Seems you're correct about 'yıldırım', but according to both Wiktionary and DDG, 'şimşek' also means lightning, specifically in meteorology it's lightning between clouds. And apparently 'Şimşek' can be a name too, both first and last one.

Yeah, but what is that letter? I've seen them like ì ï ī î í, but not the undotted ones.