We will create a new generation of superconducting nanowire single-photon detectors (SNSPDs) with widths below 50 nm, thus improving dramatically on demonstrated detector efficiency at both near- and mid-infrared wavelengths. We will develop and test narrower wires by using improved nanofabrication and low-temperature characterization. We will simultaneously develop improved understanding of the superconductive nanowire device physics to enable further advances in nanowire-photodetector technology.
Intellectual Merit Superconducting nanowires are an interesting physical system for studying quantum mechanical, thermodynamic and electromagnetic interactions. Structures at the widths that will be explored in this work will provide new insights as the quantum mechanical wavefunction of the superconducting charge carriers is restricted from two, toward just one dimension. Additionally, this work will stress the limits of nanofabrication, requiring large-area structures to be developed with tens-of-nanometer widths and line-width uniformity of a few-nanometers. This work will also provide near- and mid-infrared single-photon detectors, and even photon-number-resolving detectors, with exquisite timing resolution that will advance research in quantum optics, remote sensing and astronomy.
Broader Impacts Development of SNSPDs with increased efficiencies, both in the near- and mid-infrared, will provide sensors for applications ranging from astronomy to environmental monitoring. Also, this work will provide valuable educational opportunities in nanofabrication, nano-optics, and solid-state physics to graduate students and undergraduates in the classroom and in the laboratory. Finally, this project will create an opportunity for the general public to learn about one of the important applications of nanotechnology through a public lecture.
A single photon is the smallest amount of energy that quantum mechanics allows an optical field to contain. In the visible portion of the spectrum, the dark-adjusted human eye can perceive light pulses containing on the order of a few hundred photons, but detecting single photons at high speeds requires dedicated opto-electronic devices called "single-photon detectors". Sensing single photons, especially single-photons in the infrared--invisible light, like the light used to transmit signals in a TV remote control--could enable applications such as sensing distant molecules by using spectroscopy, ultra-long range (such as deep-space) communications, astronomy, or sensing the light emitted by an integrated circuit so that one can identify bugs in circuit designs. But existing single-photon detectors are not fast or sensitive enough for many applications. Superconducting nanowire single-photon detectors, the target of our research under this program, are the most sensitive single-photon detectors available, and can provide critical timing information on the order of billionths to trillionths of seconds. Superconductors are like normal metals, but must be cooled to low temperatures--in our case three degrees above absolute zero, or -270 degrees celcius--and exhibit no resistance to electrical current in this temperature range. Until this project, these detectors had only been optimized for performance in the visible portion of the optical spectrum, or near the important telecommunications infrared spectral region (1550 nm). We were interested (for the applications mentioned above among others) in developing a detector technology that could work at longer wavelengths, specifically out to 5 millionths of a meter optical wavelength. At these wavelengths, the photon energy is so low, our conventional designs (based on nanowires 100 billionths of a meter wide and 4 billionths of a meter thick) were not sensitive enough. So we had to develop methods by which we could shrink the nanowires further--to widths as narrow as 10 billionths of a meter. The figure uploaded with this report shows some examples of the narrowest nanowires that we made under the program. Unfortunately, when you make nanowires narrower, they become more better at sensing photons that are absorbed, but less likely to absorb a photon in the first place. The nanowires also get longer, and as a result take a longer time to switch. To address this issue, we developed a nanoantenna that we built and integrated around the nanowire to enhance the optical absorptance. The nanowire antenna was analogous to antenna designs optimized for much longer wavelengths (like radio waves) but worked well at this nanoscale. The results was a fast, high-efficiency detector that could cover a large area and not be slowed down by the length of the nanowire. Finally, we performed a set of measurements up to 2.2 millionths of a meter optical wavelength that showed that these detectors had saturated efficiency in this wavelength region. These results provided strong evidence that the nanowires should be capable of sensing with high efficiency in the middle of the infrared region (near five millionths of a meter optical wavelength). This work was primarily performed by graduate students, but it also involved a number of undergrad students, and a visiting professor from a small undergraduate teaching college, who was able to take some of the techniques he learned in our laboratory and incorporate them into his own teaching. The PI was also able to incorporate content from this research into graduate courses that he teaches. In addition, a student from the Principle Investigator's research group started a company based on some of the ideas developed in this work. As a result, this research has had direct educational and economic impact on society.
