Inspired by a concept for discovering exoplanets with a giant space telescope, a team of researchers is developing holographic lenses that render visible and infrared starlight into either a focused image or a spectrum. The experimental method could be used to create a lightweight flexible lens, many meters in diameter, that could be rolled for launch and unfurled in space.
Two spherical waves of light are used to produce the hologram, which gives fine control over the diffractive grating recorded on the film, and the effect it has on light — either separating light with super sensitivity or focusing it with high resolution. The researchers believe this model could be useful in applications that require extremely high spectral resolution spectroscopy, such as analysis of exoplanets.
Telescopes that must be launched into space (to benefit from a view unimpeded by Earth’s atmosphere) are limited by the weight and bulk of the glass mirrors used to focus light. These mirrors can realistically span only a few meters in diameter. By contrast, the lightweight flexible holographic lens — more properly called a “holographic optical element” — used to focus light could be dozens of meters across. Such an instrument could be used to directly observe an exoplanet, a leap over current methods that detect exoplanets based on their effect on light coming from the star they orbit.
“To find Earth 2.0, we really want to see exoplanets by direct imaging — we need to be able to look at the star and see the planet separate from the star. And for that, we need high resolution and a really big telescope,” said Professor Heidi Jo Newberg.
The holographic optical element is a refined version of a Fresnel lens, a category of lenses that use concentric rings of prisms arrayed in a flat plane to mimic the focusing ability of a curved lens without the bulk. The concept of the Fresnel lens — which was developed for use in lighthouses — dates to the 19th century, with modern-day Fresnel lenses of glass or plastic found in automobile lamps, micro-optics, and camera screens.
But while Fresnel holographic optical elements — created by exposing a light-sensitive plastic film to two sources of light at different distances from the film — are not uncommon, existing methods have been limited to lenses that could only focus light, rather than separating it into its constituent colors.
The new method allows the designers to either focus light onto a single point or disperse it into its constituent colors, producing a spectrum of pure colors. The method uses two sources of light, positioned very close to one another, which create concentric waves of light that — as they travel toward the film — either build or cancel each other out. This pattern of convergence or interference can be tuned based on formulas developed by visiting Professor Mei-Li Hsieh. It is printed, or “recorded,” onto the film as a holographic image and, depending on how the image is structured, light passing through the holographic optical element is either focused or stretched.