Researchers at<a href="http://engineering.columbia.edu/" id="isPasted" rel="nofollow" target="_blank">&nbsp;Columbia Engineering</a> have developed a new class of integrated photonic devices--&ldquo;<a href="https://www.nature.com/articles/s41565-023-01360-z" rel="nofollow" target="_blank">leaky-wave metasurfaces</a>&rdquo;--that can convert light initially confined in an optical waveguide to an arbitrary optical pattern in free space. These devices are the first to demonstrate simultaneous control of all four optical degrees of freedom, namely, amplitude, phase, polarization ellipticity, and polarization orientation--a world record. Because the devices are so thin, transparent, and compatible with photonic integrated circuits (PICs), they can be used to improve optical displays, LIDAR (Light Detection and Ranging), optical communications, and quantum optics.&nbsp;<blockquote>Our work points to new ways to create hybrid systems that utilize the best of both worlds--free-space optics for shaping the wavefront of light and integrated photonics for optical data processing--to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.&nbsp; NANFANG YU, ASSOCIATE PROFESSOR OF APPLIED PHYSICS AND APPLIED MATHEMATICS</blockquote><section id="isPasted">&ldquo;We are excited to find an elegant solution for interfacing free-space optics and integrated photonics--these two platforms have traditionally been studied by investigators from different subfields of optics and have led to commercial products addressing completely different needs,&rdquo; said Nanfang Yu, associate professor of applied physics and applied mathematics who is a leader in research on nanophotonic devices. &ldquo;Our work points to new ways to create hybrid systems that utilize the best of both worlds--free-space optics for shaping the wavefront of light and integrated photonics for optical data processing--to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.&rdquo;</section><section><h2>Bridging free-space optics and integrated photonics</h2><img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1683758808062-leaky-wave-metasurface-operation.png" style="width: 766px;" class="fr-fic fr-dib">Left: Schematic showing the operation of a leaky-wave metasurface. Right: A 2D array of optical spots forming a Kagome pattern that is produced by a leaky-wave metasurface. Credit: Heqing Huang, Adam Overvig, and Nanfang Yu/Columbia Engineering&nbsp;<section id="isPasted">The key challenge of interfacing PICs and free-space optics is to transform a simple waveguide mode confined within a waveguide--a&nbsp;thin ridge defined on a chip--into a broad free-space wave with a complex wavefront, and vice versa. Yu&rsquo;s team tackled this challenge by building on their invention last fall of &ldquo;<a href="https://www.engineering.columbia.edu/news/optical-magic-new-flat-glass-enables-optimal-visual-quality-augmented-reality-goggles" rel="nofollow" target="_blank">nonlocal metasurfaces</a>&rdquo; and extended the devices&rsquo; functionality from controlling free-space light waves to controlling guided waves.&nbsp;Specifically, they expanded the input waveguide mode by using a waveguide taper into a slab waveguide mode--a sheet of light propagating along the chip. &ldquo;We realized that the slab waveguide mode can be decomposed into two orthogonal standing waves--waves reminiscent of those produced by plucking a string,&rdquo; said Heqing Huang, a PhD student in Yu&rsquo;s lab and co-first author of the study, published&nbsp;today&nbsp;in&nbsp;Nature Nanotechnology. &ldquo;Therefore, we designed a &lsquo;leaky-wave metasurface&rsquo; composed of two sets of rectangular apertures that have a subwavelength offset from each other to independently control these two standing waves. The result is that each standing wave is converted into a surface emission with independent amplitude and polarization; together, the two surface emission components merge into a single free-space wave with completely controllable amplitude, phase, and polarization at each point over its wavefront.&rdquo;</section><section><h2>From quantum optics to optical communications to holographic 3D displays</h2>Yu&rsquo;s team experimentally demonstrated multiple leaky-wave metasurfaces that can convert a waveguide mode propagating along a waveguide with a cross-section on the order of one wavelength into free-space emission with a designer wavefront over an area about 300 times the wavelength at the telecom wavelength of 1.55 microns. These include:A leaky-wave metalens that produces a focal spot in free space. Such a device will be ideal for forming a low-loss, high-capacity free-space optical link between PIC chips; it will also be useful for an integrated optogenetic probe that produces focused beams to optically stimulate neurons located far away from the probe.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1683758834906-kagome-lattices-leaky-wave-metasurfaces.png" style="width: 766px;" class="fr-fic fr-dib">Left: Photo of two leaky-wave metasurfaces for generating Kagome lattices.&nbsp;Right: SEM image of a portion of a leaky-wave metasurface, which is composed of nano-apertures etched into a polymer layer on top of a silicon nitride thin film. Credit: Heqing Huang, Adam Overvig, and Nanfang Yu/Columbia Engineering&nbsp;<section id="isPasted">A&nbsp;leaky-wave optical-lattice generator that can produce hundreds of focal spots forming a Kagome lattice pattern in free space. In general, the leaky-wave metasurface can produce complex aperiodic and three-dimensional optical lattices to trap cold atoms and molecules. This capability will enable researchers to study exotic quantum optical phenomena or conduct quantum simulations hitherto not easily attainable with other platforms, and enable them to substantially reduce the complexity, volume, and cost of atomic-array-based quantum devices. For example, the leaky-wave metasurface could be directly integrated into the vacuum chamber to simplify the optical system, making portable quantum optics applications, such as atomic clocks, a possibility.A leaky-wave vortex-beam generator that produces a beam with a corkscrew-shaped wavefront. This could lead to a free-space optical link between buildings that relies on PICs to process information carried by light, while also using light waves with shaped wavefronts for high-capacity intercommunication.A leaky-wave hologram that can displace four distinct images simultaneously: two at the device plane (at two orthogonal polarization states) and another two at a distance in the free space (also at two orthogonal polarization states). This function could be used to make lighter, more comfortable augmented reality goggles and more realistic holographic 3D displays.</section><section><h2>Device fabrication</h2>Device fabrication was carried out at the Columbia Nano Initiative cleanroom, and at the Advanced Science Research Center NanoFabrication Facility at the Graduate Center of the City University of New York.</section><section><h2>Next steps</h2>Yu&rsquo;s current demonstration is based on a simple polymer-silicon nitride materials platform at near-infrared wavelengths. His team plans next to demonstrate devices based on the more robust silicon nitride platform, which is compatible with foundry fabrication protocols and tolerant to high optical power operation. They also plan to demonstrate designs for high output efficiency and operation at visible wavelengths, which is more suitable for applications such as quantum optics and holographic displays.</section></section></section>

Left two figures: Two holographic images produced by a leaky-wave metasurface at two different distances from the device surface. Right four figures: Four distinct holographic images produced by a single leaky-wave metasurface at two different distances from the device surface and at two orthogonal polarization states. Credit: Heqing Huang, Adam Overvig, and Nanfang Yu/Columbia Engineering

New class of integrated nanophotonic devices--a world record in simultaneous control of all four optical degrees of freedom--can convert light initially confined in an optical waveguide to an arbitrary optical pattern in free space.

Leaky-wave Metasurfaces: A Perfect Interface Between Free-space and Integrated Optical Systems

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Holography &ndash; the use of light projection to display images &ndash; can be used to construct holograms, which are three-dimensional structures of light that depict true-to-life objects and scenes. While today&rsquo;s technologies can&rsquo;t yet produce the incredibly lifelike holograms of science fiction movies, researchers from the&nbsp;<a href="https://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS) have made a significant leap toward this goal.A team of scientists and engineers led by&nbsp;<a href="https://seas.harvard.edu/person/federico-capasso" target="_blank">Federico Capasso</a>, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, has reported in&nbsp;<a href="https://www.nature.com/articles/s41566-023-01188-y" target="_blank">Nature Photonics</a> an entirely new way of creating holograms using threads of light layered to construct 2D sheets containing tightly-controlled arrays of light. These sheets can then be tightly stacked &ndash; like a pile of deli cheese &ndash; to depict 3D objects.&ldquo;This research leverages in an innovative way the widely used and commercially established technology of spatial light modulators to shape light as it propagates, creating an entirely new class of holograms,&rdquo; says Capasso. &ldquo;I foresee that this holography method will have impact on virtual and augmented reality, biological imaging, volumetric displays, human-computer interactions, interactive educational tools, and more.&rdquo;Traditional holography methods arrange light in planes moving away from the eye of the viewer like a series of sequential dominoes, which is problematic because the layers farthest away become harder to see. That can disrupt the viewer&rsquo;s depth perception and ultimately make 3D objects appear in diminished detail. To solve this problem, Capasso&rsquo;s team and collaborators from University of S&atilde;o Paulo and Brazil&rsquo;s State University of Campinas devised a holography method to propagate slices of light&nbsp;perpendicularly to the optical display, so that the viewer is looking along the full length of the slices.&ldquo;By doing so, we have mitigated the depth perception problem that has traditionally affected holography,&rdquo; says Ahmed Dorrah, first author on the paper and a research associate in the Capasso group. &ldquo;We can also achieve uniformly spaced sheets of light while maintaining low crosstalk between the sheets and high resolution of depicted images and objects.&rdquo;Dorrah says the method could someday be used to depict any 3D hologram, viewable from any angle with continuous depth, as if it&rsquo;s physically present. &ldquo;At the heart of this development is a special kind of light beam called a Bessel beam,&rdquo; he says. Unlike more commonly used laser beams which diffract and grow less intense as they move farther from their source, Bessel beams maintain their intensity and can even &ldquo;heal&rdquo; themselves after encountering obstacles by winding around them.&ldquo;Bessel beams spread outward in a ring-like pattern. Combining more than one Bessel beam &ndash; a process called superposition &ndash; provides precise control over the intensity of light along any given point through interference,&rdquo; says Michel Zamboni-Rached, professor of electrical engineering at Brazil&rsquo;s State University of Campinas and senior collaborator on this work. &ldquo;You can selectively illuminate desired parts of these light sheets,&rdquo; he adds, without losing the depth of field crucial for depicting images and objects crisply.By stacking these superimposed beams together to form sheets of light, it&rsquo;s possible to additively create a &ldquo;3D-printed&rdquo; projection made of precisely structured light. Capasso&rsquo;s team is especially interested in leveraging this new method to improve biological research.&ldquo;I think the selective control we can exert over light&rsquo;s intensity could be combined with microscopy methods to have greater control over where light is sent into a biological sample,&rdquo; says Dorrah. That&rsquo;s important because laser beams used in microscopy can damage living cells through a process known as phototoxicity, limiting how much imaging a tissue sample can withstand.Dorrah is also keen to adapt the method for optogenetics research, in which brain circuits are genetically engineered so that tissue activity can be triggered on or off in response to light, allowing scientists to study how certain cells impact behavior. With light sheet holography, &ldquo;you could activate targeted cells or networks of cells within optogenetic circuits with a very high degree of accuracy and specificity,&rdquo; he says.The new holography method was discovered rather serendipitously by Capasso&rsquo;s team. Dorrah and his collaborators from Brazil had been developing techniques to trap particles in focal points using Bessel beams of light, so that atoms in specific spots of space and time could be better studied. That&rsquo;s when the team started to wonder if stacking these beams in sheet shapes could be used to build holograms.The group was awed when they tested the technique and saw how well it worked. &ldquo;Usually, we develop a new theory, we test it with an experiment, the results are flawed, and then we rethink our approach,&rdquo; says Leonardo Ambrosio, professor of electrical engineering at the University of S&atilde;o Paulo and another senior collaborator. &ldquo;But this time, our initial experiments far exceeded even our most optimistic expectations. We were amazed by the complexity of images and objects we could make.&rdquo;Harvard&rsquo;s Office of Technology Development has protected the intellectual property arising from the Capasso Lab&rsquo;s new holography technique and is exploring commercialization opportunities.Additional authors include Priyanuj Bordoloi, Vinicius S. de Angelis, and Jhonas O. de Sarro.This work was supported by the Natural Sciences and Engineering Research Council of Canada (award no. PDF-533013-2019), the Office of Naval Research under the MURI program (grant no. N00014-20-1-2450), the Air Force Office of Scientific Research (grant no. FA9550-21-1-0312), the National Council for Scientific and Technological Development (grant nos. 140270/2022, 309201/2021-7, and 306689/2019-7), and the Sao Paulo Research Foundation (grant nos. 2021/15027-8, 2020/05280-5, and 2021/06121-0).

By stacking 2D sheets of tightly controlled rays of light, researchers in the lab of Federico Capasso can depict 3D holographic objects.

Method can depict holograms – viewable from any angle as if physically present – with continuous depth

New light sheet holography overcomes the depth perception challenge in 3D holograms

Researchers have developed new technology that could usher in the &quot;next-generation&quot; of thinner, higher-resolution and more energy efficient screens and electronic devices. The international team from Nottingham Trent University in the United Kingdom, The Australian National University (ANU) and UNSW Canberra has&nbsp;created nanoparticles called &quot;metasurfaces&quot; that perform better than current displays like LCDs and LEDs. LCDs and LEDs rely on liquid crystal cells to create a display on TVs and other screens. The research team&#39;s metasurfaces are 100 times thinner than liquid crystal cells, offer a tenfold greater resolution and consume 50 per cent less energy. The researchers believe their technology is compatible with modern electronic displays. &quot;We have paved the way to break a technology barrier by replacing the liquid crystal layer in current displays with a metasurface, enabling us to make affordable flat screens liquid crystal-free,&quot; lead researcher Mohsen Rahmani, Professor of Engineering at Nottingham Trent University, said. &quot;The most important metrics of flat panel displays are pixel size and resolution, weight and power consumption. We have addressed each of these with our meta-display concept. &quot;Most importantly, our new technology can lead to a huge reduction of energy consumption - this is excellent news given the number of monitors and TV sets being used in households and businesses every single day. We believe it is time for LCD and LED displays to be phased out in the same way as former cathode ray tube (CRT) TVs over the past ten to 20 years.&quot; Dragomir Neshev, Director of the ARC Centre for Excellence in Transformative Meta-Optical Systems (TMOS) and ANU Professor in Physics, said: &quot;The capability of conventional displays has reached its peak and is unlikely to significantly improve in the future due to multiple limitations. &quot;Today there is a quest for fully solid-state flat display technology with a high-resolution and fast refresh rate. We have designed and developed metasurface pixels that can be ideal for the next-generation display. &quot;Unlike liquid crystals, our pixels do not require polarised lights for functioning, which will halve screens&#39; energy consumption.&quot; Khosro Zangeneh Kamali, a PhD scholar at ANU and the first author of the study, said: &quot;Metasurfaces are proven to exhibit extraordinary optical behaviour. &quot;However, inventing an effective way to control them is still a subject of heavy research. We have proposed electrically programmable silicon metasurfaces, which is a versatile platform for programmable metasurfaces.&quot;&nbsp; Dr Lei Xu, a team member from Nottingham Trent University, said: &quot;There is significant room for further improvements by employing artificial intelligence and machine learning techniques to design and realise even smaller, thinner and more efficient metasurface displays.&quot; Professor Andrey Miroshnichenko, a team member from UNSW Canberra, said: &quot;Our pixels are made of silicon, which offers a long lifespan in contrast with organic materials required for other existing alternatives. Moreover, silicon is widely available, CMOS compatible with mature technology, and cheap to produce.&quot; Professor Miroshnichenko said the team hoped their development could generate a frontier technology in new flat displays with a global market value of about $117 billion in 2020. The work is reported in the journal&nbsp;<a href="https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanu.us2.list-manage.com%2Ftrack%2Fclick%3Fu%3D73b3c4bf45063d7aa04d62036%26id%3D7b14896d75%26e%3Da3d1be9eda&data=05%7C01%7Cjames.giggacher%40anu.edu.au%7C3aa53e73db6f43b787c308db15138a89%7Ce37d725cab5c46249ae5f0533e486437%7C0%7C0%7C638126945905851498%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=1XgS6ghU7I9hrvV%2BPDdflpeNRGWeOEOf4kRtvlLVivo%3D&reserved=0" target="_blank">Light: Science &amp; Applications</a>. &nbsp;

PhD scholar Khosro Zangeneh. Photo: Jamie Kidston/ANU

Researchers have developed new technology that could usher in the "next-generation" of thinner, higher-resolution and more energy efficient screens and electronic devices.

New tech could create cheaper and thinner flat screens

This article was first published on <a href="https://news.mit.edu/2023/vertical-stacked-color-microscopic-leds-0201" id="isPasted" rel="noopener noreferrer" target="_blank">MIT News</a>. &nbsp; &nbsp;&nbsp;<hr>Take apart your laptop screen, and at its heart you&rsquo;ll find a plate patterned with pixels of red, green, and blue LEDs, arranged end to end like a meticulous Lite Brite display. When electrically powered, the LEDs together can produce every shade in the rainbow to generate full-color displays. Over the years, the size of individual pixels has shrunk, enabling many more of them to be packed into devices to produce sharper, higher-resolution digital displays.But much like computer transistors, LEDs are reaching a limit to how small they can be while also performing effectively. This limit is especially noticeable in close-range displays such as augmented and virtual reality devices, where limited pixel density results in a &ldquo;screen door effect&rdquo; such that users perceive stripes in the space between pixels.Now, MIT engineers have developed a new way to make sharper, defect-free displays. Instead of replacing red, green, and blue light-emitting diodes side by side in a horizontal patchwork, the team has invented a way to stack the diodes to create vertical, multicolored pixels.Each stacked pixel can generate the full commercial range of colors and measures about 4 microns wide. The microscopic pixels, or &ldquo;micro-LEDs,&rdquo; can be packed to a density of 5,000 pixels per inch.&ldquo;This is the smallest micro-LED pixel, and the highest pixel density reported in the journals,&rdquo; says Jeehwan Kim, associate professor of mechanical engineering at MIT. &ldquo;We show that vertical pixellation is the way to go for higher-resolution displays in a smaller footprint.&rdquo;&ldquo;For virtual reality, right now there is a limit to how real they can look,&rdquo; adds Jiho Shin, a postdoc in Kim&rsquo;s research group. &ldquo;With our vertical micro-LEDs, you could have a completely immersive experience and wouldn&rsquo;t be able to distinguish virtual from reality.&rdquo;The team&rsquo;s results are&nbsp;<a href="https://www.nature.com/articles/s41586-022-05612-1" target="_blank">published today</a> in the journal&nbsp;Nature. Kim and Shin&rsquo;s co-authors include members of Kim&rsquo;s lab, researchers around MIT, and collaborators from Georgia Tech Europe, Sejong University, and multiple universities in the U.S, France, and Korea.<h2>Placing pixels</h2>Today&rsquo;s digital displays are lit through organic light-emitting diodes (OLEDs) &mdash; plastic diodes that emit light in response to an electric current. OLEDs are the leading digital display technology, but the diodes can degrade over time, resulting in permanent burn-in effects on screens. The technology is also reaching a limit to the size the diodes can be shrunk, limiting their sharpness and resolution.&nbsp;<iframe id="63c8866da848540008f1a233" width="250" class="specs-iframe" height="400" src="https://wevolver.com/iframe/specs/maxim-integrated-max25603evkit-evaluation-kit?iframe=true" frameborder="0" scrolling="true" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>For next-generation display technology, researchers are exploring inorganic micro-LEDs &mdash; diodes that are one-hundredth the size of conventional LEDs and are made from inorganic, single-crystalline semiconducting materials. Micro-LEDs could perform better, require less energy, and last longer than OLEDs.But micro-LED fabrication requires extreme accuracy, as microscopic pixels of red, green, and blue need to first be grown separately on wafers, then precisely placed on a plate, in exact alignment with each other in order to properly reflect and produce various colors and shades. Achieving such microscopic precision is a difficult task, and entire devices need to be scrapped if pixels are found to be out of place.&ldquo;This pick-and-place fabrication is very likely to misalign pixels in a very small scale,&rdquo; Kim says. &ldquo;If you have a misalignment, you have to throw that material away, otherwise it could ruin a display.&rdquo;<h2>Color stack</h2>The MIT team has come up with a potentially less wasteful way to fabricate micro-LEDs that doesn&rsquo;t require precise, pixel-by-pixel alignment. The technique is an entirely different, vertical LED approach, in contrast to the conventional, horizontal pixel arrangement.Kim&rsquo;s group specializes in developing techniques to fabricate pure, ultrathin, high-performance membranes, with a view toward engineering smaller, thinner, more flexible and functional electronics. The team previously developed a method to grow and peel away perfect, two-dimensional, single-crystalline material from wafers of silicon and other surfaces &mdash; an approach they call 2D material-based layer transfer, or 2DLT.In the current study, the researchers employed this same approach to grow ultrathin membranes of red, green, and blue LEDs. They then peeled the entire LED membranes away from their base wafers, and stacked them together to make a layer cake of red, green, and blue membranes. They could then carve the cake into patterns of tiny, vertical pixels, each as small as 4 microns wide.&ldquo;In conventional displays, each R, G, and B pixel is arranged laterally, which limits how small you can create each pixel,&rdquo; Shin notes. &ldquo;Because we are stacking all three pixels vertically, in theory we could reduce the pixel area by a third.&rdquo;As a demonstration, the team fabricated a vertical LED pixel, and showed that by altering the voltage applied to each of the pixel&rsquo;s red, green, and blue membranes, they could produce various colors in a single pixel.&ldquo;If you have a higher current to red, and weaker to blue, the pixel would appear pink, and so on,&rdquo; Shin says. &ldquo;We&rsquo;re able to create all the mixed colors, and our display can cover close to the commercial color space that&rsquo;s available.&rdquo;The team plans to improve the operation of the vertical pixels. So far, they have shown they can stimulate an individual structure to produce the full spectrum of colors. They will work toward making an array of many vertical micro-LED pixels.&ldquo;You need a system to control 25 million LEDs separately,&rdquo; Shin says. &ldquo;Here, we&rsquo;ve only partially demonstrated that. The active matrix operation is something we&rsquo;ll need to further develop.&rdquo;&ldquo;For now, we have shown to the community that we can grow, peel, and stack ultrathin LEDs,&rdquo; Kim says. &ldquo;This is the ultimate solution for small displays like smart watches and virtual reality devices, where you would want highly densified pixels to make lively, vivid images.&rdquo;This research was supported, in part, by the U.S. National Science Foundation, the U.S. Defense Advanced Research Projects Agency (DARPA), the U.S. Air Force Research Laboratory, the U.S. Department of Energy, LG Electronics, Rohm Semiconductor, the French National Research Agency, and the National Research Foundation in Korea.<hr>&quot;Reprinted with permission of&nbsp;<a href="https://news.mit.edu/2023/vertical-stacked-color-microscopic-leds-0201" id="isPasted" rel="noopener noreferrer" target="_blank">MIT News</a>&quot; &nbsp;

MIT engineers have developed a new way to make sharper, defect-free displays. Instead of patterning red, green, and blue diodes side by side in a horizontal patchwork, the team has invented a way to stack the diodes to create vertical, multicolored pixels. Image: Illustration by Younghee Lee

Stacking light-emitting diodes instead of placing them side by side could enable fully immersive virtual reality displays and higher-resolution digital screens.

Engineers invent vertical, full-color microscopic LEDs

Prof. Becky Peterson led a team that has developed a scalable, manufacturable method for developing thin film transistors (TFTs) that operate at the lowest possible voltage. This is particularly important for TFT integration with today&rsquo;s silicon complementary metal-oxide semiconductors (CMOS), which are used in the vast majority of integrated circuits.&ldquo;We&rsquo;re essentially developing a less complicated device that operates at lower voltage,&rdquo; said ECE PhD student Tonglin (Tanya) Newsom, who is first author on the paper. &ldquo;With this steep sub-threshold swing device, we could significantly lower the energy dissipation of our circuitry, meaning there&rsquo;s less energy loss. This could help everyone who uses electronic devices.&rdquo;TFTs enable the operation of modern displays, acting as switches that control the light at each individual pixel. Switching efficiently between the on and off states allows for lower voltage operation, and results in a more energy-efficient system. One of the key manufacturing challenges to achieving a highly efficient switch is the requirement of a super clean interface between the different material layers within the TFT. A clean interface means electrons can flow in the channel to turn the pixel &ldquo;on&rdquo; or &ldquo;off&rdquo; without getting trapped.&ldquo;The key technology that we were trying to develop is an ultra-clean interface between the semiconductor and the gate insulator, so the switching process between the on and off states would be very sharp,&rdquo; Peterson said. &ldquo;We were able to achieve switching at the fastest rate possible, according to fundamental physical limits at room temperature.&rdquo;To do this, Peterson&rsquo;s group partnered with Mechanical Engineering Professor<a href="https://dasgupta.engin.umich.edu/" target="_blank">&nbsp;</a><a href="https://dasgupta.engin.umich.edu/" target="_blank">Neil Dasgupta</a>, with whom they developed an atomic layer deposition technology for zinc tin oxide, which is a wide bandgap semiconductor that can be used for electronic and energy devices, such as TFTs, versatile sensors, and solar cells. Peterson&rsquo;s team took this technology one step further by directly integrating the atomic layer deposition processes for two different key parts of the transistor &ndash; the gate capacitor and the semiconductor channel.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1674827663044-EDL-chip-content.jpg" style="width: 766px;" class="fr-fic fr-dib">Microchip containing thin film transistors having record sub-threshold slope, made using the in situ atomic layer deposition process. Photo by Silvia Cardarelli, Michigan ECE.&nbsp;&ldquo;We get the best results by depositing the two layers back-to-back within the same tool, without breaking vacuum,&rdquo; Peterson said. &ldquo;This approach demonstrates a straightforward method to achieve optimal TFT performance.&rdquo;Peterson&rsquo;s team focused on amorphous oxide semiconductors, which are a category of materials that are commercialized in displays and enable us to individually control pixels. They&rsquo;re important for achieving low power operation, high pixel density screens, touch screens, and haptic displays, which generate tactile effects &ndash; such as vibrations &ndash; for a variety of applications, including wearables and AR/VR devices.&ldquo;I truly hope more people will become interested in doing this kind of fundamental science and engineering,&rdquo; Newsom said &ldquo;because this work is not just about discovering new possibilities &ndash; it&rsquo;s about improving technology to help the world.&rdquo;&quot;This work is not just about discovering new possibilities &ndash; it&rsquo;s about improving technology to help the world.&quot; Tanya Newsom<hr>The research, &ldquo;<a href="https://ieeexplore.ieee.org/document/9937198" target="_blank">59.9 mV&middot;dec-1 Subthreshold Swing Achieved in Zinc Tin Oxide TFTs With In Situ Atomic Layer Deposited Al2O3 Gate Insulator</a>,&rdquo; was selected as an Editor&rsquo;s Pick in IEEE Electron Device Letters. In addition to Newsom, Peterson, and Dasgupta, co-authors include ECE PhD alum Christopher Allemang and Mechanical Engineering PhD student Tae H. Cho.Fabrication work was accomplished in the<a href="https://lnf.umich.edu/" target="_blank">&nbsp;</a><a href="https://lnf.umich.edu/" target="_blank">Lurie Nanofabrication Facility</a>.

ECE PhD student Tonglin (Tanya) Newsom in the lab of Prof. Becky (R.L.) Peterson, measuring the electrical characteristics of thin film transistors. Photo by Silvia Cardarelli, Michigan ECE.

Led by Prof. Becky Peterson, the research focuses on a category of materials important for low power logic operations, high pixel density screens, touch screens, and haptic displays.

Scalable method to manufacture thin film transistors achieves ultra-clean interface for high performance, low-voltage device operation

Novel fluorescent dyes developed by ETH researchers are relatively simple and inexpensive to produce. The dyes are polymers with a modular structure. They consist of a different number of subunits depending on their colour. The subunits used are chemically simple molecules that are either commercially available or can be produced by chemists in one reaction step.Now, scientists led by Yinyin Bao have succeeded in using the new approach to produce a wide range of colours, including red, which was previously difficult to produce. Bao is a senior scientist in the groups of ETH professors Jean-Christophe Leroux and Chih-Jen Shih. Together with scientists from RMIT University in Melbourne, the team developed artificial intelligence algorithms that help decide which molecule subunits are needed in what numbers for a particular colour.Potential applications for the fluorescent inks include UV-activated security inks for banknotes, certificates, passports or for encrypting information. The method can also be used to produce inks that change colour after prolonged UV illumination. In their new work, which the scientists published in the scientific journal Chem, they demonstrated this using the example of two initially red fluorescent inks, one of which turns blue after several minutes of UV illumination, while the other remains red. This property can also be used for security features.Other applications for the new fluorescent molecules are in solar power plants, or they could one day be combined with semiconducting molecules to produce low-cost organic light-emitting diodes (OLEDs) for displays.<h2>Reference</h2>Ye S, Meftahi N, Lyskov I, Tian T, Kumar S, Christofferson AJ, Winkler DA, Shih CJ, Russo S, Leroux JC, Bao Y: Machine Learning-Assisted Exploration of a Versatile Polymer Platform with Charge Transfer-Dependent Full-Color Emission, Chem, 2 January 2023. doi: <a href="https://dx.doi.org/10.1016/j.chempr.2022.12.003" target="_blank">external page10.1016/j.chempr.2022.12.003call_made</a>

Polymer fluorescent inks can now also be produced in red. (Photograph: ETH Zurich)

ETH researchers have developed a modular system for the simple and inexpensive production of security inks. It is based on polymers and could also be used in solar power plants and screens in the future.

Entire colour palette of inexpensive fluorescent dyes

It was a simple idea &mdash; maybe even too simple to work.Research scientist James Ponder and a team of Georgia Tech chemists and engineers thought they could design a transparent polymer film that would conduct electricity as effectively as other commonly used materials, while also being flexible and easy to use at an industrial scale.They&rsquo;d do it by simply removing the nonconductive material from their conductive element. Sounds logical, right?The resulting process could yield new kinds of flexible, transparent electronic devices &mdash;&nbsp;things like wearable biosensors, organic photovoltaic cells, and virtual or augmented reality displays and glasses.&ldquo;We had this initial idea that we have a conductive element that we&#39;re covering with a nonconductive material, so what if we just get rid of that,&rdquo; said Ponder, who earned a Ph.D. in chemistry at Georgia Tech and returned as a research scientist in mechanical engineering. &ldquo;It&#39;s a simple idea, and there were so many points where it could have failed for different reasons. But it does work, and it works better than we expected.&rdquo;To make a plastic film that can carry an electric charge, chemists start with a known polymer backbone &mdash; in this case, a popular polymer called PEDOT that&rsquo;s used in industry in certain formulations. It&rsquo;s great for conducting electricity, but difficult to use in its bare form because it&rsquo;s insoluble. However, when side chains are added to the PEDOT, it can be dissolved and used like a printable ink or a spray paint. That makes it easy to use and apply. Unfortunately, those side chains are essentially waxy material, and wax isn&rsquo;t so great at electrical conductivity.&ldquo;If you think about electrical conductivity, imagine a copper wire: it&#39;s nice and conductive. Then you cover it with wax, and it&#39;s not as conductive; you have a barrier,&rdquo; Ponder said. &ldquo;The idea was, we really want both: we want the side chains for processing, but we don&#39;t want them in our final material. So, we add side chains that, once we&#39;re done with the processing, we can knock off and wash away.&rdquo;In other words, Ponder and his collaborators create the polymer with side chains, print or spray it to apply, chemically cleave the side chains, and wash them away with common industrial solvents. After a final conversion step, the result is a flexible, highly conductive film that&rsquo;s stable and now impervious to water or other solvents.The research team spanned mechanical engineering, chemistry and biochemistry, and materials science and engineering. They&rsquo;ve published their work in a pair of studies this year in two prestigious chemistry journals: first <a href="https://doi.org/10.1021/jacs.1c11558" target="_blank">describing the idea and proving it could work in the Journal of the American Chemical Society</a>, and in recent weeks, <a href="https://doi.org/10.1002/anie.202211600" target="_blank">optimizing the design for maximum conductivity in a study in the top German chemistry journal, Angewandte Chemie.</a><img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1670396326668-PEDOT-polymer-both.webp" style="width: 766px;" class="fr-fic fr-dib">Left: A bluish, semi-transparent strip of the PEDOT polymer before the final processing step. Right: The flexible, highly conductive PEDOT(OH) polymer after the final doping. (Photos Courtesy: James Ponder)&nbsp;&ldquo;This idea that we&#39;ve come up with a way to make a polymer that has a conductivity of over 1,000 siemens per centimeter, that is able to be processed using simple industrial printing methods and solvents that the industrial people like, and that, on top of conductivity, have this optical transparency is just so expansive to me,&rdquo; said&nbsp;<a href="https://chemistry.gatech.edu/people/john-reynolds" target="_blank">John Reynolds</a>, professor in the&nbsp;<a href="https://chemistry.gatech.edu/" target="_blank">School of Chemistry and Biochemistry</a> and one of the co-authors of the two papers. &ldquo;I just get very excited about it.&rdquo;Reynolds was Ponder&rsquo;s Ph.D. advisor. When Ponder came back to Georgia Tech after a postdoctoral fellowship, he joined Associate Professor&nbsp;<a href="https://www.me.gatech.edu/faculty/yee" target="_blank">Shannon Yee&rsquo;s</a> lab in the&nbsp;<a href="https://www.me.gatech.edu/" target="_blank">George W. Woodruff School of Mechanical Engineering</a>. Because of those connections, Ponder became the bridge that cemented the collaboration. The team developed the molecules through chemistry. They measured their effectiveness with engineering.&ldquo;James basically put his feet in both camps and served as the conduit between the groups,&rdquo; said Reynolds, who also is jointly appointed in the School of Materials Science and Engineering. &ldquo;This multidisciplinary approach to research is the reason I made the move to Georgia Tech 11 years ago. I was excited about the ability to easily cross between the College of Sciences and the College of Engineering. Having collaborations such as these with Shannon are really important, and this is what makes Georgia Tech tick.&rdquo;The team already is attracting attention for their material, which they call PEDOT(OH). They have a patent application in process and are meeting with industry collaborators interested in licensing the technology because of a few key advantages of the films.<blockquote>&ldquo;These polymers we&rsquo;ve designed are mechanically flexible. There&#39;s an entire area called bioelectronics, where people are putting electronic devices onto skin and into implantable devices, where mechanical flexibility is very important. That&#39;s where these kinds of materials will shine.&rdquo;JOHN REYNOLDS Professor, School of Chemistry and Biochemistry</blockquote>One of the most widely used transparent conductors for flat panel displays, photovoltaics, smart windows, and other applications is indium tin oxide. However, the material has some drawbacks, Reynolds said.&ldquo;It is quite difficult to make curved and flexible devices using indium tin oxide because it&rsquo;s a brittle material that cracks,&rdquo; he said. &ldquo;These polymers we&rsquo;ve designed are mechanically flexible. There&#39;s an entire area called bioelectronics, where people are putting electronic devices onto skin and into implantable devices, where mechanical flexibility is very important. That&#39;s where these kinds of materials will shine.&rdquo;Another advantage? Indium tin oxide must be used in thin films to balance how it&rsquo;s prepared with maximum conductivity and transparency. The Georgia Tech team&rsquo;s material, on the other hand, can be easily processed to thick films that maintain their conductivity.&ldquo;One real benefit here is that you have a lot of control over how you process the material,&rdquo; said Ponder, who now works for the U.S. Air Force Research Laboratory. &ldquo;Industrially, the biggest benefit to this in my mind is that if you want a 20-nanometer film, you can do that. Or if you want a one-micron film&nbsp;&mdash; which is 500 times thicker &mdash; you can do that, too. You really have a lot more control.&rdquo;Ponder said other researchers have experimented with breaking off the side chains of polymers to boost conductivity, but their work usually only removed a few of the chains. Plus, that process wasn&rsquo;t the main thrust of their research.&ldquo;It&#39;s combining the right type of polymer backbone with the right type of breakable linkage for the right application&rdquo; &mdash; high electrical conductivity, in this case, Ponder said. &ldquo;For the most part, other researchers haven&rsquo;t been doing this; they didn&rsquo;t cleave off enough chains and use a well-designed backbone.&rdquo;Reynolds said the simplicity of the team&rsquo;s polymer was key: &ldquo;Being able to make a very simple, straightforward backbone to this polymer is what really has led to the high level of conductivity.&rdquo;

The polymer created by Georgia Tech researchers is initially a bluish tint when it's cast as a film and not transparent. Further processing results in a flexible, highly conductive, transparent plastic. (Photo Courtesy: James Ponder)

Chemists and engineers collaborate on process that washes away nonconductive side chains from a robust polymer backbone to create a powerful conductive plastic.

Going Back to Basics Yields a Printable, Transparent Plastic That's Highly Conductive

XTPL developed an unique an totally new approach of printing functional materials with unparallel precision and repeatability. Technology called Ultra-Precise Deposition (UPD) is a nanodispensing method capable to print high density and high viscous materials with the resolution down to 1 &micro;m in feature size and with high ratio of width to height after single pass. For this method material extrusion is controlled by a pressure, which means it is not supported with high electric field. Thanks to this there are no limitation if the substrate is conductive or dielectric.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjY3OTQ0NDk1MzczLUltZy0xLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 736px;" class="fr-fic fr-dib">The combination of unique parameters of the UPD approach opens new possibilities, previously not covered by any other printing methods. First of all, the resolution of the conductive traces can reach 1/1 &micro;m L/S. What is more, similar resolution can be achieved even on complex, 3D topographies. This enables printing high density interconnections through even 90&deg; steps (for example wire-bonding alternative for chip interconnections). Two more unique features were originally tested for conductive structures, but can have a huge benefit for printing other materials like QD ink. These features are micro-bumps deposition and via/micro-cavity filling with the highest repeatability comparing to other printing methods. The biggest needs for QD Color Conversion are ultra-high resolution, possibility of using high solid content inks for high efficiency and repeatability for high homogeneity of the display.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1667944549431-Img-2.jpg" style="width: 764px;" class="fr-fic fr-dib">The precise microdots deposition capability can be used for deposition of micro-bumps for flip chip application. Currently the results presented on the slide below are made using highly-concentrated silver paste (CL85). The dot diameter is around 8 &micro;m, and the dot height is around 800 nm, which gives a relatively high aspect ratio compared to printed electronics techniques, like inkjet or EHD, and competitive repeatability. The cross-section analysis of the dots shows parabolic geometry, which, for example, simplifies the deposition of subsequent layers without the risk of cracks. A photo of Albert Einstein made of hundreds of thousands of 8 &micro;m micro-bumps presents also the stability of the process over the time.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1667944610806-Img-3.jpg" style="width: 750px;" class="fr-fic fr-dib">The ability of UPD to extrude ink with femtoliter precision allows not only to print microdots but also to fill microcavities. This feature may help manufacture high-resolution multilayers or through silicon via interconnections. To fill a microcavity, one has to precisely position the printing nozzle and dispense a certain amount of material. In the slide below you can see microcavity with a depth of 10 &mu;m and a diameter of 25 &mu;m, which we uniformly fill with a silver paste (85 wt.% of solid content). Similar capabilities of precise microcavity filling can be used in deposition of QD material into structures like photoresist windows or nanorings to significantly increase color conversion efficiency (CCE). Compared to other digital additive manufacturing techniques like inkjet and EHD, UPD technology allows to fill the structures with high uniformity and repeatability with use of inks with higher concentration of QDs. Combination of unique capabilities of the UPD printing method provides a solution for efficient fabrication of QD color conversion for next generation Micro-LED displays.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1667944645211-Img-4.jpg" style="width: 751px;" class="fr-fic fr-dib">

A unique new approach of printing functional materials with unparallel precision and repeatability. Technology called Ultra-Precise Deposition (UPD) is a nanodispensing method capable to print high density and high viscous materials with the resolution down to 1 µm in feature size and with high ratio of width to height after single pass. For this method material extrusion is controlled by a pressure, which means it is not supported with high electric field. Thanks to this there are no limitation if the substrate is conductive or dielectric.