The development of innovative terahertz (THz) imaging systems has recently moved in the focus of scientific efforts due to the ability to screen substances through textiles or plastics. The invention of THz imaging systems with high spatial resolution is of increasing interest for applications in the realms of quality control, spectroscopy in dusty environment and security inspections. To realize compact THz imaging systems with high spatial resolution it is necessary to develop lenses of minimized thickness that still allow one to focus THz radiation to small spot diameters with low optical aberrations. In addition, it would be desirable if the lenses offered adaptive control of their optical properties to optimize the performance of the imaging systems in the context of different applications. Here we present the design, fabrication and the measurement of the optical properties of spectrally broadband metamaterial-based gradient index (GRIN) lenses that allow one to focus THz radiation to a spot diameter of approximately one wavelength. Due to the subwavelength thickness and the high focusing strength the presented GRIN lenses are an important step towards compact THz imaging systems with high spatial resolution. Furthermore, the results open the path to a new class of adaptive THz optics by extension of the concept to tunable metamaterials.
We present a gradient-index (GRIN) metamaterial based on an array of annular slots. The structure allows a large variation of the effective refractive index under normal-to-plane incidence and thus enables the construction of GRIN devices consisting of only a small number of functional layers. Using full-wave simulations, we demonstrate the annular slot concept by means of a 3-unitcell thin GRIN lens for the terahertz (THz) range. In the presented realizations, we achieved an index contrast of ∆n = 1.5 resulting in a highly refractive lens suitable for focusing THz radiation to a spot size smaller than the wavelength.In the last ten years, metamaterials have emerged to be powerful tools for the manipulation of light on the subwavelength scale. From the very first day of metamaterials, the scientific interest has been driven by the possibility of guiding light by tailoring the effective material parameters. In this respect, various concepts have been proposed, ranging from gradient index materials [1][2][3], to invisibility cloaks [4][5][6][7] and other transformation optical structures [8][9][10][11][12]. Most of the concepts have only been experimentally realized in the microwave regime where comparatively simple fabrication techniques allow a large freedom in the metamaterial design. However, when approaching higher frequencies as the THz or the optical regime, the experimental realization becomes more challenging since the standard fabrication methods, such as photo-or electron beam lithography, allow only the fabrication of planar structures with a very limited number of layers. In order to compensate the layer restriction in the high frequency range, it is therefore important to use metamaterials that provide a high refractive index contrast. However, most of the metamaterial structures are very lossy and allow only a moderate change of the refractive index within an acceptable transmission window. In particular, this is the case for resonant elements which are usually associated with high intrinsic losses. One possibility to create a non-resonant element is to choose the operation frequency well below the resonance frequency and, hence, operate in the non-resonant region of the structure. This approach has been applied in Ref. 2 − 4 where the achieved index contrast was reported to be in the range of ∆n = 0.7−0.9. In this paper, we present an alternative approach for a non-resonant, polarization-independent structure which allows a very large variation of the refractive index of about ∆n = 1.5 with high transmission and reasonable bandwidth for practical applications.The unit cell of the proposed metamaterial consists of a 3 µm wide annular slot within a square metal patch as shown in Fig. 1(a). The metal layer is embedded in a cubic dielectric matrix with an edge length of 60 µm. The radius of the annular slot was varied in order to alter the effective index of refraction. We analyzed the electromagnetic properties of the metamaterial by fullwave time-domain simulations (CST Microwave Studio).To consider ...
A full view spherical camera exploits its extended field of view (FOV) to map its complete environment onto a 2D image plane. Thus, with a single shot, it delivers a lot more information about the surroundings than one can gather with a normal perspective or plenoptic camera, which are commonly used in light field imaging. However, in contrast to a light field camera, a spherical camera does not capture directional information about the incident light, and thus a single shot from a spherical camera is not sufficient to reconstruct 3D scene geometry.In this paper, we introduce a method combining spherical imaging with the light field approach. To obtain 3D information with a spherical camera, we capture several independent spherical images by applying a constant vertical offset between the camera positions and combine the images in a Spherical Light Field (SLF).Our approach differs from its related work in terms of expanded FOV and reduced acquisition time: Taguchi et al.[2] used an array of spherical mirrors to model catadioptric cameras for wide angle light field rendering, which implies decreasing tangential resolution close to the mirror borders and limits the FOV to 150 • × 150 • . Unger et al.[4] employed a fisheye-camera translated on a plane to capture hemispherical HDR images of a scene. The total acquisition time of up to 12 hours for a single scene restricts the application scenario to constantly illuminated indoor environments. Our proposed approach for SLF acquisition uses spherical cameras as shown in Figure 1(a) and allows to capture scenes within a few minutes, making it applicable to outdoor scenes.A convenient description of this camera type is provided by Torii et al. [3], who consider a spherical camera to consist of a camera center C with a surrounding unit sphere acting as projection surface. This definition implies that no intrinsic parameters such as focal length or distortion values known from perspective imaging need to be considered (Figure 1(b)). By applying the Mercator projection [1], the spherical image is conformally mapped to an image on a cylinder surface Π (Figure 1(c)) allowing for epipolar plane image (EPI) reconstruction.To describe a SLF, we define a new parametrization for the camera domain and the surrounding spherical 2D mapped image.We take the cylinder surface Π and denote the center line with Ω. The cylinder surface Π is parametrized by the image coordinates (φ , θ ) ∈ Π. The line Ω contains the focal points t ∈ Ω of all possible camera positions in vertical direction. A Spherical Light Field can then be described by a functionwhere L(t, φ , θ ) defines the intensity of the incident light ray on the image plane (φ , θ ) passing through the focal point t. To estimate the disparity, we address a 2D slice Σ φ * of the SLF by setting φ to a fixed value φ * . The restriction of the light field to such a slice defines an EPI, being formally given asAssuming a Lambertian scene, the EPI yields information about the disparity of a scene point in the form of orientated lines. To...
We present different metamaterial-based optical components that open exciting new ways to deliberately manipulate the spatial or spectral properties of terahertz (THz) radiation. The central scope of the paper is the design, fabrication and optical characterization of a 3-layer gradient index (GRIN) lens that allows one to strongly focus THz radiation to a spot diameter in the order of one wavelength. The GRIN lens is based on a specific metamaterial structure that is fully embedded in a polymer background matrix and thus provides a free-standing optics with high transmission and minimal Fresnel reflection. THz lenses with such strong focusing capabilities are especially intriguing with respect to the resolution enhancement of THz imaging systems
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