Multispectral detection or simultaneous collection of signals from different infrared bands provides enhanced target discrimination and identification, and has attracted increased interest. In this region, quantum cascade detector (QCD) is a very competitive candidate, but barely exploited.

Confined by the ladder-shaped subbands arrangement, the photo-excited transitions in QCDs are complicated. It allows the absorption of several colors to occur in a single active region. As a consequence of the bound-to-bound optical transition, each photoresponse peak is sharp, clear and has no overlap with others. Conventional multi-color detectors have to work with external bias, and therefore, the signals from different channels have to be output in sequence. However, thanks to the asymmetric intersubband structure, QCDs are photovoltaic devices, and the signals of several channels can be output independently as well as simultaneously. In addition, operating with zero-bias avoids mutual electrical interference among different channels, which is beneficial to reduce the crosstalk. Moreover, according to the intersubband (ISB) selection rule, infrared absorption of QCDs is possible only when the electric field vector of the radiation has a component perpendicular to the quantum well layers, (i.e., QCDs are generally unresponsive to normal incidence,) which is good news for multispectral detection with a single active region, as we can realize wavelength selection utilizing specific optical coupling structure under normal incidence, and then separate the operation regimes. Compared with conventional multi-color detectors with multiple single-color active regions grown in stacks, QCDs can effectively simplify the material growth and manufacture.

Recent years, coupling to patch antenna resonators (PARs) has drastically redefined the perspective of the ISB pathway and attracted a lot of interests. For the patch antenna shown in Fig. 1, due to the strong impedance mismatch between the single-metal and double-metal regions which carry the TM_NM0 mode, a lateral Fabry-Perot effect arises and hence the formation of the standing wave pattern. The resonant features can be identified with TM_NM0 modes confined under the metallic patch, with frequencies provided by the equation:

where s is the size of the patch, and n_eff = 3.3 is the effective refractive index of the resonator. For TM₁₀₀ mode or TM₀₁₀ mode, considering that c = λ·v, eq. (1) is simplified as:

λ_r = 2n_eff·s

Therefore, for a given dielectric material, the resonant wavelength of a patch antenna is only related to its dimensions. Utilizing a dual-color active region design, switchable operating modes will be achieved by two electrically independent patch-antenna arrays nested with each other.

Fig. 1. Illustration of a patch antenna. The figure shows the simulated vertical component of the distribution of the electric field E_z in the dielectric area.

Building upon this theoretical foundation, the research team has presented a patch-antenna-array enhanced quantum cascade detector with freely switchable operating modes of mid-wave, long-wave, and dual-color. At 77 K, the 5.7-μm channel achieved a peak responsivity of 34.6 mA/W and exhibited a detectivity of 2.0×10¹⁰ Jones, while the 10.0-μm channel achieved a peak responsivity of 87.5 mA/W, giving a detectivity of 5.0×10¹⁰ Jones. The crosstalk of the mid-wave and the long-wave operating mode was 1:12.4 and 1:5.7, respectively. By modulating the polarization angle of the incident light, the crosstalk reaches the minimum at 90° for both the 5.7-μm channel and the 10.0-μm channel, and was 1:22.5 and 1:7.6, respectively. Such all-in-one device also shows potential of working as single or dual-color photodetectors at room temperature. This work has proposed a novel approach in making multi-color infrared imagers and for the first time, successfully realized the signals of two channels output independently as well as simultaneously. Relevant research results were recently published in Photonics Research, Volume 12, No. 2, 2024. [ Yixuan Zhu, Shenqiang Zhai, Kun Li, Kai Guo, Qiangqiang Guo, Jinchuan Zhang, Shuman Liu, Lijun Wang, Fengqi Liu, Junqi Liu. Mode-switchable dual-color infrared quantum cascade detector[J]. Photonics Research, 2024, 12(2): 253 ]

Fig. 2. (a) Scanning electron microscope image of the detector, with partial enlargements of the electrically connected patch antennas. (b) Conduction band diagram and relevant energy levels of one period active core.

The device architecture and material energy band diagram are described in Fig. 2. As shown in Fig. 2(a), to separate the two channels, two arrays of metal-dielectric-metal microcavity patches, which provide a strong sub-wavelength electric field confinement and act as antennas, are nested within each other, and respectively connected to their own top metal electrode (B and C). They share the Ti/Au ground plane as the bottom metal electrode (D). By separately reading out the electrical signals between the metal electrodes B-D, C-D, and BC-D, the operating mode can be freely switched among long-wave, mid-wave, and dual-color. To match the device design, the active region of the wafer composed of 5 periods of infrared dual-band quantum cascade active cores was optimized to room temperature operation. The conduction band diagram and relevant energy levels of one period active core is shown in Fig. 2(b). In this design, the dual-color detection is achieved by a vertical-transition in the active quantum well (E₀→E_T) and a diagonal-transition across the adjacent quantum wells (E₀→E_mini).

In the future, the team will further optimize the device structure, for example, by refining the material and dimensions of the connecting wires between patch antenna platforms. This will ensure the performance of dual-color detection while reducing crosstalk between the two detection channels, laying the foundation for achieving high-performance infrared dual-color detectors.

The corresponding author of the study, Professor Jun-Qi Liu remarked, "Dual-color infrared detection technology can assist in acquiring more information about objects, enabling target recognition in complex environments, thus holding broad prospects for application in various fields such as optical imaging and optical communication. Early dual-color detection technology primarily relied on detector units composed of multiple pixel points with different response spectra, leading to image distortion due to poor spatial consistency. Meanwhile, the currently more mature back-to-back structure, hindered by the sequential output requirement for dual-color signals, suffers from poor temporal consistency, thereby limiting its effectiveness in detecting moving objects. To address the higher demands of practical applications for infrared dual-color detectors, our team has developed a novel dual-color quantum cascade detector. This detector utilizes a dual-metal patch antenna array to achieve seamless switching between mid-wave, long-wave, and dual-color operating modes. Moving forward, our team will continue to explore the feasibility of applying such structures to infrared imaging."