Jungho Mun

Amino-acid- and peptide-directed synthesis of chiral plasmonic gold nanoparticles

Understanding chirality, or handedness, in molecules is important because of the enantioselectivity that is observed in many biochemical reactions1, and because of the recent development of chiral metamaterials with exceptional light-manipulating capabilities, such as polarization control2–4, a negative refractive index5 and chiral sensing6. Chiral nanostructures have been produced using nanofabrication techniques such as lithography7 and molecular self-assembly8–11, but large-scale and simple fabrication methods for three-dimensional chiral structures remain a challenge. In this regard, chirality transfer represents a simpler and more efficient method for controlling chiral morphology12–18. Although a few studies18,19 have described the transfer of molecular chirality into micrometre-sized helical ceramic crystals, this technique has yet to be implemented for metal nanoparticles with sizes of hundreds of nanometres. Here we develop a strategy for synthesizing chiral gold nanoparticles that involves using amino acids and peptides to control the optical activity, handedness and chiral plasmonic resonance of the nanoparticles. The key requirement for achieving such chiral structures is the formation of high-Miller-index surfaces ({hkl}, h ≠ k ≠ l ≠ 0) that are intrinsically chiral, owing to the presence of ‘kink’ sites20–22 in the nanoparticles during growth. The presence of chiral components at the inorganic surface of the nanoparticles and in the amino acids and peptides results in enantioselective interactions at the interface between these elements; these interactions lead to asymmetric evolution of the nanoparticles and the formation of helicoid morphologies that consist of highly twisted chiral elements. The gold nanoparticles that we grow display strong chiral plasmonic optical activity (a dis-symmetry factor of 0.2), even when dispersed randomly in solution; this observation is supported by theoretical calculations and direct visualizations of macroscopic colour transformations. We anticipate that our strategy will aid in the rational design and fabrication of three-dimensional chiral nanostructures for use in plasmonic metamaterial applications.Chirality can be ‘encoded’ into gold nanoparticles by introducing chiral amino acids or peptides during the growth process, leading to the formation of helicoid morphologies.

Polarization-sensitive tunable absorber in visible and near-infrared regimes

A broadband tunable absorber is designed and fabricated. The tunable absorber is comprised of a dielectric-metal-dielectric multilayer and plasmonic grating. A large size of tunable absorber device is fabricated by nano-imprinting method. The experimental results show that over 90% absorption can be achieved within visible and near-infrared regimes. Moreover, the high absorption can be controlled by changing the polarization of incident light. This polarization-sensitive tunable absorber can have practical applications such as high-efficiency polarization detectors and transmissive polarizer.

Accordion-like plasmonic silver nanorod array exhibiting multiple electromagnetic responses

AbstractWe prepared a high-density array of “accordion-like” plasmonic silver nanorods over a large area (2.5 × 2.5 cm2) that exhibited multiple electromagnetic responses to visible and near-infrared (NIR) wavelengths. This array of “accordion-like” silver nanorods was fabricated by confining the lamellae-forming polystyrene-block-poly (methyl methacrylate) copolymer (PS-b-PMMA) inside the cylindrical pores of an aluminum oxide (AAO) template grafted with thin neutral brush layers. PS and PMMA lamellar nanodomains with sizes of 15 nm were alternatively stacked along the nanorod direction. After the AAO template was removed, a 5-nm-thick layer of silver was thermally deposited on only the PS nanodomains. Owing to the multiple resonances exhibited in the visible and NIR regimes, the array could be used for multi-analyte detection. Furthermore, this concept of fabricating sophisticated nanoscale architectures by utilizing block copolymer self-assembly and incorporating plasmonic metals into one nanodomain could be applied to realize large-scale metamaterials that function under visible and NIR wavelengths.Plasmonics: Nano-accordions playing the music of lightA simple technique for creating large array of nanorods that manipulate both visible and near-infrared light has been devised by researchers in South Korea. Metal nanostructures can dramatically influence light. These so-called plasmonic effects usually only occur at specific wavelengths when the light’s frequency matches the natural collective oscillation of surface electrons. The resonant wavelength depends on the dimensions of the nanostructure. Jin Kon Kim, Junsuk Rho, and co-workers from Pohang University of Science and Technology (POSTECH) fabricated nanorods that combine many metal nanostructures of differing dimensions and support multiple resonances. Their structures comprised a stack of alternating layers of polystyrene and poly(methylmethacylate). The outer edges of the polystyrene layers were coated in silver, making the nanorods look like tiny accordions. Arrays of these structures covering several square centimeters were constructed using block copolymer self-assembly.A high density of “accordion-like” silver nanorod array over a large area (~cm2) was fabricated by confining lamellar-forming polystyrene-block-poly (methyl methacrylate) copolymer (PS-b-PMMA) inside cylindrical pores of aluminum oxide (AAO) template grafted by thin neutral brush layers. After removing the AAO template, a 5 nm thick layer of silver was thermally deposited on only PS nanodomains. Owing to combination of hemispherical head and side silver rings, multiple resonances exhibited in the visible and NIR regimes. This sophisticated fabrication utilizing block copolymer self-assembly could be applied to multi-analyte detection and realization of large-scale metamaterials working at visible and NIR wavelengths.

Fabrication of three-dimensional suspended, interlayered and hierarchical nanostructures by accuracy-improved electron beam lithography overlay

Nanofabrication techniques are essential for exploring nanoscience and many closely related research fields such as materials, electronics, optics and photonics. Recently, three-dimensional (3D) nanofabrication techniques have been actively investigated through many different ways, however, it is still challenging to make elaborate and complex 3D nanostructures that many researchers want to realize for further interesting physics studies and device applications. Electron beam lithography, one of the two-dimensional (2D) nanofabrication techniques, is also feasible to realize elaborate 3D nanostructures by stacking each 2D nanostructures. However, alignment errors among the individual 2D nanostructures have been difficult to control due to some practical issues. In this work, we introduce a straightforward approach to drastically increase the overlay accuracy of sub-20 nm based on carefully designed alignmarks and calibrators. Three different types of 3D nanostructures whose designs are motivated from metamaterials and plasmonic structures have been demonstrated to verify the feasibility of the method, and the desired result has been achieved. We believe our work can provide a useful approach for building more advanced and complex 3D nanostructures.

Metasurfaces-Based Absorption and Reflection Control: Perfect Absorbers and Reflectors

In the past decade, the realisation of negative index materials has initiated extensive research into metamaterials. Perfect absorbers and reflectors are of particular interest as their usefulness is endless in a range of different fields and devices. Since it was originally shown that a device can achieve unity absorption of electromagnetic waves, it has become a hot area of research to develop perfect absorbers based on polarisation independence and incident angle independence, at a range of frequencies from microwave to optical ones. The amazing performance, flexibility, and tunability of these metamaterials will be discussed here, by presenting the different designs and working mechanisms that have been realised up to now. Their limitations and shortcomings will be addressed and future plans for perfect absorbers and reflectors will be suggested.

A Compact THz FEL at KAERI: the Project and the Status

A new compact THz free electron laser driven by a microtron is being developed recently at KAERI. It uses a hybrid electromagnetic undulator. A novel scheme of injection/extraction/outcoupling is developed. The machine is partially assembled and commissioned. Characteristic features and current state are described in the paper.

Surface-enhanced spectroscopy: Toward practical analysis probe

ABSTRACT Surface-enhanced spectroscopy (SES) is a consequence of extreme electromagnetic fields and chemical interactions near a surface. SES is highly sensitive and selective and has been exploited in chemistry, physics, biology, and medicine. It is a rapidly developing technique and is expected to become an important analysis tool. This review introduces theories and concepts of SES techniques including surface-enhanced (SE) Raman scattering, SE infrared absorption, SE chiroptical spectroscopy, and SE fluorescence. Then recent research and applications are discussed to indicate current challenges and future directions.

Demonstration of steering acoustic waves by generalized Eaton lens

We demonstrate an acoustic generalized Eaton lens that steers an acoustic wave to a desired angle, to propose a design method for gradient-index devices using the sub-wavelength structure to shape an acoustic wave. Based on investigations on the effective parameter with several methods: the S-parameter retrieval method, Floquet-Bloch calculations, and multiple scattering theory (MST) for the cylindrical rigid rod structure, we speculate the design process to realize inhomogeneous refractive index distribution. For realization of a high effective index (∼2), the MST design inevitably fails, whereas the Floquet-Bloch calculations and S-parameter retrieval give identical results. By appropriately designing a two-dimensional array of cylindrical PMMA rods, we experimentally verify the acoustic generalized Eaton lenses for steering angles of 15° and 45°. Furthermore, the use of non-resonant metamaterials enables the proposed devices to work in the broad frequency range from 4 to 8 kHz.We demonstrate an acoustic generalized Eaton lens that steers an acoustic wave to a desired angle, to propose a design method for gradient-index devices using the sub-wavelength structure to shape an acoustic wave. Based on investigations on the effective parameter with several methods: the S-parameter retrieval method, Floquet-Bloch calculations, and multiple scattering theory (MST) for the cylindrical rigid rod structure, we speculate the design process to realize inhomogeneous refractive index distribution. For realization of a high effective index (∼2), the MST design inevitably fails, whereas the Floquet-Bloch calculations and S-parameter retrieval give identical results. By appropriately designing a two-dimensional array of cylindrical PMMA rods, we experimentally verify the acoustic generalized Eaton lenses for steering angles of 15° and 45°. Furthermore, the use of non-resonant metamaterials enables the proposed devices to work in the broad frequency range from 4 to 8 kHz.

Towards 3D metamaterials at optical frequencies

Recent development of hierarchical fabrication techniques for three-dimensional metamaterial and plasmonic structures based on ultra-accurate and ultra-precise electron-beam lithography overlay will be discussed in this article.

Waveguide-Mode Terahertz Free Electron Lasers Driven by Magnetron-Based Microtrons

We have developed small-sized terahertz free-electron lasers by using low-cost and compact microtrons combining with magnetrons as high-power RF sources. We could stabilize the bunch repetition rate by optimizing a modulator for the magnetron and by coupling the magnetron with an accelerating cavity in the microtron. By developing high-performance undulators and low-loss waveguide-mode resonators having small cross-sectional areas, we could strengthen the interaction between the electron beam and the THz wave inside the FEL resonators to achieve lasing even with low-current electron beams from the microtron. We used a parallel-plate waveguide in a planar electromagnet undulator for our first THz FEL. We try to reduce the size of the FEL resonator by combining a dielectric-coated circular waveguide and a variable-period helical undulator to realize a table-top THz FEL for applying it to the security inspection on airports.