Doctoral thesis: Counting Events in Superconducting Thin-Film Strips up to 20 K

What: Counting Events in Superconducting Thin-Film Strips up to 20 K

When: Wednesday, May 6 2:00 PM – 3:30 PM EDT

Where: Haus Room (36-428) (or Zoom)

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Abstract:

Detection of single infrared photons at wavelengths beyond the silicon bandgap is necessary to enable quantum communication, dark matter search, deep space communication, and deep imaging of biological tissues. Superconducting nanowire single photon detectors (SNSPDs) could fill this technological need. However, existing SNSPDs require low operating temperatures which limit their utility in environments with constraints on space, power, or operating cost. Moving to material platforms with higher transition temperatures could open up a broader application space, but fabrication challenges and poorly-understood material and device physics have been major obstacles to reproducible and scalable operation of SNSPDs at elevated operating temperatures.

In this thesis, we study novel fabrication techniques for SNSPDs in magnesium diboride (MgB2) and reveal the simple mechanism for unintuitive device behaviors at high fractions of Tc, demonstrating paths forward to 1) more reliably fabricate MgB2 devices with operating temperatures 10-20~K and 2) engineer any material to operate at higher fractions of Tc. 

Towards the first goal, we simulate and perform irradiation of a micron-wide MgB2 detector with 30-keV helium ions. The device is at first insensitive to photons, but after irradiation it detects 1550-nm light at 1000~cps at 10~K with few-photon sensitivity. Towards the second goal, we incorporate a realistic thermal Langevin term and spatial variations of Tc based on grain distribution measurements into a numerical TDGL solver and simulate dark count rates in devices of different geometries, then compare the results to experimental data.  We demonstrate that spatial inhomogeneity can quantitatively predict switching current suppression and that simple Langevin mechanics can explain the unintuitive decrease of switching rate at high temperatures. This model can be used to make quantitative predictions about material engineering for improved device performance, potentially enabling increases in both operating current and temperature with respect to their critical values.

Details

• Date: Wednesday, May 6
• Time: 2:00 pm - 3:30 pm
• Category: Thesis Defense
• Location: Haus Room (36-428)