Physicists at Harvard University have demonstrated an optical antenna with potential applications in chemistry, biology and medicine. Known as a quantum cascade laser nanoantenna, the device was developed by Federico Capasso and others in Harvard’s School of Engineering and Applied Sciences. Their research findings were reported as the cover feature in the 22 October issue of Applied Physics Letters.
Capasso’s nanoantenna consists of two gold rods separated by a nanometre-sized gap, integrated with a quantum cascade laser. The laser emits infrared light with a wavelength of 7 μm, which is in the mid-infrared part of the electromagnetic spectrum where most molecules have their signature absorption lines. The antenna produces a concentrated spot of light some 50–100 times smaller than the laser wavelength, and this spot can be scanned across a prepared sample such as biological tissue to generate chemical maps of the surface with sub-wavelength resolution.
Existing Fourier transform infrared microscopes are diffraction limited to resolutions of a couple of microns. Aperture-less near-field scanning microscopes can significantly exceed the diffraction limit in the mid-infrared spectral region, but suffer from interference between the near and background fields, making it difficult to obtain the quantitative images necessary for detailed chemical analysis. They also require bulky lasers with limited tunability and wavelength coverage.
“There is currently a major push to develop powerful tabletop microscopes with spatial resolution much smaller than the wavelength that can provide images of materials, and in particular biological specimens, with chemical information on a nanometric scale,” says Capasso.
To use Capasso’s nanoantenna-based microscope, one would first impregnate a tissue sample with a chemical polymer such as paraffin to make it rigid, and slice the sample to below 100 nm in thickness with a device called a microtome. The microscope head then scans across the surface of the sample, recording absorption contrasts spot by spot. In this way one can build up a detailed chemical image of the sample.
Quantum cascade lasers were first demonstrated over a decade ago by Capasso and his group at Bell Labs. These millimetre length semiconductor devices are now commercially available, and are constructed by stacking nanometre thick layers of semiconductor materials on top of each other. By varying the thickness of the layers, the wavelength of the laser can be tuned across the entire molecular absorption spectrum. Applications of quantum cascade lasers include pollution monitoring, chemical sensing, medical diagnostics and security screening.
The new development is the antenna, but Capasso says that there is much work to do before it can be used to build a practical, sub-wavelength microscope. “We have another year of work before we can think of building a microscope, so it’s premature to think of partnering yet with others. But the ultimate product would be such a microscope.”
Capasso’s research plan is now to optimise the antenna. This will include the use of bowtie antennas, which consist of two triangular pieces of wire or flat metal plates arranged in a bowtie shape, with the feed point at the gap between the triangle apexes. “Bowtie antennas allow more concentration of intensity in the main illumination spot, and almost eliminate extraneous spots at the antenna ends,” says Capasso. “So we will work on optimising the antenna and reducing the stray light going through.”
Quantum cascade laser nanoantennas will enable chemical imaging at the biological cell level, and the study of viral structures below a micron in size. So far there has been little work done in this area, and Capasso believes that this is potentially the most exciting development.
“Near-field techniques are beginning to revolutionise imaging,” says Capasso. “The challenge is to make them quantitative, and this will take a lot of development.”
Further reading: Plasmonic quantum cascade laser antenna, Yu et al., App. Phys. Lett. 91, 173113 (2007).
Figure: Harvard University applied physicist Federico Capasso with quantum cascade laser nanoantenna study lead author Nanfang Yu. Superimposed are an atomic force microscope topographic image of the antenna, and an optical image obtained with a near-field scanning optical microscope showing the localised light spot in the antenna gap (source: Nanfang Yu, Ertugrul Cubukcu and Federico Capasso/Harvard University).
Article first published in Nanomaterials News.