Hybrid Superlens for Superfocusing

Superlens can achieve imaging with subwavelength resolution via negative refraction. Recently, plasmonic superlens has been investigated for nanolithography. However, the relationship between focal length and resolution is seldom discussed but has been studied in left-handed media based superlens [1]. In this study, we propose a hybrid superlens structure to break the symmetric ray trajectory. Unusual focusing properties are revealed and discussed.

The hybrid superlens is composed of two stacks of metal-dielectric multilayers with different metals, shown in Fig. 1(a). Fig. 1(b) shows focusing characteristics of two waveguide modes. Each point represents a specific wavelength. An unusual behavior between focal length and resolution (full width at half maximum, FWHM) is found for second waveguide mode. The opposite trends of these two modes can be explained by different behaviors of transmittance when varying wavelength, shown in Fig. 2.

Fig. 1: (a) Illustration of hybrid superlens with ray trajectories of different incident angles. (b) Focusing characteristics for 1st and 2nd waveguide modes at different wavelengths.

The focal point moves further away from the lens as the x-component of wave vector increases, which can be understood by the ray trajectories indicated in Fig. 1(a). The transmittance varies with incident angle and focal length can be determined by the k_x components with higher transmittance. The unique property, longer focal length with better resolution, found for second waveguide mode has not been observed for general superlens. Therefore, our proposed hybrid superlens possesses the potential of achieving long focal length with small resolution.

Fig. 2: Transmittance v.s. k_x for (a) 1st and (b) 2nd waveguide modes at different wavelengths.

Reference

[1] V. A. Podolskiy and E. E. Narimanov, Opt. Lett. 30, 75 (2005).

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