Matthew D. McMahon

3D Particle Trajectories Observed by Orthogonal Tracking Microscopy

We demonstrate high-resolution, high-speed 3D nanoparticle tracking using angled micromirrors. When angled micromirrors are introduced into the field of view of an optical microscope, reflected side-on views of a diffusing nanoparticle are projected alongside the usual direct image. The experimental design allows us to find the 3D particle trajectory using fast, centroid-based image processing, with no nonlinear computing operations. We have tracked polystyrene particles of 190 nm diameter with position measurement precision <20 nm in 3D with 3 ms frame duration (i.e., at an imaging rate >330 frames per second). Because the image processing requires only approximately 1 ms per frame, this technique could enable real-time feedback-controlled nanoparticle assembly applications with nanometer precision.

A technique for producing ordered arrays of metallic nanoclusters by electroless deposition in focused ion beam patterns

Ordered arrays of Au nanoclusters have been prepared on Si substrates using a combination of focused ion beam (FIB) surface processing and electroless deposition. Particles varying in size from approximately 30 to 100 nm have been produced in regular grid patterns whose geometry is controlled to high precision by the FIB. Potential applications range from engineering of the surface plasmon resonance for numerous optical applications to building structures for tethering organic molecules in specific geometric arrangements.

Effects of Surface Asymmetry on Femtosecond Second-Harmonic Generation from Metal Nanoparticle Arrays

We describe experiments aimed at distinguishing possible mechanisms of second-harmonic generation (SHG) in lithographically prepared arrays of metal nanoparticles. It is well-known that even-order harmonics cannot be generated by electric dipole-dipole interactions in centrosymmetric systems. The experiment employs two basic sample geometries. In our first geometry, as in our previous work, the NPs are left exposed to air, producing an asymmetric local dielectric environment with ITO on one side and air on the other. In the second geometry, we propose coating the arrays with the same material as they are created on, thus producing a centrosymmetric environment in which any SHG observed can not be due to asymmetry in the medium, but to nonlocal or retardation mechanisms in the particles. The arrays are fabricated using focused ion-beam lithography and vapor deposition of the metal, followed by standard lift-off protocols. This procedure yields typical NP dimensions between 60 nm and 200 nm in diameter, and between 15 nm and 30 nm in height, as characterized by scanning electron and atomic-force microscopy. By tuning the NP resonances to the excitation wavelength the SHG signal can be substantially enhanced. Surface melting effects are minimized by the use of ultra-short (50-fs) pulses which give high intensity while allowing us to work at relatively low fluence.

Time-resolved Second-harmonic Generation from Gold Nanoparticle Arrays

We have studied the effects of planar inversion symmetry and particle-coupling of gold nanoparticle (NP) arrays by angle dependent second-harmonic generation (SHG). Time- and angle- resolved measurements were made using a mode-locked Ti:sapphire 800 nm laser onto gold NP arrays with plasmon resonance tuned to match the laser wavelength in order to produce maximum SHG signal. Finite-difference time domain simulations are used to model the near-field distributions for the various geometries and compared to experiment. The arrays were fabricated by focused ion-beam lithography and metal vapor deposition followed by standard lift-off protocols, producing NPs approximately 20nm high with various in-plane dimensions and interparticle gaps. Above a threshold fluence of ~ 7.3 × 10-5 mJ/cm2 we find that the SHG scales with the third power of intensity, rather than the second, and atomic-force microscopy shows that the NPs have undergone a reshaping process leading to more nearly spherical shapes.

Second harmonic generation from centrosymmetric arrays of gold nanoparticles

We present experimental results from second-harmonic generation studies of lithographically-prepared arrays of centrosymmetric gold nanorods, extending a previous treatment. The arrays serve as diffraction gratings, allowing control over the emission directions. The intrinsic radiation patterns from the nanoparticles are superimposed on the diffraction pattern, creating a unique angular distribution of second-harmonic light. The surface plasmon resonance mode of the particles is tuned to match the wavelength of the ultrafast Ti:sapphire excitation laser, dramatically enhancing the second-harmonic intensity but also increasing photodesorption effects. The details of the diffracted peak intensities depend sensitively on the geometry of the system and require a complex normalization of the data.