Massively multi-level optical data storage using
subwavelength-sized nano-grating structures

Fred Thomas1, Hubert Kostal2 and Jian Jim Wang2
1 Iomega Corporation, 1821 West Iomega Way, Roy, Utah 84067
Phone: 801-332-4662 Email: thomasf@iomega.com
2 NanoOpto Corporation, 1600 Cottontail Lane, Somerset, NJ 08873-5117
Phone: 732-627-0808 Email: hkostal@nanoopto.com; jwang@nanoopto.com

Abstract: Can 3D reflective nano-grating structures molded in plastic optical ROM media be
interrogated by a diffraction-limited focused spot for the retrieval of massively multilevel
information? Empirical data for nano-grating encoded data states is presented.

 2005 Optical Society of America
OCIS codes: (050.0050) Diffraction and gratings, (210.0210) Optical data storage, (260.0260) Physical optics

1. Introduction

Nanostructures— structures with one or more dimensions measured in less than a hundred nanometers—
produce a broad range of important and often unexpected optical effects. By operating in the subwavelength realm,
nanostructure-based optical structures can reach, and sometimes cross, the boundary between classical and quantum
optics. These functions include polarization, phase, wavelength, and refractive index filtering or modification. Thus,
nano-optical structures offer the capability to create ROM and potentially WORM optical data elements (ODEs) for
which data is encoded in a massively multi-level format.
At the 2002 and 2003 ODS conferences, papers1, 2 on using sub-optical wavelength 3-D structures,3 for ROM
data storage were presented by one of the authors of this paper. The primary method of encoding introduced then was
the creation of unique spatial light patterns, or states, via reflection from nanostructures that were approximately a
quarter of the size (370 nm) of the optical stylus used in a DVD drive. Arrays of these nanostructures formed data
elements and were termed ODEs. The changes in reflective orientation of one or multi-beam paths from an ODE when
detected by an optical positional sensitive sensor hence provide for massively multi-level encoding of information. In
the 2003 paper finite difference time domain (FDTD) simulations were used to show that multi-level information
content could be extracted from reflective nanostructures. This technology was termed AO-DVD (articulated optical
digital versatile disc). Recent reference to this form of data encoding is found in Scientific American magazine4.
Recently a U.S. Patent5 covering this nanostructure-based encoding method has received notice of issuance. Previous to
these ODS papers, several papers6 in recent years have addressed amplitude domain multi-level recording.




Fig. 1. Sub-optical wavelength metal nano-grating structures
produced by NanoOpto Corp. The grating has a period of 150 nm,
with linewidth of 70 nm and depth of 150 nm.



Fig. 2. Birefringent Phase Encoded NG-DVD Media
In this paper, we introduce a new form of sub-wavelength optical data storage which is based upon
subwavelength nano-grating technology7. Figure 1 shows such a structure. Different embodiments of these nano-
grating ODEs for encoding massively multi-level information are presented. We have designated this new optical
data storage format NG-DVD (nano-grating digital versatile disc). Test data on both multi-level amplitude and phase
encoding of data using nano-gratings is shared. Figure 2 illustrates one such data storage media implementation.
Also discussed is nano-manufacturing technology8, which is capable of producing commercial quality ROM data
storage media with these nano-grating ODE structures with high throughput and low-cost.
2. Subwavelength Nano-Grating Physical Optical Effects

A photonic grating structure with feature size larger than wavelength of light generates high-order
diffraction for both transmission and reflection beams. As feature size approaches the wavelength of light, the
number of the high-order diffractive beams decreases proportionately. In the regime where the feature size is smaller
than the wavelength of light, only normal direction transmission and reflection, i.e., the zero-order diffraction
modes, exist and all high-order diffraction modes become evanescent.
Fabrication of subwavelength optical structures is actually re-engineering the material’s optical property,
such as the refractive index7. The refractive index of subwavelength optical structures can be calculated by the
effective index theory7. The refractive index can be engineered by selecting the index of base material, the index of
filled-in material and the volume ratio of the two, which is also called duty cycle. By engineering these three
parameters, ODEs with various index profiles, and therefore different modes of optical multi-level information
encoding, can be created. In addition, one can tailor the shape of subwavelength optical features, which opens an
additional dimension for new reflective state encoding. Nano-devices have been in research for many years.7-14
As noted previously, both nano-grating structures for phase and amplitude encoding of multi-level
information are examined7, 8. One-dimensional nano-structured gratings are made of homogeneous materials’ breaks
in-plane symmetry of the material, which leads to artificial birefringence property, i.e., form-birefringence13. This
structure is the basis for the phase-encoded subwavelength optical data storage media. Figure 3 illustrates this basic
structure. Reflective wire nano-grating polarizers can be fabricated via deposition of a thin film of metal on top of
the grating structure14. Figure 4 shows reflected amplitude data taken from such a grating illuminated with 633 nm
linearly polarized light. The theoretically predicted amplitude curve for grating rotation angle relative to input beam
polarization plane is also shown. This grating has a pitch of 150 nm, a depth of 150 nm and a duty cycle of ~50%.



3. Storage System for Massively Multi-level Encoding of Data

Figure 5 shows physical illustrations of two different data states for multi-level NG-DVD media. Each
illustrated element is a single ODE, which is 740 nm square, roughly the size of a DVD laser stylus. Both amplitude
encoded data states and phase encoded states are shown.
For the amplitude encoded ODEs, data is encoded in a multi-level fashion via changes in relative orientation of
the nano-grating relative to the interrogating laser stylus. Beam reflected amplitude is modulated as shown in Figure 4.
For the phase-encoded ODEs, multi-level data is created via the vectorized phase retardation of reflected light
from a 1/4 wave plate (two pass 1/8 WP) when the OED is rotated relative to the interrogating linearly polarized laser
stylus. Each ODE has its fast and slow axis oriented slightly differently for each multi-level data state. Figure 2 shows a
graphic illustration of the general topography of three data tracks of media for this phase-encoded embodiment of NG-
Reflective layer
Core
Nano-structured
grating with fill.
ARC - top
Overcoat


Substrate
Fig. 3. Schematic of the nanostructure based true
zero-order reflective quarter-wave retarder design
Fig. 4 Reflected optical amplitude from a
rotating metal nanowire grating structure
0
0.1
0.2
0.3
0.4
0.5
0.6
-100 -80 -60 -40 -20
0
20
40
60
80 100
Angle (degree)
R


DVD. One will note the orthogonal push-pull sampled servo sectors to facilitate track following shown in the Figure 2.
A real-time nano-grating based polarimeter is envisioned as the read back mechanism for this type of encoding.


Fig. 5 Example NG-DVD Optical Data Elements

Combining these multi-level state mechanisms (amplitude and phase) with positional data encoding, much like
that described for the AO-DVD1,2 format, is possible via application of blazing to the nano-grating structures. Further
data storage capacity enhancement for this technology may also include arrayed structures in a single ODE producing
multi-spot data states. Inherent with this multi-level encoding approach are also significant transfer rate increases.
4. Nano-Replication of Media

Nano-optic structures, in both their discrete and integrated forms, are all manufactured in single uniform
process: nano-imprint manufacturing8. This process is combination of printing and semi-conductor manufacturing –
both of which are high volume, highly repeatable, and highly scalable processes8. Manufacturing capability, as well as,
shipping optical elements with grating line widths down to 50 nm are presented.
5. Conclusions

Subwavelength periodic nano-grating reflective structures in media are shown to be capable of encoding an
interrogating focused laser beam into multi-level data states upon reflection with empirical data. The ability to
hybridize NG-DVD/AO-DVD technology within a DVD drive makes this ROM technology an appealing alternative for
the future of low-cost optical distribution media. This technology is scalable to future shorter wavelength optical drive
technologies. Capacities in the terabyte range for low-cost 120 mm discs are envisioned.
6. 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. OSA/ODS: F. Thomas, "Exploring optical multi-level information storage using subwavelength-sized media structures," in Optical Data
Storage 2003, N. Miyagawa and M. O’Neill, Proc. SPIE Vol. 5096, (Optical Society of America, Washington, D.C., 1900), pp. 391-399.
3. C. Wu, “HEBS Glass Gray Scale Lithography” Canyon Materials, Inc..: Product Information No. 01-88.
4. J.R. Minkel, “More Bits in Pits,” Scientific American, pp. 30, February 2005.
5. F. Thomas, USPTO. Notice of Issuance, “Method and Apparatus for Optical Data Storage,” App. # 10/076,016, August 14, 2004.
6. T.L. Wong and M..P. O’Neill, “Multilevel Optical Recording,” J. Mgn. Soc. Jpn. A 25, 433 (March 2001).
7. J. Wang, X. Deng, L. Chen, P. Sciortino, J. Deng, F. Liu, A. Nikolov, A. Graham, and Y. Huang, “Innovative nano-optical devices, integration and
nano-fabrication technologies (invited paper)”, Proc. of SPIE (Passive components and fiber-based devices, edited by Y Sun, S. Jian, S. Lee, K.
Okamoto), Vol. 5623, pp. 259 – 273, (2005).
8. J. Wang, L. Chen, S. Tai, D. Deng, P. Sciortino, J. Deng, and F. Liu, “Wafer based nano-structure manufacturing for integrated nano-optic
devices,” J. Lightwave Technology, Vol. 23, No. 2, February 2005.
9 D. C. Flanders and A. E. White, “Application of ~ 100 Angstrom linewidth structures fabricated by shadowing techniques”, J. Vac. Sci. Technol. 19,
pp. 892 – 900, 1981.
10. D. C. Flanders, “Submicronmeter periodicity gratings as artificial anisotropic dielectrics”, Appl. Phys. Lett., 42, pp. 492 – 494, 1983.
11. T. K. Gaylord, W. E. Baird, and M. G. Mohoram, Appl. Opt. 25, 4562, 1986.
12. J. R. Wendt, G. A. Vawter, R. E. Smith, and M. E. Warren, J. Vac. Sci. Technol. B15, 2946, 1997.
13. M. Born and E. Wolf, Principle of Optics, 6th ed., Chapter 14, Macmillan, New York, 1964.
14. Z. Yu, P. Deshpande, W. Wu, J. Wang, and S. Y. Chou, “Reflective polarizer based on a stacked double-layer subwavelength metal grating
structure fabricated using nanoimprint lithography”, Appl. Phys. Lett., 77 (7), 927 (2000).