Invited Paper AS-WeM4
Enhanced Infrared Spectroscopy and Near-Field Microscopy with Infrared Antennas

Wednesday, October 31, 2012, 9:00 am, Room 20

Infrared (IR) spectroscopy allows for the investigation of chemical, structural or electronic properties of a sample material by directly probing molecular, crystal lattice or charge carrier oscillations. Combined with scattering-type near-field optical microscopy (s-SNOM), which relies on the scattering of light at a sharp metallic tip, it is possible to obtain such spectroscopic information in images with strongly subwavelength spatial resolution [1-4] of typically about 20-30 nm. Currently, the main limitation of this technique comprises of the low signals that demand tunable laser sources and restrict the spectral range of operation.

Recently, new broadband IR light sources enabled s-SNOM near-field spectroscopy on different polar crystals [5], semiconductor nanostructures [6] as well as biominerals [7]. The majority of these experiments has been performed on samples which provide a resonant optical interaction between the sample and the probing tip, thus resulting in comparably strong signals. For the detection of weak molecular vibrations like polymers and proteins however, the SNOM signals either have to be enhanced or stronger IR light sources have to be developed.

Here we present a way to enhance the near-field probing process by suitable substrates [8], increasing both signals and contrasts in infrared s-SNOM when probing thin sample layers. In a next step, we investigate the use of resonant nanostructures (“infrared antennas”, [9,10]) to enhance s-SNOM sensitivity even further. Additionally, we compare enhanced near-field spectra with the corresponding far-field spectra obtained by diffraction-limited FTIR-Microscopy. We will also present our lasted results obtained with a new powerful broadband IR laser source that is currently developed at the Fraunhofer ILT.

References

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[2] R. Hillenbrand, T. Taubner, F. Keilmann, Nature 418, 159 (2002).

[3] A. Huber et al., Nano Letters 7, 774 (2006).

[4] B. Knoll, F. Keilmann, Applied Physics Letters 77, 3980 (2000).

[5] S. Amarie, T. Ganz & F. Keilmann, Optics Express 17, 21794 (2009).

[6] F. Huth et al., Nature Materials 10, 352 (2011).

[7] S. Amarie et al., Beilstein J. Nanotechnol.3, 312 (2012).

[8] J. Aizpurua et al., Optics Express 16, 1529 (2008).

[9] F. Neubrech et al., Physical Review Letters 101, 157403 (2008).

[10] R. Adato et al., PNAS 106, 19227 (2009).