Free Electron Relaxation Time Research Articles

Temperature dependence of the anisotropy of the infrared dielectric properties and phonon-plasmon coupling in n-doped 4H-SiC

Polarized infrared reflectivity spectroscopy is used to measure the temperature dependence of the anisotropy of the infrared dielectric properties and the coupling between phonon and plasmon modes in n-doped 4H-SiC. The advantage of polarized infrared spectroscopy over the commonly used Raman spectroscopy is that it enables dielectric, plasmonic, and transport properties to be probed along and perpendicular to the c-axis, which is impossible with Raman spectroscopy due to its selection rules. In addition, the infrared spectrum, comparatively rare for Raman spectroscopy, can be described at high temperatures with simple classical theory, providing more precise information on the properties of the material being measured. Analysis of the s- and p-polarized infrared reflectivity spectra of an n-doped 4H-SiC substrate shows that the strength and damping of the oscillators, the response of the bound electrons to an infrared excitation, the effective ion mass, and the transverse frequency and anharmonicity of the phonons of n-doped 4H-SiC are temperature-sensitive and strongly anisotropic. It also shows that the plasmonic properties of n-doped 4H-SiC, namely the relaxation time and the collective oscillation frequency of free electrons, are also highly anisotropic and, due to the long relaxation time of free electrons in the direction parallel to the c-axis, only the axial longitudinal phonon-plasmon modes split into low-frequency and high-frequency modes at temperatures above 700 K.

Improvement Mechanism of Thermal Conductivity in T6/T7‐Treated Al–Si–Mg Cast Alloy: Experimental Studies and Density Functional Theory Calculations

In this article, high strength and high thermal conductivity are achieved in T6/T7‐treated Al–9%Si–0.8%Mg alloy. The improvement mechanism of thermal conductivity is discussed by experimental studies and density functional theory calculations. The experimental results indicate that the T7‐treated Al–9%Si–0.8%Mg alloy shows the good overall performances: thermal conductivity of 177.34 W m−1K−1, yield strength of 288 MPa, tensile strength of 314 MPa, and elongation of 9.63%. The β″ precipitates at peak‐aged condition show higher strengthening effect than that of β′ at overaged condition, while the overaged condition is more favorable for improvement of thermal conductivity. The decrease in concentration of solution atoms by precipitation of β″ and β′ enhances periodic field of localized matrix lattice and increases the relaxation time of free electrons in the localized lattice. The decrease in concentration of solution atoms and increase in the number density of β″ and β′ precipitates not only markedly strengthen the mechanical properties but also reduce the scattering positions in Al lattice, resulting in the improvement of thermal conductivity.

Theoretical and experimental research of femtosecond laser processing fused silica

The ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. Based on tracking the spatiotemporal distribution of the free electron density, free electron temperature, and laser intensity, the electron dynamics as well as the transient optical and thermophysical properties of femtosecond laser irradiated fused silica are quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimentally measured ones at a wavelength of 800 nm. The free electron relaxation time at different laser fluences and pulse durations throughout the photoionization process and impact ionization process are probed. In the present work, the findings are as follows. 1) The electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically. The fused silica becomes opaque for the case of laser irradiation with fluence higher than the ablation threshold. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies with the femtosecond laser ablation of fused silica. 2) By using the proposed model, the experimentally observed saturation of ablation depth at high laser fluence is elucidated by the significant change of optical reflectivity and absorption coefficient. Both the results of theoretical simulation and experimental observation indicate that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with the increase of laser fluence for femtosecond laser irradiation, compared with that for picosecond laser irradiation. 3) With the increase of femtosecond laser fluence, the ablation depth removal efficiency and ablation efficiency are both saturated, followed by slight decrements. The peak of ablation depth removal efficiency peak occurs at the femtosecond laser fluence close to 1.4 times of the ablation threshold. While the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter femtosecond laser pulse. For the femtosecond laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range can achieve accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads the processing quality to degrade, compared with the scenario of femtosecond laser fluence close to the ablation threshold.

Spin relaxation induced by interfacial effects in GaN/Al0.25Ga0.75N heterostructures

Spin relaxation induced by the interfacial effects in GaN/Al0.25Ga0.75N heterostructures was carefully investigated using a photon-energy-dependent time-resolved Kerr rotation spectrum. The existence of the interfacial localized states with potential fluctuations at the GaN/AlGaN heterointerface leads to photoluminescence peaks showing blue and S-shaped shifts owing to the excitation power and temperature, respectively. Photoexcited electrons in the localized states show a spin relaxation time longer than 1 ns because of the suppression of the D'yakonov–Perel’ (DP) scattering, while the spin relaxation time of free electrons was approximately only 10 ps because of the giant Rashba spin-orbit coupling induced by the interfacial polarization field under the framework of the DP scattering mechanism. Furthermore, it is found that the high electron mobility at the heterointerface results in a long spin diffusion length of 300 nm at high temperatures, which is promising for the development of GaN-based spintronic devices.

Temporal-spatial measurement of electron relaxation time in femtosecond laser induced plasma using two-color pump-probe imaging technique

The femtosecond (fs) laser is a powerful tool to study ultrafast plasma dynamics, especially electron relaxation in strong ionization of dielectrics. Herein, temporal-spatial evolution of femtosecond laser induced plasma in fused silica was investigated using a two-color pump-probe technique (i.e., 400 nm and 800 nm, respectively). We demonstrated that when ionized electron density is lower than the critical density, free electron relaxation time is inversely proportional to electron density, which can be explained by the electron-ion scattering regime. In addition, electron density evolution within plasma was analyzed in an early stage (first 800 fs) of the laser-material interaction.

Electron energy transfer effect in Au NS/CH3NH3PbI3-xClx heterostructures via localized surface plasmon resonance coupling.

Localized surface plasmon resonance coupling effects (LSPR) have attracted much attention due to their interesting properties. This Letter demonstrates significant photoluminescence (PL) enhancement in the Au NS/CH3NH3PbI3-xClx heterostructures via the LSPR coupling. The observed PL emission enhancement is mainly attributed to the hot electron energy transfer effect related to the LSPR coupling. For the energy transfer effect, photo-generated electrons will be directly extracted into Au SPs, rather than relaxed into exciton states. This energy transfer process is much faster than the diffusion and relaxation time of free electrons, and may provide new ideas on the design of high-efficiency solar cells and ultrafast response photodetectors.

Metal Adsorbate-Induced Plasmon Damping in Gold Nanorods: The Difference Between Metals

We presented a single particle study on the metal adsorbate-induced plasmon damping in Au nanorods (AuNRs) through adsorbing clusters of different metals including Pt, Au and Ag. AuNRs with different longitudinal surface plasmon resonance (LSPR) wavelength were measured and investigated individually. Linewidth broadening, plasmon shift and reduction of plasmonic resonance of single AuNRs were studied and compared between Pt, Au and Ag adsorbates. The measured linewidths perfectly match the theoretical predictions of the billiard model with increased scattering coefficients resulted from the metal adsorbates. The results indicate that the plasmon damping in case of Ag is significantly weaker than Pt and Au, which can be attributed to longer relaxation time of free electrons in Ag and therefore less loss of the oscillating plasmon electrons. In contrast to the red shift observed from Au and Pt, blue shift of the LSPR is observed in case of Ag. It suggests that plasmonic properties brought by the metal adsorbates can exert dramatic influence on the nanoparticle that is adsorbed with. We believe that our study not only provides important understanding on plasmon damping but pave the road for the fabrication of complex nanostructures with two or more metal elements.

Thickness-dependent free-electron relaxation time of Au thin films in near-infrared region

The free-electron relaxation time is a crucial property to be considered in the design of optical devices, because it determines the dielectric function. Thus, an accurate understanding of this relaxation time is essential for design optimization. Some simulations showed that the relaxation times of Au thin films with thicknesses below 30 nm are different from those of the bulk material. Therefore, we deposited films with four different thicknesses below this value and used near-infrared spectroscopic ellipsometry to show that the relaxation time is dependent on the film thickness. We fitted the ellipsometry spectrum of Au thin films with a thickness <30 nm and found the imaginary part of the dielectric function of the thin films to vary with the film thickness in the near-infrared region. Furthermore, different relaxation times were used to simulate the reflectance of a Fabry–Perot absorber and a plasmonic metamaterial absorber. The simulation results indicated that the obtained relaxation time enables a more reliable evaluation of optical device design.

Optical metrology of Ni and NiSi thin films used in the self-aligned silicidation process

The thickness-dependent optical properties of nickel metal and nickel monosilicide (NiSi) thin films, used for self-aligned silicidation process, were characterized using spectroscopic ellipsometry. The thickness-dependent complex dielectric function of nickel metal films is shown to be correlated with the change in Drude free electron relaxation time. The change in relaxation time can be traced to the change in grain boundary (GB) reflection coefficient and grain size. A resistivity based model was used as the complementary method to the thickness-dependent optical model to trace the change in GB reflection coefficient and grain size. After silicidation, the complex dielectric function of NiSi films exhibit non-Drude behavior due to superimposition of interband absorptions arising at lower frequencies. The Optical models of the complete film stack were refined using x-ray photoelectron spectroscopy, Rutherford backscattered spectroscopy, and x-ray reflectivity (XRR).

Prediction of crater shape in femtosecond laser ablation of dielectrics

This paper reports on a new model to investigate the femtosecond laser ablation of dielectrics by considering the time and space dependent optical properties of highly ionized dielectrics. A quantum treatment is employed to determine the free electron heating and the free electron relaxation time in the dense plasma. The proposed model greatly improves the prediction precision of ablation depth and can predict the crater shape of femtosecond laser ablation of dielectrics. The predicted ablation depths are in agreement with published experimental data for the example of barium aluminium borosilicate. Some interesting phenomena observed experimentally can be well explained by the proposed model.

Investigation of the monochromatic emissivity of liquid copper

The monochromatic normal emissivity of copper at a wavelength of 0.656 mem is experimentally investigated in the temperature range from 1300 to 1700 K. The experimental data are obtained for samples of different purity, which are indicative of the influence of the chemical composition of metal on its emissivity in the molten state. A change in the emissivity is found during the transition of metal from the solid phase into the liquid one. It is shown that the model including the effect of the relaxation time of free electrons of metal produces the best agreement with the experiment in the liquid phase.

Spontaneous switching in cavityless optically bistable systems

Relaxation cavityless optical bistability is studied when a semiconductor is exposed to light quanta with an energy exceeding the transition energy and the free-electron relaxation time depends on temperature. The numerical analysis of dynamic equations for temperature and free-electron density demonstrates that 11 regimes of light-semiconductor interaction are possible. The regimes are classified in terms of the features of temperature-intensity characteristic curves, namely, by the number and arrangement of instability and quasistability segments and by the presence or absence of bistability. It is found that both the upper and the lower branch of a bistable characteristic curve may be unstable, which results in spontaneous switchings between the respective states. Similarly, if the upper state is unstable and the lower state is stable, a small perturbation of the parameters (free-electron temperature and density) near the former causes the transition to the oscillatory regime. If the upper state is unstable and a small perturbation of the lower (stable) state excites a damped oscillation, spontaneous switchings result if the fluctuating temperature exceeds some critical value for the lower state.

Optical Reflectance Anisotropy Spectroscopy of the Au(110) Surface in Electrochemical Environment

The Au(110) surface in electrochemical environment (0.1 M Na2SO4 solution) has been studied by in situ reflectance anisotropy spectroscopy (RAS) and by in situ scanning tunneling microscopy (STM). Three ranges of applied potential could be determined: negative potential (≈ —0.6 V vs. Ag/AgCl) where a clear (1 × 3) reconstruction is obtained; positive potential (≈0.6 V) where the surface is unreconstructed; and intermediate potential where the surface displays both mixed (1 × 3) and (1 × 2) domains. The significant differences in the RAS curves for the differently reconstructed Au(110) surfaces are attributed both to the local-field effect acting on the interband transitions at the surface, and to the changes of free electron relaxation time in the [001] direction (perpendicular to the atomic rows) due to different corrugations of the surface.

Aging histories of optical constants of thermally evaporated silver and copper films

Optical constants of thermally evaporated Ag and Cu films show temporal changes after evaporation. These variations have been measured by an attenuated total reflection technique. The temporal changes in the optical constants have been explained through a change in the relaxation time of free electrons due to structural defects.

Optical bistability produced by resonant Joule's heating of semiconductors

The thermal hysteresis of Joule's absorption of infrared radiation in then-type semiconductor magnetoplasma is analysed. Such bistability is connected with the temperature dependence of the relaxation time of free electrons, their density being constant. The resonant Joule's absorption bistability of the pumping infrared wave near a Langmuir resonance of free electrons is discussed as well as the hysteresis cross-modulation of an infrared probing wave by means of a pumping microwave. The possibilities of smooth detuning of the hysteresis regime due to a magnetic-field control and low energy thresholds of these regimes are illustrated for InSb thin films in relatively weak magnetic fieldsH≦1–2 kG.

Laser-Stimulated Exoelectron Emission

Laser-stimulated exoelectron emission is discussed as a new technique to measure the electron affinity, the electron concentration in the conduction band, and the relaxation time of free electrons in exicted states. A simple theory is presented and applied to NaCl exposed to CO2-laser photons of 10.6 µm wavelength during a thermally stimulated relaxation experiment.

Frequency‐Dependent Relaxation Time of Electrons in PbSe from Infrared Measurements

AbstractInfrared magneto‐reflectivity data for n‐PbSe are analysed and interpreted using a frequency‐dependent momentum relaxation time of free electrons. A peculiar variation of the relaxation time with frequency suggests a dissipation mechanism consisting in short‐wavelength plasmon generation.

Diagnosis of the optical properties and structure of lanthanum hexaboride thin films

The application of the typical metallic refractory compound lanthanum hexaboride as a thin-film material has been studied. Film properties are estimated by using bulk optical constants derived from a Kramers-Kronig analysis of the bulk material. Films with thicknesses of 130–2000 Å have been prepared by sputtering and electron-beam evaporation onto substrates with temperatures between 100 and 1200 °C. The effective optical constants of the films are derived by means of a computer program and used together with the modified Drude theory to give the free-electron relaxation time. Conductivity measurements give the electron mean free path. The optical and electrical film data are compared with the bulk properties of the material. The observed differences can be explained by the influence of crystal imperfections. The explanation is in qualitative agreement with information from x-ray diffractometer scans.

The high temperature resistivity of dilute alloys

Recently measured anomalies in the high temperature resistivity of dilute alloys (transition metals in copper) are interpreted on the basis of the Anderson model. Assuming that the anomalies are caused by phonons the Anderson hamiltonian extended to the case of many impurities is treated, electron-phonon coupling being included. Using the equation-of-motion technique for theGreen's functions, the free electron relaxation time, and the resistivity are calculated. The approximations consist in neglecting impurity correlations and taking into account only terms linear in the temperature and the impurity concentration. We find that in most cases the calculated additional resistivity caused by the impurities show the measuredT-dependance as related to the occurrence of magnetic moments of the impurities in the host metal.