For these types of campaigns, I try to think as someone in my target audience would.
Which books would I read? Which blogs would I follow? Are there any relevant thought-leaders/celebrities in the industry I can target? Magazines? Events? Build a list of interest out of these categories and test them.
Then, focus on building content that is tailored to these people. Create ads that make it evidently clear who the ad is for and, most importantly, **who it is not for.**
Don’t worry too much if there are some users in your audience who don’t fit your exact requirements.
If you’re running conversion-based campaigns, Facebook will eventually figure out who’s interacting the most with your ads and start delivering to similar people.
Which leads to the last point: try and keep your audiences not too narrow, even when targeting very specific niches (such as physicians). Again, Facebook has gotten pretty good at figuring out who it should deliver ads to.
So, focus on creating great ads instead, great landing pages, and on your funnel, and you should see physicians coming in 🙂