Surface plasmon-polaritons in thin metal films: Relaxation dynamics and optical field strength enhancement
Abstract
In this thesis, the electron dynamics of Surface Plasmon-Polaritons (SPPs) in thin films were studied using picosecond and femtosecond laser pulses. With moderate optical pulse energy from an ultrashort pulse laser and a novel Attenuated-Total-internal Reflection (ATR) optical coupling scheme, optical excitation and probing of SPP electron dynamics is possible. When SPP was optically excited, several order of magnitudes of optical field enhancement was found in the vicinity of the metal interfacial boundaries. This optical field enhancement is larger for the Long-Range Surface Plasmon mode (LRSP) than for the single-metal boundary surface plasmon mode or the short range surface plasmon mode. Nonlinear optical second-harmonic generation (SHG) making use of this optical field enhancement was demonstrated first in noncentrosymmetric media--crystal quartz, and then in an otherwise centrosymmetric media--bulk silver. The electric field discontinuity and the structural discontinuity are the source of SHG at the metal-dielectric interfaces. In conjunction with the SHG studies, energy relaxation of SPP and subsequent ultrafast heating of the conduction electrons in thin metal films was investigated using subpicosecond pump-probe thermomodulation spectroscopy. Nonequilibrium electron heating was achieved by irradiating the metal film with ultrashort laser pulses. The energy relaxation time of the transient temperature rise was found to depend strongly on the energy transfer coupling coefficient between the electrons and the lattice, the laser pulse width, and the heating laser fluence. The long propagation length of the LRSP mode has attracted a great deal of attentions from laser scientists. This long propagation length has been demonstrated in a nonlinear optical waveguide, but has never been directly observed in metal films with the excitation of LRSPs. In this thesis, optical probing of the LRSP decay length using several techniques have been performed. However, discrepancy between experimental results and theory was observed which may be attributed to the surface roughness of the metal film and the limitation of beam collimation. A new experimental technique has been proposed to carry the investigation further.
Degree
Ph.D.
Advisors
Smith, Purdue University.
Subject Area
Condensation
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