Hey there, /u/cicada_knows! This question has come up a lot recently, and /u/restricteddata has a pretty good answer that you can read here.

In short, the idea of an atomic bomb had been around since the 1910s and was a science fiction trope of moderate popularity. Even Winston Churchill referenced the idea during a speech in the mid-1920s.

You should probably turn off smoothing, as it's distorting your picture a lot. Not for the question you're asking (it was mentioned from time to time before Hiroshima), but the graph you're using smooths out the changes to erroneously show a gradual increase of interest starting in 1940 when there was actually an enormous spike between 1944 and 1945 and then even more in 1946.