Direct measurement of local field factors for second-harmonic generation from quantum dot Langmuir monolayers compressed through the metal-insulator transition

The frequency-dependent complex optical dielectric function epsilon(omega) was determined in situ in order to calculate local electric field factors r esponsible for large (500-fold) enhancements in second-harmonic generation (SHG) observed from two-dimensional superlattices of silver quantum dots co mpressed through a reversible metal-insulator transition. For all films stu died, the local field contribution amounted to, at most, a few per cent of the total SHG enhancement observed. This indicates that the enhancement is dominated by changes in the intrinsic optical nonlinearity chi((2)) of the system and not by linear optical effects.