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Dive into the research topics where Brian P. Mehl is active.

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Featured researches published by Brian P. Mehl.


Nano Letters | 2013

Direct imaging of free carrier and trap carrier motion in silicon nanowires by spatially-separated femtosecond pump-probe microscopy.

Michelle M. Gabriel; Justin R. Kirschbrown; Joseph D. Christesen; Christopher W. Pinion; David F. Zigler; Erik M. Grumstrup; Brian P. Mehl; Emma E. M. Cating; James F. Cahoon; John M. Papanikolas

We have developed a pump-probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼10(4) cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales.


Journal of Physical Chemistry A | 2010

Direct imaging of optical cavity modes in ZnO rods using second harmonic generation microscopy.

Brian P. Mehl; Ralph L. House; Abhineet Uppal; Amanda Reams; Chuan Zhang; Justin R. Kirschbrown; John M. Papanikolas

Images of second harmonic generation (SHG) in needle-shaped ZnO rods obtained from individual structures show areas of enhanced second harmonic intensity along the longitudinal axis of the rod that are periodically distributed and symmetrically situated relative to the rod midpoint. The spatial modulation is a direct consequence of the fundamental optical field coupling into standing wave resonator modes of the ZnO structure, leading to an enhanced backscattered second harmonic condition that cannot be achieved in bulk ZnO. A more complicated second harmonic image is observed when excitation is below the band gap, which is attributed to whispering gallery modes. This physical phenomenon, which extends beyond just ZnO to many other optical materials, could pave the way to new applications that exploit the nonlinear optical properties of individual structures.


Journal of Physical Chemistry B | 2013

Pump-probe microscopy: spatially resolved carrier dynamics in ZnO rods and the influence of optical cavity resonator modes.

Brian P. Mehl; Justin R. Kirschbrown; Michelle M. Gabriel; Ralph L. House; John M. Papanikolas

Femtosecond pump-probe microscopy is used to investigate the charge recombination dynamics at different points within a single needle-shaped ZnO rod. Recombination in the tips of the rod occurs through an excitonic or electron-hole plasma state, taking place on a picosecond time scale. Photoexcitation in the larger diameter sections of the interior exhibit dramatically slower recombination that occurs primarily through defects sites, i.e., trap mediated recombination. Transient absorption imaging shows that the spatial variation in the dynamics is also influenced by the cavity resonances supported within the hexagonal cross section of the rod. Finite element simulations suggest that these optical resonator modes produce qualitatively different intensity patterns in the two different locations. Near the end of the rod, the intensity pattern has significant standing-wave character, which leads to the creation of photoexcited carriers in the core of the structure. The larger diameter regions, on the other hand, exhibit intensity distributions in which the whispering gallery (WG) mode character dominates. At these locations, the photoexcited carriers are produced in subsurface depletion zone, where the internal fields separate the electrons and holes and lead to a greater degree of trap recombination on longer time scales.


Journal of Physical Chemistry B | 2012

Base-induced phototautomerization in 7-hydroxy-4-(trifluoromethyl)coumarin.

Brittany C. Westlake; Jared J. Paul; Stephanie E. Bettis; Shaun D. Hampton; Brian P. Mehl; Thomas J. Meyer; John M. Papanikolas

Excited-state proton-transfer dynamics between 7-hydroxy-4-(trifluoromethyl)coumarin and 1-methylimidazole base in toluene were studied using ultrafast pump-probe and time-resolved emission methods. Charge-transfer excitation of the hydroxycoumarin shifts electron density from the hydroxyl group to the carbonyl, resulting in an excited state where proton transfer to the base is highly favored. In addition to its the photoacid characteristics, the shift in the hydroxycoumarin electronic distribution gives it characteristics of a photobase as well. The result is a tautomerization process occurring on the picosecond time scale in which the 1-methylimidazole base acts as a proton-transfer shuttle from the hydroxyl group to the carbonyl.


Proceedings of SPIE | 2009

Investigation of ultrafast carrier dynamics in ZnO rods using two-photon emission and second harmonic generation microscopy

Ralph L. House; Brian P. Mehl; Chuan Zhang; Justin R. Kirschbrown; Scott C. Barnes; John M. Papanikolas

The demand for novel optoelectronic and photonic technologies has fueled an intense research effort to synthesize and characterize nanostructured semiconductor materials with unique properties that lend themselves to technological innovation. Zinc Oxide has emerged as an attractive candidate for a variety of applications, due in part to a large second order nonlinear susceptibility, its wide band-gap and large exciton binding energy. We have used time-resolved nonlinear two-photon emission and second harmonic generation microscopy to characterize the optical properties and excited state dynamics of individual rods. Ultrafast emission microscopy is used to follow the trapping dynamics of photoexcited charge carriers. Our results show a time-dependent red-shift in the trap emission band that is interpreted as arising from carrier percolation through trap states. In a second series of experiments, second harmonic generation (SHG) microscopy illustrates the connection between the optical mode structure of the object and its nonlinear mixing efficiency. Images show a periodic modulation in the SHG efficiency that is symmetrically situated relative to the rod midpoint. This phenomenon arises when the fundamental optical field couples into standing wave resonator modes of the structure and is a direct manifestation of the tapered shape of the rod.


Journal of the American Chemical Society | 2010

Energy transfer dynamics in metal-organic frameworks.

Caleb A. Kent; Brian P. Mehl; Liqing Ma; John M. Papanikolas; Thomas J. Meyer; Wenbin Lin


Journal of Physical Chemistry C | 2008

Exciton Dynamics and Biexciton Formation in Single-Walled Carbon Nanotubes Studied with Femtosecond Transient Absorption Spectroscopy

David J. Styers-Barnett; Stephen P. Ellison; Brian P. Mehl; Brittany C. Westlake; Ralph L. House; Cheol Park; Kristopher E. Wise; John M. Papanikolas


Journal of Physical Chemistry Letters | 2011

The End Is Different than The Middle: Spatially Dependent Dynamics in ZnO Rods Observed by Femtosecond Pump–Probe Microscopy

Brian P. Mehl; Justin R. Kirschbrown; Ralph L. House; John M. Papanikolas


Journal of Physical Chemistry C | 2013

Triplet Excitation Energy Dynamics in Metal–Organic Frameworks

Jiaxing Lin; Xiangqian Hu; Peng Zhang; Andre van Rynbach; David N. Beratan; Caleb A. Kent; Brian P. Mehl; John M. Papanikolas; Thomas J. Meyer; Wenbin Lin; Spiros S. Skourtis; Marios Constantinou


Journal of Physical Chemistry C | 2011

Characterizing the Ultrafast Charge Carrier Trapping Dynamics in Single ZnO Rods Using Two-Photon Emission Microscopy

Ralph L. House; Brian P. Mehl; Justin R. Kirschbrown; Scott C. Barnes; John M. Papanikolas

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John M. Papanikolas

University of North Carolina at Chapel Hill

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Justin R. Kirschbrown

University of North Carolina at Chapel Hill

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Ralph L. House

University of North Carolina at Chapel Hill

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Michelle M. Gabriel

University of North Carolina at Chapel Hill

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Thomas J. Meyer

University of North Carolina at Chapel Hill

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Brittany C. Westlake

University of North Carolina at Chapel Hill

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Caleb A. Kent

University of North Carolina at Chapel Hill

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Chuan Zhang

University of North Carolina at Chapel Hill

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Scott C. Barnes

University of North Carolina at Chapel Hill

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