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Ultrafast Optical Studies of Phonon Polaritons, Squeezed Modes and High Frequency Diamagnetism in Metamaterials.
[摘要] The coupling of the electromagnetic field with polar lattice vibrations of a solid, whichgives rise to what is traditionally known as phonon polaritons, is investigated both throughspontaneous and stimulated Raman scattering. Experimental results relative to polaritonmodes excited in several semiconductors are presented to explore their dependence on thecrystal symmetry, temperature, excitation wavelength and measuring techniques.In GaAs we find discrepancies between spontaneous and Impulsive Stimulated RamanScattering (ISRS) which are attributed to the presence of free carriers interacting with theelectric field of the longitudinal phonon mode. In GaSe, we successfully excite two distinctfrequencies 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 oneinduced by the beam reflected at the back surface of the sample. Moreover, it is shown howthe reduced value of the scattering cross section retrieved in the time domain experimentsis attributable to the polariton field spatial distribution, estimated in accordance with theCherenkov radiation theory. In CdSe we identify another polariton mode that is presentwhenever the dielectric constant of a medium becomes negative: the surface plasmon po-lariton.Besides coherent phonons, squeezed phonons are studied, discussing their generation anddetection 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 thewell established sphere-in-a-host model that is throughly described with particular emphasison its conditions of validity. Several samples are fabricated and measured in the microwaverange where the extracted permeability turns out to be as low as 0.4. Experimental dataadhere closely to the numerical simulations except for the bigger sphere samples: a plausibleexplanation to interpret this apparent mismatch is proposed and justified.
[发布日期]  [发布机构] University of Michigan
[效力级别] Metamaterials [学科分类] 
[关键词] Coherent Phonons;Metamaterials;Physics;Science;Applied Physics [时效性] 
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