J. Kevin Erwin

Static tester for characterization of phase-change, dye-polymer, and magneto-optical media for optical data storage.

We have designed and built a static tester around a commercially available polarized light microscope. This device employs two semiconductor laser diodes (at 643- and 680-nm wavelengths) for the purpose of recording small marks on various media for optical data storage and for the simultaneous monitoring of the recording process. We use one of the lasers in the single-pulse mode to write a mark on the sample and operate the other laser in the cw mode to monitor the recording process. The two laser beams are brought to coincident focus on the sample through the objective lens of the microscope. The reflected beams are sent through a polarizing beam splitter and thus divided into two branches, depending on whether they are p or s polarized. In each branch the beam is further divided into two according to the wavelength. The four beams thus produced are sent to four high-speed photodetectors, and the resulting signals are used to monitor the reflectance as well as the polarization state of the beam on reflection from the sample. We provide a comprehensive description of the testers design and operating principles. We also report preliminary results of measurements of phase-change, dye-polymer, and magneto-optical samples, which are currently of interest in the areas of writable and rewritable optical data storage.

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.

Phase and amplitude apodization induced by focusing through an evanescent gap in a solid immersion lens microscope

We characterize phase and amplitude apodization induced when a converging beam passes through an evanescent gap. The apodization is caused by vector transmission and reflection properties. General characteristics are described for solid immersion lens indices from nSIL51.5 to 3.1, gap heights from h50 to 300 nm, and marginal ray angles a m50.7 or 0.84, where a m is the direction cosine of the marginal ray angle inside the solid immersion lens. A small amount of defocus is found to be a good compensator of the phase apodization for low nSIL and h. After proper defocus is applied, asymmetry of focus in the spot may remain, primarily due to the uncompensated amplitude apodization. Simplification of the phase and amplitude characteristics is accom- plished by applying a Jones-matrix expansion to the transmission coef- ficient through the gap. Simulation and experiment quantify the effect with a simple solid immersion lens geometry.

Design of a Near-Field Probe for Optical Recording Using a 3-Dimensional Finite Difference Time Domain Method.

Tapered dielectric near-field probes are designed for optical recording by means of a 3-Dimensional finite difference time domain (FDTD) method. Probe fabrication is attempted with a double-exposure holographic technique.

Variation on Zernike’s phase-contrast microscope

We describe the design, construction, and testing of a variant of Zernikes phase-contrast microscope. The sample is illuminated with a white-light source through an annular aperture, which is projected onto the entrance pupil of the objective lens. In the return path the light diffracted by the sample and appearing in the interior of the objectives aperture (i.e., the test beam) is separated from the light returning in the annular region near the rim of the objective (i.e., the reference beam). The separated beams are relatively phase shifted and then combined to create an interferogram of the samples surface on a CCD camera. It is fairly straightforward to use this system as a conventional bright-field or dark-field microscope, but its most interesting application is as a Zernike phase-contrast microscope with adjustable amplitude ratio and phase shift between test and reference beams. The ability to continuously adjust the phase of the reference beam also enables quantitative measurement of the phase imparted by the sample to the incident beam.

Versatile polychromatic dynamic testbed for optical disks

A dynamic testbed for the evaluation of optical disks has been designed and constructed. The system is achromatic within the wavelength range 440-690 nm, allowing any light source in this range to be utilized for read-write-erase experiments. In addition, the system accepts disks with substrate thicknesses ranging from 0 to 1.7 mm. The polarization handling capabilities of the testbed are such that, with the turn of a knob, one can generate either linearly polarized or circularly polarized light at the disk surface. This feature permits the testing of both magneto-optical and phase-change disks, in addition to compact disks and digital versatile disks, without any modifications to the system. A leaky polarizing beam splitter (LPBS) has been specially designed and built for this tester. The LPBS allows continuous adjustment of the ratio between p- and s-polarized components of the reflected beam that reach the detectors. This feature is especially useful for magneto-optical disks, where one can achieve an optimum signal-to-noise ratio by adjusting the relative amounts of the two components of polarization at the detection module. Focus-error detection is based on the astigmatic method, and the primary track-error detection scheme is the push-pull method, although other focusing and tracking schemes may also be implemented. The rf data signal and the focusing and tracking servo signals are all derived from the same detectors, thus allowing the optical power returning from the disk to be used in its entirety for these multiple purposes. The detection channel consists of two high-speed quad detectors mounted on the two arms of a differential detection module. By combining the various outputs of these detectors it is possible to generate the astigmatic focus-error signal, the push-pull track-error signal, the differential magneto-optical readout signal, the conventional sum signal for phase-change disk readout, and the differential edge-signal for mark-edge detection on various types of optical media.

Measuring the wavelength dependence of magnetooptical Kerr (or Faraday) rotation and ellipticity: a technique.

A technique is proposed that allows accurate determination of the magnetooptic rotation and ellipticity angles. This method requires only a few standard optical elements and is expected to be effective over a relatively wide range of wavelengths.

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.

Determination of optical constants of thin films and multilayer stacks by use of concurrent reflectance, transmittance, and ellipsometric measurements

Using measurements of reflectance, transmittance, and the ellipsometric parameter D, we have determined the thickness, refractive index, and the absorption coefficient of various thin films and thin-film stacks. (D, the relative phase between the p- and s-polarized components, is measured for both reflected and transmitted light.) These optical measurements are performed with a specially designed system at the fixed wavelength of lambda = 633 nm over the 10 degrees -75 degrees range of angles of incidence. The examined samples, prepared by means of sputtering on fused-silica substrates, consist of monolayers and trilayers of various materials of differing thickness and optical constants. These samples, which are representative of the media of rewritable phase-change optical disks, include a dielectric mixture of ZnS and SiO(2), an amorphous film of the Ge(2)Sb(2.3)Te(5) alloy, and an aluminum chromium alloy film. To avoid complications arising from reflection and transmission losses at the air-substrate interface, the samples are immersed in an index-matching fluid that eliminates the contributions of the substrate to reflected and transmitted light. A computer program estimates the unknown parameters of the film(s) by matching the experimental data to theoretically calculated values. Although our system can be used for measurements over a broad range of wavelengths, we describe only the results obtained at lambda = 633 nm.

Leaky polarizing beam splitter with adjustable leak ratio for operation in the wavelength range of 440-690 nm

We discuss the optomechanical design and fabrication of a novel wideband (440-690-nm), leaky polarizing beam splitter with an adjustable leak ratio. This beam splitter is an important component of a multiwavelength dynamic testbed that we have constructed for testing optical disks. The multilayer thin-film structure of the beam splitter is essentially a stacked pair of narrow-band dielectric reflectors that have been fine tuned for optimal performance. The characteristics of the fabricated device are in good agreement with our theoretical calculations.