White-light interferometry with polarization phase-shifter at the input of the interferometer

Abstract

In white-light interferometry the surface profile is determined by measuring the fringe contrast function of the white-light interferogram. One of the techniques proposed for the measurement of the fringe contrast function is the use of the phase shifting technique with the help of an achromatic phaseshifter. The available phase-shifters employ a rotating polarization component at the output end of the interferometer. Using a rotating polarization component at the input end, rather than at the output end, has certain advantages. In this paper we investigate a rotating half-wave plate phase-shifter at the input end for its applications to a white-light interferometer.     

Optimal phase contrast in common-path interferometry

We have developed an analytical model for the design and optimization of common-path interferometers (CPI's) based on spatial filtering. We describe the mathematical analysis in detail and show how its application to the optimization of a range of different CPI's results in the development of a graphical framework to characterize quantitatively CPI performance. A detailed analytical treatment of the effect of curvature in the synthetic reference wave is undertaken. We show that it is possible to improve the linearity and fringe accuracy of certain standard interferometers by a modification of the Fourier filter, and we propose and analyze a dual CPI system for the unambiguous mapping of phase to intensity over the complete input phase range.     

Midinfrared broadband achromatic astronomical beam combiner for nulling interferometry

Integrated optic beam combiners offer many advantages over conventional bulk optic implementations for astronomical imaging. To our knowledge, integrated optic beam combiners have only been demonstrated at operating wavelengths below 4 μm. Operation in the midinfrared wavelength region, however, is highly desirable. In this paper, a theoretical design technique based on three coupled waveguides is developed to achieve fully achromatic, broadband, polarization-insensitive, lossless beam combining. This design may make it possible to achieve the very deep broadband nulls needed for exoplanet searching.     

Thin-film achromatic phase shifters for nulling interferometry: design approach

Nulling interferometry in the thermal IR is the most promising technique for direct detection of Earth-like exoplanets. This technique requires a pi phase shifter for the parent star of the planet to be completely extinguished by destructive interference. We investigate how thin films can be used to design pi achromatic phase shifters. The design approach that we propose works on reflection and can be carried out by two steps, namely, the design of a mirror and an antireflection structure with no constraint on the phase properties of the thin film stacks. Phase-shift accuracy is derived analytically, and a numerical example illustrates this concept.     

Application of bacteriorhodopsin film for polarization phase-shifting interferometry

Photoinduced anisotropy in bacteriorhodopsin (BR) film is based on photoanisotropic selective bleaching of BR molecules under linearly polarized excitation light. It is modulated by the polarization orientation of the linearly polarized light. The anisotropic information recorded in the BR film is read by a circularly polarized light, which is in turn converted into an elliptical polarized light by the BR film. The rotation angle and the ellipticity of the elliptical polarized light are dependent on the polarization orientation of the linearly polarized excitation light. A phase-shifting interferometer based on the photoinduced anisotropy of BR film is presented theoretically and experimentally. Phase shift is controlled by the polarization orientation of the external excitation light, thus, the phase shift can be controlled without moving parts inside the interferometer, which contributes to the mechanical stability of the system.     

Grating-based real-time polarization phase-shifting interferometry: error analysis

A phase Ronchi grating-based real-time polarization phase-shifting method can be efficiently used for dynamic phase measurement in optical interferometry. A thorough error analysis is required for exhibiting how error sources influence phase-measurement results. We analyze the phase-measurement errors that are induced by the retardation error and azimuth angle error of the quarter-wave plate, the azimuth angle error of polarizers, the phase and intensity aberrations of diffractive wave fronts, and pixel mismatch of the interferometric patterns. The results will also be useful for evaluating the phase-measurement accuracy of other similar systems.     

Use of dual polarization interferometry as a diagnostic tool for protein crystallization

The use of dual polarization interferometry (DPI) as a tool for probing the different possible outcomes of protein crystallization experiments is described. DPI is a surface analytical technique used for the characterization of structure and interactions of molecular layers on an optical waveguide surface for a wide range of applications, including protein-protein interactions and conformational changes. The application of this technique provides a "signature" of crystallization events, thus predicting if there will be protein crystal formation, amorphous precipitate, or clear solution. The technique was demonstrated on a number of model proteins, and it also produced meaningful results in the case of two problematic target proteins. DPI in conjunction with a dialysis setup, allows changes in the protein solution above the waveguide surface to be monitored simultaneously with continuous control of its precipitant content. DPI has the potential to be used as a powerful method for discovering crystallization conditions, for obtaining information on the crystallization process, and as an aid in crystal optimization. It has also provided what is, to the best of our knowledge, the most direct observation to date of salting-in behavior in a protein-salt solution.     

Dynamic, electronically switchable surfaces for membrane protein microarrays

Microarray technology is a powerful tool that provides a high throughput of bioanalytical information within a single experiment. These miniaturized and parallelized binding assays are highly sensitive and have found widespread popularity especially during the genomic era. However, as drug diagnostics studies are often targeted at membrane proteins, the current arraying technologies are ill-equipped to handle the fragile nature of the protein molecules. In addition, to understand the complex structure and functions of proteins, different strategies to immobilize the probe molecules selectively onto a platform for protein microarray are required. We propose a novel approach to create a (membrane) protein microarray by using an indium tin oxide (ITO) microelectrode array with an electronic multiplexing capability. A polycationic, protein- and vesicle-resistant copolymer, poly(l-lysine)-grafted-poly(ethylene glycol) (PLL-g-PEG), is exposed to and adsorbed uniformly onto the microelectrode array, as a passivating adlayer. An electronic stimulation is then applied onto the individual ITO microelectrodes resulting in the localized release of the polymer thus revealing a bare ITO surface. Different polymer and biological moieties are specifically immobilized onto the activated ITO microelectrodes while the other regions remain protein-resistant as they are unaffected by the induced electrical potential. The desorption process of the PLL-g-PEG is observed to be highly selective, rapid, and reversible without compromising on the integrity and performance of the conductive ITO microelectrodes. As such, we have successfully created a stable and heterogeneous microarray of biomolecules by using selective electronic addressing on ITO microelectrodes. Both pharmaceutical diagnostics and biomedical technology are expected to benefit directly from this unique method.     

Surface plasmon resonance sensor based on polarization interferometry and angle modulation

A surface plasmon resonance (SPR) sensing technique based on polarization interferometry and angle modulation is presented. Its sensitivity is not a direct function of variation of reflection intensity, nor of phase shift. Rather, it is a function of the complex reflection coefficient. A three times standard deviation detection limit of 5.1 x 10(-7) refractive index units in a 2 Hz bandwidth is obtained with our experimental setup. A theoretical analysis shows that this technique can provide a wide linear measurement range. Moreover, the sensitivity is insensitive to the thickness of gold films over approximately 5 nm. This SPR sensing technique is suitable for physical, chemical, and biological research.     

Measurement and differentiation of ligand-induced calmodulin conformations by dual polarization interferometry

In early drug discovery, knowledge about ligand-induced conformational changes and their influence on protein activity greatly aids the identification of lead candidates for medicinal chemistry efforts. Efficiently acquiring such information remains a challenge in the initial stages of lead finding. Here we investigated the application of dual polarization interferometry (DPI) as a method for the real-time characterization of low molecular weight (LMW) ligands that induce conformational changes. As a model system we chose calmodulin (CaM), which undergoes large and distinct structural rearrangements in response to calcium ion and small molecule inhibitors such as trifluoperazine (TFP). We measured concentration-dependent mass, thickness, and density responses of an immobilized CaM protein layer, which correlated directly with binding and conformational events. Calcium ion binding to CaM induced an increase in thickness (≤0.05 nm) and decrease in density (≤-0.03 g/cm(3)) whereas TFP induced an increase in both thickness (≤0.05 nm) and density (≤0.01 g/cm(3)). The layer measurements reported here show how DPI can be used to assess and differentiate ligands with distinct structural modes of action.     

Effects of birefringence on Fizeau interferometry that uses a polarization phase-shifting technique

Interferometers that use different states of polarization for the reference and the test beams can modulate the relative phase shift by using polarization optics in the imaging system. Thus the interferometer can capture simultaneous images that have a fixed phase shift, which can be used for phase-shifting interferometry. As all measurements are made simultaneously, the interferometer is not sensitive to vibration. Fizeau interferometers of this type have an advantage compared with Twyman-Green-type systems because they are common-path interferometers. However, a polarization Fizeau interferometer is not strictly common path when both wavefronts are transmitted by an optic that suffers from birefringence. The two polarized beams see different phases owing to birefringence; as a result, an error can be introduced in the measurement. We study the effect of birefringence on measurement accuracy when different polarization techniques are used in Fizeau interferometers. We demonstrate that measurement error is reduced dramatically and can be eliminated if the reference and test beams are circularly polarized rather than linearly polarized.     

Adaptive nanowires for switchable microchip devices

This paper demonstrates for the first time the use of adaptive functional nickel nanowires for switching on-demand operation of microfluidic devices. Controlled reversible magnetic positioning and orientation of these nanowires at the microchannel outlet offers modulation of the detection and separation processes, respectively. The former facilitates switching between active and passive detection states to allow the microchip to be periodically activated to perform a measurement and reset it to the passive ("off") state between measurements. Fine magnetic tuning of the separation process (postchannel broadening of the analyte zone) is achieved by reversibly modulating the nanowire orientation (i.e., detector alignment) at the channel outlet. The concept can be extended to other microchip functions and stimuli-responsive materials and holds great promise for regulating the operation of microfluidic devices in reaction to specific needs or unforeseen scenarios.     

Fast polarization reversal of proton spins in solid-state targets

The method of polarization reversal of proton spins in solid-state targets, by using the effect of spin superradiance is analysed theoretically. The main aim is to find out the most accurate way of calculating the time of reversal and the final reversal polarization. Three approaches are compared: one, based on the standard Bloch equations; another, using numerical simulations for finite-particle model; and an approach based on effective equations taking into account local spin fluctuations. The latter method is shown to be the most accurate. The minimal reversal time can be of the order of 10⁻⁵ – 10⁻⁴ s. The maximal final polarization can reach 80–90%, but can never be 100% complete. The proposed method only works from negative to positive polarization.     

Edge localization of subwavelength structures by use of polarization interferometry and extreme-value criteria

A polarization interferometric method is presented for the quantitative microscopy of topographical structures with subwavelength linewidths. A liquid-crystal phase shifter is inserted into the imaging optics of a reflected-light microscope, and the principles of phase-shifting interferometry are applied to measuring the phase and the contrast of the TE-polarized image (E parallel edge) with the TM-polarized image (E perpendicular edge) as the reference. This common-path interferometric method provides selective edge detection for line structures because the polarization difference is localized at the structure edges. Two different threshold criteria for linewidth determination are discussed: distance of the contrast minima and distance of the points of the steepest phase change. Linewidths as small as 300 nm were measured at a 635-nm wavelength. The dependence on the illumination numerical aperture, as well as on the material, the width, and the depth of the structure, is investigated both experimentally and by rigorous numerical simulations.     

Fast polarization switching panel with high brightness and contrast ratio for three-dimensional display

We propose a polarization switching device using optically compensated pi cell for polarization-glass-type three-dimensional display. This device shows good optical properties such as high transmittance and low cross-talk ratio because of its fast dynamic response characteristics. To improve the brightness and contrast ratio on the right- and left-hand sides, we attach optical retardation films on each side of the polarization glasses instead of attaching the films on the polarization switching panel. From the calculation and experiment, we obtain high contrast ratios, over 200:1, on both sides and a high brightness using only one film on each side.     

Liquid-crystal-based switchable polarizers for sensor protection

Linear polarizers are generally employed in conjunction with advanced liquid-crystal filters for the protection of human eyes and optical sensors. For detection sensitivity under a no-threat condition to be maximized, the polarizer should remain in a clear state with a minimum insertion loss. When threats are present, it should be quickly switched to function as a linear polarizer with a high extinction ratio. Two types of switchable polarizer for sensor protection are demonstrated. The polarization conversion type exhibits a high optical efficiency in its clear state, a high extinction ratio in the linear polarizer state, and a fast switching speed, except that its field of view is limited to approximately ±10°. In contrast, an improved switchable dichroic polarizer functions effectively over a much wider field of view. However, its extinction ratio and optical efficiency in its clear state are lower than those of the polarization conversion type.     

Precision, accuracy, and range of perceived achromatic transparency

How accurately do human observers perceive the properties of an achromatic transparent filter with both reflective and transmissive components? To address this question, a novel six-luminance stimulus was employed, consisting of three transparent layer luminances set against three background luminances, which satisfied the conventional constraints of perceptual transparency. In one experiment, subjects adjusted one of the three layer luminances to complete the impression of a uniform transparent disk. It was found that the luminance-based formulation of Metelli's episcotister model and a model based on ratios of Michelson contrasts best predicted the subjects' settings, which were both accurate and precise. In another experiment, pairs of stimuli selected from a range with various values of the adjustable layer luminance were presented in a series of forced-choice trials. A modified implementation of the pair comparisons method was employed to recover the distribution that describes each subject's preference pattern. Results showed that there exists a reasonably wide range of stimuli that give rise to at least some degree of perceived transparency.     

All-prism achromatic phase matching for tunable second-harmonic generation

Achromatic phase matching (APM) involves dispersing the light entering a nonlinear optical crystal so that a wide range of wavelengths is simultaneously phase matched. We constructed an APM apparatus consisting of six prisms, the final dispersion angle of which was optimized to match to second order in wavelength the type I phase-matching angle of beta barium borate (BBO). With this apparatus, we doubled tunable fundamental light from 620 to 700 nm in wavelength using a 4-mm-long BBO crystal. An analogous set of six prisms after the BBO crystal, optimized to second order in second-harmonic wavelength, realigned the output second-harmonic beams. Computer simulations predict that adjustment of a single prism can compensate angular misalignment of any or all the prisms before the crystal, and similarly for the prisms after the crystal. We demonstrated such compensation with the experimental device. The simulations also indicate that the phase-matching wavelength band can be shifted and optimized for different crystal lengths.     

Beam-splitting cube for fringe-projection, holographic, and shearographic interferometry

We describe a simple method in which the techniques of fringe-projection, holographic, and shearographic interferometry may be readily realized with a commercial beam-splitting cube, a laser source, and a beam expander. With the use of a computer algorithm, moiré fringes may be derived digitally from the fringes that are projected onto a reference surface and onto an object surface. Successful use of the beam-splitting cube for these optical methods is attributed to the refraction of the two split beams that exit from the two adjacent faces of the cube. By careful orientation of the cube, the two refracted light beams will converge and interfere, resulting in the formation of Young's interference fringes for the fringe-projection method. When the hypotenuse of the beam-splitting cube is placed nearly normal to an illuminated object, both the image and the mirror image of the object will be formed behind the cube where a camera is placed. This optical setup thus forms the basis for double-exposure holography when these two images are fully overlapped and for shearography when the images are slightly laterally displaced.