Ultrafast dynamic reflectivity of vanadium pentoxide
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Marshall Onellion Department of Physics, University of Wisconsin—Madison, Madison, Wisconsin 53706
Thomas F. Georgeb) Office of the Chancellor and Center for Nanoscience, Departments of Chemistry & Biochemistry and Physics & Astronomy, University of Missouri—St. Louis, St. Louis, Missouri 63121
Dániel Szentesi, Sándor Szatmári, and László Nánai Department of Experimental Physics, University of Szeged, Dóm tér 9, H-6720 Szeged, Hungary (Received 20 April 2007; accepted 18 June 2007)
The ultrafast dynamic reflectivity of vanadium pentoxide is measured using 40 fs pulses from a self-mode-locked Ti:sapphire laser. The laser pulses excite acoustic vibrations at wave numbers of 145 and 103 cm−1. The amplitudes of the induced oscillations depend strongly on the orientation between the linear polarization of the laser pulses and the crystal axes, with the largest oscillations observed for an orientation of 45°. The higher-frequency oscillation is induced immediately upon arrival of the laser pulse, while the lower-frequency oscillation appears a few picoseconds later. The oscillations persist for approximately 10 ps after the arrival of the pulse. The oscillations are attributed to transverse acoustic modes propagating along the a-axis of the crystal.
I. INTRODUCTION
Vanadium pentoxide has been studied extensively both theoretically and experimentally.1–8 It is useful for sensitive gas detectors, rechargeable batteries, and highspeed electronics. Laser-grown vanadium pentoxide nanotubes have extremely large surface-area-to-volume ratios, which makes them ideally suited for sensitive gas sensors.9,10 When sodium or lithium is added between layers of vanadium pentoxide crystals, the resulting material makes an excellent cathode for rechargeable batteries.11 Vanadium dioxide has been shown to have a metal-to-insulator phase transition that occurs in approximately 100 fs.12 Ultrafast processes in materials such as this could lead to higher speed electro-optic devices.13 Although an ultrafast phase transition has not been reported for vanadium pentoxide, a slower laser-induced photochromic transition has been observed using excimer laser pulses.6 As is shown in this paper, vanadium pentoxide exhibits ultrafast dynamics.
a)
Present address: Kapteyn-Murnane Labs, Inc., 1855 South 57th Court, Boulder, CO 80301. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0039 308
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 2, Feb 2008 Downloaded: 02 Apr 2015
Femtosecond laser pulses can excite vibrations in materials if the duration of the laser pulse is less than the vibrational period.14,15 In the impulsive-stimulated Raman technique, a higher-energy “pump” pulse is used to stimulate the vibrations, and then a lower-energy “probe” pulse is reflected from the excited sample after a controllable time delay. By measuring small changes in the probe light reflected from the sample as a function of the time delay between the pump and probe pulses, we can extract information about
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