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Register a new userAmplifying Evanescent Waves by Dispersion-induced Plasmons: Defying the Materials Limitation of Superlens
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Breaking the diffraction limit is always an appealing topic due to the urge for a better imaging resolution in almost all areas. As an effective solution, the superlens based on the plasmonic effect can resonantly amplify evanescent waves, and achieve subwavelength resolution. However, the natural plasmonic materials, within their limited choices, usually have inherit high losses and are only available from the infrared to visible wavelengths. In this work, we have theoretically and experimentally demonstrated that the arbitrary materials, even air, can be used to enhance evanescent waves and build low loss superlens with at the desired frequency. The operating mechanisms reside in the dispersion-induced effective plasmons in a bounded waveguide structure. Based on this, we constructed the hyperbolic metamaterials and experimentally verified its validity in the microwave range by the directional propagation and imaging with a resolution of 0.087 wavelength. We have also demonstrated that the imaging potential can be extended to terahertz and infrared bands. The proposed method can break the conventional barriers of plasmon-based lenses and bring new possibilities to the field of superresolution imaging from microwave to infrared wavelengths.
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There has been significant interest in imaging and focusing schemes that use evanescent waves to beat the diffraction limit, such as those employing negative refractive index materials or hyperbolic metamaterials. The fundamental issue with all such schemes is that the evanescent waves quickly decay between the imaging system and sample, leading to extremely weak field strengths. Using an entropic definition of spot size which remains well defined for arbitrary beam profiles, we derive rigorous bounds on this evanescent decay. In particular, we show that the decay length is only $w / pi e approx 0.12 w$, where $w$ is the spot width in the focal plane, or $sqrt{A} / 2 e sqrt{pi} approx 0.10 sqrt{A}$, where $A$ is the spot area. Practical evanescent imaging schemes will thus most likely be limited to focal distances less than or equal to the spot width.
Abbes resolution limit, one of the best-known physical limitations, poses a great challenge for any wave systems in imaging, wave transport, and dynamics. Originally formulated in linear optics, this Abbes limit can be broken using nonlinear optical interactions. Here we extend the Abbe theory into a nonlinear regime and experimentally demonstrate a far-field, label-free, and scan-free super-resolution imaging technique based on nonlinear four-wave mixing to retrieve near-field scattered evanescent waves, achieving sub-wavelength resolution of $lambda/15.6$. This method paves the way for application in biomedical imaging, semiconductor metrology, and photolithography.
The scattering of electromagnetic wave by a periodic array of nanowires is calculated by the boundary element method. The method is extended to the infinite grating near the interface between two dielectrics. A special Green function is derived that allows to study the evanescent wave. The Rayleigh--- Woods anomalies are found in the period-to-wavelength dependence of the average Pointing vector in the wave zone. For thin wires the calculations are shown to agree with the two-dimensional coupled dipole approximation.
Based on transformation optics, a strategy is proposed to expose the inner one-dimensional space of a wave field inside a beam volume to the surface of the propagation medium and extend the space from one-dimensional to two-dimensional, allowing the corresponding field distribution to be detected directly and more subtly, which is important in optical signal processing. The method is applied to the quadratic graded index lens to construct a new graded index lens, and its enhanced chirpyness detection ability is demonstrated by numerical simulation.
The coordinate transformation technique is applied to the design of perfect lenses and superlenses. In particular, anisotropic metamaterials that magnify two-dimensional planar images beyond the diffraction limit are designed by the use of oblate spheroidal coordinates. The oblate spheroidal perfect lens or superlens can naturally be used in reverse for lithography of planar subwavelength patterns.
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