## 3 Experimental Results and Discussion  
Schematic of an experimental setup together with the data attesting an observation of very narrow angular plasmon resonance in the light reflection from the aforementioned PC + Ag layer structure are presented in Figure 2. For the measurements of LRSPP, standard Kretschmann configuration employing quartz 45\({}^{0}\) prism on which the structure at question has been \"glued\" via a droplet of phase matching oil has been used [28]. Standard p-polarized He-Ne laser light (beam diameter around 5 mm) has been focused on the structure studied with 10 cm focal length lens, and the angular structure of the reflected light has been recorded observing the light spots at distances of 162 and/or 405 cm (both recordings of the light intensity with CMOS (complementary metal-oxide semiconductor) camera and processing of color digital photographs of the corresponding light spots were used leading to essentially the same results). Surface plasmon excitation has been observed for conditions quite close to the normal incidence of the incoming light beam onto the prism entrance face: the corresponding angle was equal to 0.3\({}^{\circ}\) which results in the incidence angle onto the prism hypotenuse \(\vartheta=44.8^{0}\) giving the propagation parameter \(\rho=1.04\). Deviation of this value from the theoretical one is due to the deviations of actual refraction indices and layer thicknesses from the (theoretical or \"searched for\") values used in calculations, and this is quite common for the field. Another indication of the narrowness of the plasmon resonance in our structure has been obtained when we, in the same configuration, used unfocused He-Ne laser beam: for an optimal adjustment, the intensity of reflected light dropped from 5.9 mW (out of resonance) to 2.6 mW (in resonance) thus attesting that more than one half of an incoming radiation power has been transferred into the surface wave. The same experiment also was used to demonstrate the polarization dependence of the observed reflection minimum: the difference in the intensity of light reflected for out of- and in-resonance conditions well followed the content of the \(p\)-polarized light in the incoming laser beam (itwas changed by the rotation of He-Ne laser having the polarization ratio not smaller than 50:1 without a polarizer), and such a difference was completely undetectable for the pure \(s\)-polarization case. The width of the angular resonance curve in Figure 2 is equal to ca. \(\Delta\theta_{air}=8\cdot 10^{-4}rad\) which corresponds to the surface plasmon propagation length \(l\) of approximately 180 microns. (Standard formula \(l=\frac{\lambda}{2\pi\Delta\theta_{pr}n_{pr}\cos\theta_{pr}}\) was used. Here, the subscript \(pr\) corresponds to the values inside the prism; the value of \(\Delta\theta_{pr}\) can be recalculated from the experimentally measured \(\Delta\theta_{air}\) by applying [PERSON]'s law. For our case of almost normal incidence of light onto the prism cate \(\Delta\theta_{air}\simeq n_{pr}\Delta\theta_{pr}\)). Comparable, but slightly worse results with the angular width somewhat exceeding \(10^{-3}\) rad were obtained when using a semiconductor cw fiber Bragg grating laser diode at the wavelength of 636.7 nm (QPhotonics LLC, Ann Arbor, MI, USA). This is the main result of our research, and it unambiguously attests the first realization of LRSPP for PC-supported silver nanofilms. The long-term stability of the prepared structures was noticed: the reflection parameters remained virtually the same during nine months of observations; the sample has been kept in usual room conditions in a polyethylene packet (not hermetically sealed). The experimentally observed value \(\rho=1.04\) enables to determine the actual distribution of the square (\(EE^{*}\)) of electric field of SPP both inside an external medium (air, for which we take the refraction index equal to unity) and PC.