Lenses and their focusing mechanisms can be relatively bulky components in electro-optical systems, but a new development and demonstration based on metalens technology may offer an alternative. A team at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS), led by graduate student Alan She, has built a controllable metalens that is a flat, extremely thin, electronically controlled artificial eye.
The adaptive metalens simultaneously supports real-time control of three of the major contributors to blurry images: focus, astigmatism, and image shift (Fig. 1). Applications include embedded optical zoom and autofocus for cell-phone cameras, eyeglasses, and virtual/augmented-reality hardware.
1. Each discrete cell of the metalens contains a metasurface element that provides the required phase shift to the incident light, allowing it to reconstruct the desired wavefront (middle column; dashed line: optical axis). That, in turn, determines the subsequent beam shaping (right column). Original: metasurface without stretch (a); defocus: metasurface with uniform and isotropic stretch (b); astigmatism: metasurface under asymmetric stretch (c); shift: metasurface displaced laterally in the x,y plane (d). (Source: Alan She/ Harvard SEAS)
Metalenses are a good example of using nanoparticles to create structures that can’t be fabricated through any existing techniques. They use a dense pattern of nanostructures, each smaller than a wavelength of light, to focus light and eliminate path-related spherical aberrations. As described in the very detailed paper “Adaptive metalenses with simultaneous electrical control of focal length, astigmatism, and shift,” published in the February 23 issue of Science Advances, the researchers explained the design and fabrication of metalenses up to centimeters or more in diameter (Fig. 2), using processes analogous to those for standard ICs.
2. In the actual device, the adaptive metalens (center) is controlled by embedded electrodes made of carbon nanotubes. (Source: Alan She/ Harvard SEAS)
“This research combines breakthroughs in artificial muscle technology with metalens technology to create a tunable metalens that can change its focus in real time, just like the human eye,” said She. “We go one step further to build the capability of dynamically correcting for aberrations such as astigmatism and image shift, which the human eye cannot naturally do.”
Earlier metalenses were about the size of a single piece of glitter, so the team had to develop techniques to support development of these larger metalenses. “Because the nanostructures are so small, the density of information in each lens is incredibly high,” said She. “If you go from a 100-micron-size lens to a centimeter-size lens, you will have increased the information required to describe the lens by 10,000. Whenever we tried to scale up the lens, the file size of the design alone would balloon up to gigabytes or even terabytes.”
To overcome this tools and production barrier, they created an algorithm which shrinks the file size. This made it compatible with IC-fabrication technology.
The next challenge was finding a way to attach the large metalens to an artificial muscle without affecting the lability of the lens to focus. They found a thin, transparent dielectric elastomer with low optical loss, but needed to develop a platform to transfer and adhere the lens to the soft surface. (In the human eye, the lens is surrounded by ciliary muscle, which stretches or compresses the lens, changing its shape to adjust its focal length.) To replicate this function, the team worked with David Clarke, Extended Tarr Family Professor of Materials at SEAS, who is an expert in dielectric elastomer actuators (DEAs), often referred to artificial muscles.
3. In the DEA metalens device design, five addressable electrodes are combined to allow for electrical control over the strain field of the metasurface (a); optical microscope images (scale bars are 20 μm) at no voltage (i), 2.5 kV applied to the center electrode (V5) (ii), and 2.75 kV applied to tune x-axis astigmatism (concurrently, V1 and V3) (iii). (The dark spots are defects—either missing or tilted silicon posts—introduced during the transfer process.) The corresponding two-dimensional Fourier transforms (FTs) of (i) to (iii) are shown in (iv) to (vi), respectively (normalized amplitudes).
The elastomer is controlled by an applied voltage in the low-kilovolt range; as it’s stretched by the voltage, the nanopillars on the surface of the lens shift position. The metalens is tuned by controlling the position of these pillars in relation to their neighbors and the overall total displacement of the structures (Fig. 3). The researchers also demonstrated that the lens can simultaneously perform three critical functions: adjust focus, control aberrations caused by astigmatisms, and implement image shift.
The flat metalens with its “muscle” is only 30 microns thick, which means that it can be easily incorporated into the optical path. She pointed out that “because the adaptive metalens is flat, you can correct those aberrations and integrate different optical capabilities onto a single plane of control.” The researchers plan to further improve the functionality of the lens and decrease the voltage required to control it.
The research was co-authored by Shuyan Zhang and Samuel Shian. The research was supported in part by the Air Force Office of Scientific Research and by the National Science Foundation. This work was performed in part at the Center for Nanoscale Systems (CNS), which is supported by the National Science Foundation.
Optics Express, Vol. 26, Issue 2 (2018), “Large area metalenses: design, characterization, and mass manufacturing.”