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New Insights in Nano-Optics Using a Synchrotron Light Source

Date: 
Wednesday, March 30, 2016
Contact: 

Luciana Noronha - Communication | Tel. +55 (19) 3512-1107 | luciana.noronha@cnpem.br

An international collaboration between scientists from the Brazilian Synchrotron Light Laboratory (LNLS) and the University of Colorado, Boulder (CU-Boulder) brings new understanding on fundamentals of infrared interferometry for scattering-type Scanning Near-Field Optical Microscopy (s-SNOM). The team, composed of the LNLS researchers Raul Freitas and Francisco Maia, and the CU-Boulder researchers Markus Raschke and Benjamin Pollard, applied a new detection scheme for s-SNOM which provides enhanced performance compared to conventional detection geometries for such experiments. The study, published in the Nano Letters journal and titled “Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry,” describes the new detection geometry and its application to the study of a variety of materials.
 

Self-referenced s-SNOM setup with a symmetric Michelson interferometer for the synchrotron broadband IR source or using a tunable quantum cascade laser as source.

Fourier transform infrared spectroscopy (FTIR) is an established technique for analyzing molecular vibrational properties of materials. In the last few decades, the use of ultra-bright IR light sources, such as synchrotrons and lasers, has improved dramatically the signal-to-noise ratio (SNR) of FTIR and consequently has increased sensitivity. Moreover, and especially inside synchrotron facilities, the combination of optimal focusing optics and IR area detectors has allowed important breakthroughs in micrometer-scale chemical analysis (µ-FTIR) in several areas of chemistry, biology, medicine, forensics, geology and physics.
 
Despite the opportunities triggered by µ-FTIR, many questions are still unanswered in the realm of sub-micron scale chemistry. For instance, analyzing internal chemical reactions related to drug delivery within a single cell is still quite an active area of research. In fact, µ-FTIR is nowadays at its conceptual limit of spatial resolution since it is a diffraction limited technique. This limit, often referred as the Abbe diffraction limit of light, defines the minimum size of a focal point when light is focused by a given optical system. Quantitatively speaking, the smallest focal point produced by an optical system is approximately half of the wavelength of the incident light. Therefore, in the mid-IR range, µ-FTIR has a spatial resolution limited to 1 to 5 µm in the best conditions.
 
s-SNOM is a scanning probe technique which provides optical properties of materials with spatial resolution beyond the Abbe diffraction limit. In s-SNOM, light is focused onto a nano-antenna (a metallic scanning probe tip) that concentrates a significant portion of the incident radiation at its apex. The greatly enhanced radiation field at the tip apex is confined in a volume which is no longer wavelength-dependent, but instead defined by the tip size (typically 25 nm radius). Therefore, the technique allows measurement of a tip-sample interaction that probes simultaneously topography and IR scattering from the sample surface.
 
The figure above depicts the s-SNOM setup used in the published work using two different sources, a tunable narrow band quantum cascade laser (QCL) and the synchrotron broadband IR beam. As an innovation brought by the research, the synchrotron setup used a classic symmetric Michelson interferometer for Fourier processing the data as an alternative to the standard asymmetric scheme typically used in s-SNOM. Better SNR related to increased optical power at the sample and higher mechanical stability highlight the instrumental advances of the research. The work explored the interferometric properties of the far-field background arising from tip-sample geometrical scattering and developed a model for retrieving the near-field phase, and consequently the absorption properties, of a variety of samples such as poly(methyl methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS), explosive polymers 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and pentaerythritol tetranitrate (PETN), and Bovine Serum Albumin (BSA).
 
In summary, the work presents an alternative detection scheme for s-SNOM with significant improvements in mechanical stability and optical power at the sample. These improvements are related to better SNR and consequently to increased spectral sensitivity for s-SNOM. Therefore, these innovations open new opportunities for applying IR nanospectroscopy in the analysis of systems with weak scattering signals like the polymers and biological films studied in the paper. Moreover, the research describes an algorithm for model-extracting the absorption spectrum of samples from the self-referenced scattering intensity, yielding spectra comparable to far-field absorption measurements from established techniques such as FTIR and ellipsometry.

 

Benjamin Pollard, Francisco C. B. Maia, Markus B. Raschke and Raul O. Freitas. Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry, Nano Letters 16, 55-61 (2016).

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