Tunable and multiwavelength superlenses for infrared-optical subwavelength imaging
Physik | nano optics | near-field optics | superlens | subwavelength imaging
Recently, new metamaterial-based imaging concepts, such as a perfect lens, a superlens and a hyperlens, offer great possibilities to realize sub-diffraction-limited optical imaging. However, most of these achievements have focused on the visible wavelength range. This thesis aims to develop new designs of infrared superlenses that could be used for future infrared microscopy and spectroscopy.First, I have provided an experimental study of the basic imaging mechanism of an infrared silicon carbide (SiC) superlens. I have determined the dispersion relation of a 500-nm-thick SiC superlens through real-space optical imaging of the launched surface phonon polariton modes. The experimentally determined dispersion relation 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. Moreover, an imaging resolution of down to λ/30 (λ, the free-space wavelength) has been achieved in the same experiment. This resolution is about two times better than that reported in previous superlens experiments.Second, I have achieved the superlensing of dielectric objects with a resolution of about λ/18. It is a significant experimental advance, because so far all reported superlens experiments are based on the imaging of metallic or strongly reflective objects that usually exhibit much stronger contrasts. Additionally, the obtained spectroscopic imaging results have also revealed that the superlensing can identify the material properties of the imaged objects. This is the first demonstration that the superlens can be used for infrared index-sensing and spectroscopic applications. However, the SiC superlens is not suitable for broadband applications owing to a very narrow wavelength bandwidth. In order to overcome this limitation, I have proposed two new designs of infrared superlenses. The first idea is a multiwavelength superlens consisting of multiple polar dielectrics. The presented numerical investigations have demonstrated that the superlensing at multiple wavelengths can be achieved by increasing the number of phonon resonant dielectrics in the proposed multilayered superlens system. In other words, a subwavelength object can be imaged at many different wavelengths by just one single lens. Considering the abundance of polar dielectrics, the proposed multi-wavelength superlens can cover a wavelength range from infrared to terahertz (THz) by choosing suitable materials.The second idea is a wavelength-tunable graphene lens that can work at different wavelengths. This graphene lens due to its non-resonant enhancement of evanescent fields yields new promising properties including broad intrinsic bandwidth and low influence by the optical losses, 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 broad infrared wavelength range. As a proof of this theory concept, I have 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.Moreover, I have also investigated the near-field optical imaging through a natural hyperbolic material, hexagonal boron nitride (hBN). I 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 hyperbolic phonon polaritons propagating highly directional in hBN.The results presented in the thesis would be useful for the development of superlens-based super-resolution infrared microscopy and spectroscopy. In particular, combing the (graphene) superlens with a conventional near-field optical microscope could have the great potential for subsurface imaging of buried subwavelength-sized objects.