High resolution microdisplays that use liquid-crystal-on-silicon (LCoS) technology are commonplace these days. They are found across various optical modulation applications including projectors, telecommunication switches and head-mounted displays. However, with the rise in photonics, a pixel’s surface area is underutilized and the potential for the phenomena associated with plasmonic nanostructures is far reaching.
A team of Researchers from the University of Cambridge, UK, have demonstrated a plasmonic metapixel, which offers a unique spectral (full RGB) and polarization control, as novel pixel elements for state-of-the-art high-resolution reflective microdisplays.
Modern day technology commonly utilizes liquid-crystal-on-silicon (LCoS) technology, using pixels that utilize reflective surfaces on top of electrical backplanes. Whilst many devices are considered state-of-the-art, each pixel in use in these technologies are confined to a single fixed color that will only modulate the amplitude of light.
A single pixel is commonly composed of linear polarizers, RGB pigment-based color filters and an electrically switchable waveplate, which is a liquid crystal layer which sits on top of a mirror-quality reflector- a square aluminum electrode, roughly 10 μm2 in size, which is connected to electronic circuitry.
However, this size is very rarely utilized to its full potential and nanophotonics has been stipulated to be the next logical step to achieving a pixel’s full potential, but have been subsequently limited in commercial applications by a low reflectance stemming from plasmonic losses and sub-optimal design schemes.
The team of Researchers have used a combination of experimental and computational approaches to develop plasmonic metapixels which incorporate in-plane 2D amplitude functions on the pixel itself, which are encoded within the nanostructures.
The Researchers designed Gaussian-profile plasmonic pixels to specifically tailor the color properties on the sub-pixel level. The design introduced non-rectangular pixels where the polarization controlled the color spatial functions and were compatible with liquid crystal waveplates. The approach was different to most, which only utilize square pixels that encode amplitude only.
To realize this, the used a combination of silver (30 nm), silicon dioxide (100 nm), aluminum (100 nm) and bulk silicon layers. All of which are compatible with current LCoS technologies and can be easily integrated.
The Researchers designed the devices using MATLAB and used remote plasma RF sputtering and electron beam lithography (EBL, Nanobeam Ltd.) methods to create the layered structure. The device was characterized using optical microscopy (Olympus BX-51 polarizing optical microscope), ultraviolet-visible spectrometry (UV-Vis, Ocean optics UV-VIS HR2000+) and scanning electron microscope (SEM, Carl-Zeiss).
The Researchers varied the grating width, designs and degree of polarization to produce plasmonic metal-insulator-metal (MIM) pixels, which utilized the aluminum reflectors as back reflectors, colloquial to that of a traditional MIM pixel. These new pixels also offer a unique spectral (full RGB) color filtering from the excitation of resonant modes and polarization control from the anisotropic nanostructures within the layered device.
The developed plasmonic metapixel was found to permit a high reflection capability whilst producing vivid, polarization switchable, wide color gamut filtering. The ultra-thin geometries within the device were found to produce an excitation of the hybridized absorption modes within the visible spectrum, and included surface plasmons and quasi-guided modes. It was found to be possible to tailor the absorption modes to target specific wavelengths, which resulted in pixels that produced a multicolour reflection on mirror-like surfaces.
The Researchers extended the concepts to incorporate 1D and 2D nanostructures, which led to dual resonant behavior and vivid color profiles. The Researcher also tailored 2D gaussian profiles across the pixels with the aid of isolated nanostructures, resulting in unique 2D functions at different wavelengths. These effects were found to be controlled by the polarization and therefore compatible with existing technologies.
The work produced has removed the need for additional input polarizers and pigment-based color filters and instead utilizes in-plane pixel color functions which are not limited by the conventional square/rectangular form commonly seen.
The choice of materials and dimensions used also lend this design to be highly compatible with a range of scalable manufacturing methods. Including extreme-UV photolithography and nanoimprint lithography, and is therefore an excellent candidate for commercial scale applications– especially as novel pixel elements in state-of-the-art high-resolution reflective microdisplay technologies.
“Nanostructured plasmonic metapixels”- Williams C., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-08145-0
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