Exploring the World Inside the Body with Shining Fibers
[POSTECH research team led by professors Chulhong Kim and Junsuk Rho succeeds in imaging photoacoustic signals obtained with near-field for the first time]
[The team applied fiber optics instead of lenses to open a new horizon of super-resolution photoacoustic microscopes]
Microscopes allow us to explore tiny worlds that are invisible to the naked eye. In particular, photoacoustic microscopy – that uses vibrations generated when light is absorbed – that captures images of cells or blood vessels has been recently developed. This development has increased the possibility of peering inside the body without contrast agents. However, obtaining super-resolution images has been limited due to the difficulty of focusing light into a small, single point with a lens. Recently, a POSTECH research team has opened a new horizon in the development of a super-resolution photoacoustic microscopy system by using fibers instead of lenses.
A POSTECH research team led by Professor Chulhong Kim (Department of Convergence IT Engineering ‧ Electrical ‧ Mechanical Engineering), Dr. Byullee Park (Department of Convergence IT Engineering), and Ph.D. candidate Moongyu Han, in collaboration with Professor Junsuk Rho (Department of Mechanical and Chemical Engineering) and Ph.D. candidate Hongyoon Kim (Department of Mechanical Engineering) have successfully acquired images of photoacoustic signals using a near-field optical fiber for the first time.
When using a photoacoustic microscopy, light is usually collected into the lens at a certain distance between the light source and the sample; however, owing to diffraction limitations*1, collecting light at a single point has been challenging. To overcome this issue, the research team developed a microscopy system that maintains the distance between the light source and the sample in the near-field range (tens of nm) where the diffraction phenomenon does not occur by using a tapered fiber with a diameter of tens of nanometers (nm, 1 nm = 1 billionth of a meter).
This is the first study to apply a noncoated fiber to the photoacoustic microscopy. As the metallic component had an impact on the photoacoustic signals, conventional metallic-coated tapered fibers could not be used for photoacoustic microscopy.
As a result of the study, evanescent wave*2 was generated at the end of the optical fiber, and the images of the sample were acquired with a resolution of 1.0 ± 0.3 micrometers (μm, 1 μm = 1 millionth of a meter). This resolution is sufficient for cellular imaging, including red blood cells.
The findings from the study are an achievement that will be the cornerstone for the development of a super-resolution photoacoustic microscopy using optical tapered fibers. It is anticipated to be used as a research instrument to study the fundamental life phenomena that form the basis of various diseases, including cardiovascular disease and cancer.
Recently published in Laser and Photonics Reviews, this study was conducted with the support from the National Research Foundation of Korea’s Mid-level Technology, BRIDGE Convergence R&D Program, Pioneer Program of Future Technology, Global Frontier Program, Korea Medical Device Development Fund grant, Industrial Innovation Talent Growth Support (R&D), and the BK21 Project.
1. Diffraction Limit
Diffraction refers to the phenomenon wherein a wave expands and travels following an encounter with a slit or an obstacle. Moreover, the diffraction limit refers to the limitation of the resolution of an optical microscope due to the diffraction phenomenon of light.
2. Extinction Wave
A wave that carries high-resolution information below the diffraction limit, which exists only at extremely short distances.