Square-wave retarder for polarization computer-generated holography.

An etched calcite square-wave retarder is designed, fabricated, and demonstrated as an illuminator for an interlaced polarization computer-generated hologram (PCGH). The calcite square-wave retarder enables alternating columns of orthogonal linear polarizations to illuminate the interlaced PCGH. Together, these components produce a speckled, tangentially polarized PCGH diffraction pattern with a measured ratio of polarization of 84% and a degree of linear polarization of 0.81. An experimental alignment tolerance analysis is also reported.

Polarization synthesis by computer-generated holography using orthogonally polarized and correlated speckle patterns.

An interlaced polarization computer-generated hologram (PCGH) is designed to produce specific irradiance and polarization states in the image plane. The PCGH produces a tangentially polarized annular pattern with correlated speckle, which is achieved by a novel application of the diffuser optimization method. Alternating columns of orthogonal linear polarizations illuminate an interlaced PCGH, producing a ratio of polarization of 88% measured on a fabricated sample. The demonstrated technique can be applied to designs for arbitrary irradiance and polarization states in the image plane.

Vector diffraction analysis by discrete-dipole approximation.

The discrete-dipole approximation is applied to vector diffraction analysis in a system with large-numerical-aperture (NA) optics and subwavelength targets. Distributions of light diffracted by subwavelength dielectric targets are calculated in a solid angle that corresponds to a NA of 0.9, and their dependence on incident polarization, target shape, and target size is studied. Electric field distributions inside the target are also shown. Basic features of the vector diffraction are clearly demonstrated. This technique facilitates understanding of the vectorial effects in systems that are expected to be applied in the future to optical data storage.

High-performance readout and recording by a combination aperture.

A solid immersion lens combined with a conical dielectric tip exhibits good resolution and efficiency in reading and recording data marks on optical storage media. We demonstrate a combination aperture that produces ~200-nm full-width 1/e(2) spot size and achieves 50% optical efficiency in an edge-scan experiment. A comparison of recording with the combination aperture, with an unmodified solid immersion lens, and with a far-field system is made.

Effects of object roughness on partially coherent image formation.

Phase perturbations in the object plane of a partially coherent imaging system are found to produce artifacts in the aerial image. It is demonstrated that phase perturbations of as little as lambda/30 rms can produce visible deformation in the final image for modest coherence factors, such as sigma(c) = 0.4. A combination of simulation and experiment is used to demonstrate the effects. Application to line-edge roughness in lithography is described.

Pupil-plane filtering for improved signal detection in an optical data-storage system incorporating a solid immersion lens.

A pupil-plane filtering technique is applied to data-signal detection in an optical data-storage system that uses a solid immersion lens (SIL) and a four-layered phase-change recording medium. We have confirmed by numerical calculations and experiment that the technique improves signal contrast and makes the contrast less sensitive to the gap width between the bottom surface of the SIL and the top surface of the recording medium. Light that is incident upon the objective lens that is used with the SIL is linearly polarized, and the full vectorial feature of the light is considered in the calculations.

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.

New way to describe diffraction from optical disks.

An efficient decomposition of the diffraction pattern from optical disks that yields insight into the origin and the characteristics of various signals is described. The Babinet principle is used to separate components that describe the data signal, servo signals, and three types of cross talk. The construction of a basis set that yields efficient calculation for optimization studies is described. Two media types are considered as examples. Several applications are also described, including an explanation for the origin of the differential phase-detection tracking signal that is used with DVD-ROM media.

Heating mechanisms in a near-field optical system.

A finite-difference-time-domain and two finite-difference-thermal models are used to study various heating mechanisms in a near-field optical system. It is shown that the dominant mechanism of sample heating occurs from optical power that is transferred from the probe to a metallic thin-film sample. The optical power is absorbed in the sample and converted to heat. The effects of thermal radiation from the probe 's coating and thermal conduction between the probe and the sample are found to be negligible. In a two-dimensional waveguide with TE polarization, most of the optical power is transferred directly from the aperture to the sample. In a two-dimensional waveguide with TM polarization, there is significant optical power transfer between the probe 's aluminum coating and the sample. The power transfer results in a wider thermal distribution with TM polarization than with TE polarization. Using computed temperature distributions in a Co -Pt film, we predict the relative size of thermally written marks in a three-dimensional geometry. The predicted mark size shows a 30 % asymmetry that is due to polarization effects.

High-numerical-aperture effects in photoresist.

Two-beam and three-beam vector interference in thin photoresist films is used to illustrate the striking differences between s -polarized and p-polarized high-numerical-aperture illumination. Both simulations and experiments are performed for several cases, including undyed photoresist on silicon, dyed photoresist on silicon, and the addition of an antireflective layer between the photoresist and the silicon. A 0.85 numerical-aperture system is examined. The major differences between s- and p-polarized illumination include elliptical versus rectangular features and lower contrast for p-polarized images.

Mechanism for improving the signal-to-noise ratio in scanning optical microscopes.

We demonstrate an improved signal-to-noise ratio in a scanning optical microscope used to read out information from a magneto-optical data storage layer. By placing a shading band in the return path of the optical system we can reduce noise by as much as 3 dB in certain spatial frequency ranges. The signal-to-noise ratio improvement arises from differences in the signal and noise distributions in the pupil of the optical system. Although the experimental results are shown only in one dimension, the concept is applicable to two-dimensional scanning of low-contrast samples.

Optical data storage readout with quadrant pupil detection.

A novel detection scheme that uses combinations of quadrant signals derived in a pupil of the optical system is described for optical storage devices. The signals arise because of an asymmetry in the reflected light distribution when the focused spot scans data with a nonzero tracking offset. Theoretical and scalar diffraction characterization indicates that the signals may be useful for improved data density by reducing intertrack interference (cross talk). The signals may also be useful for providing a tracking error signal.

Detection of probe dither motion in near-field scanning optical microscopy.

The probe-to-sample separation in near-field scanning optical microscopes can be regulated by a noncontact shear-force sensing technique. The technique requires the measurement of a minute dither motion applied to the probe. We have characterized an optical detection method for measuring this motion to determine the optimum detection configuration in terms of sensitivity and stability. A scalar diffraction model of the detection method is developed for calculating sensitivity, and experimental results are found to be in good agreement with the theoretical predictions. We find that maximum sensitivity and stability cannot be achieved simultaneously, and it may be desirable in practice to trade sensitivity for enhanced stability.

Compensation of chromatic errors in high na molded objective lenses.

High NA molded glass objective lenses are commercially available for use in the 680 nm to 850 nm range. We show that a simple positive lens can be used with an objective lens to compensate for wavelength-induced aberration in the 400 nm to 700 nm range. There is a direct relationship between the optimum power of the corrector lens and the wavelength of interest. We also show that a single positive lens and a holographic optical element can be used with an objective lens to compensate for wavelength-induced aberration over a wide range of wavelengths.

Differential wax-wane focus servo.

We describe a differential wax-wane focus servo technique for use in optical data storage. A combination of scalar diffraction modeling and experiment is used to quantify performance. Our results indicate that the differential technique is superior to the single-detector wax-wane technique with respect to gain, linearity, and lock-on range. We present modeling results that show the effects of aberrations and detector misalignment. The differential system was found to be robust. It can also reject many common pattern noise effects such as tracking cross talk, which was reduced from 0.7 µm in a single channel to < 0.1 µm in the optimized differential channel.

Analysis of superresolution in magneto-optic data storage devices.

A mathematical framework is derived for transfer-function analysis of magneto-optic data storage devices. The characteristics of the magneto-optic medium define what portions of the optical system transfer function are important. Several ways to improve the transfer function are analyzed, and several optical systems are compared in respect to relative contrast level versus frequency. Estimates of edge responses and two-point resolution are compared. The system that exhibits the best edge and two-point resolution is a self-masking medium. However, shading bands in the collection optics show excellent response.

Technique for simultaneous alignment and collimation of a laser diode in an optical data storage head.

An autocollimatoris used to align and detect collimation of a laser diode in an optical data storage head. High wavefront quality is achieved without interferometric equipment.

A full-field modular gamma camera.

A modular gamma ray camera is described that gives useful image information over its entire crystal face. The lack of dead area on the periphery of the camera is made possible by a unique application of digital electronics and optimal position estimation using maximum likelihood (ML) estimates. The ML estimates are calculated directly from photomultiplier tube responses and stored in a lookup table, so the restriction of calculating the position estimates in separate circuitry is removed. Each module is designed to be optically and electronically independent, so that many modules can be combined in a large system. Results from a prototypical module, which has an active crystal area of 10 cm X 10 cm, are presented.

Image reconstruction from coded data: II. Code design.

A strategy is given for the design of coded apertures with respect to a given class of objects that are to be imaged. Previous knowledge of the first- and second-order statistics for the object class is assumed. The object class is characterized by its Karhunen-Loève eigenvectors and eigenvalues, whereas the imaging system is characterized by its singular-value decomposition. We introduce the concept of alignment in which the aperture parameters are adjusted until the system is tuned to measure the given object class well. A mean-square-error figure of merit that indicates degree of alignment is given, and alignment is performed by standard optimization techniques. We illustrate this technique with a simple proof-of-principle experiment. These concepts are general and may be applied to any linear imaging system.