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

Abstract

An interferometric method is used to enhance the weak scattered signal in apertureless near-field scanning infrared microscopy. The method involves introducing a homodyning reference field, and amplifies the desired signal field by the magnitude of the reference field. This method markedly improves the signal-to-noise ratio of the detected signal, over the nonhomodyned experiment. A model for the dependence of the near-field signal, as a function of the normal distance of the tip from the surface, is discussed. Application of a model in which the tip is represented by two spherical scatterers, one large and one small, indicates the electromagnetic field enhancement is 90-fold greater at the sharp apex of the metallic probe tip.

Infrared near-field microscopy with the Vanderbilt free electron laser: overview and perspectives

“…High-quality LiF films (thickness % 800 nm) were grown by thermal evaporation on untreated silicon substrate 13 at 250°C. The films total thickness, measured by a stylus profilometer after the growth, was about 800 nm.…”

“…The main motivation for the development of scanning near-field optical microscopy (SNOM) is beating the lateral resolution diffraction limit of conventional far-field microscopy [1][2][3][4][5][6][7][8][9][10][11][12][13]. This is of course particularly important for long wavelengths in the infrared (IR) range.…”

“…We note that a different ''apertureless'' SNOM approach was also developed (see Refs. [10][11][12][13]19]) that images different samples with nanometric resolution. Apertureless SNOM is very interesting for its potential applications to IR imaging in biology.…”

Nanoscale Conductivity Contrast by Scattering-Type Near-Field Optical Microscopy in the Visible, Infrared and THz Domains

“…5 Mid-infrared near-field microscopy of highly-doped semiconductors Much of the advancement of s-SNOM has been attained using the CO 2 and CO lasers, in the 9-11µm and 5-6µm wavelength regions, respectively [5,6,8,10,11,20,21,24,[27][28][29][30][31][32][33][34][35][36][37]. This came first of all because mid-infrared can be used to exploit chemical fingerprints for recognizing chemical composition, now at nanometric resolution [8,31,33,37,38], but furthermore for technical reasons.…”

“…Metallic materials were investigated with the first heterodyne interferometric s-SNOM [9] that resulted, for the scattering contrast of Au relative to Si, in an amplitude ratio of 2.5±0.1, and a phase difference of 60°±10°. Nanostructured metal samples have since served as a standard in s-SNOM testing [19][20][21][22][23][24].…”