Imagine creating novel devices withamazing and exotic optical properties not found in nature — by simplyevaporating a droplet of particles on a surface.By chemically building clusters of nanospheres from a liquid, a team ofHarvard researchers, in collaboration with scientists at RiceUniversity, the University of Texas at Austin, and the University ofHouston, has developed just that.The finding, published in the latest issue of Science, demonstratessimple scalable devices that exhibit customizable optical propertiessuitable for applications ranging from highly sensitive sensors anddetectors to invisibility cloaks.Using particles consisting of concentric metallic and insulating shells,Jonathan Fan, a graduate student at the Harvard School of Engineeringand Applied Sciences (SEAS), his lead co-author Federico Capasso, RobertL. Wallace Professor of Applied Physics and Vinton Hayes Senior ResearchFellow in Electrical Engineering at SEAS, and Vinothan Manoharan,associate professor of chemical engineering and physics in SEAS andHarvard’s Physics department, devised a bottom-up, self-assemblyapproach to meet the design challenge.“A longstanding challenge in optical engineering has been to find waysto make structures of size much smaller than the wavelength that exhibitdesired and interesting properties,” says Fan. “At visible frequencies,these structures must be nanoscale.”In contrast, most nanoscale devices are fabricated using top-downapproaches, akin to how computer chips are manufactured. The smallestsizes that can be realized by such techniques are severely constrainedby the intrinsic limits of the fabrication process, such as thewavelength of light used in the process. Moreover, such methods arerestricted to planar geometries, are expensive, and require intenseinfrastructure such as cleanrooms.“With our bottom-up approach, we mimic the way nature creates innovativestructures, which exhibit extremely useful properties,” explainsCapasso. “Our nanoclusters behave as tiny optical circuits and could bethe basis of new technology such as detectors of single molecules,efficient and biologically compatible probes in cancer therapeutics, andoptical tweezers to manipulate and sort out nano-sized particles.Moreover, the fabrication process is much simpler and cheaper to carryout.”The researchers’ self-assembly method requires nothing more than a bitof mixing and drying. To form the clusters, the particles are firstcoated with a polymer, and a droplet of them is then evaporated on awater-repellent surface. In the process of evaporation, the particlespack together into small clusters. Using polymer spacers to separate thenanoparticles, the researchers were able to controllably achieve a twonanometer gap between the particles — far better resolution thantraditional top-down methods allow.Two types of resulting optical circuits are of considerable interest. Atrimer, comprising three equally-spaced particles, can support amagnetic response, an essential property of invisibility cloaks andmaterials that exhibit negative refractive index.“In essence, the trimer acts as a nanoscale resonator that can support acirculating loop of current at visible and near-infrared frequencies,”says Fan. “This structure functions as a nanoscale magnet at opticalfrequencies, something that natural materials cannot do.”Heptamers, or packed seven particle structures, exhibit almost noscattering for a narrow range of well-defined colors or wavelengths whenilluminated with white light. These sharp dips, known as Fanoresonances, arise from the interference of two modes of electronoscillations, a “bright” mode and a non-optically active “dark” mode, inthe nanoparticle.“Heptamers are very efficient at creating extremely intense electricfields localized in nanometer-size regions where molecules and nanoscaleparticles can be trapped, manipulated, and detected. Molecular sensingwould rely on detecting shifts in the narrow spectra dips,” says Capasso.Ultimately, all of the self-assembled circuit designs can be readilytuned by varying the geometry, how the particles are separated, and thechemical environment. In short, the new method allows a “tool kit” formanipulating “artificial molecules” in such a way to create opticalproperties at will, a feature the researchers expect is broadlygeneralizable to a host of other characteristics.Looking ahead, the researchers plan to work on achieving higher clusteryields and hope to assemble three-dimensional structures at themacroscale, a “holy grail” of materials science.“We are excited by the potentially scalability of the method,” saysManoharan. “Spheres are the easiest shapes to assemble as they can bereadily packed together. While we only demonstrated here planar particleclusters, our method can be extended to three-dimensional structures,something that a top-down approach would have difficulty doing.”Fan, Capasso, and Manoharan’s co-authors included Chihhui Wu and GennadyShvets of University of Texas at Austin; Jiming Bao of the University ofHouston; and Kui Bao, Rizia Bardhan, Naomi Halas, and Peter Norlander,all of Rice University.The researchers was supported by the National Science Foundation;the Air Force Office of Scientific Research; the U.S. Department ofDefense; the Robert A. Welch Foundation; and the Center for AdvancedSolar Photophysics, a U.S. Department of Energy Frontier ResearchCenter. The work was carried out at the Center for Nanoscale Systems atHarvard, a member of the National Nanotechnology Infrastructure Network.