Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump bacteriorhodopsin. Light-adapted bacteriorhodopsin was excited with a 60-femtosecond pump pulse at 618 nanometers, and the transient absorption spectra from 560 to 710 nanometers were recorded from -50 to 1000 femtoseconds by means of 6-femtosecond probe pulses. By 60 femtoseconds, a broad transient hole appeared in the absorption spectrum whose amplitude remained constant for about 200 femtoseconds. Stimulated emission in the 660- to 710-nanometer region and excited-state absorption in the 560- to 580-nanometer region appeared promptly and then shifted and decayed from 0 to approximately 150 femtoseconds. These spectral features provide a direct observation of the 13-trans to 13-cis torsional isomerization of the retinal chromophore on the excited-state potential surface. Absorption due to the primary ground-state photoproduct J appears with a time constant of approximately 500 femtoseconds.

Cesium lead halide perovskite quantum dots (PQDs) have emerged as a promising new platform for lighting applications. However, to date, light emitting diodes (LED) based on these materials exhibit limited efficiencies. One hypothesized limiting factor is fast nonradiative multiexciton Auger recombination. Using ultrafast spectroscopic techniques, we investigate multicarrier interaction and recombination mechanisms in cesium lead halide PQDs. By mapping the dependence of the biexciton Auger lifetime and the biexciton binding energy on nanomaterial size and composition, we find unusually strong Coulomb interactions among multiexcitons in PQDs. This results in weakly emissive biexcitons and trions, and accounts for low light emission efficiencies. We observe that, for strong confinement, the biexciton lifetime depends linearly on the PQD volume. This dependence becomes sublinear in the weak confinement regime as the PQD size increases beyond the Bohr radius. We demonstrate that Auger recombination is faster in PQDs compared to CdSe nanoparticles having the same volume, suggesting a stronger Coulombic interaction in the PQDs. We confirm this by demonstrating an increased biexciton binding energy, which reaches a maximum of about 100 meV, fully three times larger than in CdSe quantum dots. The biexciton shift can lead to low-threshold optical gain in these materials. These findings also suggest that materials engineering to reduce Coulombic interaction in cesium lead halide PQDs could improve prospects for high efficiency optoelectronic devices. Core-shell structures, in particular type-II nanostructures, which are known to reduce the bandedge Coulomb interaction in CdSe/CdS, could beneficially be applied to PQDs with the goal of increasing their potential in lighting applications.

We demonstrate that a combination of prisms and diffraction gratings can provide not only quadratic but also cubic phase compensation of ultrashort optical pulses. We obtain compressed pulses as short as 6 fsec.

No abstract

Optical second-harmonic studies show that the electronic structure in the top 75-130 A of a crystalline Si surface loses cubic order only 150 fsec after the Si is excited by an intense 100-fsec optical pulse. This suggests that atomic disorder can be induced directly by electronic excitation, before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of femtoseconds. PACS numbers: 78.47.+p, 61.80.Ba, 71.38.+i With use of ultrashort laser radiation pulses, it is possible to excite the electronic states of a solid before appreciable energy is transferred to the lattice vibrational states. Here, we explore the question posed several years ago by Van Vechten, Tsu, and Saris, 1 whether or not crystalline Si can be driven to disorder by electronic excitation without the lattice modes' equilibrating at a temperature above the melting temperature. This possibility has obvious practical importance in laser-assisted semiconductor processing and surface photochemistry in general. It is of fundamental interest because the dynamics of electron-phonon equilibration may directly effect the evolution of the phase transition. Indeed, a recent picosecond time-resolved study of graphite after pulsed laser excitation reported observation of an intermediate nonequilibrium phase. 2 The physical picture of how Si might be disordered by nonequilibrium electronic excitaton is appealing. Silicon is well described by a tight-binding model, and therefore electronic excitation might directly drive disorder by a process akin to photodissociation in molecules. In a more delocalized picture, the electronic excitation might weaken the interatomic bonding and thus lower the vibrational energy required to produce disorder. However, previous linear reflection 3,4 and second-harmonic generation 5 (SHG) experiments on laser-induced disorder of silicon can be interpreted in the normal picture of cascade electron-phonon relaxation and subsequent lattice melting in « 1 psec. Here, we report new time-resolved optical measurements. We find that the SH reflection changes abruptly in a way that is consistent with the top 75-130 A of the silicon surface's losing cubic symmetry in < 150 fsec. Because 150 fsec is less than two electron-phonon relaxation times, the result suggests that the laser induces disorder before the phonons equilibrate above the melting temperature. In addition, the data indicate that the electronic properties of the equilibrium molten phase are not obtained on this short time scale but rather with a > 300-fsec time constant. This suggests that considerable vibrational excitation is required to obtain the interatomic distance and/or correlations of the liquid.The experiments are very similar to those reported in Refs. 4 and 5. Laser pulses of 75 fsec duration (110 fsec FWHM autocorrelation) were obtained by dispersion compensation of the amplified output of a colliding-pulse mode-locked laser operating at 610 nm. To make the excitation...

The development of microstructured fibres offers the prospect of improved fibre sensing for low refractive index materials such as liquids and gases. A number of approaches are possible. Here we present a new approach to evanescent field sensing, in which both core and cladding are microstructured. The fibre was fabricated and tested, and simulations and experimental results are shown in the visible region to demonstrate the utility of this approach for sensing.

Perovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 10 GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm. We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process.

No abstract