Recently, Prof. Weida Hu, Prof. Xiaoshuang Chen and Prof. Wei Lu, from the State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, have made progress in the study of room temperature nanowire single photon detectors. Researchers used the photogating effect of nanowires for the first time to detect single photon at room temperature. The related results were published on Nano Letters (DOI: 10.1021/acs.nanolett.8b01795, if =12.08) and the title is "Room-Temperature Single-Photon Detector Based on Single Nanowire". Dr. Wenjin Luo is the first author of this article.

Single-photon detectors rely on their extremely sensitive detection capabilities to realize individual photons—a quantum unit, which is widely used in many fields, such as precision analysis, bioluminescence, high-energy physics, astronomical ranging, etc., especially in quantum key distribution. Although single-photon detectors have been developed for many years and have a wide practical application, there are still many limitations. First, the operating conditions of traditional single-photon detectors are relatively harsh. For example, photomultiplier tubes and avalanche diodes operating in Geiger mode require driving voltages of tens of volts or even kilovolts, while superconducting single-photon detectors require extremely low temperature of several Kelvin. Secondly, most single-photon detectors rely on high-quality material growth, which requires molecular beam epitaxy (MBE) and other expensive equipments, thus the device has high preparation cost. In addition, few single-photon detectors have photon-resolving ability, and this resolvability is critical to many protocols for communication technology. Therefore, there is an urgent need to study photodetectors with room temperature, low cost and simultaneous photon-number resolution to meet the needs of single photon detection in many fields of modern science and engineering.

What are the advantages of nanowires for single-photon detection at room temperature?

Semiconductor nanowires are quasi-one-dimensional quantum structures with large surface-to-volume ratios and special physical and chemical properties. In recent years, many works has reported that the performance of nanowire photodetectors can be improved by manipulating the surface states of nanowires. Among them, the photogating effect is widely concerned. This effect is a special photoconductive gain in the quasi-one-dimensional quantum structure, which can help the nanowire detector to obtain high responsibility at room temperature.

In this work, the core-shell CdS nanowires were grown by chemical vapor deposition (CVD) method. The core of nanowires is a single crystal with perfect lattice quality, high carrier mobility, and the shell has a large number of "localized" surface states. This structure provides a suitable condition for forming of photogating effect, even making the device sensitive to a single photon at room temperature.

What is the Photogating effect?

Photogating effect, as its name implies, is a phenomenon in which the device seems to be applied an additional local gate when under illumination. In traditional photoconductive or photovoltaic devices, the carrier concentration just slightly increases under illumination. For nanowire phototransistors with the "core-shell" structure, the incident photons not only increase the number of carriers in the core, but also make the surface states of the shell layer to capture some photo-generated carriers. This further enhances the conductance of the core, so that the nanowire transistor can generate a high current gain even under few photons irradiation. In this work, the background carrier concentration is firstly reduced by the negative back-gate voltage of the transistor. The photogenerated electrons migrate rapidly in the core, and the photogenerated holes are trapped by the surface state to generate gate-like voltage, realizing single-photon detection. Since 2014, the research group has published many articles in international journals about photogating effect-based nanowire high-gain photodetectors, including ACS Nano, 2014, 8, 3628-3635, Advanced materials, 2014, 26 , 8203-8209, Nano Letters, 2016, 16, 6416-6424, Advanced Science, 2017, 4, 1700323 (Invitation Review), Small, 2017, 13, 1700894 and ACS Nano, 2018, 12, 7239-7245.

Specific highlights of this work

The highlight of this work is that the device can simultaneously detect the intensity and polarization state of a single photon, which will provide the helpful guide for the on-chip single-photon detector. In addition, there are other significant features, such as room temperature operation, photon-number resolution (1～3), low operating voltage (0.1V), low dark count rate (1.87×10^-3 Hz), high detection efficiency (23%), easy to operate, easy to manufacture, etc.

Currently, in the field of quantum communication, photon polarization decoding is mainly determined by a polarization analyzer in front of a single-photon detector, and then the single photon detector is used to detect the intensity information of incident photon. This is unfavorable for the development of detectors in the future miniaturization and highly integrated devices, so it is crucial to develop a single-photon detector that can simultaneously identify intensity and polarization information of the same photon. This work provides a new idea for room temperature operation and photon-number resolvable single-photon detection technology. It is expected to bring a new type of photon-decoding detection methods to lightweight, on-chip integrated detection applications and quantum communication.

Article link：https://pubs.acs.org/doi/10.1021/acs.nanolett.8b01795

Figure: Diagram of room temperature single-photon detector based on single nanowire and the corresponding time-resolved of current measurements at room temperature.