S. G. Han

Studies of resonant and preresonant femtosecond degenerate four-wave mixing in unoriented conducting polymers

Degenerate four‐wave mixing with femtosecond time resolution is used to measure the magnitude and transient response of the third‐order nonlinear optical susceptibility χ(3) (ω;ω,−ω,ω) at 620 nm in nonoriented conducting polymers including polydiacetylene, polyacetylene, polyaniline, polydiethynylsilane, polythiophene, and polythiophene derivatives. Resonant and nonresonant excitations influence the magnitude and transient response of χ(3). The electronic response is instantaneous for preresonant excitation, but for resonant excitation it has ultrafast and slow components which illustrate photoexcitation dynamics. The magnitude of χ(3) for all of the polymers is in the range from 10−10 to 5×10−8 esu depending on the energy difference between the laser excitation and the polymer absorption maximum.

Femtosecond dynamics of the nonlinear optical response in polydiethynylsilane

We have measured the femtosecond dynamics of the nonlinear optical response χ(3) in polydiethynylsilane (C4H2SiBu2)x, a novel class of π‐conjugated polymers incorporating Si, using degenerate four‐wave mixing and photoinduced absorption techniques. In resonance conditions at 620 nm we found χ(3)=3×10−9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at high laser intensities. This material is an excellent candidate for nonlinear optical devices in the sub‐THz frequency range.

Theoretical structural characterization of polydiethynylsilane - a new nonlinear optical material

Abstract Recently, a novel π-conjugated polymer, polydiethynylsilane (PDES), with excellent nonlinear optical properties has been reported. The structures of several isomeric forms for PDES have been characterized theoretically. The results show that the four-membered ring structure is the most plausible and some dialkyl, ethynylsilane groups may insert in the backbone of PDES as sequential copolymers. These results were obtained using the molecular orbital method at the ab initio level (3–21 G ∗ modified) and molecular mechanics method with potentials derived from ab initio calculations. We have also calculated the second-order polarizability.

Optical properties of polydiethynylsilanes: A quasi (AB)x polymer

Abstract We have measured optical absorption, resonant Raman scattering (RRS), cw photomodulation (PM), picosecond transient PM and degenerate four-wave mixing, in I 2 -doped and undoped polydiethynylsilanes (PDES): [(HC = C) 2 SiR 2 ] x , where R = alkyl, phenyl, etc. Our measurements show that the PDES backbone structure consists of units of four-membered rings alternatively connected to CH and SiR 2 units and thus it can be regarded as an almost degenerate ground state (AB-AB) x conducting polymer. As such, it supports A and B solitons as primary excitations; these have been observed in doping and photoexcitation spectra. The non-linear x (3) value at 625 nm was measured to be 3 × 10 −9 esu, one of the largest measured in conjugated polymers.

Femtosecond dynamics of quasi-particles in YBa/sub 2/Cu/sub 3/O/sub 7- delta / superconductor films

The transient electronic response of epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7- delta / superconductor thin films in the femtosecond time domain was investigated using transient photoinduced reflectivity ( Delta R) with 60-fs time resolution. For temperatures T>T/sub c/, only a bolometric signal was observed with Delta R>0. For T 0. The results for T<T/sub c/ are explained in terms of quasi-particle (QP) electronic response giving Delta R<0. Thus, the femtosecond rise time is interpreted as avalanche multiplication of QP across the gap 2 Delta , and the subsequent picosecond relaxation is interpreted as QP recombination. The QP optical response is explained within the framework of the two-fluid model.

Femtosecond degenerate-four wave mixing studies of third order electronic nonlinearities in conjugated polymers

Abstract The technique of degenerate four wave mixing with femtosecond time resolution has been used to measure the magnitude and dynamical response of the third order susceptibility ( χ 3 ( ω )) of polydiacetylene (PDA-4BCMU), polyacetylene ((CH) x ), polydiethynylsilane (Si(Bu) 2 -PDES), polyaniline, polythiophene and its derivatives. We found that the value and transient response of χ 3 ( ω ) depends on the position of the excitation wavelength with respect to that of the absorption maximum of the polymer studied. The electronic χ 3 ( ω ) values for all of these polymers have been measured to be in the range 10 −10 − 10 −8 esu. The transient χ 3 ( ω ) response for non-resonant excitation is instantaneous (

Studies of photocarrier relaxation processes in polysilane alloys from subpicosecond to metastability

The photocarrier relaxation processes in polysilane alloys, a-Si:H/a-(SiH2)n, have been studied using the picosecond transient and cw photomodulation (PM) techniques and light-induced ESR. For below gap excitation, the picosecond decay is exponential with an increasing decay rate at lower temperatures. This is interpreted in terms of e-h geminate recombination in the small clusters (≈ 10 A) of a-Si:H embedded in the polysilane matrix. The cw PM spectrum contains a single photoinduced-absorption band, which is interpreted as due to photocarriers trapped in dangling bond (DB) defects. This interpretation is based on the temperature and excitation intensity dependencies, as well as on the reduction of the ESR signal under illumination. From the analysis of the DB optical transitions, the DB effective correlation energy Ueff = 0.3 eV and its relaxation energy Δ Er = 0.4 eV are estimated.

Optical probes of polydiethynylsilanes

Abstract We have applied a variety of optical probe techniques for studying the ground-state properties, photoexcitations, and optical nonlinearities of the novel class π-conjugated polymers incorporating Si (or Ge), the polydiethynylsilanes (PDES): [(HCC) 2 SiR 2 ] x , where R = alkyl, phenyl, etc. Our measurements imply that the PDES backbone structure consists of units of four-membered rings alternately connected to CH and SiR 2 units; thus it can be regarded as an almost degenerate ground state (AB) x with a band gap at 2 eV. The PDES are therefore the second class of conducting polymers that can support soliton excitations. Using the photomodulation spectroscopy technique, we have found in PDES photogeneration of charged solitons similar to that in trans -(CH) x , as predicted by theory. However, due to the short average conjugation length in PDES films, the soliton energy levels are split by end-effects confinement. We have also found at 2 eV χ (3) = 3 × 10 − 9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at higher laser intensities.

Studies of picosecond carrier dynamics in polysilane alloys: Evidence for geminate recombination in small hydrogenated amorphous silicon clusters

The ultrafast photocarrier dynamics in polysilane alloys, amorphous (SiH2)n, has been studied using the picosecond photoinduced absorption (PA) technique. For excitation below the optical gap, the PA response decays exponentially and is faster at low temperatures. This is interpreted in terms of e‐h geminate recombination in the small clusters (∼10 A) of a‐Si:H embedded in the polysilane matrix. The PA response with above‐gap excitation is similar to that of conventional a‐Si:H; it decays much slower in the form of a power law t −β (β<1), independent of spectral range between 1.4 and 2.2 eV. This shows that the e‐h distance after above‐gap photogeneration is larger than the size of the a‐Si:H clusters.

Picosecond and CW photomodulation spectroscopy of polysilane alloys

Abstract The CW and Picosecond Photomodulation techniques have been applied to thin films of polysilane-alloys. The Cw response shows a single photoinduced absorption band which is due to carriers trapped at DB defects. The picosecond decays are relatively fast exponentials in contrast to the slow power-low decays in regular a-Si:H. This is interpreted as fast geminate recombination in the a-Si:H clusters embedded in the polysilane matrix.