Quantitative polarimetry of plasmon resonant spheroidal metal nanoparticles: A Mueller matrix decomposition study

Quantitative polarization characteristics of plasmon resonant spheroidal metal (silver) nanoparticles are investigated using polar decomposition of scattering Mueller matrices. The decomposition analysis on the scattering matrices (computed using T-matrix approach) of preferentially oriented spheroidal silver nanoparticles showed enhanced linear retardance (delta) and diattenuation (d) effects in their surface plasmon resonance bands, with intriguing spectral characteristics. While the magnitude of delta (lambda) peaked around the spectral overlap region of the transverse and the longitudinal dipolar plasmon resonance bands, d (lambda) peaked at wavelengths corresponding to the peaks of these two bands. The observed linear retardance and diattenuation effects were attributed to the inherent differences in phases and amplitudes respectively, of the two orthogonal dipolar plasmon polarizabilities of the spheroidal metal nanoparticles, which were confirmed further by studying their angular dependence. Further, when averaged over all possible (random) orientation, addition of the individual retardance matrices having random orientation of axes manifests as stronger depolarization (Delta) in spheroidal metal nanoparticles as compared to their dielectric counterparts. Consequently, the spectral variations of Delta (lambda) for the randomly oriented nanoparticles resembled that for the variation of delta (lambda) for the preferentially oriented ones. Derivation, quantification and unique interpretation of the intrinsic plasmon polarization characteristics (and their spectral behavior) aided by Mueller matrix decomposition may have important implications for contrast enhancement in nanoparticle-based biomedical imaging, as well as in other applications involving nano-plasmonics. (C) 2011 Elsevier B.V. All rights reserved.