Circular Dammann grating under high numerical aperture focusing

Circular Dammann grating (CDG) under high numerical aperture (NA) focusing is described based on Richards-Wolf vectorial diffraction theory in this paper. Several CDGs are presented under the condition of NA=0.9 with the illumination of circularly polarized plane-wave laser beams. Numerical results show that the sizes of these circular patterns with equal-intensity are in the wavelength scale, and doughnut-shaped central spots and dark rings are in the subwavelength width. To verify this kind of CDG, a binary pure-phase three-order CDG is fabricated to produce a dark center pattern surrounded by three concentric bright rings. The corresponding intensity distribution of the pattern on the focal plane of a high-NA objective (NA=0.9) is measured, and the results agree well with theoretical simulations. This kind of CDG with annular patterns of equal-intensity in the wavelength scale should be highly interesting for its potential applications in optical trapping, stimulated emission depletion (STED) microscopy, and the study of singular optics, as well as annular array illumination.     

Stress birefringent, space-variant wave plates for vortex illumination

We describe the use of stress birefringence in the creation of vortex illumination. A trifold symmetric stress pattern will provide an annular region that exhibits polarization vortices when illuminated with linearly polarized light and scalar vortices when illuminated with circularly polarized light. A finite element plane-stress model is used to analyze the space-variant anisotropy.     

New azobenzene polymers for light-controlled optical elements

Three new azobenzene polymers have been synthesized by polymerization of cyanazobenzene monomers with different numbers of methylene groups attached to the cyanazobenzene dye. The rearrangement of the anisotropic azobenzene moieties in them was initiated by illumination with linearly polarized light (488 nm), and the optical anisotropy due to this arrangement has been investigated. The photobirefringence can be erased by illumination with circularly polarized light and re-induced to the same value with linearly polarized light. We show that films from these polymers can be used as optically controlled reversible polarization elements.     

Polarization effects in 4Pi confocal microscopy studied with water-immersion lenses

We studied the effect of electric field orientation on the point-spread function (PSF) of a 4Pi microscope. We show that in a standard 4Pi arrangement the orientation of the field can be used for changing between constructive- and destructive-mode 4Pi microscopy. The effect is counteracted by introduction of a phase shift of pi into one of the half-arms. This compensation is compulsory during illumination with unpolarized or circularly polarized light. By performing our experiments with 1.2-N.A. water-immersion lenses, we demonstrate that water immersion is suitable for 4Pi confocal microscopy. At a two-photon excitation wavelength of 1064 nm, the water 4Pi confocal PSF features an axial lobe of 40% above and below the focal plane, which, by linear filtering, can be unambiguously removed. The measured axial full width at half-maximum of the PSF is 240 nm. This is 4.3 times narrower than its single-lens confocal counterpart. The 4Pi confocal microscope sets a new resolution benchmark in three-dimensional imaging of watery samples.     

Real-time objects recognition by photoanisotropic copies

The method of object recognition is described by the example of objects of amplitude transparent type. The method is to obtain a photoanisotropic copy of object images on polarization-sensitive material. At consequent illumination of the photoanisotropic copy with a parallel circularly polarized beam of nonactinic light, the transmitted light becomes elliptically polarized. It is shown that the characteristics of the summary polarization ellipse in the Fraunhofer diffraction region uniquely identify the given object. The real-time determination of the characteristics of the summary polarization ellipse is made by means of diffraction gratings of anisotropic profile and by comparison of these characteristics with etalon from the database.     

Effects of turbid media optical properties on object visibility in subsurface polarization imaging

We studied the effectiveness of using polarized illumination and detection to enhance the visibility of targets buried in highly scattering media. The effects of background optical properties including scattering coefficient, absorption coefficient, and anisotropy on image visibility were examined. Both linearly and circularly polarized light were used in the imaging. Three different types of target were investigated: scattering, absorption, and reflection. The experimental results indicate that target visibility improvement achieved by a specific polarization method depends on both the background optical properties and the target type. By analyzing all polarization images, it is possible to reveal certain information about target or the scattering background.     

Angular-scattering characteristics of ferroelectric liquid-crystal electro-optical devices operating in the transient-scattering and the extended-scattering modes

The angular distribution of forward-scattered light in transient-scattering-mode (TSM) and extended-scattering-mode (ESM) ferroelectric liquid-crystal (FLC) devices was evaluated by use of circularly polarized incident light. For both modes the intensity and the distribution of forward-scattered light depended primarily on the FLC birefringence, spontaneous polarization, and the cell path length. In the FLC materials examined, the forward-scattering intensity under ESM drive conditions increased with longer FLC pitch lengths, whereas under TSM conditions stronger forward scattering was observed with increasing FLC spontaneous polarization. Although both TSM and ESM drive conditions displayed a similar angular distribution for forward-scattered light, the intensity of ESM scattering over a 0 degrees -6 degrees range was considerably smaller than that observed in earlier experiments with linearly polarized incident light.     

Weak value amplification of an optical Faraday differential refraction effect

In the presence of a longitudinal magnetic field B, a beam of linearly polarized light incident from a Faraday medium of Verdet constant V refracts at its interface with a medium of negligible Verdet constant and emerges as two opposite circularly polarized beams that are separated by a small divergence angle δ that is proportional to the product BV. Judicious postselection of the polarization state of the emergent light can be used to amplify the measured value of δ by several orders of magnitude. This technique makes it possible to optically measure either very small V values when B is known or small magnetic fields when V is known.     

Polarization holographic gratings in side-chain azobenzene polyesters with linear and circular photoanisotropy

We investigate thin phase polarization holographic gratings recorded with two waves with orthogonal linear polarizations in materials in which illumination with linearly/circularly polarized light gives rise to linear/circular birefringence. The theoretical analysis shows that the presence of circular photoanisot-ropy changes significantly the diffraction characteristics of the gratings. The intensities of the waves diffracted in the +1 and -1 orders of diffraction and their ratio depend substantially on the reconstructing-wave polarization. Experiments with films of side-chain liquid-crystalline azobenzene polyester that is a photoanisotropic material of the considered type confirm the unusual polarization properties. It is shown that polarization holography may be used for real-time simultaneous measurement of photoinduced linear and circular birefringence.     

Deformed chiral-nematic networks obtained by polarized excitation of a dichroic photoinitiator

Photo-initiated polymerization of chiral-nematic monomers leads to helicoidal networks with well-defined and stable reflection bands. Exposure of the monomer mixtures to polarized UV light in the presence of a dichroic photoinitiator causes photo-induced diffusion simultaneously with the polymerization process. The diffusion is on a length scale equal to half of the cholesteric pitch and deforms the chiral-nematic helix. The films that are produced exhibit extraordinary optical properties such as in-plane birefringence, polarization conversion and polarization dependent higher order reflections. When combined with photo-induced diffusion on the length scale of the film thickness, wideband polarizing reflectors are obtained that transmit linearly polarized light rather than circularly polarized light as is the case for conventional chiral-nematic networks.     

Stokes-parameter analysis of the polarization of light transmitted through a chiral nematic liquid-crystal cell

A spectroscopic Stokes polarimeter is used to directly measure the linearly, circularly, and randomly polarized components of light obtained on transmission of unpolarized light through thick chiral nematic liquid-crystal cells in the stop band. The Stokes parameters are simulated to fit the experimental data by use of the Berreman 4 x 4 transfer matrix by means of the Jones and Stokes vectors and taking into account multiple reflections at the interfaces of the cell. Excellent agreement is obtained. The transmitted light through a commercial cell is mainly circularly polarized at normal incidence, but a significant linearly polarized component is also observed. The model shows that this results from refractive-index mismatching at the liquid-crystal-alignment-layer interface, but a small linearly polarized component remains even with optimized index matching. An improved device configuration incorporating random defects at the exit boundary of the liquid crystal gives a highly circularly polarized output with virtually no linear or unpolarized components.     

Thin film coatings which inhibit spin relaxation of polarized potassium atoms

A new technique is being developed at Argonne National Laboratory to produce a beam of polarized deuterium atoms. A part of the apparatus consists of a small cell which contains potassium vapor. The potassium atoms are polarized by circularly polarized light from a high-power cw laser. A port feeds deuterium into the cell and the polarization of the potassium is transferred to the deuterium by spin-exchange scattering. It is important that the potassium and deuterium atoms do not lose polarization by scattering from the walls of the exchange cell. We have tried various coatings of the exchange walls in order to inhibit losses from spin relaxation. Methods used to produce these coatings as well as their success will be described.     

Circularly polarized optical heterodyne interferometer for optical activity measurement of a quartz crystal

Phase retardation between two orthogonal circularly polarized light waves that propagate in an optical active medium is proportional to its optical activity. The measurement of optical activity of a quartz depolarizer in terms of the phase difference of two orthogonal circularly polarized waves is proposed. A circularly polarized optical heterodyne interferometer with a Zeeman laser to measure the optical activity of a quartz crystal is demonstrated experimentally. The accuracy of the measurement is discussed. In addition, the effect of elliptical polarization and nonorthogonality of linearly polarized light waves of a Zeeman laser on the optical activity measurement is analyzed.     

Light-induced optical activity in optically ordered amorphous side-chain azobenzene containing polymer

Abstract

We report the appearance of circular birefringence (optical activity) in amorphous side-chain azobenzene polymer films on illumination with circularly polarized light. The effect is observed only if an optical axis is previously created in the film with the help of linearly polarized light. The photoinduced optical activity is believed to be due to changes in the polymer structure initiated by a circular momentum transfer from the circularly polarized light to the azobenzene chromophores.     

Polarization discrimination of coherently propagating light in turbid media

We describe the use of degree of polarization to discriminate unscattered and weakly scattered light from multiply scattered light in an optically turbid material. We use spatially resolved measurements of the degree of polarization to compare how well linearly and circularly polarized light survives in a sample. Experiments were performed on common tissue phantoms consisting of polystyrene and Intralipid microsphere suspensions and on adipose and arterial tissue. The results indicate that polarization is maintained even after unpolarized irradiance through each sample has been extinguished by several orders of magnitude. The results also show that polarized light propagation in common tissue phantoms is distinctly different from polarized light propagation in the two tissues investigated. Further, these experiments illustrate when polarization is an effective discrimination criterion and when it is not. The potential of a polarization-based discrimination scheme to image through the biological and nonbiological samples investigated here is also discussed.     

Mapping the Spin-Injection Probability on the Atomic Scale

A spin-polarized electron current is injected into the GaAs(110) surface at 100 K by using a polycrystalline ferromagnetic Ni scanning tunneling microscopy (STM) tip. The injected electrons recombine to the valence band and emit circularly polarized radiation whose degree of light polarization is related to the polarization of the conduction-band electrons at the instant of recombination. When the polarized electrons are injected into clean, flat terraces an average polarization for the emitted radiation is found to be 6.79%, while over the 10-nm step region the polarization is reduced to 0.46%. This step scattering effect is studied further by varying the tunneling gap through adjusting the tunneling current. As the distance between the STM tip and sample decrease the spin-scattering effect of the step edge is enhanced.     

Generation of multifrequency laser emission quasi equally spaced throughout the entire visible region

A circularly polarized, monochromatic laser beam is focused into a Raman cell, which contains hydrogen to generate rotational stimulated Raman emission. After linear polarization, this two-color (separated by 587 cm(-1)) laser beam is focused several times into a second Raman cell that is filled with hydrogen to generate a multifrequency laser emission. Many rotational and vibrational lines are generated efficiently by this multipass effect. Eighteen colors that are quasi equally spaced with a rather flat intensity distribution are generated throughout the entire visible region. The present multifrequency laser emission may be advantageously used for illumination in a higher-grade display, such as a laser light show.     

Symmetrically coated pellicle beam splitters for dual quarter-wave retardation in reflection and transmission

A trilayer pellicle that consists of a high-index center layer that is symmetrically coated on both sides by a low-index film can be designed to produce differential reflection and transmission phase shifts of +/- 90 degrees at oblique incidence and equal throughput for the p and the s polarizations. Such a device splits a beam of incident linearly polarized light into two orthogonal circularly polarized components that travel in well-separated angular directions. Examples of infrared dual quarter-wave retarders that use a symmetrically coated Ge pellicle at 77 degrees angle of incidence are presented. A 50-50% splitter requires a symmetric pellicle with at least five layers. Error analysis shows that the thicknesses of the high-index layers must be tightly controlled. These circular polarization beam splitters are intended for operation with a well-collimated light source and can be used as the basis of a novel circular polarization Michelson interferometer.     

Possibility of an optical focal shift with polarization masks

The polarization phase shift (PPS) has emerged as an important analytical tool in optical metrology. The present study utilizes the concept of controlling the polarization phase in applications such as focal shift and automatic focusing. When elliptically polarized light, in general, is incident upon a circularly symmetric polarization mask consisting of circular and annular zones with each zone having a unique linear polarizability, the polarization-phase difference introduced between the polarization-masked zones is also circularly symmetric. With the mask at the lens aperture, the polarization phase introduced is multiplicative with the lens function and is shown to result in a shift of the Gaussian focus plane. Because the polarization phase can be controlled by variation of the polarization parameters, the effective focal length of the imaging system can be varied within a small range. A study of the point-spread functions at the shifted focal planes has shown that the quality of the focal patch in these planes is comparable with that produced by a diffraction-limited imaging system at Gaussian focus. The shift of focus can be achieved by control of the polarization of the input beam. It is anticipated that this technique may find application in areas for which dynamic focusing within a small range is required.     

Backscattering target detection in a turbid medium by polarization discrimination

We describe a method for increasing target contrast within a turbid medium by means of the polarization state of the scattered light. The backscattered Mueller matrices for various concentrations of 0.1-mum spherical scatterers were measured with and without a painted metal target. Simple discrimination based on detecting cross-polarized intensities is shown to be more effective than the use of total intensity information. As a result, the choice of polarization state is dictated primarily by the requirement to maximize depolarization at the target. This in general means that circularly polarized light is the optimum choice.