Flat Optics of the Future: How Metal Lenses Are Replacing Thick Glass Lenses

Modern smartphones can capture stunning photos, but their optical performance comes at the cost of a significant design compromise: massive, protruding camera modules on the back of the device. The reason isn’t electronics, but the pure physics of traditional optics. To achieve a sharp, distortion-free image, we must arrange several curved glass or plastic lenses in a row.

However, this more than century-old approach to manipulating light is reaching its limits. A new technology from the field of nanotechnology—metalenses—is advancing toward radical miniaturization. These are completely flat optical elements capable of directing light using microscopic structures on their surface layer.

Why are traditional lenses so thick?

A classic lens works on the principle of light refraction. When a light beam passes through media of different densities—for example, from air into curved glass—it changes direction. To focus light rays at a single point, the glass must be precisely ground to a curved thickness.

However, curved glass suffers from optical errors, such as chromatic aberration—a condition in which different wavelengths of light (red, green, blue) are refracted at slightly different angles. To correct this error, engineers must add additional lenses made of different types of glass to the lens. The result is complex, heavy, and thick optical systems consisting of six to eight separate elements.

How does a metal lens work?

A metamaterial lens does not use the thickness of the glass to bend the path of light. Instead, it utilizes metamaterials and nanostructures created on a microscopically thin, completely flat substrate.

The surface of a metamaterial lens is covered with billions of microscopic pillars or protrusions (called nanopillars) made of special dielectric materials, such as silicon dioxide or silicon nitride. These protrusions are on the order of nanometers in size—that is, they are smaller than the wavelength of visible light itself.

When a light wave strikes the flat surface of the metal lens:

1. Phase shift at the nanometer level: Each individual nanopillar slows down and shifts the phase of the passing light by a precisely defined amount depending on its height, width, and shape.
2. Wave-by-wave combination: By strategically arranging nanopillars of various shapes across the surface, an engineer can precisely model a phase profile that bends light rays just as effectively (or more precisely) as a thick, curved lens.
3. Focusing to a Single Point: Light passing through a flat plate thus converges precisely at the desired focal point, while the total thickness of the lens does not exceed a fraction of a millimeter.

Key Advantages of Flat Nano-Optical Elements

The transition from mechanical glass grinding to lithographic printing of nanostructures brings about a fundamental shift in the entire field of optics:

• Radical space and weight savings: An assembly of multiple glass lenses can be replaced by a single or two flat layers. Camera modules can thus be completely flush with the device body.
• Higher light transmission and elimination of distortion: Thanks to the precise design of nanostructures, advanced achromatic metal lenses can focus the entire spectrum of visible light without chromatic aberrations or geometric distortions.
• Semiconductor-style manufacturing: Unlike traditional glass grinding and polishing, metal lenses are manufactured using the same photolithography process as that used in the production of computer chips. This enables the mass production of thousands of optical elements simultaneously on a single silicon or glass wafer.

Where will metal lenses change the game?

Although the development of perfectly achromatic metal lenses for full-color cameras is still underway, the technology is already gaining traction in specialized applications:

• Smart glasses and augmented reality (AR/VR): The biggest obstacle to comfortable AR glasses is the weight and size of the display optics. Metal lenses make it possible to create extremely lightweight, transparent displays built directly into standard eyeglass lenses.
• Healthcare and endoscopy: The microscopic dimensions of metal lenses open the door to new diagnostic devices. Endoscopes thinner than a human hair can provide highly detailed images of the interior of blood vessels and tissues without invasive procedures.
• 3D Sensors and LiDAR: Single-channel metal lenses are already an exceptionally effective solution for transmitting infrared light in facial recognition sensors and wireless measurement systems.

Obstacles on the Path to Mass Adoption

Despite its enormous potential, flat optics faces challenges. The greatest of these is efficiency across the full color spectrum (RGB). Designing a nanostructure that functions with equal precision for blue, green, and red light simultaneously requires exceptionally complex computational models and extremely precise manufacturing tolerances.

However, the transition from classical optics to nanotechnology points the way forward. Just as microelectronics replaced bulky vacuum tubes with silicon chips, metal-lens structures are gradually replacing heavy glass lenses with lightweight, flat layers created through precision engineering at the molecular level.