Interaction of a phonon with evanescent wave

[physicsninja]

When exciting a phonon mode with an evanescent wave possessing a wavevector ~100-200X larger than freespace but still ~10X smaller than fermi wavevector, would one expect a different resonance frequency for the phonon mode? I would think not.. but experiment seems to say yes.

More specifically: I am dealing with the main infrared active mode of SiO2 at 1080cm-1. I bring an evanescent probe near the surface which is capable of generating light of any frequency 900cm-1 to 1250cm-1 (enough to make a spectrum) with wavevector equal to roughly 1/probe radius ~ 1/10nm. This ends up being something like 100-200X larger than the free space wavevector. Experimentally I am seeing a large shift and I am not sure why.

Any ideas, thoughts or suggestions are welcome. Thanks!

 

[BigJDubs]

Yes, it is possible to expect a different resonance frequency for the phonon mode when exciting it with an evanescent wave possessing a wavevector much larger than freespace. This is because the evanescent wave has different boundary conditions which can affect the resonance frequency of the phonon mode. The larger wavevector can also cause a shift in the resonance frequency due to the increased wave-matter interactions. In addition, the evanescent wave may interact differently with the material near the surface, leading to a different resonance frequency.