Near-Field Microscopy Through a SiC Superlens

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Science  15 Sep 2006:
Vol. 313, Issue 5793, pp. 1595
DOI: 10.1126/science.1131025


The wave nature of light limits the spatial resolution in classical microscopy to about half of the illumination wavelength. Recently, a new approach capable of achieving subwavelength spatial resolution, called superlensing, was invented, challenging the already established method of scanning near-field optical microscopy (SNOM). We combine the advantages of both techniques and demonstrate a novel imaging system where the objects no longer need to be in close proxim-ity to a near-field probe, allowing for optical near-field microscopy of subsurface objects at sub-wavelength-scale lateral resolution.

Diffraction limits the spatial resolution of classical optical microscopy to about one-half of the illuminating wavelength. In scanning near-field optical microscopy (SNOM) (1), the use of a nanofocus generated by a tiny hole (aperture) in the metallic coating of a tapered glass fiber tip that is raster-scanned over the object of interest can overcome the diffraction limit. Alternately, in its apertureless (2) or scattering (3) version (s-SNOM), the nanofocus is defined by the presence of strongly confined electromagnetic fields at the apex of a metallic probe tip. In both cases, the probe must be very close to the investigated object, which limits SNOM applications to surface studies. We combine near-field microcopy with a phenomenon called “superlensing” that allows for subwavelength-scale resolved imaging of buried objects.

A superlens uses a thin slab of a material with a negative permittivity, ϵ, to form a high-resolution optical image at the opposing side of the slab (46). In our experiment, we placed a SiC superlens (7) between the scanning probe tip of an infrared s-SNOM and objects located 880 nm below the tip. The superlens (Fig. 1A) is a 440-nm-thick single-crystalline SiC membrane coated on both sides with 220-nm-thick SiO2 layers (7). The two surfaces of the sandwich correspond to the object and the image planes of the lens, respectively. The object plane is covered by a Au film patterned with holes of different diameters and separations (Fig. 1B). In contrast to the first indirect demonstration of superlensing of a SiC slab concluded from enhanced transmission experiments (7), we directly map the image plane with a s-SNOM (3), recording both the amplitude and the phase of the optical field distribution (8). In our experiment, the superlens is operated in reflection mode where illumination and detection are carried out from the same side of the superlens. This extends its applicability to samples located on opaque substrates.

Fig. 1.

Near-field microscopy through a 880-nm-thick superlens structure: (A) Experimental setup. (B) Scanning electron micrograph (mirrored) of the object plane, showing holes in a 60-nm-thick Au film. (C) Infrared amplitude in the image plane at λ = 10.85 μm where superlensing is expected. (D) Infrared phase contrast (λ = 11.03 μm). (E) Control image showing infrared amplitude at λ = 9.25 μm (no superlensing). (F) Fourier tranforms of line scans taken from amplitude images of a grating (≈3 μm period, averaged over 26 scan lines), normalized to unity for zero frequency. High spatial frequencies (up to the grating's fourth harmonic) are imaged by the superlens/s-SNOM system around λ ≈ 10.84 μm wavelength.

At mid-infrared wavelengths, λ close to 11 μm, where superlensing is expected (7), the infrared amplitude (Fig. 1C) and phase images (Fig. 1D) resolve the 1200-nm and 860-nm holes. Even the smaller 540-nm holes exhibit sufficient optical contrast to allow for the detection of λ/20-sized objects 880 nm away from the near-field probe. A control image is taken at a wavelength of λ = 9.25 μm, where the superlensing condition (8) is not satisfied. The image (Fig. 1E) does not exhibit optical contrast demonstrating that s-SNOM alone cannot resolve the objects 880 nm below the surface.

To quantify the resolution enhancement provided by the superlens, a grating with a ≈3-μm period and a 440-nm slit width was imaged. The Fourier transforms of line scans taken perpendicularly to the slits (Fig. 1F) show that high spatial frequencies, up to the grating's fourth harmonic, can be recovered only in a narrow spectral range (around λ ≈ 10.84 μm), demonstrating the resonant character of the imaging process by a superlens. In resonance the spatial resolution of the subsurface features is enhanced by a factor of four compared with near-field imaging without superlensing of the SiC slab (e.g., at λ = 9.47 μm).

Superlens-based near-field microscopy is neither restricted to SiC lenses nor to infrared wavelengths. It can be applied from visible to terahertz frequencies where superlensing can be provided by flat metal films, thin slabs of polar crystals, or even by artificial media (9) such as metamaterials and photonic crystals. It thus opens the door for high-resolution optical imaging of various manmade or biological nanostructures not directly accessible by a near-field probe.

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