TECHNOLOGY FOR CHANGE

Bottom-Up Nanomaterials Enable Optical Circuits, Invisibility Cloaks

R. Colin Johnson | Date: 06-17-10 | Comments

Engineers want to move from top-down methods of building optical circuits today to a bottom-up approach that would enable a new era of molecular-sized optical circuits, including Harry Potter-like invisibility cloaks.

Engineers want to move from today's top-down methods of building optical circuits to a bottom-up approach that would enable a new era of molecular-sized optical circuits including Harry Potter-like invisibility cloaks.

Applied physicists are creating the building blocks for a new class of optical circuits inspired by nature and enabling customizable optical properties at the nanoscale, such as sensors that can identify individual molecules and invisibility cloaks that divert visible light around objects.

Today, optical circuitry for communications, sensors and the "metamaterials" that enable invisibility cloaks (at radar and infrared frequencies) are fashioned using a top-down methodology. This method starts with a bulk material and then sculpts out the needed structures using masking and etching. Now researchers are experimenting with a bottom-up approach that starts with individual molecules and then uses self-assembly techniques inspired by nature to fabricate molecular-scale patterns impossible to achieve from the top down.

The researchers at Harvard University include doctoral candidate Jonathan Fan working under Harvard physics professors Federico Capasso and Vinothan Manoharan in collaboration with scientists at Rice University, the University of Texas at Austin and the University of Houston. These scientists have taken the first steps toward these tiny optical circuits by precipitating clusters of nanospheres out of liquid compounds, which exhibit the customizable properties needed for advanced optical applications.

Materials that operate at optical frequencies, such as cloaks that divert visible light around objects, have been difficult to construct using traditional top-down methods. This difficulty arises because the patterns necessary to divert visible light must be cast at nanoscale sizes smaller than the wavelength of light being cloaked. Traditional lithographic techniques cannot reach these resolutions even using the most expensive fabrication equipment and cleanrooms.

The researchers' technique sidesteps these traditional constraints by harnessing methods whereby nanoscale structures self-assemble out of liquid solutions. To form the tiny structures, the researchers coated tiny particles with polymers, then disolved them in solutions that were placed on a water-repellent surface. During the evaporation process, the particles packed together into clusters of the desired shape and size. Using polymer spacers between these nanoparticles, the researchers were able to demonstrate repeatable patterns with resolutions as fine as 2 nanometers, a scale unachievable with traditional top-down approaches.

Schematics of two types of optical circuits: The three-particle trimer functions as a nanoscale magnet, while the seven-particle heptamer exhibits almost no scattering for a narrow range of wavelengths due to interference.

For the current demonstration, the researchers created two types of patterns: "trimers" and "heptamers." Trimers�three-sided particles that exhibit a magnetic response�could be used to craft invisibility cloaks that work in the ordinary visible light wavelengths, unlike top-down cloaks that only work at radar and microwave frequencies. Heptamers, on the other hand, could be used to create intense electric fields in nanometer-sized regions capable of trapping, manipulating and identifying substances from samples as small as individual molecules.

Next, the researchers want to create a "tool kit" of techniques for fabricating massive arrays of such materials so that their nanoscale optical properties can be used at the macroscale�the "holy grail" of materials science.