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Zhongming Li, Kyle Aleshire, Masaru Kuno , and Gregory V. Hartland, “Super-Resolution Far-Field Infrared Imaging by Photothermal Heterodyne Imaging,” J. Phys. Chem. B, Article ASAP, DOI: 10.1021/acs.jpcb.7b06065

Abstract: Infrared (IR) imaging provides chemical-specific information without the need for exogenous labels. Conventional far-field IR imaging techniques are diffraction limited, which means an effective spatial resolution of >5 μm with currently available optics. In this article, we present a novel far-field IR imaging technique based on photothermal heterodyne imaging (IR-PHI). In our version of IR-PHI, an IR pump laser excites the sample, causing a small temperature rise that is detected by a counterpropagating visible probe beam. Images and spectra of several different types of soft matter systems (polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells) are presented to demonstrate the sensitivity and versatility of the technique. Importantly, the spatial resolution in the IR-PHI measurements is determined by the visible probe beam: a spatial resolution of 0.3 μm was achieved with a 0.53 μm probe wavelength and a high numerical aperture focusing objective. This is the highest spatial resolution reported to date for far-field IR imaging. Analysis of the experiments shows that for polymer beads in a dry environment, the magnitude of the IR-PHI signal is determined by the scattering cross section of the nano-object at the probe wavelength. This is in contrast to conventional PHI experiments in a heat-transfer medium, where the signal scales as the absorption cross section. This different scaling can be understood through the optical theorem. Our analysis also shows that both thermal expansion and changes in the refractive index of the material are important and that these two effects, in general, counteract each other.

Kuai Yu, Tuphan Devkota, Gary Beane, Guo Ping Wang, and Gregory Hartland, “Brillouin oscillations from single Au nanoplate opto-acoustic transducers,” ACS Nano, Articles ASAP, DOI: 10.1021/acsnano.7b02703.

Abstract: Brillouin oscillations, which are GHz frequency waves that arise from the interaction of light with acoustic waves, are experiencing increasing applications in biology and materials science. They provide information about the speed of sound and refractive index of the material they propagate in, and have recently been used in imaging applications. In the current study, Brillouin oscillations are observed through ultrafast transient reflectivity measurements using chemically synthesized Au nanoplates as opto-acoustic transducers. The Au nanoplates are semitransparent, which allows the Brillouin oscillations to be observed from material on both sides of the plate. The measured frequencies are consistent with the values expected from the speeds of sound in the different materials, however, the attenuation constants are much larger than those reported in previous studies. The increased damping is attributed to diffraction of the acoustic wave as it propagates away from the excitation region. This effect is more significant for experiments with high numerical aperture objectives. These results are important for understanding the relationship between frequency and spatial resolution in Brillouin oscillation microscopy.

Our perspectives article on hot electrons effects in photocatalysis with metal nanoparticles has just been published in ACS Energy Letters.  This paper is a collaboration between the Hartland group at the University of Notre Dame and Sasha Govorov at Ohio University.

Gregory V. Hartland, Lucas V. Besteiro, Paul Johns and Alexander O. Govorov, “What’s so Hot about Electrons in Nanoparticles?” ACS Energy Letters, DOI: 10.1021/acsenergylett.7b00333.

Abstract: Metal nanoparticles are excellent light absorbers. The absorption processes create highly excited electron-hole pairs and recently there has been interest in harnessing these hot charge carriers for photocatalysis and solar energy conversion applications. The goal of this Perspectives article is to describe the dynamics and energy distribution of the charge carriers produced by photon absorption, and the implications for the photocatalysis mechanism. We will also discuss how spectroscopy can be used to provide insight into the coupling between plasmons and molecular resonances. In particular, the analysis shows that the choice of material and shape of the nanocrystal can play a crucial role in hot electron generation and coupling between plasmons and molecular transitions. The detection and even calculation of many-body hot-electron processes in the plasmonic systems with continuous spectra of electrons and short lifetimes are challenging, but at the same time very interesting from the point of view of both potential applications and fundamental physics. We propose that developing an understanding of these processes will provide a pathway for improving the efficiency of plasmon-induced photocatalysis.

Congratulations to Paul Johns, our latest Ph.D.

Congratulations to Zhongming (Jeremy) Li who successfully defended his thesis today.  Jeremy will start a Postdoc at MIT in June.

Paul Johns, Gary Beane, Kuai Yu, and Gregory V. Hartland, “Dynamics of Surface Plasmon Polaritons in Metal Nanowires,” J. Phys. Chem. C., 2017 121, 5445–5459.

Abstract: Metal nanostructures have found extensive use in a variety of applications in chemistry, including as substrates for molecular sensing and surface enhanced spectroscopy and as nanoscale heaters for photothermal therapy. These applications depend on the strong absorption and field enhancements associated with localized surface plasmon resonance (LSPR). This has led to a number of studies of how the LSPR line width, which measures energy losses for the coherent electron motion, depends on the size and shape of different types of metal nanoparticles, and the environment around the particle. Extended metal nanostructures, such as nanowires and nanoplates, display propagating surface plasmon modes, termed surface plasmon polaritons (SPPs), in addition to LSPRs. These modes are important for applications where metal nanostructures are used as waveguides. However, less is known about the damping of the propagating SPPs compared to the LSPRs. The energy losses for the propagating SPP modes can be investigated by measuring propagation lengths. The goal of this Feature Article is to review recent experiments that have provided quantitative information about the propagation lengths of SPPs in metal nanostructures, and to provide a physical understanding of the important factors in SPP damping.