GaussianBeam for TFST technique

I found that if the FWHM width of the Gaussian beam was less than the wavelength of the light wave, the TFSF caused great reflection.

can anyone explain that for me?

Please talk precise!
what do TFSF and FWHM stand for?

TFSF: total field/scattered field
FWHM: full wide half maxim indicates the wide of Gaussian beam.

Need to specify more exactly what "reflection" is of interest.

Possible explanations:
1. there are different definitions for the Gaussian Beam. Commercial softwares seldom give the details of their implementation. Therefore the "reflection" might not be the same as you expect.

2. As the beam width goes smaller, more "reactive" energy is carried by the beam, causing more ambiguity in the "reflected" and "transmitted" power.

whatever the definition of Gaussian Beam is, What I only concern is only at the interface of scattered field and total field.

in fact, I want to whether there is a limit to use Gaussian beam in TFSF interface?

i need 2D STFT plane wave for Ex,Hy,Ez can somebody help me i really need it pls send my mail afsharibawili@gmail.com