Ultrafast tuning of the focusing efficiency of a nonlinear atomically thin lens

Light is the tool we use to see the world, to capture images, and to transmit information. For centuries, the devices that bend and focus this light have relied on bulk and weight. Traditional glass lenses are thick, curved pieces of material that take up space and cannot easily be shrunk down to fit inside the tiny chips that power our modern electronics. To make optical systems smaller and faster, scientists have turned to flat surfaces made of microscopic patterns, known as metasurfaces, which can guide light without the need for heavy curvature. Among these, a specific design called a Fresnel zone plate acts like a lens by using a series of concentric rings to concentrate light into a single point. While these flat lenses can be made incredibly thin, they have historically been static objects; once built, their ability to focus light is fixed, and changing their performance requires slow, bulky mechanical adjustments or electrical currents that generate heat.

A team of researchers has now demonstrated a way to make these ultra-thin lenses active and responsive, capable of changing how they focus light in a fraction of a second. They created a lens from a single layer of atoms, a material so thin it is essentially two-dimensional, and showed that they could switch its focusing power on and off using a second beam of light. This achievement solves a long-standing problem in the field: previous methods to control such lenses were either too slow to be useful for high-speed data or too weak to make a noticeable difference. By using the unique properties of this atomic material, the researchers achieved a modulation of the focusing efficiency that is both rapid and strong, opening a path toward optical devices that can be reconfigured instantly without moving parts.

The lens at the heart of this discovery is made from a material called tungsten diselenide, which exists as a stable sheet just one atom thick. The researchers patterned this sheet into a lens shape consisting of twenty-two concentric rings, creating a structure only one hundred micrometers wide. In its natural state, this lens is transparent and does not focus light well because the material is so thin that most light passes right through it. However, the material has a special property: when struck by light of a specific color, it can generate a new color of light at exactly double the frequency, a process known as second-harmonic generation. The researchers designed their experiment so that the lens would focus this newly generated light rather than the light that simply passed through.

To test the lens, the team shone a beam of light at it, which caused the atomic layer to emit a second, higher-frequency beam. They measured where this new beam came to a focus and found that the lens successfully concentrated the light into a spot roughly two hundred micrometers away. The efficiency of this focusing was surprisingly high for such a thin material, proving that the lens worked as intended. The real breakthrough came when they introduced a second beam of light, acting as a control switch. This control beam was tuned to excite the electrons within the atomic layer, effectively changing the material's optical properties for a brief moment.

When the control beam hit the lens, it caused the electrons to rearrange, which immediately reduced the material's ability to generate and focus the second beam of light. This change happened almost instantly, within a timeframe measured in picoseconds, which is a trillionth of a second. The researchers observed that the intensity of the focused light dropped by nearly thirty percent when the control beam was active. This is a significant change, especially considering that the control beam was relatively weak, requiring less than ten microjoules of energy per square centimeter. In contrast, when they tried to modulate the lens using the same control beam but without the special frequency-doubling effect, the change in focus was less than one percent, showing that the new method is vastly more effective.

The speed of this switching is determined by how quickly the excited electrons in the material settle back down to their resting state. The researchers measured this recovery time and found that the lens returned to its original state in about ten picoseconds. This speed is orders of magnitude faster than previous methods that relied on electricity or heat to tune lenses, which often took milliseconds to respond. The ability to switch the lens on and off so quickly suggests that these devices could be used in future technologies that require rapid adjustments, such as steering laser beams for communication or creating ultra-fast optical switches for computing.

The study confirms that by combining the extreme thinness of atomic materials with the physics of nonlinear light generation, it is possible to create optical components that are not only small but also dynamically controllable. The researchers did not just observe a theoretical possibility; they built the device, measured the light, and verified the timing of the response. They showed that the lens could be tuned with a large effect using a very fast mechanism, a combination that was previously difficult to achieve. This work establishes a new platform for active nanophotonics, where the focus of light can be manipulated with the same speed and precision as the electronic signals that drive our modern world, but using light itself as the control mechanism.