Stephan Link, PhD , Rice University
Project 1: One-photon luminescence of single gold nanoparticles
While two-photon luminescence in gold nanoparticles yields a strong signal, one-photon luminescence is generally regarded to be much weaker and has seldom been employed for optical nanoparticle detection. We investigated one-photon luminescence of gold nanospheres and nanorods using single particle spectroscopy with excitation at 514 and 633 nm. We characterized the polarization dependence, determined the quantum yield, and proposed a mechanism describing one-photon luminescence, which suggests that the luminescence occurs via emission by a surface plasmon and that fast interconversion between surface plasmons and hot electron-hole pairs plays an important role. By recording single particle luminescence spectra using polarization sensitive excitation and detection, we found that one-photon luminescence of gold nanospheres and nanorods closely follows the scattering spectrum. Regardless of the excitation wavelength and polarization, the major intensity of the luminescence always occurs polarized along the long axis of the gold nanorods through emission of longitudinal plasmons, which can only be explained by fast interconversion between hot electron-hole pairs and surface plasmons. Especially for 514 nm excitation, transverse surface plasmons and interband absorption create hot electron-hole pairs that subsequently decay into longitudinal plasmons causing an apparent depolarization of the absorption dipole moment. These luminescence results therefore reveal the important interplay between hot electron-hole pairs and surface plasmons, which could be exploited for plasmon assisted surface photochemistry. For gold nanorods, we further determined that direct excitation at the longitudinal surface plasmon resonance at 633 nm yields larger luminescence quantum yields. We also found that the absorption and emission dipoles are collinear for excitation of the longitudinal plasmon resonance. In contrast, absorption and emission dipoles for luminescence excited at 514 nm are not collinear.
Project 2: Coupling between substrate images and collective nanoparticle plasmons
The interaction between adjacent metal nanoparticles within an assembly induces interesting collective plasmonic properties. Using dark-field imaging of plasmon scattering, we investigated rings of gold nanoparticles and observed that the images were dependent on the substrate. In particular, for nanoparticles assembled on carbon and gold substrates intensity line sections of the ring revealed a significant broadening beyond the optical resolution accompanied by an intensity dip in the middle of the line profile. Overall this appears in the image as ‘splitting’ into two offset circles along the direction of the scattered light polarization. This effect was not observed for a substrate with a low permittivity. By varying the substrate as well as selecting different detected wavelengths and polarization components of the excitation light we were able to confirm that the observed effect is due to coupling of collective plasmon modes with their induced image charges in the supporting substrates. By isolating different polarization components of the excitation light we showed that the coupling responsible for this effect is strongest for polarization perpendicular to the substrate. We interpret the splitting into offset rings as the coupling between collective plasmon modes of the ring and induced images charges in the substrate as verified by single particle measurements and wavelength resolved dark-field imaging. The strength of this coupling depends on the nature of the substrate as no effect was seen for nanoparticle rings on a low permittivity substrate such a glass. The experimental results for the optical response of individual and assembled gold nanoparticles on the carbon compared to the gold film suggest a similar interaction strength, which is important to consider when using carbon coated TEM grids as nanoparticle substrates. Our results show that it is possible to spatially modulate scattering from plasmonic nanostructures by changing the permittivity of the substrate through external control with electrical or optical signals. One can furthermore use the effect discussed in this work to sense changes in the dielectric properties of the supporting substrate.
Project 3: Intensity modulation of nanorod plasmons
Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogs of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100 % using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogenous to a twisted nematic phase of the liquid crystal covering the nanorods. Even our non-optimized prototype devices show a surprisingly robust long-term stability enduring several thousand reversible switching cycles at 1 Hz for an hour without a change in scattering intensity, initial nanorod orientation, and nanorod position. We anticipate that the performance of our devices can be further improved in terms of operating voltage and response time by carefully selecting other liquid crystal solvents. While plasmonic nanorods represent the simplest form of an optical antenna, the described strategy should directly translate to more complex antenna architectures and other plasmonic elements for the electrical manipulation of visible light in structures with nanoscale dimensions.