Site Updated:
09/02/2008 |
Negative Refraction
| Metamaterials exhibit many interesting optical properties and bring about new exciting applications that cannot be obtained from nature materials, such as terahertz and optical magnetism, negative refraction, superlensing and cloaking. Recent shift of the focus from microwave to optical frequencies has resulted in considerable progress in optical metamaterials. However, major challenges still lay in the way to practical applications of metamaterials in the optical frequencies, namely, fabrication and loss. To tackle these issues, our group is vigorously working on novel design, fabrication and characterization techniques to develop low loss, bulk metamaterials that can be used for real device applications. At optical frequencies, the plasmonic effect starts to kick in; therefore, a deep understanding of the localized surface plasmon polaritons (LSPP) is key to the design of the optical metamaterials. In our group, much effort has been put into the investigation of the interactions among the plasmonic particles, as well as the new functionalization introduced by these interactions in the optical metamaterials. We are also interested in the nonlinear processes in optical plasmonic metamaterials, such as second harmonic generation and four-wave mixing. |
Negative Refraction in Indefinite Materials
We observed negative refraction in bulk metamaterials composed of silver nanowires embedded in alumina at optical frequencies (Fig. 1A). A porous alumina template was prepared by electrochemical anodization, into which silver nanowires were electrochemically deposited. A 1µm wide slit, etched through a 250nm-thick silver film coated on the metamaterials, was illuminated by a collimated diode laser beam at different incident angles (see left panel of Fig 1A). The transmitted light was mapped by scanning a tapered optical fiber at the bottom surface of the metamaterial. The results are shown in Fig. 2 for incident light at a wavelength of 660nm and 780nm, respectively. |
When the incident angle is 30°, the transmitted beam is shifted to the left for TM-polarized light, which corresponds to the negative refraction. The subwavelength composite forms an effective medium with opposite signs of electrical permittivities along and perpendicular to the wires . The hypobolic dispersion enables negative light refraction even though the phase velocity remains positive. Conversely, the TE-polarized light undergoes positive refraction. Fig. 1D shows the dependence of refraction angles on a range of incident angles at 780nm. The group refractive indices of the metamaterial are shown to be -4.0 and 2.2 for TM and TE light, respectively. The phase refractive index of the metamaterial remains positive, in contrast to left-handed metamaterials. For normal incidence, the light intensity only decays ~0.43/μm in the medium at 780nm wavelength, showing loss a few order of magnitude lower than that of single layer metamaterials reported at the same wavelength . Further calculations show that the negative refraction in this nanowire composite exists for longer wavelengths and also does not depend on the orientation of the nanowire lattice. |
Fig. 1, (Upper)
Schematic of negative refraction from air into the silver nanowire metamaterials.
(Lower) Nanowires embedded in an alumina matrix, as well as scanning electron
microscopy images showing the top and side view of the nanowires (60-nm wire
diameter and 110-nmcenter-to-center distance). |
As the dielectric response in this metamaterial does not require any resonance, the negative refraction has low loss and occurs in a broad spectral range, for all incident angles. Moreover, such bulk metamaterials have the potential of supporting propagation of large wave vectors which are evanescent in air or dielectrics, enabling manipulation of visible light at subwavelength scale. This can significantly impact applications such as waveguiding, imaging and optical communication.
Fig. 2, (Left and center) Measured beam intensity at the existing surface of the metamaterial slab at the
wavelength of 660 nm and 780 nm. The lateral displacement of TM
polarized light shows the negative refraction in the metamaterial at both
wavelengths, whereas TE light undergoes positive refraction. (Right) The dependence of
refraction angles on incident angles and polarizations at 780-nm wavelength.
The negative refraction occurs for broad incident angles. The experiment data
agree well with calculations (solid curves) using the effective medium theory.
Jie Yao, Zhaowei Liu, Yongmin Liu, Yuan Wang, Cheng Sun, Guy Bartal, Angelica Stacy and Xiang Zhang, "Optical Negative Refraction in Bulk Metamaterials", Science, Vol.321, 930, 2008 |
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