When should I send thank you letter or email to my thesis committee members?
After the defense will be fine since by then the committee members have completed their job. What happens between that and when you have the final paper work done is irrelevant. Pointing at the discussion you have just had and finishing with a few personal comments/reflections will be fine. But, do not overdo it since long and talkative mails can be either too long to read or in the worst case seem constructed. So be yourself and be happy for what you have achieved ( I take it you will or have passed?)