Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3\mathrm{Bi}_2\mathrm{Se}_3

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents

Energy transfer within the hydrogen bonding network of water following resonant terahertz excitation

Energy dissipation in water is very fast and more efficient than in many other liquids. This behavior is commonly attributed to the intermolecular interactions associated with hydrogen bonding. Here, we investigate the dynamic energy flow in the hydrogen bond network of liquid water by a pump-probe experiment. We resonantly excite intermolecular degrees of freedom with ultrashort single-cycle terahertz pulses and monitor its Raman response. By using ultrathin sample cell windows, a background-free bipolar signal whose tail relaxes monoexponentially is obtained. The relaxation is attributed to the molecular translational motions, using complementary experiments, force field, and ab initio molecular dynamics simulations. They reveal an initial coupling of the terahertz electric field to the molecular rotational degrees of freedom whose energy is rapidly transferred, within the excitation pulse duration, to the restricted translational motion of neighboring molecules. This rapid energy transfer may be rationalized by the strong anharmonicity of the intermolecular interactions

Intrinsic response time of graphene photodetectors

Graphene-based photodetectors are promising new devices for high-speed optoelectronic applications. However, despite recent efforts, it is not clear what determines the ultimate speed limit of these devices. Here, we present measurements of the intrinsic response time of metal-graphene-metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses. We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers. This time translates into a bandwidth of ~262 GHz. Moreover, we investigate the dependence of the response time on gate voltage and illumination laser power

Ultrafast tilting of the dispersion of a photonic crystal and adiabatic spectral compression of light pulses

We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photonic-crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11%. We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide.Publisher PDFPeer reviewe

Ultrafast adiabatic manipulation of slow light in a photonic crystal

We demonstrate by experiment and theory that a light pulse propagating through a Si-based photonic-crystal waveguide is adiabatically blueshifted when the refractive index of the Si is reduced on a femtosecond time scale. Thanks to the use of slow-light modes, we are able to shift a 1.3-ps pulse at telecom frequencies by 0.3 THz with an efficiency as high as 80% in a waveguide as short as 19 mu m. An analytic theory reproduces the experimental data excellently, which shows that adiabatic dynamics are possible even on the femtosecond time scale as long as the external stimulus conserves the spatial symmetry of the system.Publisher PDFPeer reviewe

Ultrafast tunable optical delay line based on indirect photonic transitions

We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tunable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 mu m in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency.Publisher PDFPeer reviewe

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) similar to c/10 to a waveguide in which it is v(g) similar to c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results. (C) 2011 Optical Society of AmericaPublisher PDFPeer reviewe

Slow-light photonic crystal switches and modulators

We discuss the performance of slow-light enhanced optical switches and modulators fabricated in silicon. The switch is based on photonic crystal waveguides in a directional coupler geometry, and the dispersion of the device is engineered to allow a switching length as short as 5 µm and rerouting of optical signals within 3 ps. The 3 ps switching time is demonstrated using free carriers in the silicon generated by the absorption of a femtosecond pump pulse. The modulator is based on a Mach-Zehnder interferometer configuration, with photonic crystal waveguides in each arm to act as phase-shifters. A flat-band slow-light region has been engineered in the phase-shifters to provide an extinction ratio in excess of 15 dB over the entire 11 nm bandwidth of the modulator device