W. T. Lu

Super-resolution imaging using a three-dimensional metamaterials nanolens

Super-resolution imaging beyond Abbe’s diffraction limit can be achieved by utilizing an optical medium or “metamaterial” that can either amplify or transport the decaying near-field evanescent waves that carry subwavelength features of objects. Earlier approaches at optical frequencies mostly utilized the amplification of evanescent waves in thin metallic films or metal-dielectric multilayers, but were restricted to very small thicknesses (⪡λ, wavelength) and accordingly short object-image distances, due to losses in the material. Here, we present an experimental demonstration of super-resolution imaging by a low-loss three-dimensional metamaterial nanolens consisting of aligned gold nanowires embedded in a porous alumina matrix. This composite medium possesses strongly anisotropic optical properties with negative permittivity in the nanowire axis direction, which enables the transport of both far-field and near-field components with low-loss over significant distances (>6λ), and over a broad spectral ra...

Storing light in active optical waveguides with single-negative materials

We show that a nonresonant planar waveguide consisting of conventional dielectric cladded with single-negative materials supports degenerate propagating modes for which the group velocity and total energy flow can be zero if the media are lossless. Absorptive losses will destroy the zero-group velocity condition for real frequency/complex wave vector modes. We show that by incorporating gain G into the core dielectric, there exists a critical gain value Gc at which we can recover the condition of zero group velocity, so that light pulses can be stopped and stored. This structure is simpler to achieve than double-negative metamaterials, has small footprint, and can be incorporated into ultracompact on-chip optoelectronics.

Nanoengineering of a negative-index binary-staircase lens for the optics regime

We show that a binary-staircase optical element can be engineered to exhibit an effective negative index of refraction, thereby expanding the range of optical properties theoretically available for future optoelectronic devices. The mechanism for achieving a negative-index lens is based on exploiting the periodicity of the surface corrugation. By designing and nanofabricating a planoconcave binary-staircase lens in the InP /InGaAsP platform, we have experimentally demonstrated at 1.55 m that such negative-index concave lenses can focus plane waves. The beam propagation in the lens was studied experimentally and was in excellent agreement with the three-dimensional finite-difference time-domain numerical simulations.

Beating the diffraction limit using a 3D nanowires metamaterials nanolens

Super-resolution imaging using a three-dimensional metamaterials nanolens has been recently reported [B. D. F. Casse et al. Appl. Phys. Lett. 96, 023114 (2010)]. This nanolens, consisting of bulk gold nanowires embedded in alumina template, can transport with low-loss object details down to λ/4 (λ, wavelength) length scales, over significant distances of the order of 6λ. Here, we present validation of the super-resolution imaging by the nanolens through extensive control experiments. We also analytically show that the nanowire array medium supports a quasi transverse electromagnetic mode (TEM) with flat isofrequency contours, which is a requirement for super-resolution imaging. We numerically compute the optical transfer function to quantify the imaging quality of the lens and show that the theoretical resolution of this nanolens is λ/5. Additionally, we demonstrate the broadband nature of the lens in the spectral region 1510 nm to 1580 nm. Finally, imaging of a large object (~ 52λ in diameter), with subwavelength features, is presented.

Slow light using negative metamaterials

A general overview of slow light waveguide structures made of negative metamaterials is presented. We discuss the conditions and the parameter space to achieve zero total energy flow and zero group velocity due to the degeneracy of forward and backward waves in waveguides cladded with single negative metamaterials. Absorptive loss plays a severely limiting role and can prevent achieving the zero group velocity condition. Gain can be introduced either in dielectric or negative metamaterials to restore the zero group velocity condition. This type of slow light waveguide has a large delay bandwidth product and is suitable for use in integrated optoelectronic circuits.