A hyperlens-based device for nanoscale focusing of light

Abstract

To resolve the problem of missed evanescent waves in a beam focusing system, a hyperlens-based beam focusing device is proposed in this letter. This device can convert the evanescent waves into propagating waves, and then a super-resolution spot is formed at the center of the hyperlens. The working principle of the device is presented, and the way in which the material and structural parameters of the hyperlens affect the resolution and transmission is analyzed in detail. A multibeam focusing device is optimally designed, and the simulated results verify that a nanoscale spot with a diameter of 15.6 nm (corresponding to λ0/24, where λ0 is the working wavelength in vacuum) is achieved, which is far less than the diffraction limited resolution with a value of 625 nm (1.7λ0). The device is expected to find numerous applications in optical data storage and nano-photolithography, among others.OCIS codes: 230.0230, 160.3918, 220.3630, 350.4238. doi: 10.3788/COL201210.042302. In an optical data storage and nano-lithography area, an ultra-small beam spot is required for information recording. With the phase transformation of an ideal positive lens, an input plane wave can be converted into a converging spherical wave and then eventually focused into a point at the focal plane. However, limited by diffraction, the ideal focusing point is through an Airy pattern. This can be explained by the fact that when the beam is focused toward the focal plane, the conservation of the angular momentum forces the tangential wave vector k θ to increase toward the center, however, light with k θ > k 0 corresponds to the evanescent wave that cannot reach the focal plane, where k 0 is the wavenumber in vacuum. According to the equation ∆x∆k ≈ 2π [1] , where ∆x is the beam size and ∆k is the bandwidth of the angular spectrum, the maximum ∆k that could be delivered is determined by the maximum transverse wavenumber k 0 ×NA(∆k = k 0 ×NA and NA is the numerical aperture of the focusing lens) and the minimum beam size ∆x is λ 0 /NA, which corresponds to the diffraction limit. To further reduce the beam size, we should find materials that allow waves with transverse wavenumber exceeding k 0 ×NA to propagate in it with a propagating wave mode, instead of an evanescent wave mode. Conventional isotropic materials such as optical glasses do not show such characteristics, whereas artificial materials such as metamaterials [2] hold such potential. By optimization design [3−5] , metamaterials can achieve the desired effective dielectric permittivity ε and magnetic permeability µ and have been theoretically shown to support propagating waves with very large wavenumbers. Lens fabricated by metamaterials, such as near-field planar superlens [6,7] , far-field superlens [8] , and hyperlens [9−12] , can support the propagation of evanescent waves and thus are capable of imaging an ultra-small object far below the diffraction limit. In this letter, we concentrate on another application and report on how a hyperlens-based device can work for super-resol...

Twisted-light-induced intersubband transitions in quantum wells at normal incidence

“…This is the case of intersubband transitions in quantum well and quantum disk structures at normal incidence, which is the situation we focus on here. Moreover, advances in optical hyperlenses suggest that highly collimated beams (with large q r ) can be obtained [27]. Optical transitions induced by the xy-component of the TL beam in bulk [15,18], quantum wells [18], and quantum dots [16] have been studied recently.…”