Near-Field Scanning Optical Microscopy: Single Channel Imaging of Selected Gold Nanoparticles through Two Photon Induced Photoluminescence

Near-field scanning optical microscopy (NSOM) is known to be a technique of choice to investigate nanometric materials and their properties beyond far-field diffraction limit resulting high spatial, spectral and temporal resolution. Here in this report, a state of art facility, aperture-NSOM was used to probe single nanoparticle, dimer, trimer and small nanoaggregate of gold nanoparticles. Shear force topography and two photon induced photoluminescence (TPI-PL) images captured simultaneously by the system facilitated to clarify the correlation between the local geometry and the emitted photon of TPI-PL. Small gold aggregates including trimer showed strong optical confinement of TPI-PL with reference to that of single gold nanoparticles. It was also evident that the interparticle gap does have a great influence in localized electromagnetic (EM) field mediated optical confinement of TPI-PL. Such observations were also supported by finite different time domain (FDTD) analysis keeping the simulation parameter nearly identical to that of experiment. FDTD simulation demonstrated that incident excitation parallel to the interparticle axis induces strong near-field distribution at the interstitials whereas out of plane excitation modifies such confinement depending on the nanometric geometry of the nanoaggregates. Such an observation is indispensable to understand the localized EM field-mediated optical confinement in surface-enhanced spectroscopy.

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Simulation of Electromagnetic Field Distribution at a Nanowire Probe for Near Field Scanning Optical Microscopy

ASME 2008 First International Conference on Micro/Nanoscale Heat Transfer, Parts A and B ◽

10.1115/mnht2008-52078 ◽

2008 ◽

Author(s):

Nathan P. Malcolm ◽

Alex J. Heltzel ◽

Li Shi ◽

John R. Howell

Keyword(s):

Optical Microscopy ◽

Signal To Noise Ratio ◽

Near Field ◽

Three Dimensional ◽

Field Enhancement ◽

Fdtd Simulation ◽

Plasmonic Nanoparticle ◽

Excitation Laser ◽

Scanning Optical Microscopy ◽

Near Field Scanning

This work studies a new design of a near field scanning optical microscopy (NSOM) probe based on a ZnO nanowire sub-wavelength waveguide terminated with a plasmonic gold nanoparticle. Three-dimensional finite difference time domain (FDTD) simulation is used to visualize light guiding in the nanowire and near field coupling between the plasmonic nanoparticle and the substrate. The simulation results reveal local field enhancement at the gap between the nanoparticle and a gold substrate when the nanowire axis is tilted from the substrate normal by a small angle. The enhancement occurs only along the cross section plane that is parallel to the polarization of the excitation laser beam. The regime of field enhancement is much smaller than the diameter of the 100 nm plasmonic particle, making the nanowire probe well suited for NSOM with superior spatial resolution and signal to noise ratio compared to the state of the art.

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