Nanoscopy with Broadband Infrared Radiation

Traditional light microscopy is limited by its wavelength-related resolving power. Fortunately, near-field methods (SNOM) can fully overcome this limit and routinely achieve 20 nm spatial resolution, even with long-wavelength infrared light.

1. Our aim is to exploit the rich "fingerprint" contrasts offered by vibrational resonances, by operating a near-field microscope with broadband infrared radiation spanning the octave from 6 to 12 µm wavelength. This new nanoscopy technique is to be applied to medical nano-imaging of bone, and expected to solve questions of biomineralization.

For this aim we have equipped the recently introduced NeaSNOM (neaspec.com) with a mid-infrared continuum source based on difference-frequency mixing within a 10-fs TiS laser spectrum. This resulted in a convincing proof-of-concept, demonstrating the powerful information lying in spectrally resonant features to optimally couple the laser beam to the AFM tip and to distinguish and suppress artifacts.

2. Yet scanning a scene with a spectrum at each pixel has not been feasible owing to the low power of 5 µW that necessitated ca. 1 min. acquisition time per spectrum. A subsequent replacement with a dual output Er-fiber laser (toptica.com) has recently generated as much as 25 µW mid-infrared continuum power. This allows a real-time system optimization using a live near-field spectrum, and the recording of artifact-free spectra of a great number of materials, especially also of biominerals such as calcium carbonate and calcium phosphate.

3. Biocomposite materials such as bone, teeth, and shells have highly controlled heterogeneous nano-architectures, with crystals of 10 nm to 1 µm size, assembled in complex protein matrices. Their biological formation and decay is of great fundamental and applied interest, e.g. in osteoporosis. SEM and AFM techniques have given much insight, but our infrared s-SNOM promises additional unique information, namely the chemical recognition of the proteins, plus both the chemical and structural recognition of the mineral components.

Key Publications:

1. F. Keilmann and R. Hillenbrand, "Near-field nanoscopy by elastic light scattering from a tip," in Nano-Optics and Near-Field Optical Microscopy, ed. by A. Zayats and D. Richard, ISBN 978-1596932838 (ArtechHouse, 2009).

2. S. Amarie, T.Ganz, and F. Keilmann, "Mid-infrared near-field spectroscopy", Opt. Express 17, 21794 (2009).

3. S. Amarie and F. Keilmann, in preparation.