Physicists realize time-varying strong coupling in a magnonic system

Time-varying systems, materials with properties that change over time, have opened new possibilities for the experimental manipulation of waves. Contrarily to static systems, which exhibit the same properties over time, these materials break so-called temporal translation symmetry. This in turn prompts the emergence of various fascinating phenomena, including time reflection, refraction and diffraction.

Most time-varying materials studies so far are optical systems, or in other words, systems designed to manipulate light in specific ways. More recently, however, some physicists have started exploring the possibility of realizing time-varying magnonic systems, which are made up of collective wave-like electron spin excitations in magnetic materials and can carry information with low energy loss.

Researchers at ShanghaiTech University, Shandong University, Shanghai Institute of Technical Physics, Chinese Academy of Sciences and Zhejiang University have devised a new strategy for the experimental realization of time-varying strong coupling in magnonic systems.

Their approach, outlined in a paper published in Physical Review Letters, relies on a technique called time-resolved frequency-comb spectroscopy, which can be used to detect the rapidly changing spectral variation of coupled magnon modes.

"We discovered a pump-induced magnon mode (PIM) under microwave drive in 2023. The PIM is unique because it responds readily to extremely weak microwave fields (around nanotesla level that is 10,000 times weaker than Earth's magnetic field)," Bimu Yao and Wei Lu, who directed this study, told Phys.org.

"Motivated by this, we asked: what happens if the continuous drive is replaced with pulses? As a result, our experiments revealed chirped Rabi-like oscillations, evidencing time-varying strong coupling between magnon modes. This effectively realizes a time-varying medium for spin waves—a task usually hindered by the difficulty of fast magnon-dispersion modulation, but that is made feasible by our approach."

The key objective of the recent work by Yao and his colleagues was to experimentally realize the breaking of temporal symmetry in a chip-based system made of spin waves, or magnon modes. To achieve this, they had to achieve a time-dependent strong coupling between magnons in a co-planar waveguide.

"We also wanted to create time interfaces and time slits for magnons to demonstrate time diffraction," said Jinwei Rao and Lihui Bai, professors from Shandong University who carried out this study at ShanghaiTech University. "Finally, we set out to develop a technique capable of resolving nanosecond-scale spectral variations, revealing previously unobservable time-varying coupling strengths of magnons."

To create their magnonic system, the researchers placed a ferrimagnet (i.e., a material made up of populations of atoms with opposing magnetic moments) on a coplanar waveguide, a transmission line in which conductors lie in a single plane on a substrate. They then sent periodic microwave pump pulses through the system to excite the magnon modes in the ferrimagnet.

"The rapid formation and waning of the PIM at the pulse edges modulate the coupling between the PIM and the other magnon mode," explained Rao.

"The frequency variation of the magnon modes occurs on the nanosecond scale, far exceeding the acquisition speed of commercial analyzers. To capture this ultrafast dynamic process, we developed a novel time-resolved frequency comb spectroscopy (trFCS) technique.

"The periodic microwave pulses correspond to a frequency comb comprising numerous discrete, equally spaced components in the frequency domain. When applied to a ferrimagnet, these components simultaneously probe the resonant response of magnon modes across a broad frequency range."

Using the trFCS technique they developed, the researchers were able to detect spectral variations of magnon modes on a nanosecond scale, which is orders of magnitude faster than widely employed techniques. Such resolution was essential for observing the sudden changes in magnon dispersion that form "time interfaces."

To produce time slits (i.e., sharp changes in coupling at specific instants) in their system, the researchers shortened the external energy source used to modulate interactions between magnon modes, also known as the "pump."

"The fast turn-on and turn-off create two adjacent time interfaces (a time slit). Using two short pulses makes a double time slit," said Yao. "The spectrum shows sidebands whose spacing scales inversely with slit separation—the time-domain analog of Young's double-slit."

This study introduces a new viable and practical strategy to realize time-varying strong coupling in a chip-based magnonic system, without reconfiguring it. Using their approach, the researchers were able to demonstrate the double-slit time-diffraction of magnon modes for the first time.

In the future, other research teams could devise similar methods for the nanosecond, broadband spectroscopy of microwave systems and apply them to the realization of time-varying magnonic systems.

"Our work demonstrates the potential to enable efficient magnon multiplication and programmable control, thereby enhancing spin-wave conversion efficiency, enabling all-magnetic mixers and on-chip GHz sources for low-loss computing and quantum hybrid systems," concluded Wei Lu.

"The trFCS technique serves as a versatile tool for studying dynamic microwave systems. Next, we will shorten slits/pulses to capture ultrafast behavior of temporal refraction/diffraction and integrate multi-slit strong coupling systems on-chip towards the 'grating-programmed magnonics.'"

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