Metasurface Light-Emitting Diodes with Directional and Focused Emission.

Phased-array metasurfaces enable the imprinting of complex beam structures onto coherent incident light. Recent demonstrations of photoluminescent phased-array metasurfaces highlight possibilities for achieving similar control in electroluminescent light-emitting diodes (LEDs). However, phased-array metasurface LEDs have not yet been demonstrated owing to the complexities of integrating device stacks and electrodes within nanopatterned metasurfaces. Here, we demonstrate metasurface LEDs that emit directional or focused light. We first design nanoribbon elements that achieve the requisite phase control within typical LED device constraints. Subsequently, we demonstrate unidirectional emission that can be engineered at will via phased-array concepts. This control is further exhibited in metasurface LEDs that directly emit focused beams. Finally, we show that these metasurface LEDs exhibit external quantum efficiencies (EQEs) superior to those of unpatterned LEDs. These results demonstrate metasurface designs that are compatible with high-EQE metal-free LED devices and portend opportunities for new classes of metasurface LEDs that directly produce complex beam structures.

Heterogeneous layered structures for improved terahertz generation.

One of the most effective ways of generating terahertz (THz) radiation involves the conversion of short-pulsed IR or visible laser light into THz pulses at significantly lower frequencies. This conversion can be accomplished using organic crystals with nonlinear optical crystal (NLO) properties for IR to THz conversion through optical rectification. Due to the high refractive indices of organic crystals, pump laser light as well as generated THz radiation is lost from reflections at crystal surfaces. Here we report a structure composed of a layered series of materials with intermediate refractive indices designed to reduce reflective losses and improve the THz generation from organic crystals. This structure increases the transmission coefficients for both infrared pump input and THz output. We combine simple theoretical calculations with experimental data to show that a structure composed of materials with intermediate refractive indices can be used to increase generated THz intensity by nearly 50%.

Terahertz generation and optical characteristics of P-BI.

We present the structural and THz generation characteristics of the molecular salt crystal (E)-2-(4-(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium iodide (P-BI) using optical rectification with IR pump wavelengths. P-BI shows a peak-to-peak field ∼6 times greater than inorganic crystal GaP, and a broader THz spectrum. Data were obtained from 0-6 THz showing a significant dip in generation at 1.8 THz, similar to what has been observed with the THz generation crystal DAST at 1 THz. We characterized the power dependence of P-BI at different IR wavelengths, with optimal THz generation at the 1550-nm pump wavelength. To model THz generation as a function of P-BI crystal thickness, we measured the THz complex refractive index and the IR group index; modeling shows that imperfect phase matching leads to spectral narrowing centered at ∼2.5 THz as the crystal thickness is increased. P-BI could provide a useful alternative to inorganic THz generation crystals such a GaP.

Light-emitting metalenses and meta-axicons for focusing and beaming of spontaneous emission.

Phased-array metasurfaces have been extensively used for wavefront shaping of coherent incident light. Due to the incoherent nature of spontaneous emission, the ability to similarly tailor photoluminescence remains largely unexplored. Recently, unidirectional photoluminescence from InGaN/GaN quantum-well metasurfaces incorporating one-dimensional phase profiles has been shown. However, the possibility of generating arbitrary two-dimensional waveforms-such as focused beams-is not yet realized. Here, we demonstrate two-dimensional metasurface axicons and lenses that emit collimated and focused beams, respectively. First, we develop off-axis meta-axicon/metalens equations designed to redirect surface-guided waves that dominate the natural emission pattern of quantum wells. Next, we show that photoluminescence properties are well predicted by passive transmission results using suitably engineered incident light sources. Finally, we compare collimating and focusing performances across a variety of different light-emitting metasurface axicons and lenses. These generated two-dimensional phased-array photoluminescence waveforms facilitate future development of light sources with arbitrary functionalities.