Changes In Dielectric Function Research Articles

Electron dynamics and surface plasmon resonance nonlinearities in metal nanoparticles

The ultrafast surface plasmon resonance nonlinearities and their connection with the conduction band electron dynamics are discussed in metal nanoparticles in the light of the results of high sensitivity femtosecond pump-probe experiments in silver nanoparticles embedded in a glass matrix. The optical response is interpreted in terms of frequency shift and broadening of the surface plasmon resonance and is related to the changes of the metal nanoparticle dielectric function induced by ultrafast perturbation of the electron distribution. Alteration of the interband absorption is found to be responsible for the observed red shift and very short time delay broadening of the surface plasmon resonance, in agreement with numerical simulations and with measurements in silver films. On a longer time scale, a new nonlinear mechanism due to increase of the electron scattering off the surfaces is demonstrated. This mechanism, specific to confined system, plays an important role in the ultrafast nonlinear optical response of small nanoparticles.

Local Density Information From Low Energy-Loss Peels Measurements

It is known that radiation produces microscopic and macroscopic changes in silicas and other silicate glasses that result in densifications of several percent [1], This change in density is matched by a corresponding linear change in refractive index via the Lorentz-Lorenz relation such that where R is the molar polarizablilty, V the molar volume and n the refractive index.- Measurements of neutron irradiated bulk a-Si02 yield an exact correlation between the refractive index and density as shown by Fig. 1 such that dn/dp =185 m3/kg [ 1 ].Utilizing a VG HB603 STEM fitted with a parallel EELS, measurement of irradiation-induced density changes in silicas were attempted using the 0-100 eV low-loss region of the electron energy-loss spectrum. The dielectric function and refractive index changes were extracted from these data using a modified Kramers-Kronig analysis (Fig. 2).

Magneto-optical Kerr effect and optical properties of amorphous Co1−xSix (0.59⩽×⩽0.77) alloy films

The Kerr spectra and optical constants of amorphous Co1−xSix (0.59⩽×⩽0.77) alloy films were measured in the spectral range 1.6–3.5 eV. It was found that there is rather considerable Kerr rotation near the high-energy side of 3.5 eV for the films (0.67⩽×⩽0.75), and there is a maximum value of −0.35°, which is comparable with that of pure Co, at x=0.75. The considerable Kerr rotation was ascribed to the following two causes: (i) the finite, but nonzero, nondiagonal element of dielectric function; (ii) the large optical constant effect due to the appropriate diagonal element of dielectric function. These changes of dielectric function originated from the metal-to-semiconductor transition controlled by Si concentration. There may be potential magneto-optical application in blue light range for such kinds of material.

Surface and bulk effects in ex-situ hydrogenated α-SiC thin films

Abstract The effect of ex-situ hydrogenation on the bulk and surface properties of amorphous SiC (α-SiC) thin films has been investigated using a number of bulk- and surface-sensitive characterisation techniques. The diffusion coefficient of deuterium in α-SiC is found to be equal to 3–4 × 10 −15 cm 2 s −1 . It is shown that atomic hydrogen causes drastic changes in the dielectric function, due to preferential Si etching from the film surface, and the formation of a carbon surface layer rich in sp 2 bonds. Furthermore, the atomic hydrogen diffuses in the bulk and causes an increase of the optical band gap and a concurrent decrease in conductivity, due to partial passivation of dangling bonds.

Surface dielectric change of GaAs determined from surface photo-absorption spectra

We compare surface photo-absorption spectra observed for various surface structural transformations on GaAs (001) and (111)B surfaces. Surface conversions investigated here are (i) from (2×4) to (3×1) caused by Ga deposition, (ii) from (2 × 4) to (1 × 1) caused by H-adsorption on GaAs (001) surface, and (iii) from (2 × 2) to (√19 × √19) caused by As desorption on (111)B surface. The change in surface dielectric function is determined for each of the surface conversions using curve fitting based on the harmonic oscillator model of the dielectric function. Common peaks that coincide with the critical points of the dielectric function of GaAs are found in the spectra of the surface dielectric change thus determined. This indicates that the effect of bulk electronic states modified by the surface is significant in the surface photo-absorption spectra.

Microwave dielectric function of charge density wave conductors

A calculation of the dielectric function for the charge density wave conductor NbSe 3 is presented. It is based on experimental results of the frequency dependent conductivity showing a roll-off at high frequencies, and applies one of the Kramers-Kronig relations. As a consequence of the roll-off, the dielectric function changes sign at the lower end of the microwave range of frequencies. Consequences of this behaviour on microwave harmonic mixing are indicated.

Shift of surface-plasmon energy across the phase transitions of silicon reconstructed structures for several crystal planes

By electron-energy-loss spectroscopy, energy shifts of the surface plasmon were observed across the phase transitions for several reconstructed structures of silicon surfaces. The surface-plasmon energy shifts by 1.0-1.6 eV, depending on the phase transitions for Si(111) and Si(110) surfaces. A bulk plasmon and other peaks associated with the interband transitions between localized electronic states did not exhibit any notable change. For the Si(100) surface, no shift occurred, corresponding to the nonexistence of a phase transition. The energy shift of the surface plasmon was found to reflect the surface structure change. Two kinds of qualitative interpretation of this shift are presented. One is a change of dielectric function and the other is a change of the electronic density distribution at the surface due to the phase transition.

Chemisorption-induced changes of the surface dielectric function for W(110)

The change in reflectance due to adsorption of ${\mathrm{H}}_{2}$, ${\mathrm{O}}_{2}$, and CO on W(110) was measured for photon energies in the range 0.5 to 6.0 eV. The changes in dielectric function and joint density of states were obtained and analyzed in terms of vertical transitions in the projected band structure of the W(110) surface Brillouin zone. A new surface resonance is found above the Fermi level.

Metal‐Nonmetal Transition in Metal‐Ammonia Solutions

AbstractThe properties of metal‐ammonia solutions in the concentration range of the metal‐nonmetal transition between 0.1 and 10 mol percent metal (MPM) are discussed. The high‐frequency electrical properties show a number of characteristic features in this intermediate concentration range, such as additional dielectric relaxations at about 0.1 MPM and 0.7 MPM for LiNH3 and NaNH3, respectively, and a resonance‐like behavior at about 1.0 MPM and 1.5 MPM for LiNH3 and NaNH3, respectively. For both LiNH3 and NaNH3 the real part of the high‐frequency dielectric function changes to large negative values around 2.5 MPM, indicating a transition to metallic behavior at high frequencies. The high‐frequency properties, which are also discussed in terms of electron conductivity, are explained quantitatively as a function of concentration, temperature, and frequency using a model of polaron hopping. The metal‐nonmetal transition is related to structural changes in the solutions which lead to phase separations in LiNH3 and NaNH3 at lower temperatures. The structural properties of the solutions are investigated using sound velocity and ultrasonic attenuation data. Concentration and temperature dependence of the ultrasonic attenuation in LiNH3 and NaNH3 can be described quantitatively in terms of structural fluctuation.

High-frequency Franz-Keldysh effect on excitons

When the effective characteristic electro-optic energy ħ|Λ H|, which is obtained by including the electron-hole interaction in the solid, is very large compared to ħω of the modulating electric field, the line shapes of the field-induced change of dielectric function are the same as those of Franz-Keldysh effect. However, in the low-field and high-frequency limit Δ∈ 2(Ω, E is nearly proportional to the first derivative of the unperturbed dielectric function. It is different from the low-field and low-frequency limit case where Δ∈ 2(Ω, E ) is proportional to the third derivative of the unperturbed dielectric function.

Magnetoreflection Measurements on the Noble Metals

A technique is described which allows the change in optical dielectric function of a solid due to an applied magnetic field to be measured. Results are presented for Cu, Ag, and Au in the photon energy range 2-5.5 eV.