperature at the later stage. Hence, one should expect a higher damage threshold for the multilayer samples. Comparison of the experimental results with the predictions of the Two-Temperature Model (TTM) is presented. The damage threshold of the single layer gold film corresponds to the melting threshold predicted by the model. In contrast to the single layer gold film, the multi-layer sample damaged at almost one third the damage threshold predicted by the TTM model. Possible damage mechanisms leading to the early onset of damage for the multilayer films are discussed.
A review of time-resolved reflection high-energy electron diffraction studies of approximately 100-ps laser heated surfaces is given. The dynamic structural behavior is shown to be strongly dependent on surface orientation and is different from that observed under slow heating conditions. Surface superheating and melting of single crystals of lead are described in addition to surface reconstruction of germanium.
We review time-resolved reflection high-energy electron diffraction studies of 200-ps laser heated metal surfaces. The dynamic structural behavior is shown to be strongly dependent on surface orientation and is different from that observed under slow heating conditions. Single crystals of lead and bismuth are used as examples.
We present a detailed description of ultrafast electron diffraction and its applications to study photoinduced molecular dynamics at single crystal surfaces. Experimental investigations for a new design of an ultrashort pulsed laser activated electron gun ((tau) < 5 ps) for time- resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time-resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. The total experimental temporal resolution is discussed in terms of the electron pulse width and the time difference between an electron scattered at the front edge of the sample to an electron scattered at the trailing edge of the sample.
Time-resolved techniques are applied to issues of vibrational energy transfer at
surfaces. Primary attention is given to the relaxation of vibrationally excited
diatomic adsorbates on metals. The sensitivity of the vibrational decay rate to the
number of metal atoms in the solid is demonstrated. Preliminary results for the
transient response of CO on Pt(111) are also reported. This latter measurement
demonstrates the recently developed capability of monitoring adsorbate energy
transfer processes at well defined surface sites. The observations appear to be
consistent with relaxation through the excitation of electron/hole pairs in the