Stéphane Durant

Theory of the transmission properties of an optical far-field superlens for imaging beyond the diffraction limit

A conventional optical superlens for imaging beyond the diffraction limit produces images only in the near-field zone of the superlens. In contrast, an optical far-field superlens (FSL) device has a remarkable transmission property that leads to a one-to-one relationship between the far-field and the near-field angular spectra. This property makes the device suitable for imaging beyond the diffraction limit from far-field measurement. This specific FSL is composed of a properly designed periodically corrugated metallic slab-based superlens. Through the numerical design and parameter study, we show that the transmission property of this FSL is based on a specific strong-broadband wavenumber excitation of surface-plasmon polaritons supported by the nanostructured metallic grating.

Experimental studies of far-field superlens for sub-diffractional optical imaging.

Contrary to the conventional near-field superlensing, subwavelength superlens imaging is experimentally demonstrated in the far-field. The key element is termed as a Far-field SuperLens (FSL) which consists of a conventional superlens and a nanoscale coupler. The evanescent fields from the object are enhanced and then converted into propagating fields by the FSL. By only measuring the propagating field in the far-field, the object image can be reconstructed with subwavelength resolution. As an example of this concept, we design and fabricate a silver structured one dimensional FSL. Experimental results show that feature resolution of better than 50nm is possible using current FSL design.

Deep subwavelength nanolithography using localized surface plasmon modes on planar silver mask

The development of a near-field optical lithography is presented in this paper. By accessing short modal wavelengths of localized surface plasmon modes on a planar metallic mask, the resolution can be significantly increased while using conventional UV light source. Taking into account the real material properties, numerical studies indicate that the ultimate lithographic resolution at 20nm is achievable through a silver mask by using 365nm wavelength light. The surface quality of the silver mask is improved by adding an adhesion layer of titanium during the mask fabrication. Using a two-dimensional hole array silver mask, we experimentally demonstrated nanolithography with half-pitch resolution down to 60nm, far beyond the resolution limit of conventional lithography using I-line (365nm) wavelength.

Tuning the far-field superlens: from UV to visible.

A far-field optical superlens, which is able to form sub-diffraction- limited images in the far field at UV wavelength, was recently demonstrated. In current work we present two methods to tune the working wavelength from UV to visible by tuning either the permittivity of the surrounding medium or that of the metal. A practical design is provided for each method. The tunable far-field superlens enables possible applications of the far-field superlens in sub-diffraction-limited imaging and sensing over a wide range of wavelength.

Near-field Moiré effect mediated by surface plasmon polariton excitation

We have demonstrated a surface plasmon polariton mediated optical Moiré effect by inserting a silver slab between two subwavelength gratings. Enhancement of the evanescent fields by the surface plasmon excitations on the silver slab leads to a remarkable contrast improvement in the Moiré fringes from two subwavelength gratings. Numerical calculations, which agree very well with the experimental observation of evanescent-wave Moiré fringes, elucidate the crucial role of the surface plasmon polaritons. The near-field Moiré effect has potential applications to extend the existing Moiré techniques to subwavelength characterization of nanostructures.

Theory of optical imaging beyond the diffraction limit with a far-field superlens

Recent theoretical and experimental studies have shown that imaging with resolution well beyond the diffraction limit can be obtained with so-called superlenses. Images formed by such superlenses are, however, in the near field only, or a fraction of wavelength away from the lens. In this paper, we propose a far-field superlens (FSL) device which is composed of a planar superlens with periodical corrugation. We show in theory that when an object is placed in close proximity of such a FSL, a unique image can be formed in far-field. As an example, we demonstrate numerically that images of 40 nm lines with a 30 nm gap can be obtained from far-field data with properly designed FSL working at 376nm wavelength.

Comment on “Submicron imaging with a planar silver lens” [Appl. Phys. Lett. 84, 4403 (2004)]

Melville, Blaikie, and Wolf reported a submicron imaging using a 120-nm-thick silver slab. Diffraction-limited features as small as 350 nm sat a 700 nm period d were imaged onto the photosensitive material with the silver slab. The image resolution is said to be improved compared to a control experiment with 120 nm of poly smethylmethacrylate d sPMMAd between the mask and the photoresist wi hout silver filmd. Despite an extensive and intended connection to the superlens theory 2 throughout the letter and the following news thereof, 3 this experiment in our opinion is irrelevant to the referred superlens theory. This comment clarifies the interpretation of the experiment reported by Melville et al. Regarding the work reported in Ref. 1, we believe that the 120 nm silver film does not help to record an image of the mask. As we will show, the transmission behavior through this thick silver film shows that incident waves are strongly attenuated for all near field features including propagating and evanescent waves, thus the system does not enhance any evanescent waves as a superlens does. The blurred image recorded by the control experiment is in our opinion likely due to an excessive exposure. The basic theoretical idea suggested by Pendry 2 is based on the potential of a silver film to transmit and strongly enhance evanescent waves which carry subwavelength informationsoriginally scattered by the mask d. This so-called superlensing effect may be used to rebuild a subwavelength image. 4 But because the minimum feature size of the mask in this experiment is about the incident wavelength, the imaging reconstruction of the mask does not require a subwavelength imaging method, in other words, is not relevant to a superlens that significantly enhances evanescent waves scattered by the object. In Fig. 1, we calculate the transmission transfer function of TM wavesslike in the experiment Fig. 1 of Ref. 1 d for a 120 nm silver at 365 nm wavelength, which is the amplitude ratio of the transmitted H field over the incident H field as a function of the transverse wave number. It can be seen that the 120 nm silver film actually acts like an attenuator. For instance, for the 700 nm period object, the corresponding incident propagating waves have only less than 1% of the transmitted intensity. More importantly, this 120 nm silver film does not enhance evanescent waves. Since a broadband wavelength of 300–450 nm is used in Ref. 1 as described in detail in Ref. 5, we further found that the transmission for 300–450 nm has very similar behavior as 365 nm. A superlens effect is expected only for wavelength longer than 400 nm for the system used in Ref. 1 for a very narrow band of transverse wave number corresponding to evanescent waves that cannot be generated by the mask in the experiment of Ref. 1. A blurred optical image shown Fig. 2 sad of Ref. 1 is obtained by a control experiment using the same thickness of PMMA s120 nmd but without silver film. In contrast, the image obtained using a silver film sandwiched between two 60 nm layers of PMMA is in agreement with the mask topography. According to Melvilleet al., the image is blurred because of diffraction effect in the PMMA. In our opinion, the blurred image is likely due to an overexposure. We note that a comparable exposure time of 2 min is used in both the experiment with silver and the control experiment. In the control experiment, the transmission will be significantly higher than the experiment with silver where less than 1% of the energy is transmitted through this silver film. We think that the appropriate exposure time used in the control experiment should be drastically reduced to obtain a good image. Therefore, the blurred image in the control experiment cannot serve as a trustworthy verification or confirmation in Ref. 1. Despite the very low transmission factor of waves through this thicks120 nmd silver, the recorded image shown in Fig. 2sbd of Ref. 1 is in agreement with the mask topog-