Ultrafast electronic and acoustic effects in conducting polymers

Journal ArticleThe polarized picosecond pump and probe technique with 50 fsec resolution has been used to generate and detect electronic excitations and propagation of ultrasonic phonons in thin films of oriented and unoriented polyacetylene and polythiophene. We found that the photoexcitations are instantaneously generated followed by a power law decay, which is interpreted as fast geminate recombination; only up to about 2% of the intrachain carriers are able to escape fast recombination

Phonon spectroscopy in π-conjugated polymers; the role of the excited electronic states

Journal ArticleThe Raman active vibrational modes in 7r-conjugated polymers are known to have different frequencies in different types of experiments. The best known examples are the resonant Raman scattering (RRS) and doping or photoinduced absorption infrared active vibrations (IRAV) measurements, in which the same modes appear at different energies. Other examples are the phonon side bands, accompanying the polarons, bipolarons, and solitons absorption bands, in which the same modes give rise to different side band energies. In this work we discuss RRS and IRAV measurements and correlate their experimentally observed frequencies with the relevant electronic state that is active in this type of experiment. We show that the IRAV frequencies are lower than those of RRS since the relevant energy levels of charged solitons and/or polarons are closer to the ground state than the optically allowed 1BU state of the neutral polymer. Thus a new insight into the meaning of the "pinning" potential, which is associated with the lowest frequency IRAV mode is obtained

Photoinduced IR active vibrations in trans-(CD)x: a three-mode system

Journal ArticleIn a recent Letter Blanchet et al.? have extended photoinduced absorption measurements of frYms-polyacetylene to lower energy than obtained before 2 for both isotopes, (CH)x and (CD)X. While in (CE)X three strong ir-active vibrations (IRAV) are photoinduced, only two IRAV were observed in (CD)X, at ?1 = 400 cm-? and at ?2 = 1045 cm-? . This appeared strange, since from doping-induced IRAV and from resonant Raman scattering (RRS) it is known that (CD)X is also a three-mode system.3'4 Because of the asymmetry of the mode at 1045 cm'?, it was suggested that two vibrations may be contained in this line, but high-resolution examination of the line failed to resolve it.

Optical probes of photoexcitations in fullerene thin films from femtoseconds to milliseconds

Journal ArticleWe have studied photoexcitations in various fiillerene thin films using transient photomodulation and photoluminescence from 100 fs to 50 ms and absorption-detected magnetic resonance (ADMR). We show that singlet Frenkel type excitons are the primary photoexcitations; their recombination kinetics in the picosecond time domain are dispersive as a result of inhomogeneity. The long-lived photoexcitations, however, are shown to be triplet excitons and charged polarons, identified by the correlation found between their associated optical transitions and ADMR signals with spin 1 and Vz, respectively

Ultrafast response of surface electromagnetic waves in an aluminum film perforated with subwavelength hole arrays

Journal ArticleThe ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength arrays of holes was studied in the visible spectral range by the technique of transient photomodulation with ~100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the optical response of nanohole arrays. The blueshift is inconsistent with plasmonic mechanism of extraordinary transmission and points to the crucial role of interference in the formation of transmission bands. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron-phonon relaxation dynamics, involving nonequilibrium hot electrons and quasiequilibrium phonons

Mechanically stretchable and reversibly deformable liquid metal-based plasmonics

pre-printWe demonstrate that liquid metals are attractive materials for active plasmonic devices at terahertz frequencies. Using a liquid metal injected into an elastomeric mold, we measure the static and stretched transmission properties of aperture arrays

Optical studies of metallodielectric photonic crystals: bismuth and gallium infiltrated opals

Journal ArticleWe have fabricated three-dimensional metallodielectric photonic crystals that consist of bismuth and gallium metals infiltrated into porous synthetic opals that have 300 nm diam silica balls. The specular reflectivity was measured in a broad spectral range from 0.3 to 25 mm using a variety of light sources, detectors and spectrometers. In addition to broadening of the original Bragg stop bands in the visible spectral range that give rise to iridescence colors, we also found a dramatic decrease of reflectivity in the visible/near-infrared spectral range. For frequencies below a cut-off frequency in the mid-infrared range the reflectivity increases to its normal bulk value in the respective metal, as predicted by theory and fitted by a model calculation

Midinfrared optical response and thermal emission from plasmonic lattices on Al films

Journal ArticleWe studied the midinfrared optical transmission and thermal light emission spectra of subwavelength hole arrays in the form of square lattice with 4 µm periodicity (plasmonic lattice) in aluminum films deposited on silicon substrates. The optical transmission of these films showed temperature independent resonance bands and antiresonance dips in the midinfrared spectral range, which could be explained by a model involving light coupling to surface plasmon polaritons (SPPs) on the two film interfaces. We fitted the transmission spectrum using a dielectric response function that is based on an effective plasma frequency determined by the individual holes and resonant modes associated with the reciprocal vectors in the lattice structure factor; subsequently, we also calculated the absorption spectrum. We found that the absorption spectrum shows bands opposite in phase compared to those in the transmission spectrum, where peaks are replaced by dips and vice versa. The thermal emissivity spectrum of the heated perforated films were measured at elevated temperatures and showed resonant bands similar to those in the transmission spectrum rather than the absorption spectrum, in apparent contradiction to Kirchhoff's law of radiation. We thus conclude that the perforated films act primarily as radiation filters, where thermal emission is suppressed for frequencies outside the resonant transmission bands. This optical filtering is characteristic of weak photonic crystals, where the photon density of states in the heated metal film is only weakly modified by the perforated interfaces, suggesting weak interaction between the SPP excitations on the metal/dielectric interfaces and the photonic states inside the metal film

Optical and magneto-optical studies of two-dimensional metallodielectric photonic crystals on cobalt films

Journal ArticleWe studied the optical transmission and magneto-optical effect through a subwavelength hole array fabricated on a ferromagnetic cobalt (Co) thin film in comparison to a control unperforated Co film having the same thickness. We found that the perforated film sustains extraordinary transmission bands through the hole array, which can be well explained as due to light coupling to surface plasmons on the two film interfaces. We also found that due to resonant coupling to the surface plasmons, the magneto-optical Kerr effect in the spectral range of the anomalous transmission bands of the perforated Co film is much smaller than that in the control Co film

Ultrafast dynamics in metallic and semiconducting carbon nanotubes

Journal ArticleTransient polarized pump/probe transmission changes upon pulse photoexcitation at 1.6 eV were measured in chiral enriched carbon nanotubes ensembles of both metallic and semiconductor characters, in a very broad probe spectral range from mid-IR to visible, combining several ultrafast laser systems. We identify a photoinduced absorption band at ~0.7 eV as due to hot Dirac fermions in the metallic tubes and measure its dynamics. The hot carriers equilibrate within the electronic system in ~400 fs and with the lattice within ~2 ps; in agreement with hot Dirac fermions thermalization in graphene. We also assign excitonic transitions in the semiconducting tubes, mainly of (6,5) chirality; in particular, an intraexciton transition at ~0.4 eV represents a lower limit for the exciton binding energy