For years many engineers have assumed that you cannot encode more than one holographic image in a single surface — at least without losing resolution.

By creating a silicon platform that reflects light in a unique way, a team from the California Institute of Technology made the idea – multiple encoded holograms – a reality.

Lead researcher Andrei Faraon, assistant professor of applied physics and materials science in Caltech’s Division of Engineering and Applied Science, and first author Seyedeh Mahsa Kamali, Ph.D candidate at Caltech, spoke with Tech Briefs about how the thin, patterned surface could lead to new kinds of augmented- and virtual reality applications.

This research is published in a paper titled "Angle-Multiplexed Metasurfaces: Encoding Independent Wavefronts in a Single Metasurface under Different Illumination Angles " in Physical Review X.

Tech Briefs: What are the limitations of displaying holograms?

Professor Faraon: From the fundamental aspect, it has been generally accepted that a thin hologram can only project one holographic image. This property, assumed to be a result of a law in optics known as the “optical angular memory effect,” limits the functionalities one could expect from devices employing thin diffractive devices, including holograms. Thin holograms will project the same image regardless of the angle of illumination.

Tech Briefs: So, how were you able to encode multiple images?

Kamali: Our metasurface platform can encode multiple distinct holographic images and project each of them selectively, when light shines on it from different directions.

In this work, we were able to pattern a surface with millions of tiny silicon posts, structures each thinner than a hundredth of a human hair, that reflect light differently based on the angle of incoming light.

Tech Briefs: What do the posts do?

A scanning electron microscope image of a portion of the fabricated device. The final device is composed of tens of millions of the above silicon posts.­ For scale, the distance between two adjacent posts in this image is less than one hundredth of a human hair thickness. (Credit: Caltech)

Professor Faraon: Each post acts as a pixel for multiple holograms. For example, it works as a black pixel if incoming light strikes the surface at 0 degrees, and a white pixel if incoming light strikes the surface at 30 degrees. By changing the shape and size of the posts, we can span the intensity in grayscale for different illumination angles. In this way, we are able to encode different images in a single surface without losing any resolution.

Tech Briefs: What is possible with this kind of technology?

Kamali: This newly developed technology opens the path towards a new category of ultra-compact, multi-functional flat optical elements not feasible otherwise. One promising application could be the development of novel augmented reality (AR) head-mounts that can project different holographic images in various parts of a scene, and control each of them separately.

The other application could be encoded holograms with more secure antifraud protections. In the future, we can think of encoding more functionalities into a single surface, to expand the range of applications for nano-engineered surfaces.

Tech Briefs: How is the platform made?

Professor Faraon: To fabricate these devices, we start with a silicon wafer, similar to what is used in the fabrication of electronic integrated circuits. We deposit multiple thin films on the silicon wafer through various deposition techniques, including a metallic layer, which acts as a mirror, and a silicon layer, which acts as the active layer for generating the holographic images. After that, we create our desired pattern with nanometer feature sizes in the deposited silicon layer through electron beam lithography and dry etching.

Tech Briefs: What will you be working on next?

Kamali: As a next step, we are exploring the degrees of freedom and limitations of a single surface, to increase the number of encoded images and their quality. We would like to find answer to an important, yet unanswered question: How many images can ultimately be encoded in such a surface? We would also like to find ways to realize the surfaces with the maximum number of functionalities, without sacrificing performance. We are also looking for the possible applications of this platform in head-mounted 3D projectors.

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