It is always a good idea to send a thank you letter to everyone who takes the time to construction interview you. It’s a courtesy like thanking a host who has you over for dinner, or who invites you to their party. In the letter you want to thank them for their time and let them know you enjoyed meeting them and learning about the opportunity. Let them know you also thought about the meeting and have decided that the job opportunity is one you are interested in pursuing, and that you will be following up with them with any questions.