Joseph A. Miragliotta

High quality self‐nucleated AlxGa1−x N layers on (00.1) sapphire by low‐pressure metalorganic chemical vapor deposition

High quality AlxGa1−xN alloy films with x<0.4 have been prepared on self‐nucleated (00.1) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. It has been shown that the lattice constant of the films varies linearly with alloy composition x (Vegard’s law is obeyed) and that homogeneous and inhomogeneous strain and alloy clustering are minimized in these self‐nucleated AlxGa1−xN films. Consistent with their reduced strain and chemical uniformity, the derived optical band gaps of these AlxGa1−xN films also show a linear dependence on alloy composition x, yielding a bowing parameter b≊0 eV.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy.

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications.

TRANSIENT PHOTOCURRENT INDUCED IN GALLIUM NITRIDE BY TWO-PHOTON ABSORPTION

We have studied the subband gap induced, transient photocurrent in an epitaxial GaN film immersed in an electrolyte solution. For photon energies near the midgap position, one‐ and two‐photon (TP) contributoins were observed in the photocurrent. The one‐photon term exhibited a sublinear intensity dependence and was attributed to carrier generation from traps in the gap. The TP current was negligible for energies below Egap/2. Above this energy, the dispersion was consistent with previous calculations of the TP absorption coefficient [β(ω)] in direct gap semiconductors. A relationship between the TP photocurrent and β(ω) determined a value for the latter of ∼1.5 cm/GW at photon energies above Egap/2.

Thermally annealed GaN nucleation layers and the device‐quality metal organic chemical vapor deposition growth of Si‐doped GaN films on (00.1) sapphire

The effect of epitaxial growth temperature (985–1050 °C) on the properties of Si‐doped GaN layers on self‐nucleated (00.1) sapphire has been investigated. Several device‐related properties monotonically improve with increasing growth temperature, including (a) carrier density and (b) volume fraction of heteroepitaxial domains. However, a number of equally important device‐related properties show a local maximum and include (a) optical second‐harmonic generation intensity, (b) structural coherence, and particularly (c) surface morphology. The antecedents of the first class lie in increases in surface and bulk diffusion and reductions in film defect incorporation and stress at the GaN/GaN (nucleation layer)/α‐Al2O3 heterointerface. The second class arises from the quite limited range over which the thermally annealed GaN nucleation layer stimulates pseudo‐two‐dimensional growth of the GaN overlayer.

Mine field detection and identification using terahertz spectroscopic imaging

The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz - 70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.

Overview: MURI Center on spectroscopic and time domain detection of trace explosives in condensed and vapor phases

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.

Detection of microwave emission from solid targets ablated with an ultrashort pulsed laser

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).

ASSIST - Automated System for Surgical Instrument and Sponge Tracking

Every surgical item used during surgery (e.g., sponges) must be accounted for after surgery to ensure that none of these items is left inside the patient. Despite the numerous precautions in place, in approximately 1 in 1500 cases, something gets left behind inside the patients body. This paper presents ASSIST, an automated system for surgical instrument and sponge tracking that increases the safety of surgical procedures. ASSIST utilizes RFID (Radio Frequency Identification) technology to aid in accounting for all items used during surgery. The design takes into account safety, simplicity, ease of deployment, and ease of use. An initial evaluation utilizing RFID-tagged sponges demonstrates that ASSIST can reliably track surgical sponges with minimal impact to current operating room procedures. Sources of error that can impact the reliability of the system are also discussed.

Neutron irradiation of sapphire for compressive strengthening. II. Physical properties changes

Abstract Irradiation of sapphire with fast neutrons (0.8–10 MeV) at a fluence of 1022/m2 increased the c-axis compressive strength and the c-plane biaxial flexure strength at 600 °C by a factor of ∼2.5. Both effects are attributed to inhibition of r-plane twin propagation by damage clusters resulting from neutron impact. The a-plane biaxial flexure strength and four-point flexure strength in the c- and m-directions decreased by 10–23% at 600 °C after neutron irradiation. Neutron irradiation had little or no effect on thermal conductivity, infrared absorption, elastic constants, hardness, and fracture toughness. A featureless electron paramagnetic resonance signal at g=2.02 was correlated with the strength increase: This signal grew in amplitude with increasing neutron irradiation, which also increased the compressive strength. Annealing conditions that reversed the strengthening also annihilated the g=2.02 signal. A signal associated with a paramagnetic center containing two Al nuclei was not correlated with strength. Ultraviolet and visible color centers also were not correlated with strength in that they could be removed by annealing at temperatures that were too low to reverse the compressive strengthening effect of neutron irradiation.

An SOS MEMS interferometer

The function of a large number of MEMS and NEMS devices relies critically on the transduction method employed to convert the mechanical displacement into electrical signal. Optical transduction techniques have distinct advantages over more traditional capacitive and piezoelectric transduction methods. Optical interferometers can provide a much higher sensitivity, about 3 orders of magnitude, but are hardly compatible with standard MEMS and microelectronics processing. In this paper, we present a scalable architecture based in silicon on sapphire (SOS) CMOS 1 for building an interferometric optical detection system. This new detection system is currently being applied to the sense the motion of a resonating MEMS device, but can be used to detect the motion of any object to which the system is packaged. In the current hybrid approach the SOS CMOS device is packaged with both vertical cavity surface emitting lasers (VCSELs) and MEMS devices. The optical transparency of the sapphire substrate together with the ultra thin silicon PIN photodiodes available in this SOS process allows for the design of both a Michelson type and Fabry Perot type interferometer. The detectors, signal processing electronics and VCSEL drivers are built on the SOS CMOS for a complete system. We present experimental data demonstrating interferometric detection of a vibrating device.