Full-field parallel interferometry coherence probe microscope for high-speed optical metrology.

Parallel detection of several achromatic phase-shifted images is used to obtain a high-speed, high-resolution, full-field, optical coherence probe tomography system based on polarization interferometry. The high enface imaging speed, short coherence gate, and high lateral resolution provided by the system are exploited to determine microbump height uniformity in an integrated semiconductor chip at 50 frames per second. The technique is demonstrated using the Linnik microscope, although it can be implemented on any polarization-based interference microscopy system.

Liquid crystal high-resolution optically addressed spatial light modulator using a nanodimensional chalcogenide photosensor.

The fabrication and performance of an optically addressed spatial light modulator (OASLM) based on nematic liquid crystal and nanodimensional amorphous arsenic trisulfide (a-As2S3) chalcogenide glassy films are described. The photoconductive a-As2S3 layers are used both as photoalignment material and as a photosensor. The use of the OASLM as a color converter is demonstrated in the transmission mode. The phase retardation dynamic range is over 3π. Diffraction efficiency measurements show a high resolution (150 lp/mm at 50% MTF). A wide variety of materials from the chalcogenide glass (ChG) family are useful for simple fabrication of high-resolution OASLMs depending on the desired wavelength.

Figure-of-merit enhancement of surface plasmon resonance sensors in the spectral interrogation.

We show that adding a thin dielectric layer with high refractive index on top of the metallic layer in surface plasmon resonance sensors in the Kretschmann-Raether configuration in the spectral mode causes a redshift of the resonance wavelength, narrowing of the resonance dip, and an enhancement to the spectral sensitivity. Surprisingly, together with the sensitivity enhancement, the dip becomes much narrower and the figure of merit is considerably improved, particularly in the IR range.

Does human skin truly behave as an array of helical antennae in the millimeter and terahertz wave ranges?

The sweat ducts of the human perspiration system are helically shaped tubes, filled with a conductive aqueous solution. Recent studies have claimed that these ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the subterahertz region. Using local homogenization theory for the skin embedded with sweat ducts, we found that multiple interference effects from the skin layers play the major role in determining the skin electromagnetic characteristics in the millimeter and terahertz regions without the need for the assumption of the sweat ducts acting as low-Q helical antennae.

Tunable optical filter having a large dynamic range.

A novel (to our knowledge) concept for a tunable birefringent filter with a high dynamic range is proposed and demonstrated by using a stack of liquid-crystal retarders having different thicknesses following an arithmetic sequence. Each retarder is between crossed polarizers with its optic axis oriented azimuthally at 45 degrees to the polarizer axis. The tunable filter was fabricated by using nematic liquid crystals demonstrating tunability over a range of 800 nm, covering the visible and the near IR; hence it has a potential for use in biomedical optical imaging applications.

Liquid crystal Lyot tunable filter with extended free spectral range.

Extension of the dynamic range of liquid crystal tunable Lyot filter is demonstrated by incorporating with it a liquid crystal variable retarder as an eliminator for the third and fourth order peaks. The filter is continuously tunable in the range 500 nm to 900 nm with a nominal width in the range 50nm-100nm. Design procedure is described including the exact solution to the LC director profile and the suitability for biomedical optical imaging applications. Flexibility in the design is proposed by applying different voltages to the different liquid crystal retarders thus compensating for small thickness deviations from the nominal values and obtaining the high dynamic range.

Liquid-crystal polarization rotator and a tunable polarizer.

A liquid-crystal (LC) voltage-controlled linear polarizer is demonstrated using two LC retarders stacked with two quarter-wave plates and an intermediate linear absorptive polarizer. The device was examined experimentally using unpolarized light and was found to be in accordance with the theoretical prediction. Under certain conditions the device acts as a polarization rotator with possibility for simultaneous amplitude modulation. Hence it has a potential application in high-dynamic-range polarimetric imaging.

Sensitivity enhancement of guided-wave surface-plasmon resonance sensors.

It is demonstrated theoretically and experimentally that, by using the guided-wave surface-plasmon sensor configuration with a top layer of dielectric thin film (10-15 nm) having a high value of the real part of the dielectric function, it is possible to improve the sensitivity of the sensor up to 1 order of magnitude. The stability is improved because the thin nanolayer acts as a protection layer for the metal. The enhancement is due to the increase in the interaction volume and the evanescent field enhancement near the top layer-analyte interface.

Effect of the number of sublayers on axial optics of anisotropic helical structures.

The transition from the continuously rotating dielectric ellipsoid approach to the discrete periodic collection of wave plates is investigated by comparing an exact analytic solution found using a Bloch-Lyaponov transformation on the 4x4 matrix form of Maxwell's equations and the numerical solution. The validity of the analytic solution depends only on the number of sublayers within the unit cell and not on the number of periods in the sample. The circularly polarized Bragg-type selective reflection peak is shown to exist with helices containing as few as N(l)=3 sublayers in a single period, but due to the appearance of higher Fourier harmonics in the dielectric tensor, additional selective reflection peaks appear. The case of N(l)=4 corresponds to the folded Solc-type filter case with a twist angle of pi/4, but it acts as a reflection filter for unpolarized light. Hence using this fact with liquid crystalline structures, polarization insensitive tunable filters can be built.

Simplified optical scatterometry for periodic nanoarrays in the near-quasi-static limit.

Scatterometry is now proven to be a very powerful technique for measurement of subwavelength periodic structures. However it requires heavy numerical calculations of the scattered optical waves from the structure. For periodic nanoarrays with feature size less than 100 nm, it is possible to simplify this using the Rytov near-quasi-static approximation valid for feature periods only few time less than the wavelength. The validity is investigated by way of comparison with exact numerical results obtained with the eigenfunctions approach. It is shown to be adequate for the determination of the structure parameters from the specularly reflected or transmitted waves and their polarization or ellipsometric properties. The validity of this approach is applied to lamellar nanoscale grating photoresist lines on Si substrate. The high sensitivity of the signals to the structure parameters is demonstrated using wavelengths of only few times the period.

Optical leaky waveguide biosensors for the detection of organophosphorus pesticides.

Organophosphorus (OP) pesticides can be rapidly detected by integrating organophosphorus hydrolase with an optical leaky waveguide biosensor. This enzyme catalyses the hydrolysis of a wide range of organophosphorus compounds causing an increase in the pH. Thus, the direct detection of OP is possible by monitoring of the pH changes associated with the enzyme's activity. This article describes the use of an optical, leaky waveguide clad with absorbing materials for the detection of OP pesticides by measuring changes in refractive index, absorbance and fluorescence. In the most effective configuration, a thick sensing layer was used to increase the amount of immobilized enzyme and to increase the light interaction with the sensing layer, resulting in a greatly enhanced sensitivity. The platforms developed in this work were successfully used to detect paraoxon and parathion down to 4 nM concentrations.

Unique optical properties of anisotropic helical structures in a Fabry-Perot cavity.

Anisotropic helical structures in Fabry-Perot (FP) cavities are shown to exhibit unique properties. The FP peaks are unpolarized, consisting of doublets corresponding to Bloch-Lyaponov eigenwaves with increased splitting as the local anisotropy increases and as they become closer to the photonic bandgap. The Bragg-type selective reflection peak existing in a free-cavity sample is inhibited when the helical medium is inside a cavity, and it has no effect on the cavity modes. Hence the unique characteristics are obtained even with a single period of the helix, and consequently thin films of helical liquid crystals can be used to build fast tunable polarization insensitive FP filters.

Photoelastically induced light modulation in gradient-index lenses.

A new photoelastic light modulator is demonstrated based on the modulation of the birefringence and of the index profile in graded-index lenses. Using the birefringence modulation, we obtained a 35% modulation depth in a quarter-pitch lens and a 65% modulation depth in a half-pitch lens at acoustic frequencies up to the megahertz range. Using the index profile modulation in a half-pitch lens as a fiber-to-fiber connector, we obtained 15% modulation without the incorporation of a polarizer.