# Explaining Surface Plasmon Excitation by Light

• geo_alchemist
In summary, surface plasmons are generated when light with a normal component to the surface is incident on a material with high conductivity. This condition is necessary due to the conservation of momentum and the different boundary conditions for electric fields parallel and normal to the surface. The electric field parallel to the surface is small due to the high mobility of charges, while the electric field normal to the surface is hindered and can build up outside the metal.

#### geo_alchemist

Surface Plasmons ?

Well, to begin with, I am not a physicist and therefore please don't redirect me to some physics tutorials.
now the question:
Why is it necessary for surface plasmon exitation by light that the incident light must have component normal to the surface?
I know that this condition can be fulfilled only in case of p-polarized light. but my questioin is about necessity of normal component.
Pls, if anyone can explain this to me, I would be much obliged.

Isn't it just conservation of momentum?
If you want plasmons to be generated to the left of a surface then the light coming in from the right must reflect to the right.

Sorry, I can't follow the point.
may be for you it looks as A, B, C, but for me it's far more complex.
can you tell me in more details, what you mean by "If you want plasmons to be generated to the left of a surface then the light coming in from the right must reflect to the right"?

Inside the metal, all electric fields are very small, as conductivity is high. Now from the Maxwell equations there arise two different boundary conditions for the electric field parallel and normal to the surface. The electric field component parallel to the surface has to be continuous, while normal to the surface, D has to be continuous. Now even if E normal to the surface inside the metal is small, D=E*epsilon is much larger as the absolute value of epsilon is high (roughly equal to the conductivity of the metal).
Roughly speaking, the electric field parallel to the surface approximately vanishes due to the high mobility of the charges parallel to the surface. In direction normal to the surface, the mobility of the charges is hindered (by the surface) so that higher fields can build up, at least outside the metal.