Bridging the 100 GHz - 10 THz domain with unipolar quantum optoelectronics

The challenge of Unipolar Quantum Optoelectronics (UQO) is to bring reliable technology in the mid-infrared and terahertz domains with dozens of GHz bandwidth and room-temperature operation. The semiconductor devices based on this novel technology rely on two-dimensional electronic states localized in the conduction band, which implies that electrons are the only charge carriers involved. Though UQO technology has been proven useful for emission (quantum cascade lasers) and detection (quantum cascade detectors), it is still underdeveloped for other applications, like high-speed modulation. In this paper, we will review our recent results with a full transmission system UQO in the 8 to 14 μm atmospheric window, composed of a quantum cascade (QC) laser, an external modulator and a QC detector, all optimized for operation at 33 THz optical wavelength. Dynamics down to a few dozens of picoseconds are observed, which allow us demonstrating data rate transmission of 10 Gbps without any signal processing. In addition, the paper aims at discussing further applications of UQO in particular for radio over free-space. The basic principle for producing microwave carriers is based on an optical heterodyne beating technique taking advantage of the high-bandwidth potential of QC detectors. Then, the microwave signal is transmitted through a point-to-point wireless link by using radiofrequency antennas. With UQO, microwave signals of dozens of GHz can be achieved. To sum, this paper highlights the importance of using UQO devices operating at a few dozens of THz optical wavelength for both free-space optics and microwave photonics targeting 100 GHz radiofrequencies.