Sociality does not drive the evolution of large brains in eusocial African mole-rats

The social brain hypothesis (SBH) posits that the demands imposed on individuals by living in cohesive social groups exert a selection pressure favouring the evolution of large brains and complex cognitive abilities. Using volumetry and the isotropic fractionator to determine the size of and numbers of neurons in specific brain regions, here we test this hypothesis in African mole-rats (Bathyergidae). These subterranean rodents exhibit a broad spectrum of social complexity, ranging from strictly solitary through to eusocial cooperative breeders, but feature similar ecologies and life history traits. We found no positive association between sociality and neuroanatomical correlates of information-processing capacity. Solitary species are larger, tend to have greater absolute brain size and have more neurons in the forebrain than social species. The neocortex ratio and neuronal counts correlate negatively with social group size. These results are clearly inconsistent with the SBH and show that the challenges coupled with sociality in this group of rodents do not require brain enlargement or fundamental reorganization. These findings suggest that group living or pair bonding per se does not select strongly for brain enlargement unless coupled with Machiavellian interactions affecting individual fitness.The Czech Science Foundation (14–2758 S, to P.N.), Grant Agency of Charles University (325515, to K.K.) and the European Social Fund and the state budget of the Czech Republic (CZ.1.07/2.3.00/30.0022, to S.O.).http://www.nature.com/srepam2018Mammal Research InstituteZoology and Entomolog

Propagation of terahertz pulses in photoexcited media: Analytical theory for layered systems

Optical pump-terahertz probe spectroscopy has become a widely used experimental tool for the investigation of the ultrafast far-infrared response of polar systems. In this paper the authors present an analytical method of calculating the propagation of ultrashort terahertz pulses in photoexcited media. The transient terahertz wave form transmitted through the sample is equal to a product of the incident terahertz field (at a mixed frequency), transient susceptibility, and a so called transfer function which depends on the properties of the sample in equilibrium. The form of the transfer function is derived for general layered systems and for specific cases including one-dimensional photonic crystals, thin films, and bulk samples. Simplified expressions directly applicable to the analysis of the experimental results related to the most common sample geometries are shown and discussed. (C) 2007 American Institute of Physics

Charge transport in nanostructured materials for solar energy conversion studied by time-resolved terahertz spectroscopy

Spectra of far-infrared conductivity contain useful information on charge transport at nanoscopic length scales. However. decrypting the mechanisms and parameters of charge transport from the measured spectra is a complex task in nanostructured systems in particular, the conductivity is strongly influenced by charge carrier interaction with surfaces or interfaces between constituents of the composite material as well as by local field effects Here we review our work on transient far-infrared conductivity in polymer fullerene bulk heterojunctions and in bare and dye-sensitized semiconductor nanoparticles Measurements performed by time-resolved terahertz spectroscopy are complemented by Monte-Carlo calculations which clearly link the charge transport properties and the terahertz conductivity spectra. (C) 2010 Elsevier B.V. All rights reserve

Far-infrared response of free charge carriers localized in semiconductor nanoparticles

A Monte Carlo method is employed to calculate the dynamical conductivity in the terahertz range of free charge carriers localized in semiconductor nanoparticles. The shape of the conductivity spectrum is essentially determined by the probability of carrier transition through interparticle boundaries and by the ratio of the nanoparticle size and carrier mean free path in the bulk. It is shown that the conductivity spectrum exhibits similar features as the classical extension of the Drude conductivity of electrons proposed by Smith [Phys. Rev. B 64, 155106 (2001)]. We find and discuss the link of this model to the results of our simulations which suggests an interpretation of the phenomenological parameters of the Drude-Smith model

Fast one-dimensional photonic crystal modulators for the terahertz range

Optically controlled one-dimensional photonic crystal structures for the THz range are studied both theoretically and experimentally. A GaAs:Cr layer constitutes a defect in the photonic crystals studied; its photoexcitation by 800 nm optical femtosecond pulses leads to the modulation of the THz beam. Since the THz field can be localized in the photoexcited layer of the photonic crystal, the interaction between photocarriers and THz light is strengthened and yields an appreciable modulation of the THz output beam even for low optical pump fluences. Optimum resonant structures are found, constructed and experimentally studied. The dynamical response of these elements is shown to be controlled by the lifetime of THz photons in the resonator and by the free carrier lifetime. The time response of the structures studied is shorter than 330 ps. (c) 2007 Optical Society of America

Ultrafast opto-terahertz photonic crystal modulator

We present an agile optically controlled switch or modulator of terahertz (THz) radiation. The element is based on a one-dimensional photonic crystal with a GaAs wafer inserted in the middle as a defect layer. The THz electric field is enhanced in the photonic structure at the surfaces of the Ga-As wafer. Excitation of the front GaAs surface by ultrashort 8 10 nm laser pulses then leads to an efficient modulation of the THz beam even at low photocarrier concentrations (approximate to 10(16) cm(-3)). The response time of the element to pulsed photoexcitation is about 130 ps. (c) 2007 Optical Society of America

Charge carrier dynamics in alternating polyfluorene copolymer: Fullerene blends probed by terahertz spectroscopy

Time-resolved terahertz spectroscopy is used for investigation of photoinduced charge carrier dynamics in blends of a polyfluorene copolymer (poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3-benzothi adiazole)]) and an electron acceptor ([6,6]-phenyl-C-61-butyric acid methyl ester). The transient far-infrared response appears instantaneously after photoexcitation. We show that the transient conductivity spectrum is dominated by two major contributions: response of separated charge carriers and response of coupled polaron pairs

Ultrafast conductivity in a low-band-gap polyphenylene and fullerene blend studied by terahertz spectroscopy

Time-resolved terahertz spectroscopy and Monte Carlo simulations of charge-carrier motion are used to investigate photoinduced transient conductivity in a blend of a low-band-gap polyphenylene copolymer and fullerene derivative. The optical excitation pulse generates free holes delocalized on polymer chains. We show that these holes exhibit a very high initial mobility as their initial excess energy facilitates their transport over defects (potential barriers) on polymer chains. The conductivity then drops down rapidly within 1 ps, and we demonstrate that this decrease occurs essentially by two mechanisms. First, the carriers loose their excess energy and they thus become progressively localized between the on-chain potential barriers-this results in a mobility decay with a rate of (180 fs)(-1). Second, carriers are trapped at defects (potential wells) with a capture rate of (860 fs)(-1). At longer time scales, populations of mobile and trapped holes reach a quasiequilibrium state and further conductivity decrease becomes very slow.

Sub-Picosecond Time-Dependent Mobility in Low-Band-Gap Polyphenylene:Fullerene Blend Probed by Terahertz Spectroscopy

Time-resolved terahertz spectroscopy is used to investigate photoinduced dynamics of charge carriers in a polymer heterojunction. We directly observe instantaneous generation of highly mobile charge carriers followed by a rapid drop in their mobility. (C) 2008 Optical Society of Americ

Influence of the Electron-Cation Interaction on Electron Mobility in Dye-Sensitized ZnO and TiO2 Nanocrystals: A Study Using Ultrafast Terahertz Spectroscopy

Charge transport and recombination in nanostructured semiconductors are poorly understood key processes in dye-sensitized solar cells. We have employed time-resolved spectroscopies in the terahertz and visible spectral regions supplemented with Monte Carlo simulations to obtain unique information on these processes. Our results show that charge transport in the active solar cell material can be very different from that in nonsensitized semiconductors, due to strong electrostatic interaction between injected electrons and dye cations at the surface of the semiconductor nanoparticle. For ZnO, this leads to formation of an electron-cation complex which causes fast charge recombination and dramatically decreases the electron mobility even after the dissociation of the complex. Sensitized TiO2 does not suffer from this problem due to its high permittivity efficiently screening the charges