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Images of the detector. (a) A low magnification scanning electron microscope (SEM) micrograph of a SNSPD on a tip of a fiber (colored for clarity): the detector (green), the gold contacts (yellow), and the fiber (purple circle). (b) A higher magnification image. The thin 120 nm wide lines are folded in a meander form. The meander covers an area of . (c) A photo of a mounted device.
An optical image of a detector, obtained using IR sensitive camera. The nanowire is not visible, due to the optical resolution limitation (see Fig. 1(b) for higher resolution). (a) The device under external illumination. (b) The external illumination is lowered and light is injected from the fiber, behind the detector (see Fig. 1(c)). The high accuracy alignment of the light spot to the detector is demonstrated; the center of the light Gaussian beam overlaps the area of the detector.
The coupling efficiency () between the Gaussian light beam with diameter and the detector with size , where the center-center-misalignment is . We assume (SMF-28), and detector fill factor of 1. (a) vs. for circular detectors with , and 15 μm in solid-blue, dashed-green, and dashed-dotted-red, respectively. (b) vs. . In solid-blue (dotted-blue), for a circular (rectangular) detector with diameter (edge) of . In dashed-green, . In dashed-dotted-red: the light loss due to relative to the case, i.e., the solid-blue curve divided by the dashed-green curve.
(a) The experimental setup. The device is connected in parallel to a load resistor, , to prevent latching.16 We use CW monochromatic laser at 1550 nm as a light source. The light is attenuated at room temperature to 10 nW and sent into a cryostat where the device is cooled to 3.5 K. Current is sourced from a computer-controlled battery. The current enters the direct current (DC) port of a bias-T and into the detector. The output signal is sent from the RF port of the bias-T through a cryogenic amplifier (1–8 GHz bandwidth, gain) and up to room temperature using a semirigid coax cable. The signal is further amplified at room temperature (1–10 GHz bandwidth, +24 dB gain)and sent to a pulse counter (Stanford Research SR-400). (b) The system detection efficiency () of one of our devices as a function of the bias current. The error-bars represent the standard deviation for the measurement. (c) An electrical pulse from the detector after an event of a single photon detection. Ringing in the pulse appears due to high pass filtering by the amplifiers.
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