Tuning NLO Susceptibility in Functionalized DNA

The reversible tunability of the third‐order nonlinear optical susceptibility of a deoxyribonucleic (DNA)‐based thin film, doped with a photochromic molecule, by light is shown. This tunability is demonstrated by optical third‐harmonic generation (THG) experiments at the excitation wavelength of 1064 nm. It is based on the light control of the two photon resonance when shuttling from the merocyanine (MR) to the spiropyran (SP) form and on the changes in the π electron conjugation length. A total decrease of the THG intensity by a factor of 25, that is the NLO susceptibility decreases by a factor of 5, is observed when a complete transformation from the MR to the SP form is achieved. This observation not only applies to THG experiments, the NLO susceptibility tunability is also obvious in other NLO processes that implicate the interaction of 4 photons. Second harmonic generation experiments show the same results, but are less obvious for practical application. The potential applications of the effect in photonics are also discussed.