How to use my iMac as second display with laptop

Hi there,

So I own a 2011 iMac running Windows 7 and have an Acer Notebook for Work currently running Win 7 (maybe soon upgrading to Win10). I want to connect my Notebook with the iMac to us it as a bigger screen.

So far I used a HDMI to Mini DP (Display Port) but get know connection. I dont know which kind of cable will do the trick, so has anyone tried this yet?

Thanks for the help in advance,

Delorian110