Richard Gaughan, Contributing Editor
A team at the University of Alberta in Edmonton, Alberta, Canada, recently evaluated novel designs for near-field scanning optical microscope probes that provide both high throughput and high resolution. The results of the work suggest that throughput may be increased by orders of magnitude, offering benefits in biological imaging and near-field nanolithography.
Near-field scanning optical microscopy captures images with a resolution well beyond the diffraction limit. The key to the resolution is in the design of the optical probe, the scanning end of the microscope. A typical probe is a waveguide structure with an aperture much smaller than the wavelength of the illumination source. When the probe is brought close to the surface of a specimen, the size of the illuminated spot is governed by the aperture size rather than by the wavelength of the source. If the aperture is smaller than the diffraction limit, the excitation will be confined to a region that also is smaller than the diffraction limit.
Although the promised resolution is high, near-field scanning optical microscopy has been constrained by some practical considerations. First and foremost is low-intensity transmission through the aperture. For example, when 800-nm radiation is coupled through a 50-nm aperture, the throughput is less than 0.01 percent. Specialized fiber-based probes have intrinsically high resolution but also even lower throughputs — on the order of 0.001 percent or less. Throughput increases with aperture size, but increasing the aperture size reduces the resolution. So in practice, near-field scanning optical microscopy probe design is a trade-off between resolution and throughput.
Needlelike probe
Various schemes have been developed to optimize the trade-off. One is to use an apertureless probe, a hollow, needlelike structure that comes to a point a few nanometers in radius. The radiation injected into the waveguide that is formed by the interior hollow space couples with surface plasmon modes in the probe tip. The small size of the tip locally enhances the electromagnetic field.
Outside the tip, in the region brought near to the sample, the surface plasmon couples to radiative modes. If the tip is brought to within 25 nm or less of the sample surface, a subdiffraction-limited region is excited. Unfortunately, the surface plasmon coupling process is not very efficient, and the basic trade-off between resolution and throughput remains.
Additional modification of the optical probe geometry can improve the situation by orders of magnitude, according to models of the new tip designs. Abdulhakem Y. Elezzabi and his research team at the university’s department of electrical and computer engineering adopted a hybrid approach, combining elements of a small-aperture probe with surface plasmon coupling.
The scientists began with a modified atomic force microscope (AFM) probe. A typical AFM probe consists of a silicon chip platform, held on one end in cantilever fashion. The opposite end has a small needlelike or pyramidal structure that extends from the plane of the cantilever.
For an initial probe design, they chose a tapered cone constructed from a 50-nm-thick layer of silver. To provide mechanical support, the investigators surrounded the silver with a 150-nm-thick layer of chromium. The end of the cone was truncated, leaving a small aperture at the pointed end. Radiation coupled into surface plasmon modes in the silver, enhancing the transmission through the aperture.
Because throughput is a function of aperture size relative to wavelength, Elezzabi realized that an additional high-index layer could increase the coupling. A 1-μm-thick layer of GaP with an index of refraction of 3.36 essentially shortens the wavelength by a factor of the index.
Numerical models predicted that the throughput of 800-nm radiation would be enhanced by a factor of from three to more than 100, with the greatest improvement at the smallest aperture size. The models also indicated that scattering loss was a factor that could reduce the throughput.
Another design addressed the problem of loss. It featured the same hollow, truncated Ag/Cr cone, but instead of adding a coating layer, the scientists wedged a 10-μm-diameter SiO2 bead into the conical opening. The effects of this embedded solid immersion lens were even more dramatic. For a 50-nm aperture at 800 nm, the probe’s modeled throughput is 800 times greater than that of an unfilled one.
Practical tips based on the designs have been fabricated at the university’s micromachining and nanofabrication facility. The researchers have not yet had the opportunity to test them, but they are confident that the probes will perform as predicted because the physics involved is simple electromagnetics, Elezzabi said. They have not yet approached industrial partners but are interested in marketing the technology.
Elezzabi’s team is working on other nano-optics projects, such as applying near-field optical microscopy to the study of ultrafast time-resolved phenomena to create a novel technique for examining femtosecond dynamics on the nanometer scale. The scientists are designing femtosecond-resolution AFMs, in which 10-fs laser pulses are coupled into the near-field scanning optical microscope tips. The extremely short duration of these pulses will enable the probes to obtain measurements with high spatial and temporal resolution.