Far-field superlens imaging at visible wavelengths
The optical microscope is one of the most important and widely used imaging tools due to its noninvasive nature, low cost, and versatility. But despite its remarkable achievements in the history of science and technology, the conventional optical microscope is generally not considered a competent nanoscale imaging apparatus because its spatial resolution originates from the diffraction of light and is limited to a few hundred nanometers. Therefore, the development of an optical microscope that circumvents the diffraction limit and can fulfill the demands of modern nanotechnology is widely desired.
The diffraction limit is determined by evanescent waves, which carry information about the smaller features of objects and decay exponentially with distance away from the object surface. Another kind of wave, propagating waves, imparts information about the larger features of objects, and can travel long distances and be collected by an optical lens. Consequently, the conventional optical microscope only forms images of large object features. The near-field scanning optical microscope (NSOM)1,2 improves the resolution of an optical system by using a sharp probe placed close to the object that records the evanescent waves from the object within a fraction of a light wavelength distance. The tradeoff is that the NSOM sacrifices its imaging speed by using a time-consuming, point-by-point scanning process. It can take a few minutes to get one image, which prevents NSOM from undertaking real-time dynamic imaging.
Recent progress in the fabrication of subwavelength imaging superlenses, including the so-called hyperlens and the far-field superlens, has attracted significant attention because they can project subwavelength images into the far field without sacrificing imaging speed.3–10 The far-field superlens enhances evanescent waves and converts them into propagating waves, so that the high-resolution information carried by evanescent waves can be captured in the far field and a subwavelength image can be reconstructed. The optical far-field superlens microscope is realized by inserting a far-field superlens between an object and the objective of a conventional optical microscope. Our previous work has experimentally proved the concept of far-field superlens microscopy for a 1D object at a UV wavelength.11
Recently, we reported that the optical far-field superlens microscope can also be used at visible wavelengths for 2D subwavelength imaging.12 The 2D image is obtained by accumulating measurements from six orientations of the far-field superlens. We used a test object composed of eight circles with a radius of 40nm (see Figure 1, left). The subwavelength features are not distinguishable under a conventional microscope at a wavelength of 405nm (see Figure 1, center). However, the far-field superlens image clearly resolves the object with a resolution better than 100nm (see Figure 1, right), the smallest center-to-center distance. The resolution can be further improved with different designs, although it is ultimately limited by the loss of object material.
The new far-field superlens used in the 2D imaging described above can be seen in Figure 2 and is made of a silver conducting-nonconducting material matrix (Ag-dielectric multilayer and a 1D Ag-dielectric subwavelength grating). The new design removes the constraint of having a practical object present in the previous design so therefore the amplitudes of the propagating components of the object spectrum have to be comparable with, or smaller than, those of the evanescent components. In addition, unlike the previous far-field superlens, which works at a UV wavelength, the new far-field superlens can work over the entire range of visible wavelengths.
The practical requirements to experimentally realize 2D imaging by the optical far-field superlens microscope are the fabrication of the new far-field superlens, rotational control of the far-field superlens or the grating, and the precise measurement of both the phase and amplitude of the diffraction waves from the far-field superlens, which are all feasible considering current manufacturing and calibration techniques. We predict that the optical far-field superlens microscope will find wide-ranging applications in optical nanoscale imaging and sensing.
This work was supported by the Army Research Office Multidisciplinary University Research Initiative (MURI) program (grant 50432- PH-MUR) and the National Science Foundation (NSF) Nanoscale Science and Engineering Center under award DMI0327077.
