fourier-transform spectroscopy principle


A polychromatic light source is split in two by a beamsplitter. The beams are recombined with an optical path difference d that varies thanks to a mobil mirror. For each position d the detector records the signal coming from the sample.  For all the explored d-values one gets the interferogramme : the intensity as a function of d.  By Fourier-transform  one obtains the intensity as a function of the incident energy of the photons w, that is a spectra. Owing the range of energy, the light source, the beamsplitter and the detector have to be adapted.  




The resulting spectra contains informations on the dynamic of the sample. Crudely, in transmission, if the energy of an incident photon matches the energy of a phonon (lattice vibration) then the photon is absorbed by the sample exciting the phonon. Thus, the spectra will present a deficit of intensity at the energy that corresponds to the phonon. The deficit can be quantified by comparison with a spectra obtained through a hole which diameter is identical to the sample one.


To realize optical spectroscopy measurements over a wide energy range from sub-THz to visible, two Fourier-transform spectrometer will be used :

   - the «VERTEX» : commercial spectrometer dedicated to far infrared to visible studies.

   - the «MARTIN-PUPLETT» : spectrometer developed at the Néel Institute dedicated to THz studies.




 

OPTICAL SPECTROSCOPY

  1.     STAKES

  2.     PRINCIPLE

  3.     THE VERTEX

  4.     THE MARTIN-PUPLETT