Plasmonic nano-structures for optical data storage.

We propose a method of optical data storage that exploits the small dimensions of metallic nano-particles and/or nano-structures to achieve high storage densities. The resonant behavior of these particles (both individually and in small clusters) in the presence of ultraviolet, visible, and near-infrared light may be used to retrieve pre-recorded information by far-field spectroscopic optical detection. In plasmonic data storage, a very short (approximately few femtoseconds) laser pulse is focused to a diffraction-limited spot over a small region of an optical disk containing metallic nano-structures. The digital data stored in each bit-cell, comprising multiple bits of information, modifies the spectrum of the incident light pulse. This spectrum is subsequently detected, upon reflection/transmission, with the aid of an optical spectrum analyzer. We present theoretical as well as preliminary experimental results that confirm the potential of plasmonic nano-structures for high-density optical data storage applications.

Surface-wave interferometry on single subwavelength slit-groove structures fabricated on gold films.

We apply the technique of far-field interferometry to measure the properties of surface waves generated by two-dimensional (2D) single subwavelength slit-groove structures on gold films. The effective surface index of refraction n(surf) measured for the surface wave propagating over a distance of more than 12 mum is determined to be n(surf) = 1.016+/-0.004, to within experimental uncertainty close to the expected bound surface plasmon-polariton (SPP) value for a Au/Air interface of n (spp) = 1.018. We compare these measurements to finite-difference-time-domain (FDTD) numerical simulations of the optical field transmission through these devices. We find excellent agreement between the measurements and the simulations for n(surf). The measurements also show that the surface wave propagation parameter k(surf) exhibits transient behavior close to the slit, evolving smoothly from greater values asymptotically toward k (spp) over the first 2-3 mum of slit-groove distance x(sg). This behavior is confirmed by the FDTD simulations.

Single-beam trapping of micro-beads in polarized light: Numerical simulations.

Using numerical solutions of Maxwell's equations in conjunction with the Lorentz law of force, we compute the electromagnetic force distribution in and around a dielectric micro-sphere trapped by a focused laser beam. Dependence of the optical trap's stiffness on the polarization state of the incident beam is analyzed for particles suspended in air or immersed in water, under conditions similar to those realized in practical optical tweezers. A comparison of the simulation results with available experimental data reveals the merit of one physical model relative to two competing models; the three models arise from different interpretations of the same physical picture.

Transmission of light through periodic arrays of sub-wavelength slits in metallic hosts.

Using Bloch modes to study the extraordinary transmission of light through a periodic array of slits in a metallic host, we discuss the differing roles of surface plasmon polaritons and Wood's anomalies in the observed behavior of such structures. Under certain circumstances, the first few excited modes appear to play a decisive role in determining the transmission efficiency of the array. Surface plasmon excitations tend to reduce the transmissivity of a semi-infinitely thick slit array, yet, paradoxically, the same reduction can account for enhanced transmission in an array of finite thickness tau, provided that tau is tuned to a Fabry-Perot-like resonance between the entrance and exit facets of the slit array. At the Wood anomaly, power redistribution produces sharp peaks in the diffraction efficiencies of various reflected and transmitted orders of the semi-infinite structure. With skew incidence, the degenerate states split, resulting in two peaks and two valleys, as observed by Wood in his 1902 experiments.

Transmission of light through small elliptical apertures.

The results of computer simulations based on the Finite Difference Time Domain method with local space and time grid refinement, are presented for an elliptical aperture in a thin metal film illuminated by a normally incident, monochromatic plane wave. Both cases of incident polarization parallel and perpendicular to the long axis of the ellipse are considered. An intuitive description of the behavior of the electromagnetic fields is developed in each case, and simulation results that exhibit patterns similar to those expected from this qualitative analysis are presented. The simulations reveal, in quantitative detail, the amplitude and phase behavior of the E- and B-fields in and around the aperture.

Effects of high-numerical-aperture focusing on the state of polarization in optical and magneto-optic data storage systems.

Using vector diffraction theory and an exact method for computing reflection coefficients for multilayer structures, we analyze the effects of high-numerical-aperture focusing on the state of polarization in optical data storage systems. The focused incident beam is decomposed into a spectrum of plane waves, and the reflected beam is obtained by the superposition of these plane waves after they are independently reflected from the multilayer. Plots of polarization rotation angle and ellipticity for several disk structures are presented.

Analysis of astigmatic focusing and push-pull tracking error signals in magnetooptical disk systems.

Methods of geometrical optics are used in deriving a simple expression for the focus error signal in terms of the parameters of an astigmatic system. The extent of validity of this result is then examined by comparison with diffraction calculations. Diffraction analysis also permits the study of push-pull tracking error signal from pregrooved disk surfaces.

Evaluation of partial-response maximum-likelihood detection for phase-change optical data storage.

We describe the application of partial-response (PR) maximum-likelihood (ML) detection in rewritable phase-change optical data storage. The input to this detector, which is simulated in software, is the actual signal (without any equalization), reproduced from reading of the recorded sequence on an optical disk. The detection algorithm involves the extraction of the impulse response from the readout signal, PR equalization, the adjustment of gain and recovery of clock, ML sequence estimation with the Viterbi algorithm, and analysis of PRML performance. With a laser wavelength of 0.69 microm and an objective lens with a numerical aperture of 0.6, three linear densities are examined: 0.35 and 0.31 microm/bit without modulation code and 0.2 microm/bit with the (1, 7) modulation code. The equalized signal exhibits good eye patterns, especially at the densities of 0.35 and 0.31 microm/bit. Analyses of noise and bit-error rate indicate that jitter, rather than noise, is the main obstacle to realizing ultrahigh density in phase-change media with PRML detection. We also briefly discuss the problem of the inherent nonlinear effect in phase-change readout.

Sources of noise in erasable optical disk data storage.

Noise sources in the readback signal for phase-change and magneto-optical disks at red, green, and blue wavelengths are examined, and a simple model is presented to explain the observed noise spectra. For phase-change disks the media noise, which corresponds to ~0.4% fluctuation in the disk's amplitude reflection coefficient, is the limiting performance factor for the conventional detection scheme. In magneto-optical media the depolarization noise, whose fluctuations are ~0.05% of the disk's reflection coefficient, is the major contributor to the media noise in the differential detection scheme. In phase-change optical disks the main sources of noise are the roughness of the groove profiles and the graininess of the polycrystalline recording layer. In nongrooved regions of the disk the media noise measured with green light is found to be nearly the same as that obtained with the red light. In magneto-optical disks the scattering of light from the rough groove profiles, as well as media inhomogeneities, gives rise to depolarization. Measurements on nongrooved regions of a magneto-optical disk indicate that the media noise obtained with the green light is somewhat higher than that obtained with the red light.

Effects of substrate birefringence on focusing and tracking servo signals in magneto-optical disk data storage.

Using computer simulations based on the diffraction theory of high-N.A. systems, we examine the effects of substrate birefringence on servo signals in magneto-optical disk drives. Our attention is confined to systems that use the methods of push-pull tracking and astigmatic focus-error detection. We show that the amounts of birefringence typically observed in polycarbonate substrates do not in themselves cause problems for the servo channels. However, the presence of residual aberrations in the optical path (astigmatism in particular) can have devastating effects on the magnitude of the track-error signal and the stability of the focusing servo. We show that the combination of substrate birefringence and residual beam astigmatism can either improve or deteriorate the performance of the focusing servo, depending on the orientation of the aberration relative to the principal axes of the substrate.

Effect of tilted ellipsoid of birefringence on readout signal in magneto-optical disk data storage.

In the 2× format for second-generation magneto-optical (MO) disks, there are recording tracks next to preformat pits. It has been observed that the MO signals from the tracks adjacent to the preformat pits can be affected. We present numerical modeling results on the effect of a tilted ellipsoid of substrate birefringence on the MO readout signal. Theoretical calculations show that the observed effects can be explained if a certain tilt of the ellipsoid of birefringence exists around the preformat pits.

Measurement of the thermal conductivity of erasable phase-change optical recording media.

We describe a method to estimate the thermal conductivity of the substrate, the dielectric layer, the phase-change (PC) layer, and the reflective layer of PC optical recording media. The method relies on the amorphous-to-crystalline phase transition that occurs in the PC layer and takes advantage of the difference in the thermal diffusion behavior under different-sized focused spots. All the results obtained here are reliable with better than ?5% accuracy, which is within the margin of our experimental error.

Investigation of substrate birefringence effects on optical-disk performance.

Substrates for magneto-optical disks are produced economically and efficiently by injection molding of polycarbonate plastics. Unfortunately, most plastics are birefringent, with different refractive indices in the plane of the disk (lateral birefringence) and perpendicular to the plane of the disk (vertical birefringence). One manifestation of media birefringence is the existence of a focus offset between the two distinct best-focus positions for data detection and tracking. This focus offset degrades overall system performance as the compromise focus position between the two best-focus points reduces the operating margin of the individual data and tracking channels. We present detailed numerical modeling results on the role of substrate birefringence in causing this focus offset.

Design approaches with a lenslet array and a single, high-numerical-aperture annular-field objective lens for optical data storage systems that incorporate large numbers of parallel read-write-erase channels.

To achieve very high data rates (>10(9) bits/s) in optical data storage systems it is necessary to employ a large number of laser beams for parallel read-write-erase operations. Bringing all these beams to diffraction-limited focus with a high-numerical-aperture objective lens (while maintaining focus and tracking) requires techniques that are fundamentally different from those that are currently practiced in the field of optical data storage. We present two possible solutions to the problem of designing an objective lens for such systems, one involving an array of high-quality lenslets and the other based on a single, high-numerical-aperture annular-field-of-view conventional lens. Both approaches have advantages and disadvantages, on which we elaborate in the course of our discussions.

Causes of focus-error feedthrough in optical-disk systems: astigmatic and obscuration methods.

High-density magneto-optical recording systems require sensitive and robust focus position sensors that are immune to transient changes in the amplitude and the phase of the light diffracted from pregrooved media during the seek operation. The false focus-error signal produced by track crossing during seeking is termed feedthrough. Total immunity to feedthrough is never achieved, although some focus-error detection methods, notably the obscuration method, perform better in this regard. The astigmatic focus-error detection method is usually operated with a large astigmatic foci separation distance to facilitate detector alignment and to permit push-pull tracking, which increases pattern noise and contributes to its poor resistance to feedthrough. Pattern noise is caused by the projection of the intensity pattern at the exit pupil of the objective lens onto the detector plane, at which it produces false focus-error signals. The obscuration method, a diffraction-limited method of focus-error sensing, evens out his pattern noise and is therefore more resistant to feedthrough. We present numerical modeling results that compare the feedthrough performance of the astigmatic and the obscuration methods of focus-error detection.

Measuring distribution of the ellipsoid of birefringence through the thickness of optical disk substrates.

We have measured the birefringence of polycarbonate optical disk substrates, using ellipsometry. For a more comprehensive characterization, the probe beam was incident upon substrates in a wide range of polar angles and from different azimuths relative to track direction (ø). Our measurements show that the ellipsoid of birefringence is tilted in the plane of radial (r) and normal (z) directions. The tilt angle varies through thickness, with a maximum value of approximately 10°. For beams passing through the substrate in the ø-z plane and at large incident angles, this tilt causes significant conversion (up to 100%) between p- and s-polarized components. Distributions of other parameters, such as the values of in-plane and vertical birefringence, are obtained simultaneously in the measurements.

Measurement of the wavelength dependence of birefringence for optical disk substrates.

The in-plane and vertical birefringence of polycarbonate plastic substrates of optical disks are measured for wavelengths between 360 and 860 nm, which covers the full range of interest for blue as well as for the current red and infrared recording. It is found that the birefringence generally decreases as the measurement wavelength is increased. In a typical case, the in-plane birefringence, Δn‖ goes from 1.7 × 10(-5) to 1.2 × 10(-5), and the vertical birefringence, Δn⊥, drops from 7.5 δ 10(4-) to 5.7 × 10(4-) in the wavelength range from 360 to 860 nm.