Experimental Observation of Surface Exciton Polaritons

Abstract

Surface exciton polaritons are observed in a semiconductor for the first time. The modes are excited in ZnO crystals by the method of attenuated total reflection. Changing the angle of incidence of the ultraviolet radiation gives an experimental dispersion relation for surface exciton polaritons.Surface excitons have been treated theoretically by several authors, 1 " 5 Polariton dispersion relations and the reflectivity of crystals for the attenuated-total-reflection (ATR) method have been calculated. 4 ' 2 The surface exciton frequencies always lie between the transverse and longitudinal resonance frequencies oo T and oo L of the bulk excitons for a given wave vector. Because of spatial dispersion, surface excitons have among surface excitations the unique property of coexistence with a bulk mode in the energy region between oo T and ix) L . Consequently, energy transfer between these modes is possible. 3 Surface modes can be excited by an ATR arrangement. This method uses a prism to couple electromagnetic waves of wave vector k > oo/c vac across a thin spatial gap with the sample surface. The ATR method has been applied successfully to detect surface phonon polaritons and surface plasmons in various media. 6 Extending ATR to the shorter wavelengths of surface excitons is difficult because of the stringent requirements on control of the spacing between crystal and prism and because of exciton-free surface layers. 4 ' 7 According to calculations by Maradudin and Mills, 2 the reflectivity changes &R/R in an ATR experiment are expected to be of the order of 10 " 5 . All these reasons may account for the fact that experiments on surface excitons have not been reported until now. However, a direct observation of surface excitons would offer new possibilities for obtaining information about exciton-free surface layers and spatial dispersion.In this paper we report the first optical excitation of surface exciton polaritons in a semiconductor. We used an arrangement of attenuated total reflection at helium temperatures. Our experimental results give the dispersion relation of surface exciton polaritons in the short-wavelength region near the band gap of ZnO crystals.A careful choice of the crystal material is necessary to obtain the best conditions for the experimental excitation and observation of surface excitons. It is known for some materials that image charges or field-induced ionization of excitons creates an exciton-free surface layer at least 1 to 2 exciton Bohr radii thick. 7 ' 8 The exciton-free surface layer has to be as thin as possible in order to excite excitons by an ATR technique which uses the evanescent waves "leaking out" of a prism while light is totally reflected inside. Only for thin exciton-free surface layers are the exponentially decaying electromagnetic waves still sufficiently intense at the crystal regions where excitons exist. From the free-exciton reflectance spectra it is known that the exciton-free surface layer of ZnO is less than about 30 A, 9 probably the smallest one of the II-VI semi...

Surface exciton polaritons in individual Au nanoparticles in the far-ultraviolet spectral regime

“…Experimental verifications of the existence of SEPs in such a gaplike regime have been reported for ZnO, 11,12 CuBr, 13 and organic semiconducting ␥-cyclo-propyl-bis ͑1,3,3-trimethylindolenine-2-yl͒ pentamethinium-tetrafluoroborate ͑CTIP͒ 14 in the visible spectral regime, where all these materials show a sharp oscillator strength with i2 ӷ ͉ r2 ͉Ϸ0 as expected. [4][5][6] Indeed, it had been documented that onsets of the transverse, single-particle excitonic absorption T could lead to the formation of delocalized Wannier-type excitons in the bulk of materials, readily building up a relevant longitudinal, collective bulk mode at a higher frequency P .…”

“…In Fig. 1͑a͒, the small k r of the antisymmetric mode at d Ͻ 10 nm, k r Ͻ k rϱ Ϸ k 0 , 4,5 indicates that this mode can be directly excited by light, whereas excitations of the symmetric mode at the given d showing k r Ͼ k rϱ would require designated optical techniques [4][5][6][11][12][13][14] or other tools, such as EELS, that provide the extra needed momentum. 26 At finite d ͓Ͻ ϳ 5 nm, Fig.…”

“…For a very thin sample, the surface wave fields from both interfaces start to couple to each other, leading to the splitting of SPPs into symmetric and antisymmetric SPP modes that are distinguished by the respective symmetry of the electric field distributions at the interfaces. 1,7,8 In distinct contrast to SPPs, the excitations of surface exciton polaritons ͑SEPs͒, which are collective oscillations of delocalized excitons at the surface of materials, [9][10][11][12][13][14] had been shown to be correlated with sharp transverse excitonic onsets in the materials, i2 ӷ ͉ r2 ͉Ϸ0. 4,5 A larger excitonic absorption i2 results in a narrower SEP spectral feature and the sign of r2 does not affect the excitations of SEPs, [4][5][6] both at odds with the conventional wisdom of SPPs.…”

Exciton-plasmon coupling interactions: from principle to applications

“…The surface of solid that is illuminated by a beam of incident light will produce a bound pair of electron hole due to the absorption of photons [49,50]. In the meantime, the electrons of the conduction band and the holes of the valence band form a new bound state due to the Coulomb interaction, which is often called exciton.…”

Optical Field-Enhancement and Subwavelength Field-Confinement Using Excitonic Nanostructures

“…19 Surface polaritons based on excitonic materials have been known for many years. Early work involved materials that needed low temperatures, both inorganic solids: ZnO, 19 CuBr, 20 ZnSe, 21 CuCl, 22 and one organic solid, anthracene. 23 Some organic dye molecules also exhibit strong, narrow excitonic absorption bands, for example J-aggregated dye molecules.…”