M. S. Yeganeh

Femtosecond coherent transient infrared spectroscopy of CO on Cu(111)

Femtosecond infrared coherent transients have been measured for the stretch vibration of CO on Cu(111). The free induction decay exhibits a dephasing time (T2) of 2±0.3 ps (and 2±0.1 ps assuming a single exponential decay between 2 and 3 ps). The decay was best fit by exponential relaxation, thereby suggesting that the CO vibrational band is almost entirely homogeneously broadened. The surface sum frequency spectrum was also measured at two coverages (0.10 and 0.45 L) using spectrally narrowed pulses. Interferences were observed leading to a determination of the relative phase and amplitude of the resonant and nonresonant second‐order susceptibility in this system. The magnitude of the nonresonant susceptibility was only weakly dependent on coverage, suggesting that the nonresonant polarizability originates in the bulk Cu. Time and frequency domain results were in good agreement.

Interfacial electronic trap lifetimes studied by the photomodulation of second-harmonic generation processes

Second-harmonic generation and linear photomodulation are combined to study free-charge trapping mechanisms at ZnSe–GaAs(001) heterointerfaces. The variation of second-harmonic intensity as a function of charged-trap density at the buried junction is analyzed quantitatively and used with time-dependent measurements to determine interfacial charge-trap lifetimes.

Light scattering studies of ZnSe/GaAs heterostructures

Raman studies using the allowed and the forbidden longitudinal optical (LO) Frohlich interactions are employed to investigate the heterostructure of ZnSe/GaAs as a counterpart to three‐photon mixing or second harmonic generation studies. Our samples are an undoped ZnSe/GaAs (001) heterojunction with a ZnSe overlayer varying from 50 to 5000 A. The scattering intensities for the allowed and the forbidden LO interactions at both resonant and nonresonant incident frequencies for ZnSe are investigated. The nonresonant forbidden LO phonon shows the effect previously observed for the pseudomorphic region of ZnSe thickness as well as for thicker films. The allowed LO intensities show a clear interference effect similar to recent interference effects observed in second harmonic generation for identical ZnSe/GaAs heterostructures. At resonant incident frequency in ZnSe, forbidden LO phonon scattering shows strong enhancement and is an order of magnitude larger than the allowed scattering due to wave‐vector‐dependen...

Midgap states observed by nonlinear optical spectroscopy of metal:GaAs junctions

We present detailed second-order nonlinear optical spectroscopic studies of the interface electronic structure in metal:GaAs thin films and interfaces. The interface spectra from Au:GaAs, Ga-rich n-type systems exhibit two resonance features in the near infrared at 0.715 eV and 0.731 eV. A single resonance feature at 0.715 eV is observed in Au:GaAs, As-rich n-type interfaces. Similar single resonance features at 0.715 eV were observed in As:GaAs n-type samples. No resonances however were observed in oxide:GaAs and metal:GaAs p-type systems. All of the spectral features are sharp one-photon resonances. They provide compelling evidence for the existence and symmetry of atomic displacement-induced defect states just below the buried interface.