Nile F. Hartman

Antireflection gold surface-relief gratings: experimental characteristics

A systematic procedure using the effective index method and impedance matching has recently been developed Appl. Opt. 26, 3123 (1987)] for the design of antireflection high-spatial-frequency rectangulargroove gratings on lossy materials including high conductivity metals. The design procedure in turn can be used as a starting point to design antireflection metallic gratings with lower spatial frequencies using rigorous coupled-wave analysis. These lower spatial-frequency gratings have the advantage of being easier to fabricate. In the present work, a particular antireflection gold grating design (having a period of 1.0 microm, a filling factor of 50%, and a groove depth of 147.5 nm for use at a freespace wavelength of 500 nm, normal incidence, and polarization parallel to the grooves) was fabricated and its diffraction characteristics experimentally measured. The grating indeed showed very nearly zero specular reflection in the blue region of the spectrum. Unlike previously reported antireflection anomalies, the effect is broadband occurring over a broad range of wavelengths and angles of incidence, and for both orthogonal polarizations. This work clearly shows that the systematic design of zero specular reflection grating surfaces is possible.

Optical system-on-a-chip for chemical and biochemical sensing: the chemistry

Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. Optically combining one guided sensing beam with a reference beam in an interferometric configuration generates measurable signals. Applying a chemically selective film over the sensing arm of the interferometer provides the basis for a chemical sensor. Tailored chemistries can be passive (e.g.; inducing swelling or dissolution in a film) or active (e.g.; containing reactive or binding sites). Fast and reversible chemistries are the goal, in most cases for both gaseous and liquid applications. Passive mechanisms are used when the target analyte is relatively inert, i.e. aromatic and chlorinated hydrocarbons. Active chemistries developed include tailoring the acid-base strength of the sensing film for pH or ammonia response, and antibody-antigen binding. Currently the integrated optic waveguide platform consists of thirteen interferometers on a 1 X 2-cm glass substrate. A different sensing film deposited on each channel allows for multiple analyte sensing, interferant cancellation, patterned outputs for analyte identification, or extended dynamic range. Sensitivities range from the low ppm to low ppb for both vapor and aqueous applications, 0.01 pH units and ng/mL for biologicals.

Integrated optic gaseous NH3 sensor for agricultural applications

An integrated optic sensor for monitoring NH3 volatization as related to agricultural fertilizer applications is described. The sensor is capable of monitoring NH3 levels over a range from less than 100 parts per billion by volume to levels approaching 1000 parts per million by volume. The sensor is based on a planar waveguide operating in an interferometric mode. The device functions by monitoring a refractive index change resulting from a reversible chemical reaction occurring on the waveguide surface.

Phase stability of ferroelectric liquid crystals upon repeated switching and static temperature characteristics

Surface-stabilized ferroelectric liquid crystals (FLCs) are promising materials for semiconductor integrated-circuit-based spatial light modulators. For coherent optical processing applications, phase stability upon repeated switching is critically important. The phase characteristics of an FLC device were measured at switching rates of up to 1 kHz and found to be very stable. The change in the total optical path length through the cell was found to be < 0.0025λ at a wavelength of 632.8 nm. The static optical characteristics were measured for a range of temperatures at and above room temperature in order to be able to identify any temperature-induced phase changes upon switching. The temperature of the FLC cell was externally varied, and changes in the birefringent optical path difference, the optical path length, and the optic axis tilt angle were measured. However, because of the observed phase stability of the FLC, the change of temperature caused by switching was determined to be < 0.046°C. It is clearly shown that FLCs can exhibit the stability needed for critical coherent and incoherent optical data-processing applications.

Beam diameter threshold for polarization conversion photoinduced by spatially oscillating bulk photovoltaic currents in LiNbO 3 : Fe

Polarization conversion that is photoinduced by means of spatially oscillating photovoltaic currents in bulk LiNbO3:Fe is studied both experimentally and theoretically. We measured nearly complete ordinary-to-extraordinary conversion for input ordinary beam diameters greater than ∼200 μm and almost no conversion for beam diameters less than ∼60 μm. The extraordinary light was scattered perpendicular to the optic axis into a bow-tie-shaped distribution. To analyze the effect, we use the bulk photovoltaic model to derive coupled-wave equations for arbitrary-direction two-beam coupling (one ordinary wave, one extraordinary wave) and multiple-beam coupling (one ordinary wave, multiple extraordinary waves). We include the effect of beam diameter by using an overlap integral of the interacting beam profiles. The theory correctly models both the experimental three-dimensional distribution of extraordinary scattered light and the total polarization conversion as a function of beam diameter.

Micromechanically based integrated optic modulators and switches

The integration of micromachined devices into integrated optic systems offers the potential for both miniaturization and improved performance of these systems. In this paper, an electromechanical switch integrated with an optical waveguide is described that is suitable for phase and intensity modulation and thus modulation or switching functions. The electromechanical switch is fabricated using surface micromachining techniques and consists of a multilevel polyimide platform, some regions of which are suspended 2 - 3 micrometers above the waveguide surface and other region 15 micrometers above the waveguide surface. The platform is free to move in the vertical direction. Application of voltage between the platform and substrate brings the platform into intimate contact with the waveguide, thus changing the waveguide transmission characteristics. Extensions of this technique to multiple platforms in series to create multibit digital modulation is easily envisioned.

Rapid response biosensor for detection and identification of common foodborne pathogens

An integrated optic biosensor for detecting foodborne pathogens is described. The sensor is based on a planar waveguide operating in an interferometric mode. The device functions by detecting the direct binding of an antigen molecule to a functionalized waveguide surface. It is capable of detecting biomolecules at subnanogram/milliliter concentrations and has been used to detect proteins specific to Salmonella.

Optical alignment and tilt-angle measurement technique based on Lloyd’s mirror arrangement

An optical alignment and tilt-angle measurement technique is presented. The method is noncontacting, easy to use, and has an angular sensitivity of better than 0.01°.

Reversible integrated optic evanescent field biosensor using chemical amplification for added sensitivity

Planar waveguide interferometers provide an attractive sensing platform for biosensor applications. Advantages include small size, real-time sensing, multiple analyte detection on a chip, performance independent of wavelength and optical power, and nulling of thermal and mechanical noise. Limitations include slow diffusion time of the analyte to the functionalized surface, interference from non-specific binding and bulk index of refraction changes and a lack of reversibility. Combining certain techniques used in affinity chromatography and enzyme-linked immunosorbent assays and with an amplifying chemoselective film on the waveguide produces a sensor that is versatile, reusable and overcomes most of the above limitations. Work will be presented using an optical pH and ammonia sensor for detection.

Coherent optical characterization of magnetooptical spatial light modulators

A precision optical system, developed for characterizing the amplitude and phase properties of spatial light modulators, was used to characterize a 48 x 48 pixel magnetooptic spatial light modulator (MOSLM). Considerable variations in the amplitude (+/-25%) and phase transmittance (+/-50%) over the area of a given pixel were observed with coherent light illumination. The pixel-to-pixel variations in the average amplitude (+/-5%) and average phase (+/-6%) were considerably less. The contrast ratio and the polarization rotation for full frame monochromatic illumination were ~10:1 and 11.25 degrees , respectively. For illumination within a single pixel, the contrast ratio and polarization rotation were ~100:1 and 14.0 degrees , respectively. A theoretical model is presented showing that the reduced values for full frame illumination may be described by the coherent addition of light of unrotated polarization (transmitted between pixels and around the edges) with the polarization rotated light. The amplitude and phase characteristics of the MOSLM were found to be a very stable with repeated switching of the pixel and with switching of neighboring pixels. This stability is a central requirement in coherent optical information processing.