Practical holography at SPIE Photonics West

I’m in San Francisco this week for SPIE Photonics West , “the world’s leading photonics, laser, and biomedical optics event,” according to the conference slogan. The conference is huge. In fact, it consists of five separate but contemporaneous conferences: BiOS (1771 research papers), LASE (662), MOEMS-MEMS (202), OPTO (1320), and Green Photonics (270).

Faced with such a cornucopia, I chose to spend my first morning attending an OPTO session entitled “Scientific Holography, Applications and Experimental Techniques I.” Here, I thought, was a session that represents what Photonics West is all about: an interesting and important application of light.

My favorite talk of the morning was by Tokyo University’s Naoya Tate. He and his colleagues are using nanotechnology to embed information on the nanoscale within information on the macroscale.

That goal is hard to reach if, as is the case with Tate’s scheme, the information is to be retrieved optically. “Nanoscale” is a somewhat loose term, but it usually refers to features that are 1 to 100 nm long. Visible light, which ranges in wavelength from 380 nm (violet) to 750 nm (red), can’t ordinarily resolve subwavelength features.

However, if you bring your probe into the near-field region—that is, within one wavelength of an object’s surface—you can resolve subwavelength features. In Tate’s scheme, which he calls a nanophotonic hierarchical hologram , the subwavelength features belong to a nanoscale metallic grid-like structure embedded within a sandwich of holographic gratings.

When illuminated from a macroscopic distance, the gratings project a hologram of a three-dimensional, macroscale object. When illuminated and viewed from a nanoscale distance, the nanoscale grid reveals the information encoded in its structure.

Security is one possible application. The nanoscale grid could serve as a covert watermark on a hologram. Besides a near-field microscope, no special equipment would be needed to check it.

In his talk, Tate noted that the idea of embedding information on short length scales in a larger image has been used before. Last year, scientists examining the Mona Lisa discovered that Leonardo da Vinci had written tiny letters on Lisa del Giocondo’s pupils.