FEMTOSECOND PULSE LASER 2ND-HARMONIC GENERATION ON SEMICONDUCTOR ELECTRODES

Laser pulses of approximately 300 fs duration (300 x 10(-15) s) were a pplied for second harmonic generation (SHG) at Si electrodes, for the first time. The frequency doubling by reflection at surfaces of centro -symmetric materials like Si is generated only at the surface due to t he breaking of the crystal symmetry. This method is particularly inter esting because even buried solid state interfaces can be studied. SHG as a non-linear optical effect needs high illumination power densities . This has restricted its use mainly to metal electrodes. Our recent s tudies with ns laser pulses showed that even semiconductor electrodes can be probed with SHG if the duration of the light pulses is shorter than any electron transfer reaction. A further advantage of fs pulses is the fact that illumination fluences well below the damage threshold , but still with sufficient power density, can be chosen. In contrast, the use of ns pulses required an operation near the damage threshold. Si electrodes in contact with aqueous electrolytes are normally cover ed by an oxide layer of SiO2 with a transition layer of non-stoichiome tric suboxide SiO(x) (1 < x < 2). The extension of this layer as well as its growth mechanism and structure are still under discussion. Surf ace SHG promises to provide direct insight into this buried interface since the SH signal is generated exactly there. An oxide covered Si(11 1) surface yields an azimuthal SHG dependence as expected from the (11 1)C3nu symmetry. Removal of the oxide causes a drop in SH intensity wi th a subsequent rise ascribable to changes of the static electric fiel d in the transition layer. At an oxide covered Si(111) electrode the S H signal can be modulated by an externally applied potential.