A Reflection Near-Field Scanning Optical Microscope Technique for Subwavelength Resolution Imaging of Thin Organic Films

Method to map individual electromagnetic field components inside a photonic crystal

We present a method to map the absolute electromagnetic field strength inside photonic crystals. We apply the method to map the dominant electric field component Ez of a two-dimensional photonic crystal slab at microwave frequencies. The slab is placed between two mirrors to select Bloch standing waves and a subwavelength spherical scatterer is scanned inside the resulting resonator. The resonant Bloch frequencies shift depending on the electric field at the position of the scatterer. To map the electric field component Ez we measure the frequency shift in the reflection and transmission spectrum of the slab versus the scatterer position. Very good agreement is found between measurements and calculations without any adjustable parameters.

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.

Determination of local refractive index variations in thin films by heterodyne interferometric scanning near-field optical microscopy

We report on a heterodyne interferometric scanning near-field optical microscope developed for characterizing, at the nanometric scale, refractive index variations in thin films. An optical lateral resolution of 80 nm (lambda/19) and a precision smaller than 10(-4) on the refractive index difference have been achieved. This setup is suitable for a wide set of thin films, ranging from periodic to heterogeneous samples, and turns out to be a very promising tool for determining the optical homogeneity of thin films developed for nanophotonics applications.

Modulation imaging in reflection-mode near-field scanning optical microscopy

A simple implementation of modulation of the near-field optical signal of near-field probes based on the shear-force feedback system is demonstrated in a reflection-mode near-field optical microscope. The modulation exhibits a derivative type of dependence on the near-field signal and no sensitivity to topography. It is shown that the modulation image can be calculated directly from the derivative of the conventional near-field scattering image. This type of near-field modulation is an excellent way to reject far-field artifacts from the near-field signal.

Scanning near-field fluorescence resonance energy transfer microscopy

A new microscopic technique is demonstrated that combines attributes from both near-field scanning optical microscopy (NSOM) and fluorescence resonance energy transfer (FRET). The method relies on attaching the acceptor dye of a FRET pair to the end of a near-field fiber optic probe. Light exiting the NSOM probe, which is nonresonant with the acceptor dye, excites the donor dye introduced into a sample. As the tip approaches the sample containing the donor dye, energy transfer from the excited donor to the tip-bound acceptor produces a red-shifted fluorescence. By monitoring this red-shifted acceptor emission, a dramatic reduction in the sample volume probed by the uncoated NSOM tip is observed. This technique is demonstrated by imaging the fluorescence from a multilayer film created using the Langmuir-Blodgett (LB) technique. The film consists of L-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers containing the donor dye, fluorescein, separated by a spacer group of three arachidic acid layers. A DPPC monolayer containing the acceptor dye, rhodamine, was also transferred onto an NSOM tip using the LB technique. Using this modified probe, fluorescence images of the multilayer film reveal distinct differences between images collected monitoring either the donor or acceptor emission. The latter results from energy transfer from the sample to the NSOM probe. This method is shown to provide enhanced depth sensitivity in fluorescence measurements, which may be particularly informative in studies on thick specimens such as cells. The technique also provides a mechanism for obtaining high spatial resolution without the need for a metal coating around the NSOM probe and should work equally well with nonwaveguide probes such as atomic force microscopy tips. This may lead to dramatically improved spatial resolution in fluorescence imaging.