Non-canonical statistics of finite quantum system

The canonical statistics describes the statistical properties of an open system by assuming its coupling with the heat bath infinitesimal in comparison with the total energy in thermodynamic limit. In this paper, we generally derive a non-canonical distribution for the open system with a finite coupling to the heat bath, which deforms the energy shell to effectively modify the conventional canonical way. The obtained non-canonical distribution reflects the back action of system on the bath, and thus depicts the statistical correlations through energy fluctuations

Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications

Journal ArticleWe used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.15 to 2.25 eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ~20 ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent

Exciton dynamics in single-walled nanotubes: transient photoinduced dichroism and polarized emission

Journal ArticleUltrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, t, follow a power law, std−a with a in the range of 0.7 to 1. Whereas the PA bands in S-NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition S-NTs in D2O solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum

Nonlinear optical spectroscopy of excited states in disubstituted polyacetylene

Journal ArticleWe used a variety of nonlinear optical (NLO) spectroscopies for studying the excited states order and primary photoexcitations in disubstituted polyacetylene (DPA). The NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of 0.2-2.5 eV, two-photon absorption in the range of 3.2-4.2 eV, and electroabsorption (EA) in the range of 2.3-4.1 eV. For completeness we also measured the linear-absorption and photoluminescence spectra. We found that the primary photoexcitations in DPA are singlet excitons with three photoinduced absorption (PA) bands, PA0 at 0.6 eV, PA1 at 1 eV, and PA2 at 1.6 eV, respectively, and a stimulated emission band at ~2.5 eV. This is in contrast to other rr-conjugated polymers where the singlet excitons are characterized by two PA bands and a stimulated emission (SE) band. We conclude that in addition to the four essential excited states known to determine the NLO spectra of rr-conjugated polymers, namely, 1Ag, 1Bu, mAg, and nBu; in DPA there is another important complex of states at ~3.3 eV that contains almost degenerate states with odd and even parity symmetries (dubbed here transverse excited state or TES) that is probably reminiscent of the 2Ag state in unsubstituted trans-polyacetylene, which lies in between the 1Bu and mAg of which dipole moment has a strong component perpendicular to the chain direction. In support for this model we found that the depolarization ratio of the EA spectrum drastically changes around the TES from a value of ~2 for һω3.3 eV

Infrared ultrafast optical probes of photoexcitations in ∏-conjugated organic semiconductors

Journal ArticleWe measured the ultrafast dynamics of photoexcitations in a variety of semiconductor ∏-conjugated polymer films and solutions, in the spectral range from 0.13 eV to 1.05 eV and time domain from 100 fs to 800 ps. The measurements were made in the low signal regime, where the relative changes in transmission, ΔT/T <10-5. In pristine poly [2-methoxy-5(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) solution we found that the primary photoexcitations are excitons with a photoinduced absorption (PA) band PA1 at 1.0 eV. In pristine MEH-PPV film, however we found two PA bands, PA1 at 1.0eV and P1 at 0.3eV. We consider P1 to be due to a polarons that are photogenerated intrinsically via defects and/or impurities. When following the transient dynamics of PA1 in C60-doped PPV-based films we could easily measure the dynamics of exciton dissociation from the polymer chains onto the C60 molecules. We found the charge transfer time to range from about 100 fs to 100 ps, depending on the concentration of C60 molecules in the films

Comment on "Large optical nonlinearity of semiconducting single-walled carbon nanotubes under resonant excitations"

Journal ArticleIn a recent Letter Maeda et al. studied the optical nonlinearities of single-walled carbon nanotubes (SWNT) films using both ??Z-scan?? and ??pump and probe?? (PP) techniques with ~100 fs time resolution [1]. They found that SWNTs have a large resonant third-order nonlinear optical susceptibility (X(3)), which was interpreted in terms of ??coherent process rather than by saturation of absorption,?? possibly due to ??optical Stark effect and stimulated emission.?? This interpretation was based on degenerate PP measurements, in which an ultrafast response was found for t < tp (pulse duration), superimposed on a slower response with ~1 ps time constant. Importantly, the ultrafast response was not found in nondegenerate PP measurements [1,2]

Ultrafast spectroscopy of excitons in semiconducting carbon nanotubes

Journal ArticleUltrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D 2 0 solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t)"a, with a in the range of 0.7 to 1. The PL emission of S-NTs in D2 0 solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation