Photoacoustic study of phase correlation of surface plasmons in a diffraction grating

Abstract

.Nonradiative surface plasmons may be excited in a metal-coated diffraction grating by either p­ or s-polarized photons if the ruling is oriented neither perpendicular nor parallel to the plane of photon incidence. Phase correlation between these surface plasmons in a 520-line/mm Ag-coated diffraction grating was observed by measuring absorption of elliptically polarized 633-nm photons at the plasmon resonance angle. The absorption measurements were made by using a photoacoustic technique. The experimental results were analyzed by a first-order perturbation theory of photoab­ sorption in a rough surface. This allowed us to determine the phase difference between surface plasmons excited by p- and s-polarized photons. I. INTRODUCTION In contrast to conventional spectroscopic methods in which light intensities are measured outside the sample under study, photoacoustic methods 1 measure absorption of photons by probing heat generation occurring inside the sample as a result of nonradiative dissipation of excited states created by incident photons. This makes possible new experimental studies which have been difficult to per­ form by conventional experimental methods. In recent years we have used 2 - 6 this photoacoustic technique to study excitation and relaxation of surface plasmons 7 in metals by using the attenuated-total reflection (ATR) and grating methods. Most recently, we have investigated 6 plasmon resonance absorption in a Ag-coated diffraction grating as a function of the ruling orientation relative to the plane of incidence. Such a study has been hitherto dif­ ficult to perform by conventional spectroscopic methods because photons are diffracted out of the plane of in­ cidence. Surface plasmons may be excited in a diffraction grating by both p- and s-polarized photons if the ruling is neither perpendicular nor parallel to the plane of in­ cidence. These surface plasmons are excited at the same angle of photon incidence for both p and s photons and propagate in the same direction along the grating surface and are thus coherent to each other. Therefore, if they are excited simultaneously by photons having both p and s components in a certain phase relation, the phase correla­ tion of the plasmons may be observed directly in the reso­ nance absorptance values. In this paper we report experi­ mental results of this phase correlation for surface plasmons in a Ag-coated shallow diffraction grating. The plasmon resonance absorption was measured by the same photoacoustic method as in our previous studies 2 - 6 as a function of the relative phase between p and s components of elliptically polarized incident photons. The experimen­ tal results were analyzed numerically with the use of a first-order perturbation theory 8 of photoabsorption in a rough surface. This allowed us to determhle the phase difference between surface plasmons excited by p- and s­ polarized photons.