Quantum cascade detector

A quantum cascade detector (QCD) is a photodetector sensitive to infrared radiation. The absorption of incident light is mediated by intersubband transitions in a semiconductor multiple-quantum-well structure. The term cascade refers to the characteristic path of the electrons inside the material bandstructure, induced by absorption of incident light. QCDs are realized by stacking thin layers of semiconductors on a lattice-matched substrate by means of suitable epitaxial deposition processes, including molecular-beam epitaxy and metal organic vapor-phase epitaxy. The design of the quantum wells can be engineered to tune the absorption in a wide range of wavelengths in the infrared spectrum and to achieve broadband operation: QCDs have been demonstrated to operate from the short-wave to the long-wave infrared region and beyond. QCDs operate in photovoltaic mode, meaning that no bias is required to generate a photoresponse. For this reason, QCDs are also referred to as the photovoltaic counterpart of the photoconductive quantum well infrared photodetectors (QWIPs). Since the vibrational modes of organic molecules are found in the mid-infrared region of the spectrum, QCDs are investigated for sensing applications and integration in dual-comb spectroscopic systems. Moreover, QCDs have been shown to be promising for high-speed operation in free-space communication applications.

History In 2002, Daniel Hofstetter, Mattias Beck and Jérôme Faist reported the first ever use of an InGaAs/InAlAs quantum-cascade-laser structure for photodetection at room temperature. The specific detectivity of the device was shown to be comparable to the detectivity of more established detectors at the time, such as QWIPs or HgCdTe detectors. This pioneering work stimulated the search for bi-functional optoelectronic devices embedding both lasing and detection within the same photonic architecture. The term quantum cascade detector was coined in 2004, when L. Gendron and V. Berger demonstrated the first operating cascade device fully devoted to photodetection purposes, employing a GaAs/AlGaAs heterostructure. This work was motivated by the necessity to find an alternative intersubband infrared photodetector to QWIPs. Indeed, while manifesting high responsivity enhanced by photoconductive gain, QWIPs suffer from large dark current noise, which is detrimental to in room-temperature photodetection. In the subsequent years researchers have explored a variety of solutions leading to an enhancement of the device performances and functionalities. New material platforms have been studied, such as II-VI ZnCdSe/ZnCdMgSe semiconductor systems. These compounds are characterized by a large conduction band offset, allowing for broadband and room-temperature photodetection. Moreover, QCDs based on GaN/AlGaN and ZnO/MgZnO material platforms have also been reported with the aim to investigate photodetection operation at the very edges of the infrared spectrum.

Innovative architectures have been designed and fabricated. Diagonal-transition quantum cascade detectors have been proposed to improve the mechanism of electronic extraction from the optical well. While in conventional QCDs the transition is hosted in a single well (vertical transition), in diagonal-transition QCDs the photoexcitation takes place in two adjacent wells, in a bound-to-bound or bound-to-miniband transition scheme. The motivation behind the realization of this architecture lies in the opportunity to improve the extraction efficiency towards the cascade, even though at the expense of the absorption strength of the transition. Since early 2000s up to more recent years, QCDs embedded in optical cavities operating in the strong light–matter interaction regime have been investigated, aiming to further improvement of the device performances.

Working principle

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