Few-photon detection using InAs avalanche photodiodes

<p>Abstract: An avalanche photodiode with a ratio of hole to electron ionization coefficients, <i>k</i> = 0, is known to produce negligible excess noise irrespective of the avalanche gain. The low noise amplification process can be utilized to detect very low light levels. In this work, we demonstrated InAs avalanche photodiodes with high external quantum efficiency of 60% (achieved without antireflection coating) at the peak wavelength of 3.48 µm. At 77 K, our InAs avalanche photodiodes show low dark current (limited by 300 K blackbody background radiation), high avalanche gain and negligible excess noise since InAs exhibits <i>k</i> = 0. They were therefore able to detect very low levels of light, at 15-31 photons per 50 µs laser pulse at 1550 nm wavelength. These correspond to the lowest detected average power by InAs avalanche photodiodes, ranging from 19 to 40 fW. The measurement system’s noise floor was dominated by the pre-amplifier. Our results suggest that, with a lower system noise, InAs avalanche photodiodes have high potential for optical detection of single or few-photon signal levels at wavelengths of 1550 nm or longer.</p><p><br></p><p>Measurements were carried out on the InAs APDs to obtain the dark current versus voltage characteristics (forward and reverse bias), responsivity versus wavelength at a fixed reverse bias, avalanche gain versus reverse bias, and weak light detection. For all measurements, the device-under-test (DUT) was placed in a low temperature probe station (model ST-500 of Janis Research).<br></p><p><br></p><p>Spectral response measurements are carried out using a monochromator-tungsten lamp combination which provided monochromatic light from 600 to 4000 nm on our InAs APDs and a reference diode (Judson J12-18C-250U).</p><p><br></p><p>For the avalanche gain measurements, the light source was a 1550 nm wavelength laser modulated at 10 kHz with 50% duty cycle ratio. The optical path included two fixed optical attenuators (26.5 dB each) and a calibrated variable optical attenuator (0-80 dB) to obtain the required optical attenuation. The DUT’s photocurrent was amplified and converted into a voltage signal by a low-noise current preamplifier (model SR570 of Stanford Research Systems), before being measured by a lock-in amplifier. The avalanche gain was defined as the ratio of the photocurrent measured using a lock-in amplifier to the unity gain photocurrent at <i>V<sub>r</sub> = -</i>1.0 V. </p><p><br></p><p> The measurements for weak light detection used an identical setup as the avalanche gain measurements, except that the lock-in amplifier was replaced with a spectrum analyzer (model SR760 of Stanford Research Systems). <br></p>