Abstract Body

Advanced microwave technologies constitute the foundation of a wide range of modern sciences, including microwave integrated circuits, quantum computing, microwave photonics, spintronics, etc. To facilitate the design of chip-based microwave devices, there is an increasing demand for state-of-the-art microscopic techniques capable of characterizing the near-field microwave distribution and performance. In this work, we integrate Josephson junctions onto a nano-sized quartz tip, forming a highly sensitive microwave mixer on-tip ^[1]. This allows us to conduct spectroscopic imaging of near-field microwave distributions with high spatial resolution. Leveraging its microwave-sensitive characteristics, our Josephson microscopy achieves a broad detecting bandwidth of up to 300 GHz, as well as remarkable frequency and intensity resolutions. To verify its functionality for applications, our study marks the first implementation of superconducting Josephson probe microscopy for near-field microwave detection on multiple voltage-controlled oscillators ^[2]. Focusing on spectrum tracking, various phenomena, such as stray spectrums and frequency drifts, were found under non-steady operating states. Parasitic electromagnetic fields, originating from power supply lines and frequency divider circuits, were identified as sources of interference between units. The investigation further determined optimal working states by analyzing features of microwave distributions. Our research not only provides insights into the optimization of circuit design and performance enhancement in microwave oscillators, but also emphasizes the significance of non-destructive near-field microwave microscopy as a pivotal tool in characterizing advance integrated microwave devices.

References

[1] Ping Zhang, Jingjing Lv, etc., National Science Review (2024) (in press)

[2] Ping Zhang, Yang-Yang Lyu, etc., Nano Letters 24(18), 5453–5459 (2024)

Acknowledgment

This work was supported by the National Key Research and Development Program of China (2021YFA0718802), the National Natural Science Foundation of China (61727805), and Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves.

Figure 1. System setup and application of the Josephson microscope. (a) Microscope assembly. When the probe approaches the chip, Josephson junctions on the apex receive subtle near-field microwaves emitted from the chip surface, causing oscillations of Cooper pairs. The probe is biased to perform coherent detection and characterizations of microwave distribution and spectrum tracking. (b) SEM image of the probe. The image shows the formation of weak-link Josephson junctions on top separated by grooves. (c) Photo of the whole oscillator chip. The red dashed line marks the imaging area in (d). Part of the surrounding pins provides bias voltages required for the normal operation of the chip. (d) Near-field microwave imaging above the oscillator. The probe's voltage signals are spatially tracked during the scanning process, showing a circular microwave distribution. The inset shows the design schematic corresponding to the imaging area. (e) Restored spectra of the oscillator. When the oscillator is in a dysfunctional state, the center frequency shifts and sidelobes appear in its frequency spectrum.

Keywords: near-field probing, microwave microscopy, Josephson junction, frequency mixing