Tag Archives: Optical Society of America

Chinese scientists develop a novel 3D fabrication technique for bio-inspired hierarchical structures

An April 14, 2016 news item on phys.org describes a new 3D fabrication technique devised by Chinese scientists,

Nature is no doubt the world’s best biological engineer, whose simple, exquisite but powerful designs have inspired scientists and engineers to tackle the challenges of technologies for centuries. Scientists recently mimicked the surface structure of a moth’s eye, a unique structure with an antireflective property, to develop a highly light-absorbent graphene material. This is breakthrough [sic] in solar cell technology. Rice leaves and butterfly wings also have unique self-cleaning surface characteristics, which inspire scientists to develop novel materials resistant to biofouling. The bio-inspired periodic multi-scale structures, called hierarchical structures, have recently caught broad attention among scientists in various applications such as solar cells, Light-emitting diodes (LEDs), biomaterials and anti-bacterial surfaces.

An April 14, 2016 Optical Society of American news release (also on EurekAlert), which originated the news item, provides more detail,

Although a number of techniques for fabricating bio-inspired hierarchical structures already exist, most conventional methods either involve complicated processes or are highly time-consuming and low cost-efficiency for industrial applications. Now, a team of researchers from Changchun University of Science and Technology, China, have developed a novel method for the rapid and maskless fabrication of bio-inspired hierarchical structures, using a technique called laser interference lithography.

Specifically, the researchers use the interference pattern of three-and four-beam lasers to fabricate ordered multi-scale surface structures on silicon substrates, with the pattern of hierarchical structures controllable by adjusting the parameters of incident light. In accordance with the theoretical and computer analysis, the researchers have experimentally demonstrated the novel technique’s potential in large-area, low-cost and high-volume 3D fabrication of micro and nanostructures. …

“We presented a flexible and direct method for fabricating ordered multi-scale 3D structures using three- and four-beam interference lithography,” said Zuobin Wang, the primary author and a professor of International Research Centre for Nano Handling and Manufacturing of China at the Changchun University of Science and Technology, China. “Compared with other patterning technologies, our method is simple and efficient in terms of obtaining bio-inspired hierarchical structures.”

Wang mentioned that for certain complicated surface structures, conventional techniques such as electron beam lithography may take several hours or a day to fabricate the pattern, while the laser interference approach only takes several minutes to generate the structure, which makes the technique suitable for high-volume industrial production.

“Laser interference lithography is a maskless patterning technique that uses the interference patterns generated from two or several coherent laser beams to fabricate micro and nanometer periodic patterns over large areas,” Wang said. Different from conventional patterning techniques like electron beam lithography, the laser interference technique enables fabricating the entire substrate surface with one single exposure or one-step lithography.

For example, in Wang’s experiment, the one-dimension multi-scale structure, that is, one-dimension oriented arrangement with the sinusoidal grooves covered with periodic line-like structures was fabricated by exposing the silicon substrate to three or four interfered beams for one time. The resultant surface pattern, though arranged in one direction, has three-dimension spatial structure. To obtain more complicated structures such as two-dimension oriented multi-scale structures, the researchers simply rotated the substrate by 90 degrees in the plane and applied second laser exposure to the surface.

“Laser interference lithography is capable of fabricating homogeneous micro and nanometer structured patterns over areas more than one square meter, which is either impossible or highly time or cost consuming for conventional techniques,” Wang said. These features make laser interference lithography superior to other techniques in terms of efficiency and high-volume production.

According to Wang, their experimental process is simple: a high power laser beam was split into three or four equal beams, which then were directed by mirrors to generate interference patterns to fabricate the surface structures. The laser parameters such as incident angle and azimuthal angle of each beam were adjusted by beam splitters and mirror positions. Other optical devices such as quarter-wave plates and polarizers were used to select the polarization mode and control the energy of laser beams.

“The laser beam parameters are selected according to the desired surface structure and corresponding interference energy distribution calculated from theoretical simulation. In other words, the shapes or patterns of hierarchical structures in our method are controllable by adjusting the parameters of each incident beams,” Wang noted.

According to Wang, the proposed technique could be used to fabricate optical or medical devices such as solar cells, antireflective coatings, self-cleaning and antibacterial surfaces and long-life artificial hip joints.

The researchers’ next step is to develop functional surface structures with controllable wettability, adhesion and reflectivity properties for optical, medical and mechanical applications.

Here’s a link to and a citation for the paper,

Bio-inspired hierarchical patterning of silicon by laser interference lithography by Yaowei Hu, Zuobin Wang, Zhankun Weng, Miao Yu, and Dapeng Wang. Applied Optics Vol. 55, Issue 12, pp. 3226-3232 (2016) doi: 10.1364/AO.55.003226

I believe this paper is behind a paywall.

The researchers have provided this image as an illustration of their concept,

 Caption: This is a Scanning Electron Microscope (SEM) image of a moth eye. Credit: Zuobin Wang/Changchun University of Science and Technology, China

Caption: This is a Scanning Electron Microscope (SEM) image of a moth eye. Credit: Zuobin Wang/Changchun University of Science and Technology, China

Fireflies and their jagged scales lead to brighter LEDs (light emitting diodes)

According to the Jan. 8, 2013 news item on ScienceDaily, scientists have used an observation about fireflies to make brighter LEDs (light emitting diodes),

The nighttime twinkling of fireflies has inspired scientists to modify a light-emitting diode (LED) so it is more than one and a half times as efficient as the original.

Researchers from Belgium, France, and Canada studied the internal structure of firefly lanterns, the organs on the bioluminescent insects’ abdomens that flash to attract mates. The scientists identified an unexpected pattern of jagged scales that enhanced the lanterns’ glow, and applied that knowledge to LED design to create an LED overlayer that mimicked the natural structure. The overlayer, which increased LED light extraction by up to 55 percent, could be easily tailored to existing diode designs to help humans light up the night while using less energy.

The Optical Society of America’s Jan. 8, 2013 news release, which originated the news item, describes how the scientists came to make their observations,

“The most important aspect of this work is that it shows how much we can learn by carefully observing nature,” says Annick Bay, a Ph.D. student at the University of Namur in Belgium who studies natural photonic structures, including beetle scales and butterfly wings.  When her advisor, Jean Pol Vigneron, visited Central America to conduct field work on the Panamanian tortoise beetle (Charidotella egregia), he also noticed clouds of twinkling fireflies and brought some specimens back to the lab to examine in more detail.

Fireflies create light through a chemical reaction that takes place in specialized cells called photocytes. The light is emitted through a part of the insect’s exoskeleton called the cuticle.  Light travels through the cuticle more slowly than it travels through air, and the mismatch means a proportion of the light is reflected back into the lantern, dimming the glow. The unique surface geometry of some fireflies’ cuticles, however, can help minimize internal reflections, meaning more light escapes to reach the eyes of potential firefly suitors.

In Optics Express papers, Bay, Vigneron, and colleagues first describe the intricate structures they saw when they examined firefly lanterns and then present how the same features could enhance LED design. Using scanning electron microscopes, the researchers identified structures such as nanoscale ribs and larger, misfit scales, on the fireflies’ cuticles. When the researchers used computer simulations to model how the structures affected light transmission they found that the sharp edges of the jagged, misfit scales let out the most light. The finding was confirmed experimentally when the researchers observed the edges glowing the brightest when the cuticle was illuminated from below.

“We refer to the edge structures as having a factory roof shape,” says Bay.  “The tips of the scales protrude and have a tilted slope, like a factory roof.” The protrusions repeat approximately every 10 micrometers, with a height of approximately 3 micrometers. “In the beginning we thought smaller nanoscale structures would be most important, but surprisingly in the end we found the structure that was the most effective in improving light extraction was this big-scale structure,” says Bay.

Here’s how the scientists applied their observations to LEDs (from the news release),

Human-made light-emitting devices like LEDs face the same internal reflection problems as fireflies’ lanterns and Bay and her colleagues thought a factory roof-shaped coating could make LEDs brighter. In the second Optics Express paper published today, which is included in the Energy Express  section of the journal, the researchers describe the method they used to create a jagged overlayer on top of a standard gallium nitride LED. Nicolas André, a postdoctoral researcher at the University of Sherbrooke in Canada, deposited a layer of light-sensitive material on top of the LEDs and then exposed sections with a laser to create the triangular factory-roof profile. Since the LEDs were made from a material that slowed light even more than the fireflies’ cuticle, the scientists adjusted the dimensions of the protrusions to a height and width of 5 micrometers to maximize the light extraction.

“What’s nice about our technique is that it’s an easy process and we don’t have to create new LEDs,” says Bay.  “With a few more steps we can coat and laser pattern an existing LED.”

Other research groups have studied the photonic structures in firefly lanterns as well, and have even mimicked some of the structures to enhance light extraction in LEDs, but their work focused on nanoscale features. The Belgium-led team is the first to identify micrometer-scale photonic features, which are larger than the wavelength of visible light, but which surprisingly improved light extraction better than the smaller nanoscale features. The factory roof coating that the researchers tested increased light extraction by more than 50 percent, a significantly higher percentage than other biomimicry approaches have achieved to date. The researchers speculate that, with achievable modifications to current manufacturing techniques, it should be possible to apply these novel design enhancements to current LED production within the next few years.

For those who care to investigate further,

Both articles (HTML version) are open access; PDF versions were not checked.

It’s a bird. It’s a plane. No, it’s a laser!

I couldn’t resist the Superman reference although it really should have been a Morpho butterfly or a jewel beetle reference since these are two other animals/insects that also display unusual optical properties courtesy of nanoscale structures.

Top: Male eastern bluebird (Sialia sialis, Turdidae). Credit: Ken Thomas (image in public domain). Published in Soft Matter, 2009, 5, 1792-1795. E.R. Dufresne et al., “Self-assembly of amorphous biophotonic nanostructures by phase separation.” Royal Society of Chemistry. http://dx.doi.org/10.1039/B902775K

According to the Oct. 12, 2011 news item on Nanowerk,

Researchers at Yale University are studying how two types of nanoscale structures on the feathers of birds produce brilliant and distinctive colors. The researchers are hoping that by borrowing these nanoscale tricks from nature they will be able to produce new types of lasers—ones that can assemble themselves by natural processes. The team will present their findings at the Optical Society’s (OSA) Annual Meeting, Frontiers in Optics (FiO) 2011, taking place in San Jose, Calif. next week. [It starts Sunday, Oct. 16, 2011.]

Devin Powell, in a May 13, 2011 article for Science News provides some additional detail,

The barbs of these feathers [from bluebirds, blue jays, and parrots] contain tiny pockets of air. Light striking the tightly packed air bubbles scatters, bringing out deep shades of blues and ultraviolet (which birds can see but humans can’t).

“Birds use these structures to create colors that they can’t make in other ways,” says Richard Prum, an  ornithologist at Yale University who discovered the mechanism behind this color.

To make a two-dimensional imitation of a bird feather, Yale physicist Hui Cao and her colleagues punched holes into a thin slice of gallium arsenide semiconductor. The holes were arranged like people in a crowd — somewhat haphazardly but with small-scale patterns that dictate roughly how far each hole is from its neighbor.

“The lesson we learned from nature is that we don’t need something perfect to get control,” says Cao, whose team describes their laser in the May 6 [2011] Physical Review Letters.

The latest work being presented is described this way in an Oct. 2011 news release (why aren’t people putting dates on their news releases????) from the Optical Society of America,

Inspired by feathers, the Yale physicists created two lasers that use this short-range order to control light. One model is based on feathers with tiny spherical air cavities packed in a protein called beta-keratin. The laser based on this model consists of a semiconductor membrane full of tiny air holes that trap light at certain frequencies. Quantum dots embedded between the holes amplify the light and produce the coherent beam that is the hallmark of a laser. The researchers also built a network laser using a series of interconnecting nano-channels, based on their observations of feathers whose beta-keratin takes the form of interconnecting channels in “tortuous and twisting forms.” The network laser produces its emission by blocking certain colors of light while allowing others to propagate. In both cases, researchers can manipulate the lasers’ colors by changing the width of the nano-channels or the spacing between the nano-holes.

What makes these short-range-ordered, bio-inspired structures different from traditional lasers is that, in principle, they can self-assemble, through natural processes similar to the formation of gas bubbles in a liquid. This means that engineers would not have to worry about the nanofabrication of the large-scale structure of the materials they design, resulting in cheaper, faster, and easier production of lasers and light-emitting devices.

Here’s an image of a ‘feather-based laser’,

Top: A laser based on feathers with the sphere-type nanostructure. This laser consists of tiny air holes (black) in a semiconductor membrane; each hole is about 77 nanometers across. (Scale bar = 5 micrometers.) Credit: Hui Cao Research Laboratory / Yale University.

As for the Morpho butterfly and jewel beetle, I last posted about gaining inspiration from these insects (biomimicry) in my May 20, 2011 posting in the context of some anti-counterfeiting strategies.

I first came across some of this work on the optical properties of nanostructures in nature in a notice about a 2008 conference on iridescence at Arizona State University. Here’s the stated purpose for the conference (from the conference page),

A unique, integrative 4–day conference on iridescent colors in nature, Iridescence: More than Meets the Eye is a graduate student proposed and organized conference supported by the Frontiers in Life Sciences program in Arizona State University’s School of Life Sciences. This conference intends to connect diverse groups of researchers to catalyze synthetic cross–disciplinary discussions regarding iridescent coloration in nature, identify new avenues of research, and explore the potential for these stunning natural phenomena to provide novel insights in fields as divergent as materials science, sexual selection and primary science education.