Identification of chemical compounds by vibrational spectroscopy at infrared wavelengths requires macroscopic samples: the spatial resolution is diffraction-limited to a scale of about half the wavelength, or about five micrometres. The scanning near-field optical microscope1,2, however, can reveal sub-wavelength detail because it uses near-field probing rather than beam focusing. Here we demonstrate the use of the aperture-less approach to scanning near-field optical microscopy3,4,5,6 to obtain contrast in vibrational absorption on a scale of about 100 nanometres, about one-hundredth of a wavelength. We record infrared scattering from the tip of an atomic force microscope scanned over a composite polymer film. At the boundary between different polymers we observe contrast changes owing to changes in vibrational absorption. The contrast is strongly enhanced in the near field of the probe tip, which we interpret as evidence of surface-enhanced infrared absorption7. When extended to multi-wavelength operation, this approach should enable imaging of chemical composition at nanometre resolution.

Near-field probing of vibrational absorption for chemical microscopy

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

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Scanning near-field optical microscopy with aperture probes: Fundamentals and applications

Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM-IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM-IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM-IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM-IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.

https://journals.sagepub.com/doi/pdf/10.1366/12-06804

AFM-IR: Combining Atomic Force Microscopy and Infrared Spectroscopy for Nanoscale Chemical Characterization

The early eighties have experienced a revolution in the perception of physical phenomena. This revolution is the birth of a new generation of imaging systems based on the detection of non-radiating fields. The near field optical microscope is the latest of this family. Like its prestigious brothers, the STM and the AFM, it allows one to see the physical world with new eyes. The objective of this article is to provide an overview concerning the physical mechanisms and paradoxes taking place in non-radiative detection. We will first explain the connection between Heisenberg uncertainty relations and the Rayleigh criterion. The fundamental role of evanescent fields will be pointed out through the plane wave expansion. Finally a catalogue of the different configurations currently in use will be given and illustrated with some experimental results.

https://iopscience.iop.org/article/10.1088/0034-4885/57/10/002/pdf

Near field microscopy and near field optics

The development of near-field optics theory is reviewed. We first recall that near-field optics is not limited to near-field microscopy. Broadly speaking, it concerns phenomena involving evanescent electromagnetic waves. The importance of such waves was ignored for a long time in optical and surface physics until the emergence of scanning near-field optical microscopes. Taking evanescent waves into account prevents the use of any simple approximation in the set of Maxwell's equations. The various theoretical approaches of near-field optics are discussed from the point of view of their ability to assess evanescent electromagnetic waves. We discuss the main results of the application of the various practical schemes which all rely on a numerical procedure. This review was received in February 1996.

https://iopscience.iop.org/article/10.1088/0034-4885/59/5/002/pdf

Near-field optics theories

Imaging and local infrared spectroscopy with a near field optical microscope

Locally resolved infrared spectroscopy

One of the most promising extension of the photon scanning tunneling microscope (PSTM) is its use as a locally resolved spectroscopic tool. The infrared (IR) spectral window offers a wide variety of interesting applications since many materials have a well-defined signature in this range. We have undertaken the experimental modifications that are necessary to operate the existing PSTM’s in the IR domain. In this work, we present preliminary data obtained with two IR light sources: an extended source (Globar type) and the free electron IR laser source (CLIO) available at LURE (Orsay). In both cases, the evanescent field has been detected with a fluoride glass optical fiber and the characteristic decay is reproducibly observed. On the basis of these results, we discuss the current limits of spatial and spectral resolutions. Mainly, we conclude that a spectral resolution of about 10 cm−1 over an area of 1μm × μm is possible to achieve.

C. Licoppe

Papers

Imaging and local infrared spectroscopy with a near field optical microscope

Locally resolved infrared spectroscopy

First Specifications of a PSTM Working In the Infrared