Controlling the optical resonance in applications is akin to raising or lowering a dimmer switch. While it is possible to weaken an optical resonance or shift the wavelength slightly, complete on-off switching of a resonance is not yet possible because the resonator remains coupled with the light. Researchers at Ludwig Maximilian University of Munich (LMU) and Monash University achieved control over the presence and absence of resonances by using ultrafast optical pumping on an asymmetrically designed metasurface. Control over resonance creation and annihilation could enable purely optical switches for telecommunications and optical data processing, and could advance research into quantum phenomena. The precise control over the presence and absence of resonances could represent a turning point in the engineering of an ultrafast optical switch for active photonics and quantum applications. Courtesy of Daniel Hellweg. To allow precise, fast, complete control of the way light interacts with a material, the researchers developed a metasurface with two asymmetrical silicon nanorods. Because the nanorods have different geometric shapes, they respond differently to light of various wavelengths and polarizations. At a specific wavelength, the optical responses of the nanorods cancel each other out, concealing the metasurface from light and thereby turning off the resonance. But when one of the nanorods is excited with an ultrafast laser pulse, its optical properties are temporarily altered, causing the resonance to be switched on. By using selective Mie-resonant pumping on portions of the nanorods, the researchers can modify the dipole balance between rods, causing the structure to couple with light and become visible. When the resonance is switched on, the researchers can create or annihilate resonances and tune the linewidth, amplitude, and near-field enhancement of the structure. Central to this approach is the use of restored symmetry-protected bound states of continuum (RSP-BICs), which enable photonic symmetry in systems with broken geometric symmetry. The researchers leveraged RSP-BICs to allow selective resonant pumping with the laser. “The centerpiece of our work is this deliberate symmetry breaking on extremely short timescales,” professor Andreas Tittl, who led the research, said. “We generate a perfect optical balance in a structurally asymmetric system. “By deliberately disrupting this equilibrium with a laser pulse, we gain a completely new level of freedom for controlling the light-matter interaction. We can generate a resonance at will, quench it, or precisely adjust its bandwidth as with a control knob.” After designing the metasurfaces and manufacturing them in a cleanroom, the team used spectroscopy to optically measure the temporal behavior of the metasurfaces. “Only with the aid of our time-resolved spectroscopy approach were we able to experimentally capture these ultrafast processes and watch in real time how the resonance appears within picoseconds and then disappears again,” professor Leonardo de S. Menezes, who led the spectroscopic experiments, said. “Our measurements showed a huge increase in the coupling with light, while there were scarcely any unwanted energy losses in the material itself.” The team demonstrated four different switching operations — generation of a resonance out of a dark state, complete quenching of an existing resonance, the targeted broadening of a light response, and the targeted sharpening of a light response. In sharpening mode, the researchers increased the quality factor (Q-factor) of the resonance by more than 150%. The researchers achieved precise control at a speed of 200 femtoseconds without disruptive losses. In addition to enabling low-loss, all-optical switching for communications and computing, the ultrafast optical control of resonances could offer advantages for many active nanophotonics applications, like integrated photonics. The breakthrough could help advance quantum technologies, including phenomena like time crystals — exotic states of matter that are believed to operate outside classical time constraints. “It opens up real possibilities for high-speed, low-loss optical computing and communications and for advancing quantum technologies where light control is critical,” professor Stefan Maier said. The new approach to optically controlling resonances is not limited to silicon, but can be extended to other materials and even faster switching mechanisms, which could expand its potential for future applications even further. “This work represents a real turning point in how we can approach the complex problem of making an ultrafast optical switch,” Maier said. “We’ve gone from being able to nudge these light-matter interactions, to now being able to switch them from a state where the nanostructures are almost completely invisible to light to a state where we can control the attenuation of light of a particular color to a very high degree.” The research was published in Nature (www.doi.org/10.1038/s41586-025-09363-7).
