Far-infrared absorption by small silver particles in gelatin

The far-infrared (FIR) properties of a composite material consisting of {~}100-{}-diameter silver particles imbedded in a gelatin host are discussed in detail. This material was studied by both transmission electron microscopy---so that the morphology of the particles is well characterized---and FIR spectroscopy. The volume fraction f of Ag is adjustable over a broad range (0{≤}f{≤}0.34). Samples with either well-dispersed or deliberately agglomerated particles were prepared. The procedures for preparation and characterization of this novel composite material are presented. The role of an absorbing host in effective-medium theories of composite materials is examined because gelatin absorbs FIR radiation. The FIR complex dielectric function of gelatin is determined. The Bruggeman model provides agreement with both the frequency and volume-fraction dependences of the absorption coefficient for low-f material (f{≤}0.15) with well-dispersed particles. The results are used to place bounds on the possible enhancement of the absorption by Ag particles. The Bruggeman model does not describe the data quantitatively for f>0.15. The observed increase in absorption is more gradual than predicted. The Bruggeman model is qualitatively consistent with the low-frequency dielectric constant of the Ag-gelatin composite, which was determined using an interference effect. Samples with deliberately agglomerated particles show enhanced absorption. A simple clustering model predicts enhanced absorption, even though absorption by the gelatin dominates absorption by the Ag particles by at least 2 orders of magnitude for f=0.05. .AE