Logicaly speaking, they'd have to be composed of nuclear energy. If they were infact light captured into a body, they'd have a bulb at the top, which they don't. So they must be some denser form such as nuclear energy. How they managed to capture so much velocity of power such as that into a component such as the saber itself is anybody's guess, and it's an even larger question of how they manage to keep a magnetic force in the saber that restricts the nuclear energy infused to it down to a cearain size that doesn't cause it to just burst up untill it stretches beyond it's reach and works almost like a laser instead..
We've known the anatomy of a lightsaber since the Star Wars Visual Dictionary was released to document episodes IV through VI.
Also, for more information, and a shot of Maul's lightsaber opened up...
Always a bit of a hot potato this one; lots of different interpretations over time.
Spelljammer, 'logically' speaking it is at least 15 times more complex than you say; expert sabre geek Bob Brown had eight different possible technological explanations for the sabre, each one more sad than the last. Yay!