Multi-frequency near-field scanning optical microscopy.

We demonstrate a new multi-frequency approach for mapping near-field optically induced forces with subwavelength spatial resolution. The concept relies on oscillating a scanning probe at two different frequencies. Oscillations at one frequency are driven electrically to provide positional feedback regulation.

Optical-force-induced artifacts in scanning probe microscopy.

In the practice of near-field scanning probe microscopy, it is typically assumed that the distance regulation is independent of the optical signal. However, we demonstrate that these two signals are entangled due to the inherent action of optically induced force. This coupling leads to artifacts in both estimating the magnitude of optical fields and recording topographic maps.

Intrinsic detection of scattering phase with near-field scanning optical microscope.

We show that the interferometric interaction between the tip and the sample is inherently measured by a near-field scanning optical microscope (NSOM) operating in reflection mode. This is demonstrated by measuring the phase of the sample reflectivity on a standard multilayer system with variable thickness. The demonstrated intrinsic sensitivity to the phase has implications in the interpretation of images collected by using reflection mode NSOM.

Structural control of nonlinear optical absorption and refraction in dense metal nanoparticle arrays.

The linear and nonlinear optical properties of a composite containing interacting spherical silver nanoparticles embedded in a dielectric host are studied as a function of interparticle separation using three dimensional frequency domain simulations. It is shown that for a fixed amount of metal, the effective third-order nonlinear susceptibility of the composite chi((3))(omega) can be significantly enhanced with respect to the linear optical properties, due to a combination of resonant surface plasmon excitation and local field redistribution. It is shown that this geometry-dependent susceptibility enhancement can lead to an improved figure of merit for nonlinear absorption.

Numerical study of surface plasmon enhanced nonlinear absorption and refraction.

Maxwell Garnett effective medium theory is used to study the influence of silver nanoparticle induced field enhancement on the nonlinear response of a Kerr-type nonlinear host. We show that the composite nonlinear absorption coefficient, beta(c), can be enhanced relative to the host nonlinear absorption coefficient near the surface plasmon resonance of silver nanoparticles. This enhancement is not due to a resonant enhancement of the host nonlinear absorption, but rather due to a phase shifted enhancement of the host nonlinear refractive response.