Near-Field Microscopy with Superlenses
Sub-Wavelength Resolution in the IR Using Perovskite Oxides
- Fig. 2: Near-field images. (a) For SrRuO3 objects on a SrTiO3 substrate we image near-field signals using a CO2 laser (λ = 10.6). The objects appear bright, which certain maxima due to plasmonic excitation related to the geometry of the structure. (b) On the superlens we observe a contrast beyond the diffraction limit for λ = 14.6 μm. Here, the objects appear equally bright, due to details in the illumination . (scalebars are 10 μm). Figure adapted from . Reprinted with permission from Nature Communications, Copyright (2011) .
- Fig. 1: Sketch of the near-field setup including the superlens and the geometry at the near-field probe (blue). The superlens consists of a layer of SrTiO3 with a thickness of 400 nm sandwiched by two layers of BiFeO3 (thickness 200 nm). The objects to be imaged are SrRuO3 patterns on a SrTiO3 substrate. All constituents of the superlens are perovskite oxides that match in their crystalline structure resulting in low scattering at the highly crystalline interfaces. The near-field tip probes the evanescent fields on the image side of the lens. The superlens is excited by an IR free- electron laser, which is precisely tunable in the range from 4 to 250 μm. Figure adapted from . Reprinted with permission from Nature Communications, Copyright (2011) .
- Fig. 3: Spectral response of the symmetric superlens. Near-field spectrum as well as near-field images for selected wavelengths (scalebars are 10 μm). (a) For λ = 13.9 to 15.9 μm, an imaging contrast exists due to the localized polariton mode. (b) Shows the near- field spectrum for a fixed distance of z = 20 nm with the results between 13.5 μm and 16.25 μm being multiplied by a factor of 4 in the plot. The red and green curves correspond to the near-field signals on areas with and without SrRuO3 objects on the opposite side of the lens, respectively. (c) For λ = 16.8 to 18.4 μm, no imaging contrast is observed although the near- field signal is enhanced by the non-localized polariton mode. Figure adapted from . Reprinted with permission from Nature Communications, Copyright (2011) .
- Fig. 4: Normalized cross sections on an asymmetric superlens without the top-layer of BiFeO3. The horizontal range in all cross sections is 12 μm (scalebars are 6 μm). The topography of the sample and the position of the SrRuO3 object is reflected by the dark areas at the bottom of the figures. The localized evanescent fields on the objects show a maximum at a certain distance, which increases with smaller wavelength. This effect corresponds to a superlens-enhanced coupling of probe and object . Figure adapted from . Reprinted with permission from Nature Communications, Copyright (2011) .
Conventional imaging devices are limited in their resolution and, hence, restrict the insights into structures smaller than the wavelength. Near-field microscopy overcomes this limitation by probing evanescent fields resulting in a wavelength-independent resolution. A superlens is a planar device, which transforms these fields to an image plane. We study such lenses for the infrared based on perovskite oxides, which might be applicable to imaging of highly-damped samples e.g. in liquids.
Breaking the diffraction limit becomes possible, when studying the fields close to an object. As these so called evanescent fields do not propagate they are lost in conventional microscopy. However, in near-field microscopy (NSOM), a probe placed close to the objects transforms the evanescent fields into propagating ones, which can be detected in the far-field . The resulting signal carries sub-wavelength information about the objects with a resolution, which in particular does not depend on the wavelength (λ) but rather on the size of the probe and its distance to the sample.
Typically, NSOM is based on scanning probe microscopy this is so that the distance between probe and objects can be controlled, which conveniently results in topographical information of the sample. In addition the near-field signature allows for studying the sample‘s local optical properties. Hence, it becomes possible to probe material-induced optical contrasts  and optical anisotropy , as well as field distributions of nanoparticles and even viruses .
In general evanescent fields are bound to the object. However, with a superlens it becomes possible to transform this information to an image plane by reconstructing the evanescent fields and, hence, the sub-λ resolution on the opposite side of the lens . This becomes possible when exciting polariton modes at both interfaces of a planar slab with permittivity ε ≅ -1. At the superlensing wavelength λSL both polariton modes are coupled and localized creating a sub-λ image.
Many different materials naturally show ε ≅ -1 at certain wavelengths, e.g.
metals in the visible regime  and dielectrics in the infrared (IR) . For designing an efficient superlens with low losses, it is desirable to choose a material with low absorption at λSL and low surface roughness in order to avoid scattering of the polariton modes.
We study perovskite-based superlenses in the IR with λSL around 14.5 μm. As objects, we examine structured strontium ruthenate (SrRuO3) on a strontium titanate (SrTiO3) substrate. The actual superlens is grown directly on these samples and consists of an active superlensing layer of SrTiO3, which is sandwiched between two spacer layers of bismuth ferrite (BiFeO3) with ε ≅ εair = 1 at λSL (fig. 1). All materials were grown by means of pulsed laser deposition. As all materials show similar crystal structure they grow epitaxially on top of each other resulting in highly crystalline interfaces. In addition to their optical response, the intriguing properties of perovskites such as colossal magnetoresistance, ferroelectricity, and multiferroic coupling are of interest as they might lead to further applications concerning the tunability of superlenses by external electric or magnetic fields. Here, we focus on the general behavior of perovsite-based superlenses in NSOM such as resolution, distance dependence and spectral response.
Near-field Microscopy on Perovskite-based Superlenses
We study the structured objects with and without superlens with an AFM-based near-field microscope (fig. 1+ 2). In order to address the polariton modes of the superlens, we use the free-electron laser at Helmholtz Zentrum Dresden Rossendorf (www.hzdr.de) as a precisely tunable IR light-source from 4 to 250 μm .
When scanning the surface of our samples laterally, we are able to resolve structures smaller than λ/14 on both sample types (fig.2). It is remarkable that through the superlens the signal is larger than without the superlens, and the objects appear homogeneously bright, which is a characteristic of this illumination process (for details, please see ).
In addition to the resolution of our superlens, we study the characteristic spectral response of such a lens. Therefore, we probe the evanescent-field image for different wavelengths (fig. 3). For wavelength from 14.1 to 15.2 μm we clearly observe a sub-λ image. In addition to the superlensing effect, at longer wavelength we observe non-localized polariton modes on the sample surface. These modes result in a strong near-field signal for 17.5 to 18.5 μm, carrying no information about the objects on the opposite side of the lens but are equally strong on the whole sample surface. The lateral field distribution shows the object structure with sub-λ resolution. But what about the vertical field distribution of the superlensed image? In order to study this distribution more closely, we examine a sample without the top layer of BiFeO3, which allows us to probe the non-localized polariton modes more closely. Figure 4 shows cross sections for selected lambda with one SrRuO3 object in the center. For large wavelengths we observe an enhanced near-field signal on the whole sample corresponding to the non-localized polariton mode. Close to λSL, a maximum forms at the position of the SrRuO3 object only. This maximum is located on the surface for λ = 14.5 μm, but moves away from the sample surface with smaller wavelength. This effect was described in detail in . The superlens controls the coupling between probe and object and depending on the wavelength; this near-field coupling shows a maximum at a specific distance resulting in enhanced scattering of the system.
With a superlens, it becomes possible to transform evanescent fields and its sub-wavelength information to the opposite side of the lens. When studying these images by means of near-field microscopy we observe sub-wavelength resolution around the superlensing wavelength. Moreover, the superlens mediates an enhanced near-field coupling between probe and objects, resulting in a maximum of the near-field signal at certain distanced between probe and sample surface.
Using a superlens in near-field microscopy allows for remote-examination of samples, which are difficult to probe directly with an NSOM. This might be in particular of interest e.g. for objects in liquids or materials with high damping. In addition, the superlens-controlled coupling of probe and objects could be applied for active coupling of several objects which might find applications in plasmonics or metamaterial-based multifunctional circuits .
This work was supported by the German Academic Exchange Service DAAD. The samples were grown at UC Berkeley supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy. The author acknowledges technical assistance of the FELBE team at Helmhotz-Zentrum Dresden-Rossendorf and thanks them for their dedicated support. She highly appreciates the contributions of Yongmin Liu to the general understanding of this work and of Lukas M. Eng to the idea for this project. Moreover, she thanks Lane Martin and Pu Yu for the sample preparations, the groups of Manfred Helm and Lukas M. Eng for the NSOM measurements, and Ramamoorthy Ramesh for hosting this project.
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Dr. Susanne C. Kehr
School of Physics and Astronomy
University of St. Andrews