PHOTODIODES- Resonant-tunneling detector counts photons

While crucial for quantum-cryptographic systems, single-photon detectors have other uses that extend from medical diagnosis to laser ranging. Avalanche photodiodes and photomultiplier tubes work as photon counters; however, their avalanche-multiplication schemes, which provide only indirect detection of single photons, can have associated problems. Trapped carriers in avalanche photodiodes, for example, limit the maximum data-transmission rate of quantum-cryptographic systems.

Researchers at Toshiba Research Europe and the University of Cambridge (both of Cambridge, England) have now developed a quantum-dot detector that directly senses single-photon-excited carriers, allowing low-noise detection.1 In the device, a single-photon-produced hole in a layer of quantum dots causes a change in the resonant tunnel current passing through an adjacent double-barrier structure.

The gallium arsenide (GaAs)-based semiconductor-diode device contains a GaAs intrinsic region with a double-barrier tunnel structure adjacent to a layer of indium arsenide quantum dots populated to a density of about 100/µm². To achieve a small tunnel-junction area, a cross-wire geometry was fabricated by etching two semiconductor wires such that the top wire was freestanding except for a 1-µm-square contact between the wires (see figure). For experimental comparison, large-area (with sides of 5 to 50 µm) devices were also fabricated.

While the large-area structures showed a steady increase in tunnel current upon illumination, the smaller-area devices showed abrupt discrete steps. Each step, the researchers believe, is the result of a charging or discharging of a single quantum dot responding to a photon-produced hole. A monochromatic attenuated source of 550-nm light producing a known rate of photons allowed the researchers to measure quantum efficiency, which reached about 11% for the particular setup. A compromise can be made between the dark-count rate and the detection efficiency—a 12.5% efficiency for a dark-count rate of 2 × 10^-3 s-1 (about one dark count every ten minutes), and 5% for a dark-count rate of 3 × 10^-4 s-1 (about one dark count per hour).

Thinner is faster

The thickness of the barrier layers determines the magnitude of the tunnel current, with the strength of the single-photon signal greatly increased by the use of thinner barriers (3 vs. 10 nm). “The barrier thickness essentially determines the speed of the device,” notes Andrew Shields, a Toshiba researcher. “The device with 10-nm barriers is good for measuring photon fluxes for which the average time separation between photons is several seconds, while the 3-nm barrier sample is useful for measuring photons with microsecond separation times.”

The device is most sensitive to wavelengths between 400 and 820 nm. Adding high-bandwidth differentiating electronics converts each rapidly occurring step into a pulse, allowing photons to be counted with conventional electronics. The transient response of the detector to a periodic train of 684-nm laser pulses was measured, showing a jitter of about 150 ns—low enough to enable measurement of single-photon pulses arriving at a rate of 5 MHz.

The sensor is well suited for quantum cryptography, which has typical count rates of 10-3 to 10-5 per clock cycle. With improvement, the detector may find use in medical imaging and diagnosis, chemical analysis, atmospheric monitoring, scientific research, and fiber­optic strain and temperature sensors.

REFERENCE

1. J. C. Blakesley et al., Phys. Rev. Lett. 94, 067401 (Feb. 18, 2005).