Fig. 8. Experimental reflectance of the surface plasmon mode versus angle of incidence in a silver film 560 A thick on a crown glass prism z at the He-Ne laser wavelength 6328 A AN GLE。F| NCIDENCE minima. When viewed from the hypotenuse face, too may then be compared with Eq (1). The observed back thick a film will appear opaque while too thin a film will bending in the near ultraviolet of this experimentally de- appear semitransparent; however, the apex edge of the termined dispersion curve is a result of the damping of prism should be visible through the metal film. If the sur- the surface plasmon. o Another experiment is to use other face plasmon resonance is not observed, the prism should metals which can be easily evaporated, for example be recleaned and the evaporation process repeated again. aluminum, gold, and copper. The relevant parameter for Since this process requires only a few minutes, new sam- estimating the strength of the surface plasmon resonance ples of varying thickness may be easily prepared until the is the magnitude of the ratio ey/E2. For silver in the near desired effect is achieved. Film thickness may be deter- infrared, this ratio is larger than that of the other met- mined by placing a microscope slide adjacent to the prism als. 4 The sharpest reflectivity minima will now occur for transmission of the coated slide. For the 560-A film used film, Turbadar2 gives a thickness of 125 a at a in this experiment, the transmission of the slide was ap wavelength of 5500 A. The dispersion of surface plas proximately 1% at 6328 mons in gold films has been studied by Barker. 5 The The numerical calculations for the curves presented in opportunity exists to make original observations of the Figs. 2-6 were performed on a PDP-11 time-sharing surface plasmon resonance in many metals by use of the computer. The complex dielectric constant of the silver technique we have described here film2 was E=-18.3+i0. 4 at the He-Ne laser In conclusion, we have given a theoretical description wavelength(6328 A), and the index of refraction of the of the effect of the surface plasmon mode on the reflectiv- crown glass prism was taken to be n =1.52. The optical ity of thin metal films and have demonstrated how such roperties of the silver film are sensitive to the conditions films may be prepared with a modest apparatus. The of film preparation (especially at our vacuum pressure of dramatic change in reflectivity of a silver film should not 10-3 Torr) and to the growth of silver sulfide layers on only provide a stimulus in the laboratory for physics stu the film in air; therefore, the value of e2 was allowed to dents, but should also provide a dramatic effect in a lec- take on a wide range of values to test the sensitivity of ture demonstration for a more general audience the reflectance to this parameter. We see from Fig. 4 that the qualitative features of the reflectivity minimum should be observable over a large range of eg. The results dis played in Fig. 8 were repeatable with the same film main- tained in the ambient laboratory environment over a "For a review of surface plasmon physics, see R.H.Ritchie, Surf.Sci period of several weeks. Although the films should 34,1(1973) evaporated under high vacuum 9 Torr) and remain inT.Turbad S, soc vacuum during the experiment, such stringent conditions 'A. Otto, Z. Phys. 216, 398(1968); Phys. Status Solidi 42, K37 are not necessary for the qualitative observation of surfac plasmon phenomena There are several interesting extensions of this ex ment. One experiment is to replace the He-Ne laser C. Kittel, Introduction to Solid State Physics (Wiley, New York source, which was chosen solely for its experimental con- venience, with a collimated monochromator. By measuring K, w. Chiu and JJ. Quinn, Nuovo Cimento B 10,1(1972).This the angle at which the reflectivity minimum occurs as a function of wavelength, one can experimentally determine dispersion relation the dispersion relation for the surface plasmon. This curve Borm and E. Wolf, Principles of Optics (MacMillan, New York Am, J. Phys. VoL, 43, No. 7, July 1975 Simon, Mitchell, and Watson /635