IEEE study reveals breakthroughs in high-performance photon detectors

From high-speed communication to quantum computing and sensing, the detection, transmission, and manipulation of light (photons) have transformed modern electronics. Central to these systems are photon detectors, which detect and measure photons. One notable type is the superconducting nanowire single-photon detector (SNSPD). SNSPDs utilize ultra-thin superconducting wires that quickly transition from a superconducting state to a resistive state when a photon strikes, allowing for ultra-fast detection. The wires in these detectors are arranged in a Peano arced-fractal pattern, which remains consistent across various scales. This unique design enables the detector to detect photons regardless of their direction or polarization (the orientation of the photon's electric field). Due to these advantages, arced-fractal SNSPDs (AF SNSPDs) are crucial in applications such as light detection and ranging, quantum computing, and quantum communication.

In a recent study published on 25 December 2024, in the IEEE Journal of Selected Topics in Quantum Electronics, Professor Xiaolong Hu and Dr. Kai Zou from Tianjin University, China, provide a comprehensive guide to fabricating high-quality AF SNSPDs. The paper outlines the necessary materials and techniques for constructing these detectors and addresses challenges associated with their complex fractal design.

“This paper aims to present the nano- and micro-fabrication developments of high-performance fractal SNSPDs, with particular emphasis on the important experimental details that are key to the success of these devices,” says Prof. Hu.

AF SNSPDs consist of three components: nanowires for photon detection, optical microcavities to capture photons, and keyhole-shaped chips that house and align the detector with the optical fiber. The fabrication process begins with creating the optical microcavity by coating a silicon wafer with six or eight alternating layers of silicon dioxide (SiO₂) and tantalum oxide (Ta₂O₅) using ion-beam-assisted deposition (IBD) to form a bottom-distributed Bragg reflector, followed by the addition of a SiO₂ defect layer. Next, a 9-nm niobium-titanium nitride (NbTiN) superconducting film is deposited on the defect layer using reactive magnetron sputtering, creating the photon-sensitive surface. Titanium-gold electrodes are then fabricated on this surface using optical lithography and lift-off processes.

The nanowires are patterned into a fractal design using scanning-electron-beam lithography and then transferred to the NbTiN layer through reactive-ion etching. The microcavity is completed by depositing a top SiO₂ defect layer and additional alternating layers of Ta₂O₅/SiO₂ using aligned optical lithography and IBD. The chip is shaped into its keyhole form using optical lithography, inductively coupled plasma etching, and the Bosch etching process, and packaged for optical fiber connections.

The authors also provided suggestions for optimizing the fabrication processes of nanowires, optical microcavities, and keyhole-shaped chips. Some of their recommendations include: Applying a 5-nm silicon or 3-nm SiO₂ layer as an adhesion promoter to improve bonding between the resist patterned into nanowires and the NbTiN material, using auxiliary AF nanowire patterns to ensure consistent nanowire widths, and a careful design of the layout and spacing for optical microcavities to minimize photoresist deformation. They also suggested using accurate alignment markers for keyhole-shaped chips and gradually applying heat during the curing process to enhance photoresist stability and minimize etching defects.

In conclusion, the researchers were able to develop SNSPDs with impressive sensitivity and system detection efficiency.  “These advancements will help simplify the fabrication of fractal SNSPDs enabling the development of more advanced devices with additional functionalities,” says Prof. Hu.

Such steady improvements in SNSPD design and fabrication can pave the way to breakthroughs in quantum computing, telecommunications, and optical sensing. The future of photonics sure looks bright!

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Reference

DOI: 10.1109/JSTQE.2024.3522176

Authors: Kai Zou and Xiaolong Hu    

Affiliations: School of Precision Instrument and Optoelectronic Engineering, Tianjin University; the Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, Tianjin, China