Photoconductivity in a Pb2CrO5 ceramic disk with surface electrodes

The photoconductive behavior of a Pb₂CrO₅ ceramic disk with one pair of electrodes on one surface has been studied at room temperature. When illuminated with light in the visible region, the device exhibits photocurrent which is much larger than the dark current. The current-voltage characteristics and the variation of photocurrent with (i) applied field, (ii) light intensity, and (iii) wavelength are presented with transient behavior.     

Photoelectronic and electrical properties of Tl2InGaS4 layered crystals

Tl₂InGaS₄ layered crystals are studied through the dark electrical conductivity, space charge limited current and illumination- and temperature-dependent photoconductivity measurements in the temperature regions of 220-350 K, 300-400 K and 200-350 K, respectively. The space charge limited current measurements revealed the existence of a single discrete trapping level located at 0.44 eV. The dark electrical conductivity showed the existence of two energy levels of 0.32 eV and 0.60 eV being dominant above and below 300 K, respectively. The photoconductivity measurements reflected the existence of two other energy levels located at 0.28 eV and 0.19 eV at high and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 330 K. The illumination dependence of photoconductivity is found to exhibit supralinear recombination in all the studied temperature ranges. The change in recombination mechanism is attributed to exchange in the behavior of sensitizing and recombination centers.     

LiTaO3 as holographic storage material

The optical storage properties of LiTaO₃:Fe are investigated and compared with those of the isomorphous compound LiNbO₃:Fe. Absorption, photocurrent, photoconductivity and holographic measurements are reported. In the case of photovoltaic writing similar results for LiTaO₃- and LiNbO₃-crystals are obtained. However, in the case of photoconductive writing using external electric fields LiTaO₃:Fe-crystals yield much better results due to large photoconductivity values. Considering the recording sensitivity and the extremely large storage time LiTaO₃:Fe turns out to be one of the most promising materials for photorefractive storage of volume phase holograms.     

Entanglement of the l- and the r-photons in an anisotropic crystal

Light incident onto an anisotropic crystal is divided into the ordinary and the extraordinary waves which vibrate in two perpendicular directions. Because of the tensor property of the dielectric constant, the direction of the electric displacement is not parallel to the vector of the incident electric field. An optical torque is induced by incidence of the linearly polarized light and propagating through the crystal. The optical torque tends to rotate the directions of eigenvibration which results in self-modulation of the ordinary and the extraordinary waves, and causes an energy splitting of the left (l)-, and the right (r)-handed circularly polarized waves in the crystal. The l- and the r-photons are correlated through the optical torque, which are found to be in an entanglement state.     

Electrically controlled optical bandgap in a structurally chiral material

The propagation of light along the axis of non-homogeneity of a structurally chiral medium (SCM) under the influence of a low-frequency (dc) electric field aligned along the same axis is investigated. The Pockels effect is assumed to occur, and the SCM is taken to possess locally a point group symmetry. The frequency-domain Maxwell curl equations are cast in a 4 × 4 matrix representation, and the Oseen transformation is invoked. The band structure is analyzed, as are also the eigenvectors as well as the transfer matrix. Finally, the reflection and transmission of a plane wave due to a SCM of finite thickness is considered. The low-frequency electric field is shown to control the bandgap.     

Torsion induced circular dichroism in hexagonal CdS

The influence of the torsion on the absorption properties of hexagonal CdS has been studied in our experiments. The undeformed CdS does not possess circular dichroism (CD) but it manifests linear dichroism (LD). An appearance of CD and a change of LD can be observed in the torsion deformed CdS crystals. The mentioned absorption phenomena occur in the vicinity of the isotropic point (i.p.), the crossing point of the curves of the refractive indicesn ₀() andn _e (). The sign of CD is connected with the direction of the torsional momentM. The magnitude ofM influences the shape of CD spectra as well as the values of LD. The appearance of CD and the change of LD are explained by the torsion induced gyrotropy in the CdS crystals, i.e. by the created chirality in the hexagonal CdS.     

Enhancement of the far-field output power and the properties of the very-small-aperture lasers

We report on a VSAL structure fabricated by a 650 nm edge emitting laser diode with an Au-coated facet and an aperture size of 250 × 500 nm. The far field output power can maintain at 1 mW and the power density is 7.5 mW/μm². Some properties of the VSAL including the threshold current change, the red-shift of the spectral position, and the strong relative-intensity-noise are presented. The physical mechanisms responsible for these phenomena are also discussed, which may contribute to the understanding and application of the potential device for near-field optics.     

Generalized Oseen transformation for and enhancement of Bragg characteristics of electro-optic structurally chiral materials

The Oseen transformation is generalized to define a non-electro-optic structurally chiral material, wherein propagation along the axis of chirality is equivalent to that in an electro-optic SCM with local point group symmetry. This generalization shows that the exploitation of the Pockels effect amounts to an enhancement of the effective local birefringence, which in turn can enhance the characteristics of the circular Bragg phenomenon. Electro-optic SCMs can therefore serve as efficient and electrically controllable circular- and elliptical-polarization rejection filters.     

Observation of dark dot splitting pattern in terbium gallium garnet

We have observed a new polarisation pattern in terbium gallium garnet which is both laser excited and mechanically stressed. The pattern can be characterized by a splitting from a “one dot” structure to a “two dots” structure. We have found that the new pattern is very sensitive to the orientation and the strength of the external forces applied to the sample. Thanks to this sensitivity and to the image simplicity, this new effect may be used in a vectorial force sensor and actuator. Received: 25 March 1999 / Accepted: 20 April 1999 / Published online: 25 August 1999     

High-frequency response and wavelength dispersion of the linear electro-optic effect in PZN-PT (88/12) single crystal

We have characterized the wavelength dispersion of the high- and low-frequency values of the linear electro-optic coefficient r _c = r ₃₃–(n _o/n _e)³ r ₁₃ of poled 0.88 Pb(Zn_1/3Nb_2/3)O₃–0.12 PbTiO₃ single crystal. The measurements have been investigated as a function of the laser wavelength from 0.465 to 1.32 m. Clamped r ^S and unclamped r ^T values of the electro-optic coefficient as well as the acoustic contribution r ^a have been established. We obtained a ratio of r ^S/r ^T = 30%, which is independent of the wavelength of the light beam. The value of r ^S _c at the wavelength of 633 nm was found to be equal to 50±5 pm/V, which is a rather high value.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Electrically controlled reflection and transmission of obliquely incident light by structurally chiral materials

The solution of a boundary-value problem for the reflection and transmission of obliquely incident plane waves due to a slab of a structurally chiral material (SCM) displaying the Pockels effect with a point group symmetry indicates the enhancement of circular Bragg phenomenon by the application of a dc voltage. The enhancement suggests that thinner SCMs can be used as devices such as polarization-rejection filters if the Pockels effect is exploited, for both normally and obliquely incident light.     

Refractive index and dielectric constants of GaxIn1−xP: Disorder effect

A pseudopotential scheme, which incorporates compositional disorder as an effective potential, is used so as to calculate the optical properties of Ga_xIn_1−xP ternary alloys in the zinc-blende structure. Generally, the agreement between our results and the existing data in the literature is reasonable. The composition dependence of the studied features showed a non-monotonic behaviour for most studied features. Reasons for these findings are discussed. Moreover, the effect of the compositional disorder on the features of interest has been examined and found to be important for the calculation of optical properties of Ga_xIn_1−xP.     

On the contribution of nonlinear scattering to optical limiting in C60 solution

We present experimental results which bring out the contribution of nonlinear scattering to the energy limiting of 527 nm, 30 ns pulses in C₆₀ solution. To perform these measurements we used a specific experimental arrangement to separate the effects of nonlinear refraction and scattering. Our results show that scattering reduces the output significantly and contributes to limiting in C₆₀ solution.     

Wide band polarizer with suspended silicon grating

In this paper, we propose an ultra broadband polarizer operating in the telecommunication wavelength band; this device consisting a single silicon suspended resonant grating layer is designed with using the inverse mathematical method and rigorous vector diffraction theory. It is shown from our calculations that the device presents extremely high reflection (R > 98%) for TE polarization light and high transmission (T > 98%) for TM polarization over ∼330 nm wavelength range; moreover, the extinction ratio is ∼100 in the central wavelength 1550 nm. Furthermore, it is found with Rigorous Coupled Wave Analysis (RCWA) and near field distribution that the extremely wide band property for TE polarization is due to the excitation of strong modulation guided modes in the design wavelength range.     

Optical and structural parameters of the ZnO thin film grown by pulsed filtered cathodic vacuum arc deposition

Transparent conductive ZnO film was deposited on glass substrate by pulsed filtered cathodic vacuum arc deposition (PFCVAD). Optical parameters such as absorption coefficient α, the refractive index n, energy band gap E_g and dielectric constants have been determined using different methods. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. The spectra of the dielectric coefficient were used to calculate the energy band gap and the value was 3.24 eV. The experimental energy band gap was found to be 3.22 eV for 357 nm thick ZnO thin film. The envelope method was also used to calculate the refractive index and the data were consistent with K-K relation results. The structure of the film was analyzed with an x-ray diffractometer and the film was polycrystalline in nature with preferred (002) orientation.     

Evolution of Photocurrent during Coadsorption of Cs and O on GaAs （100）

The photocurrent and spectral response characteristics of gallium arsenide （GaAs） are obtained by a multiinformation measurement system, and the evolution of the photocurrent versus the Cs：O flux ratio is investigated. The experimental results show that the photocurrent increases approximately exponentially after the first exposure to Cs until a maximum sensitivity is reached, the detailed evolution process and the ultimate photocurrent are different for different samples. These differences are analysed, and according to the process of coadsorption of Cs and oxygen on GaAs, an equation is presented to explain the increase of photocurrent.     

Magnetogyrotropy in twisted CdS crystals

In this paper the optical activity (gyrotropy) induced by a simultaneous action of a magnetic field and a torsion in hexagonal CdS crystals has been investigated. The crystal having undergone to both external influences is placed between two crossed polarizers. The spectral course of transmittance of this system near the isotropic point of CdS (the wavelength where the birefringence changes its sign) is studied in the experiment. An explanation of the experimental results is given by using the model of elliptical birefringence in anisotropic gyrotropic crystals. Taking into account the superposition of both external influences, it has been proved that the torsion generates gyrotropy whose parameter changes its sign in the isotropic point.     

Photoluminescence of gold, copper and niobium as a function of temperature

A specially constructed instrument for measuring the low intensity photoluminescence emission spectra of metals is described. It uses low luminescence optical components and dedicated sample mounting techniques. Room temperature measurements agree closely with literature spectra for high-purity gold and are found to be sensitive to 100 ppm impurities. Detailed spectra are presented, which are weakly temperature dependent, for gold, copper and unpolished niobium between room temperature and 100 K. We conclude that this work provides accurate luminescence data for Au from 300 K down to 100 K. Although the (variable temperature) luminescence data for Cu are consistent both with the room temperature experimental data in the literature and theory, we conclude the role of surface adsorbates and/or oxides cannot be ruled out. Theory suggests that Nb has a factor ∼50 lower luminescence intensity than Au and Cu because the real part of the refractive index is a factor ∼5 higher and the density of states ∼2 eV below the Fermi energy is a factor of ∼4 lower than Au and Cu. Measurements are presented for unpolished Nb, but given the lack of signal detection for polished Nb and that theory predicts very weak signals, we conclude that the luminescence signals from pure Nb still remain below the sensitivity of our instrument.     

Double groove silicon grating polarizer in telecommunication wavelength band

In this paper, we propose an ultra broad band polarizer operating in the telecommunication wavelength band, this device consisting a double groove silicon grating is designed with using the inverse mathematical method and rigorous vector diffraction theory. It is shown from our calculations that the device presents extremely high reflection (R > 95%) for TE polarization light and high transmission (T > 95%) for TM polarization over ∼400 nm wavelength range, moreover, the extinction ratio is ∼30 in the central wavelength 1550 nm. Furthermore, it is found with rigorous coupled wave analysis (RCWA) that the extremely wide band property for TE polarization is due to the excitation of strong modulation guided modes in the design wavelength range.