Abstract
We present a new background-free photothermal spectroscopy based on thermal lens microscope (TLM) to realize single molecule counting. For single molecule counting, approximately 20 times increase of sensitivity is necessary compared with conventional TLMs. For this purpose, a new differential interference contrast thermal lens microscope (DIC-TLM) was developed. The basic principle was verified, and sensitivity was proved to be 5 times higher than conventional TLMs. We believe single molecule counting will be possible with some improvements.