Ultrafast Optical Studies of Phonon Polaritons, Squeezed Modes and High Frequency Diamagnetism in Metamaterials.

Abstract

The coupling of the electromagnetic field with polar lattice vibrations of a solid, which gives rise to what is traditionally known as phonon polaritons, is investigated both through spontaneous and stimulated Raman scattering. Experimental results relative to polariton modes excited in several semiconductors are presented to explore their dependence on the crystal symmetry, temperature, excitation wavelength and measuring techniques. In GaAs we find discrepancies between spontaneous and Impulsive Stimulated Raman Scattering (ISRS) which are attributed to the presence of free carriers interacting with the electric field of the longitudinal phonon mode. In GaSe, we successfully excite two distinct frequencies of the lower phonon polariton branch. In the transparent regime, this is accom- plished combining in the same experiment backward and forward scattering, the latter one induced by the beam reflected at the back surface of the sample. Moreover, it is shown how the reduced value of the scattering cross section retrieved in the time domain experiments is attributable to the polariton field spatial distribution, estimated in accordance with the Cherenkov radiation theory. In CdSe we identify another polariton mode that is present whenever the dielectric constant of a medium becomes negative: the surface plasmon po- lariton. Besides coherent phonons, squeezed phonons are studied, discussing their generation and detection in regard to the ISRS theory. In particular we introduce a novel phenomenon, named “phonon echo”, occurring whenever a squeezed phonon field is induced in a crystal through a double pump excitation. Simulations are shown to validate the theoretical pre- dictions and pave the way to future experiments. Shifting to the metamaterial field, we consider a viable technique to achieve artificial dia- magnetism (the magnetic permeability μ is < 1). The proposed approach is based on the well established sphere-in-a-host model that is throughly described with particular emphasis on its conditions of validity. Several samples are fabricated and measured in the microwave range where the extracted permeability turns out to be as low as 0.4. Experimental data adhere closely to the numerical simulations except for the bigger sphere samples: a plausible explanation to interpret this apparent mismatch is proposed and justified.