Hot electron and phonon dynamics of gold nanoparticles embedded in a gel matrix Mona B. Mohamed, Temer S. Ahmadi, Stephan Link, Markus Braun, Mostafa A. El-Sayed * Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA Received 15 March 2001; in ®nal form 11 May 2001 Abstract Using pump-probe technique, the dynamics of the hot carriers in metallic nanodots induced by femtosecond laser pulses are investigated in gold nanoparticles embedded in hydrogel and in organic gel and compared to that in aqueous solution. We found that changing the surrounding matrix from aqueous solution to hydrogel and then to organic gel leads to a large increase in the relaxation time of both the electron±phonon e±ph) and the phonon±phonon ph±ph) coupling. Furthermore, the ph±ph relaxation time becomes sensitive to the type of the organic solvent trapped in the gel network. This indicates that the relaxation dynamics depend on the thermal conductivity, chemical structure and the molecular dynamics of the surrounding medium. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Metallic nanoparticles are of great importance because of their unique electronic, magnetic, and optical properties [1±4]. Comprehensive under- standing of the electron dynamics and the relax- ation processes in these systems is required because of their fast optical response, which is very im- portant for many potential applications such as optical switching and electronic devices [5,6]. Moreover, better understanding of some other properties such as electrical and thermal conduc- tivity and superconductivity of metallic nanopar- ticles could be achieved by studying the relaxation dynamics. In metallic nanoparticles, because of the large dierence in the heat capacity of the electrons and the phonons, they can be treated as two coupled subsystems. Before excitation, the electrons oc- cupy the energy states below the Fermi level. Upon excitation of metallic thin ®lms, or metallic nano- particles by ultrafast laser pulses, the energy is transferred to the electrons by absorption of photons via interband and intraband transition. During this process, the electrons are excited to energy states higher than the Fermi level. This electron distribution is a non-thermal distribution according to Fermi±Dirac statistic. The relaxation processes start to thermalize the electrons via electron±electron scattering and electron scattering with the surface. The occupied electronic states tend to a hot Fermi±Dirac distribution with a temperature depending on the intensity of the ex- citing laser pulse. This thermalization process oc- curs in a few hundreds of femtoseconds [7,8]. The 27 July 2001 Chemical Physics Letters 343 2001) 55±63 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-404-894-0294. E-mail address: mostafa.el-sayed@chemistry.gatech.edu M.A. El-Sayed). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)00653-4