Farbod Shafiei

A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance

We demonstrate that a subwavelength plasmonic metamolecule consisting of four nanoparticles supports a magnetic response spectrally overlapped with the electric dipole resonance. Small structural asymmetries lead to interference and thus a Fano resonance in scattering.

Manipulating Coupling between a Single Semiconductor Quantum Dot and Single Gold Nanoparticle

Using atomic force microscopy nanomanipulation, we position a single Au nanoparticle near a CdSe/ZnS quantum dot to construct a hybrid nanostructure with variable geometry. The coupling between the two particles is varied in a systematic and reversible manner. The photoluminescence lifetime and blinking of the same quantum dot are measured before and after assembly of the structure. In some hybrid structures, the total lifetime is reduced from about 30 ns to well below 1 ns. This dramatic change in lifetime is accompanied by the disappearance of blinking as the nonradiative energy transfer from the CdSe/ZnS quantum dot to the Au nanoparticle becomes the dominant decay channel. Both total lifetime and photoluminescence intensity changes are well described by simple analytical calculations.

Controlled AFM manipulation of small nanoparticles and assembly of hybrid nanostructures.

We demonstrate controlled manipulation of semiconductor and metallic nanoparticles (NPs) with 5-15 nm diameters and assemble these NPs into hybrid structures. The manipulation is accomplished under ambient environment using a commercial atomic force microscope (AFM). There are particular difficulties associated with manipulating NPs this small. In addition to spatial drift, the shape of an asymmetric AFM tip has to be taken into account in order to understand the intended and actual manipulation results. Furthermore, small NPs often attach to the tip via electrostatic interaction and modify the effective tip shape. We suggest a method for detaching the NPs by performing a pseudo-manipulation step. Finally, we show by example the ability to assemble these small NPs into prototypical hybrid nanostructures with well-defined composition and geometry.

Second-harmonic microscopy of strain fields around through-silicon-vias

Through-Silicon-Vias (TSVs)—10 μm-diameter conducting rods that connect vertically stacked silicon layers—provide three dimensional circuit integration, but introduce strain in the surrounding silicon when thermally cycled. Here, we noninvasively probe strain fields around Cu TSVs in Si(001) using optical second-harmonic generation (SHG) microscopy. Results are compared with micro-Raman spectra of the strained regions. We find that SHG probes strain fields more quickly than Raman spectroscopy, while maintaining comparable sensitivity and spatial resolution, and avoiding the need for spectral analysis. Moreover, SHG is selectively sensitive to axial shear components uiz (i = x, y) of the strain tensor that are often neglected in Raman analysis. Thus, SHG complements Raman spectroscopy.

Manipulation of coupling between individual nanoparticles

We investigate the emission dynamics of a single semiconductor quantum dot near a single Au nanoparticle. Due to coupling between the nanoparticles, the quantum dot exhibits a strong polarization dependence and reduced blinking.