A handwritten not would be nice, but a simple email would do the trick. Just make sure you always ask for a business card, and an email sort of like this should work fine :
Dear BLAH BLAH
I just wanted to write you to say Thank You for the time you took this morning to discuss the BLAH BLAH position available at BLAH BLAH. The interview was very informative and enjoyable. I am very excited about having just a possibility to have a position with such BLAH BLAH company. I am confident that with my abilities and skills, I would be a great fit for one of the position. I am enthusiastic about how much I could learn at BLAH BLAH and can assure you of my commitment and motivation in this regard.
Thank you again for your time and the efforts you made to arrange my interview. I look forward to hearing your final decision.
and of course you would fill in the BLAH's with the appropriate information.