A design has been proposed for a photodetector that would exhibit a high quantum efficiency (as much as 90 percent) over a wide wavelength band, which would typically be centered at a wavelength of 1.55 μm. This and similar photodetectors would afford a capability for detecting single photons — a capability that is needed for research in quantum optics as well as for the practical development of secure optical communication systems for distribution of quantum cryptographic keys.
The proposed photodetector would be of the hot-electron, phonon-cooled, thin-film superconductor type. The superconducting film in this device would be a meandering strip of niobium nitride. In the proposed photodetector, the quantum efficiency would be increased through incorporation of optical components, described below, that would increase the electromagnetic coupling between the input optical field and the meandering superconducting film.
The meandering niobium nitride strip would be fabricated on top of a dielectric (e.g., silicon) optical waveguide on a silicon dioxide substrate (see figure). The input end face of the waveguide would be cut, polished, and antireflection-coated to maximize in-coupling efficiency. The thickness of the waveguide would be chosen so that at the design wavelength, there would be a single through-the-thickness electromagnetic mode, the evanescent tail of which would overlap with the niobium nitride strip. Because the waveguide would exhibit little optical loss over the length of the strip, there would be a high probability of absorption of photons by the strip. The width of the waveguide would be chosen to accommodate multiple widthwise electromagnetic modes, thereby increasing the interaction of light with the niobium nitride strip.
Light would be brought to the photodetector via an optical fiber. A point-to-line-focusing diffractive optical element would couple the light from the output end of the optical fiber into the waveguide. The diffractive optical element would be specially designed and fabricated to collimate as well as possible in the width dimension and to focus as well as possible in the thickness dimension in order to maximize the coupling into the desired waveguide electromagnetic mode.
This work was done by Deborah Jackson, Daniel Wilson, and Jeffrey Stern of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-40163
This Brief includes a Technical Support Package (TSP).
Wide-Band, High-Quantum-Efficiency Photodetector
(reference NPO-40163) is currently available for download from the TSP library.
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Overview
The document is a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in wide-band, high-quantum-efficiency photodetectors, particularly focusing on superconducting detectors. The primary goal is to enhance the performance of hot-electron superconducting detectors, which are crucial for applications such as quantum key distribution (QKD) and other high-speed optical communication systems.
The document outlines the fabrication process of superconducting thin films, specifically NbTiN, which are deposited and patterned on waveguides. Metal contact pads are created to connect to co-planar waveguides, facilitating high-frequency operation. The design allows for multiple meander sections, each with its own amplifier, enabling better impedance matching and signal amplification. A diffractive optical element (DOE) is also designed to optimize the coupling of light from optical fibers into the waveguide, enhancing the detector's efficiency.
Key challenges addressed include optimizing the geometry of the detectors to achieve better radio frequency (RF) impedance matching and optical coupling while maintaining a small thickness (t_m) of the superconducting film, which is critical for high-speed performance. The document references the work of Gol’tsman et al., who established optimal thickness values for device recovery time, and emphasizes the need for precise fabrication techniques, such as electron-beam lithography and deep-UV lithography, to create uniform structures.
The document also discusses the potential for using silicon-on-insulator waveguide wafers for operation at 1550 nm, with considerations for other materials like LiNbO3, SiON, or GaN for visible wavelengths. The end faces of the waveguides are to be polished and coated to maximize in-coupling efficiency, and adiabatic mode converters may be fabricated for enhanced performance.
Overall, the document highlights JPL's commitment to leveraging advanced materials and fabrication techniques to produce high-efficiency superconducting detectors capable of operating at multi-gigabits per second bandwidths. The research aims to significantly improve the quantum efficiency and operational capabilities of these detectors, paving the way for their application in cutting-edge technologies.
