Ultrasensitive laser spectroscopy in solids: Optical detection of a single dopant molecule

Laser FM spectroscopy with double modulation was used for single-molecule detection (SMD). Detection of the optical absorption in the regime N̄H ≤ 1 is reported which was achieved by recording spectra far out in the wings of inhomogeneous line. An interfering background from residual amplitude modulation had to be removed; hence the sample absorption was modulated with externally applied electric fields at 5 kHz (FM/Stark) or stress fields at 2 MHz (FM/ultrasound). Spectra were observed that are due to single molecules of pentacene in p-terphenyl. Of particular interest is that the triplet bottleneck broadening that occurs near line center is weak in the SMD region.