Macro to nanoscale imaging using planar lenses at visible wavelengths A novel fabrication process from the Capasso lab develops highly efficient planar metalenses that allow for diffraction-limited focusing, unprecedented imaging quality, and the ability to resolve chirality. The bulky nature of optical components has long hindered their convenient integration into our daily lives. Digital single-lens reflex (DSLR) cameras, for example, require multiple sets of bulky lenses that must be interchanged depending on the desired imaging parameters. Analogous to the miniaturization of silicon chips that has revolutionized electronic devices over the past few decades, lightweight and planar optical components are key factors for enabling the development of easy-to-carry or wearable devices. Traditionally, optical devices shape the wavefront of light via propagation through a bulky medium, like a lens. Furthermore, to improve the image quality or extract additional information (e.g., polarization), optical components must be cascaded, resulting in heavy and bulky systems. In this scenario, metasurfaces1–4—which enable wavefronts to be controlled without requiring volumetric propagation—open up a new frontier in optics. Multifunctional, high-performance metasurfaces have been reported, but the materials that are commonly used (i.e., noble metals5 or silicon6, 7) are lossy in the visible spectrum. As a result, practical demonstrations in this region are lacking. We have recently developed a fabrication approach based on the atomic-layer deposition (ALD) of titanium dioxide (TiO ), yielding high-aspect-ratio nanostructures with negligible material and scattering losses.8 With this TiO -based metasurface platform,8–11 we have fabricated metalenses that operate at visible wavelengths with unprecedented imaging quality.9 Our metalenses are also capable of resolving chirality, thereby opening up potential applications in drug development, where distinguishing isomeric (i.e., chiral) compounds is essential.10 The building block of our metalenses—a TiO nanofin, deposited on a glass substrate—is shown in Figure 1(A) and (B). The phase of circularly polarized incident light is controlled by means of the Pancharatnam-Berry phase12, 13 via rotation of the nanofins: see Figure 1(B). The metalens operates in transmission mode and focuses collimated incident light onto a spot: see Figure 1(C).9 We fabricated three distinct metalenses, with numerical apertures (NAs) of 0.8, to operate at wavelengths of 660, 532, and 405nm (i.e., red, green, and blue light, respectively). A scanning-electron-microscope image of a fabricated metalens is shown in Figure 1(D), and the measured focal spots are shown in Figure 2(A–C). Corresponding vertical cuts of these focal spots—see Figure 2(D–F)—reveal symmetric and subwavelength-focusing features. Full width at half-maximums (FWHMs) of the focal spots for these metalenses are 280, 375, and 450nm, respectively. We also achieved measured efficiencies of 86, 73, and 66% at the three respective wavelengths. Figure 1. Metalens design and fabrication. ) nanofin on a glass substrate. Phase is imparted via the rotation of each nanofin by the angle θ. (C) The metalens, composed of arrays of rotated nanofins, focuses collimated incident light to a spot. (D) Scanning electron microscope (SEM) image of a portion of a metalens. Scale bar: 600nm. Metalens design and fabrication. 9 (A) Side view and (B) top view of our metalens building block, a titanium dioxide (TiO) nanofin on a glass substrate. Phase is imparted via the rotation of each nanofin by the angle θ. (C) The metalens, composed of arrays of rotated nanofins, focuses collimated incident light to a spot. (D) Scanning electron microscope (SEM) image of a portion of a metalens. Scale bar: 600nm. Figure 2. Diffraction-limited focal-spot intensity profiles for three metalenses with design wavelengths of (A) 660, (B) 532, and (C) 405nm, Diffraction-limited focal-spot intensity profiles for three metalenses with design wavelengths of (A) 660, (B) 532, and (C) 405nm, 9 and (D–F) the corresponding full width at half-maximum (450, 375, and 280nm, respectively). All metalenses have the same diameter (240μm) and focal length (90μm), resulting in a numerical aperture (NA) of 0.8. To demonstrate the imaging capability of our design, we fabricated a metalens with a diameter of 2mm and focal length of 0.725mm. Figure 3(A) shows images that we obtained using the metalens. The smallest features on the resolution chart (lines with widths and center-to-center distances of 2.2 and 4.4μm, respectively) are well resolved. We repeated this experiment at different wavelengths. Figure 3(B–E) shows that the metalens can resolve these micron-sized lines across the visible range. We also fabricated arrays of nanoscale holes—see Figure 3(F)—by focused ion beam to further test the resolving power of our design. The metalens formed a clear image of the H-shaped object with all holes resolved, as shown in Figure 3(G). Moreover, the quality of this image is comparable to that formed by a commercial objective lens: see Figure 3(H). We tested the resolution limit of the metalens by imaging four holes with a subwavelength gap of ∼450nm: see Figure 3(I). Figure 3. Images obtained with a metalens Images obtained with a metalens 9 of diameter 2mm and focal length 0.725mm. The metalens is designed to operate at a wavelength of 532nm. (A) Image of the 1951 United States Air Force (USAF) resolution chart (Thorlabs Inc.) formed by the metalens at a wavelength of 530nm. Scale bar: 40μm. Images of the smallest feature size—the highlighted region in Figure 3(A)—on the USAF resolution chart at wavelengths of (B) 480, (C) 530, (D) 590, and (E) 620nm. Scale bars: 5μm. The differences in the image size occur as a result of the wavelength-dependent focal length. For example, a magnification of 167× is obtained at a wavelength of 620nm, compared to a magnification of 138×at 532nm. (F) SEM image of the nanoscale H-shaped target (fabricated by a focused ion beam). The minimum gap between two neighboring holes is ∼800nm. Image of the H-shaped target formed by (G) the metalens and (H) a commercial objective with the same NA (0.8). Scale bars: 10μm. (I) Image showing the ability of the metalens to resolve nanoscale holes with subwavelength gaps of 450nm. Scale bars: 500nm. Additionally, we have realized a chiral metalens (CML)—which forms two images with opposite helicities—by exploiting the circular-polarization sensitivity of the nanofins.10 The CML is composed of interlaced arrays of two nanofin arrangements, shown in Figure 4(A–C). To determine its performance, we used the CML in a series of imaging experiments. First, we imaged a facet of a single-mode fiber. To control the polarization state of the incident beam, we placed a linear polarizer and a quarter-waveplate between the fiber and the CML. The fiber was pumped by a tunable laser with a center wavelength and bandwidth of 550 and 100nm, respectively. The images, captured by a color camera—see Figure 4(D–F)—show that the metalens can simultaneously focus and disperse the broadband incident light. The rainbow-like images are obtained as a result of the dispersion-engineering design of the CML (accomplished by suitably selecting the off-axis focusing angle of the CML). As is expected for linearly polarized light (i.e., the result of superposition between left-circularly polarized and right-circularly polarized components with equal intensities), two identical images are formed within the camera's field of view. Figure 4. Multispectral chiral imaging. Multispectral chiral imaging. 10 (A) The building block of the chiral metalens (CML), consisting of two nanofins on a glass substrate. SEM images showing (B) the top view and (C) the side view of the fabricated CML. Of the two false-colored interlaced arrays of nanofins, the blue nanofins impart the phase required to focus incident right-circularly polarized light, while the green nanofins focus left-circularly polarized light. Scale bars: 600nm. (D) The CML forms two images of a single-mode fiber facet in the field of view of a color camera. Light is linearly polarized. (E) Image formed by the CML when the polarization state is left-circularly polarized. (F) Image formed under right-circularly polarized illumination. Scale bars are 500μm. (G) Two side-by-side images of a beetle, formed within the same field of view by the metalens. Illumination was provided by a green LED paired with a band-pass filter with a 10nm bandwidth centered at 532nm. We examined the chiral response of the CML by changing the polarization of the incident light to left-circular. For this polarization, fading of the right-side image—see Figure 4(E)—is accompanied by an increase to the intensity of the left-side image. By switching the chirality of the incident light, the opposite effect is observed, as shown in Figure 4(F). We also used our CML to image a biologically chiral specimen (Chrysina gloriosa, a beetle with inherent circular dichroism, CD). As shown in Figure 4(G), the CML simultaneously forms two images of the beetle on the camera chip. These two images possess opposite chirality, and their contrast showcases the very strong CD of the beetle's exoskeleton. In summary, we have developed a metasurface platform for visible wavelengths based on ALD-prepared TiO . In doing so, we have fabricated high-performance metalenses with a high efficiency and NA that allow for g focusing and high-resolution imaging. Futhermore, we have demonstrated the flexibility of this platform by realizing a multifunctional metalens with a planar and compact configuration. Together with its simple fabrication (by one-step lithography), this high-performance and versatile platform could find many applications in optics, ranging from imaging and spectroscopy to laser-fabrication processes. In our next step, we plan to realize an achromatic metalens that is suitable for white-light imaging. This development will greatly extend the scope of applications from single-wavelength imaging and spectroscopy to general microscopy, as well as potential mainstream-camera imaging devices. Harvard University Federico Capasso is the Robert Wallace Professor of Applied Physics. He joined Harvard University in 2003 after 27 years at Bell Labs, where his career advanced from postdoctoral fellow to vice president for physical research. He and his group have made wide-ranging contributions to optics, photonics, and nanotechnology, including pioneering the bandgap-engineering technique, the invention of the quantum cascade laser, the first measurement of the repulsive Casimir force, and (more recently) research on metasurfaces and their applications (including flat lenses and the generalized Snell's law). He is a member of the National Academy of Sciences, the National Academy of Engineering, the American Academy of Arts and Sciences, and a foreign member of the Accademia dei Lincei. His awards include the SPIE Gold Medal, the IEEE Edison Medal, the American Physical Society Arthur L. Schawlow Prize, and the King Faisal Prize. 4. S. B. Glybovski, S. A. Tretyakov, P. A. Belov, Y. S. Kivshar, C. R. Simovski, Metasurfaces: from microwaves to visible, Phys. Rep. 634, p. 1-72, 2016. 7. A. Arbabi, Y. Horie, M. Baheri, A. Faraon, Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission, Nat. Nanotechnol. 10, p. 937-943, 2015. 8. R. C. Devlin, M. Khorasaninejad, W. T. Chen, J. Oh, F. Capasso, High efficiency dielectric metasurfaces at visible wavelengths, arXiv 1603.02735 [physics.optics], 2016. 9. M. Khorasaninejad, W. T. Chen, R. C. Devlin, J. Oh, A. Y. Zhu, F. Capasso, Metalenses at visible wavelengths: diffraction-limited focusing and subwavelength resolution imaging, Science 352(6290), p. 1190-1194, 2016. 10. M. Khorasaninejad, W. T. Chen, A. Y. Zhu, J. Oh, R. C. Devlin, D. Rousso, F. Capasso, Multispectral chiral imaging with a metalens, Nano Lett. 16, p. 4595-4600, 2016. 11. R. C. Devlin, A. Ambrosio, D. Wintz, S. L. Oscurato, A. Y. Zhu, M. Khorasaninejad, J. Oh, P. Maddalena, F. Capasso, Spin-to-orbital angular momentum conversion in dielectric metasurfaces, arXiv 1605.03899 [physics.optics], 2016. 12. S. Pancharatnam, Generalized theory of interference and its applications, Proc. Ind. Acad. Sci. 44, p. 398-417, . 13. M. V. Berry, The adiabatic phase and Pancharatnam's phase for polarized light, J. Mod. Opt. 34, p. 1401-1407, 1987.
Antireflective coatings are used to cut surface glare in everything from eyeglasses and camera lenses to solar cells, TV screens and LED devices. Now, inspired by the eyes of moths, researchers from the Research Institute for Nuclear Problems of Belarusian State University in Belarus and Institut Jean Lamour-Université de Lorraine in France have developed a novel, low-cost, ultra-lightweight material that can act as an effective anti-reflective surface for microwave radiation. The eyes of moths are covered with a periodic, hexagonal pattern of tiny bumps smaller than the wavelength of the incident light. They act as a continuous refractive index gradient, allowing the moths to see at night and avoid nocturnal predators, like bats. This structure also makes the moth eye one of the most effective antireflective coatings in nature. It has already successfully been mimicked by scientists to produce high-performance antireflective coatings for visible light, albeit coatings that are often expensive to fabricate and difficult to customize. This new material cuts down reflections from microwaves rather than from visible light; blocking microwave reflection is important for conducting precise microwave measurements. As a consequence, the coating may be used as a radar-absorbing material in stealth technology, making an airplane invisible to radar, or in police traffic radar that uses microwaves to measure the speed of passing cars. Described in Applied Physics Letters, the new technology is based on a monolayer of hollow carbon spheres packed in two dimensions. The researchers have demonstrated that this monolayer is able to achieve almost perfect microwave absorption – near 100% absorption of microwaves in the Ka-band (26–37 gigahertz) frequency range, the first antireflective material to achieve this. "Based on the experimental and modeling results, we found that using hollow carbon spheres with larger spherical diameters and optimal shell thickness it is possible to achieve almost perfect microwave absorption," said Dzmitry Bychanok, the primary author and a researcher at the Research Institute for Nuclear Problems of Belarusian State University in Belarus. The novel coating material they produced can also be completely derived from biological resources, he added, which may make it greener, lower-cost, easier to fabricate and ultra-lightweight compared to conventional antireflective coatings. Hollow carbon spheres with a uniform diameter can be used to produce ordered periodic structures. To mimic the structure of moth eyes, the researchers compactly packed the hollow carbon spheres in two dimensions to form a hexagonal-patterned monolayer. This monolayer can then act as a strong, electrically conductive coating material. "You can picture the geometry of the hollow sphere monolayer as that of Christmas cake decoration balls compactly filled in a Petri dish – filling a flat surface with identical balls will lead to a spontaneous hexagonal self-ordering," Bychanok explained. "The spatial distribution of the hollow sphere monolayer is ideally hexagonal, but in practice it is more in-between cubic and hexagonal. The thickness of the monolayer is in the range of one to two millimeters." In the experiment, carbon hollow spheres were fabricated by a template method that utilized fish eggs or sugar-based polymer beads with certain diameters. Specifically, the researchers coated the bio-based template spheres with sugar, then ‘pyrolysed’ them – a chemical modification that involves thermally decomposing the resultant spheres in an inert atmosphere. This heating converts the sugar coating into char, while the inner template sphere is largely destroyed and decomposed into gas, leaving a hollow carbon sphere. Using theoretical modeling based on long-wave approximation and experimental measurements, the team studied the electromagnetic properties of monolayers produced by hollow spheres with different parameters, focusing on the Ka-band (microwave) frequency. Their results showed that, for electromagnetic applications requiring high absorption, the most effective hollow spheres are those with larger radii or diameters. Additionally, each value of hollow sphere radius has an optimum shell thickness to achieve the highest absorption coefficient. "Our study showed that the monolayer formed by spheres with a radius of 6mm and a shell thickness of about 5µm enables the highest microwave absorption coefficient, which is more than 95% at 30 gigahertz," said Bychanok. Bychanok said the work pointed out that moth-eye-like two-dimensional ordered structures based on hollow conducting spheres are promising systems for microwave radiation absorption applications. The team's next step is to investigate and develop three-dimensional periodic structures that can effectively manipulate microwave radiations. This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
News Article | August 24, 2016
Charles McLaren, a Ph.D. candidate in materials science and engineering at Lehigh University, arrived last fall for a semester of research at the University of Marburg in Germany with his language skills lagging significantly behind his scientific prowess. “It was my first trip to Germany, and I barely spoke a word of German,” he confesses. With the help of his new German colleagues, he got past the point-and-eat phase of the international experience in no time. “The group members there were very welcoming. They showed me around and helped me learn enough vocabulary to order some food, at least.” The main purpose of McLaren’s exchange study in Marburg was far from culinary, however. He was there to learn more about a complex process involving transformations in glass that occur under intense electrical and thermal conditions. New understanding of these mechanisms could lead the way to more energy-efficient glass manufacturing, and even glass supercapacitors that leapfrog the performance of batteries now used for electric cars and solar energy. “This technology is relevant to companies seeking the next wave of portable, reliable energy,” says Himanshu Jain, the T. L. Diamond Distinguished Chair in Materials Science and Engineering at Lehigh and director of its International Materials Institute for New Functionality in Glass. “A breakthrough in the use of glass for power storage could unleash a torrent of innovation in the transportation and energy sectors, and even support efforts to curb global warming.” In his doctoral research, McLaren discovered that applying a direct current field across glass reduced its melting temperature. In lab experiments, he and Jain placed a block of glass between a cathode and anode, and then exerted steady pressure on the glass while gradually heating it. Together with colleagues at the University of Colorado, the Lehigh researchers reported their results last fall in Applied Physics Letters. The implications for the finding were intriguing. In addition to making glass formulation possible at lower temperatures and reducing energy needs, designers using electrical current in glass manufacturing would have a tool to make precise manipulations not possible with heat alone. “You could make a mask for the glass, for example, and apply an electrical field on a micron scale,” says Jain. “This would allow you to deform the glass with high precision, and soften it in a far more selective way than you could with heat, which gets distributed throughout the glass.” Though McLaren and Jain had isolated the phenomenon and determined how to dial up the variables for optimal results, they did not yet fully understand the mechanisms behind it. McLaren and Jain had been following the work of Bernard Roling at the University of Marburg, who had discovered some remarkable characteristics of glass using electro-thermal poling, a technique that employs both temperature manipulation and electrical current to create a charge in normally inert glass. The process imparts useful optical and even bioactive qualities to glass. Roling invited McLaren to spend a semester at Marburg to analyze the behavior of glass under electro-thermal poling, to see if it would reveal more about the fundamental science underlying what McLaren and Jain had observed in their Lehigh lab. McLaren’s work in Marburg revealed a two-step process in which a thin sliver of the glass nearest the anode, called a depletion layer, becomes much more resistant to electrical current than the rest of the glass as alkali ions in the glass migrate away. This is followed by a catastrophic change in the layer, known as dielectric breakdown, which dramatically increases its conductivity. McLaren likens the process of dielectric breakdown to a high-speed avalanche, and uses spectroscopic analysis with electro-thermal poling as a way to see what is happening in slow motion. “The results in Germany gave us a very good model for what is going on in the electric field-induced softening that we did here. It told us about the start conditions for where dielectric breakdown can begin,” says McLaren. “Charlie’s work in Marburg has helped us see the kinetics of the process,” Jain says. “We could see it happening abruptly in our experiments here at Lehigh, but we now have a way to separate out what occurs specifically with the depletion layer.” McLaren, Jain, Roling, and his Marburg team members published their findings in the September 2016 issue of the Journal of Electrochemical Society. “The Marburg trip was incredibly useful professionally and enlightening personally,” says McLaren. “Scientifically, it’s always good to see your work from another vantage point, and see how other research groups interpret data or perform experiments. The group in Marburg was extremely hard-working, which I loved, and they were very supportive of each other. If someone submitted a paper, the whole group would have a barbecue to celebrate, and they always gave each other feedback on their work. Sometimes it was brutally honest — they didn’t hold back — but they were things you needed to hear.” “Working in Marburg also showed me how to interact with a completely different group of people. You see differences in your own culture best when you have the chance to see other cultures close up. It’s always a fresh perspective.”
Erica Calman and Chelsey Dorow align optics required to collect measurements from a molybdenum disulfide sample. Credit: Calman A team of physicists from the University of California, San Diego and The University of Manchester is creating tailor-made materials for cutting-edge research and perhaps a new generation of optoelectronic devices. The materials make it easier for the researchers to manipulate excitons, which are pairs of an electron and an electron hole bound to each other by an electrostatic force. Excitons are created when a laser is shone onto a semiconductor device. They can transport energy without transporting net electric charge. Inside the device the excitons interact with each other and their surroundings, and then convert back into light. This makes them attractive for new technology. Inside the device the excitons interact with each other and their surroundings, and then convert back into light that can be detected by extremely sensitive charge-coupled device (CCD) cameras. Most of the team's previous work involved structures based on gallium arsenide (GaAs), which is a material commonly used throughout the semiconductor industry. Unfortunately, the devices they've developed come with a fundamental limitation: They require cryogenic temperatures (below 100 K)—ruling out any commercial applications. So the team made a radical material change to bring their excitonic devices up to room temperature. They report their results in Applied Physics Letters. "Our previous structures were built from thin layers of GaAs deposited on top of a substrate with a particular layer thickness and sequence to ensure the specific properties we wanted," said Erica Calman, lead author and a graduate student in the Department of Physics, University of California, San Diego. To make the new devices the physicists turned to new structures built from a specially designed set of ultrathin layers of materials—molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN)—each a single atom thick. These structures are produced via the famous "Scotch tape" or mechanical exfoliation method developed by the group of Andre Geim, a physicist awarded a Nobel Prize in physics in 2010 for his groundbreaking work regarding the two-dimensional material graphene. "Our specially designed structures help keep excitons bound more tightly together so that they can survive at room temperature—where GaAs excitons are torn apart," explains Calman. Impressively, excitons can form a special quantum state known as a Bose-Einstein condensate. This state occurs within superfluids and enables currents of particles without losses. The team discovered a similar exciton phenomenon at cold temperatures with GaAs materials. "The results of our work suggest that we may be able to make new structures work all the way up to room temperature," said Calman. "We set out to prove that we could control the emission of neutral and charged excitations by voltage, temperature, and laser power ... and demonstrated just that." Explore further: Physicists find patterns in new state of matter More information: E. V. Calman et al. Control of excitons in multi-layer van der Waals heterostructures, Applied Physics Letters (2016). DOI: 10.1063/1.4943204
In the new technique, researchers start with a silicon substrate. They top that with a layer of single-crystal titanium nitride, using domain matching epitaxy to ensure the crystalline structure of the titanium nitride is aligned with the structure of the silicon. Researchers then place a layer of copper-carbon (Cu-2.0atomic percent C) alloy on top of the titanium nitride, again using domain matching epitaxy. Finally, the researchers melt the surface of the alloy with nanosecond laser pulses, which pulls carbon to the surface. If the process is done in a vacuum, the carbon forms on the surface as graphene; if it is done in oxygen, it forms GO; and if done in a humid atmosphere followed by a vacuum, it forms as rGO. In all three cases, the carbon's crystalline structure is aligned with the underlying copper-carbon alloy. "We can control whether the carbon forms one or two monolayers on the surface of the material by manipulating the intensity of the laser and the depth of the melting," says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and senior author of a paper describing the work. "The process can easily be scaled up," Narayan says. "We've made wafers that are two inches square, and could easily make them much larger, using lasers with higher Hertz. And this is all done at room temperature, which drives down the cost." Graphene is an excellent conductor, but it cannot be used as a semiconductor. However, rGO is a semiconductor material, which can be used to make electronic devices such as integrated smart sensors and optic-electronic devices. "We have already patented the technique and are planning to use it to develop smart biomedical sensors integrated with computer chips," Narayan says. The paper, "Wafer Scale Integration of Reduced Graphene Oxide by Novel Laser Processing at Room Temperature in Air," was published Sept. 9 in the Journal of Applied Physics. Explore further: New technique controls crystalline structure of titanium dioxide More information: Anagh Bhaumik et al, Wafer scale integration of reduced graphene oxide by novel laser processing at room temperature in air, Journal of Applied Physics (2016). DOI: 10.1063/1.4962210