High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [χ (2) > 10-6 m V-1] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10-6 m V-1]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 1013 W m-3). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.
- 2.02 ELECTRICAL ENG, ELECTRONIC ENG
- 2.05 MATERIALS ENGINEERING
- 1.03 PHYSICAL SCIENCES AND ASTRONOMY
- 1.03.UB PHYSICS, APPLIED