H. W. K. Tom

Self-organized phase-matched harmonic generation in optical fibers

We generated second-harmonic light from a single-mode fiber after a few minutes of seeding with 532-nm harmonic light along with 1.064-microm pump light. This experiment supports a mechanism for photoinducing an alternating second-order nonlinearity in the fiber.

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

Dramatic changes were observed in the optical second-harmonic generation from the Rh(111) crystal surface as the surface was covered with alkali atoms. For low alkali coverage, these changes are explained qualitatively in terms of optical transitions between alkali atomic-derived electronic states and, for high coverage, in terms of the plasmon resonance of the alkali layer. The results establish the surface specificity of second-harmonic generation to the topmost 1 or 2 atomic layers of alkali metals.

Preparation of long-coherence-length second-harmonic-generating optical fibers by using mode-locked pulses

We have prepared an optical fiber that produces efficient (0.24%) second-harmonic conversion by using modelocked 1.064-microm laser pulses along with a harmonic seeding beam. The effective coherence length increased to 35 cm as compared with only a few centimeters in fibers prepared with mode-locked Q-switched pulses. We show that the input bandwidth of the pump pulse imposes a fundamental limit on the effective coherence length and that this length can be severely reduced by competing nonlinear effects.

Study of soft-x-ray generation by laser-heating solid and gaseous tantalum plasmas with subpicosecond pulses

The efficiency of x‐ray emission in the 17–35 nm region from laser‐heated solid and gaseous density Ta plasmas has been compared. An x‐ray conversion efficiency of 0.4% from a solid density Ta plasma requires a laser fluence of 200 J/cm2 whereas the same efficiency in a gaseous Ta plasma with optimal density profile requires only 3 J/cm2 . This is due to ultrarapid thermal diffusion in the solid and to the shorter attenuation length and duration of x rays emitted from the heated solid density plasma.

Coherent phonon spectroscopy of GaAs surfaces using time-resolved second-harmonic generation

Abstract The general theory and experimental considerations are presented for a novel all-optical time-domain technique for measuring low frequency vibrational modes of surfaces (phonons or adsorbate vibrational modes). A pump-laser pulse impulsively drives an initial displacement of the surface atoms. The subsequent free-induction decay of the coherent phonon modes of the surface atoms is detected by time-resolved second-harmonic generation (SHG). The spectral features are recovered by fitting the time-domain data to exponentially decaying sinusoids. This all-optical probe has advantages over inelastic particle scattering techniques because it can be applied at buried interfaces. It has signal to noise advantages over linear and spontaneous Raman techniques. This technique is demonstrated by measurement of surface optical phonon spectra on the clean GaAs (110)–relaxed–(1×1), GaAs (100)–(1×6), and GaAs (100)–(4×1) in UHV and of a local monolayer-scale interfacial mode at the buried native-oxide-covered GaAs (100). The chemical sensitivity is demonstrated by in situ oxidation of the GaAs (100)–(4×6). The general mechanism of generating coherent surface phonons is discussed in light of the symmetry selection rules.

Picosecond soft-x-ray pulse-length measurement by pump–probe absorption spectroscopy

We have demonstrated a system for subpicosecond, soft-x-ray-continuum, pump–probe absorption spectroscopy. By using a correlation measurement of visible and soft-x-ray photons we report the temporal profile of laser-generated x-ray pulses near 90 eV.

Effect of laser pulse duration on short wavelength emission from femtosecond and picosecond laser‐produced Ta plasmas

The efficiency, duration, and spectral content of the emission from laser‐produced Ta plasmas in the 10–71 nm spectral region have been measured for laser pulse durations ranging from 100 fs to 70 ps. Efficiencies from 0.3% for 100 fs pulses to 2.65% for 70 ps pulses, x‐ray pulse durations from less than 10 ps for 100 fs excitation pulses to 105 ps for 70 ps excitation pulses, and spectral content peaking in the 17–35 nm region were measured for 1 mJ pulses at intensities from 1011 to 5×1014 W/cm2.

PROPERTIES OF SURFACE-MODIFIED COLLOIDAL PARTICLES

The reactivity of colloidal particles is regulated by their surface properties. These properties affect the wettability, flocculation-dispersion characteristics, ion exchange, sorption capacities and transport of inorganic colloids. Most studies have focused on hydrophilic, charged-particle surfaces, often ignoring the alterations in surface properties produced by the adsorption of natural organic matter, surfactants and other compounds. Adsorption of these substances can potentially render a surface substantially more hydrophobic. Nevertheless, comparatively little is known about changes in surface properties and reactivity of minerals upon sorption of hydrophobic organic compounds. In this study, the properties of four minerals (kaolinite, pyrophyllite, montmorillonite and Min-U-Sil®) and two inorganic materials (X-ray amorphous Al hydroxide and X-ray amorphous Si oxide) were compared before and after treatment with the common silylating agent, trimethylchlorosilane (TMCS). The samples were characterized by measurements of total carbon, cation exchange capacity (CEC), particle size, specific surface area (SSA), electrophoretic mobility, contact angle, particle aggregation, and by X-ray diffraction and diffuse reflectance infrared spectroscopy. For the layer silicates, surface coverage was limited to ∼2% trimethyl silane (TMSi). TMSi covered 7.5% of the Min-U-Sil® surface and 33% of the X-ray amorphous Si oxide. Treatment did not affect the structure of the minerals but reduced the CEC, SSA and electrophoretic mobilities. Water contact angles increased to between 18 and 114° with treatment. While the apolar characteristic of the surfaces decreased minimally with treatment, the Lewis acid/base properties were substantially reduced and interfacial free energy shifted from positive to negative values indicating a more hydrophobic surface character. For all the samples except kaolinite, these changes affected the stability of the colloids in suspension depending upon solution pH. Although the grafting of TMSi altered colloidal mineral surface properties and increased their hydrophobicity, these changes were not sufficient to predict colloid aggregation behavior.

Large area epitaxial germanane for electronic devices

We report the synthesis and transfer of epitaxial germanane (GeH) onto arbitrary substrates by electrochemical delamination and investigate its optoelectronic properties. GeH films with thickness ranging from 1 to 600 nm (2–1000 layers) and areas up to ~1 cm2 have been reliably transferred and characterized by photoluminescence, x-ray diffraction, and energy-dispersive x-ray spectroscopy. Wavelength dependent photoconductivity measurements on few-layer GeH exhibit an absorption edge and provide a sensitive characterization tool for ultrathin germanane materials. The transfer process also enables the possibility of integrating germanane into vertically stacked heterostructures.

Nanosecond time-resolved multiprobe imaging of laser damage in transparent solids

We report the nanosecond time-scale and micron length-scale dynamics of elastic wave, plastic deformation, melt-front and crack propagation during laser-induced damage in the bulk of an optically transparent crystalline solid. Time-delayed probe pulses with wavelengths corresponding to RGB-color filters are used to obtain multiple images during a single damage event. This technique enables “real-time” dynamical studies of complex transient phenomena.