- Nanostructured Material Exhibits Negative Refractive Index in the Visible
Daniel S. Burgess
A team of investigators at the University of Manchester in the UK, the Institute of Microelectronics Technology in Chernogolovka, Russia, and Aston University in Birmingham, UK, has reported a real part of the refractive index of −0.7 at visible frequencies of 700 THz in an array of gold nanopillars on glass. The scientists hope to develop a three-dimensional design for a lens made of the material that would produce perfect images with subwavelength accuracy.
Nearly 40 years ago, Victor G. Veselago posited that materials with a negative dielectric constant and permeability would have a negative refractive index, refracting an incident ray by the same magnitude but in the opposite direction from the normal line as a typical material. If they exist, he suggested, the “left-handed substances” — noting the fact that a left-hand equivalent of the right-hand rule would govern the direction of electric and magnetic fields with respect to the direction of propagation of an electromagnetic wave passing through the materials — would enable the design of novel optical elements, including flat-surface lenses.
Based on these predictions, John B. Pendry of Imperial College London published a series of equations in 2000 that suggested that a slab of material with a refractive index of −1 could be used to create a “superlens” with a diffraction limit of zero.
In the short time since then, numerous experiments have established the existence of negative refraction and of the ability of slabs of left-handed materials to focus electromagnetic radiation at microwave frequencies (see “Photonic Crystal Enables Flat-Lens Imaging,” January 2004, page 27). Arrays of parallel gold nanorods have exhibited a refractive index of −0.3 in the near-IR.
Alexander N. Grigorenko of the University of Manchester’s department of physics and astronomy explained that the new work demonstrates that it is possible to create media with a negative dielectric constant and permeability in the visible range. And although the absorption of the material is too high to produce a superlens, the refractive index of −0.7 at 700 THz is tantalizingly close to the −1 required. Other potential applications of visible-wavelength left-handed materials, he said, might include ultrathin narrow-bandwidth filters, high-frequency optical modulators, optical couplers, and biological and chemical sensors.
The researchers employed high-resolution electron-beam lithography to deposit pairs of the gold pillars on glass in arrays with a lattice constant of 400 nm (Figure 1). They investigated variations of the material structure and discovered that the best results were obtained from arrays of 140-nm-diameter, 80-nm-tall pillars separated by 200 nm and of 110-nm-diameter, 90-nm-tall pillars separated by 140 nm.
Figure 1. The left-handed material comprises an array of paired nanopillars. Images courtesy of Alexander N. Grigorenko.
The material’s negative dielectric constant and permeability are the result of the excitation of an antisymmetric plasmon resonance in the interacting pairs of nanopillars that depends on the orientation of the electric field component of the incident light, causing it to exhibit different plasmon resonances under TE- and TM-polarized white light (Figure 2). Grigorenko explained that electrons in the pillars vibrate in antiphase to incident light of higher frequencies than the plasmon resonance.
He noted that the scientists did not observe negative refraction in this round of experiments, despite the negative refractive index of the material. The beam displacement from the thin samples simply was too small and the absorption in the material too large.
Figure 2. The left-handedness of the material is the result of the excitation of an antisymmetric plasmon resonance that depends on the orientation of the electric field component of the incident light. The material exhibits different plasmon resonances under TE- and TM- polarized white light (top and bottom, respectively), resulting in a marked shift in the reflection spectrum.
They plan to improve the performance of the material with an optimized geometry of the pairs of pillars and by employing different dielectrics for the substrate. Doing so, Grigorenko said, should improve the magnetic response and shift the plasmon resonance to lower frequencies, reducing the absorption of the left-handed material at visible frequencies. They also are developing a 3-D design, he said, in hopes of realizing a superlens with “a reasonable focal length.”
- A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
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