Vibrational Anharmonicity and Energy Relaxation in Nanoscale Acoustic Resonators.

The fundamental and n = 3 overtones of Au nanoplate thickness vibrations have been studied by transient absorption microscopy. The frequencies of the n = 3 overtone are less than 3× the frequency of the fundamental. This anharmonicity is explained through a continuum mechanics model that includes organic layers on top of the nanoplate and between the nanoplate and the glass substrate. In this model, anharmonicity arises from coupling between the vibrations of the nanoplate and the organic layers, which creates avoided crossings that reduce the overtone frequencies compared to the fundamental. Comparison of the experimental and calculated quality factors shows that coupling occurs to the top organic layer. Good agreement between the measured and calculated quality factors is obtained by introducing internal damping for the nanoplate. These results show that engineering layers of soft material around metal nanostructures can be used to control the vibrational lifetimes.

Photothermal heterodyne imaging of micron-sized objects.

Micron-sized dye-doped polymer beads were imaged using transmitted/reflected light microscopy and photothermal heterodyne imaging (PHI) measurements. The transmitted/reflected light images show distinct ring patterns that are attributed to diffraction effects and/or internal reflections within the beads. In the PHI experiments pump laser induced heating changes the refractive index and size of the bead, which causes changes in the diffraction pattern and internal reflections. This creates an analogous ring pattern in the PHI images. The ring pattern disappears in both the reflected light and PHI experiments when an incoherent light source is used as a probe. When the beads are imaged in an organic medium heat transfer changes the refractive index of the environment, and gives rise to a ring pattern external to the beads in the PHI images. This causes the beads to appear larger than their physical dimensions in PHI experiments. This external signal does not appear when the beads are imaged in air because the refractive index changes in air are very small.

Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires.

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm-1 for 2.5-3.9-µm-long NWs reveal a series of resonances due to the Fabry-Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry-Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry-Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Polymer dependent acoustic mode coupling and Hooke's law spring constants in stacked gold nanoplates.

Metal nanoparticles are excellent acoustic resonators and their vibrational spectroscopy has been widely investigated. However, the coupling between vibrational modes of different nanoparticles is less explored. For example, how the intervening medium affects the coupling strength is not known. Here, we investigate how different polymers affect coupling in Au nanoplate-polymer-Au nanoplate sandwich structures. The coupling between the breathing modes of the Au nanoplates was measured using single-particle pump-probe spectroscopy, and the polymer dependent coupling strength was determined experimentally. Analysis of the acoustic mode coupling gives the effective spring constant for the polymers. A relative motion mode was also observed for the stacked Au nanoplates. The frequency of this mode is strongly correlated with the coupling constant for the breathing modes. The breathing mode coupling and relative motion mode were analyzed using a coupled oscillator model. This model shows that both these effects can be described using the same spring constant for the polymer. Finally, we present a new type of mass balance using the strongly coupled resonators. We show that the resonators have a mass detection limit of a few femtograms. We envision that further understanding of the vibrational coupling in acoustic resonators will improve the coupling strength and expand their potential applications.

Approaches to mid-infrared, super-resolution imaging and spectroscopy.

This perspective highlights recent advances in super-resolution, mid-infrared imaging and spectroscopy. It provides an overview of the different near field microscopy techniques developed to address the problem of chemically imaging specimens in the mid-infrared "fingerprint" region of the spectrum with high spatial resolution. We focus on a recently developed far-field optical technique, called infrared photothermal heterodyne imaging (IR-PHI), and discusses the technique in detail. Its practical implementation in terms of equipment used, optical geometries employed, and underlying contrast mechanism are described. Milestones where IR-PHI has led to notable advances in bioscience and materials science are summarized. The perspective concludes with a future outlook for robust and readily accessible high spatial resolution, mid-infrared imaging and spectroscopy techniques.

Chemical interface damping for propagating surface plasmon polaritons in gold nanostripes.

Leakage radiation microscopy has been used to examine chemical interface damping (CID) for the propagating surface plasmon polariton (PSPP) modes of Au nanostripes-nanofabricated structures with heights of 40 or 50 nm, widths between 2 and 4 µm, and 100 µm lengths. Real space imaging was used to determine the propagation lengths LSPP of the leaky PSPP modes, and back focal plane measurements generated ω vs k dispersion curves, which yield the PSPP group velocities vg. The combination of these two experiments was used to calculate the PSPP lifetime via T1 = LSPP/vg. The difference in T1 times between bare and thiol coated nanostripes was used to determine the dephasing rate due to CID ΓCID for the adsorbed thiol molecules. A variety of different thiol molecules were examined, as well as nanostripes with different dimensions. The values of ΓCID are similar for the different systems and are an order-of-magnitude smaller than the typical values observed for the localized surface plasmon resonances (LSPRs) of Au nanoparticles. Scaling the measured ΓCID values by the effective path length for electron-surface scattering shows that the CID effect for the PSPP modes of the nanostripes is similar to that for the LSPR modes of nanoparticles. This is somewhat surprising given that PSPPs and LSPRs have different properties: PSPPs have a well-defined momentum, whereas LSPRs do not. The magnitude of ΓCID for the nanostripes could be increased by reducing their dimensions, principally the height of the nanostructures. However, decreasing dimensions for the leaky PSPP mode increases radiation damping, which would make it challenging to accurately measure ΓCID.

Ultrafast measurements of the dynamics of single nanostructures: a review.

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.

Long Lifetime and Coupling of Acoustic Vibrations of Gold Nanoplates on Unsupported Thin Films.

Radiation losses due to energy transfer to acoustic waves in the surroundings lead to short vibrational lifetimes of plasmonic nanoresonators and severely limits their applications in quantum technologies and mass sensing. Here we demonstrate a general method for improving the acoustic vibration lifetimes of Au nanoplates (flat structures with edge lengths of several microns and thicknesses of 10-40 nm) by using unsupported thin films instead of thick substrates, which blocks the outgoing acoustic waves. We experimentally confirm the vibrational quality factor improvement of Au nanoplates on Si3N4 and Al2O3/Si3N4 thin films with thicknesses less than 100 nm. Through this substrate phonon engineering, we successfully observe strong vibrational coupling between two Au nanoplates. Furthermore, the increased vibrational lifetimes allow us to study the vibrations of the nanoplates relative to the substrate and, for stacked nanoplates, relative to each other. The frequencies of these relative vibrational modes are well described by a simple coupled mass model, which provides the effective spring constants between interfaces separated by surface bonded molecules. We also demonstrate that the continuous thin films can be used for small plasmonic nanostructures (dimensions of several hundred nm), with similar increases in the vibrational lifetimes.

Making waves: Radiation damping in metallic nanostructures.

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

On the measurement of relaxation times of acoustic vibrations in metal nanowires.

The mechanical resonances of metal nanostructures are strongly affected by their environment. In this paper the way the breathing modes of single metal nanowires are damped by liquids with different viscosities was studied by ultrafast pump-probe microscopy experiments. Both nanowires supported on a glass substrate and nanowires suspended over trenches were investigated. The measured quality factors for liquid damping for the suspended nanowires are in good agreement with continuum mechanics calculations for an inviscid fluid that assume continuity in stress and displacement at the nanowire-liquid interface. This shows that liquid damping is controlled by radiation of sound waves into the medium. For the nanowires on the glass surface the quality factors for liquid damping are approximately 60% higher than those for the suspended nanowires. This is attributed to a shadowing effect. The nanowires in our measurements have pentagonal cross-sections. This produces two different breathing modes and also means that one of the faces for the supported nanowires is blocked by the substrate, which reduces the amount of damping from the liquid. Comparing the supported and suspended nanowires also allows us to estimate the effect of the substrate on the acoustic mode damping. We find that the substrate has a weak effect, which is attributed to poor mechanical contact between the nanowires and the substrate.