Nanocrystals appear as versatile building blocks for the design of low-cost optoelectronic devices. The design of infrared sensors based on nanocrystals is currently facing a key limitation: the short carrier diffusion length resulting from hopping transport makes that only a limited part of the incident light is absorbed. In order to enhance the device absorption, we use Guided Mode Resonance (GMR). The method appears to be quite versatile and is applied to both PbS and HgTe nanocrystals presenting respectively cutoff wavelengths at 1.7 and 2.6 µm. The designed electrodes present a large enhancement of the material responsivity around a factor of ≈250, reaching external quantum efficiency of 86% for PbS and 340% for HgTe. This increase of the response can be deconvoluted in a factor of 3 for the enhancement of the absorption and a factor of 80 for the photocurrent gain. The method can also be suited to finely tune the cutoff wavelength of the material thanks to geometrical parameters at the device level. The obtained devices are now only limited by the material noise.
Infrared applications remain too often a niche market due to their prohibitive cost. Nanocrystals offer an interesting alternative to reach cost disruption especially in the shortwave infrared (SWIR, λ<1.7 µm) where material maturity is now high. Two families of materials are candidate for SWIR photoconduction: lead and mercury chalcogenides. Lead sulfide typically benefits from all the development made for wider bap gap such as the one made for solar cells, while HgTe takes advantage of development relative to mid wave infrared detector. Here we make a fair comparison of the two material detection properties in the SWIR and discuss the material stability. At such wavelengths, studies have been mostly focused on PbS rather than on HgTe, therefore we focus in the last part of the discussion on the effect of surface chemistry on the electronic spectrum of HgTe nanocrystals. We unveil that tuning the capping ligands is a viable strategy to adjust the material from p-type to ambipolar. Finally, HgTe nanocrystals are integrated into multi pixel devices to quantize spatial homogeneity and onto read out circuits to obtain fast and sensitive infrared laser beam profile.
We report nearly perfect optical transmission (87%) through freestanding metallic gratings with narrow slits, as the experimental demonstration of the theoretical prediction by Porto et al. [Phys. Rev. Lett. 83, 2845 (1999)10.1103/PhysRevLett.83.2845]. In addition, we show that the Fano line shape of transmission spectra reveals the interplay between localized and propagating surface plasmon resonances, and allows us to determine the nonradiative losses. It provides the limits for the transmission efficiency and resonance quality factor. As an illustration, a mosaic of various bandpass filters has been achieved in a single membrane.
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