Tom D. Milster

Roles of propagating and evanescent waves in solid immersion lens systems

The electromagnetic field incident on the thin-film layers in a solid immersion lens (SIL) system is decomposed into contributions from homogeneous and inhomogeneous waves, which are commonly referred to as propagating and evanescent waves, respectively. The homogeneous and the inhomogeneous parts have different properties with respect to the field distribution in the gap and inside the recording layers. The homogeneous part is shown to diffract like a focused wave with a numerical aperture of 1, and the inhomogeneous part decays exponentially away from the bottom of the SIL. Two examples are discussed in detail, and the concept of a vector illumination system transfer function, which includes effects of the recording layers, is introduced.

Digital Position Estimation for the Modular Scintillation Camera

Digital implementation of position estimation for the modular scintillation camera is presented. The modular camera system, introduced at the 1983 IEEE Nuclear Science Symposium, comprises an array of small, mechanically and electronically independent, all-digital scintillation cameras. This paper addresses how to best determine the event position given the PMT signals. Two Bayesian estimators are investigated, namely maximum-likelihood (ML) and minimum-meansquare-error (MS) estimators. Both estimators performed similarly. The more important aspect of the estimation problem was found to be the shape of the detector responses as a function of position.

Effects of structured mid-spatial frequency surface errors on image performance

Optical designers are encouraged to adopt aspheric and free-form surfaces into an increasing number of design spaces because of their improved performance. However, residual tooling marks from advanced aspheric fabrication techniques are difficult to remove. These marks, typically in the mid-spatial frequency (MSF) regime, give rise to structured image artifacts. Using a theory developed in previous publications, this paper applies the fundamentals of MSF modeling to demonstrate how MSF errors are evaluated and toleranced in an optical system. Examples of as-built components with MSF errors are analyzed using commercial optical design software.

Near-field optics: a new tool for data storage

Evanescent energy can be used to produce extremely small optical spots. Two practical implementations that use evanescent energy are aperture probes and solid immersion lenses (SILs). For data storage, the optical near field is defined in terms of evanescent coupling between the system used to read data and the recording layer. Because of the small spot size, near-field techniques are applied to optical data storage systems in order to increase recording density. Both aperture-type systems and SIL systems show good promise of achieving densities of more than 150 Gb/in/sup 2/. The characteristics and performance of several systems are compared, and future near-field technologies are discussed.

Minimum detectable displacement in near‐field scanning optical microscopy

The probe‐to‐sample separation in near‐field scanning optical microscopes can be regulated by a noncontact atomic shear force sensing scheme that allows simultaneous acquisition of optical and shear force images. We have measured the minimum detectable displacement that can be achieved with a scheme based on diffracting a focused laser beam from the vibrating probe. The minimum detectable displacement determines the smallest resolvable change in force acting on the probe. The measured shot‐noise‐limited value is 2.8×10−3 Arms/√Hz, and the practical sensitivity is limited by thermal vibration noise to 7×10−3 Arms/√Hz. These values compare well with those calculated theoretically.

Polarization switching control in vertical-cavity surface-emitting lasers

Orthogonal polarizations within vertical-cavity surface-emitting lasers (VCSEL) lase at slightly different wavelengths. We describe the use of optical feedback to confine polarization variations to reproducible injection currents. An external cavity is used to select specific wavelengths that reflect back into the VCSEL, hence, changing the cavity Q for the different polarization states. With this control, we can change the polarization state of the laser output.

Super-Resolution by Combination of a Solid Immersion Lens and an Aperture

A solid immersion lens is combined with various aperture shapes in order to improve resolution. Both metallic and dielectric apertures are investigated, and optimum shapes for each are determined. Fabrication techniques for each type of combination probe are discussed and implemented, and a simple experiment with a dielectric combination probe scanning a reflective grating demonstrates improved resolution compared to a solid immersion lens alone.

Modular Scintillation Cameras: A Progress Report

Modular scintillation cameras are gamma cameras with relatively small crystal faces, a small number of photomultiplier tubes (PMTs), and independent processing electronics. Our prototypical module has a 10 cm square crystal face, four PMTs, and digital processing electronics. Scintillation event information is transferred to images by mapping digitized PMT response combinations to optimal position estimates of event locations. In our prototype, a look-up table is used to perform this mapping. To encode scintillation event information more effectively, we use nonlinear compression of each of the PMT signals. Also introduced are logarithmic matched filtering and likelihood windowing, two processing techniques that result from exploitations of the Poisson model of the distribution of photopeak events. Logarithmic matched filtering is a method of obtaining estimates of mean detector response functions having greater accuracy than that indicated by the digitization of the PMT responses. Likelihood windowing is the utilization of a likelihood threshold, rather than the familiar energy window, as a discriminant of photopeak and scatter events. We have implemented each of the above on our prototypical module. Performance characteristics of this module include energy resolution of 10% full width at half maximum (FWHM) at 140 keV and spatial resolution of better than 4mm FWHM over 90% of the crystal.

Horizons for optical data storage

By now, everyone is familiar with the products of the first two generations of commercially successful optical data storage: compact discs for audio and software distribution and digital versatile discs for video. This article reviews these optical disc technologies and looks ahead to successive generations, which will exhibit finer resolution, higher data rate and increased capacity.

HIGH PRECISION ASTROMETRY WITH A DIFFRACTIVE PUPIL TELESCOPE

Astrometric detection and mass determination of Earth-mass exoplanets requires sub-µas accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must however overcome astrometric distortions which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a teo-dimensional grid of regularly spaced small dark spots added to the surface of the primary mirror (PM). Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the star’s immediate surrounding. Because the diffraction spikes are created on the PM and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars, and are therefore calibrated. We describe the technique, detail how the data collected by the wide-field camera are used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4 m diameter telescope, 0.3 deg 2 field we adopt as a baseline design achieves 0.2 µas single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-µas astrometry without relying on the accurate pointing, external metrology or high stability hardware required with previously proposed high precision astrometry concepts. Subject headings: astrometry — telescopes — techniques: high angular resolution — planets and satellites: detection