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15 Sep 2006
Researchers have obtained the first direct near-field optical images using a so-called superlens made of negative refractive index metamaterials. The development could enhance near-field microscopy.
The first direct near-field optical images from a superlens have been obtained by researchers in Germany and the US. Superlenses are made of negative refractive index metamaterials and this breakthrough could lead to their use in enhancing near-field microscopy for imaging biological samples and materials used in electronics (Science 313. 1595).
The resolution of conventional optical microscopes is restricted by near-field effects; as a result, they are unable to image objects below approximately a wavelength in size, which limits their use. This is because the near-field portion of the light -- which contains the object's sub-wavelength spatial details -- decays or evanesces quickly with distance compared to the "far-field" portion, which is easily refocused by a conventional lens.
Scanning near-field optical microscopes (SNOMs) are able to probe beneath this limit, by operating in close proximity to the surface of the sample. This has, however, limited SNOMs to surface studies and (in for example the case of fragile biological samples) risked mechanical contact.
Now researchers at the Nanophotonics group at Germany's Max Planck Institute for Biochemistry, working with physicists at the University of Texas, have obtained direct sub-wavelength images of objects by fitting a conventional SNOM with a superlens.
"We have demonstrated, for the first time, that the image created by a superlens can be recorded by an optical method instead of using lithographical recording, as reported previously," lead researcher, Rainer Hillenbrand of the Max Planck Institute, told physicsweb.org. "Thanks to the superlens, near-field optical microscopy is no longer restricted to surface studies. It opens up new applications since the probe no longer has to be in such close proximity to the object."
The experiment also provides further insight into how superlenses work: showing, for the first time, the amplitude and phase of the optical field distribution from a superlens and directly quantifying the resolution enhancement.
Possible applications cited by the team include the imaging of biological objects in their natural environment. In addition, an infrared superlens could find application in the semiconductor industry where it could probe metallic interconnects buried under layers of glass or other dielectrics.
The SiC superlens is 880 nm thick and comprises a 440 nm SiC crystal sandwiched between two 220 nm thick SiO2 layers. The superlens was located between the tip of a scanning infrared SNOM and the objects, which consisted of a gold film patterned with holes of different diameters. The researchers carried out infrared illumination and detection from the same side of the superlens and found that the setup could resolve a 540 nm hole using light with a wavelength of about 11 µm (the hole being approximately one twentieth the size of the incident wavelength).
"We were able to image objects that could previously only be imaged if the tip of the near-field optical microscope (NSOM) was within 50 nm or so from the object. In our experiment, by using a superlens we increased the separation between the tip and the object to almost 1 µm," Gennady Shvets, based at the University of Texas, told physicweb.org.
The researchers now want to develop thinner superlenses to increase the resolution and also apply the method to the visible light regime by using silver superlenses.
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