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Superresolution imaging and superfocusing with negative-index metamaterials

Nanoscale metamaterials enable development of novel nano-optical lenses.
19 March 2010, SPIE Newsroom. DOI: 10.1117/2.1201002.002661

Developments in higher-resolution optics have often led to rapid scientific and technological progress in the life sciences and engineering. In 1873, Ernst Abbe showed1 that the spatial resolution of optical-imaging instruments is limited by diffraction because of the finite wavelength of light. Abbe's diffraction-limit theory implies that the resolving power of optical components cannot be smaller than half the wavelength of the incident light.

Nanoscale optical elements that can mold the flow of light offer the potential of entirely new modalities of superresolution imaging. With a vision to build superlenses capable of resolving subwavelength details, in the last decade the photonics community has focused on fabricating tiny metallic inclusions (or, closely related, plasmonic architectures) characterized by a negative index of refraction.2 Unfortunately, such metamaterials suffer from material losses enhanced by resonances, leading to a degradation of functionality. Instead, our group has demonstrated successful performance of highly functional nanostructured metamaterials in ‘low-loss’ dielectric platforms such as metal-dielectric composites and III-V semiconductors (composed of indium phosphide/indium gallium arsenide phosphide: InP/InGaAsP).

We recently reported the experimental realization of superresolution imaging with a low-loss 3D nanolens,3 comprised of gold nanowires embedded in nanoporous alumina: see Figure 1 (right). This 3D nanolens, manufactured using a combination of bottom-up self-assembly and electrochemical process, transmits subwavelength details down to λ/4 (λ: Wavelength) and over a significant distance of more than 6λ. We show an example of the superresolution imaging achieved in Figure 1 (left). The nanolens guided modes do not penetrate the nanowires, so that most of the energy propagates between them. Consequently, image reconstruction can be done with low loss and over a broad spectral range. The nanolens possesses a figure of merit (a metric for functionality) that is four times higher than the best fabricated metallic-based metamaterial4 at telecommunications wavelengths. Manufacturable on large scales, the nanolens has immediate applications in contact-lithography techniques or optical projection printing.

Figure 1. (left) Superresolution imaging using a 3D metamaterial nanolens. A metallic film with the void letters ‘NEU’ (with arms of subwavelength dimensions 0 .4 λ) is illuminated using light with a wavelength λ=1550nm. The image is formed at the other end of the lens, at a distance >6λ. (right) (a) Bulk metamaterial (pink circle) manufactured on large scale (diameter >10mm). (b) 3D illustration of the nanoscale-lens architecture. (c) The metamaterial nanolens consists of aligned gold nanowires (diameter: 12nm, lattice spacing: 25nm), embedded in a porous-alumina template matrix. This scanning-electron-microscope (SEM) image3 (top view) shows the tips of the gold nanowires. (d) SEM image3 of the cross section of the 10μm-thick nanoporous-alumina template without the gold nanowires. (e) Anisotropic optical property of the metamaterial. Negative permittivity in the z direction (Re εz <0) and positive permittivity in the (x, y) plane (Re εx, y <0). (Reprinted with permission.3 Copyright 2010, American Institute of Physics.)

We have also developed nano-optical lenses using III-V semiconductors capable of superlensing and superfocusing at λ=1550nm using negative refraction. We designed flat and planoconcave lenses using dispersion-engineering principles. We subsequently fabricated them nanolithographically in InP/InGaAsP semiconductor heterostructure platforms. We achieved flat-lens imaging of a point source with a photonic-crystal superlens5 consisting of air holes in a dielectric medium of 290nm diameter and with a lattice spacing of 470nm: see Figure 2 (left). By designing an appropriate lens-surface termination, we achieved an image spot size of 0.12λ2—see Figure 2 (right)—demonstrating superlens imaging with a resolution well below Abbe's diffraction limit of 0.5λ2.

Figure 2. (left) Imaging of a point source by a flat lens with an effective negative index of refraction.5(right) By designing a suitable lens-surface termination, we achieved an image spot size of 0.12λ2, demonstrating superlens imaging with subwavelength resolution well below Abbe's diffraction limit (0.5λ2). (a), (b), and (c) show SEM images5 of the various designs, overlaid with the corresponding near-field scanning-optical-microscope (NSOM) scans at λ=1 .52μm in the object and image planes, respectively. (a) Original design (A) with extra padding dielectric layers produced a focused spot of 1.4λ full width at half maximum (FWHM). T: Transverse direction. (b) Design B, with a cut through the middle of the air holes at the surface termination of the object plane, yielded a focused spot of 1 .0λFWHM. (c) Optimized design (C) with cuts through the middle of the air holes on both sides and waveguide-tapered to 50nm. The focused spot achieved is 0 .4 λFWHM. (Reprinted with permission.5 Copyright 2009, Optical Society of America.)

We also realized a nano-optical microlens with an effective negative index of refraction (−0.7), shown in Figure 3 (left), and with a world-record ultrashort focal length (∼8λ).6 This unique lens also possesses superior properties compared to positive-index planoconvex lenses, including a compact footprint, near diffraction-limited spot size (~0.68λ), larger numerical aperture (close to unity), and reduced aberrations. We also demonstrated successful performance of a planoconcave binary-staircase lens in the InP/InGaAsP platform—see Figure 3 (right)—that achieves superfocusing by surface engineering of a bulk medium.7 By exploiting the periodicity of the surface corrugation, the optical element can exhibit an effective negative index of refraction and focus plane waves. Potential applications are in high-density pixel digital imaging including integrated optics for focal-plane arrays, night-vision goggles, and laser-beam shaping.

Figure 3. (left) NSOM image of light focusing by negative-index planoconcave lenses at 1530nm. Focusing is observed ∼12μm from the concave face. The FWHM of the beam-spot size is ∼0.68λ. The white dashed lines indicate the beam path. Note that the picture consists of a SEM image of the lens superposed onto an NSOM scan. (right) NSOM image of the binary-staircase lens, obtained at λ=1550nm near the focal point. (Reprinted with permission.6,7 Copyright 2008, American Institute of Physics.)

In summary, we have designed, fabricated and characterized nanoscale negative-index metamaterials-based optical components that offer the prospect of revolutionary developments in imaging and optoelectronics. Low-loss platforms include III-V optoelectronics semiconductors and metal-dielectric nanowire composites. The ability of these nanoscale metamaterials to beat the diffraction limit offers tremendous opportunities in biomedical imaging, nanolithography, high-capacity storage systems, transformation optics, and IR imaging applications. Our future efforts will focus on imaging biomolecules in vitro and optimizing the 3D nanolens for other spectral ranges. Other ongoing work is focusing on metamaterials-based lithography at UV frequencies.

This work was done in collaboration with W. T. Lu, Y. J. Huang, and L. Menon. We acknowledge support from the Air Force Research Laboratories at Hanscom Air Force Base (grant FA8718-06-C-0045) and the National Science Foundation (grant PHY-0457002).

Bernard Didier F. Casse
Northeastern University
Boston, MA

Didier Casse is a lead research scientist. He received his PhD in physics from the National University of Singapore in 2007 and has published more than 30 articles in the areas of tissue engineering, microelectromechanical systems, and nanophotonics. His current research interests include engineering of metamaterials for superresolution imaging, optoelectronics, and defense applications.

Srinivas Sridhar
Electronic Materials Research Institute
Northeastern University
Boston, MA

Srinivas Sridhar is Arts and Sciences Distinguished Professor of physics and chair of the Physics Department. He is the director of Nanomedicine Science and Technology, an Integrative Graduate Education and Research Traineeship program funded by the National Cancer Institute and the National Science Foundation. He is also the founding director of the Electronic Materials Research Institute.