I. L. Shumay

Inhomogeneous deformation of silicon surface layers probed by second-harmonic generation in reflection

Semiconductor crystals possessing inversion symmetry (Si, Ge) are known to have a rather weak second-order nonlinearity of the quadrupole type [ Phys. Rev. Lett.51, 1983 ( 1983)], since the electric-dipole contribution is forbidden by symmetry. We report the experimental observation of anomalously highly efficient second-harmonic generation (SGH) in reflection from the surface of Si under inhomogeneous deformation. This effect is believed to be due to an electric-dipole contribution to the second-order susceptibility induced in the near-surface layer by inhomogeneous mechanical stress. This fact is consistent with theoretical calculations based on the molecular sp3-orbital model. Experimentally we observed an increase in the second-harmonic intensity by more than 2 orders of magnitude and a modification of the second-harmonic intensity dependence on crystal orientation with respect to the surface normal in the case of ion-implanted, pulsed-laser-annealed Si (111) samples. A similar effect was observed with thermally oxidized Si wafers and silicide-on-cSi structures. These results demonstrate the sensitivity of SHG in reflection to the presence of inhomogeneous stress in Si surface layers, which enables one to use SHG for nondestructive monitoring of stress in semiconductor structures.

Laser nonlinear-optical probing of silicon/SiO2 interfaces: Surface stress formation and relaxation

A nonlinear optical technique based on optical second harmonic generation in reflection is shown to provide information on the surface layer structure of semiconductor crystals, thin films and layered systems. The second harmonic intensity is sensitive to inhomogeneous stress in centrosymmetric materials via spatial selection rules and the appearance of an electric dipole contribution to the second order nonlinear optical susceptibility. The technique is used to monitor mechanical stress relaxation in the SiO2/Si interface during several annealing procedures.

Time-resolved second-harmonic study of femtosecond laser-induced disordering of GaAs surfaces.

An abrupt (less than 100 fs) decrease in the second-harmonic intensity reflected from the surface of a GaAs (110) wafer has been observed experimentally. The linear reflectivity was found to increase on a time scale of ~1 ps. Thus the concept of fast atomic disorder induced by electronic excitation within a relatively cold lattice is given new experimental support.

Observation of the saturation effect in continuous-wave coherent anti-Stokes Raman spectroscopy of liquid nitrogen

We report the first observation to our knowledge of the saturation effect in a Raman transition of a liquid, liquid N2. Using cw coherent anti-Stokes Raman spectroscopy, Raman line broadening by a factor of ≃2 is observed. This corresponds to vibrational excitation of up to 30% of the N2 molecules in the interaction volume.

Femtosecond laser-induced melting of GaAs probed by optical second-harmonic generation

Primary steps of melting (disordering) of GaAs surface layers after femtosecond pulse excitation have been measured through second-harmonic generation in reflection. A numerical fit of the experimental data yield a characteristic time delay between excitation and lattice melting of about 75 fs.

Femtosecond dynamics of laser-induced phase transition of the GaAs surface layer to a centrosymmetric phase

Abstract Recently we observed ultrafast laser-induced phase transition (on a time scale less than 100 fs) to a centrosymmetric semiconductor-like phase at the GaAs surface [1,2] by using time-resolved second harmonic generation in reflection. A phenomenological model describing this phase transition is developed. The new phase exists during the first 1 ps after laser excitation due to high plasma carrier density and lattice stress, the lattice temperature remaining well below the melting threshold.

Second-harmonic generation in reflection from crystalline metals: the influence of structure disordering

We report theoretical and experimental studies of the influence of crystalline structure on the nonlinear-optical response of monocrystalline metals. We show that the lattice disordering leads to a decrease of both the anisotropy and the absolute value of the quadrupole second-order nonlinear susceptibility, in agreement with experiments on optical second-harmonic generation in reflection from the surfaces of various Al samples.

Second-harmonic and sum-frequency generation in reflection: probing of GaAs surface structure and laser-induced phase transitions

The dynamics of pulsed-laser annealing of an ion-implanted GaAs surface is studied by second-harmonic generation in reflection. The results are explained in terms of surface melting. Sum-frequency and second-harmonic generation in reflection are shown to be sensitive, convenient techniques having high temporal and spatial resolution for studying the structure of GaAs surfaces that have been laser annealed or randomized by ion implantation and of laser-induced phase transitions. The tensor nature of the second-order nonlinearity provides an opportunity to make polarizational measurements that are sensitive to small structural changes.

CARS probing of the Si Raman mode transformation during strong picosecond pulsed-laser irradiation

Abstract We report on picosecond coherent anti-Stokes Raman scattering (CARS) probing of the Si optical phonon spectrum transformation during picosecond pulsed-laser irradiation. CARS spectra were obtained in reflection at various laser fluences up to melting. The observed spectrum broadening is theoretically explaned in terms of phonon heating and laser-induced mechanical stress build-up on a time scale of 10ps.

Femtosecond laser diagnostics of ultrafast laser-induced melting of the GaAs surface

We report here the experimental results on the SHG and linear reflectivity studies of the dynamics of laser-induced melting of a GaAs surface layer under subpicosecond pulsed laser excitation. We find that the SH intensity drops in a time less than 100 fs after excitation, while the linear reflectivity reaches a value characteristic of molten GaAs on a time scale of about 1 ps. Thus, the experimental results unambiguously indicate that ultra-fast phase transition to a new solid phase with structural properties different both from that of initial material and that of molten GaAs takes place in the surface layer under strong femtosecond laser excitation. This transition occurs on a time scale of 100 fs which is about an order of magnitude less than the time required for the electron relaxation to the bottom of the conduction band.