Do I need to put a "-" between "non" and an adjective? As an example in physics we say "a non isolated photon", "non tight photon"... The context is very formal (paper publications and similar). Is there a general rule? Are there some differences between countries?
Yes, a two-word modifier (like this one) requires a hyphen, except that the commonly held convention is that adverbs ending in "ly" don't (like that one). See this table in the Chicago Manual of Style.
As an addendum to Monica's fine answer, I'd like to add that there is a third possibility: fusing "non" with the word it precedes. A typical example would be "nonrelativistic", which seems to be Merriam-Webster's choice.
Similarly, one reads nonnegative, nonmagnetic, nonferrous, etc.
I find many words in dictionaries have a definition (necessary: definition follows), but others do no (unnecessary: not necessary) as we see here I still have to look up the word necessary to find out what not necessary means. Using the word nonsense as another example I will show sense (has definitions) but nonsense (has definitions but they are not meanings held to the prefix NOT applying without fail. In this case it is easy for me to see that nonsense versus non-sense where nonsense has acquired a new meaning SEPARATE from the applied non(not) prefix meaning should only be.
So I myself, contrary to all I have read by so called 'Usage Panels' of experts who list such reasonings as 60% agree or only 39% find this acceptable or blah... from that I know this: "If you do not have an absolute rule and answer, then there isn't one, it is simply one of anyone's preference".
That said, I then choose to put a hyphen between any word I choose... when I am using the implied meaning of the prefix (whether it is non(not) or otherwise) because at least I choose to make it known and not up to interpretation by any reader... what my intent is, and this is whenever I question the meanings that exist by definition that the reader may choose from. If there are too many interpretative choices in a non-hyphened usage to my liking, I will include the hyphen.
Note: I can't recall at the moment, hyphenating suffixes because they generally do not change meanings or include new definitions, but rather they usually only provide characterization of an object or its action as applies to tense, inflection, mood, etc, but if used, use it the same way as a prefix.