Written by Patrick Gill, Principal Research Scientist, Rambus Labs
As a kid, I can remember the rare treat of visiting a house where two mirrors were mounted to face each other on opposite walls.
Any object between them would be copied a myriad of times, usually curving off into the distance as they acquired the greenish tinge characteristic of the wavelengths the mirrors reflect best. It was easy to make zillions of copies of candies, although most of them appeared too far away even for a kid to be motivated to gather them.
The multiple reflections effect has been harnessed on a microscopic scale in a recent paper with lead authors Jiong Yang and Zhu Wang out of the Australian National University in Canberra. The paper describes how they made similar parallel mirror sheets, albeit on a nano scale. The key to this endeavor is the discovery of a material know as molybdenum disulfide, which has an unusually high refractive index (4.4 to 5.2, depending on thickness) and can be formed in thin films just a small number of layers thick.
It should be noted that the refractive index of a material denotes how much it slows light relative to light in a vacuum. Glass might have an index around 1.45, and one of the optically densest commonly known materials is diamond (whose index is 2.42), meaning molybdenum disulfide’s 4s – 5s is truly monstrously large. Essentially, this high index means any light entering layers of molybdenum disulphide is trapped there for multiple reflections; its optical path is multiplied just like light passing between parallel mirrors.
Image Credit: Jiong Yang and Zhu Wang, Australian National University in Canberra
Simply put, the top and bottom layers of the molybdenum disulphide form a pair of parallel mirrors. Although the two are only 0.7 nm apart in real space, they induce an effective optical path length difference of about 20 nm. Stack a few layers on top of each other in different places and you can make an optic a few molecules thick that nonetheless has the optical properties of a piece of glass approximately 60 times thicker.
At Rambus, the work I do with Lensless Smart Sensors (LSS) involves designing special diffraction gratings that optically slow down light in some regions compared to others.
While the work we’re doing now is for applications where a grating of about 1500 nm tall for visible light applications (or 8000 nm tall for thermal sensing) is still small enough, it’s fun to see that when we need to miniaturize our LSS optical thickness by another factor of 60, folks like Jiong Yang and Zhu Wang have got my back.