The photonic cannon was an imaginary type of directed energy cannon, a weapon of mass destruction, created by the USS Voyager's ECH, after he added daydreaming subroutines to his program. This weapon was supposedly impervious to all conventional sensors, and had enough power to destroy a Borg sphere with a single volley of two lightning-like energy beams.
Species 8472 bioships, seen in "Scorpion" and "Scorpion, Part II", were armed with a similar real weapon with the same level of destructive power.
http://en.memory-alpha.org/wiki/Photonic_cannon
I am surprised VOY didn't try to develop a weapon similar to what species 8472 had. They could have asked when they became friendly at the end of "In The Flesh".