Dalton Abdala's webpage

nano-FTIR beamline

 

Infrared spectroscopy (FTIR) is one of the most established techniques for chemical 

analysis. The IR spectrum covers the energy levels associated to vibrational and rotational modes of molecules, which are particular spectral signatures of the functional groups of molecules. These  natural  vibrational  modes  of  molecules  allow  applying FTIR  on  the analysis  of  composition  of  almost  all  materials.  However,  many fundamental  properties and  functions  of  materials  are  described  by  means  of structural-chemical  phases  in domains or interfaces in the nanometer to few microns scale. In this scenario, techniques able to perform FTIR in sub-micron spatial resolution are highly demanded for developing cutting  edge  research  in  new  synthetic materials  or  for  the  understanding  of  basic properties of natural materials. 

 

The  infrared  beamline  of  the  LNLS  (IR1)  is  dedicated  to  FTIR  experiments  with 

lateral resolution down to few tens of nanometers (nano-FTIR) in the mid-IR range. The 

highly  intense  broadband  IR,  up  to  1000  times  more  intense  than  conventional thermal sources, is generated from both bending magnetic and edge radiation, extracted from two consecutives 1.67 T dipoles in the 1.37 GeV storage ring. The extracted IR is then focused and collimated after a set of 5 mirrors, as illustrated in the optics layout of Figure 1. 

 

 

In  order  to  achieve  nanoscale  spatial  resolution,  the  IR1  beamline  handles  the  IR 

light  in  the  near-field  regime.  It  is  well  understood  that  light  focusing  is  ruled by  the diffraction limit and that means focal points cannot be smaller than ~λ/2. Therefore in the mid-IR range, microscopy techniques work with a typical pixel size of 4 µm. In the case of the IR1, light is focused towards a metallic tip of an atomic force microscope (AFM). The antenna effect of the elongated metallic AFM tip causes an enhancement of the incident field at the end of the tip, creating an intense evanescent field at the apex of the tip. The result is a new source with a size comparable to the tip apex diameter. The technique is called appertureless-Scanning Near-Field Microscopy (a-SNOM) (Keilmann & Hillenbrand, 2004) and a scheme of the tip-sample with the incident and scattered fields is showed in Figure 2. 

 

The  incident  and  the  evanescent  waves  at  the  tip  apex  interact  with  the  sample 

surface during AFM tapping-mode topography scans generating two images: one from the mechanical topography and other from the optical interaction of the near-field, as shown in Figure 3. The back-scattered optical signal is lock-in detected in the frequency of the tip in order to separate the near-field from the far-field signal.