We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 -250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical phonon frequency, however, owing to the presence of hot carriers, optical phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.PACS numbers: 78.67. Wj, 63.22.Rc, 78.47.jd Graphene, consisting of a single atomic layer of carbon atoms in a hexagonal lattice, exhibits a unique band structure with zero energy gap and linear energy dispersion, the so-called Dirac cone. The band structure gives rise to several remarkable properties, some of which are highly attractive for novel optoelectronic and photonic devices [1]. Of key importance are the dynamics of electronic relaxation, in particular due to the interaction with the phonon system. During the last years many insights into the relaxation dynamics have been obtained from single-color [2-5] and two-color [6-10] pump-probe experiments. Thermalization of the nonequilibrium electron distribution via electron-electron scattering on a sub-100 fs timescale and efficient scattering via optical phonons on a 100 fs -few ps timescale have been identified. Common to previous pump-probe experiments is an excitation energy of ∼1.5 eV, i.e. high above the Dirac point. Also most graphene photonic applications demonstrated so far involve near infrared or visible light [11,12]. The nature of graphene as a gapless material with constant absorption in the range, where the band structure is well described by Dirac cones, suggests to expand the studies into the mid infrared and terahertz range. In particular, it is crucial to obtain thorough insights into the relaxation dynamics in the range of the optical phonon energy and below, i.e., close to the Dirac point, where Coulomb as well as optical and acoustic phonon processes can be significant and were both interband and intraband processes are relevant. Here graphene serves as a model system to understand the relevance of electron-electron and electron-phonon interaction for both intraband and interband relaxation in materials with small or vanishing energy gap.In this Letter, we study the carrier dynamics close to the Dirac point by varying the excitation energy E by more than an order of magnitude (245 meV -10 meV). An optical phonon bottleneck is observed in the range E = 245 -30 meV, with decay times increasing from sub-ps to several 100 ps. Microscopic calculations based on the density matrix formalism explain these time constants by optical phonon scattering and additionally reveal contributions due to Coulomb-and acoustic phonon-induced processes. For E < 30 meV, a striking and unexpected sign reversal of the pump-probe signal is found. The in...

We present a planar large-area photoconducting emitter for impulsive generation of terahertz ͑THz͒ radiation. The device consists of an interdigitated electrode metal-semiconductor-metal ͑MSM͒ structure which is masked by a second metallization layer isolated from the MSM electrodes. The second layer blocks optical excitation in every second period of the MSM finger structure. Hence charge carriers are excited only in those periods of the MSM structure which exhibit a unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to THz electric field amplitudes up to 85 V / cm when excited with fs optical pulses from a Ti:sapphire oscillator with an average power of 100 mW at a bias voltage of 65 V applied to the MSM structure. The proposed device structure has a large potential for large-area high-power THz emitters. 6 Several attempts have been made to increase the THz emission efficiency by employing photoconductors with laterally structured emitter electrodes. 7,8 The driving forces for further developments of impulsive THz emitters are applications which require a large bandwidth and/or high THz electric field amplitudes. The performance of THz emitters based on photoconductors ͑PC͒ is limited by several conditions. In order to achieve a high bandwidth the electric field applied to the PC should be high in the 100 kV/ cm range, i.e., close to the breakdown voltage of the PC. On the other hand the optically excited area should be large to prevent local heating of the PC with the exciting laser pulses which would have a detrimental effect on the carrier mobility and thus decrease the bandwidth. Therefore the solution of using pin-diodes has advantages over lateral electrical contacts to the PC, since in a pin-diode high electric fields exist in the whole area of the diode. Yet the acceleration of carriers in a pin-diode is perpendicular to the surface which results in a bad outcoupling efficiency of the dipole radiation with the main intensity being emitted parallel to the surface. Hence the acceleration of carriers in the plane of the PC would be favorable. However, large lateral electric fields applied over large areas, i.e., large electrode spacing, require pulsed high voltage sources in the kV range which one tries to avoid because of the electronic interference with sensitive electronic equipment in the laboratory. 9 Another limitation of PC emitters with lateral carrier excitation is the emission of the radiation via coupling to a microstructured dipole antenna, which typically exhibits a narrower bandwidth than the intrinsic carrier acceleration in the PC.Here we investigate the THz radiation from photoconductive MSM structures modified in a way to achieve unidirectional carrier acceleration on a large active area for high excitation powers. The emission is based on the intrinsic acceleration of carriers in the PC and hence does not require a narrow-band antenna. High electric fields of the order of 100 kV/ cm are achieved by convenient dc voltages of abo...

ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.

We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 um to 220 um. The detector rise time is 50 ps in the wavelength range from 30 um to 220 um. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g. in two-color pump-probe experiments