22E2 Reflectance of the surface plasmon mode 2 ness and variable absorption col angle of incidence in this region the plasmon resonance as the absorption increases. Note In Fig. 3 we display the normalized amplitude of the that the angular half-width of these resonant curves in surface plasmon mode electric field at the plasmon angle creases with absorption. For large values of absorption evaluated at the metal-air boundary as a function of metal the plasmon resonance is heavily damped and the reflec film thickness for several values of Eg. Note the amplitude tivity minimum is no longer sharp The effect of damping scale is logarithmic. For zero absorption the amplitude of on the surface plasmon resonance has been discussed by this field increases exponentially with the thickness of the several authors. 10 The reflectivity minimum at the plas- film; however, for nonzero absorption this intemal field mon resonance is due to the increased absorption of the amplitude reaches a maximum, which may be an order enhanced internal fields throughout the volume of the of magnitude larger than the incident field. By compari- metal film. Integrating the power absorption density over son the normalized amplitude of the other field in the the irradiated volume of the film and normalizing by the metal film for zero absorption is identically zero, as pre- incident power, we have the enhanced absorptance due to viously discussed; however, for finite absorption this field the surface plasmon, given by is nonzero, although it is still several orders of magnitude smaller than the surface plasmon mode field shown in r= wLexp(2kd)-1]cosB2IE2' (14) The quantity of experimental interest is the reflectance which is shown in Fig. 4 as a function of the angle of incidence for a fixed film thickness. Again for zero ab- The above quantity is plotted in Fig. 5. Clearly, the max- sorption the reflectivity is unity but decreases sharply near ima in the absorptance match the minima in the reflec- thickness and variable ANGLE OF INCIDENCE e Am J, Phys, Vol 43, No. 7, July 1975 /63