Quantum dot LEDs emitting broadband vortex beams

Abstract

The past few years have witnessed impressive developments in optical sources capable of emitting structured forms of light, such as optical vortices or vector beams. Because structured beams result from carefully engineered interferences, their synthesis requires coherent light and all the sources demonstrated so far rely on coherent lasing cavities-usually pumped with external optical schemes. Here, we introduce non-lasing sources emitting directional vortex beams upon electrical injection. Their architecture consists of colloidal PbS quantum dot LEDs that integrate a photonic environment with two complementary functions: to make the emitters populate radial photonic modes with extended spatial coherence, and to structure the leakage of these modes into free space. Our electrically-pumped sources exhibit phase singularities across the electroluminescence spectrum of the quantum dots, leading to vortex light emission with a bandwidth of 300 nm in the near-infrared.

Fig. 1. QD LEDs operating with Au spirals.

a Schematic drawing of the LED, with half of the upper layers not represented so as to visualize the Au pattern. HTL and ETL stand for hole transfer layer and electron transfer layer, respectively, while NCs stands for nanocrystals. b Top view image of a fabricated device. The four straight bars that are visible have been added to improve the structural integrity of the e-beam resist after development of the spiraling pattern. c Near-infrared image of the EL emitted by the device (top) and measured EL spectrum (bottom). The red dashed circle added to the top panel marks the outer perimeter of the Au spiral. d Experimental dispersion relation of the EL at 6 V (negative values of k_///k₀) compared to the computed reflection of a two-dimensional linear grating that approximates the behavior of the real structure (positive values of k_///k₀). Labels 1a, 1b, and 2 on the experimental map identify the different branches discussed in the text. e Computed instantaneous electric field of the TE mode at λ = 1407 nm and k_///k₀ = 0.1, and computed instantaneous magnetic field of the TM mode at λ = 1520 nm and k_///k₀ = 0.16 supported by the two-dimensional linear grating. The Greek letters on the bottom map identify the different layers of the model: α: Al, β: TiO₂, γ: Au, δ: PMMA, ε: effective layer merging the n-doped QDs, p-doped QDs, and ITO, ζ: air.

Fig. 2. Experimental demonstration of the phase singularity.

a Design principle of the four interferometric LEDs, which each feature the same Au spiral superimposed with a different bullseye grating. All bullseye gratings have the same periodicity but a different phase at the origin. b Scanning electron micrograph of the resulting Au patterns. The yellow scale bar represents 2 µm. Note that the very center of each pattern has been intentionally omitted in the fabrication process to ensure proper electrical injection. c Back focal plane images of the EL emitted by each interferometric LED. The blue cross represents the center of the images. All the information is contained in a circle with a radius k/_//k₀ = 0.65, corresponding to the numerical aperture of our microscope objective. d Experimental phase deduced from the four interferometric patterns shown in panel c. The white dashed circle is a guide for the eyes with a radius k/_//k₀ = 0.3. e Calculated phase obtained with an analytical model assuming that the LEDs operate with scalar radial waves launched at the very center of the spirals.

Fig. 3. Improving the robustness of electrical pumping.

a Current-voltage characteristics of the LEDs presented in this study. Each group of curves represents four cycles from 0 V to 2 V and back to 0 V. Yellow: LED presented in Fig. 1, before extensive characterization. Red: LED presented in Fig. 1 shortly before failure. Gray: LED presented in Fig. 3, before extensive characterization. Blue: same as yellow, but after extensive characterization. b Schematic of the LED operating with an a-Si spiral. The different layers are the same as before, except that Au has been replaced by a-Si. c Scanning electron microscope image of an a-Si spiral on top of the TiO₂ electron injection layer (tilted view). As before, the very central part of the spiral is omitted to allow for proper electrical injection. d Experimental phase of the EL measured with four interferometric devices. e Experimental EL dispersion relation at 10 V (negative values of k_///k₀) compared to the experimental dispersion relation in photoluminescence (positive values of k_///k₀). The photoluminescence data have been obtained by pumping the center of the device with a focused laser spot. f Cross-section of the EL dispersion relations for the LEDs operating with an Au spiral (top) and a-Si (middle). The bottom panel represents a cross-section of the dispersion relation of the photoluminescence emitted by a device operating with a-Si, but without the bottom electrode (the full characterization of this device is plotted in Supplementary Fig. 8). These cross-sections have been evaluated at wavelengths close to the maximum of luminescence (1405 nm for the Au pattern and 1330 nm in the case of a-Si).