Inter-subband optical absorption in space-charge layers on semiconductor surfaces

The resonance energy and the lineshape of absorption for inter-subband transitions are calculated in an approximation scheme based on the density-functional formulation inn-channel accumulation and inversion layers on the (100) surface of Si. The exciton-like effect almost cancels the resonance screening (depolarization) effect found recently by Allen et al., and the resonance energy is rather close to subband energy-separation. The agreement between the theory and experiments is satisfactory both for the resonance energy and for the lineshape. The theory is applied to accumulation layers on the (111) and (110) surface, and explains characteristic change of the lineshape observed experimentally.A research fellow of the Allexander von Humboldt Foundation     

Anti-parallel ferroelectric domains in surface space-charge layers of BaTiO3

It is shown on simplified models of a surface space-charge layer of a single crystal of BaTiO ₃ that in the surface region, in which the direction of the electric field is opposite to the direction of spontaneous polarization inside the crystal, anti-parallel domains can exist. The dimensions of these domains are determined. The use of the results obtained is proposed for interpreting the microrelief of the etch figures and for explaining the different phases of the polarization reversal process of a single-domain single crystal of BaTiO ₃.

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BatiO₃

BaTiO₃ , , , . . BaTiO₃.

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The author would like to thank J. Kaczer, Z. Málek and V. Dvoák of the Institute of Physics, Czechoslovak Academy of Sciences, for valuable remarks on this paper and H. Havlíková for carefully carrying out the numerical calculations.     

Optical absorption in unordered systems

The coefficient of interzone absorption is calculated for the tail region in an unordered semiconductor at low temperatures with consideration of spin-orbital interaction. Together with the exponential portion, the absorption coefficient contains a Gaussian addition, which under certain circumstances may be comparable to the exponential term.     

Optical rectification in surface layers of germanium

In this Letter, we have demonstrated significant electric field induced(EFI) optical rectification(OR) effects existing in the surface layers of germanium(Ge) and measured the distributions of EFI OR signals along the normal directions of surface layers of Ge samples. Based on the experimental results, the ratios of the twoe-order susceptibility components χ2 effTzzz∕χe2 effTeffective secondzxxfor Ge(001), Ge(110), and Ge(111) surface layers can be estimated to be about 0.92, 0.91, and 1.07, respectively. The results indicate that the EFI OR can be used for analyzing the properties on surface layers of Ge, which has potential applications in Ge photonics and optoelectronics.     

Intersubband cyclotron combined resonances in 2D systems with anisotropic and tilted valleys

Lineshape of intersubband transitions is calculated in a surface space-charge layer on semiconductor whose constant-energy surfaces are ellipsoids arbitrarily oriented with respect to the surface. The case when magnetic field is normal to the surface is considered. Combined intersubband cyclotron transitions are allowed when ellipsoids are tilted with respect to the surface. The depolarization effect affects the combined transition differently from the main transition.     

Optical and electronic properties of anisotropic parabolic quantum disks in the presence of tilted magnetic fields

In the present work, the electronic and optical properties of anisotropic parabolic quantum disks are studied in the presence of an applied magnetic field. For this goal, we first obtain the electron energy levels of an anisotropic parabolic quantum disk under axial, tilted, and in-plane magnetic fields. According to the results obtained for the energy levels reveal that there is no degeneracy at zero magnetic field due to symmetry breaking. With increasing the anisotropy, the energy level spacing increases. At a constant anisotropy, the energy levels splitting decreases with increasing tilt angle of magnetic field. The total refractive index changes decrease when the tilt angle of magnetic field and the anisotropy increase. Also, the total absorption coefficients increase as the tilt angle of magnetic field and anisotropy increase.     

Surface-acoustic-wave-induced space-charge waves in electron-hole systems

Space-charge waves in an electron-hole system are studied, which are excited by a moving grating provided by a surface acoustic wave (SAW). The SAW induces a constant current that may change its sign, when a constant electric field is applied opposite to the wave propagation direction. Current resonances are predicted to appear, when the SAW wavelength and frequency match the ones of the space-charge wave.     

Optical surface absorption by surface-adsorbate charge-transfer excitations

Metal-insulator-silver junctions are well suited to measure genuine surface optical excitations, especially electronic transitions to the lowest unoccupied molecular orbitals (LUMO) of adsorbates on silver. This is demonstrated by the increase of internal photoemission current after monolayer coverage with C₂H₄ (which has π^*-LUMO) by a factor of about 2.5, but a missing change in the case of C₂H₆ (which has no π^*-LUMO ). Received: 26 November 1999 / Published online: 8 March 2000     

Microwave power absorption in space-charge region of high-resistivity semiconductors

The distribution of a microwave electric field in the space charge region (SCR) of semiconductors is found from a joint solution of the diffusion-drift equation and the Poisson equation. Large enriching bends of the bands lead to screening of the microwave field by SCR free carriers, with the result that the microwave power absorption is limited when the bends become larger. An expression is found for the relation between the quality factor Q_g of the capacitive gap of the measuring resonator and the size of the bends. Plots of maximum error of measurement of the conducitivity of semiconductors due to neglect of absorption in the SCR against measured values of Q_g are obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 18–22, November, 1981.     

Optical absorption by surface plasmons in deep sub-wavelength channels

The interaction of visible light with surface plasmons in a periodic array of silver channels is studied experimentally and theoretically. Channels of width 130 nm and depth 280 nm on a pitch of 468 nm were fabricated using electron beam lithography. Measurements of the reflectance of the channels show strong absorption for a specific angle of incidence at a given wavelength of the light. It is shown using a simple geometric model that this is consistent with the excitation and propagation of surface plasmons in the deep sub-wavelength channels. A numerical simulation of the electromagnetic fields excited in the channels confirms this result.     

Optical rotatory power in tilted smectic phases

Optical Rotatory Power measurements have been done on different chiral tilted smectics which lead to reproducible results. The high-temperature uniaxial SmA and SmC phases do not show any ORP due to the lack of in-plane anisotropy at the mesoscopic level (i.e. a few layers). At lower temperature one gets the synclinic C^* and the anticlinic C phases with a helical precession around the layer normal which is right-handed for C^* and left-handed for C . These phases are separated in a finite temperature range by a set of two so-called "ferrielectrics” SmC and SmC , which are not necessarily new phases, between which the handedness changes sign at a given temperature, while the change from synclinic to anticlinic tilt angle organization is spread out over the "ferrielectric” temperature range. In order to reconcile all experimental observations, otherwise apparently incompatible, it is proposed that the "ferrielectric” region is a diphasic one where the in-plane organization changes smoothly from the synclinic type to the anticlinic one. Received 7 January 2000 and Received in final form 7 April 2000     

Surface quantization effect on the macroscopic characteristics of semiconductor space-charge layers

A numerical study of the dependence of the space-charge density Q_sc on the surface potential Ψ_s in a two-dimensional electron inversion layer on Si(100) is presented. The dependence Q_sc(Ψ)_s being a macroscopic characteristic of semiconductor space-charge layers is found to be considerably influenced by the surface quantization in a wide temperature range up to room temperature. The conditions for the appearance of appreciable differences in the behaviour of the Q_sc(Ψ)_s curves in the quantum-mechanical and quasi-classical cases are determined and discussed.     

NMR line shape in anisotropic superconductors in a tilted magnetic field

NMR line shape has been constructed for anisotropic type-II superconductors in tilted magnetic fields, with inclusion of vortex-lattice magnetic-field nonuniformities and of the skin effect near the superconductor surface. The NMR line shape parameters are shown to change considerably when the external magnetic field changes direction. This makes it possible to obtain more detailed information about the characteristics of a superconductor, in particular, its anisotropy parameter. Fiz. Tverd. Tela (St. Petersburg) 40, 993–997 (June 1998)     

Optical properties of composite systems based on anisotropic fibers with axisymmetric stacking

Simulation of optical properties of composite systems in the form of bundles consisting of anisotropic fibers with axisymmetric stacking is carried out in the ray-optics approximation. It is shown that such composite systems observed by the interference-polarization method for the case of crossed Nicol prisms produce figures in the form of a system of concentric light and dark rings, in the center of which there is a dark cross. The results of simulations applied to microsections of the human tooth taking into account the numerical values of the constituent components of the dentin (the sizes and optical properties of the dentinal tubules and of the matrix, etc.) are compared with the experiment. A good qualitative agreement between the calculation and the experiment for the observed patterns is found in the cases of some samples (molar and premolar).     

Optical system design with conformal decentered and tilted elements

We investigate the aberration properties of the conformal optical system with decentered and tilted elements by vector aberration theory.By decentering and tilting the window and corrector of the system,two elements are effectively used together in a particular manner by aberration compensation to achieve off-axis imaging.A conceptual design is performed with a half-field of 2° ,the F# of 4,and the wavelength ranging of 3700-4800 nm.The imaging quality can reach the optical diffraction limit and satisfy corresponding requirements.