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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 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

A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB) and the other one is related to the Risken-Nummedal-Graham-Haken instability predicted in the 1960s. A model that can account for coherent phenomena, a saturable absorber, and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with a narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have a weaker saturable absorber, do not exhibit a Rabi splitting and their multimode regime is governed by SHB. This experimental phenomenology is well-explained by our theoretical model. The temperature dependence of the multimode regime is also presented

Stable trains of ultrashort light pulses with large instantaneous intensities from mode-locked lasers are key elements for many important applications such as nonlinear frequency conversion [1-3], time-resolved measurements [4, 5], coherent control [6, 7], and frequency combs [8]. To date, the most common approach to generate short pulses in the mid-infrared (3.5-20 µm) molecular "fingerprint" region relies on the down-conversion of short-wavelength mode-locked lasers through nonlinear processes, such as optical parametric generation [9-11] and four-wave mixing [12]. These systems are usually bulky, expensive and typically require a complicated optical arrangement. Here we report the unequivocal demonstration of mid-infrared mode-locked pulses from a semiconductor laser.

Metal–organic frameworks (MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Yet, the realization of their proof‐of‐concept devices remains a daunting challenge, attributed to their poor electrical properties. Following recent work on a semiconducting Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11‐triphenylenehexathiol) 2D MOF with record‐high mobility and band‐like charge transport, here, an Fe3(THT)2(NH4)3 MOF‐based photodetector operating in photoconductive mode capable of detecting a broad wavelength range from UV to NIR (400–1575 nm) is demonstrated. The narrow IR bandgap of the active layer (≈0.45 eV) constrains the performance of the photodetector at room temperature by band‐to‐band thermal excitation of charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 108 cm Hz1/2 W−1 are achieved under 785 nm excitation. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are commensurate to those obtained with the first demonstrations of graphene‐ and black‐phosphorus‐based photodetectors. This work demonstrates the feasibility of integrating conjugated MOFs as an active element into broadband photodetectors, thus bridging the gap between materials' synthesis and technological applications.

We investigate the polarization dependence of the carrier excitation and relaxation in epitaxial multilayer graphene. Degenerate pump-probe experiments with a temporal resolution of 30 fs are performed for different rotation angles of the pump-pulse polarization with respect to the polarization of the probe pulse. A pronounced dependence of the pump-induced transmission on this angle is found. It reflects a strong anisotropy of the pump-induced occupation of photogenerated carriers in momentum space even though the band structure is isotropic. Within 150 fs after excitation an isotropic carrier distribution is established. Our observations imply the predominant role of collinear scattering preserving the initially optically generated anisotropy in

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