Scattering characteristics of a beam mode in dielectric-slab optical waveguide

Scattering of the light beam propagating through a dielectric-slab optical waveguide occurs due to refractive index inhomogeneities of the thin film region and due to boundary irregularities of the filmsubstrate and the film-air interfaces. The quantity and direction of the scattered light are evaluated by means of a perturbation method together with the use of a stationary phase method, for a variety of correlation lengths and variances and the thin film thicknesses. The results show that for a slab waveguide of t/=10.0,t and being the thickness of thin film and the wavelength of light, the effect of refractive index inhomogeneities is pronounced in comparison with that of waveguide wall irregularities. In this case, therefore, the scattering pattern is determined mainly by the correlation length of refractive index inhomogeneities.     

Optical characteristics of implanted silicon films

The first data are presented on the change (following implantation) in the refractive index and the band structure of silicon on sapphire films. The implantation was effected with phosphor ions of 40 keV and doses from 10¹² to 10¹⁶ cm^–2. An increase following implantation of the refractive index and the energy of the first direct allowed transitions E₁ is noted, indicating changes in the second coordination sphere. The profile E₁(x) is studied pointing to heterogenization effects. The films were annealed with ruby laser pulses of 0.2 J/cm². The same laser was used to study the lux dependence of the injection level _n and surface photo-emf V. Hysteresis in the V(_n) dependence (after the use of maximum intensity of the laser beam) is noted indicating irreversible straightening of the bands at the film surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–43, May, 1984.     

Suppressed transmission in aperiodically modulated multilayered dielectric structures

We investigate the transmission of electromagnetic plane waves through 1D binary dielectric multilayered structures that exhibit aperiodic incommensurated sequences of refractive indices. The aperiodicity is introduced by considering the sequence of refractive indices to follow a sinusoidal function whose phase varies as a power-law of the layer index, ∝i^ν. For ν>1, the resulting sequence is effectively uncorrelated leading to the Anderson localization of most of the electromagnetic modes, except at the Bragg resonances. The crossover from a uniform structure at ν=0 to a quasi-periodic structure at ν=1 is signaled by a minimum at the spectrally averaged transmission. We perform a spectral analysis of the refractive index sequence to show its close connection to the main features exhibited by the averaged optical transmittance. Our results suggest that aperiodically modulated dielectric structures can potentially be used in the development of wide-band filters.     

The effect of interdiffusion on the change of refractive index of an AlGaAs/GaAs quantum well structure

The change of the refractive index due to quantum well (QW) disordering is calculated for light propagating normal to the Al_0.3Ga_0.7As/GaAs QW layers (i.e. along the QW growth direction). A hyperbolic function is used to model the above QW confinement profile after disordering, i.e. thermal interdiffusion of trivalent atoms across the well-barrier interfaces. The refractive index difference (n) is evaluated for two cases, where case I refers to the difference between a partially disordered QW and a more extensively disordered QW, while case II refers to the difference between an as-grown QW and a partially disordered QW. The results demonstrate that good photon confinement (large n > 0) can be achieved for both cases, where n increases with increasing QW width and decreases with annealing time for case I while for case II it increases with annealing time. In comparing the two cases, a shorter annealing time is required to achieve the same value of n if the case II structures are used. The change of refractive index obtained here demonstrates a larger value of n than that produced by the variation of the concentration of free carriers in the bulk material.     

Design and optimisation of high-brightness 980 nm laser diodes with distributed phase correction

Designs for 980 nm Al _x Ga_1–x As/In_0.2Ga_0.8As/GaAs high-power, high-brightness semiconductor lasers/amplifiers with distributed phase correction and short-cavity lengths (i.e. cavity lengths 560–1040 m) are presented. The proposed lasers/amplifiers employ a single mode feed waveguide coupled to a power amplifier with a laterally graded effective refractive index (GRIN) profile to control the lateral mode shape and phase. The lateral index of the power amplifier has a hyperbolic secant (HYSEC) profile, which can be approximately realised by tailoring the effective refractive index of the amplifier using a series of discrete etches. The epitaxial and structure designs of the laterally discretised GRIN lasers/amplifiers are presented. Finally, a method for improving the effective refractive index discretisation is described.     

Measurements of optical parameters of antimony trisulphide films

In the course of a systematic investigation of thin absorbing films, the optical constants of antimony trisulphide (Sb₂S₃) films evaporated on glass were determined for a fairly wide range of thicknesses. The measurements were performed at the wavelength of _Na=5893 Å by the ellipsometric method on the one hand and by intensity measurements of reflected and transmitted light on the other. Independently, the film thickness was measured by the Tolansky interference method. In this way it was possible in both cases to determine both the refractive index of the film and the absorption index using a method currently employed with transparent dielectric films: that of the interpolation of tabulated values. The results of measurements revealed a change in both the refractive and the absorption index with film thickness. The ellipsometric and the intensity measurements gave results in very close agreement. The paper is dedicated to the memory of Dr. A. Vaíek, Professor of the J. E. Purkyn University in Brno, who died on November 16, 1966.     

Optically induced index change in GaAs by laser diode: Measurement method and analysis to realize an all-optical switch

In this paper the possibility to realize an all-optical switch on GaAs, using a semiconductor laser as a pump beam, is investigated. An estimate for the mean value of the refractive index change (n) of the material is calculated as a function of the pump power. A method for the measurement of n is proposed, based on the mathematical relationships between the propagation characteristics of the guided mode and the refractive index change.     

“Anomalous” refractive index dispersion curves—a relativistic interpretation

The Einstein field equations (R =0) are seen as possible candidates for a set of unified field equations. Three solutions of these field equations are used for a new interpretation and reformulation of the refractive index of an isotropic material medium. The new formulation explains the basic features of anomalous refractive index dispersion curves. It also predicts that the refractive index is a function of the angle of incidence when the plane in which the measurement is made is not tangential to the surface of the spherical gravitating mass, thereby providing a suitable test for the theory and hence of general relativity.     

Spectral dependence of optical constants of heavily doped Si in region of plasma resonance of charge carriers

The semiclassical relationships for the complex refractive index are used to find the regions of integration of the Kramers-Kronig dispersion relation required to yield the spectral dependence of the optical properties of semiconducting materials in the vicinity of the plasma resonance of the charge carriers with a prescribed accuracy. The refractive index n and the absorption coefficient of heavily doped Si in the vicinity of the plasma resonance of the charge carriers are determined by the Kramers-Kronig method and by the method of two media. The values of n and obtained by these methods are in good agreement with values calculated on the basis of the Drude-Lorentz semiclassical theory of dispersion.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 11–14, February, 1973.     

On the refractive index for neutrons

The commonly used result for the neutron refractive index does not include effects due to binding of the nuclei or multiple scattering. We show how these effects can be incorporated in the theory of coherent neutron scattering and present an expression for the refractive index, which adds to the familiar result correction terms quadratic in the scattering length. Examples discussed in more detail include a calculation of the refractive indexn for H₂ gas. Corrections to 1 —n are of the order 10^–3. Results from our theory, which takes into account the actual dynamics of the scatterers, can differ substantially from those of the static approximation, in which the nuclei are held at fixed positions in space.     

Designing a graded index depressed clad non-zero dispersion shifted optical fiber for wide band transmission system

Non-zero dispersion shifted fibers (NZ-DSFs) find extensive use in wavelength division multiplexed (WDM) system as it reduces the non-linear effects like four-wave mixing (FWM) generation. A depressed clad graded index fiber with a central dip in the refractive index profile, as well as without dip, has been modeled to perform as an NZ-DSF using the spot size optimization technique. The performance characteristics of the proposed NZ-DSF have been studied by changing different fiber parameters; such as inner core radius (a), relative refractive index differences (Δ⁺), normalized end position of depressed clad (C), depression parameter (ρ), etc. for a given value of Petermann-2 spot size . By suitably adjusting the fiber parameters, the effective core areas (A_eff) as simulated here are very large (80 μm²) so that the effect of non-linearities upon them can be minimized. These NZ-DSFs have also been optimized for WDM transmission system. The dispersion slopes of the proposed fibers with and without dip have been estimated which are comparable with the reported results.     

Ultrafast beam-deflection method and its application for measuring the transient refractive index of materials

We represent an ultrafast beam-deflection method as a simple and powerful tool for the time-resolved measurement of induced changes of the refractive index in the order of n=10^–5. The method is applied for measuring the changes of components of the refractive index parallel and perpendicular to the pump-pulse polarization on a femtosecond time scale. Fused silica and CS₂ are used as samples for demonstrating our method.     

Large near resonance third order nonlinearity in GaN

We have studied third order nonlinearities, including two-photon absorption coefficient and nonlinear refractive index n ₂, of GaN in below bandgap ultraviolet (UV) wavelength regime by using UV femtosecond pulses. Two-photon absorption was investigated by demonstrating femtosecond UV pulsewidth autocorrelation in a GaN thin film while femtosecond Z-scan measurements revealed information for both n ₂ and . The distribution of n ₂ versus wavelength was found to be consistent with a model described by the quadratic Stark effect, which is the dominant factor contributed to the nonlinear refractive index near the bandgap. Large on the order of 10 cm/GW and large negative n ₂ with a magnitude on the order of several 10^–12 cm²/W were obtained. The at near mid-gap infrared (IR) wavelength was also found to be on the order of several cm/GW by using two-photon-type autocorrelations in a GaN thin film. Taking advantage of the large two-photon absorption at mid-gap wavelengths, we have demonstrated excellent image quality on two-photon confocal microscopy, including two-photon-scanning-photoluminescence imaging and two-photon optical-beam-induced current microscopy, on a GaN Hall measurement sample and an InGaN green light emitting diode.     

Possible use of a gas-discharge plasma in an oscillating field to modulate electromagnetic radiation

The conditions for resonance and signal cutoff are determined from the dispersion relation for the complex refractive index for a plane electromagnetic wave propagating in a gas-discharge plasma when a magnetic field H = H₀ + H₁e^jt is imposed on the plasma. The refractive index and absorption are calculated for circularly polarized waves as functions of the alternating magnetic field with _p, _b, andv as parameters. Possible observation of modulation of the transmitted signal is demonstrated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 94–99, April, 1971.     

Tuning of the photonic band gaps and the reflection spectra of a one-dimensional photonic crystal based on silicon and a liquid crystal

The reflection spectra and the photonic band gaps of a one-dimensional composite photonic crystal are calculated for different filling factors (the ratio of the thickness of a silicon layer to the lattice constant A). A change in the refractive index of the filler in a photonic crystal leads to the shift of both edges for certain side photonic bands. It is shown that, with a change in the refractive index from 1.49 to 1.69, the shift of the edges (the tuning effect) can reach the value A/λ = 0.04 for the main photonic band and 0.08 for a side band. An additional criterion for designing a photonic crystal is the estimation of the sharpness of the edges of photonic bands in the reflection spectra.     

Effect of pressure on intersubband optical absorption coefficients and refractive index changes in a V-groove quantum wire

In this paper, the effect of hydrostatic pressure on the intersubband optical absorption and the refractive index changes in a GaAs/Ga_1−xAl_xAs ridge quantum wire are studied. We use analytical expressions for the linear and third-order nonlinear intersubband absorption coefficients and refractive index changes obtained by the compact-density matrix formalism. The linear, third-order nonlinear, and total intersubband absorption coefficients and refractive index changes are investigated at different pressures as a function of photon energy with known values of width wire (b$b$), the incident optical intensity (I$I$), and the angle θ$\theta$. According to the results obtained from the present work, we have found that the pressure plays an important role in the intersubband optical absorption coefficients and refractive index changes in a V-groove quantum wire.     

Laser damage in silicon photodiodes

Thermal damage of silicon photodiodes exposed to intense optical radiation is investigated. Damage thresholds of Si photodiodes irradiated by 1.06m laser pulses are reported for values of irradiation time,, ranging from 10^–8 to 1s. Threshold laser irradiation produces visible microscopic damage and a permanent degradation in photoresponse. The loss of responsivity is associated with degradation of the detector diode characteristics due to laser-induced heating. The time and wavelength dependence agree with the predictions of a thermal model which treats a semi-infinite material irradiated by a Gaussian laser beam. The energy density thresholds are independent of for short irradiation times and asymptotically approach a limiting behaviour which increases as for long times. They are given by the empirical relationE ₀=65[1+217/tan^–1(258)^1/2] J cm^–2 for 1.06m radiation. The thresholds at short irradiation times of detectors damaged by 1.06m radiation are about 25 times larger than those of detectors exposed to 0.6943m radiation. The greater susceptibility at 0.6943m is attributed to a larger optical absorption coefficient.