Nanophotonics of surface states in photonic crystals
Research joined under the name “Nanophotonics of surface states in photonic crystals”, carried out in the laboratory, are aimed to the study of properties of surface electromagnetic waves (SEWs), Tamm plasmon polaritons (TPPs), guided modes, which arise inside or at the surface of photonic crystals. Photonic crystals are media with artificially modulated dispersion and their main feature is existence of the photonic bandgap. In our studies we use different techniques such as optical and nonlinear-optical spectroscopy, cross-correlation spectroscopy with a femtosecond temporal resolution, photonic-force microscopy in optical tweezers.
Practical interest about the study of surface states properties is stimulated by the possibility of their application in constructing new types of compact sensors and lasers. Fundamental interest about the study of the surface states is associated with the opportunity of the new optical phenomena observation. Excitation of the surface state leads to the localization of the electromagnetic field near the surface of a photonic crystal. Experimentally it manifests itself as a narrow resonance dip in the reflectance spectrum of the structure. Goos-Hanchen effect is the phenomenon of lateral shift of trajectory of the totally internally reflected beam comparing to the beam, reflected from a perfect mirror. This shift is a result of different phase shifts which different components of the spatially-limited beam obtain under reflection. Goos-Hanchen shift of the beam at the dielectric interfaces reaches the order of few wavelengths. It was shown that Goos-Hanchen effect can be significantly enhanced in the vicinity of the SEW resonance. We observed the SEW-induced enhancement of the Goos-Hanchen effect, which considerably exceeds the values observed on the interfaces of dielectrics or surfaces of metals in presence of the surface plasmon polaritons.
Effects of the second harmonic generation (SHG) enhancement in micro- and nanostructures due to increase of the local electromagnetic field intensity are well-known. Particularly, this enhancement takes place in the regions of surface (or localized) plasmon polaritons or microcavity modes excitation. According to the theory arise of the TPP leads to the localization of the electromagnetic field at the interface of metal and PC. Hence one can expect the amplification of the SHG in the region of the TPP excitation. In our laboratory, the experimental evidence of the SHG enhancement in gold/PC structure was obtained. Now we keep on working on the study of the nonlinear-optical properties of the surface states.
The possibility of study effects on the femtosecond time scale came into sight with the development of the ultrafast laser systems. These investigations allow to determine temporal properties of the excitation and relaxation of the surface states. The interesting problem is the measuring the temporal constants of the TPP excitation and propagation. We observed the modification of the cross-correlation functions of the femtosecond laser pulses, which were reflected from the gold/PC structure under conditions of the TPP excitation, and designated the lifetime of the TPP.
Study of the force characteristics of the surface states is performed with the optical tweezers. In optical tweezers highly focused laser beam is used to hold or move dielectric particles. In the case of small displacements of the particle from the center of the optical trap, the restoring force acting to the particle is proportional to the electromagnetic field gradient, which determines the possibility of using the optical tweezers as a dynamometer. Force measurements carried out in the optical tweezers are called “photonic-force microscopy”. We study the force characteristics of the surface states in photonic crystals. It was shown that the sensitivity of the photonic-force microscopy is very high and reaches the order of femtonewtons. This means that one can not only measure the force acting to the particle from the surface state, but also compare it to the force acting to the particle when no surface state is excited.