Optical third harmonic generation study of the hydration of DNA films.

Optical third harmonic generation (THG) has been observed for the first time from DNA films. The THG signal is observed from NaDNA films exposed to relative humidities (RHs) between 0% and 98%. A strong enhancement (approximately 5x) of the THG signal from NaDNA is observed at 84% RH; no enhancement is observed for RbDNA. The most likely mechanism for such an enhancement is an increased coherence length. A model calculation using estimates of the refractive indices at both the fundamental and third harmonic frequencies supports this interpretation. The observed THG signal has the same polarization as the incident (fundamental) light. For the A conformation, the THG signal polarized perpendicular to the helical axis is approximately twice as strong as the signal polarized parallel to the helical axis. No such anisotropy is observed for either the disordered conformation (below about 50% RH) or the B conformation (above 92% RH).