- Three joint research teams, including Incheon National University, first detected a single gold nanoparticle with millimeter waves

- Successful detection with optical clamping technology and nano resonator... Published by Nano Letters

Left in the back row) Kim Tae-hoon (Fourth year of physics department at Incheon National University), Professor Lee Deok-hyung (research professor at UNIST Department of Physics), Dr. Lee Ji-ye (Samsung Semiconductor Research Institute), and Kim Seong-hoon (Master of Physics Program at Incheon National University)

Left in front row) Professor Park Young-mi (adjunct professor of physics at Incheon National University), Professor Kim Dae-sik (special training professor of physics at UNIST), Professor Seo Min-ah (Korea Institute of Science and Technology and Professor of KU-KIST at Korea University)

Professor Park Young-mi's team from the Department of Physics at Incheon National University (President Park Jong-tae) worked with Professor Kim Dae-sik's team at UNIST, the Korea Institute of Science and Technology, and Professor Seo Min-ah of Korea University to detect single gold nanoparticles with millimeter waves using light-gathering technology for the first time in the world.

It focuses millimeter waves to a nanometer size with a Bowtie nanoantenna, a nano-resonant, and captures gold nanoparticles in the focused area using light collection technology to detect gold nanoparticles as millimeter waves in real time.

THz waves (terahertz waves), which have a frequency of 100 GHz – 10 THz and a millimeter wavelength, are a frequency band for 6G mobile communication and are sensitive to natural vibrations of biochemical molecules, making them highly likely to be used in non-destructive biosensing fields. However, it is very difficult to detect micrometer or nanometer-sized particles with terahertz waves due to the wavelength limit characteristics of electromagnetic waves that are difficult to detect materials of a size much smaller than the wavelength.

Research has been conducted to overcome the wavelength limit by using a terahertz nano-resonant that can focus electromagnetic waves at a nanoscale, but it was difficult to accurately position nanomaterials where electromagnetic waves were focused. Therefore, in previous studies, a large amount of nanoparticles had to be sprayed into a nano resonator, and it was impossible to detect a specific single nanoparticle.

To overcome this problem, the research team 1) manufactured a Bowtie-shaped nano-resonant that focuses terahertz waves to a nanometer size, and 2) placed a single gold nanoparticle in a strong concentration of terahertz waves in the Bowtie nano-resonant using light-capturing technology that captures specific nanoparticles at a desired location. 

The research team succeeded in detecting a single metal particle with a millimeter wavelength through a terahertz electromagnetic wave reflection experiment by combining terahertz nanoplasmonics, a research field that overcomes the wavelength limit of electromagnetic waves using a metal nanoscale resonator, and light clamp technology that places a single nanoparticle in a desired place using light.

This study is expected to be used in biotechnology, photochemical, medical, and terahertz active meta-elements for 6G communication by suggesting a method to improve the sensitivity of molecular sensing technology using nanoparticles. Professor Park Young-mi, who led the study, said, "A breakthrough tool has been invented to control the interaction between nanoparticles and resonators according to their relative positions," adding, "A new electricity has also been prepared in light-material interaction research."

The study was published online on May 6 in Nano Letters, a world-renowned international journal. The research was carried out with the support of the Korea Research Foundation (NRF) and the Information and Communication Planning and Evaluation Institute (IITP) of the Ministry of Science and ICT.

(the name of the thesis: Optical Tweezing Terahertz Probing for a Single Metal Nanoparticle)

https://pubs.acs.org/doi/10.1021/acs.nanolett.4c01439

Overview of research results

1. Research background

 Gold nanoparticles have been utilized in various applications such as molecular detection, drug delivery, and bioimaging. In particular, the plasmonic effect of gold nanoparticles enables sensitive detection of biochemical molecules. Terahertz waves, on the other hand, have non-destructive properties that have less heat damage or ionization effects on living bodies thanks to small photon energy (~4 meV), and there is a rotation/vibration mode of biochemical molecules in the terahertz (THz) band, so biochemical molecule detection studies using terahertz waves have been actively conducted. Therefore, detecting gold nanoparticles with terahertz waves is expected to eventually pave the way for sensitive detection of biochemical molecules. However, gold nanoparticles are not easy to detect because they are 10,000 times smaller than the wavelength of terahertz waves. Studies have reported that the detection of gold nanoparticles is possible by using a terahertz nano-resonant that focuses terahertz waves at the nanometer level, but for full-scale use of gold nanoparticles, a technology to precisely position a single gold nanoparticle in the desired place of the terahertz nano-resonant was required.

2. Contents of the study

 The research team combined light collector technology and nano resonator-based terahertz time-resolved spectroscopy. Light clamping technology is a technology that uses light to place specific nanoparticles where they want to be. In this study, gold nanoparticles were detected in real time by precisely adjusting the position of gold nanoparticles with optical tongs, placing terahertz waves in the gap part of the bowtie antenna where they are focused to the nanometer size, and measuring changes in the terahertz bowtie antenna reflection spectrum. A Bowtie antenna with a gap of several hundred nanometers in the center generates a strong electric field in the gap by terahertz resonance, and gold nanoparticles placed in the gap dramatically reduce this electric field, so gold nanoparticles are detected by overcoming the diffraction limit. These experimental results were also confirmed through simulation calculations. Simulation calculations also confirmed that biochemical molecules were detected through terahertz reflection signal changes when gold nanoparticles coated with several nanometers of biochemical molecules were placed in the Bowtie antenna gap portion with light tongs.

3. expectation effectiveness 

 The combination of the light collector system and the nano-resonator-based terahertz spectroscopy presented in the study allows nanoparticles to be placed freely near the terahertz resonator, presenting a new breakthrough in the development of terahertz wave detection technology for trace biochemical molecules using nanoparticles. It will also maximize the plasmonic effect of nanoparticles and terahertz resonators, which can also be utilized for the study of strong photo-matter interactions.