J. P. Callan

THREE-DIMENSIONAL OPTICAL STORAGE INSIDE TRANSPARENT MATERIALS

We present a novel method for three-dimensional optical data storage that has submicrometer size resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials. Bits are recorded by use of a 0.65-N.A. objective to focus 100-fs laser pulses inside the material. The laser pulse produces a submicrometer-diameter structurally altered region with high contrast in index of refraction. We record binary information by writing such bits in multiple planes and read it out with a microscope objective with a short depth of field. We demonstrate data storage and retrieval with 2-microm in-plane bit spacing and 15-microm interplane spacing (17 Gbits/cm(3)). Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.

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.

Phase Transitions Induced By Femtosecond Pulses

Optical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes places before the electronic system has time to thermally equilibrate with the lattice. The excitation of a critical density of valence band electrons destabilizes the covalent bonding in the crystal, resulting in a structural phase transition. The deformation of the lattice leads to a decrease in the average bonding-antibonding splitting and a collapse of the band-gap. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase transition.

From semiconductor to metal in a flash: Observing ultrafast laser-induced phase transformations

The authors use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.5--3.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. The authors studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased--lattice heating, lattice disordering, and a semiconductor-to-metal transition.

3-D Optical Storage and Engraving Inside Transparent Materials

We present a novel method for 3-D optical data storage and internal engraving that has sub-micron resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials.

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.

Ultrafast laser-induced structural changes in semiconductors

We present experimentally determined values of the dielectric function of GaAs following femtosecond laser excitation. The data at photon energies of 2.2 and 4.4 eV show that the response of the dielectric function tot he excitation is dominate by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric function indicates a drop in the average bonding-antibonding splitting of GaAs following the excitation, which leads to a collapse of the band gap. The changes in the electronic band structure results from a combination of electronic screening, many-body effects, and structural deformation of the lattice caused by the destabilization of the covalent bonds. Broadband measurement of the dielectric function over the range 1.5-3.5 eV reveals ultrafast laser-induced heating at low fluence, disordering at intermediate fluence, and an ultrafast semiconductor-to-metal transition at high fluence.