Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio

Near-field scanning optical microscopy (NSOM) technique has been widely used to satisfy the increasing demand for spatially resolving objects, especially at the nanoscale level. NSOM enables the user to observe the light-matter interaction beyond the diffraction limit thus leading to better spectroscopic analysis, optical imaging and chemical identification. Furthermore, it can be operated in a vacuum environment and hence does not require the presence of a medium.

Presently, two main types of NSOM commonly used are the aperture type and the scattering type. The former uses a tiny aperture connected to the tip apex of atomic force microscope probe whereby the size of the aperture determines the optical resolution. On the hand, there is no aperture in the NSOM scattering type. Instead, it consists of a sharp tip probe made of metal or semiconductor materials. The scattering type has better optical resolutions as compared to the aperture type.

However, these two types of NSOM still experience challenges that limit their workability. For example, the interference experienced between the scattered light and the illuminated light samples causes disturbances that lead to inaccurate measurements. Today, several approaches have been put forward towards the realization of NSOM that is capable of providing better optical throughput, signal to noise ration and spatial resolution beyond that provided by the aperture tips and scattered tips.

Researchers at National Tsing Hua University in Taiwan led by Professor Ta-Jen Yen developed a high efficiency plasmonic near-field scanning optical microscopy, with the aim of enhancing signal-to-noise ratio, resolution and throughput. The design is also capable of supporting Fabry-Perot resonance and radially polarized excitation for the focusing modes. Their research work is published in the journal, Nano Letters.

Ruei-Han Jiang and colleagues observed that the developed plasmonic tip outperformed the conventional NSOM aperture and scattering tips. For instance, it produced a topographic and optical resolution of 10nm, the throughput of 3.28% and a signal-noise ratio of 18.2.

The plasmonic tip (p-tip) was able to efficiently interfere with the surface plasmon polaritons (SPP) at the apex tip due to the presence of the subwavelength annular gratings on the facet of the tip. This shows that the p-tip can be used as a simplification of the conventional NSOM scattering tip by expanding its measurements and also eliminating the complex convolution and high-order analysis. Additionally, the high signal to noise ratio obtained in the experiment is attributed to the interference and propagation of the surface waves through plasmonic lens characterization.

Considering the fact that the p-tip can offer a near-background-free p-NSOM measurement, the authors are optimistic that it can be employed in various applications such as nanolithography, near-field optics and tip-enhanced spectroscopy among other several applications.