Quantum communication breakthrough First technology to integrate silicon-based single photon sources
Experts around the world are working on the implementation of quantum information technologies. One important path involves light: single light quanta (or photons) that will in the future allow the transmission of data that is both encoded and efficient; for this, new photonic sources are needed that emit single light quanta in a controlled manner and on demand. Until recently, scientists have discovered that silicon can host single photon sources with properties suitable for quantum communication. However, so far no one knew how to integrate these photonic sources into modern photonic circuits.
The team at the Helmholtz Centre for Research Dresden Rosendorf (HZDR) has presented for the first time a production technique using silicon nanopillars: capable of producing thousands of nanopillars per square millimeter at state-of-the-art photonic circuit spacing, without the radiation damage defects associated with manufacturing. Their research results were published in the Journal of Applied Physics under the title "Metal-assisted chemical etching of silicon nanopillars carrying telecommunication photonic emitters" [1].
01Handling silicon on a chip: MacEtch etching technology
"Silicon and single-photon sources in telecommunications have long been the missing link in accelerating the development of fiber-optic quantum communications, and now we have created the necessary prerequisites for it." Dr. Yonder Berencén of the HZDR Institute for Ion Beam Physics and Materials, who led the research, explained that while single-photon sources have been fabricated in materials such as diamond, only silicon-based sources can produce light particles of the right wavelength to diffuse in optical fibers; this is a considerable advantage for practical use.
The researchers achieved this technological breakthrough by choosing a wet etching technique - known as MacEtch (metal-assisted chemical etching) - over the traditional dry etching technique for silicon on chips. These standard methods that allow the creation of silicon photonic structures use highly reactive ions that induce luminescence defects caused by radiation damage to the silicon. However, they are randomly distributed and overlay noise on the desired optical signal; metal-assisted chemical etching, on the other hand, does not produce these defects: instead, the material is chemically etched away under a metal mask.
Scanning electron microscopy (SEM) images of MACEtched nanopillars taken at an angle of 45 degrees. (a) Array of dies in a silicon chip containing silicon nanopillars of different diameters fabricated by MACEtch. (b) Arrays of nanopillars with diameters of approximately 300, 500, 700, 900, and 1100 nm. The central structure serves as a control. (c) A silicon nanopillar with a diameter of 700 nm and a height of about 1200 nm.
02New nanofabrication process compatible with photonic circuits
Using the MacEtch method, the researchers initially fabricated the simplest form of a potential optical waveguide structure: a silicon nanopillar on a chip. They then bombarded the completed nanopillars with carbon ions, much like bombarding a giant block of silicon, to create a photonic source embedded in the pillars. The use of the new etching technique means that the size, spacing and surface density of the nanopillars can be precisely controlled and tuned to be compatible with modern photonic circuits. In each square millimeter of chip, thousands of silicon nanopillars conduct and bind light from a light source by vertically guiding light through the pillars.
Schematic diagram of the process of integrating a telecommunication photon emitter into a single silicon nanopillar. (a) Spin coating of photoresist (Spin coating). (b) Design of the pattern of the two-dimensional nanopillar array by electron beam lithography and gold deposition. (c)Metal-assisted chemical etching (MACEtch). (d)Photoresist-based mask (mask) removal using ultrasound. (e) Wide-beam carbon implantation with an average projection range of about 600 nm for the carbon ions and an average height of 1200 nm for the pillars.
The researchers changed the diameter of the pillars "because we had hoped that this would mean we could do single-defect creation on thin pillars and actually produce a single-photon source on each pillar." Berencén explains, "The first work wasn't perfect. By comparison, our carbon bombardment dose was too high even for the thinnest pillars; now it's just a small step toward a single-photon source."
03The future, integrated silicon photonic structures
In this experiment, the team proposed a low-cost, top-down nanofabrication process that is scalable and capable of producing thousands of nanopillars per square millimeter, each carrying a telecommunication photon emitter. These results hold great potential for hosting and scaling single spin color centers and single photon emitters coupled to silicon pillars.
The new technology also unleashes a number of races for future applications.
Berencén said, "My dream is to integrate all the basic building blocks, from single-photon sources through photonic elements to single-photon detectors, on a single chip, and then connect many chips through commercial optical fibers to form a modular quantum network."
Reference links：
[1]https://aip.scitation.org/doi/pdf/10.1063/5.0094715
[2]https://phys.org/news/2022-09-silicon-nanopillars-quantum.html
