Optical contrast in apertureless microscopy

Abstract

We report on optical image contrast for a specific apertureless near-field optical microscope. We demonstrate that the main part of the optical image's contrast results from the sample's topography. The coupling mechanism is analyzed, and we show that the microscope can be regarded as an interferometer that sensitively detects near-field components. However, in the basic configuration the reference field of the interferometer is coupled to the topography. Finally, it is demonstrated that, by controlling the phase of the reference field, one can largely decorrelate the optical image from the topography.

Nanometer-Scale Optical Imaging of Epitaxially Grown GaN and InN Islands Using Apertureless Near-Field Microscopy

“…Because of the strongly anharmonic response of E NF with respect to the harmonic oscillation of the probe, compared to the nearly harmonic response of E T , this background term can be selectively removed by recording the signal at the higher harmonics (2Ω, 3Ω, ...) of the probe oscillation: 16,17 The relative phase between E NF and E S , ∆, is ill-defined, and it has been shown to cause serious topographic artifacts in ANSOM images as well. 6,18 By coherently mixing E SC in eq 1 with a stronger reference beam (E ref ) that has a well-defined phase (homodyning), we can override the interference between E NF and E S with the stronger interference between E NF and E ref to obtain a nonambiguous, artifact-free signal:…”

Enhancement of the weak scattered signal in apertureless near-field scanning infrared microscopy

“…10 This work has taken place along with notable developments in apertured near-field methods and other new optical and infrared microscopies. [11][12][13][14][15][16][17][18][19][20] In apertureless near-field IR microscopy, the development of several imaging techniques, including constant height imaging, detection of the scattered signal at higher harmonics of the tip motion, heterodyne detection, and homodyne detection, [21][22][23][24] in addition to the availability of new IR, tunable sources, have set the stage for significant advances. One of the primary challenges in apertureless nearfield microscopy is the detection of the weak scattered field.…”

An atomic force microscope tip as a light source

“…This exponential decay is useful because it allows illumination of a small area under the AFM tip, and therefore it is useful for SNOM. In comparison to conventional apertureless SNOM ͑using elastic scattering͒, the inelastic-scattering method is easier to interpret because one does not need to worry about signals arising from the sample 6 ͑provided that the sample is not photoluminescent͒. By simple addition of an illuminating source, a filter, and a light detector, one can convert a commercial AFM into an SNOM using mass-produced silicon nitride cantilevers.…”

“…The contrast mechanism in apertureless SNOM is still a subject of debate, but it is thought that contrast arises from interference between scattering from the tip and the sample. 6 By measuring the signal from the tip only, we aim to eliminate this interference. We show that highresolution optical images can be captured by analyzing the inelastically scattered light when a sample is scanned under an illuminated tip.…”