Photoinduced Absorption Signal Research Articles

Synthesis and optical properties of a novel polyfluorene derivative

We report the synthesis and optical properties of a novel polyfluorene derivative: poly(3,5-dimethylphenyl ester of 4-(9-methyl-9H-fluorene-9-yl)-butane-1-sulfonic acid) (PFS). Optical spectroscopies such as absorption, photoluminescence, cw and transient photoinduced absorption (PA) techniques were used to study the optical properties. The absorption spectrum shows a weaker band at 3.5 eV and also contains strong bands peaked at 3.95, 4.1 and 4.3 eV. The PL spectrum shows a structureless band centered at 2.8 eV. The PA spectrum at 80 K shows a broad high energy (HE) band peaked at 2.1 eV and three low energy (LE) bands located at 0.5, 0.71 and 1.05 eV. The recombination dynamics of the photoexcitations at 2.1 eV was studied by measuring the intensity and frequency dependence of the PA signal and by fitting the results using a bimolecular recombination (BR) process. We have also studied the temperature dependence of the transient decay of the HE PA band from 80 to 290 K. The BR coefficient was approximately constant from 80 to 220 K and showed an Arrhenius temperature dependence with an activation energy of 0.24 eV for T>240 K.

The photophysics of triplet excitons in substituted polycarbazolyldiacetylenes

Absorption and photoinduced absorption (PIA) spectra of thin films of polycarbazolyldiacetylenes carrying alkyl (polyDCHD-HS) or acyl (polyDPCHD) substitutions are reported. The absorption spectra are dominated by an excitonic absorption followed by a vibronic progression, and are quite insensitive to the temperature. The PIA spectra show the typical spectral feature of a triplet excitation whose rise and decay times are different. The PIA signal shows a strong sublinear pumping intensity dependence, which also affects the rise time.

Infrared photo-induced absorption spectroscopy of porous silicon

The quantum confinement model, which assigns some of the luminescence features in porous silicon to size quantization in Si nano-crystallites, also predicts quantization of both the conduction and valence bands into sub-levels. In order to resolve this effect we have used a new experimental technique called “photo-induced infrared absorption spectroscopy”. Here, a pump, visible laser, optically induces carriers in the conduction/valence band. Optical transitions between the quantized sub-levels are resolved by a probe, infrared beam in the energy range 50–300 meV. A broad photo-induced absorption signal has been observed in the 60–250 meV spectral range, in agreement with the prediction of the quantum confinement model. However, the photo-induced absorption signal decreases with the decreasing temperature, resolving activation energy of about 10 meV. This behavior can be understood if the allowed optical transitions are from the exciton singlet state only. Also, we found additional features in the photo-induced absorption spectrum that are correlated with the Si=O vibrational modes. Our results indicate a strong coupling between bulk excitonic states and surface states in small Si nano-crystallites.

Optical studies of excited-state relaxation in poly(9,9-dihexylfluorene-co-benzothiadiazole)

We have studied the optical properties of poly(9,9-dihexylfluorene-co-benzothiadiazole) using absorption, photoluminescence as well, as cw and transient photoinduced absorption (PA) techniques. The PA spectrum shows a high-energy (HE) band peaked at 1.44 eV accompanied by shoulders on both sides. To study the recombination kinetics of the photoexcitations associated with the HE band we measured the intensity and frequency dependence of the PA signal, and modeled the results by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that the experimentally observed linear intensity dependence at low pump intensities can be explained by including a monomolecular term into the rate equation, in contrast to the superlinear intensity dependence predicted for pure bimolecular recombination. Furthermore, the value of the monomolecular lifetime $\ensuremath{\tau}$ can directly be estimated from the ratio of the in-phase and out-of-phase signals in the linear regime, and we also show how to estimate the bimolecular recombination coefficient from the saturation value of the out-of-phase signal. To study the temperature dependence of the recombination process we measured the transient decay of the HE band, and by fitting the results to the full bimolecular rate equation we extracted the temperature dependence of the amplitude, the monomolecular lifetime, and the bimolecular recombination coefficient. The monomolecular lifetime decreases almost linearly with increasing temperature, and the bimolecular recombination coefficient has a very strong non-Arrhenius temperature dependence i.e., $\ensuremath{\beta}={\ensuremath{\beta}}_{0}\mathrm{exp}[{(T/T}_{0}{)}^{2}],$ with ${\ensuremath{\beta}}_{0}=1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{3}/\mathrm{s}$ and a characteristic temperature ${T}_{0}=82\mathrm{K}.$ A model for this unusual temperature dependence is proposed.

Recombination dynamics in regiorandom poly(3-hexylthiophene)

Using cw photoinduced absorption (PA) techniques we study the photoexcitation dynamics in the ms time regime, in thin films of regiorandom poly(3-hexylthiophene). The PA spectrum shows a high energy band (HE) peaked at 1.46eV and a low energy band (LE) at 0.5eV. The PA dependence on pump intensity and modulation frequency is modeled by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that in the steady-state limit (i.e. ωτ B «1), the out-of-phase PA signal saturates and becomes independent of pump intensity. This gives rise to a new possibility of directly estimating the bimolecular recombination coefficient β from the saturation value of the out-of-phase PA signal at high pump intensities. The decay of the HE band is attributed to a bimolecular recombination process with β ≃ 0.6s * 10 -14 cm 3 /s.

Bimolecular recombination in regiorandom poly(3-hexylthiophene)

Using cw and transient photoinduced absorption (PA) techniques we study the photoexcitation dynamics in the ms time regime, in thin films of regiorandom poly(3-hexylthiophene). The PA spectrum shows a high energy band (HE) peaked at 1.46 eV and a low energy band (LE) at 0.5 eV. The PA dependence on pump intensity and modulation frequency is modeled by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that in the steady-state limit (i.e., xsB � 1), the out-of-phase PA signal saturates and becomes independent of pump intensity. This gives rise to a new possibility of directly estimating the bimolecular recombination coefficient b from the saturation value of the out-of-phase PA signal at high pump intensities. The transient PA decay of the HE band shows two components having different relaxation times. The PA decay of the HE band is mainly dominated by the fast component, which is attributed to a bimolecular recombination process in good agreement with the result from the cw measurements, together yielding a bimolecular recombination constant b ¼ 1:6 � 1:4 � 10 � 14 cm 3 =s. 2002 Elsevier Science B.V. All rights reserved.

Intersubband and interminiband spectroscopy of doped and undoped CdS/ZnSe multiple quantum wells and superlattices

In cubic CdS/ZnSe type-II heterostructures collective excitations have been studied using infrared spectroscopy. The CdS/ZnSe structures were grown by solid-source molecular-beam epitaxy on semi-insulating GaAs substrates. Highly n-type-doped multiple-quantum-well and superlattice samples show strong intersubband and interminiband absorption in the midinfrared. The validity of the polarization selection rule is verified experimentally. The CdS/ZnSe conduction band offset is determined using a combination of interband and intersubband spectroscopy. Measured transition energies agree well to model calculations if many-body effects and band nonparabolicity are included. Intensity-dependent pump and probe measurements on doped and undoped samples reveal a fast increase of the photoinduced absorption signal at low pump intensities. At high pump intensities the absorption signal saturates. This behavior is explained by the existence of a subgroup of long-lived photogenerated electron-hole pairs. An observed redshift of the photoinduced interminiband transitions is explained by filling of the lowest miniband. The effective electron mass of cubic CdS is determined from thick films using infrared reflection spectroscopy.

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) ${N}_{0}$ dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons (SE's) and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-$\frac{1}{2}$ magnetic resonance conditions reduce the total polaron population. Since the sublinear ${N}_{0}$ dependences of the positive (PL-enhancing) spin-$\frac{1}{2}$ PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent \ensuremath{\pi}-conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.

Ultrafast relaxation dynamics of neutral soliton pairs in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex [Pt(en)2][Pt(en)2Br2](ClO4)4

Ultrafast optical response of photoexcited states is studied by femtosecond pump and probe spectroscopy in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex, [Pt(en)2][Pt(en)2Br2](ClO4)4 (en=ethylenediamine). We observed two kinds of photo-induced absorption (PA) signals, and both of the PA signal intensities follow non-exponential decay profiles. The signals are ascribed to two kinds of neutral soliton pairs with different configurations, that is, a spatially confined soliton and antisoliton pair and a noncorrelated soliton and antisoliton pair. Their formations are found to be closely related with both the excitation energy and the excitation density.

Investigation on relaxation dynamics of photoexcitations and spectral narrowing of photoluminescence in PPV films

Photoinduced absorption spectral feature as well as their temporal profiles on PPV thin film in femtosecond time regime exhibit a strong pump power density dependence. The increase in pump power density also results in the spectrally narrowed emission (SNE) band superimposed to the well-known photoluminescence from PPV. The dependencies on both pump- and probe densities denote much lower saturation intensity in photoinduced absorption as compared with that in stimulated emission from the identical primary photoexcitations, the singlet excitons. We have also discussed the nature of SNE, which has a correlation with an appearance of ultrafast decay component (τ1 ps) in temporal decay profile of the photoinduced absorption signal.

Photoinduced absorption measurements of triplet states in poly(p-pyridine)

Abstract We report cvv photoinduced absorption (PIA) measurements of poly( p -pyridine) (PPY). Three correlated, induced features are found below the π → π * transition. All have a strong temperature dependence and similar lifetimes of ca 10 ms. Comparing to ZINDO/S-CI calculations of the triplet manifold we find good agreement and so conclude that the PIA represents transitions within the triplet manifold. However, as yet we have not been able to definitively measure a ν −1 chop rate dependence for the PIA signals indicative of monomolecular recombination.

Solitons, Polarons, and Excitons in Quasi-One-Dimensional Halogen-Bridged Transition Metal Compounds

Abstract We report the photogeneration of solitons and polarons in the quasi-one-dimensional (1-D) halogen (X)-bridged metal (M) compounds (simply abbreviated as the MX chain compounds). The most significant feature of this system is the remarkable tunability of the charge density wave (CDW) ground states. By substituting the metals (M = Pt, Pd, and Ni), the bridging halogens (X = Cl, Br, and I), the ligand molecules and the counter anions surrounding the 1-D chains, the amplitude of CDW, the optical gap energy, and the degeneracy of CDW can be widely controlled. On the basis of these controls, we have investigated the nature of photoexcited states. By comparing the results of photoinduced absorption (PA), ESR and photoinduced ESR measurements in the degenerate CDW states with those in the non-degenerate CDW states, we clearly characterized the photoinduced gap states as solitons and polarons. In the compounds having relatively large optical gap energies (ECT), spin-solitons and polarons are photogenerated. From a comparison of the excitation profiles of the PA signals with those of the luminescence of the self-trapped exciton (STE), it was demonstrated that the luminescence process strongly competes with the dissociation to spin–soliton pairs. An analysis of the temperature dependence of the luminescence decay time revealed that the conversion from an STE to a solitonic state occurs through a finite potential barrier, the magnitude of which depends on degeneracy of CDW. With decrease of ECT, the nature of the photoexcited states changes considerably; photogenerations of charged-solitons are observed instead of spin–solitons and the STE luminescence is remarkably quenched. While referring to the theoretical expectations based upon the Peierls–Hubbard model, we will discuss the overall view of the relaxation process of the photoexcited states related to solitons, polarons, and excitons in the MX compounds.

Dynamics of photoinduced gap states and self-trapped excitons in the MX chain compounds with degenerate and non-degenerate CDW ground states

We have characterized the photoinduced gap states from the photoinduced absorption (PA) spectroscopy in the halogen (X)-bridged metal (M) complexes having degenerate CDW state, ([Pt(en) 2][Pt(en) 2X 2](ClO 4) 4 : XCl, Br) and those having non-degenerate CDW state, ([Pd(en) 2][Pt(en) 2X 2](ClO 4) 4 : XBr, I). In the former complexes, observed are the two midgap bands named as A and B, which have been assigned to spin-solitons, while in the latter complexes, photogenerations of solitons are suppressed. Photogenerations of polarons have been detected in both types of complexes. We have also investigated the excitation profiles of the luminescence from the self-trapped excitons, which are compared with those of the PA signals due to the gap states. The results demonstrate that the luminescence process strongly competes with the dissociation process to spin-soliton pairs followed by nonradiative recombinations.

Photoinduced absorption and excitation spectra studies of excited states in a cyano-derivative of poly ( p-phenylenevinylene)

We have identified in a cyano-substituted derivative of poly ( p-phenylenevinylene) a broad photoinduced absorption signal at 1.3 eV from a photoexcited state with a lifetime of 0.2 ms at 20 K, which we assign to the triplet exciton. We present here excitation spectra in the range 2–5 eV of the triplet-triplet photoinduced absorption, and photoluminescence signals, which are both broadly flat above the onset of the linear absorption. From these results we conclude that quantum efficiencies of singlet and triplet exciton generation are constant over the excitation range.

Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-type ladder polymer

The dynamics of optical excitations in a PPP-type ladder polymer have been investigated by means of femtosecond pump-probe experiments. Stimulated emission (SE) is observed around the S 1 → S 0 transition while photoinduced absorption (PA) dominates the differential transmission at lower energies. The stimulated emission is due to singlet exciton recombination. Comparative studies of the temporal behaviour of the SE and the PA demonstrate that the SE and the PA signals originate from different species. Our results provide evidence that the PA is due to spatially separated electron-hole pairs (indirect excitons).

Femtosecond Nonlinear Transmission Study of Free-Standing Porous Silicon Films

AbstractLarge photo-induced absorption signals were observed in free standing porous silicon films in the spectral range from 1.1 to 2.5 eV using a femtosecond pump and probe technique. The measured nonlinear signal has a very fast component with relaxation constants from 800 fs to tens of picoseconds superimposed on the slow-relaxing thermal background. The spectral structure and relaxation dynamics of the short-lived component of transient absorption show the presence of molecular-like Si complexes with well defined energy levels and spectrally uniform picosecond relaxation dynamics.

Photoexcitations in polycarbazolyldiacetylenes in different time regimes

We present the results of photoinduced absorption (PA) studies carried out on oligomers of 1(-N-carbazzolyl)penta-1,3-diyn-5-acetoxy (CPDA). From the temperature, laser intensity and chopper frequency dependence of the PA signals upon excitation at 488 nm, we have demonstrated that in the msec-sec range the dominant photocarries are charged bipolarons. The picosecond decay kinetics have been obtained on CPDA oligomer solutions by transient saturated absorption at 627 nm, measured by femtosecond pump and probe technique. The decay curves could be fitted to biexponential functions. These results are discussed in terms of the relaxation model proposed for other polydiacetylenes.

Ultrafast spectroscopy of conducting polymers

We have used various experimental techniques in the femtosecond and picosecond time domains to study the ultrafast time response, the optical nonlinearities and the character of the fundamental photoexcitations in conducting polymers with (polyacetylene) and without (polythiophene, polydiacetylene) degenerate ground states. Using the technique of photoinduced absorption (PA) we found that photoexcitations in conducting polymers become self trapped in about 300 fs. They form charged and neutral solitons in polyacetylene and polarons and singlet or triplet excitons in other polymers. Due to the existence of disorder and inhomogeneity in conducting polymers thin films, the dynamics of the PA signal, which is due to recombination, is in the form of a power law t −α (α < 1) or stretched exponential exp [−( t/τ) β] (β < 1). The decays are sensitive to high pressure showing that the recombination is dominated by 3D defects in the chains, which act as recombination centers. We have also measured the magnitude and dynamical response of the third order nonlinear optical coefficient χ( 3) in conducting polymers thin films,using the femtosecond degenerate four wave mixing (DFWM) technique. χ( 3) was in the range of 10 −11 to 10 −8 esu depending on the resonance excitation conditions. Also the dynamical response of χ( 3) for resonant and non-resonant conditions is categorically different. We found, however, that the dephasing time of the photoexcitations (< 50 fs) does not depend on resonance excitation conditions.

Photo-Generation of Solitons and Polarons in the Quasi-1-D MX Compounds

The effect of interchain interaction on the nonlinear excitations in the one-dimensional (1-D) Peierls-Hubbard system is investigated by the photo-induced absorption (PA) measurements of the halogen-bridged metal complexes (the MX compounds; M = Pt and X = Br or I), changing the strength of the interchain coupling via the hydrogen-bonds between the charge- density wave (CDW) states. Systematic analysis of the PA signals provides a clear evidence for photogeneration of solitons and polarons which have been theoretically predicted. From the excitation profile of the PA signals, informations are obtained useful for general understanding of the relaxation processes of photo-excited states and the excitonic effect in the 1-D CDW state

Photogeneration of solitons and polarons in 1-D halogen-bridged metal complexes

The photoinduced ir absorption spectra of the halogen-bridged metal complexes have been measured on single crystals with different dimensionalities of chain structures. A decrease of the interchain coupling via the hydrogen bonds leads to the appearance of a new band, which is attributable to photogeneration of solitons. The structures assigned to photogenerated polarons are rather insensitive to the intechain coupling. From the excitation spectra of the photoinduced absorption signals, it has been found that the excitonic effect completely quenches photogeneration of these gap states.