Michael D. Mason Assistant Professor   • B.S. (Chemistry) University of Puget Sound, Tacoma, Washington, 1994 • B.S. (Physics, minor in Mathematics) University of Puget Sound, Tacoma, Washington, 1995 • Ph.D. (Chemistry) University of California - Santa Barbara, 2000   Phone: 207-581-2344 Fax: 207-581-2323 Email: mmason@umche.maine.edu

Research Interests

Photophysics of nanoparticles and molecular nanoprobes •  single molecule imaging • time-resolved single photon spectroscopic imaging techniques

Current Research

Single Nanoparticle/Nanoprobe Photophysics

Using a combination of single molecule spectroscopic and imaging techniques we characterize and quantify the underlying photophysics of a range of new nanoprobes which exhibit potential as fluorescent single molecule reporters for applications in the biological and materials sciences.

Nanoprobe Design and Optimization for Biological/Materials Applications

Passive and reactive molecular and quantum dot (metallic and semiconductor) nanoprobes, generally referred to as fluors, have shown great promise as localized reporters in a range of in vitro biochemical and materials systems. The individual fluor represents the highest possible spatial resolution for chemical processes within a sample. However, in order to achieve sufficient signal-to-noise for single fluor imaging/spectroscopy in complicated materials and biological systems, where the main source of signal is often from background radiation, nanoprobes must be specifically designed taking into account their intrinsic photophysics as well as any potential influences of the system of interest. A broad range of techniques are being employed with the eventual goal of controlling photophysical processes of fluors such as photo-stability, excited state dynamics (i.e. lifetime and triplet dynamics), conformational fluctuations in absorption and emission properties, and environmental (chemical) sensitivity and specificity.

Time-resolved Single–molecule Imaging Spectroscopy

In all high resolution imaging techniques there is a continuing drive to increase the amount of signal, and therefore information, obtained. In general, these techniques fall into one of two categories: High quantum efficiency time-resolved single photon counting, or much lower efficiency spectroscopies using dispersion type monochrometers/spectrometers. In fact, some combination of these techniques represents the potential for the greatest information density: the temporal behavior, energy, and in a scanning format, the point of origin within a 3-dimensional sample of each photon. Recently, this effort has been advanced using single-photon counting techniques coupled with high efficiency optics providing <ns time resolution and simultaneous, though severely limited, energy resolution. By further extending these techniques to the single molecule level, where the underlying photophysics of the probe fluor are carefully characterized, the quantum mechanical nature of the fluor can be used to statistically analyse the photon stream revealing the underlying physical and chemical processes within the system of interest with a resolution not previously obtained. Unlike traditional spectroscopies, the sub-ensemble nature of the single molecule experiment is uniquely sensitive to rare events and random fluctuations which are otherwise washed out in bulk measurements due to their low relative probability and the use of experimental averaging.

Selected Publications

Mason, M.D., Ray, K., Grober, R.D., Pohlers, G., Cameron, J.F. “Single molecule acid-base kinetics and thermodynamics”. Physical Review Letters. 2004 (in press).

Mason, M.D., Ray, K., Pohlers, G., Cameron, J.F., Grober, R.D. “Probing the local pH of polymer photoresist films using a two-color single molecule nanoprobe”. J. Phys. Chem B. 2003; 107:14219-14224.

Sirbuly, D.J., Schmidt J.P., Mason M.D., Summers M.A., Buratto, S.K. “Variable-ambient scanning stage for a laser scanning confocal microscope”. Rev. Sci. Inst. 2003; 74(10):4366-4368.

Mason, M.D., Sirbuly, D.J., Buratto, S.K. “Correlation between bulk morphology and luminescence in porous silicon investigated by pore collapse resulting from drying”. Thin Solid Films. 2002; 406(1-2):151-158.

Michler, P., Imamoglu, A., Kiraz, A., Becher, C., Mason, M.D., Carson, P.J., Strouse, G.F., Buratto, S.K., Schoenfeld, W.V., Petroff, P.M., “Nonclassical radiation from a single quantum dot”. Physica Status Solidi B-Basic Research. Jan 2002; 229(1):399-405.

Schuck, P.J., Mason, M.D., Grober, R.D., et al. “Spatially resolved photoluminescence of inversion domain boundaries in GaN-based lateral polarity heterostructures”. Appl Phys Lett. August 13, 2001; 79(7):952-954.

Michler, P., Imamoglu, A., Mason, M.D., Carson, P.J., Strouse, G.F., Buratto, S.K. “Quantum correlation among photons from a single quantum dot at room temperature”. Nature. August 2000; 406(31), 968-970.

Mason, M.D., Sirbuly, D.J., Carson, P:.J., Buratto, S.K. “Investigating individual chromophores within single porous silicon nanoparticles”. Journal of Chemical Physics. May 8, 2001; 114(18):8119-8123.

Credo, G.M., Mason, M.D., Buratto, S.K. “External quantum efficiency of single porous silicon nanoparticles”. Applied Physics Letters. April 5, 1999; 74(14):1978-1980.

Mason, M.D., Credo, G.M., Weston, K.D., Buratto, S.K., “Luminescence of individual porous Si chromophores”. Physical Review Letters.  June 15, 1998; 80(24):5405-5408.