Brenden A. Magill

Upconverting nanocomposites dispersed in urea-containing acrylics

Lanthanide-doped upconverting nanoparticles (UCNPs) have the ability to convert low energy photons into high energy photons, making this material appealing for a variety of scientific pursuits, from solar energy conversion to bioimaging. A combination of polymers and nanocomposites increases the utility of these upconverting nanoparticles allowing nanoparticles to be added to any device compatible with polymer coatings. Here, trifluoroacetate salt decomposition enables Er/Yb doped NaYF4 upconverting nanoparticle synthesis. The subsequent deposition of a silica nanoshell yields polar silica-coated upconverting nanoparticles, enabling composite formation with polar urea-containing methacrylic polymers. Hydrogen bonding between urea groups in the polymer and the silica-coated nanoparticles allowed for dispersion of the upconverting nanoparticles to form upconverting composite films. These films exhibit desirable upconversion comparable to the nanoparticles dispersed in methanol. Urea-containing polymers are promising candidates for matrices in nanocomposites formed with polar silica nanoparticles due to favorable polymer–nanoparticle interactions. This architecture is superior to urea-methacrylate homopolymers, since the central low glass transition temperature block will provide critical ductility to the film, thus rendering the film to be durable for optical applications.

Photoluminescence lineshape and dynamics of localized excitonic transitions in InAsP epitaxial layers

The excitonic radiative transitions of InAsxP1−x (x = 0.13 and x = 0.40) alloy epitaxial layers were studied through magnetic field and temperature dependent photoluminescence and time-resolved photoluminescence spectroscopy. While the linewidth and lineshape of the exciton transition for x = 0.40 indicate the presence of alloy broadening due to random anion distribution and the existence of localized exciton states, those of x = 0.13 suggest that this type of compositional disorder is absent in x = 0.13. This localization is further supported by the behavior of the exciton transitions at low temperature and high magnetic fields. InAs0.4P0.6 exhibits anomalous “S-shaped” temperature dependence of the excition emission peak below 100 K as well as linewidth broadening at high magnetic fields due to the compression of the excitonic volume amid compositional fluctuations. Finally, photoluminescence decay patterns suggest that the excitons radiatively relax through two channels, a fast and a slow decay. While ...

Enhanced Multiphoton-Induced Luminescence in Silver Nanoparticles Fabricated with Nanosphere Lithography

We study multiphoton-absorption-induced luminescence (MAIL) in triangular silver nanoprisms fabricated with nanosphere lithography. We observe strong MAIL when the exciting infrared fs laser pulses overlap the main surface plasmon resonance of the particles, with significantly less signal from off-resonance structures present on the same substrate. The MAIL signal partially bleaches during imaging, but at higher illumination intensities, certain particles undergo a sudden multiplefold increase in MAIL brightness, which we show is associated with the deformation or melting of the particles into spheroids. The brightening may be due to the reduction of surface silver oxide into strongly luminescent silver nanoclusters or to the formation or activation of emission centers on the particle surface triggered by the melting of the particle. Regardless of the exact nature of the emission, silver oxide plays an important role in the process, evidenced by a twofold or threefold increase in MAIL luminosity from the triangular particles when their surface oxide is chemically reduced during imaging.

Two-photon activation of o-nitrobenzyl ligands bound to gold surfaces

We report on the use of two-photon absorption to photocleavage o-nitrobenzyl-based ligands bound to a gold surface with thiol groups. Ablation of ligands occurs at high power densities, but this can be largely avoided by reducing the optical power level, at which point two-photon mediated reactions still occur on a time scale of tens of seconds. This means that photoactive ligands can be activated at wavelengths where plasmon resonances in gold and silver nanoparticles can easily be achieved, which will allow the surface properties at the hot spots on plasmonic nanostructures to be chosen differently from the rest of the structure, with possible applications in high-efficiency Surface-Enhanced Raman Spectroscopy and bottom-up nanoassembly.

A review of the growth, doping, and applications of Beta-Ga2O3 thin films

β-Ga2O3 is emerging as an interesting wide band gap semiconductor for solar blind photo detectors (SBPD) and high power field effect transistors (FET) because of its outstanding material properties including an extremely wide bandgap (Eg ~4.9eV) and a high breakdown field (8 MV/cm). This review summarizes recent trends and progress in the growth/doping of β-Ga2O3 thin films and then offers an overview of the state-of-the-art in SBPD and FET devices. The present challenges for β-Ga2O3 devices to penetrate the market in real-world applications are also considered, along with paths for future work.

Ultrafast Anisotropic Optical Response and Coherent Acoustic Phonon Generation in Polycrystalline BaTiO3-BiFeO3

Abstract We optically study the as-yet little explored multiferroic material, BaTiO3-BiFeO3 (BTO-BFO), that has demonstrated enhanced magnetic properties, a higher DC resistance in comparison to BFO, and improved magnetoelectric coupling. Our studies include: ultrafast time resolved differential reflection, optically induced birefringence, and second-harmonic nano-imaging of the ferroic order. We observe a strong sensitivity to pump/probe polarizations, photo-induced ferroelectric poling on a picosecond timescale, as well as the generation of photo-induced coherent acoustic phonons with a frequency of ∼11

Ultrafast Magneto-Optical Spectroscopy of BiFeO3-BaTiO3 Based Structures

\sim 11

Time resolved magneto-optical studies of InAsP ternary alloys

 GHz. The second-harmonic generation nano-imaging reveals disordered but distinct ferroelectric domain order, percolating even across grain boundaries in the poly-crystalline thin film.

High-field magnetic circular dichroism in ferromagnetic InMnSb and InMnAs: Spin-orbit-split hole bands and g factors

Ultrafast optical spectroscopy can provide insight into fundamental microscopic interactions, dynamics and the coupling of several degrees of freedom. Pump/ probe studies can reveal the answer to questions like “What are the achievable switching speeds in multiferroics?”, “What is the influence of the crystallographic orientation and domain states on the available switching states?”, and “What is the effect of the hetrostructure on promoting the coupling between the varying field excitations?”. In this presentation, we report on two color (400/800nm) ultrafast pump-probe differential reflectance spectroscopy of BiFeO3-BaTiO3 structures to probe the coupling between optical and acoustic phonons to spin waves. The data presented here is a combination of different transient reflectivity measurements to probe both the carrier and spin dynamics. The (001)-BiFeO3-BaTiO3 thin films were prepared using pulsed laser deposition on vicinal SrTiO3 substrates using La0.70 Sr0.30MnO3 bottom electrodes. Crystal orientation and topography were analyzed by x-ray diffraction and atomic force microscopy. . Our results are important to developing devices on the basis of this material system. This work was supported by the AFOSR through grant FA9550-14-1-0376,NSF-Career Award DMR-0846834, and the Virginia Tech Institute for Critical Technology and Applied Science.

Highly-Stable Organo-Lead Halide Perovskites Synthesized Through Green Self-Assembly Process (Solar RRL 6∕2018)

The recent rapid progress in the field of spintronics requires extensive studies of carrier and spin relaxation dynamics in semiconductors. In this work, we employed time and spin resolved differential transmission measurements in order to probe carrier and spin relaxation times in several InAsP ternary alloys. In addition, the dynamics of the excitonic radiative transitions of InAs0.13P0.87 epitaxial layer were studied through the time-resolved photoluminescence spectroscopy.