Ellipsometric Investigation of C60 Single Crystal

The optical properties of C60 fcc single-crystal are investigated for the first time in the 1.5–5 eV energy range by spectroscopic ellipsometry. The good quality of the samples allow a careful determination of the optical band gap at 1.72 eV. Optical and dielectric functions are directly obtained from the experimental data and compared with the results for isolated clusters and for thin films. The contribution of the optical transitions forbidden in the C60 cluster is discussed. Experimental findings are compared with available theoretical calculations

Optical characterization of hierarchically self-assembled GaAs/AlGaAs quantum dots

Recently, a simple method has been developed to obtain inverted GaAs/AlGaAs quantum dots (QDs) below a quantum well (QW) via multi-step (hierarchical) self-assembly [1]. Here we report on the optical characterization of a series of structures GaAs QDs with different size. The study is performed by means of reflectance (R) down to T=15 K, photo- and thermo-reflectance (T>80 K) and by spectroscopic ellipsometry (at RT). Such measurements allow us to check the thickness and composition of the barrier layers and to complete the study of the electronic states involved in the emission properties of QDs and QW levels. QW states were well identified and their energies and shift with the size parameters well agree with the photoluminescence (PL) data. The identification of QD states seems less straightforward in R measurements and deserves further investigation

Optical characterization of hierarchically self-assembled GaAs/AlGaAs quantum dots

Recently, a simple method has been developed to obtain inverted GaAs/AlGaAs quantum dots (QDs) below a quantum well (QW) via multi-step (hierarchical) self-assembly [1]. Here we report on the optical characterization of a series of structures GaAs QDs with different size. The study is performed by means of reflectance (R) down to T=15 K, photo- and thermo-reflectance (T>80 K) and by spectroscopic ellipsometry (at RT). Such measurements allow us to check the thickness and composition of the barrier layers and to complete the study of the electronic states involved in the emission properties of QDs and QW levels. QW states were well identified and their energies and shift with the size parameters well agree with the photoluminescence (PL) data. The identification of QD states seems less straightforward in R measurements and deserves further investigation

Second-harmonic generation in reflection and diffraction by a GaAs photonic-crystal waveguide

Nonlinear reflection and diffraction measurements have been performed on a GaAs/AlGaAs photonic-crystal waveguide patterned with a square lattice: The basis in the two-dimensional unit cell consists of rings of air in the dielectric matrix. The measured angles of diffracted second-harmonic beams agree with those predicted for nonlinear diffraction conditions, Results for second-harmonic intensities as a function of incidence angle, polarization, and pump wavelength show that the reflected second-harmonic signal is dominated by the crystalline symmetry of GaAs, whereas nonlinear diffraction is determined by the photonic-crystal structure

Resonant second-harmonic generation in a GaAs photonic crystal waveguide

Second-harmonic generation (SHG) measurements in reflection on a GaAs/Al0.25Ga0.75As photonic crystal waveguide show a resonant enhancement when the pump beam is frequency and momentum matched with the photonic modes in the slab. The enhanced SH signal is observed in the form of resonant peaks, unlike in linear reflectance spectra. The observations are in very good agreement with a full 3D calculation of the anisotropic mode dispersion in the photonic crystal slab. The present results open the way towards realizing the extraordinary enhancement of SHG which was recently predicted [A. R. Cowan and J. F. Young, Phys. Rev. B 65, 085106 (2002)], and also demonstrate the potential of SHG as a nonlinear spectroscopic tool for optical studies of photonic crystals