Mid-infrared near-field superlenses to overcome the diffraction limit

CIC nanoGUNE Seminars

Peining Li, . Physikalisches Institut (IA) IR-Nano-Group RWTH Aachen University (Germany)
nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
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Mid-infrared near-field superlenses to overcome the diffraction limit New metamaterial-based imaging concepts such as perfect lenses [1], superlenses [2,3] and hyperlenses [4] offer great possibilities to realize sub-diffraction-limited optical imaging. However, most of these achievements have focused on the visible wavelength. In this talk we present and discuss the development of infrared-version superlenses (hyperlenses) that could be used for future infrared microscopy and spectroscopy. We provide an experimental study of the basic imaging mechanism of an infrared silicon carbide (SiC) superlens. We have experimentally mapped the dispersion relation of a 500-nm-thick SiC superlens through the real-space imaging of the launched surface phonon polariton modes. The obtained dispersion map clearly reveals that the so-called ‘superlensing’ effect (subdiffractive imaging using the superlens) originates from the evanescent-field enhancement enabled by the surface polariton modes. However, the SiC superlens is not suitable for broadband applications owing to a very narrow wavelength bandwidth. In order to overcome this limitation, we have also proposed a wavelength-tunable graphene lens that can work at different wavelengths [5]. This graphene lens due to its non-resonant enhancement of evanescent fields yields new promising properties including broad intrinsic bandwidth and low sensitivity to loss, together with a still good subwavelength resolution of around λ/7 for a two-layer case and over λ/10 for the multilayered configuration. Most importantly, the working wavelength of the graphene lens can be electrically tuned via the dynamical tuning of the graphene conductivity. This large tunability allows it to cover a very broad infrared wavelength range. As a proof of this theory concept, we have also experimentally demonstrated that a monolayer-graphene lens offers a 7-fold enhancement of evanescent information, improving conventional infrared near-field microscopy to resolve buried structures at a 500-nm depth with λ/11-resolution [6]. Moreover, we have also investigated the near-field imaging through a natural hyperbolic material, hexagonal boron nitride (hBN).[7] We have demonstrated that a thin, flat hBN slab exhibits wavelength-dependent multifunctional imaging operations, offering both the superlensing of single objects with down to λ/32 resolution, as well as enabling an enlarged imaging of the outline of the object. Both imaging functionalities can be explained based on the volume- confined, wavelength dependent propagation angle of hyperbolic phonon polaritons in hBN. Our results provide in-depth physical understanding and experimental proofs of different infrared superlenses and will inspire future superlens-based high- resolution infrared microscopy and spectroscopic applications. 1\. J. B. Pendry, “Negative refraction makes a perfect lens,” Physical Review Letters, 85, 3966 (2000). 2\. N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub–diffraction-limited optical imaging with a silver superlens,” Science, 308, 534–537 (2005). 3\. T. Taubner, D. Korobkin, Y. Urzhumov, G. Shvets, and R. Hillenbrand, “Near-field microscopy through a sic superlens,” Science, 313, 1595 (2006). 4\. Z. Liu, H. Lee, Y. Xiong, C. Sun, and X. Zhang, “Far-field optical hyperlens magnifying sub-diffraction-limited objects” Science, 315, 1686 (2007). 5\. P. Li and T. Taubner, “Broadband subwavelength imaging using a tunable graphene-lens”. ACS nano 6, 10107 (2012). 6\. P. Li, T. Wang, H. Boeckmann, and T. Taubner, “Graphene-Enhanced Infrared Near-Field Microscopy” Nano Lett. 4, 4400 (2014) 7\. P. Li, et al., “Hyperbolic phonon-polaritons in boron bitride for near- field optical imaging and focusing”. Nature communications 6, 7507 (2015). **Host** : Rainer Hillenbrand