We carry out a comprehensive theoretical and experimental study of charge injection in poly(3-hexylthiophene) (P3HT) to determine the most likely scenario for metal-insulator transition in this system. Wecalculate the optical-absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first- or a second-order metal-insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin-film field-effect transistors (FETs). Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal-insulator transition.

The observation and modeling of coherent transition radiation from femtosecond laseraccelerated electron bunches is discussed. The coherent transition radiation, scaling quadratically with bunch charge, is generated as the electrons transit the plasma-vacuum boundary. Due to the limited transverse radius of the plasma boundary, diffraction effects will strongly modify the angular distribution and the total energy radiated is reduced compared to an infinite transverse boundary. The multi-nC electron bunches, concentrated in a length of a few plasma periods (several tens of microns), experience partial charge neutralization while propagating inside the plasma towards the boundary. This reduces the space-charge blowout of the beam, allowing for coherent radiation at relatively high frequencies (several THz). The charge distribution of the electron bunch at the plasma-vacuum boundary can be derived from Fourier analysis of the coherent part of the transition radiation spectrum. A Michelson interferometer was used to measure the coherent spectrum, and electron bunches with duration on the order of 50 fs (rms) were observed.

The terahertz (THz) and sub-THz region of the electromagnetic spectrum bridges the infrared and the microwave. This boundary region is beyond the normal reach of optical and electronic measurement techniques normally associated with these better-known neighbors. Only over the past decade has this THz region become scientifically accessible withbroadband sources of moderate intensity being produced by ultra-fast laser pulses incident on biased semiconductors or non-linear crystals [1,2]. Very recently, a much higher power source of THz radiation was demonstrated: coherent synchrotron radiation (CSR) from short, relativistic electron bunches [3-5]. Coherent synchrotron radiation will open up new territory in the THz frequency range with intensities many orders of magnitude higher than previous sources. The energy range between microwave and the far infrared, 3 33 cm-1 (0.1 1 THz), has proven to be challenging to accesses and is therefore referred to as the THz gap . However, with the new CSR source at BESSY [3,5] we have been able to extend traditional infrared measurements down into this sub-terahertz frequency range. This source is broadband and is made up of longitudinally coherent single-cycle sub-picosecond pulses with a high repetition rate (100 s of MHz). With the combination of high intensity and short pulse duration new opportunities for scientific research and applications are enabled across a diverse array of disciplines from condensed matter physics, to medical, technological, manufacturing, space and defense industries. Imaging, spectroscopy, femtosecond dynamics, and driving novel non-linear processes are all among the potential applications. The high average power of the CSR source allows one to extend experimental conditions to lower frequencies than have been possible with thermal and conventional synchrotron sources (Figure 1). In this paper, we make use of the stable CSR THz source at BESSY for threeinitial scientific demonstration experiments.

This article describes the design rationale, a C implementation, and conformance testing of a subset of the new Standard for the BLAS (Basic Linear Algebra Subroutines): Extended and Mixed Precision BLAS. Permitting higher internal precision and mixed input/output types and precisions allows us to implement some algorithms that are simpler, more accurate, and sometimes faster than possible without these features. The new BLAS are challenging to implement and test because there are many more subroutines than in the existing Standard, and because we must be able to assess whether a higher precision is used for internal computations than is used for either input or output variables. We have therefore developed an automated process of generating and systematically testing these routines. Our methodology is applicable to languages besides C. In particular, our algorithms used in the testing code will be valuable to all other BLAS implementors. Our extra precision routines achieve excellent performance--close to half of the machine peak Megaflop rate even for the Level 2 BLAS, when the data access is stride one.