Hidemi Shigekawa

Synchrotron radiation photoemission analysis for (NH4)2Sx-treated GaAs

The chemistry of the (NH4)2Sx‐treated n‐GaAs (100) surfaces has been studied using synchrotron radiation photoemission spectroscopy. Ga 3d, As 3d, and S 2p photoemission spectra are measured before and after annealing in vacuum with a photon energy of about 210 eV, where S 2p core level spectra can be sensitively detected. It is found that Ga‐S, As‐S, and S‐S bonds are formed on the as‐treated GaAs surfaces, and that stable Ga‐S bonds become dominant after annealing at 360 °C for 10 min in vacuum. The thickness of the surface sulfide layer is reduced from about 0.5 to 0.3 nm by annealing. The surface Fermi‐ level position of the as‐treated surfaces is determined to be about 0.8 eV below the conduction band minimum, which is about 0.1 eV closer to the valence band maximum than that of the untreated surfaces. A Fermi‐level shift of 0.3 eV toward a flat band condition is also observed after annealing. It is found that the Ga‐S bonding plays an important role in passivating GaAs surfaces.

Hydrolysis of DNA and RNA by lanthanide ions: Mechanistic studies leading to new applications

A few years ago, the remarkable catalytic activity of lanthanide ions for the hydrolysis of nucleic acids was discovered. With CeIV, DNA was hydrolysed under physiological conditions. For RNA hydrolysis, the last three lanthanide ions (TmIII, YbIII, and LuIII) are superb. Furthermore, artificial restriction enzymes for site-selective scission of DNA and RNA, essential tools for the future biotechnology, have been prepared by using the lanthanide complexes. The present article emphasizes the mechanistic aspects of the catalyses of these metal ions. Both DNA hydrolysis and RNA hydrolysis involve the cooperation of acid catalysis (by metal ion and/or metal-bound water) and base catalysis (by metal-bound hydroxide). The magnitudes of contributions of these catalyses, as well as the positions where they work, are primarily governed by the relative height of the energy-barrier for the formation of the pentacoordinated intermediate and that for its breakdown. The following conclusions have been obtained on the basis of various kinetic and spectroscopic evidence: (1) for the hydrolysis of both DNA and RNA, the catalytically active species are dinuclear hydroxo-clusters, (2) CeIV enormously activates DNA and promotes the formation of the pentacoordinated intermediate, and (3) the catalysis for RNA hydrolysis is mainly ascribed to the promotion of breakdown of the pentacoordinated intermediate.

Comparison between theory and experiment of nonlinear propagation for a-few-cycle and ultrabroadband optical pulses in a fused-silica fiber

Wave-propagation equations, including effectively the second derivative in time under the condition of a small difference between the group and phase velocities and the first derivative in position /spl xi/ in the group velocity coordinate, are derived based on the slowly evolving wave approximation. These can describe ultrabroadband optical pulse propagation with not only self-phase modulation (SPM), but also induced-phase modulation (IPM) in the monocycle regime in a fiber. It is shown that linear dispersion effects can be rigorously included in the numerical calculations. Calculations including SPM in a single-mode fused-silica fiber with the Raman effect are performed and compared with experimental results. Also, calculations including IPM in the fused-silica fiber are compared with experimental results. The effects of each term in the calculations on spectra are analyzed and it is shown that inclusion of the Raman effect and the dispersion of the effective core area is important for obtaining better agreement with experiments. It is shown that inclusion of more than third-order dispersion terms is necessary for calculations of monocycle pulse propagation.

How to fabricate a defect free Si(001) surface

We demonstrate the successful fabrication of an almost defect free Si(001) surface by refining the standard annealing and flashing surface preparation method. On any desired samples, we can routinely fabricate a surface with defect densities lower than 0.1%, significantly reducing the defect density compared to surfaces fabricated by standard methodology.

Generation of intense ultrabroadband optical pulses by induced phase modulation in an argon-filled single-mode hollow waveguide

We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-muJ pulse energy by a novel, simple setup utilizing induced phase modulation (IPM) in an argon-filled single-mode hollow waveguide. Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphire laser-amplifier system are copropagated in the hollow waveguide. The effect of the delay time between the two input pulses on the IPM spectral broadening is clarified and confirmed to agree with the theoretical result. It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensated for.

Study of the adsorption structure of NO on Pt(111) by scanning tunneling microscopy and high-resolution electron energy-loss spectroscopy

The structure of nitric oxide (NO) on a Pt(111) surface was studied by scanning tunneling microscopy (STM ) and high-resolution electron energy-loss spectroscopy (HREELS ). The coexistence of two species is observed by STM after saturating the surface with NO at 70 K and annealing to 215 K. Two pairs of NMO and PtMN stretching vibrational frequencies are observed in HREELS spectra after annealing, which are assigned to the two species observed by STM. Comparison between the spectra of HREELS and infrared absorption spectroscopy (IRAS) indicates that IRAS is less sensitive to the lower-frequency mode. A new model for the adsorption structure of NO on the Pt(111) surface is proposed and discussed.

Experimental generation of an ultra-broad spectrum based on induced-phase modulation in a single-mode glass fiber

An ultra-broad spectrum over the range from 480 to 900 nm is experimentally generated by induced-phase modulation (IPM) of two 120 fs intense optical pulses copropagating in a 3-mm single-mode fused-silica fiber for the first time to our knowledge. The center wavelengths of the two pulses are 640 nm and 795 nm, respectively. The Fourier transform of this spectrum yields a transform limited 4 fs optical pulse. This IPM-induced spectrum broadening method opens the way to monocycle optical pulse generation in the near future.

Optical pulse compression to 5.0 fs by use of only a spatial light modulator for phase compensation

We experimentally demonstrate the generation of 5.0-fs optical pulses (2.5 µJ, 1-kHz repetition rate), using only a spatial light modulator for phase compensation. Pulse compression of the broadband pulse (500–1000 nm) from an argon-filled capillary fiber is achieved with a liquid-crystal spatial light modulator without any prechirp compensation. The output pulse width is found to be 4.1 fs by a fringe-resolved autocorrelator fitted with a transform-limited pulse and to be 5.0 fs by second-harmonic generation frequency-resolved optical gating with marginal correction. It is to our knowledge the shortest pulse ever generated by use of only a spatial light modulator for phase compensation.

Multiple stacking of self-assembled InAs quantum dots embedded by GaNAs strain compensating layers

We have investigated a growth technique to realize high-quality multiple stacking of self-assembled InAs quantum dots (QDs) on GaAs (001) substrates, in which GaNxAs1−x dilute nitride material was used as a strain compensation layer (SCL). The growth was achieved by atomic hydrogen-assisted rf molecular beam epitaxy, and the effect of strain compensation was systematically investigated by using high-resolution x-ray diffraction measurements. By controlling the net average lattice strain to a minimum by covering each QD layer with a 40-nm-thick GaN0.005As0.995 SCL, we obtained a superior QD structure with no degradation in size homogeneity. Further, no dislocations were generated even after 30 layers of stacking, and the area density of QDs amounted to as high as 3×1012cm−2. The photoluminescence peak linewidth was improved by about 22% for QDs embedded in GaNAs SCLs as the accumulation of lattice strain with increasing growth of QD layers was avoided, which would otherwise commonly lead to degradation of ...

Programmable chirp compensation for 6-fs pulse generation with a prism-pair-formed pulse shaper

We describe a TF5 prism-pair-formed pulse shaper for programmable pulse chirp compensation. The advantages of this kind of pulse shaper are: (1) very broad bandwidth of transmission; (2) smaller losses; and (3) no requirement for a large-size spatial light modulator (SLM) if the input spectrum is very broad. In our experiment, an ultrabroad spectral (500-1000 nm) pulse is produced by launching 1-kHz, 30-fs, 400-/spl mu/J pulses at 780 nm into an argon filled glass capillary fiber at the gas pressure of 2.0 bar. The fiber has an inner diameter of 140 /spl mu/m and a length of 60 cm. The chirped pulse is first precompressed by a pair of BK7 prisms with a separation length of 65 cm and then directed into the prism-pair-formed pulse-shaping apparatus with a 128-pixel SLM, which provides quadratic and cubic phase compensation. When the quadratic and cubic phases are -330 fs/sup 2/ and +2000 fs/sup 3/, respectively, at the wavelength of 760 nm, an ultrashort optical pulse of 6 fs (FWHM) is generated. This is, to the best of our knowledge, the shortest optical pulse ever compressed using the SLM pulse-shaping technique.