Apertureless Coherent Interferometry

Apertureless interferometric

Since the introduction of SPM, many different techniques have been invented. Among them is the Near-Field Scanning Optical Microscopy (NSOM) which was first demonstrated by Prof. A. Lewis (CEO and Founder of Nanonics) to image beyond the diffraction resolution limit.

It the classical approach, the light is confined within a cone and illuminated onto the sample through an aperture smaller than the wavelength of light hence the illumination is in the near field.

In the apertureless (aNSOM) approach, a laser spot illuminates the sample. A small metallic tip in the near vicinity of the sample enhances the localized electrical field. The tip scatters the light according to its polarizability. The presence of the sample under the metallic tip modifies the polarizability of the tip according to the dielectric function of the sample. A signal is then scattered and collected in the far-field.

In order to distinguish between the background scattering and the near-field scattering, the tip is kept oscillating above the sample in the tapping mode operation, and the near-field signal is extracted at the resonance frequency harmonics using an interferometer and a lock-in amplifier. This is due to the non-linear characteristics of the near-field in comparison to the elastic scattering of the non-near-field background signal.

By scanning the metallic probe over the sample with an AFM feedback in intermittent contact mode, one can use the scattered far-field signal, obtained through lock-in detection, to form a correlated image of topography and near-field characteristics.

Apertureless NSOM is particularly advantageous method for infrared light source & THz optical frequencies, high resolution surface imaging.

Apertureless NSOM has been successfully applied to imaging of various features in condensed phase materials, including the metal-to-insulator transition of correlated electronic materials, metallic plasmonic structures, surface phonon polaritons, graphene plasmons, peptide and proteins and many other applications.

Our Apertureless technique can be combined inside a Cryogenic or High Vacuum chamber along with simultaneous Raman & Femto-second Imaging.

 

Want to learn more about integrating IR and Raman,and scattering and apertureless NSOM?

 

Key Features

  • The only technique allowing localized illumination without out-of-focus light which is impossible in Confocal, PALM\STORM or STED.

  • Ideal for substrates in liquid mediums where far field illumination is difficult.

  • Ultrahigh Q Factor Tuning Fork Probe For Ultra Sharp Resonance Frequency For Excellence in Third Harmonic Deconvolution Of Far-field Interference

  • Highly Exposed Probe Tip With Unprecedented Distance Tens of Microns From The Cantilever For Reduced Far-field Interference

  • Parabolic Mirror for Focusing and Collecting Light

  • Pseudo Heterodyne Interferometry

  • Tuning fork feedback allows for no optical AFM feedback and no optical interference with the measurement

  • Tuning fork feedback with no jump to contact which is critical for real oscillation of the tip in the very near vicinity of the sample where the highest enhancement of the optical field occurs

  • Topographic and near-field optical data are acquired simultaneously

  • Rapid position of probe on the sample with nano-meter accuracy

  • Probe scanning allowing NSOM mapping of the SPP independent to the illumination and without moving the sample.

  • Can be combined in Cryogenic and High Vacuum environment

  • Free optical access of the NSOM head from both the top and the bottom, allowing for other optical integrations such as in situ Raman and Fsec lasers

  • Over 15 years of experience in the NSOM field.

 

Exemplary Paper

Combined Apertureless Near-Field Optical Second Harmonic Generation/Atomic Force Microscopy Imaging and Nanoscale Limit of Detection

Meyer, K. A., Ng, K. I. N. C., Gu, Z., Pan, Z., Whitten, W. B., & Shaw, R. W. (2010). accelerated paper Combined Apertureless Near-Field Optical Second-Harmonic Generation / Atomic Force Microscopy Imaging and Nanoscale Limit of Detection. Applied Spectroscopy, 64(1), 1–7.

aperturless2

Nano-wires have become of rapid interest with potential for a variety of applications such as nano-lasers and solar energy conversion materials. In this article, ZnO nanowires were investigated when exposed to CO2 gas and water vapor. The SHG spectra was imaged in high resolution in combination with the Nanonics MV4000 apertureless NSOM configuration.

SHG is an indication of the symmetry properties of crystals which allow understanding of their polarizability. Different SHG intensities were imaged when exposed to different gaseous environments. Spatial resolution was observed in agreement with the tip characteristics and superior to the optical diffraction limit.

 

Link to Abstract

About the Authors

Robert W. Shaw is the group leader of the Laser Spectroscopy and Chemical Micro-technology group in ORNL. He has published many articles in the field of spectroscopy and physiology using high resolution techniques.

Exciting Applications

  • Study ion transport into membranes by using a Calcium sensitive dye and locally illuminating on a desired location on the membrane with no background light.

  • Study absorption properties of different materials or live organisms.

  • Study IR absorption in micron resolution by introducing IR light via an apertured NSOM probe.

 

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