FEATURE SINGLE PHOTON DETECTION
fabrication process are widely used in the microelectronics foundries,” she says. The teams verified the yield
of the fabrication process at wafer scale through the measurement of the nanowire resistance at room temperature and obtained a very good uniformity (less than 1% non-uniformity). “After further optimisation, the single-photon detectors will be included in our silicon photonics process design kit in order to be used as a building block that can be integrated with conventional silicon photonics components (for light routing and manipulation) to build functional circuits,” she says. After electrical and optical packaging on a small PCB test card, teams tested the SNSPD performances in a customised cryostat operating at 2.3 Kelvin: “We have used a strongly attenuated laser at 1,550nm for which intensity is calibrated with a NIST traceable calibrated sensor and measured the fibre coupler insertion loss at cryogenic temperature in order to eliminate their contribution, so as to obtain reliable measurement of the on-chip detection efficiency.”
Innovation in magnetometry and quantum sensing Indian researchers have also been striving to advance the field of single-photon detection, presenting a paper at Photonics West on the design of a high- efficiency detector using 2D materials. A patent has been filed recently based on the work. The scientists, from the Spin Photon Electronics Lab (SPELL), Department of Physics, Indian Institute of Technology Madras, have been exploring innovations being made in defect centres in diamonds, as well as their application in magnetometry and quantum sensing. “Defects in diamonds, in particular those such as nitrogen-vacancy (NV) centres (which are also responsible for a pink colour in diamond) show excellent potential for a variety of quantum applications, especially in quantum-enhanced magnetic field sensing and quantum
28 Electro Optics March 2024
“The high efficiency and low background noise of a superconducting camera helps with the detection of quantum correlations in both time and space”
emitters, both of which we work on in our lab,” says Dr Sagar Chowdhury, Senior Research Fellow at SPELL. According to Chowdhury, for magnetic sensing, diamond- based sensors offer among the best combination of magnetic field sensitivity and spatial resolution. “They offer quantum advantage, even at room temperature and in ambient conditions,” he says. “This can be especially useful for applications in biology, where understanding the dynamic structure of biomolecules is important, and these can be studied mainly in ambient conditions. The diamond-based magnetic sensors will also be used for studying nanoscale electronic and magnetic devices.” The researchers used specific 2D semiconducting materials in the development of the single-photon detectors, which they say enhance sensitivity, especially in the infrared (IR) range. “The key parameters for a detector are: dark current, which is the intrinsic signal from a detector and unrelated to the signal of interest, and efficiency, which is related to how effectively the incoming light is converted to a current signal,” he says. “The wavelength of
operation for a detector decides the material, since the semiconductor band gap of the material should be lower than the wavelength of interest/ operation. Silicon makes a good material of choice for the visible wavelength, while III-V-based semiconductor materials are suitable for IR. Dark current is related to thermal energy of the electrons in the material. For IR materials, the dark current is larger because IR (and thus the detector’s band gap) is closer
The schematic (leſt) and packaged (right) result of CEA- LETI and CEA-IRIG’s niobium nitride superconducting nanowire single-photon detectors (SNSPDs) integrated on a silicon- based waveguide using CMOS- compatible processing
to thermal energy temperature than for visible light. Also, extremely high-quality silicon is available, resulting in high efficiencies. However, for other materials used in IR, such high quality is not as yet possible.” SPELL, along with its collaborators, are working on developing single-photon detectors using 2D materials across a broad range of wavelengths. Chowdhury explains how reducing dark current is a paramount challenge for the development of single-photon detectors, especially in the IR range: “Dark current is also volume- dependent and can be significantly reduced in a 2D material due to its low volume. The reduction of dark current leads to high sensitivity effectively in terms of signal- to-background. Additionally, 2D materials can be more easily integrated with silicon technologies. However, the same thinness that reduces the dark current also leads to lower absorption of the incoming light, thus resulting in lower efficiency and a smaller signal. We are trying to address the issue of low absorption by coupling a suitable 2D material with a photonic crystal/structure.” According to Chowdhury, one advantage of 2D materials is that they provide more flexibility than conventional semiconductors
for integration. “They are not as limited by ‘lattice matching’ issues. This allows for a wider range of 2D materials to be used. Many 2D materials and their heterostructures are available, so we can choose one for a given wavelength of operation. We are now focused on the visible wavelength, and chose molybdenum disulphide (MoS2), which has a direct band gap of 1.8eV (689nm). Our collaborators are almost ready with a manuscript of predicted performance of a 2D material in the IR spectrum.” Chowdhury elaborates on
the integration of a photonic crystal substrate and its role in enhancing light-matter interaction in 2D materials for the development of efficient quantum detectors. “The challenge is to simultaneously achieve high absorption of incident light to enable high efficiency detection, while also reducing the dark current,” he says. “Normally, light is absorbed vertically, thus requiring thick materials, which in turn increase dark current. However, by integrating the 2D material with a suitable photonic crystal slab, we can couple the light into the 2D material more effectively as a horizontally propagating wave, increasing the effective absorption length while still reducing dark current.” EO
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