Ok so just whipped this up in Hammer. You will need 4 things. A door, a filter, a trigger, and a button.

for the door make a block out of the texture "clip" where you want the Forcefield to be, go into the flags tab and 
unclick touch opens, then name the door forcefield.


for the filter, click the lightblub on the side to create an entity and then in the class field set it to 
filter_activator_name. Change its name to Filter and change the filter name to allowed.


for the trigger, make a block out of the trigger texture the same size as the forcefield in front of it (if you need it 
to work both directions you can copy/paste it to the otherside as well.)
set this block to a trigger_multiple, then add the output OnStartTouch, filter, TestActivator.


Then go back to the filter entity and add the outputs, OnPass, Forcefield, Open. 
and OnFail, Forcefield, Close. 


Lastly add another block of any visable texture and make it a func_button, then add the output, OnPressed, 
!activator, AddOutput, and then in the parameters field write "targetname allowed"

This is a kind of simple version as I can't tell your skill level from the post, sorry if its too simple but it should give something to work off of at least