How to search for files modified after a particular time?

Take a look at how Advanced Query Syntax works.

Try something like modified:>YYYY-MM-DD hh:mm:ss or datemodified:>YYYY-MM-DD hh:mm:ss

Ranges work like this: datemodified:‎YYYY-MM-DD hh:mm:ss .. ‎YYYY-MM-DD hh:mm:ss

There's also support for various other properties, Boolean operators etc.

(Note that date and time formats may differ according to your locale, system settings and so on.)