When to send thank you letter (email?) to committee members?

When is the proper time to send a thank you email to the committee members? After the defense or after getting the certificate confirmation from the registrar office? To me after defense saying thank you is such a nice gesture, just had a long conversation and question-answer session and everything is still so fresh in the mind of the members! But I would like to know about your suggestion (and probably would like to go with that too!).