Publication date: 3rd July 2020
Colloidal Quantum Dots (CQDs) are a broadly tunable building block for low-cost optoelectronic devices and particularly infrared (IR) photodetectors.1 However, due to hooping transport the carrier diffusion length remains short, typically few tens of nanometers2, which is more than one order of magnitude shorter than the absorption depth (few µm). Consequently, to efficiently collect charge, only thin film can be used and only few percent of the light is absorbed. To tackle this issue, we have designed light-matter coupling devices using plasmonic resonator to drastically increase the absorption in photoconductive devices and boost responsivity.
The light-matter coupling is based on sub-wavelength resonator and relies on guided mode resonance (GMR): a metallic grating focuses the incident light in a thin slab of nanocrystals that acts as a wave guide.3 The structure is designed to achieve 100 % light absorption at the targeted wavelength. A key challenge is to make the absorption within the CQD film and not in the metal, to avoid losses.
This structure is versatile and can be applied to different materials (PbS and HgTe) at different wavelengths (1.55 and 2.5 µm, respectively) in short wave IR and extended short wave IR.4 There is an excellent agreement between electromagnetic simulation and the measured photocurrent. The responsivity is increase of several orders of magnitude due to the enhancement of absorption (factor 3-6) and of photoconductive gain as well. This method can also be applied to a film of mixed CQDs5 (FAPI/PbS) in order to increase the absorption in a low-dark current IR system.
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