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I.B. Maradiaga Rosales
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Electrostatically-defined nanostructures in bilayer graphene (BLG), known for its tunable bandgap, have promising applications in spintronics and valleytronics, however, its thermo- transport phenomena have not yet been investigated. This thesis aims to fabricate a BLG field-effect transistor (FET) device and characterize the thermotransport phenomena (See- beck coefficient) in electrostatically defined quantum point contacts (QPCs) and quantum dots (QDs). For this purpose, a bilayer graphene flake is encapsulated in hexagonal boron nitride (hBN) with Ti/Au heaters, top gates and 100-nm-separated split gates placed on top of the upper hBN flake. To define a QD, the 100 nm wide finger gates were separated from the top gates by a 30 nm Al2O3 dielectric. However, the electric field induced by the back gate was being screened by a layer of charges somewhere between the back gate and the bilayer graphene. The origin of this layer of charges remains unknown. As a result, the channel could not be fully depleted (unless when B = 5 T) and showed features indicating an unintended charging and discharging effect somewhere in the sample. As a consequence, the formation of a QD or a QPC at B = 0 T was not possible. Despite that, thermal voltages were measured in the two-dimensional BLG, applying currents up to 50 μA to the aforementioned heaters. The estimated Seebeck coefficient (based on resistance characterizations) was in the range of μV/K (corresponding with theoretical predictions) and enabled an estimate of an induced temperature gradient of 0.5 ± 0.2 K.
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Electrostatically-defined nanostructures in bilayer graphene (BLG), known for its tunable bandgap, have promising applications in spintronics and valleytronics, however, its thermo- transport phenomena have not yet been investigated. This thesis aims to fabricate a BLG field-effect transistor (FET) device and characterize the thermotransport phenomena (See- beck coefficient) in electrostatically defined quantum point contacts (QPCs) and quantum dots (QDs). For this purpose, a bilayer graphene flake is encapsulated in hexagonal boron nitride (hBN) with Ti/Au heaters, top gates and 100-nm-separated split gates placed on top of the upper hBN flake. To define a QD, the 100 nm wide finger gates were separated from the top gates by a 30 nm Al2O3 dielectric. However, the electric field induced by the back gate was being screened by a layer of charges somewhere between the back gate and the bilayer graphene. The origin of this layer of charges remains unknown. As a result, the channel could not be fully depleted (unless when B = 5 T) and showed features indicating an unintended charging and discharging effect somewhere in the sample. As a consequence, the formation of a QD or a QPC at B = 0 T was not possible. Despite that, thermal voltages were measured in the two-dimensional BLG, applying currents up to 50 μA to the aforementioned heaters. The estimated Seebeck coefficient (based on resistance characterizations) was in the range of μV/K (corresponding with theoretical predictions) and enabled an estimate of an induced temperature gradient of 0.5 ± 0.2 K.
Superconducting nanowire single-photon detectors (SNSPDs) are characterized by their quantum limited ability to accurately detect single photons, with low jitter, high detection efficiency and low dark count rate. To achieve this, the detector is cooled to 2-3K, bringing the device in a superconductive state, and is then biased with a direct current (DC) close to its critical current. When a photon impinges the detector, the depairing of Cooper-pairs by the photon leads to local destruction of the superconductivity. The growth of this non-superconducting area, first across and then along the nanowire, leads to the development of a measurable resistance and hence the production of detection pulses. Increasing system detection efficiency (SDE) of detectors has been a long-term goal in the community. Recently ultrahigh efficiency detectors (SDE>98%) have been demonstrated. It has also been shown that the wavelengths dependence of SDE, typically defined by a quarter wavelength cavity, is modulated by fiber-detector airgap (Fabry–Pérot). Measuring such modulations and finding the optimal operation wavelength manually is a time consuming and tedious process. In this thesis a setup for automatic measurement of SDE versus wavelength was developed and benchmarked. For the tested detector, efficiencies were found ranging between 22% and 95% in the wavelengths range between 1260nm and 1650nm. For the optical circuit using Single Mode (SM) fibers only, the automated SDE measurements were unreliable due to shifts in polarisation during measurements. Using PM fibers led to efficiencies very similar to the values measured manually.
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Superconducting nanowire single-photon detectors (SNSPDs) are characterized by their quantum limited ability to accurately detect single photons, with low jitter, high detection efficiency and low dark count rate. To achieve this, the detector is cooled to 2-3K, bringing the device in a superconductive state, and is then biased with a direct current (DC) close to its critical current. When a photon impinges the detector, the depairing of Cooper-pairs by the photon leads to local destruction of the superconductivity. The growth of this non-superconducting area, first across and then along the nanowire, leads to the development of a measurable resistance and hence the production of detection pulses. Increasing system detection efficiency (SDE) of detectors has been a long-term goal in the community. Recently ultrahigh efficiency detectors (SDE>98%) have been demonstrated. It has also been shown that the wavelengths dependence of SDE, typically defined by a quarter wavelength cavity, is modulated by fiber-detector airgap (Fabry–Pérot). Measuring such modulations and finding the optimal operation wavelength manually is a time consuming and tedious process. In this thesis a setup for automatic measurement of SDE versus wavelength was developed and benchmarked. For the tested detector, efficiencies were found ranging between 22% and 95% in the wavelengths range between 1260nm and 1650nm. For the optical circuit using Single Mode (SM) fibers only, the automated SDE measurements were unreliable due to shifts in polarisation during measurements. Using PM fibers led to efficiencies very similar to the values measured manually.