Interference Gratings Research Articles

Realization of AlGaAs antidot arrays by pulsed laser interference gratings

AlGaAs antidot arrays with about 107 antidots are produced by single-shot interference processing with a pulsed high-power Nd:YAG laser system. We apply magnetotransport experiments and atomic force microscopy (AFM) to explore the electronic and geometric properties of the arrays. The size of the antidot arrays are 3 mm×3 mm and the period varies from 400 to 1000 nm. The dots are elliptic or circular and have diameters ranging from 255 to 690 nm. The magnetotransport experiments are performed at 1.5 K in van der Pauw contact configuration. The laser structuring leaves the two dimensional electron density nearly unchanged but decreases the mobility by a factor of about 30. Several maxima are detected in the low magnetic field magnetoresistivity which are discussed based on the geometric data determined by AFM.

Pulsed laser ablation of polyimide: fundamental aspects

Single-shot laser ablation of polyimide in air was investigated using focused Ar + laser UV radiation ( λ≈302 nm) and pulse lengths between 140 ns and 5 μs. The irradiated polymer surface was studied in terms of its topology and ablation depth by atomic force microscopy. The dependence of the ablation threshold on the laser pulse length and intensity can be tentatively interpreted on the basis of a thermal process and a (thermal or non-thermal) mechanism which decreases the apparent activation energy for the desorption of species from the surface. On the basis of the changes in surface topology observed, a new model for the formation of interference gratings and row doubling is suggested. © 1997 Elsevier Science S.A.

Extended resolution for lateral structuring with laser interference gratings using high-index input coupling

Extended resolution for lateral structuring with laser interference gratings using high-index input coupling

Photosensitive Guest-Host Polymer for Optical Data Storage

Methylene blue sensitized poly(methyl methacrylate) prepared by the process of dye diffusion has been shown to be efficient medium for recording interference gratings. Phase and/or amplitude holograms can be written in the methylene blue sensitized films of poly(methyl methacrylate) using a conventional source of light a He—Ne laser operating at 632.8 nm wavelength. Kinetics of photobleaching process of methylene blue is measured. Diffraction efficiencies of up to 30% were found for thick holograms. Phase conjugation and multiple holograms writing has been tested in the studied system.

Contrast sensitivity and disability glare in the middle years.

Spatial contrast sensitivity and disability glare were measured in a large sample (n = 90, 30 per decade) of middle-aged subjects, aged 21-50 years, who had clear media and were ophthalmologically normal. We found no significant differences in the contrast sensitivities as a function of age in the middle years for (1) gratings generated on a monitor; (2) interference gratings generated in the retinal plane; (3) gratings in the presence of glare; and (4) mesopic increment thresholds with and without glare. The large sample size provides sufficient statistical power (0.8) for one to conclude that contrast sensitivity, optical quality, and foveal neural sensitivity are unlikely to vary more than 0.1 log unit between the ages of 21 and 50 in ophthalmologically normal subjects with clear media.

Detection of magnetic flux with superconducting quantum interference gratings

The authors have carried out finite-inductance calculations of the critical current vs. flux (I/sub c/- Phi ) and voltage vs. flux (V- Phi ) characteristics of superconducting interferometers with many Josephson junctions in parallel. At least two features of the calculations suggest that many junction interferometers, called superconducting quantum interference gratings, might be advantageous for the detection of magnetic flux. First, the voltage noise can be reduced significantly for a given flux-to-voltage transfer coefficient, a feature which is likely to improve the magnetic flux sensitivity of both low- and high-T/sub c/ superconducting devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction grating such as enhancement and narrowing of the peaks in the I/sub c/- Phi characteristic. Specific schemes for efficiently coupling flux into the device are proposed. >

Applications of Parallel Josephson Junction Arrays for the Detection of Magnetic Flux

ABSTRACTWe have carried out finite-inductance calculations of the critical current vs, flux(Ic-φ) and voltage vs, flux (V-φ) characteristics of superconducting interferometers with many Josephson junctions (JJ's) in parallel. At least two features of our calculations suggest that interferometers with many junctions, which we call Superconducting QUantum Interference Gratings, or SQUIG's, might be advantageous for the detection of magnetic flux. First, the voltage noise can potentially be reduced significantly as compared to a dc SQUID with the same overall voltage-to-flux transfer coefficient - a feature which might reduce 1/f noise and enhance the magnetic flux sensitivity of both low and high Tc superconducting (HTS) devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction-grating like enhancement and narrowing of the peaks in the Ic-φcharacteristic, suggesting that flux uniformity, rather that critical current uniformity, is of primary importance.

The confinement of light in solar cells

The upper limit of the absorption increase due to light confinement is presented here on the basis of non-imaging optics, as well as the conditions to achieve it. It is found that the usual Lambertian only gives maximum absorption for weakly absorbed photons but how to make a device fulfilling the conditions of maximum confinement is not known so far. A reduction of the étendue of the escaping light is a way to increase the confinement. Several ways of achieving it are analyzed including the use of microconcentrating covers, of interference gratings and of large entry aperture light confining external cavities.

Bleached phase holograms exposed on Agfa-Gevaert 10E75 NAH plates

An experimental study has been made of the diffraction efficiency, scattering and stability against printout effect of bleached, photographically recorded two- beam interference gratings using Agfa-Gevaert 10E75 NAH plates. The efficiency is increased by using bleaching processes that convert the silver image into a dielectric image. Plots of the diffraction efficiency and scattering against exposure, and variation of the maximum diffraction efficiency as a function of departure from the Bragg angle, and exposure to white light, for various bleaching processes, are given. A maximum diffraction efficiency of approximately 54% has been achieved by using a potassium iodide and iodine bleach process.

Infrared recording of transient and permanent interference gratings with commercial plastics

Two commercial plastics have been used to record infrared interference patterns when a CO(2) laser is used as a light source. Recording of transient and permanent interference gratings with different spatial frequencies was carried out. The diffraction efficiency for red light as a function of time for different exposure times is shown; maximum diffraction efficiencies attained were of the order of 30%. The possibility of using one of these plastics as an optical switch is shown.

Critical coupling strength for enhanced four-wave mixing by use of moving interference gratings in photorefractive crystals

Increased four-wave mixing reflectivity in BaTiO 3 has been observed in the case of moving interference gratings earlier [Phys. Rev. Lett. 55 (1985) 821]. In diffusion dominated photo-refractive crystals this increased reflectivity is obtained only for a coupling strength above a critical value. In our analytical study we calculate this coupling strength for week input signals as a function of the pump beam intensity ratio. Then we discuss the reflectivity versus grating (temporal-) frequency dependence.

Dry polymer for holographic recording

A dry polymeric mixture rendered red sensitive with a dye has been used to record interference gratings and holograms. Diffraction efficiencies of ~10% can be attained with exposures of ~94 mJ/cm(2). If necessary the reading step can be done just after the exposure or after a fixing step performed with light from a mercury lamp.

Real-time optical image processing and polarization holography with dyed gelatin

The application of dyed gelatin to real-time non-Fourier image processing methods and polarization holography is described. Examples in contrast reversal of a transparency and subtraction and addition of two signals are presented. In the field of polarization holography interference gratings have been made with two beams having different polarization orientation between them; the polarization state of the diffracted orders has been studied.

Phase gratings formed by induced anisotropy in dyed gelatin plates

A mixture of malachite green and ammonium dichromate has been used to photosensitize gelatin films. When this material is irradiated with polarized light an anisotropy is induced with its optical axis parallel to the electric vector of the incident vibration. After proper development of the material, the parts that showed anisotropy present a change in the refractive index of the gelatin independent of the orientation of the optical axis of the exposed part. On this basis the recording material was used to record interference gratings. When the two coherent recording beams had an angle α = 90° between their relative polarizations, the spatial frequency of the resultant phase grating was twice the one presented when α = 0°.

Transient interference gratings and holograms in dyed plastic

Transient diffraction gratings were holographically made in dyed plastic by absorbing laser light. Mean energy values of 40 mJ/cm2 absorbed in a 3-mm thick layer were needed to make gratings that lasted about 10 s. The writing beams and a reading one of different wavelength were simultaneously made to fall on the plastic: the reading beam was diffracted by the grating, which produced transient diffracted beams of sufficient intensity to be seen by the naked eye or to be stored permanently by conventional photographic methods. Curves showing diffraction efficiency versus exposure with grating frequency as a parameter are presented. Applications to real-time single and double exposure holography are also discussed.

Adaptive processes controlling sensitivity of short-wave cone pathways to different spatial frequencies

The sensitivity of the short-wave cone pathway was measured with violet interference gratings on monochromatic adapting fields. Spectral field sensitivity functions for test gratings of 2,4 and 8 c/deg approximately matched Stiles' π 3 function, which presumably reflects light-adaptation of the short-wave cones. Previous work had suggested that field sensitivity varies as a function of test spatial frequency, but this now seems an artifact of spatial adaptation. (The present study shows the pathway is highly susceptible to spatial adaptation induced by fine patterns.) For various spatial frequencies that affect the short-wave pathway, signals also pass through a cancellative adaptation site where signals from the short-wave and the middle- and long-wave cones act in opposition. Transient inputs in either opponent direction polarize or desensitize the opponent site. Additional transient inputs of the opposite sign then reduce the polarization and increase sensitivity. Such effects were demonstrated with test gratings as high as 12 c/deg. Transient effects at the opponent site are particularly evident at higher spatial frequencies.

Interference gratings blazed by ion-beam erosion

The use of ion-beam erosion to transform rounded symmetric grating relief patterns into asymmetric profiles with planar facets and sharp angular features is described. Calculated profiles are confirmed by experimental profiles produced in GaAs but with significant modifications caused by redeposition. Higher efficiency interference (holographic) gratings for spectroscopic and integrated optics applications are suggested by these results.

Influence of light polarization on the diffraction efficiency of TIR holograms

Normally when forming a TIR hologram two interference gratings are stored in the recording medium. The object beam interferes with the incoming and with the reflected reference beam. An appropriate selection of the polarization orientations of the interfering light beams leads to the suppression of one of the grating systems. Even to reconstruct TIR hologram it is not all the same in which orientation the reconstructing laser beam is polarized. With the proper selection of the polarization orientations of the forming and reconstructing laser light the diffraction efficiency for TIR holograms can be optimized.