Merlin Mah

Merlin received B.S. degrees in Electrical Engineering and Computer Engineering (cum laude) from the University of Minnesota. Working with the Optical MEMS group since his undergrad days, he defended in July 2017 his Ph.D thesis on the dual topics of thermoluminescent microparticle thermometry for high-explosive detonations and the development of optical methods and instruments for characterizing glacial ice. He is currently a postdoc attached to the latter project.

Outside of the lab, Merlin enjoys (further) ventures into programming, photography, graphics and web design (including this site), a few musical instruments, and swimming.

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  • A. Das, A. K. Brown, M. L. Mah, and J. J. Talghader, “Photon diffusion in microscale solids,” Journal of Physics: Condensed Matter, vol. 31, no. 33, p. 335703, Jun. 2019.

    This paper presents a theoretical and experimental investigation of photon diffusion in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 μm and 100 μm. Optical intensities of 10 kW cm-2 and 20 kW cm-2 are used in the laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high speed pyrometer. To compare the observed temperatures, differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated in the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly-overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientific literature.

  • M. L. Mah and J. J. Talghader, “Decomposition of aberrated or turbulent wavefronts into a spatial mode spectrum using optical cavities,” Applied Optics, vol. 58, no. 16, pp. 4288–4299, Jun. 2019.

    It is shown that an aberrated wavefront incident upon a Fabry-Perot optical cavity excites higher order spatial modes in the cavity and that the spectral width and distribution of these modes is indicative of the type and magnitude of the aberration. The cavities are purely passive, and therefore frequency content is limited to that provided by the original light source. To illustrate this concept, spatial mode decomposition and transmission spectrum calculation are simulated on an example cavity; the effects of various phase delays, in the form of two basic Seidel aberrations and a composite of Zernike polynomial terms, are shown using both Laguerre-Gaussian and plane wave incident beams. The aggregate spectral width of the cavity modes excited by the aberrations is seen to widen as the magnitude of the aberrations/ phase delay increases.

  • M. L. Mah and J. J. Talghader, “The decomposition of aberrated or turbulent wavefronts into a spatial mode spectrum using optical cavities,” arXiv e-prints, p. arXiv:1901.11087, Jan. 2019.
  • M. L. Mah and J. J. Talghader, “Spectral decomposition of aberrated wavefronts from actuated micromirrors,” in 2018 International Conference on Optical MEMS and Nanophotonics (OMN), 2018, pp. 191–192.

    Aberrated optics degrade the cavity finesse and point spread function of any optical system. It is frequently overlooked that the phase errors of these aberrations cause characteristic spectral features that can be observed and measured by an optical cavity. A general theory of the spectral decomposition of aberrations is developed and applied to an electrically deformable membrane feeding a high-finesse resonator. The micromirror, which consists of a 1 cm metallized SiNxelectrostatically-actuated membrane with phase offsets of about 1.5μm, leads to optical power being distributed to over twenty higher-order Laguerre-Gaussian cavity modes.

  • P. Armstrong, M. Mah, H. Ross, and J. Talghader, “Individual Microparticle Measurements for Increased Resolution of Thermoluminescent Temperature Sensing,” IEEE Sensors Journal, vol. 18, no. 11, pp. 4422–4428, Jun. 2018.

    Typically when microsensor particles are used to monitor temperatures in harsh environments, a statistical number of particles are collected and measured in aggregate. While this method is undoubtedly the most practical method for the rapid acquisition of overall temperature data, the limitations of collective measurements of numbers of particles versus individual ones has never been explored. In this paper, the thermoluminescent (TL) magnesium borate microparticles are used to measure temperatures inside the periphery of explosions with collective and individual behavior contrasted. It is found that individual measurement indicate that some particles undergo extreme temperature while others seem to have had no exposure to high temperature at all. The microparticles were irradiated with 200Gy of gamma radiation to fill the traps in the band gap. Several grams of the irradiated microparticles were placed at various distances from the source of the detonation. After the microparticles were collected the TL curve was measured for microparticles that were in the detonation and a control group of microparticles not in the detonation. The TL curve of an individual microparticle was measured by placing the microparticle ranging in size from 25 μm to 75 μm on a microheater with an area of 300 μm \texttimes 300 μm. The microheater was then used to heat the microparticle at a linear rate while the thermoluminescence of the microparticle was measured. A summation of first-order kinetics curves were used to do a fit to the thermoluminescence curves of the microparticles that were in the detonation and the control group. By comparing the ratio of first-order kinetic curve peaks of the particles that were in the detonation to the control group the temperature that the particles in the explosion were calculated. This process was carried out for many different particles that were in the same detonation and collected from the same location in the explosive test chamber. Individual extracted temperatures from the microparticles show a large distribution ranging from room temperature to 516\textdegreeC, but in aggregate, the microparticles show a clustering of temperatures around 290\textdegreeC.

  • M. L. Mah, “Fire and Ice: Thermoluminescent Temperature Sensing in High-Explosive Detonations and Optical Characterization Methods for Glacier Ice Boreholes,” Doctoral, University of Minnesota, Minneapolis, MN, USA, 2017.

    The environment around a detonating high explosive is incredibly energetic and dynamic, generating shock waves, turbulent mixing, chemical reactions, and temperature excursions of thousands of Kelvin. Probing this violent but short-lived phenomena requires durable sensors with fast response times. By contrast, the glacier ice sheets of Antarctica and Greenland change on geologic time scales; the accumulation and compression of snow into ice preserves samples of atmospheric gas, dust, and volcanic ash, while the crystal orientations of the ice reflect its conditions and movement over hundreds of thousands of years. Here, difficulty of characterization stems primarily from the location, scale, and depth of the ice sheet. This work describes new sensing technologies for both of these environments. Microparticles of thermoluminescent materials are proposed as high-survivability, bulk-deployable temperature sensors for applications such as assessing bioagent inactivation. A technique to reconstruct thermal history from subsequent thermoluminescence observations is described. MEMS devices were designed and fabricated to assist in non-detonation testing: large-area electrostatic membrane actuators were used to apply mechanical stress to thermoluminescent Y2O3:Tb thin film, and microheaters impose rapid temperature excursions upon particles of Mg2SiO4:Tb,Co to demonstrate predictable thermoluminescent response. Closed- and open-chamber explosive detonation tests using dosimetric LiF:Mg,Ti and two experimental thermometry materials were performed to test survivability and attempt thermal event reconstruction. Two borehole logging devices are described for optical characterization of glacier ice. For detecting and recording layers of volcanic ash in glacier ice, we developed a lightweight, compact probe which uses optical fibers and purely passive downhole components to detect single-scattered long-wavelength light. To characterize ice fabric orientation, we propose a technique which uses reflection measurements from a small, fixed set of geometries. The design and construction of a borehole logger implementing these techniques is described, and its testing discussed.

  • P. R. Armstrong, M. L. Mah, K. D. Olson, L. N. Taylor, and J. J. Talghader, “Reduction of thermal emission background in high temperature microheaters,” Journal of Micromechanics and Microengineering, vol. 26, no. 5, p. 055004, 2016.

    High temperature microheaters have been designed and constructed to reduce the background thermal emission radiation produced by the heater. Such heaters allow one to probe luminescence with very low numbers of photons where the background emission would overwhelm the desired signal. Two methods to reduce background emission are described: one with low emission materials and the other with interference coating design. The first uses platforms composed of material that is transparent to mid-infrared light and therefore of low emissivity. Heating elements are embedded in the periphery of the heater. The transparent platform is composed of aluminum oxide, which is largely transparent for wavelengths less than about 8 μ m. In the luminescent microscopy used to test the heater, an optical aperture blocks emission from the heating coils while passing light from the heated objects on the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture was moved from the highly emissive heater coils at 450 \,^∘C to the largely transparent center at the same temperature. The second method uses microheaters with integrated multilayer interference structures designed to limit background emission in the spectral range of the low-light luminescence object being measured. These heaters were composed of aluminum oxide, titanium dioxide, and platinum and were operated over a large range of temperatures, from 50 \,^∘C to 600 \,^∘C. At 600 \,^∘C, they showed a background photon emission only 1/800 that of a comparison heater without the multilayer interference structure. In this structure, the radiation background was sufficiently reduced to easily monitor weak thermoluminescent emission from CaSO 4 :Ce,Tb microparticles.

  • A. Brown, M. Mah, and J. Talghader, “Power limits for non-destructive laser ablation of contaminants on micromachined structures,” in 2016 International Conference on Optical MEMS and Nanophotonics (OMN), 2016, pp. 1–2.

    The minimum and maximum laser irradiances that are effective in cleaning etch-released membranes and microstructure was measured for carbon microparticles on LPCVD silicon nitride thin films and suspended platforms. A 1064nm Nd:Yag laser was scanned across the samples to ablate the contaminants. Microscope images of the membranes showed mass removal for an irradiance as low as 600W cm-2. More thorough cleaning was achieved by increasing the irradiance. For 2.5×2.5mm, 205nm thin silicon nitride membranes, 79% of the contaminated area was removed with an exposure of 3.7kW cm-2. Catastrophic damage was seen at a power level of 8.4kW cm-2. Ablation effects were also measured as a change in optical absorption using a photo thermal common-path interferometer. Peak absorption values were decreased from over 100,000ppm to less than 20,000ppm. Silicon nitride platforms were also tested. Despite significant substrate heating, the platforms survived intact up to power levels of 8.4kW cm-2 with near perfect cleaning of carbon particles from their surfaces.

  • W. S. Chan, M. L. Mah, R. C. Bay, and J. J. Talghader, “Long-wavelength optical logging for high-resolution detection of ash layers in glacier ice,” Journal of Glaciology, pp. 1–5, Oct. 2016.

    A new instrument for high-resolution optical logging has been built and tested in Antarctica. Its purpose is to obtain records of volcanic products and other scattering features, such as bubbles and impurities, preserved in polar ice sheets, and it achieves this by using long wavelength near-infrared light that is absorbed by the ice before many scattering events occur. Longer wavelengths ensure that the return signal is composed primarily of a single or few backscattering event(s) that limit its spatial spread. The compact optical logger features no components on its body that draw power, which minimizes its size and weight. A prototype of the logger was built and tested at Siple Dome A borehole, and the results were correlated with prior optical logging profiles and records of volcanic products from collected ice core samples.

  • J. J. Talghader, M. L. Mah, E. G. Yukihara, and A. C. Coleman, “Thermoluminescent microparticle thermal history sensors,” Microsystems & Nanoengineering, vol. 2, p. 16037, Aug. 2016.

    While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared—many months in advance of a test, if desired—by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 \,^∘C range in a variety of high-explosive environments.

  • P. R. Armstrong, M. L. Mah, K. D. Olson, and J. J. Talghader, “Microheater multilayer interference to reduce thermal emission for low photon number luminescence measurement,” in 2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), Anchorage, AK, USA, 2015, pp. 924–927.

    Under low light conditions, high temperature measurements of luminescence are limited by the overlap of the thermal emission spectra and the luminescent emission spectra being measured. A solution to this is to have a heat source that can be designed not to emit in a certain wavelength range(s) by coating it with an interference multilayer. The multilayer effectively changes the emissivity of the heat source. Microheaters made from aluminum oxide platforms with platinum heating elements were coated with aluminum oxide and titanium oxide multilayers. This multilayer structure was used to measure the thermoluminescence of CaSO4:Ce,Tb up to 420\,^∘C. They also showed a thermal emission background 800 times lower at 600\,^∘C than the same microheater with no multilayer structure.

  • P. R. Armstrong, M. L. Mah, S. S. Kim, and J. J. Talghader, “Thermoluminescence of Y2O3:Tb3+ thin films deposited by electron beam evaporation,” Journal of Luminescence, vol. 148, pp. 225–229, Apr. 2014.

    Most thermoluminescent materials are created using crystal growth techniques; however, it would be of great utility to identify those few thermoluminescent materials that can be deposited using simpler methods, for example to be compatible with the early portions of a silicon integrated circuit or microelectromechanical fabrication process. In this work, thin films of yttrium oxide with a terbium impurity (Y2O3:Tb) were deposited on silicon wafers by electron beam evaporation. The source for the Y2O3:Tb was made by combining Y2O3 and Tb4O7 powders. The approximate thicknesses of the deposited films were 350 nm. After deposition, the films were annealed at 1100 \,^∘C for 30 s to improve crystallinity. There is a strong correlation between the x-ray diffraction (XRD) peak intensity and the thermoluminescent glow curve intensity. The glow curve displays at least two peaks at 140 \,^∘C and 230 \,^∘C. The emission spectra was measured using successive runs with a monochromator set to a different wavelength for each run. There are two main emission peaks at 490 nm and 540 nm. The terbium impurity concentration of approximately 1 mol% was measured using Rutherford backscattering spectrometry (RBS). The Y2O3:Tb is sensitive to UV, x-ray, and gamma radiation. The luminescent intensity per unit mass of UV irradiated Y2O3:Tb was about 2 times that of x-ray irradiated TLD-100.

  • W. S. Chan, M. L. Mah, D. E. Voight, J. J. Fitzpatrick, and J. J. Talghader, “Crystal orientation measurements using transmission and backscattering,” Journal of Glaciology, vol. 60, no. 224, pp. 1135–1139, 2014.
  • P. R. Armstrong, M. L. Mah, L. Taylor, and J. J. Talghader, “Reduced blackbody microheaters for measuring high temperature thermoluminescent glow curve peaks,” in 2014 International Conference on Optical MEMS and Nanophotonics (OMN), Glasgow, U.K., 2014, pp. 7–8.

    Infrared-transparent microheaters have been constructed to reduce the background blackbody radiation produced by the heater. Among other applications, such heaters allow one to probe the high temperature peaks of thermoluminescent(TL) materials. The microheater consists of peripheral platinum heating elements on a mid-infrared transparent alumina platform. Alumina has a relatively low blackbody signal at high temperature for wavelengths less than 8μm. To test the reduced blackbody emission, an aperture was placed over the heating coils and then the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture moved from the highly emissive heater coils at 450\,^∘C to the largely transparent center at the same temperature.

  • M. L. Mah, P. R. Armstrong, S. S. Kim, J. R. Carney, J. M. Lightstone, and J. J. Talghader, “Sensing the thermal history of high-explosive detonations using thermoluminescent microparticles,” IEEE Sensors Journal, vol. 13, no. 5, pp. 1742–1747, May 2013.
  • S. S. Kim, P. R. Armstrong, M. L. Mah, and J. J. Talghader, “Depth-dependent temperature effects on thermoluminescence in multilayers,” Journal of Applied Physics, vol. 114, no. 5, p. 053519, 2013.
  • J. J. Talghader and M. L. Mah, “Sensing Temperature Inside Explosions,” in Smart Sensors for Industrial Applications, K. Iniewski, Ed. Boca Raton, FL: CRC Press, 2013, pp. 259–272.

    Sensor technologies are a rapidly growing area of interest in science and product design, embracing developments in electronics, photonics, mechanics, chemistry, and biology. Their presence is widespread in everyday life, where they are used to sense sound, movement, and optical or magnetic signals. The demand for portable and lightweight sensors is relentless in several industries, from consumer electronics to biomedical engineering to the military. Smart Sensors for Industrial Applications brings together the latest research in smart sensors technology and exposes the reader to myriad applications that this technology has enabled. Organized into five parts, the book explores: Photonics and optoelectronics sensors, including developments in optical fibers, Brillouin detection, and Doppler effect analysis. Chapters also look at key applications such as oxygen detection, directional discrimination, and optical sensing. Infrared and thermal sensors, such as Bragg gratings, thin films, and microbolometers. Contributors also cover temperature measurements in industrial conditions, including sensing inside explosions. Magnetic and inductive sensors, including magnetometers, inductive coupling, and ferro-fluidics. The book also discusses magnetic field and inductive current measurements in various industrial conditions, such as on airplanes. Sound and ultrasound sensors, including underwater acoustic modem, vibrational spectroscopy, and photoacoustics. Piezoresistive, wireless, and electrical sensors, with applications in health monitoring, agrofood, and other industries. Featuring contributions by experts from around the world, this book offers a comprehensive review of the groundbreaking technologies and the latest applications and trends in the field of smart sensors.

  • M. L. Mah, P. R. Armstrong, and J. J. Talghader, “Alteration by repeated electrostatic MEMS actuation of the thermoluminescence of thin films,” in 2013 International Conference on Optical MEMS and Nanophotonics (OMN), 2013, pp. 55–56.

    The thermoluminescence characteristics of a thin film of terbium-doped yttrium oxide change upon repeated stress application through electrostatic actuation. A maximum 42% decrease in the intensity of two thermoluminescent peaks is seen when voltage is applied in 5V increments to 25V, translating to 0.15 μm of center deflection. While the overall intensity decreases, the higher temperature peak - corresponding to deeper traps - is affected more than the lower temperature one. Two possible physical explanations for the behavior are mechanical stress and dielectric charging.

  • J. J. Talghader and M. L. Mah, “Luminescent thermometry for sensing rapid thermal profiles in fires and explosions,” in Optical, Acoustic, Magnetic, and Mechanical Sensor Technologies, K. Iniewski, Ed. Boca Raton, FL: CRC Press, 2012, pp. 79–105.
  • M. L. Mah, P. R. Armstrong, S. S. Kim, J. R. Carney, J. M. Lightstone, and J. J. Talghader, “Thermal history sensing inside high-explosive environments using thermoluminescent microparticles,” in Sensors, 2011 IEEE, 2011, pp. 1269–1272.
  • M. Mah, P. Armstrong, J. Lightstone, and J. Talghader, “Thermal history sensing of post-detonation environments with thermoluminescent microparticles,” in APS Shock Compression of Condensed Matter Meeting Abstracts, Chicago, IL, USA, 2011, p. 1197.

    Thermoluminescent (TL) particles show promise as robust direct-contact thermal history sensors for explosive events. Research with microheaters has shown that TL microparticles can measure temperature excursions of hundreds of degrees; however, microheaters do not generate the severe pressure and shock stimuli present in post- detonation environments. To address this, TL particles were tested under conditions produced by the detonation of an aluminized explosive formulation. TLD-100 (LiF:Mg,Ti) powder was irradiated with 220 Gy of gamma radiation from a ^167Cs source before being exposed to the free field detonation of a 20 gram charge. Particles were recovered post-detonation from two separate tests and their TL glow curves measured. At least two TL emission peaks 50 ^oC apart are clearly distinguishable in both samples, with peak intensity ratios decreasing 33.7% and 60.0% from an original 8.88:1, indicative of distinct carrier traps emptying at rates depending on the trap energy. These ratios agree well with thermocouple measurements from within the post-detonation fireball.

  • M. L. Mah, M. E. Manfred, S. S. Kim, M. Prokic, E. G. Yukihara, and J. J. Talghader, “Measurement of rapid temperature profiles using thermoluminescent microparticles,” IEEE Sensors Journal, vol. 10, no. 2, pp. 311–315, Feb. 2010.
  • M. L. Mah, M. E. Manfred, S. S. Kim, M. Prokic, E. G. Yukihara, and J. J. Talghader, “Sensing of thermal history using thermoluminescent microparticles,” in 2009 IEEE/LEOS International Conference on Optical MEMS and Nanophotonics, Clearwater, FL, USA, 2009, pp. 23–24.
  • M. E. Manfred, N. T. Gabriel, M. L. Mah, E. G. Yukihara, and J. J. Talghader, “Pulsed thermal excitation of luminescent microparticles for radiation dosimetry,” in 2008 IEEE/LEOS International Conference on Optical MEMs and Nanophotonics, Freiburg, Germany, 2008, pp. 64–65.

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We are the ECE research group of Professor Joey Talghader at the University of Minnesota.

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