C.A.D. Roeser

Femtosecond time-resolved dielectric function measurements by dual-angle reflectometry

We present a technique to measure the dielectric function of a material with femtosecond time resolution over a broad photon energy range. The absolute reflectivity is measured at two angles of incidence, and e(ω) is calculated by numerical inversion of Fresnel-like formulas. Using white-light generation, the single-color probe is broadened from the near IR to the near UV, but femtosecond time resolution is maintained. Calibration of the apparatus and error analysis are discussed. Finally, measurements of isotropic, thin film, and uniaxial materials are presented and compared to reflectivity-only studies to illustrate the merit of the technique.

Chapter 4 Ultrafast dynamics and phase changes in highly excited GaAs

Publisher Summary This chapter discusses ultrafast dynamics and phase changes in highly excited gallium arsenide (GaAs). The progress of science in modern times is often driven by improvements in the precision of observations of natural systems or in the scientists ability to control systems in experiments. The first and most obvious characteristic of femtosecond lasers is that they produce pulses of shorter than five femtoseconds (fs). These short pulses can be used to observe phenomena with a time resolution 1,000 to 10,000 times greater than possible in any other experiment. The second key characteristic of femtosecond pulses is that they can deliver an enormous amount of power with just a modest quantity of total energy, because the energy is packed into such a short time. Consequently, the electrons in a solid reach extremely high temperatures while the atomic structure is still cold; this is a unique nonequilibrium excitation of a solid and can cause novel and unusual phase transitions.

Ultrafast lattice-bonding dynamics of tellurium

Summary form only given. We present a study of the ultrafast bonding dynamics of tellurium in the presence of a symmetry-preserving coherent phonon. The coherent phonon modulates the band structure at THz frequencies. We measure the dielectric function, because it is closely related to the band structure, over a broad frequency range (1.8 eV to 3.4 eV) following excitation by an 800-nm, 50-fs pump pulse. In order to measure the two independent components of the dielectric tensor of a uniaxial material (/spl epsiv//sub ord/(/spl omega/) and /spl epsiv//sub ext/(/spl omega/)), four pump-probe measurements are required. Each component describes the optical properties along certain directions in the crystal: /spl epsiv//sub ord/(/spl omega/) corresponds to the ab-plane and /spl epsiv//sub ext/(/spl omega/) to the c-axis (helical axis). The symmetry-preserving phonon mode excited in Te is confined to the ab-plane, so we expect that the ordinary dielectric function will be most sensitive to these oscillations.

Ultrafast phase transition dynamics in GeSb alloys

We measure the femtosecond time resolved dielectric function of a-GeSb after excitation with an ultrashort laser pulse. The results reveal an ultrafast transition to a new non-thermodynamic phase which is not c-GeSb as previously believed. We present the most thorough experimental study to date of laser induced ultrafast phase transitions in GeSb alloys. We investigate the changes of the material by directly monitoring the full dielectric function over a broad energy range (1.7 eV - 3.5 eV) with 100 fs time resolution.

Two-photon-absorption FROG: measuring white-light continuum pulses

Summary form only given. Frequency-resolved optical gating (FROG) is currently the most commonly used technique to fully characterize an ultrashort optical pulse. Various species of FROG have been demonstrated which utilize various optical nonlinearities, such as second harmonic generation (/spl chi//sup (2)/), polarization gate, transient grating, third harmonic generation and self diffraction (all /spl chi//sup (3)/). We devised a new FROG technique based on two-photon absorption (TPA) which is ideally suited for characterizing white-light continuum pulses. Two-photon absorption in various materials is routinely used for autocorrelation measurements of ultrashort laser pulses. Due to the resonant enhancement at the materials band gap and the lack of phasematching requirements for TPA, this /spl chi//sup (3)/ nonlinearity is an excellent candidate for ultrashort pulse characterization. To produce a FROG trace based on TPA, two pulses are spatially overlapped in the TPA crystal and the spectrum of one pulse is measured as a function of the time delay between the two pulses. The TPA FROG trace is equivalent to the trace generated by polarization gate FROG. As a test of the new technique we have measured the second harmonic generation (SHG) and TPA FROG traces of pulses generated by a standard multipass Ti:sapphire amplifier.