Second harmonic phase spectroscopy: frequency vs. time domain

Summary form only given. We quantitatively compare conventional vs. frequency-domain interferometric second harmonic (FDISH) phase spectroscopy for the first time. A Si/SiO/sub 2//Cr MOS capacitor with applied bias -9 to 10 V provided a common sample. Since the space-charge field (SCF) changes sign at the flat-band voltage (-2.45 V), the phase of electric-field-induced SH (EFISH) shifts by /spl sim//spl pi/, providing a convenient point of comparison. FDISH phase spectra acquired in seconds, agree generally with conventional phase spectra acquired over hours. Both show strong bias-dependent phase shifts near the E/sub 1/ resonance (3.3 to 3.4 eV), where EFISH generation is resonantly enhanced, and relatively featureless, bias-independent phase for 2h/spl nu/ < 3.2 eV. However, FDISH phase shifts abruptly by /spl sim//spl pi/ at V/sub flat-band/, as expected, whereas conventionally measured phase shifts more gradually. Apparatus drifts during the longer measurement partly explain the discrepancy. In addition, screening of the SCF by photo-excited carriers selectively influences the conventional measurement, because electrons and holes separate more during its 100 fs pulse than during the FDISH 15 fs pulse, thus weakening the bias dependence. Thus, in this case, FDISH clearly yields the more accurate /spl Delta//spl psi/(/spl omega/) measurement.