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Development of a microscope to discover the microscopic details ...
  • 글쓴이 : Communications Team
  • 조회 : 229
  • 일 자 : 2020-06-03


Development of a microscope to discover the microscopic details of nanostructures

Resolution of the near-field scanning optical microscope enhanced to expand the view of the nano world

 

▲ From the left, Associate Director Choi Won-shik (Professor of the Department of Physics, Korea University) of the Institute for Basic Science (IBS), Professor Kim Myung-ki (KU-KIST Graduate School of Converging Science and Technology), Research Professor Seo Eun-sung (IBS Center for Molecular Spectroscopy and Dynamics), and researcher Jin Young-ho (KU-KIST Graduate School of Converging Science and Technology)

 

When a star seen as one with the naked eye is observed through a telescope, a cluster of several stars may become apparent. This is because a telescope of higher resolution than our eyes reveals hidden information that was previously invisible. While telescopes are used to observe distant objects, microscopes are used to observe the microscopic world. In particular, the Near Field Scanning Optical Microscope (NSOM) is a tool for observing phenomena occurring in the nano world.

* Resolving power: The minimum identifiable distance or time between two points on an object is an important factor for determining the performance of optical instruments including microscopes. The higher the resolution, the more advantageous it is for observing object detail.

 

Teams headed by Associate-Director Choi Won-shik (Professor of the Department of Physics, Korea University) of the IBS Center for Molecular Spectroscopy and Dynamics and Professor Kim Myung-ki (KU-KIST Convergence Graduate School of Korea University) collaborated to improve the resolution of existing near-field scanning optical microscopes and developed an imaging technology capable of determining even the microscopic information of nanostructures that had been previously difficult to observe.

 

The near-field refers to light that has not propagated far in space and has been localized on the sample surface. Near-field scanning optical microscopes scan the sample after advancing a probe tip of small aperture to a close-range access distance of about 20 nm from the surface of the sample. Through the interaction between the probe tip and the surface, the height information of the sample is obtained and the optical signal passing through the small aperture is imaged.

 

Although near-field scanning optical microscopy is a useful tool for observing the nanoworld, it has the limitation that anything smaller than the size of the aperture of the probe tip is imperceptible. The smaller the aperture size becomes, the higher the resolution, but the lower the intensity of the optical signal, complicating measurement. This had made it impossible to observe a microstructure with an aperture size smaller than about 150 nm using a conventional near-field scanning optical microscope.

 

The researchers overcame this limitation by successfully increasing the resolution of a near-field scanning optical microscope. First, the researchers coated the glass surface with gold, and then used a focused ion beam device to draw two rectangles 50 nm apart. The ‘double nano-slit nanostructure’ prepared in this way was used as a sample for evaluating the resolution of a near-field scanning optical microscope.

 

When light is incident on the double nano-slit nanostructure at an angle, a very weak antisymmetric mode is formed due to the phase difference of the light between the nano-slits. The antisymmetric mode contains microscopic information that can determine that a double nano-slit is comprised of two individual nano-slits. However, the antisymmetric mode was difficult to image separately using existing technology because it was obscured by the stronger symmetric mode.

* Symmetric mode and antisymmetric mode: In the symmetric mode, the phases applied to the two nano-slits are the same, and in the antisymmetric mode the phases of the two nano-slits are opposite. When the phases of the two nano-slits are opposite, offset interference occurs at the midpoint between the double nano-slit and optical signal images of the now distinguishable nano-slits are obtainable.

 

The researchers used a near-field scanning optical microscope to find hidden symmetric modes using the near-field images that occur when light is incident from a varied angle. With light incident from 100 angles, the near-field was recorded, and the hidden anti-symmetric mode was visualized through calculation and image processing. Conventional microscopes image such a double nano-slit as one point, but the developed microscope was confirmed as being able to distinguish the nano-slits from each other. The resolving power was improved to the point where identification of microscopic information one third the size of the probe tip aperture was possible.

 

Professor Kim Myung-ki, co-corresponding author, said, “The calculation is like finding the solution to a simultaneous equation. Existing technology only visualized the mode with the strongest signal strength, but the developed microscope finds all the existing hidden modes, enabling the collection of more microscopic information.”

 

In general, electron microscopy is used as a tool for microscopic observation at the nanometer scale. Unlike electron microscopes, which allow the microscopic observation of samples only in a vacuum, a near-field scanning optical microscope can observe samples in normal atmospheric conditions. Therefore, it is expected that the technology developed by the researchers can broaden the field of view of the nano world by complementing the existing electron microscope.

 

Associate-Director Choi Won-shik said, “With the development of ultra-small semiconductors and nano photonics, the importance of imaging technology with nanometer-level resolution is growing. We plan to improve our technology to understand more complex microscopic nanostructures.”

 

The results of the study were published in the May 22 online edition of the international journal, Nature Communications (IF 11.878).

* Paper title: Near-field transmission matrix microscopy for mapping high-order eigenmodes of subwavelength nanostructures

* Author information: Seo Eun-sung, Jin Young-ho, Choi Won-jun, Jo Yong-hyeon, Lee Su-yeon, Song Kyung-deok, Ahn Joon-mo, Park Q-han, Kim Myung-ki, and Choi Won-shik


 

[Figure 1] Overview of the near-field optical electron microscope developed by the researchers

Overview of the near-field optical electron microscope (far-field near-field transmission matrix imaging microscope) developed by researchers at the IBS Center for Molecular Spectroscopy and Dynamics. This is a combination of a Spatial Light Modulator (SLM) that controls the wavefront and a near-field scanning electron microscope (NSOM). It was possible to measure the near-field of the sample by illuminating light from a varied angle, and thereby observe the hidden eigenmode.

 

 

[Figure 2] Working principle of the near-field scanning optical microscope developed by the researchers

The researchers illuminated plane waves of varied angle from the bottom of the nano-slits and developed an imaging system that measures the near-field by using a probe tip on the transmission surface.

 

 

[Figure 3] Symmetric mode and antisymmetric mode

The conventional near-field scanning optical microscope was able to recognize only the symmetric mode with strong intensity. The symmetric mode is the mode in which the phases are equally applied to the nano-slits, so double nano-slits are seen as only one rectangle. In contrast, the developed microscope can even recognize the antisymmetric mode. In the antisymmetric mode the nano-slits have opposite phases and can be distinguished as two individual nano-slits.

 

 

[Figure 4] Verification of experimental results through simulation

Researchers confirmed through simulation that the optical eigenmodes measured are actual existing modes. (a) A simulation under symmetric conditions was performed, (b) the spectra of the optical eigenmodes were measured, and (c) the electric field images of each eigenmode were obtained and confirmed to coincide with the experimental results.

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