Exploring optical multilevel information storage using
subwavelength-sized media structures

Fred Thomas
Advanced R&D, Iomega Corporation, 1821 West Iomega Way, Roy, Utah 84067
Phone: 801-332-4662 Email: thomasf@iomega.com

Abstract: Can complex 3-D subwavelength-sized structures molded in plastic optical ROM media
be interrogated by a diffraction-limited focused spot for the retrieval of multilevel information?
Finite-difference time-domain (FDTD) methods are used to examine the reflected intensity fields
from such structures. The results presented show that such information retrieval is possible.
 2003 Optical Society of America
OCIS codes: (050.0050) Diffraction and gratings, (210.0210) Optical data storage, (260.0260) Physical optics

1. Introduction

The general goal behind the data storage concept explored in this paper is the following: the creation of
subwavelength media structures whose property, upon reflection/diffraction of a focused laser spot, is the creation of
multiple beam paths, each of whose irradiance pattern’s centroid position in the lens aperture changes predictably
and measurably for the storage of multilevel encoded information.
The author introduced this concept for storing information in subwavelength-sized topographic media features at
this conference last year [1]. That paper used geometrical optical arguments for conceiving a high capacity and high
transfer rate ROM optical memory system that encoded multilevel information using an array of lithographically
fabricated micro-mirrors. A 2x2 array of these mirrors was termed an AO-DVD “optical data element” (ODE). Each
ODE is the size of the far-field optical drive’s laser stylus spot. For the particular example discussed, each ODE had
a square dimension of 780 nm. A DVD track width has this dimension. Leveraging this equivalent track size, an
explanation as to how a DVD/AO-DVD cross-compatible system might be implemented was provided. Each of the
four micro-mirrors in the ODE has both a tilt and rotation angle associated with its orientation relative to the
interrogating laser stylus. These four micro-mirrors hence are intended to split the reflected beam into four
independent beam paths, each with a different spatial orientation dependent on its individual micro-mirror angular
encoding. The detection sensor for the system incorporated a set of four, spot centroid position sensors for decoding
the massively parallel information being reflected from each ODE. Using geometrical analysis, it was shown that a
40X capacity increase for DVD could be obtained. This capacity increase was premised on whether a 2° separation
between angular orientation states of the micro-mirrors could be discriminated by the system.
Figure 1 shows some images of a macro-scale AO-DVD concept demonstrator and its associated micro-mirror
arrayed media. The image on the right (Fig. 1b) illustrates, using a ground glass detector plane, the nature of the
positional encoding of information described above.




(a) (b)
Fig. 1. Macro-scale concept demonstrator of media structural encoding of information – a) media with quad micro-mirror “optical data
elements,” b) positional encoding of information by a symmetric “optical data element.”


Figure 2 shows a graphic illustration of the general topography of three data tracks of AO-DVD media in the
embodiment examined by this paper.


Fig. 2. Topography of subwavelength-sized media structures for multilevel information storage

Some fundamental questions relative to the feasibility of such a system, to actual scale, are readily apparent.
These include:
•
Can complex 3-D subwavelength-sized structures molded in plastic optical ROM media be interrogated
by a diffraction-limited focused spot for the retrieval of multilevel information?
•
As the micro-mirror facets become subwavelength in size, does the focused wavefront’s interaction
with these features produce more than just a scattered blob of light?
•
What is the feature size that will produce a diffracted/scattered reflected return versus a specular
reflection?
•
Is there a transition region of feature sizes that produces a hybrid of both types of reflection?
•
Is it possible to split a diffraction limited focused spot into multiple paths with subwavelength media
topography?
•
Is this just not a complicated way of trying to extract multilevel information from the reflected signal’s
amplitude S/N excess?
This paper discusses, and most significantly presents, some FDTD modeling results, which address these curious
questions.
2. Periodic vs. aperiodic diffractive/reflective structures

Figure 3 shows the periodic structure of a blazed diffraction grating. One will note there are structural similarities to
the AO-DVD tilted micro-mirror proposal. Due to the one-dimensional periodic nature of this structure, the
prediction of the angular orientation of diffracted orders can be ascertained with the wavefield’s expansion in terms
of known eigenfunctions. Closed form analytical solutions do not exist in this manner for aperiodic subwavelength
structures such as those proposed for AO-DVD ODEs. A numerical electromagnetic technique such as the finite-
difference time-domain method (FDTD) must be employed to explore the nature of the reflected field in detail.



















Fig. 3. Periodic blazed diffractive grating structure with diffractive and refractive reflective paths

A measure of insight can be garnered from what is known about blazed grating scalar diffraction theory and
empirical observation of reflected returns from such gratings. Most noteworthy are the following facts [2]:
•
Blazed gratings are as much as 40% more efficient when the specular reflection is superimposed on the
diffractive order by altering the blaze angle.
•
The orthongonality of diffracted/reflected polarization is a function of blaze angle.
These facts reveal that light interaction with these types of structures, even in periodic format, produce reflections
that are a bifurcated interaction between diffractive and specular phenomena. The blaze angle dependence of
reflected polarization further connotes the complicated and possible information-bearing nature of these interactions.
3. FDTD modeling of subwavelength information structures
Recently a FDTD model that is capable of modeling a subwavelength reflective structure such as an AO-DVD ODE
has been developed at the University of Arizona. A paper at this conference [3] is being presented that focuses on
the validation of that model with other more empirically well-known subwavelength structures such as optical media
pits and near-field apertures.
A 0.60 NA lens is used with 405 nm (blue) circularly polarized laser source to produce a focused spot with an
Airy diameter of 0.88 microns. The simulation centers this focused spot on the apex of the four micro-mirror array,
which defines a single ODE. Each micro-mirror facet is 0.370 microns square. Three different ODE simulations
were run. The angular geometry of the ODEs for these runs is found in Table 1. The relative position of facets A, B,
C and D within an ODE is illustrated by Figure 4a. Angle θ is the tilt of the facet relative to the media plane normal
to the focused beam, the positive direction being into the plane of the media. Angle α is the angle of rotation
(12:00=0° CW=+) in the media plane of the facet’s tilt. Alpha (α) is measured rotationally relative to the axis
normal to the plane of the media and through the centroid of area of the particular facet. Each tilted micro-mirror is
positioned vertically (normal to media) such that its centroid of area intersects the focal plane of the media. ODE
surrounding mirrored flat portions of the media also lay in this media focal plane.
Table 1. Angular orientation of ODE micro-mirror facets modeled with FDTD
Facet >
A
B
C
D
Angle
(deg)
α
θ
α
θ
α
θ
α
θ
Case 1
135
5
45
20
225
12.5
315
20
Case 2
135
20
45
5
225
12.5
315
5
Case 3
315
5
225
20
45
12.5
135
20

Figure 5 illustrates the reflected near-field intensity field and phase plots for both orthogonal polarizations in
Case 1. The reflected near-field plots were all computed in the cover layer (n=1.55, thickness=0.6 mm) at a distance
of 130 nm above the focal plane of the media. The reflective aluminum layer is 20 nm thick. Figure 6 shows both
the intensity field and phase plot for this same reflected signal at the exit pupil of the objective lens. In color
versions of this paper, rainbow color scaling is used, with blue as a minimum through red as a maximum.
Interpretation of these images in black-and-white is facilitated by the papers descriptions and comments relative to
the images. Cases 2 and 3’s FDTD modeling results are illustrated in a similar manner with Figures 7 through 10.
For Case 1, we see that four fairly distinct lobes of light are reflected for the near-field intensity plot (Fig. 5a).
The brightest lobe is from the facet with a 5° tilt while the facets with the largest tilt (20°) are the dimmest. As the
tilt angles of the facets get larger, there also appears to be a more pronounced difference between the two
polarization intensity fields for those quadrants in which the corresponding facets are found. The exit pupil intensity
plot (Fig. 6a) illustrates an intensity field that is displaced from the center (right) of the field as well as containing


cyclic modulation within this field. Since the exit pupil is the Fourier transform of the final image producable from
the original object illuminated (ODE) we see that this exit pupil is indeed information bearing. Case 2, as noted in
Table 1, includes two 5° facets. The comments relative to Case 1’s near-field intensity field are similar for Case 2
(Fig. 7a), except that the two contiguous 5° facets form a figure 8 pattern of irradiance rather than the more discrete
lobes seen in Case 1. The exit pupil intensity plot for Case 2 (Fig. 8a) again shows frequency content as well as a
totally different displacement (left) of the intensity distribution then found for Case 1.
Case 1 and 2 have geometries that have a general protrusive shape (Fig. 3b) like a pyramid while Case 3’s
topography is more pit shaped (Fig. 3c). Interestingly, the near-field intensity fields for Case 3 (Fig. 9a) produce a
displaced from center, single spot, with a crescent shaped halo to one side. The quadrant that the centroid for this
spot appears in seems to be driven by the geometry of the 5° mirror facet.




(a) (b) (c)
Fig. 4. FDTD Modeled ODE Micro-mirror facet layout (a) and general protrusive (b) or pit (c) type topography for ODE


(a) (b)
Fig. 5. FDTD Case 1- reflected near-field (a) intensity field and (b) phase plot results for aperiodic AO-DVD “optical data element”


(a) (b)
Fig. 6. FDTD Case 1- reflected objective lens exit pupil (a) intensity field and (b) phase plot results for aperiodic AO-DVD ODE




(a) (b)
Fig. 7. FDTD Case 2 - reflected near-field (a) intensity field and (b) phase plot results for aperiodic AO-DVD “optical data element”

(a) (b)
Fig. 8. FDTD Case 2 - reflected objective lens exit pupil (a) intensity field and (b) phase plot results for aperiodic AO-DVD ODE


(a) (b)
Fig. 9. FDTD Case 3 - reflected near-field (a) intensity field and (b) phase plot results for aperiodic AO-DVD “optical data element”


(a) (b)
Fig. 10. FDTD Case 3 - reflected objective lens exit pupil (a) intensity field and (b) phase plot results for aperiodic AO-DVD ODE


Looking at the exit pupil intensity plot for the pit shaped Case 3 shows a more pupil centered intensity
distribution than found in Cases 1 or 2. The modulation of the field for this case is indicative again of spatial
information content being extracted from the ODE. These three FDTD modeled cases in summary, illustrate that
information from an array of subwavelength reflected media structures can be extracted simultaneously with a single
focused optical stylus (spot).
In these modeled cases the Airy illuminated area of any single micro-mirror facet is 0.130 microns2. The square-
root of this area is hence, 0.36 microns. The ratio of this micro-mirror dimensional metric to the wavelength of the
focused light (0.405 microns) can be computed, and is 0.89 (0.36/0.405). From this ratio the claim of
subwavelength-sized media structure illumination is supported.
4. Multilevel signal-to-noise (S/N) discussion

Both multilevel recording, as practiced by Calimetrics [4] in the form of gray-level encoding and run-length limited
(RLL) recording as practiced in conventional optical drive technology, are techniques that extract, in large part,
more information from excess signal-to-noise in the “amplitude domain.” Gray-level encoding is purely “amplitude
domain” dependednt while RLL encoding uses excess signal amplitude to extract higher information density from
the “temporal domain.” The AO-DVD concept is different in that it explores the extraction of information from the
spatial orientation of reflected subwavelength media features. This is a different signal domain. AO-DVD exacts
information from the “spatial domain.” This approach provides a new dimension or domain for multilevel signal
extraction from a pre-fabricated optical ROM media. Hence, the question posed earlier in this paper, “Is this just not
a complicated way of trying to extract multilevel information from the reflected signal’s amplitude S/N excess?”
can be answered, “No, it is not.” It is a fundamentally different multilevel optical data encoding technique.
To make the assertion of above more tangible and clear, the following example is provided. RLL, gray-scale
(Calimetrics) and AO-DVD encoding for a signal amplitude-limited case are explored. Figure 11 is provided to help
illustrate this example for all three cases. In all cases a focused laser spot illuminates the data track. The reflected
intensity of the spot for fully reflective media (100% reflection) at each of the drive’s detection planes has a
normalized power value of two (2). In each case the noise-equivalent power (NEP) of a detector has a value of one
(1). For the sake of the discussion, the detection threshold of a signal in each of the systems will require a S/N level
of two (2:1). This is met in all three cases for a fully reflected spot (100% reflection).
Now, let’s first look at “Case 1” illustrated in Figure 11 for RLL encoding. A simple reflectance modulated
system (CDR, CDRW) rather than a pit depth modulate ROM format is presented to make the discussion more
straight forward while not sacarficing the validity. Three different laser spot positions (A1, B1 & C1) are shown
relative to two dark (0% reflection) RLL marks on the media. This encoding method uses the transition edges
between bright and dark marks to detect the transition time between spaced marks on the media. One approach to
this edge or “temporal domain” detection is to set a threshold for the signal at about 50% of the full dynamic range
between bright and dark marks. When the laser spot is in position A1, the full signal is reflected and its 2:1 S/N at
the detector can be detected. As the spot transitions to B1 (50% signal) to C1 (no reflection), no difference in the
detected signal can be ascertained since in both cases the detected signal has fallen below the required detection
threshold.
RLL encoding uses the distance between RLL marks to increase the capacity for data storage. This is achieved
by being able to discriminate RLL marks, which are closer than a full laser spot width apart. In practice this fairly
complicated temporal convolution of the focused guassian spot with the transition edge requires an amplitude
excess to noise on the order of at least 10:1. A more direct correlation with this temporal jitter “noise” is obtained by
measurement of the carrier-to-noise ratio (CNR), but discussion of this difference adds little to this basic discussion.
Hence, to ensure that discretely different encoded states, which are closer than one spot diameter, do not “jitter” or
overlap in time with other encoded RLL states with a very high probability, then a S/N or CNR with significant
excess to the 2:1 S/N illustrated here are required. In this manner “temporal domain” encoding (RLL), takes
advantage of excesses in reflected signal amplitude to provide for sub-spot size recording resolution. In the signal-
limited case shown in Figure 11, this is not possible because there is no excess S/N in the “amplitude domain.”




Figure 11 – Multi-level encoding “domains”

“Case 2” in Figure 11 illustrates this same amplitude signal-limited case for gray-level (Calimetrics) style
encoding. Again three focused laser spot positions are shown. At position A2 the spot is fully reflected by the media
and the 2:1 S/N is detected. Two different gray-level type marks are shown in the Figure. The mark to the left is a
dark mark (0% reflection) and to the right a gray mark (50% reflection). Laser spot positions B2 and C2
respectively show the laser spot partially illuminating the gray mark and fully illuminating the gray mark. These
intermediate levels of reflection upon which gray-level type encoding extracts multiple states (>2) from and hence
its higher information density, will not work here. This is because, again, for this signal amplitude-limited case,
there is not enough excess S/N to make the technique applicable. Only two levels or data states can be recorded
here.
The final example is “Case 3” where AO-DVD type encoding is shown. For simplicity, each AO-DVD optical
data element (ODE) to be considered is a single micro-mirror facet, rather than the 4 element arrays explored in the
FDTD modeling section of this paper. The illustration shows six (6) such ODE arranged in serial fashion on a data
track. Each ODE is a fully reflective mirror (100% reflection). Only one laser spot is illustrated (A3) which falls
fully on a single AO-DVD ODE. This ODE can have a multitude of angular orientation states. If this ODE can be
tilted to about 20 degrees relative to the media plane and rotated through a full 360 degrees one sees that the number
of detectable orientations states is dependent upon the ability of the system to resolve between states. If one assumes
here that a 2.5-degree resolution in tilt and rotation of each ODE is possible, then over 1024 states (210) are
encodable within this spot-sized ODE. Each of these orientations is reflected back to an array of 32x32 detectors
each with a normalized NEP of 1. This reflected signal has a power of 2 (i.e. 100% reflection) and hence is
detected by one of the individual detectors (1024 total detectors). For this signal amplitude-limited case, the AO-
DVD encoding method was hence able to provide another order of magnitude of recording density and transfer rate
without the need for further excess signal amplitude. This was not achievable in the RLL and gray-level encoding
cases.
The example hopefully provides the insight required to understand that the AO-DVD technique opens a new
signal “domain” for optical recording not provided for by more traditional multi-level encoding techniques such as
RLL and grey-level encoding. This new “domain” is the spatial domain. By combining AO-DVD encoding with
RLL encoding, by varying individual ODE element lengths, there is the potential to tap both the amplitude, temporal
and spatial signal domains in optical data recording. The result could be increases in areal density and transfer rate
of orders of magnitude for low-cost ROM distribution media.
5. Conclusions

Subwavelength aperiodic reflective structures in media are shown by FDTD modeling to be capable of separating
the interacting focused laser beam into four separate beam paths upon reflection/diffraction. This was not an
apparent result predictable from known classical closed-form theory on the topic. The exit pupil intensity fields for
these subwavelength structures show both spatial frequency content as well as significantly differing positional
distributions within that field. These FDTD modeling results demonstrates that information can be extracted from
the reflected intensity distributions of subwavelength-sized optical ROM media structures. Further modeling using
the FDTD methods described is required to better understand the practical potential of the concept described.
6. Acknowledgements

The FDTD results presented here were produced by Dr. Armis R. Zakharian of the Department of Mathematics,
University of Arizona. The author would like to acknowledge and thank Dr. Zakharian for this central contribution.


7. References

1. ISOM/ODS: F. Thomas, "AO-DVD (Articulated Optical - Digital Versatile Disk) A 20X to 100X Performance Enhancement Path
for DVD-ROM ," presented at ISOM/ODS 2002, 7-11 July, Waikoloa, Hawaii.
2. C. Palmer, Diffraction Grating Handbook (Spectra-Physics/Richardson Gratings, 2002), Chap. 2 and 9. www.gratinglab.com
3. OSA/ODS: A. Zakharian, J. Moloney and M. Mansuripur, "Interaction of light with subwavelength structures in optical storage
media," in Optical Data Storage 2003, N. Miyagawa and M. O’Neill, Proc. SPIE Vol. 5096, (Optical Society of America,
Washington, D.C., 1900), pp. tbd.
4. T.L. Wong and M..P. O’Neill, “Multilevel Optical Recording,” J. Mgn. Soc. Jpn. A 25, 433 (March 2001).