Thomas Wilson

Marshall University, Physics, Faculty Member

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Address: Huntington, West Virginia, United States

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A high-power far-infrared NH3 laser pumped in a three-mirror CO2 laser cavity with optically-switched cavity-dumping

International Journal of Infrared and Millimeter Waves, 1993

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Bilayer lift-off process for aluminum metallization

Journal of Micro/Nanolithography, MEMS, and MOEMS, 2015

Recently published reports in the literature for bilayer lift-off processes have described recipe... more Recently published reports in the literature for bilayer lift-off processes have described recipes for the patterning of metals that have recommended metal-ion-free developers, which do etch aluminum. We report the first measurement of the dissolution rate of a commercial lift-off resist (LOR) in a sodium-based buffered commercial developer that does not etch aluminum. We describe a reliable lift-off recipe that is safe for multiple process steps in patterning thin (<100 nm) and thick aluminum devices with micron-feature sizes. Our patterning recipe consists of an acid cleaning of the substrate, the bilayer (positive photoresist/LOR) deposition and development, the sputtering of the aluminum film along with a palladium capping layer and finally, the lift-off of the metal film by immersion in the LOR solvent. The insertion into the recipe of postexposure and sequential develop-bake-develop process steps are necessary for an acceptable undercut. Our recipe also eliminates any need for accompanying sonication during lift-off that could lead to delamination of the metal pattern from the substrate. Fine patterns were achieved for both 100-nm-thick granular aluminum/palladium bilayer bolometers and 500-nm-thick aluminum gratings with 6-μm lines and 4-μm spaces.

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Evidence for Terahertz Self-Starting Mirrorless Acoustic Phonon Parametric Oscillator with Distributed Feedback

Nonlinear Optics (NLO)

Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-si... more Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-simulation (DDES) turbulence model with the Compressible Flamelet Progress Variable (CFPV) combustion model. Three different pressure instability domains are simulated: completely unstable, semi-stable, and fully stable. The different instability domains are found by varying the combustion chamber and oxidizer post length. Laminar flamelet solutions with a detailed chemical mechanism are examined. The β Probability Density Function (PDF) for the mixture fraction and Dirac δ PDF for both the pressure and the progress variable are used. A coupling mechanism between the volumetric Heat Release Rate (HRR) and the pressure in an unstable cycle is demonstrated. Local extinction and reignition is investigated for all the instability domains using the full S-curve approach. A monotonic decrease in the amount of local extinctions and reignitions occurs when pressure oscillation amplitude becomes smaller. The flame index is used to distinguish between the premixed and nonpremixed burning mode in different stability domains. An additional simulation of the unstable pressure oscillation case using only the stable flamelet burning branch of the S-curve is performed. Better agreement with experiments in terms of pressure oscillation amplitude is found when the full S-curve is used.

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Progress on: “Coherent Terahertz Acoustic Phonon Scattering: Novel Diagnostic for Erosion in Plasma Thruster Discharge Chamber Walls

Note: Phonons are quantized traveling elastic waves associated with the displacement field of ato... more

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Evidence for the Generation of Coherent Longitudinal Acoustic Phonons through the Resonant Absorption of Pulsed Far-Infrared Laser Radiation in Silicon Doping Superlattices

Chinese Journal of Physics, 2011

We report the first experimental evidence for the direct excitation of coherent high-frequency ac... more We report the first experimental evidence for the direct excitation of coherent high-frequency acoustic phonons in semiconducting doping superstructures by electromagnetic fields of the same frequency. Nanosecond pulses of acoustic phonons have been detected by a superconducting bolometer at the appropriate time-of-flight across a (100) silicon substrate for ballistic longitudinal phonons when a silicon doping superlattice is illuminated with grating-coupled nanosecond-pulsed 246-GHz laser radiation with power density of ~1 kW/mm^2. The absorbed phonon power density in the microbolometer is estimated to be ~10 μW/mm^2, in agreement with theory. The phonon pulse duration matches the laser pulse duration. The absence of any detected transverse acoustic phonon signal by the superconducting bolometer is particularly striking. We believe these observations provide the first evidence that coherent zone-folded longitudinal acoustic phonons can be generated in a silicon doping superlattice ...

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Evidence for terahertz acoustic phonon parametric oscillator based on acousto-optic degenerate four-wave mixing in a silicon doping superlattice

Physical Review B, 2018

We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless acoustic phonon param... more We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength grating was used to couple the THz laser radiation into the DSL. Superconducting granular aluminum bolometric detection, coupled with Si:B piezophonon spectroscopy, revealed excitation of THz coherent compressional and shear waves, along the <111> direction only. The Bragg scattering condition for distributed feedback, and the energy conservation requirement for the D4WM process, were both verified. A THz MAPPO could provide a testbed for studies of non-classical acoustic phonon fields.

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The fabrication and characterization of granular aluminium/palladium bilayer microbolometers

Measurement Science and Technology, 2007

Superconducting granular aluminium/palladium bilayer microbolometers can be easily fabricated usi... more Superconducting granular aluminium/palladium bilayer microbolometers can be easily fabricated using an all lift-off process with image-reversal lithography. The palladium capping layer allows for convenient and reproducible Ohmic contacts to be made to the device. We report their fabrication, resistance-current calibration, responsivity and response time. Finally, a deconvolution algorithm is used to improve the time resolution to approximately 1 ns while obtaining the incident photon and phonon fluxes in both direct photoexcitation and phonon heat pulse experiments.

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Phonon interference in crystalline and amorphous confined nanoscopic films

Journal of Applied Physics, 2017

Using molecular dynamics phonon wave packet simulations, we study phonon transmission across hexa... more Using molecular dynamics phonon wave packet simulations, we study phonon transmission across hexagonal (h)-BN and amorphous silica (a-SiO 2 ) nanoscopic thin films sandwiched by two crystalline leads. Due to the phonon interference effect, the frequency-dependent phonon transmission coefficient in the case of crystalline film (Si|h-BN|Al heterostructure) exhibits a strongly oscillatory behavior. In the case of amorphous film (Si|a-SiO 2 |Al and Si|a-SiO 2 |Si heterostructures), in spite of structural disorder, the phonon transmission coefficient also exhibits oscillatory behavior at low frequencies (up to ~ 1.2 THz), with a period of oscillation consistent with the prediction from the two-beam interference equation. Above 1.2 THz, however, the phonon interference effect is greatly weakened by the diffuse scattering of higher-frequency phonons within an a-SiO 2 thin film and at the two interfaces confining the a-SiO 2 thin film.

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Evidence of Longitudinal Acoustic Phonon Generation in Si Doping Superlattices by Ge Prism-Coupled THz Laser Radiation

IOP Conference Series: Materials Science and Engineering, 2014

We report preliminary results on the direct excitation of single-polarization (longitudinal) 246 ... more We report preliminary results on the direct excitation of single-polarization (longitudinal) 246 GHz acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation of the same frequency. The longitudinally polarized evanescent laser light field is coupled to the superlattice through a germanium prism providing total internal reflection at the superlattice interface. The ballistic phonon signal is detected by a superconducting aluminum bolometer. The sample is immersed in low-temperature liquid helium.

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Evidence for the Generation of Coherent Longitudinal Acoustic Phonons through the Resonant Absorption of Pulsed Far-Infrared Laser Radiation in Silicon Doping Superlattices

We report the first experimental evidence for the direct excitation of coherent highfrequency aco... more We report the first experimental evidence for the direct excitation of coherent highfrequency acoustic phonons in semiconducting doping superstructures by electromagnetic fields of the same frequency. Nanosecond pulses of acoustic phonons have been detected by a superconducting bolometer at the appropriate time-of-flight across a (100) silicon substrate for ballistic longitudinal phonons when a silicon doping superlattice is illuminated with grating-coupled nanosecond-pulsed 246-GHz laser radiation with power density of ∼1 kW/mm 2. The absorbed phonon power density in the microbolometer is estimated to be ∼10 µW/mm 2 , in agreement with theory. The phonon pulse duration matches the laser pulse duration. The absence of any detected transverse acoustic phonon signal by the superconducting bolometer is particularly striking. We believe these observations provide the first evidence that coherent zone-folded longitudinal acoustic phonons can be generated in a silicon doping superlattice and with negligible associated heat pulse generation, through the resonant absorption of grating-coupled pulsed far-infrared laser radiation.

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Modeling the high-speed switching of far-infrared radiation by photoionization in a semiconductor

Physical Review B, 1999

Data from an earlier study ͓T. Vogel et al., Appl. Opt. 31, 329 ͑1992͔͒ on the subnanosecond swit... more Data from an earlier study ͓T. Vogel et al., Appl. Opt. 31, 329 ͑1992͔͒ on the subnanosecond switching of 119-m radiation in high-resistivity silicon by pulsed UV laser radiation, is compared with a refined onedimensional numerical multilayer model accounting for the generation, recombination, and diffusion of the free carriers on the resulting far-infrared optical properties of the silicon. The inclusion of recent measurements for carrier-density and temperature-dependent transport parameters leads to improved agreement between experiment and theory. ͓S0163-1829͑99͒03220-8͔

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Ripple Tank Demonstration of Gaussian Beam Propagation

We have reproduced a ripple tank experimentfootnote 'Ripple Tank Studies of Wave Motion' ... more We have reproduced a ripple tank experimentfootnote 'Ripple Tank Studies of Wave Motion' by W. Llowarch (Oxford University Press, 1961) in order to illustrate the effects of focusing and diffraction of a capillary planewave by a bi-convex carbon tetrachloride 'lens'. We compare the experimental results, captured by a video camera, to the theoretical results obtained using the ABCD matrix method for the gaussian beam propagation through this paraxial 'optical' systemfootnote 'Lasers' by Anthony Siegman (University Science Book, Mill Valley, CA, 1986). The diffraction-limited focusing of the propagating wave is clearly visible; this is a feature not easily demonstrable in optics.

A calculation of nonlocal phonon heat transfer

Solid State Communications, 1984

ABSTRACT An evaluation is performed of Levinson&#39;s theory of nonlocal phonon heat transfer... more ABSTRACT An evaluation is performed of Levinson&#39;s theory of nonlocal phonon heat transfer, which applies to the propagation of phonons in a diffusive medium in the presence of anharmonic processes. The particular case described is that of a semi-infinite substrate, initially at temperature To, subjected to an imposed small change in the temperature of its surface. Appropriate formulae are both derived and numerically evaluated to illustrate their content and relevance to experimental data.

Progress on coherent generation of terahertz acoustic phonons by resonant absorption of nanosecond-pulsed far-infrared laser radiation in silicon doping superlattices

The Journal of the Acoustical Society of America, 2008

We report progress on experiments to generate and detect coherent terahertz acoustic phonons. Coh... more We report progress on experiments to generate and detect coherent terahertz acoustic phonons. Coherent phonons are generated 1 in a silicon-doping superlattice by the absorption of grating-coupled high peak-power cavity-dumped far-infared laser radiation 2 . The superlattice period is chosen to match the phonon wavelength at the excitation frequency of the laser radiation. The phonons propagate across the Si:B substrate and are detected by a novel superconducting granular aluminium/palladium bilayer microbolometer 3 with subnanosecond resolution. The phonon spectrum is obtained by piezo-phonon spectroscopy 4 via the boron impurities in the silicon substrate. (1) P.

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Superlattice ultrasonic generation

Journal of Physics: Conference Series, 2011

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Fabrication of Robust Superconducting Granular Aluminium/Palladium Bilayer Microbolometers with Sub-Nanosecond Response

Journal of Low Temperature Physics, 2008

We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoust... more We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminium bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminium film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al. [1], and direct nanosecond-pulsed photoexcitation to obtain the microbolometer's characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al. [2] to obtain the phonon flux in a heat pulse experiment with nanosecond resolution.

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Generation of high repetition rate far-infrared laser pulses

Applied Physics Letters, 1994

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Accessibility of home blood pressure monitors for blind and visually impaired people

BACKGROUND The prevalence of hypertension comorbid with diabetes is a significant health care iss... more BACKGROUND The prevalence of hypertension comorbid with diabetes is a significant health care issue. Use of the home blood pressure monitor (HBPM) for aiding in the control of hypertension is noteworthy because of benefits that accrue from following a home measurement regimen. To be usable by blind and visually impaired patients, HBPMs must have speech output to convey all screen information, an easily readable visual display, identifiable controls that are easy to use, and an accessible user manual. METHODS Data on the physical aspects and the features and functions of nine Food and Drug Administration-approved HBPMs (eight of which were recommended by the British Hypertension Society) were tabulated and analyzed for usability by blind and visually impaired individuals. Video Electronics Standards Association standards were used to measure contrast modulation in the displays of the HBPMs. Ten persons who are blind or visually impaired and who have diabetes were surveyed to determin...

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Progress towards terahertz acoustic phonon generation in doping superlattices

Progress is described in experiments to generate coherent terahertz acoustic phonons in silicon d... more Progress is described in experiments to generate coherent terahertz acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation. Future experiments are proposed that would use the superlattice as a transducer in a terahertz cryogenic acoustic reflection microscope with sub-nanometer resolution.

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