Abstract: The 9-month journey home from Mars could begin with a 9 ns laser pulse.  Ignition in rocket combustors is typically accomplished using a spark plug, a pyrotechnic charge, an injection of hypergolic fluid, or a hot gas torch. These methods involve significant mechanical complexity, increase the inert mass with ancillary subsystems, limit the potential for engine re-ignitions throughout a mission, and require additional (often toxic) propellants. Non-resonant breakdown ignition is an alternative method in which the ignition energy is provided through a focused pulse of laser light. If the local flow conditions in the vicinity of the spark are suitable, a flame will develop and stabilize within the combustor. Laser-induced spark ignition holds significant promise for rocket combustion systems because the point of energy deposition can be precisely placed at an optimum location that minimizes the ignition energy requirement.  This talk will focus on an experimental characterization ignition probability in a gaseous oxygen and gaseous methane combustor. The oxygen-centered shear co-axial injector generated a widely varying mixture field and velocity field to create significant variability in both the ignition process and outcome.  Results from time-resolved imaging diagnostics will be discussed to explain the mechanisms that manifest the final ignition probability.

Biographical Sketch: Dr. Carson Slabaugh is the Paula Feuer Associate Professor in the School of Aeronautics and Astronautics at Purdue University.  Since joining Purdue in 2015, he has developed an education and research program focused on propulsion.  Dr. Slabaugh’s laboratory is housed within the Purdue Zucrow Laboratory complex, with high pressure, high flow-rate system capabilities to enable experimental replication of the flow and flame conditions (pressure, turbulence level, thermal power density) found in the most advanced propulsion and combustion systems.  Ongoing research projects cover a wide range of topics: from the fundamental exploration of detonations and turbulent flames to the development of advanced combustion technologies for liquid rocket engines and rotating detonation engines. His group also maintains a continuous effort in the advancement of high-bandwidth (typically, laser-based) measurement techniques to non-intrusively probe the physics of these complex, reacting flows.  Support for these research projects has been provided by AFOSR, AFRL, DARPA, DOE, NASA, ONR, and numerous industrial partners.  Prof. Slabaugh has published extensively in the field and is involved with multiple national efforts to transition advanced concepts into aerospace propulsion technologies.

Abstract: Chalcogenide phase change materials (PCMs) have promising properties for photonic applications thanks to their nonvolatile and large refractive index modulation [1]. The last decade has seen a growing interest in such a combination of properties for a variety of nonvolatile programmable devices, such as metasurfaces, tunable filters, phase/amplitude modulators, color pixels, thermal camouflage, photonic memories/computing, plasmonics, etc.—giving rise to the so-called Phase-change Photonics field.[1] PCM-based devices rely on the precise switching between the amorphous and the crystalline states, which can be achieved through optical or electrical pulses via optical absorption and Joule heating, respectively. Optical pulse switching is the fastest and most precise method; however, it lacks scalability given the difficulty of on-chip pulse routing when considering many PCM cells. It is also limited to absorptive PCMs, such as Ge2Sb2Te5. As a scalable alternative, on-chip microheaters using multiple material platforms have been proposed, e.g. doped-silicon, graphene, ITO, metals, etc. Doped-silicon microheaters are particularly interesting since they are CMOS compatible and can be fabricated onto silicon-on-insulator (SOI) wafers—the same platform used for silicon photonic integrated circuits. However, this electro-thermal switching also has shortcomings. It lacks stable multi-level response due to the stochastic nature of both amorphization and crystallization processes in the typical bow-tie-like devices where the microheater heats the entire cell to an almost flat temperature [2,3]. Because of this, the focus is shifting towards the microheater’s geometry in addition to the choice of conductive material and its integration. One common goal is to control the hotspot size within the PCM cell to deterministically switch specific areas (i.e., spatially resolved amorphous domains in a crystalline cell) and, thus, achieve controllable and reproducible optical modulation [4,5]. In this talk, I will review the fundamentals of PCM for photonics and the proposed electro-thermal mechanisms. I will then focus on our current efforts to re-engineer doped-silicon microheaters for optimum performance.

[1] M. Wuttig, et, al. “Phase-change materials for non-volatile photonic applications,” Nat. Photon.11(8), 465–476 (2017).

[2] J. Feldmann, et, al. “Calculating with light using a chip-scale all-optical abacus,” Nat Commun 8(1), 1256 (2017)

[3] T. Tuma, et, al. “Stochastic phase-change neurons,” Nat. Nanotechnol.11(8), 693–699 (2016).

[4] C. Ríos, et, al “Ultra-compact nonvolatile phase shifter based on electrically reprogrammable transparent phase change materials,” PhotoniX 3(1), 26 (2022).

[5] X.  Li, et, al, “Fast and reliable storage using a 5 bit, nonvolatile photonic memory cell,” Optica 6, 1-6 (2019)

[6] Y. Zhang, et, al, “Myths and truths about optical phase change materials: A perspective.” Appl. Phys. Lett. 118 (21): 210501.

Biographical Sketch: Carlos A. Ríos Ocampo is an Assistant Professor at the University of Maryland, College Park, where he has led the Photonic Materials & Devices groups since 2021. Before joining UMD, Carlos was a Postdoctoral Associate at MIT, received a DPhil (PhD) degree in 2017 from the University of Oxford (UK), an MSc degree in Optics and Photonics in 2013 from the KIT (Germany), and a BSc in Physics in 2010 from the University of Antioquia (Colombia). Carlos’s scientific interests focus on studying and developing new on-chip technologies driven by the synergy between nanomaterials and photonics.

 

Abstract: Aerosol particles are ever-present in both natural environments, and in engineered systems.  In large part, control over aerosol particle transport hinges upon control of particle inertia, i.e. the propensity of particles to maintain a particular trajectory whilst the surrounding fluid moves in a distinct direction.  Increasing the inertia of increasingly small particles typically involves increasing particle initial velocities, such that when fluid velocities slow down, particle-fluid velocity differences are maximized.  While inertial principles have long been exploited in a wide variety of aerosol instruments, including impactors, virtual impactors, and aerodynamic particle spectrometers, application of inertially-governed aerosol systems wherein the particle Mach number (based on its velocity difference with the fluid) approaches or even exceeds unit value are much more limited.  This presentation will overview current understanding of aerosol particle behavior in high-speed systems, and subsequently discuss ongoing studies of high speed particle behavior.  Specifically, the drag force on particles as a function of both the Knudsen number and the Mach number, derived from direct simulation Monte Carlo, will first be discussed.  Subsequently, results from ongoing studies of crater formation due to microparticle impacts at up to 1 km s-1 speeds with high-speed flight-relevant materials will be presented.  Also discussed will be insights from atomistic simulations into the changes in crystallinity and defect density experienced by particles during high-speed impacts, relevant to aerosol deposition of coatings, and the optimization of virtual impactor aerosol concentrators operating near the sonic limit, capable of submicrometer particle concentration enhancement.

Biographical Sketch: Dr. Chris Hogan is the Carl and Janet Kuhrmeyer Chair Professor and the Associate Department Head in the Department of Mechanical Engineering at the University of Minnesota, Twin Cities, where has been a faculty member since 2009.  His research group focuses largely on fundamental and applied research in aerosols, including the development of theories to describe transport and reactions in aerosols, the design of new measurement principles and instruments for aerosols, and the evaluation of HVAC control technologies.  He has published more than 150 peer reviewed papers focusing in these areas, and is the editor-in-chief of the Journal of Aerosol Science.