Near-Field Scanning Optical Microscope

The basic purpose of the near-field scanning optical microscope (NSOM) is to allow optical microscopy of electromagnetic fields in nanostructured environments with a spatial resolution approximately one order of magnitude better than other microscopies that are constrained by the diffraction limit of light. By initiating and observing photoprocesses with a spatial resolution of ~20 nm, NSOM enables nanoscale spectroscopies and high-resolution imaging.

The different types of NSOM configurations we will provide are shown. Configurations a-d are aperture-based NSOM configurations, providing (a) collection mode, (b) illumination mode, (c) illumination/collection mode, and (d) collection mode using total internal reflection. Configuration (e) is for scattering NSOM using a dielectric tip, where illumination can be below or above the sample depending on sample thickness or transparency. Configuration (f) is for metal probes, where field enhancement can occur at the apex of the probe.

The basis of the NSOM apparatus is an atomic force microscope (AFM) placed onto an inverted optical microscope. An open geometry is used to accommodate a large range of configurations that enable versatility in the range of samples and  microscopies that can be performed. The available configurations are shown above, which allow for both aperture (a-d) and apertureless (e-f) geometries. The open configuration enables the mounting of a second microscope objective above the sample, so as to perform additional illumination geometries for highly scattering or thick samples, as well as provide a means to collect backscattered near-field signal. The inverted microscope allows for confocal imaging of a sample, so that areas of interest can be located for performing NSOM. The confocal imaging can also be used for single- molecule/single-particle imaging and to collect spectral information, providing critical supporting information for NSOM research.

The inverted microscope and AFM that form the basis of the NSOM equipment is shown.

The NSOM apparatus is coupled to either a continuous wave (CW) laser (Kr+ laser) or the output of a optical parametric oscillator (OPO). The OPO can operate with either 100-fs or 3-ps pulses to accommodate the narrower bandwidth requirements of near-field Raman microscopies. An optical delay line is also present for time-resolved pump-probe experiments. The OPO is broadly tunable, with output ranging from 250 to ~2300 nm to enable the initiation and probing of many light-induced effects at the nanoscale. Available detectors include a charge-coupled device spectrograph, photomultiplier tube, and avalanche photodiode.