Youngseop Lee

Biologically inspired LED lens from cuticular nanostructures of firefly lantern

Cuticular nanostructures found in insects effectively manage light for light polarization, structural color, or optical index matching within an ultrathin natural scale. These nanostructures are mainly dedicated to manage incoming light and recently inspired many imaging and display applications. A bioluminescent organ, such as a firefly lantern, helps to out-couple light from the body in a highly efficient fashion for delivering strong optical signals in sexual communication. However, the cuticular nanostructures, except the light-producing reactions, have not been well investigated for physical principles and engineering biomimetics. Here we report a unique observation of high-transmission nanostructures on a firefly lantern and its biological inspiration for highly efficient LED illumination. Both numerical and experimental results clearly reveal high transmission through the nanostructures inspired from the lantern cuticle. The nanostructures on an LED lens surface were fabricated by using a large-area nanotemplating and reconfigurable nanomolding with heat-induced shear thinning. The biologically inspired LED lens, distinct from a smooth surface lens, substantially increases light transmission over visible ranges, comparable to conventional antireflection coating. This biological inspiration can offer new opportunities for increasing the light extraction efficiency of high-power LED packages.

Bioplasmonic Alloyed Nanoislands Using Dewetting of Bilayer Thin Films

Unlike monometallic materials, bimetallic plasmonic materials offer extensive benefits such as broadband tuning capability or high environmental stability. Here we report a broad range tuning of plasmon resonance of alloyed nanoislands by using solid-state dewetting of gold and silver bilayer thin films. Thermal dewetting after successive thermal evaporation of thin metal double-layer films readily forms AuAg-alloyed nanoislands with a precise composition ratio. The complete miscibility of alloyed nanoislands results in programmable tuning of plasmon resonance wavelength in a broadband visible range. Such extraordinary tuning capability opens up a new direction for plasmonic enhancement in biophotonic applications such as surface-enhanced Raman scattering or plasmon-enhanced fluorescence.

Optically Patternable Metamaterial Below Diffraction Limit

We report an optically patternable metamaterial (OPM) for ultraviolet nanolithography below the diffraction limit. The OPM features monolayered silver nanoislands embedded within a photosensitive polymer by using spin-coating of an ultrathin polymer, oblique angle deposition, and solid-state embedment of silver nanoislands. This unique configuration simultaneously exhibits both negative effective permittivity and high image contrast in the ultraviolet range, which enables the surface plasmon excitation for the clear photolithographic definition of minimum feature size of 70 nm (≲ λ/5) beyond the near-field zone. This new metamaterial provides a new class of photoresist for ultraviolet nanolithography below the diffraction limit.

Antireflective structures for tunable liquid-filled lens

This work presents novel antireflective structures (ARS) on flexible membrane for tunable liquid-filled lens by using wafer-level MEMS fabrication. ARS on flexible membrane with enhanced optical transmittance will provide new visions for miniaturized camera systems.

Nanoplasmonic color filter on large area at visible region

This work presents plasmonic nanostructures on a large area for visible color filtering. The plasmonic nanostructures comprise alternate layers of silver nanoholes, ultrathin dielectric film, and silver nanoislands on a glass substrate. The plasmonic nanostructures were fabricated on a wafer level by using lithography-free methods such as metal dewetting of silver nanoholes, spin-coating of ultrathin dielectric film, and oblique angled deposition of silver nanoislands. The numerical results showed transmission spectra for subtractive color filtering in visible region. The plasmonic nanostructures have reflective colors from red to blue under white light illumination. This nanoplasmonic substrate can provide a new approach for fabricating practical color display or imaging devices with a low cost.

Photosensitive metamaterial for nanolithography

This work presents the novel photosensitive metamaterial (metaphotoresist) with silver (Ag) and dielectric multilayer for ultraviolet nanolithography overcoming a diffraction limit. The metamaterial consists of multilayer with Ag and photoresist (PR) layers. A plasmonic UV light coupling under diffraction limit in the metamaterial was numerically demonstrated by using a finite difference time domain (FDTD) method. Nanolithography was demonstrated by using the metaphotoresist (meta-PR) and a conventional UV (λ=365 nm) alignment system. The meta-PR was fabricated by spin-coating of PR and angled deposition of Ag nanostructures. About 150 nm nanopatterns were fabricated. We believe that the metaphotoresist provides new direction for nanolithography industries.

Hierarchically structured LED lens for wide angle and high efficiency illumination

This work presents an effective method for the wide angle and high efficiency illumination with hierarchical structures on lens surfaces, which were fabricated by a photolithography and a capillary force lithography.

Highly efficient LED lens biologically inspired from cuticle nanostructures of firefly light organs

This work reports an engineering inspiration of lanterns nanocuticles found in a firefly. It includes the first observation of antireflective structures on the lighting organ and a novel application for highly efficient LED illumination. The nanocuticular patterns contribute to highly efficient light extraction of high transmission and high out-coupling of luminescent light. The nanopatterns are employed on the lens surface for highly efficient light extraction. A bioinspired lens for highly efficient LED illumination was successfully fabricated by using colloidal lithography, RIE, and replica molding method. Transmission through a lens with nanopattern arrays is increased by 4.3%, compared to a smooth LED lens. We strongly believe this device can be effectively utilized for diverse high efficient LED lighting applications.

A plastic lens with anti-reflective structures using a nanoporous alumina template with lens curvature

This work presents a novel fabrication method for a millimeter-sized plastic lens with anti-reflective structures (ARS) for diverse illumination and imaging applications. The ARS lens was molded from a curved surface with nanoporous aluminum oxide by two-step anodization.