A high contrast transmitarray lens. Source: In press, Elsevier/Nature Communications
Scientists have been working on flat lenses for years to solve this problem, but they’ve been largely unsuccessful at creating a product that can compete with curved lenses. That is, until now. California Institute of Technology (Caltech) engineers have recently developed flat microlenses they say function similar to their curved counterparts. The technology could eventually be integrated into cameras and even microscopes.
"The lenses we use today are bulky," says Amir Arbabi, a senior researcher with the Caltech Division of Engineering and Applied Science. "The structure we have chosen for these flat lenses can open up new areas of application that were not available before."
The new lens, comprised of silicon, is called a high-contrast transmitarray. It measures a millionth of a meter in thickness and is covered in different sized silicon “posts.” The posts are there to focus various colors (also known as wavelengths) of light.
Curved vs. flat lenses
In curved lenses, the thicker parts of the lens require longer travel times for light than the thinner ones. The flat lens achieves these delays via the silicon posts; they are capable of trapping light for a certain amount of time that varies on the post’s diameter.
According to the Caltech engineers, their lens focuses approximately 80 percent of infrared light that goes through it. That’s a big jump from previous attempts, which produced significantly less efficient lenses. To put things into perspective, curved lenses are capable of focusing close to 100 percent of the light that passes through them.
The downside to curved lenses is that they tend to feature complex designs and surfaces that make them difficult to polish. In contrast, flat lenses are more versatile and can be altered to suit various applications by simply playing with the pattern of the silicon nanoposts.
The disadvantage of flat lenses
The main disadvantage to flat lenses is their limited ability to focus wavelengths, which represent different colors of the spectrum. However, the researchers say flat lenses are ideal for things like night-vision cameras and lasers, which only emit one color of light. Their monochromatic lenses could also provide multicolor images if multiple flat lenses are used (similar to how a television combines three colors and is able to produce a multitude of hues).
The lenses are currently quite expensive to produce. However, the engineers say their costs can be significantly reduced with the help of photolithography or nanoimprint lithography techniques, which integrate stamping technology.
"For consumer applications, the current price point of flat lenses is not good, but the performance is," Caltech researcher Andrei Faraon explains. "Depending on how many lenses you are making, the price can drop down rapidly."
A detailed account of the team’s research was recently published in the journal Nature Communications. For more information, visit Caltech’s website.