Long-range surface polaritons in thin layers of absorbing materials

Long-range surface polaritons (LRSPs) are electromagnetic surface modes confined at the interfaces of an thin film surrounded by a homogeneous dielectric. These modes are generally characterized by the subwavelength confinement and the long propagation length. In case of a metallic thin film, the mode is named long-range surface plasmon polaritons (LRSPPs). Because of the special optical properties mentioned above, LRSPPs have appeared in a large number of studies both fundamental and applied, including modal studies, prism experiments, field enhancement, biosensors etc.. However, much less attention has been devoted to LRSPs on strongly absorbing dielectrics. In this thesis, we present a fundamental study of the optical properties of LRSPs supported by thin layers of strongly absorbing materials. First, a detailed theoretical formulation of surface modes in arbitrary thin layers is given in chapter 2. The differences between LRSPPs, LRSPs and TM0 mode are discussed further in terms of dispersion relations and electromagnetic field profiles for the case of arbitrary thin layers and in real materials, i.e., silver and amorphous silicon. Depending on the permittivities of the thin layers, guided modes can be supported either on metal or absorbing dielectrics. The rare existing condition of LRSPs is j²2r j . ²2i (independent of the sign of ²2r ) and ²2i À 1. We also discuss the dispersion relation of LRSPs in the thin film approximation in which we use this expression in chapter 3 and chapter 6. To investigate the dependence of surface modes supported by lossy thin films to their permittivity, we show in chapter 3 that a 20 nm thin film made of chalcogenide Ge17Sb76Te7 glass can support LRSPs in the visible. This chalcogenide glass has two phases based on the different heating temperature, i.e., an amorphous and a crystalline phase. By changing the phase of this material, the real components of permittivity change the sign while the imaginary components remains large. From the dispersion relation of LRSPs on both crystalline and amorphous layers obtained from the experiment and mode calculation, we demonstrate the relative insensitivity of LRSPs to the real component of the permittivity of the thin films. In chapter 4, we illustrate the excitation of long-range surface polaritons LRSPs in ultra-thin films of amorphous silicon (a-Si) at visible and UV frequencies. As a comparison, we calculate the propagation length and confinement length of LRSPPs and LRSPs. The figure of merit of waveguides in UV and visible is given, we conclude that the LRSPs have similar characteristics to LRSPPs on gold although LRSPs are guided by thin layers of strongly absorbing materials. Excitation of longrange surface modes in absorbing Si opens the possibility to surface polariton optics compatible with standard Si processing technology. In chapter 5, we determine the dispersion of long-range guided modes in thin layers of a-Si by attenuated total internal reflectance measurements in layers with different thickness. The dispersion relation of guided modes on a-Si move away from the light cone when the thickness of the a-Si layer increases (d > 10 nm). The corresponding propagation length and confinement length are also modified by controlling the dispersion relation. An evolution between LRSPs and TM0 mode is also found when a-Si layer thickness increase from 13 to 23 nm. In chapter 6,we showthat strong optical absorption in materials can be beneficial to improve the sensitivity of surface polariton sensors. In particular, we experimentally prove that LRSPs supported by nanometric films of strongly absorbing chalcogenide glasses can be used as sensitive probes to changes in the refractive index of the surrounding medium. We also calculate the sensing figure of merit for different types of long-range surface modes on different materials. In the thin film approximation, an analytical expression of sensing figure of merit is given for LRSPs. This result provide alternative solutions for optical sensing that are CMOS compatible rather than noble metals as the sensing substrate. In Chapter 7, we demonstrate experimentally the excitation of coupled longrange surface polariton modes in multilayers of a-Si at visible frequencies. These different surface polaritons exhibit narrow resonances that result from the exclusion of the electric field from the thin absorbing layers. We model this reflectance measurement and the corresponding electric filed profile by using the transfer matrix method. Calculations of the electromagnetic field symmetries of the different resonances in the multilayer structure reveal the coupling between different LRSPs. Multilayers of strongly absorbing dielectrics may find application as surface polariton resonance sensors operating at short wavelengths or to enhance light absorption in ultra-thin layers of semiconductors.