The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

The long‐term success of photosynthetic organisms has resulted in their global superabundance, which is sustained by their widespread, continual mass‐production of the integral proteins that photocatalyze the chemical processes of natural photosynthesis. Here, a fast, general method to assemble multilayer films composed of one such photocatalytic protein complex, Photosystem I (PSI), onto a variety of substrates is reported. The resulting films, akin to the stacked thylakoid structures of leaves, consist of a protein matrix that is permeable to electrochemical mediators and contain a high concentration of photoelectrochemically active redox centers. These multilayer assemblies vastly outperform previously reported monolayer films of PSI in terms of photocurrent production when incorporated into an electrochemical system, and it is shown that these photocatalytic properties increase with the film thickness. These results demonstrate how the assembly of micron‐thick coatings of PSI on non‐biological substrates yields a biohybrid ensemble that manifests the photocatalytic activity of the film’s individual protein constituents, and represent significant progress toward affordable, biologically‐inspired renewable energy conversion platforms.

Ultrafast time-resolved differential reflectivity of Bi 2 Se 3 crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes. The deduced relaxation timescale and the sign of the reflectivity change suggest that electron-phonon interactions and defect-induced charge trapping are the underlying mechanisms for the three processes. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy.

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga 0.965 Mn 0.035 As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~T C /2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.The magnetic semiconductor GaMnAs has received considerable attention in recent years, largely because of its potential role in the development of spin-based devices 1,2 . In this itinerant ferromagnet, the collective magnetic order arises from the interaction between mobile valence band holes and localized Mn spins. Therefore, the magnetic properties are sensitive to external excitations that change the carrier density and distribution. Ultrafast pump-probe magneto-optical spectroscopy is an ideal technique for controlling and characterizing the magnetization dynamics in the magnetic materials, and has been applied to the GaMnAs system by several groups 3,4 .Although optically induced precessional motion of magnetization has been studied in 2 other magnetic systems 5 , magnetization precession in ferromagnetic GaMnAs has been observed only recently 4 and has yet to be adequately understood.In this paper, we report comprehensive temperature and photoexcitation intens ity dependent measurements of photoinduced magnetization precession in Ga 1-x Mn x As (x = 0.035) with no externally imposed magnetic field. By comparing and contrasting the temperature and intensity dependence of the precession frequency, damping, and amplitude, we identify the importance of light-induced nonlinear effects and obtain new information on the relevant physical mechanisms. Our measurements of the photoinduced magnetization show coherent oscillations, arising from the precession of collective Mn spins. Amplitude of the magnetization precession saturates above certain pump intensity is a strong indication that direction of the magnetic easy axis remains unchanged at temperatures above about half the Curie temperature (T C ). The precession is explained by invoking an ultrafast change in the orientation of the in-plane easy axis, due to an impulsive change in the magnetic anisotropy induced by the laser pulse. We also find that the Gilbert damping coefficient, which characterizes the Mn-spin relaxation, depends only weakly on the ambient temperature but changes dramatically with pump intensity. Our results suggest a general model for photoinduced precessional motion and relaxation of magnetization in the GaMnAs system under compressive strain.Time-resolved magneto-optical Kerr effect (MOKE) measurements were performed on a 300 nm thick f...

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.