Four-dimensional scanning ultrafast electron microscopy is used to investigate doping-and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured timeresolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration. When electrons undergo sufficient energy loss on their journey to the surface, dark contrast becomes dominant in the image. At negative times, however, when the electron pulse precedes the optical pulse (electron impact), the dynamical behavior of carriers manifests itself in a dark contrast which indicates the suppression of secondary electrons upon the arrival of the optical pulse. In this case, the loss of energy of material's electrons is by collisions with the excited carriers. These results for carrier dynamics in GaAs(110) suggest strong carrier-carrier scatterings which are mirrored in the energy of material's secondary electrons during their migration to the surface. The approach presented here provides a fundamental understanding of materials probed by four-dimensional scanning ultrafast electron microscopy, and offers possibilities for use of this imaging technique in the study of ultrafast charge carrier dynamics in heterogeneously patterned micro-and nanostructured material surfaces and interfaces.scanning electron microscopy | ultrafast phenomena | charge dynamics imaging R ecent advances in four-dimensional (4D) ultrafast electron microscopy (UEM) have made it possible to investigate nonequilibrium electronic and structural dynamics with atomicscale spatial resolution and femtosecond temporal resolution (1). Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2, 3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces. In these studies, image contrast was monitored as a function of time, and it was found that Si exhibits a bright contrast in the image at positive times without appreciable dynamics at negative times, whereas CdSe displays bright contrast at positive times and dark contrast at negative times (2). However, the correlation between the measured time-dependent SE intensity and electronic structure of the material of interes...

In this paper, we study nanostructure-covered laser induced periodic surface structures (NC-LIPSSs) on metals produced by femtosecond laser pulses incident at various angles. For the first time, we show that the dependence of NC-LIPSS period on the incident beam angle deviates significantly from that of regular LIPSSs studied previously using longer laser pulses. Our study shows that this deviation is due to the nanostructures on LIPSSs that can be explained by the Maxwell–Garnett theory of effective media.

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Pulsed terahertz radiation due to coherent phonon-polariton excitation in 110 ZnTe crystal J. Appl. Phys. 112, 093110 (2012) Terahertz emission from cubic semiconductor induced by a transient anisotropic photocurrent J. Appl. Phys. 112, 073115 (2012) Antenna effect in laser assisted atom probe tomography: How the field emitter aspect ratio can enhance atomic scale imaging Appl. Phys. Lett. 101, 153101 (2012) Enhanced carrier-carrier interaction in optically pumped hydrogenated nanocrystalline silicon Appl. Phys. Lett. 101, 141904 (2012) Dynamic control of directional asymmetry observed in ultrafast laser direct writing Downloaded 08 Nov 2012 to 77.236.37.83. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions 123111-3 Hwang, Vorobyev, and Guo Appl. Phys. Lett. 95, 123111 ͑2009͒

Through femtosecond laser irradiation, we produce in this work a unique type of surface nanostructure on Al that have enhanced absorption at UV and visible but a relatively small emissivity in infrared. By integrating this laser-treated Al to a solar-driven thermoelectric generator, we show that the thermoelectric generator integrated with the femtosecond laser-treated Al foil generates a significantly higher power than the ones without. Our study shows that our technique can dramatically enhance the efficiency of solar-driven thermoelectric devices that may lead to a leap forward in solar energy harnessing.