A. V. Zayats

Direct observation of surface polariton localization caused by surface roughness

Abstract Using a photon scanning tunneling microscope (operating at the wavelength of 633 nm) with shear force feedback we probe directly an optical field of surface plasmon polariton (SPP) while imaging simultaneously surface topography. We observe that near-field optical images, which are generated due to the SPP excited at a rough gold film surface, exhibit spatially localized (within 150–250 nm) intensity enhancement by up to 7 times. We find that the positions of these bright light spots do not correlate with the local surface topography and depend on the angle of exciting beam incidence. We relate the observed phenomenon to the strong localization of SPPs caused by surface roughness

Fractal surface characterization: implications for plasmon polariton scattering

Abstract We compare the scattering of surface plasmon polaritons (SPPs) at relatively smooth and rough surfaces of gold films and relate the difference in the SPP behaviour to the difference in fractality of the studied surfaces. The rough surface, which results in the strong localization of SPPs, is found to have a fractal structure with a normalized fractal dimension D ≈ 2.26 in the spatial range 80–640 nm. It is shown that the smooth surface, which supports well-pronounced propagating SPPs, cannot be viewed as a fractal structure for discretization steps exceeding 80 nm. We conclude that in order for SPPs to be localized by surface roughness, the surface should exhibit a fractal structure in a sufficiently large range of sizes around the SPP wavelength.

THE EFFECT OF THE SURFACE ENHANCED POLARITON FIELD ON THE TUNNELING CURRENT OF A STM

Abstract The influence of surface polaritons (SPs), excited on the external (fast mode) and internal (slow mode) surfaces of a gold film, on the tunneling current of a STM is investigated. The role of the surface polaritons is directly demonstrated by the angular excitation spectra of the induced tunneling current. Possible mechanisms which might give rise to the SP induced change in the tunneling current are discussed. An electromagnetic field of SPs within a tunneling gap is estimated to be surface enhanced in order to explain the value of the induced tunneling current. Images of the polariton induced signal distribution over the sample surface taken by means of different SP modes are compared in order to obtain information on the structure of the internal surface of the film.

Characterization of the linear optical properties of a multiple quantum well structure in the sheet-model approximation

Abstract The possibility of determining the linear optical response tensor of a multiple quantum well (MQW) structure from optical reflection spectra is discussed. Taking into account local-field effects, the optical parameters of the MQW system are derived directly from the sheet conductivity tensor of the individual quantum well. For an amorphous Si/SiO 2 MQW structure deposited on a crystalline Si substrate, both s- and p-polarized reflection spectra were measured at different angles of incidence in the near-infrared and visible frequency regions. It was shown that the s-polarized spectra can be characterized by means of only one (complex) parameter, whereas in general it is impossible to account for the p-polarized spectra parametrically in a full range of angles of incidence even with four angular independent parameters. Finally, it was demonstrated that Feibelmans d -parameters can only be used to describe the experimental spectra at near-normal and grazing angles of incidence.

Thickness dependence of optical second-harmonic generation from ultrathin niobium films

Abstract The optical second-harmonic generation from ultrathin niobium films embedded in a dielectric is measured as a function of the film thickness (in the range 6–42 A) for the p ω -to- p 2 ω and s ω -to- p 2 ω polarization configurations. Using a pump wavelength of 532 nm, it is found that the p ω -to- p 2 ω second-harmonic intensity exhibits a pronounced maximum for a film thickness of ∼ 15 A. The s ω -to- p 2 ω second-harmonic signal exhibits a step-like increase at the same thickness. By modeling the metal film as a symmetric quantum well, a microscopic local-field calculation of the second-harmonic generation is established, and numerical results for the thickness and angular dependencies of the second-harmonic energy reflection coefficient are presented. Using simply infinite-barrier wave functions together with a self-field approximation the theoretical calculation qualitatively accounts for the observed thickness dependence of the second-harmonic generation.

Radiative recombination in short-period a-Si/SiO2 superlattices

Abstract Photoluminescence and photoluminescence excitation spectra of short-period (⩽20 A) a-Si/SiO 2 superlattices have been investigated under continous and picosecond laser excitation. Three types of radiative transitions (intersubband luminescence in a-Si layers, cross-luminescence between states of well and barrier layers and impurity luminescence in a-SiO 2 layers) were observed in ps excited SL, as contrasted to continous excitation when only impurity recombination in a-SiO 2 layers took place. A significant increase (by about 10 4 times) in the impurity luminescence intensity in a-SiO 2 was observed in superlattices compared to that in isolated a-SiO 2 layers. It is caused by a transfer of excited carriers between SL layers. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between subbands of a-Si nad a-SiO 2 impurity states in SLs. Relaxation of excited carriers between the subbands within the same band is low. No evidence was found of the influence of quantum size effects on deep impurity states in a-SiO 2 . Fast nonradiative recombination in a-Si layers of SLs results in the absence of luminescence in these layers under continuous excitation.

Linear optical properties and second-harmonic generation from ultrathin niobium films: A search for quantization effects

We present the experimental investigations of the optical properties of ultrathin niobium films embedded in a dielectric that form quantum well structures. The linear optical properties of the Al/sub 2/O/sub 3/-Nb-Al/sub 2/O/sub 3/ structures in the near infrared and visible spectral ranges exhibit periodic dependencies on the Nb layer thickness that can be explained by the influeuce of quantum size effects on the electron scattering rate. The absorption bands that can be ascribed to the intersubband transitions have not been found in the linear optical spectra. The investigations of the optical second-harmonic generation from the Nb films show that the thickness dependencies of the second-harmonic intensity reveal resonant behavior that can be accounted for by the intersubband transitions in the niobium quantum wells. The microscopic local-field calculations qualitatively explain the thickness dependence of the SHG observed in the experiment. >

Optical second-harmonic generation from ultrathin niobium films

Experimental and theoretical investigations of the optical second-harmonic generation (SHG) from ultrathin niobium films embedded in a dielectric are presented. The dependence of the intensity of second-harmonic emission on the film thickness (in the range from 6 to 42 angstroms) and the angular dependencies of SHG are investigated for different polarization configurations. The thickness dependencies of the second-harmonic intensity reveal resonant behavior: the p(omega )-to-p2(omega ) second-harmonic intensity exhibits a pronounced maximum for a film thickness of approximately 15 angstroms while the s(omega )-to-p2(omega ) second-harmonic signal exhibits a step-like increase at the same thickness. By modeling the metal film as a symmetric quantum well, a microscopic local-field calculation of the second-harmonic generation is performed, and numerical results for the thickness and angular dependencies of the second-harmonic energy reflection coefficient are presented. Using simply infinite-barrier wave functions together with a self-field approximation the theoretical calculations qualitatively describe the observed thickness dependence of the second-harmonic generation which can be accounted for by the intersubband transitions in the quantum wells formed by the niobium films.

Weak Localization of Surface Plasmon Polaritons: Direct Observation with Photon Scanning Tunneling Microscope

Using a photon scanning tunneling microscope with shear force feedback we directly probe optical fields of surface plasmon polaritons (SPPs) excited at surfaces of two silver films with different roughness. We observe that near-field optical images exhibit interference between the excited and multiple scattered SPPs. The orientation and period of interference fringes observed relatively far from surface scatterers indicate the presence of the backscattered SPP. These fringes are found to be more pronounced for the silver film with larger roughness. We relate the observed phenomenon to weak localization of SPPs caused by multiple SPP scattering in the surface plane.

Two types of hot luminescence in Si/SiO2 superlattices

Abstract Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of short-period (⩽ 20 A ) a-Si/SiO 2 superlattices (SLs) have been investigated under picosecond excitation. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between the subbands of a-Si and impurity states of a-SiO 2 in SLs. Relaxation of excited carriers between subbands within the same band was not observed.