I know that I have seen lenses for regular flashlights to turn them into IR lights(to enhance night vision without being seen)
Like here
I have seen these work and they emit a dark red glow.
Also used for photography.
Like here
The Axim may be just that. Just a regular light through an infrared filter.
I did find
this site that tells you briefly what you have to do to make your IR stronger. But it involves changing the lense, not the diode.
Look around, someone has surely tried it. Maybe not with an axim, but with something.