image: Schematic diagram of Josephson microscope
Credit: ©Science China Press
Modern microwave technology, including the generation, manipulation, and detection of microwaves, plays a vital role in various fields, such as high-frequency integrated circuits, quantum computing, and microwave photonics. To achieve optimal performance of microwave devices, it is necessary to detect the spatially resolved microwave properties to analyze microwave interactions, promote efficient signal coupling, and address challenges including electromagnetic compatibility and signal crosstalk. Several microwave imaging techniques have been developed, including scanning near-field microwave microscopy (SNMM), nitrogen-vacancy (NV) center microscopy, atomic vapor cell microscopy, etc. However, none of these techniques can balance sensitivity, resolution, and bias magnetic fields satisfactorily. Josephson junction is a basic structure consisting of two superconductors separated by a barrier layer, which is an important device in superconducting electronics. It includes various configurations such as weak links, sandwich layers, point contacts, etc. The strong nonlinear characteristics can be used as a mixer, exhibiting extremely high sensitivity (approaching the quantum noise limit) to microwaves. When the microwave frequency is higher than the characteristic frequency of Josephson junctions, the device can function as a bolometer, capable of detecting single microwave photons. Thus, fabricating nanoscale Josephson junctions provides a new solution for near-field microwave detection and imaging.
Recently, a team led by Prof. Peiheng Wu, who is a Chinese Academician, at the Research Institute of Superconductor Electronics, Nanjing University, successfully developed a superconducting Josephson probe microscope, dedicated to near-field microwave spectroscopic imaging. This new microscope combines the sensitive microwave detection of Josephson junctions with the high spatial resolution of scanning probe microscopy. The Josephson probe microscope offers an ultra-broadband, highly sensitive, and high-resolution solution to near-field electromagnetic characterization and imaging for microwave chips with frequency ranging from microwave to millimeter-wave. It enables a variety of functionalities, including microwave detections of intensity, spectrum, and frequency-selective imaging. It is expected that this microscope would play an important role in research, including high-frequency integrated circuits, quantum computing, and magnonics. This achievement has been published in the 2024 issue of “National Science Review”, with the title “Ultra-broadband near-field Josephson microwave microscopy”. Dr. Ping Zhang is the first author of this work, Dr. Yang-Yang Lyu, Prof. Yong-Lei Wang, and Pro. Huabing Wang are the co-corresponding authors.
The research team developed the core component of the microscope system — the Josephson probe. The probe fabricated with weak-link Josephson junctions onto the nanoscale quartz tip was produced by employing grooved quartz tube drawing technology and a multi-angle evaporation process, which overcomes the weak directionality of magnetron sputtering. The research team observes clear Shapiro steps from the nanoscale Josephson probe under microwave irradiation, confirming the Josephson effects. Furthermore, based on the microwave-voltage response and noise spectral characteristics, they confirmed that the probe has high microwave sensitivity. The team also demonstrated ultra-broadband coherent detection by utilizing the highly nonlinearity of the Josephson probe. Through the signal mixing effect between local oscillations and radio frequency inputs, the unknown microwave signals can be measured from the spectra, with a frequency resolution of kHz level, as well as with a frequency range from 1 GHz to 200 GHz. Using this Josephson probe the team developed a near-field microscope system for spatial-resolved microwave imaging. They successfully examined standing waves in coplanar waveguides and ring-shaped microwave distributions in voltage-controlled oscillator chips. The spatial resolution of such microscopes is sub-micrometer level, with the potential for further improvement in future. The Josephson probe microscope exhibits excellent performance at low temperatures, providing an in-situ and non-destructive imaging tool for ultra-low-temperature quantum technologies.