A chip smaller than one square millimeter stores 160 holographic images at arbitrary 3D coordinates, with each spatial location functioning as an encryption key.
(Nanowerk Spotlight) Most storage technologies record information on surfaces. Hard drives encode data as magnetic orientations on spinning platters. Flash memory traps electrons in thin semiconductor layers. Optical discs etch patterns of pits into reflective coatings. All share a fundamental constraint: they are essentially two-dimensional.
Holographic storage takes a different approach, encoding data as interference patterns distributed throughout a three-dimensional volume. The same physical space that holds one layer of conventional data could, in principle, hold hundreds of overlapping holographic pages, each retrievable independently.
The principle is elegant. The practice is hard. Holographic systems require precise control over multiple properties of light simultaneously. They demand materials that can faithfully record subtle interference patterns without degradation. And they need methods to retrieve specific data pages without crosstalk from neighboring information. These challenges have kept holographic storage largely in laboratories despite its theoretical appeal.
Metasurfaces offer a new path forward. These ultrathin arrays of nanoscale structures called meta-atoms manipulate light through subwavelength interactions, bending, focusing, and reshaping beams within layers thinner than a human hair. When integrated with optical waveguides on a chip, metasurfaces route light and project holograms into free space without bulky external optics.
Researchers have explored multiplexing strategies using different polarizations, wavelengths, and orbital angular momentum states. But most demonstrations achieve fewer than 40 independent channels, and nearly all fix holographic images at predetermined, discontinuous spatial locations rather than allowing placement anywhere within the 3D volume.
A study published in Advanced Functional Materials (“Toward Ultra‐High‐Capacity Meta‐Optics Storage: 3D Data Cube Encrypted via Arbitrary Spatial Coordinates”) pushes past these barriers. Researchers at Wuhan University and the Wuhan Institute of Quantum Technology demonstrate a “3D data encryption cube” that stores 160 distinct holographic channels at arbitrary coordinates within a three-dimensional volume. Each channel appears only at its designated location, and those spatial coordinates function as encryption keys for secure data retrieval. The 160-channel capacity represents a fourfold increase over typical previous systems.
Schematic illustration of a 3D data encryption cube storage strategy enabled by an on-chip metasurface to define arbitrary holographic coordinates within the Fresnel region. By interacting with the metasurface, guided waves propagating along the x-direction are able to project up to 160 spatially distinct holographic images simultaneously at arbitrary designated coordinates (x, y, z) in free space. Each spatial coordinate (Key-x, Key-y, Key-z) functions as an encryption key for secure, spatially selective data retrieval. (Image: Reproduced with permission from Wiley-VCH Verlag)
The device measures 720 × 720 μm² and contains four million silicon meta-atoms, each 90 × 90 nm in cross-section and 360 nm tall. These sit atop a 190 nm thick silicon nitride waveguide on a silica substrate. Light in the fundamental transverse electric mode travels through the waveguide, interacts with the meta-atoms, and couples into free space.
The critical innovation involves controlling the phase of outgoing light through detour phase modulation. Each meta-atom occupies a periodic unit cell measuring 360 × 360 nm. Shifting a meta-atom’s position within its cell by a distance Δx introduces a phase shift equal to 2πΔx divided by the cell period. This permits continuous phase tuning from zero to 2π without varying the meta-atom’s size or shape. The total phase also incorporates the propagation phase that accumulates as the guided wave travels along the chip. Together, the detour phase and propagation phase determine the holographic wavefront.
Designing a phase pattern that produces 160 independent images at 160 arbitrary three-dimensional locations posed a substantial computational challenge. The team combined the angular spectrum method, which models how light propagates through free space, with a gradient descent algorithm that iteratively refined the phase distribution.
Starting from an initial configuration, the algorithm minimized amplitude differences between simulated and target images across all channels. It used average root mean square error as its metric. After 20 000 iterations, the phase pattern converged to a solution that faithfully reconstructed each image.
Fabrication followed standard nanolithography processes. The researchers deposited silicon nitride and silicon films onto a fused silica substrate, patterned the meta-atom array using electron beam lithography, and transferred the pattern through reactive ion etching.
To test the device, the team coupled a 450 nm laser into the waveguide and captured holographic images at different depths using a microscope objective. A motorized translation stage provided axial positioning accurate to approximately 143 nm. Lateral resolution reached about 1.2 μm.
The device reconstructed all 160 channels with minimal crosstalk. The team evaluated image quality using Pearson correlation coefficients and structural similarity index measures, both confirming close resemblance to target images. Equivalent storage density reached over 200 000 dots per inch. The spatial bandwidth product, a metric quantifying total resolvable information units in a three-dimensional optical system, measured 1.6 × 10⁷.
The architecture also provides built-in encryption. Because users can place each holographic image at any arbitrary point within the 3D volume, and because each image appears only at its specific (x, y, z) coordinate, those coordinates function as decryption keys. Without knowing the precise three-dimensional location, an unauthorized party cannot retrieve the stored information.
To demonstrate this, the researchers encrypted the message “I LOVE WHU” by assigning each letter to unique spatial coordinates within the Fresnel region, which extends in both axial directions from the metasurface. The letter “I” appeared at x = 529 μm, y = 545 μm, z = 3286 μm. The letter “W” appeared at x = 391 μm, y = 186 μm, z = -3236 μm. Retrieving the message required knowing all three coordinate values for each character.
The on-chip design eliminates another persistent problem: zero-order diffraction. In many holographic displays, unmodulated light creates a bright central spot that overwhelms the desired image. Because this metasurface couples guided waves into free space rather than transmitting a beam directly, that background noise disappears and reconstruction fidelity improves.
Challenges remain before such systems reach practical applications. Device efficiency, while sufficient for laboratory demonstration, would need improvement for commercial use. Scaling to larger channel counts and extending the approach to multiple wavelengths could further expand capacity.
Still, storing 160 independent holographic channels on a chip smaller than one square millimeter, with spatial coordinates serving as encryption keys, demonstrates that ultra-high-density secure optical storage is technically achievable. The work lays a solid foundation for future development in 3D holographic displays, high-volume optical information storage, and secure optical communications.
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