Mingzhao Liu

Optical trapping and alignment of single gold nanorods by using plasmon resonances.

We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using laser light slightly detuned from their longitudinal plasmon mode. Detuning to the long-wavelength side of the resonance allows stable trapping for several minutes, with an exponential dependence of trapping time on laser power (consistent with a Kramers escape process). Detuning to the short-wavelength side causes repulsion of the rods from the laser focus. Alignment of the long axis of the rods with the trapping laser polarization is observed as a suppression of rotational diffusion about the short axis.

Damping of acoustic vibrations in gold nanoparticles

Studies of acoustic vibrations in nanometre-scale particles can provide fundamental insights into the mechanical properties of materials because it is possible to precisely characterize and control the crystallinity and geometry of such nanostructures. Metal nanoparticles are of particular interest because they allow the use of ultrafast laser pulses to generate and probe high-frequency acoustic vibrations, which have the potential to be used in a variety of sensing applications. So far, the decay of these vibrations has been dominated by dephasing due to variations in nanoparticle size. Such inhomogeneities can be eliminated by performing measurements on single nanoparticles deposited on a substrate, but unknown interactions between the nanoparticles and the substrate make it difficult to interpret the results of such experiments. Here, we show that the effects of inhomogeneous damping can be reduced by using bipyramidal gold nanoparticles with highly uniform sizes. The inferred homogeneous damping is due to the combination of damping intrinsic to the nanoparticles and the surrounding solvent; the latter is quantitatively described by a parameter-free model.

Plasmon resonance-based optical trapping of single and multiple Au nanoparticles

The plasmon resonance-based optical trapping (PREBOT) method is used to achieve stable trapping of metallic nanoparticles of different shapes and composition, including Au bipyramids and Au/Ag core/shell nanorods. In all cases the longitudinal plasmon mode of these anisotropic particles is used to enhance the gradient force of an optical trap, thereby increasing the strength of the trap potential. Specifically, the trapping laser is slightly detuned to the long-wavelength side of the longitudinal plasmon resonance where the sign of the real component of the polarizability leads to an attractive gradient force. A second (femtosecond pulsed) laser is used to excite two-photon fluorescence for detection of the trapped nanoparticles. Two-photon fluorescence time trajectories are recorded for up to 20 minutes for single and multiple particles in the trap. In the latter case, a stepwise increase reflects sequential loading of single Au bipyramids. The nonlinearity of the amplitude and noise with step number are interpreted as arising from interactions or enhanced local fields amongst the trapped particles and fluctuations in the arrangements thereof.

Preparation and optical properties of silver chalcogenide coated gold nanorods

A homogeneous layer of silver sulfide or selenide was coated onto gold nanorods in aqueous solution by exposing Au/Ag core/shell nanorods to S2− or Se2− in an oxidizing environment. The formation of the silver chalcogenide layers was confirmed by transmission electron microscopy (TEM) and selective area electron diffraction (SAED). The longitudinal plasmon resonance of the gold nanorods shifted to the red and was attenuated. The experimental spectra agree well with a simulation based on confocal ellipsoids in the quasi-static limit. The synthetic procedure was also employed to coat silver sulfide or selenide onto gold bipyramids. A red-shift of the longitudinal plasmon resonance was also seen, although the coating was not as homogeneous. These novel composite materials may present interesting nonlinear optical properties.

Optical trapping and alignment of single gold nanorods : using plasmon resonances

We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.

Ultrafast Optical Nonlinearities of Single Metal Nanoparticles

We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena related to electron confinement and enhanced plasmon dephasing.

Optical nonlinearities of plasmons in single gold nanorods

We measure third-order optical nonlinearities of plasmons in single chemically-synthesized gold nanorods using a novel interferometric scattering technique. We observe ultrafast, coherent material nonlinearities that are enhanced by localization of light in the rods.

Metallic colloids and their plasmonic properties

Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far. Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor composite materials may present interesting nonlinear optical properties which are being currently investigated. Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be attributed entirely to the heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.

Plasmon-enhanced optical trapping of individual metal nanorods

We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods, Au/Ag core/shell nanorods, and Au bipyramids in solution, using the longitudinal surface-plasmon resonance to enhance optical forces. Laser light that is detuned slightly to the long-wavelength side of the resonance traps individual and multiple particles for up to 20 minutes; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time of individual particles depends exponentially on laser power, in agreement with a Kramers escape process. Trapped particles have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. When multiple particles are trapped simultaneously, evidence of interparticle interactions is observed, including a nonlinearly increasing two-photon fluorescence intensity, increasing fluorescence fluctuations, and changing fluorescence profiles as the trapped particle number increases.

Ultrafast optical nonlinearities of plasmons in single gold nanorods

Excitation of plasmons in a metal nanoparticle leads to localization of electromagnetic fields within the particle, which is expected to result in strong optical nonlinearities. We study ultrafast nonlinearities in optical scattering from single gold nanorods under resonant excitation at the plasmon frequency, and observe changes of as much as 20% in the scattering cross section over the 20-fs laser pulse duration. Unexpectedly, the magnitude of the ultrafast nonlinearity is the same as that due to heating of conduction electrons in the metal.We measure nonlinear optical scattering from individual Au nanorods excited by ultrafast laser pulses on resonance with their longitudinal plasmon mode. Isolating single rods removes inhomogeneous broadening and allows the measurement of a large nonlinearity, much greater than that of nanorod ensembles. Surprisingly, the ultrafast nonlinearity can be attributed entirely to heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.