Richard J. Finlay

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.

Microstructuring of silicon with femtosecond laser pulses

We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Surface femtochemistry of 02 and CO on Pt(111)

Abstract Desorption of OZ and formation of CO2 were induced with subpicosecond laser pulses on a Pt(111) surface dosed with coadsorbed OZ and CO. We report the fluence dependent yields obtained over a range of laser wavelengths from 267 to 800 nm, and pulse durations from 80 fs to 3.6 ps. The nonlinear dependence of the yield on fluence is different at different wavelengths. Two-pulse correlation measurements show two different time-scales relevant to the desorption. The results show that nonthermalized electrons play a role in the surface chemistry, and that an equilibrated pre-heating of the surface modes leads to enhanced desorption.

Surface femtochemistry of CO/O2/Pt(111): The importance of nonthermalized substrate electrons

We studied the surface femtochemistry of CO/O2/Pt(111) induced with 0.3 ps laser pulses over a wide range of wavelength and fluence. Below 10 μJ/mm2, the yields depend linearly on fluence. Above 10 μJ/mm2, the yields scale nonlinearly in the fluence. From the dependence of the yields on wavelength, we determine that the nonlinear surface femtochemistry is influenced by nonthermal substrate electrons.

REACTION PATHWAYS IN SURFACE FEMTOCHEMISTRY : ROUTES TO DESORPTION AND REACTION IN CO/O2/PT(111)

Abstract We present new data that address the chemical pathway to desorption of O 2 and formation of CO 2 in CO/O 2 /Pt(111) induced by subpicosecond laser pulses. The data show conclusively that the O 2 desorbs molecularly. We also find that if the CO oxidizes by an atomic pathway then the capture of oxygen atoms by the CO is highly efficient; if the CO oxidizes by a molecular pathway then the oxygen atoms in the CO 3 ∗ transition state are inequivalent.

Femtosecond laser activation of surface reactions

Laser induced formation of CO2 and desorption of O2 are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two-pulse correlation experiments measuring total O2 desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O2 and CO2 are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.

Femtosecond Dynamics of Molecular Reactions at Metal Surfaces

These lectures are an introduction to current research into photo-induced chemical reactions at metal surfaces. After an introduction to some qualitative quantum mechanics, we discuss the electronic and optical properties of metals, beginning from an introductory level. The Drude model is described in detail and then optical properties of matter are developed more completely by introducing band structures. The physics governing adsorption of reactants at a metal surface and other fundamental concepts in surface science are introduced. We describe the interaction with a subpicosecond laser pulse with a metal surface in preparation for discussion of some recent photochemistry experiments using subpicosecond laser pulses. The experiments address the nature of the photo-excited electrons that are responsible for chemical reaction of the adsorbates.