Mapping the Photoresponse of the Quantum-Dot Based Photon-Number-Resolving Detector

Efficient and versatile photon-number resolving detectors are critical to the development of future communication systems. The quantum-dot, optically-gated, field-effect transistor (QDOGFET) is one such detector. Utilizing quantum dots (QDs), tiny islands of semiconductor, imbedded in a transistor, QDOGFETs have been shown to exhibit single-photon sensitivity and photon-number-resolving (PNR) capabilities. A photon is detected when it photocharges a QD, which alters the amount of current flowing through the transistor by screening the gate field. Crucial to the resolving power is that each charged QD produce the same response, regardless of its location within the active area of the device. Here, we investigate the extent spatial nonuniformities in the QDOGFET’s response to light limit its ability to distinguish different numbers of photons. By using an optical-scanning microscope (OSM), contour plots of a QDOGFET’s response are acquired that show that the device exhibits localized “hotspots” where it is particularly sensitive to photons. The spatial resolution of the microscope is enhanced by capping the QDOGFET with a solid-immersion lens (SIL). We present experimental results that show how the hotspots depend on bias conditions and help decipher the root cause of the nonuniformities.