Topic
Femtosecond
About: Femtosecond is a research topic. Over the lifetime, 35106 publications have been published within this topic receiving 691405 citations. The topic is also known as: 1 E-15 s & fs.
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TL;DR: Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses through electro-optic modulations of the transient reflectivity through which the components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.
Abstract: Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses. They are observed for the first time through electro-optic modulations of the transient reflectivity. The corresponding signal oscillations are superimposed upon an additional longitudinal polarization feature which decays exponentially and whose rise time is sufficiently short to act as a driving force for the coherent phonons. The components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.
345 citations
TL;DR: In this article, the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines was observed with an increasing number of pulses per spot, up to 100, and the formation of equally oriented ripples with a spatial period close to half the laser wavelength.
Abstract: Laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics have been observed after irradiation of single-crystalline indium phosphide (c-InP) with multiple linearly polarized femtosecond pulses (130fs, 800nm) in air. With an increasing number of pulses per spot, N, up to 100, a characteristic evolution of two different types of ripples has been observed, i.e., (i) the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines and (ii), in a specific pulse number regime (N=5–30), the additional formation of equally oriented ripples with a spatial period close to half of the laser wavelength. For pulse numbers higher than 50, the formation of micrometer-spaced grooves has been found, which are oriented perpendicular to the ripples. These topographical surface alterations are discussed in the frame of existing LIPSS theories.
345 citations
TL;DR: It is shown that a true minimum on the bright S2 electronic state is responsible for the first step that occurs on a femtosecond time scale, and it is suggested that subsequent barrier crossing to the minimal energy S2/S1 conical intersection isresponsible for the picosecond decay.
Abstract: The reaction dynamics of excited electronic states in nucleic acid bases is a key process in DNA photodamage. Recent ultrafast spectroscopy experiments have shown multicomponent decays of excited uracil and thymine, tentatively assigned to nonadiabatic transitions involving multiple electronic states. Using both quantum chemistry and first principles quantum molecular dynamics methods we show that a true minimum on the bright S2 electronic state is responsible for the first step that occurs on a femtosecond time scale. Thus the observed femtosecond decay does not correspond to surface crossing as previously thought. We suggest that subsequent barrier crossing to the minimal energy S2/S1 conical intersection is responsible for the picosecond decay.
344 citations
TL;DR: It was found that LEDs fabricated with roughened ZnO and the microhole array had similar current-voltage (I-V) characteristics to those of conventional LEDs and no degrading effect was observed.
Abstract: A significant enhancement of light extraction efficiency from InGaN light-emitting diodes (LEDs) with microhole arrays and roughened ZnO was experimentally demonstrated. The roughened ZnO was fabricated using an Ar and H2 plasma treatment of ZnO films pre-coated on a p-GaN layer. When followed by a femtosecond laser direct writing technique, a periodic array of microholes could be added to the surface. The diameter of the microhole was varied by changing the output power of the femtosecond laser. Compared to conventional LEDs on the same wafer, the output power of LEDs with roughened ZnOs and a microhole (diameter of 2 μm) array was increased by 58.4% when operated with an injection current of 220 mA. Moreover, it was found that LEDs fabricated with roughened ZnO and the microhole array had similar current-voltage (I-V) characteristics to those of conventional LEDs and no degrading effect was observed.
344 citations
TL;DR: Structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein are identified.
Abstract: A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes after photon absorption. The initial step is often the photoisomerization of a conjugated chromophore. Isomerization occurs on ultrafast time scales and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein. Femtosecond hard x-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on photoactive yellow protein microcrystals over a time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.
343 citations