Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range.

Trans-polyacetylene [t-(CH)_{x}] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t-(CH)_{x}; this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1-1.5 eV) in neat t-(CH)_{x} thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t-(CH)_{x}, similar to many other nondegenerate π-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at ∼1.5 eV having similar polarization memory. The primary excitons undergo internal conversion within ∼100 fs to an even-parity (dark) singlet exciton with a PA band at ∼1.4 eV. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.

Circular photogalvanic spectroscopy of Rashba splitting in 2D hybrid organic-inorganic perovskite multiple quantum wells.

The two-dimensional (2D) Ruddlesden-Popper organic-inorganic halide perovskites such as (2D)-phenethylammonium lead iodide (2D-PEPI) have layered structure that resembles multiple quantum wells (MQW). The heavy atoms in 2D-PEPI contribute a large spin-orbit coupling that influences the electronic band structure. Upon breaking the inversion symmetry, a spin splitting ('Rashba splitting') occurs in the electronic bands. We have studied the spin splitting in 2D-PEPI single crystals using the circular photogalvanic effect (CPGE). We confirm the existence of Rashba splitting at the electronic band extrema of 35±10 meV, and identify the main inversion symmetry breaking direction perpendicular to the MQW planes. The CPGE action spectrum above the bandgap reveals spin-polarized photocurrent generated by ultrafast relaxation of excited photocarriers separated in momentum space. Whereas the helicity dependent photocurrent with below-gap excitation is due to spin-galvanic effect of the ionized spin-polarized excitons, where spin polarization occurs in the spin-split bands due to asymmetric spin-flip.

Structural, electronic, and magnetic properties of tris(8-hydroxyquinoline)iron(III) molecules and their magnetic coupling with ferromagnetic surface: first-principles study.

Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial (fac-) and meridional (mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction with ferromagnetic substrate. Our calculation results show that for the isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; both Feq3 isomers have a high spin-state of 5 µB as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3d electrons; while the standard DFT calculations produce a low spin-state of 1 µB for mer-Feq3. These magnetic behaviors can be understood by the octahedral ligand field splitting theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between Fe and Co substrate, originating from the superexchange coupling between Fe and Co mediated by the interface oxygen and nitrogen atoms. These findings suggest that Feq3 molecular films may serve as a promising spin-filter material in spintronic devices.

Exceptional gain for the stimulated resonant Raman scattering in the π-conjugated polymer poly(dioctylfluorene).

Stimulated resonant Raman scattering (SRRS) is observed in slab waveguides formed in thin films of the π-conjugated polymer poly(dioctylfluorene) (PFO). The presence of two distinct morphological phases with different electronic bandgaps in these films allows the output frequency of this process to be observed over a much broader range than in other organic materials. In particular, the SRRS peak is pronounced when it is spectrally located in the vicinity of the amplified spontaneous emission bands of the films, which peak at 449 and 463 nm, respectively, for the two different phases, allowing selective tuning in the range of 440-470 nm. Furthermore, the superposition of the SRRS spectral location with that of electronic gain produces an overall gain of 269 cm(-1), making this material extremely attractive for use as an active material in compact Raman lasers.

Organic electronics. Room-temperature coupling between electrical current and nuclear spins in OLEDs.

The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

Ultrafast intersystem-crossing in platinum containing π-conjugated polymers with tunable spin-orbit coupling.

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

Spatially mapping random lasing cavities.

A mapping technique is developed to spatially resolve random laser-emission spectra from disordered solid media with an optical gain above the threshold excitation intensity for lasing; the technique is applied to pi-conjugated polymer 1 ms. By mapping the spatial extent of emission peaks in the random laser spectrum, bright areas that correspond to naturally formed lasing microcavities are unraveled. The size of the obtained microcavities matches the size extracted from the Fourier transform analysis of the laser-emission spectrum. Mapping at increased excitation intensities shows multiple resonant microcavities that lase at increasing threshold intensities.

Magnetoconductance response in unipolar and bipolar organic diodes at ultrasmall fields.

We measured magnetoconductance (MC) response in a number of unipolar and bipolar organic diodes based on π-conjugated polymers and small molecules at fields |B|<100 mT and various bias voltages and temperatures. Similar to magneto-electroluminescence, the MC(B) response in bipolar diodes shows a sign reversal at ultrasmall |B|<1-2 mT due to interplay of hyperfine and Zeeman interactions in opposite-charge polaron pairs. Surprisingly, similar MC(B) response was also measured in unipolar devices, indicating the existence of like-charge polaron pairs, however, with a clear difference between the hyperfine interaction constants of electron polaron and hole polaron.

Terahertz spectroscopy of plasmonic fractals.

We use terahertz time-domain spectroscopy to study the transmission properties of metallic films perforated with aperture arrays having deterministic or stochastic fractal morphologies ("plasmonic fractals"), and compare them with random aperture arrays. All of the measured plasmonic fractals show transmission resonances and antiresonances at frequencies that correspond to prominent features in their structure factors in k space. However, in sharp contrast to periodic aperture arrays, the resonant transmission enhancement decreases with increasing array size. This property is explained using a density-density correlation function, and is utilized for determining the underlying fractal dimensionality, D(<2). Furthermore, a sum rule for the transmission resonances and antiresonances in plasmonic fractals relative to the transmission of the corresponding random aperture arrays is obtained, and is shown to be universal.

Below-gap excitation of pi-conjugated polymer-Fullerene blends: implications for bulk organic heterojunction solar cells.

We used a variety of optoelectronic techniques such as broadband fs transient and cw photomodulation spectroscopies, electroabsorption, and short-circuit photocurrent in bulk heterojunctions organic solar cells for studying the photophysics in pi-conjugated polymer-fullerene blends with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.

Engineering the dielectric function of plasmonic lattices.

We have systematically measured epsilon(omega) of subwavelength aperture arrays fabricated in metal films as a function of aperture size and incidence angle using terahertz time-domain spectroscopy. This approach simultaneously yields both the real and imaginary epsilon(omega) components, enabling deeper insight into the underlying mechanism of the 'enhanced optical transmission' (EOT) phenomenon. For random aperture arrays we find that epsilon(omega) has a plasma response, with an effective plasma frequency that is determined by the waveguide mode cutoff frequency of the individual apertures. However epsilon(omega) in plasmonic lattices is strongly modulated at discrete resonant frequencies that correspond to the reciprocal vectors in the structure factor that are superposed on the plasma envelope response and appear as dips in the EOT spectrum. The existence of a sum rule for the discrete resonance oscillator strengths when the aperture size or incidence angle are changed validates our approach and allows for engineering of the individual resonances in the EOT spectrum.

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

The 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 approximately 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.

Polaron spin-lattice relaxation time in pi-conjugated polymers from optically detected magnetic resonance.

We describe a method for obtaining the polaron spin-lattice relaxation time T{SL} in pi-conjugated polymers by measuring the optically detected magnetic resonance (ODMR) dynamics as a function of microwave power and laser intensity. The peculiar ODMR dynamics is well described by a spin dependent recombination model where both recombination and spin relaxation rates determine together the response dynamics. We apply this method to the spin 1/2 ODMR in films of pristine 2-methoxy-5-(2{'}-ethylhexyloxy) phenylene vinylene [MEH-PPV] polymer, as well as MEH-PPV doped with various concentrations of radical impurities. We obtained T{SL} approximately 30 micros in pristine MEH-PPV, but substantially shorter when the magnetic impurities are added.

Magnetic-field-dependent carrier injection at La2/3Sr1/3MnO3/ and organic semiconductors interfaces.

We have fabricated organic diodes utilizing several pi-conjugated organic semiconductors (OSEC) as spacer layers between La2/3Sr1/3MnO3 (LSMO) and various metallic electrodes, and measured their magnetoresistance (MR) and magnetoelectroluminescence (MEL) responses. The devices exhibit large negative high-field MR responses that resemble the MR response of the LSMO electrode, but amplified by approximately 3 orders in the resistance, and accompanied by a positive high-field MEL effect. These magnetic-field effects result from enhanced carrier injection at the LSMO-OSEC interface that is attributed to the anomalous field-dependent Fermi level shift in LSMO.

Ultrafast spectroscopy of excitons in single-walled carbon nanotubes.

We studied the femtosecond dynamics of photoexcitations in films containing semiconducting and metallic single-walled carbon nanotubes (SWNTs), using various pump-probe wavelengths and intensities. We found that confined excitons and charge carriers with subpicosecond dynamics dominate the ultrafast response in semiconducting and metallic SWNTs, respectively. Surprisingly, we also found from the exciton excited state absorption bands and multiphoton absorption resonances in the semiconducting nanotubes that transitions between subbands are allowed; this unravels the important role of electron-electron interaction in SWNT optics.

Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers.

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.

Photoinduced quantum interference antiresonances in pi-conjugated polymers.

We observed photoinduced quantum interference antiresonances between several discrete infrared-active vibrations and the lower-polaron continuous absorption band in a series of pi-conjugated polymer films having superior planar orders, where the polaron transition energy is relatively small. The photoinduced Fano-type antiresonances are well explained by extending the amplitude mode model beyond the adiabatic limit. The agreement between the data and the model confirms the presence of a continuous electronic band above the polaron state. We show that high frequency modes are strongly coupled to electrons, with implications for superconductivity.

Conjugation-length dependence of spin-dependent exciton formation rates in pi-conjugated oligomers and polymers.

We have measured the ratio, r = sigma(S)/sigma(T) of the formation cross section, sigma of singlet and triplet excitons from polarons in pi-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r(-1) depends linearly on CL-1, irrespective of the chain structure. Since r is directly related to the maximum possible electroluminescence quantum efficiency in organic light emitting diodes (OLED), our results indicate that polymers have an advantage over small molecules in OLED applications.