Has Lucas ever mentioned how he came up with the idea of the Lightsaber? I don't think it's a stolen concept as it's unique to Star Wars. Energy swords do exist in other sci-fi canons (like Halo), but those canons are newer than Star Wars.
There are prior examples of swords made of pure energy that can cut through anything.
So Star Wars wasn't the first example of such a weapon.
George Lucas has often mentioned that he draws his concepts from multiple sources. All authors and story-tellers do that to some extent, but only a few admit it or go into detail about what first triggered their imagination.
The rest of this post is just my opinion, but I think we can look though the works that influenced Star Wars and see where it leads.
#1 Flash Gordon Conquers the Universe
First on the list because Lucas made a pitch for the rights to Flash Gordon, he was quickly rejected, prompting him to create Star Wars. Flash Gordon Conquers the Universe in particular is notable for the intro-text scrolling into the sky, unique scene transitions, and "You've failed me for the last time". Due to it being made in 1940, many characteristics of the enemy were changed to make "Planet Mongo" a cross between Germany and Japan (keep that in mind), departing from the original theme.
Swords fights are an important part of Flash Gordon, even though "Planet Mongo" is a futuristic society with high-tech ray-guns, old fashion swords are at the forefront. When sneaking into an enemy base it's safest to use your sword not your gun: when Roca pulls out his high-tech ray-pistol Flash tells him not to use it "or you'll have them all on us in no time".
It's a common occurrence to see characters step off a space ship with medieval sword. (Almost always a thin sword like a saber or rapier)
#2 Lord of the Rings - (the Books, obviously not the movies made later)
It's debatable how much Lord of the Rings influenced Star Wars, Lucas mainly just used it as example of how-a-trilogy-should-be. That's probably why there's such a downer ending in Empire Strikes Back, with the characters beaten up but victorious, but with Solo / Frodo taken away not-quite-dead, a cliff-hanger that leads you to the next book/movie.
Swords are all over Lord of the Rings. One thing to note is that the Elves made swords that would glow blue when an Orc was nearby. When a character kills the last Orc in a fight, often they'll see the sword-blade fade out and know they're safe and that the battle's over.
In places like the Mines of Morea, with little light, a blue-lit sword would be very much like a lightsaber.
Jidaigeki means "period dramas" in Japanese.
Lucas said that he was greatly inspired by the works of Akira Kurosawa. Lucas visited Japan in the 1960s which is where he said he first learned the term "Jidaigeki". It's often guessed that this is where his term "Jedi" comes from. It's important to note that in the Japanese language nouns do not have a plural form, (similar to the English word "Moose"), and the plural of "Jedi" is "Jedi".
Swords were the Samurai's main weapon.
The film Seven Samurai in particular is often noted. ...Unfortunately I've never watched it so I don't have much to go on here =(
#4 Matter of Britain
The most well known order of knighthood in literature is the Knights of the Round Table. They were guardians of peace and order in Arthur's kingdom.
Swords play a huge role in Arthurian literature. Arthur's right to rule was established by pulling the Sword from the Stone. Later Arthur gets a new sword from the Lady of the Lake. In most later versions the legend ends with the battle of Camlann, where (almost) all the Knights die in battle. As the sun sets over the battlefield, it comes down to the final duel between Arthur and Mordred. Mordred wearing full black armor and a bat-winged helmet, fights Arthur wielding Arthur's old sword "Clarent", the sword-in-the-stone. In the later versions Mordred informs Arthur that he's not only Arthur's nephew but also his son (via Arthur's sister...) and is fighting to either gain Arthur's recognition as son or else claim what's rightfully his by force.
Those themes carry into the battle between Luke and Vader at the end of Empire Strikes Back.
[I thought there was a version where Mordred asks for forgiveness after Arthur pulls the spear out of Mordred's gut, but I can't find it]
All of these influences have swords as an important element. None of them would be the same without swords.
IMHO, George Lucas simply looked for a way to make swords futuristic to fit the theme.