Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas.

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 µm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.

Anomalous Absorption in Arrays of Metallic Nanoparticles: A Powerful Tool for Quantum Dot Optoelectronics.

Periodic arrays of noble metal nanoparticles are emblematic nanostructures in photonics. Their ability to sustain localized surface plasmon resonances has been used throughout the years to demonstrate a variety of passive and active functionalities such as enhanced luminescence in dipolar media and LEDs as well as higher responsivities in photoconductive detectors. Here, we show that additional magnetic resonances, associated with inductive current loops between the nanoparticles and accessible with transverse electric waves, emerge in the limit of dense arrays with subwavelength periods. Moreover, their interplay with the plasmons of the system results in spectrally sharp analogues of electromagnetically induced absorption (EIA). We use these metasurfaces to induce changes and enhancements in the emission, absorption, photoconduction, and polarization properties of active layers of PbS nanocrystals, illustrating the potential of EIA beyond the passive functionalities demonstrated so far in literature.

Identification of Two Regimes of Carrier Thermalization in PbS Nanocrystal Assemblies.

We bring fresh insight into the ensemble properties of PbS colloidal quantum dots with a critical review of the literature on semiconductors followed by systematic comparisons between steady-state photocurrent and photoluminescence measurements. Our experiments, performed with sufficiently low powers to neglect nonlinear effects, indicate that the photoluminescence spectra have no other noticeable contribution beside the radiative recombination of thermalized photocarriers (i.e., photocarriers in thermodynamic quasi-equilibrium). A phenomenological model based on the local Kirchhoff law is proposed that makes it possible to identify the nature of the thermalized photocarriers and to extract their temperatures from the measurements. Two regimes are observed: For highly compact assemblies of PbS quantum dots stripped from organic ligands, the thermalization concerns photocarriers distributed over a wide energy range. With PbS quantum dots cross-linked with 1,2-ethanedithiol or longer organic ligand chains, the thermalization concerns solely the fundamental exciton and can quantitatively explain all the observations, including the precise Stokes shift between the absorbance and luminescence maxima.