2022

<10> Antenna-based reduced IR absorbers for high-performance microbolometers

S Jagan Mohan Rao, Dai-Sik Kim, Seon Namgung, and Dukhyung Lee

Optics Letters 6305-6308 (2022)

Abstract: Absorbers for long-wavelength infrared (LWIR) are designed to have a reduced geometry fitted to a gold cross antenna and numerically studied. Compared to the square membrane geometry widely used in conventional microbolometers, the reduced geometry results in smaller thermal capacities of the vanadium dioxide (VO2) and silicon nitride (Si3N4) layers. However, near-field focusing by the cross antenna leads to a high LWIR absorption. Calculations show that the temperature change per incident energy increases with a decrease in the arm width, and the reduced absorber surpasses the square geometry for all incident angles and polarizations. The antenna-based reduced absorber studied here could serve as an alternative geometry for high-performance microbolometers.

<9> Near-maximum microwave absorption in a thin metal film at the pseudo-free-standing limit

Mahsa Haddadi. M, Bamadev Das, Jeeyoon Jeong, Sunghwan Kim & Dai‑Sik Kim

Scientific Reports 18386 (2022)

(a) Graphic description of the three-layer ultra-thin gold film, where a film of thickness h with complex refractive index n1=nm is embedded between n0 and n1. (b) The dependence of conductivity on gold film thickness (the axis of the conductivity is in the log scale). Conductivities are calculated from supplementary information S1 Eq. (2), using the value of bulk conductivity of the thick gold film 4.1×107(Ωm)−1 (dashed yellow line). Absorbance varying thickness of the thin film gold optimized for normal-incidence absorption in free standing limit n0=n2=1 where, (c) σ is the bulk conductivity, and (d) the conductivity defined by the (b) (the axis of the film thickness is in the log scale and dashed blue line indicate h = h0).

Abstract: Electromagnetic absorbers based on ultra-thin metallic film are desirable for many applications such as plasmonics, metamaterials, and long-wavelength detectors. A metallic film will achieve a maximum 50% of electromagnetic wave absorption, frequency independent, at a thickness defined by its conductivity, typically in the sub-Angstrom range for good metals if bulk conductivity is maintained throughout. This makes it extremely difficult to obtain substantial absorption from thin metal films, in contrast to 2D materials such as graphene. Luckily, however, from a practical point of view, metal conductivity is drastically reduced as the film becomes sub-100 nm, to make it a race between the thinnest possible metal thickness experimentally achievable vs the conductivity reduction. Here, we demonstrate a near-50% absorption at a gold film thickness of 6.5 nm, with conductivity much reduced from the bulk value, down to the range of 10⁶ Siemens per meter. Studying the effect of the substrate thickness, we found that the common cover glass, with its thickness much smaller than the wavelength, achieves symmetric absorption of 44%, implying that a pseudo-free-standing limit is achieved. Our work may find applications in infrared sensing as in bolometers and biomedical sensing using microwaves.

<8> Detection of Single Nanoparticles inside a Single Terahertz Resonator

Young-Mi Bahk, Kyoung-Ho Kim, Gangseon Ji, Kwang Jun Ahn, Dai-Sik Kim, Hyeong-Ryeol Park

Advanced Photonics Research (Open Access) 2200134 (2022)

a,b) Calculated field enhancement spectra of a single THz nanoresonator with (red and yellow line) and without the single gold NP (blue line), using the FEM simulation. The gap width at the top surface of the nanoresonator was 120 nm. (a) The NP with a diameter of 100 nm was placed inside the gap region, and (b) the NP with a diameter of 200 nm was placed on top of the gap. The NPs were placed at the center (red lines) and 40 μm (orange lines) from the center of the 150 μm-long resonator. c,d) Normalized current density ( J x ) distributions with: (c) the 100 nm NP embedded in the gap and (d) the 200 nm placed on top of the gap. Both of the NPs are located at the center of the long-axis length of the resonator. The black arrows describe the directions of the conducting currents through the NPs in the cross-sectional view. The current density J x is normalized to the maximum value of each figure.

Abstract: With the rapid advancement of 5G/6G communications using millimeter wavelengths, the concomitant usage of these long wavelength radiation for remote sensing and monitoring of biological and chemical agents is anticipated. However, the ability to detect and identify these agents with sizes ranging from nanometers to microns is hampered by its millimeter wavelength, which drastically reduces the interaction cross-section. Herein, it is reported that single gold nanoparticles (NPs) drop-casted on the nanoresonator can be observed by monitoring the far-field transmitting spectra of individual terahertz (THz) nanoresonators, which enhance the electric field hundreds of times on the nanoscale. Despite the enormous mismatch in length scales, full-wave 3D numerical modeling of the single THz nanoresonator is also performed to interpret the experimental results, indicating the possibility to turn off the resonance using only one NP embedded in the hotspot of the nanoresonator. Such NP detection becomes the most sensitive when the particle, whose size is comparable to the gap width, is tightly fitted into the nanoresonator. This work unveils the potential associated with refractive index sensing and hyperspectral absorption spectroscopy for detecting and fingerprinting ultra-low density of bio/chemical molecules such as viruses, lipid vesicles, and explosives.

<7> Broadband high-performance terahertz polarizer based on a dense array of 5 nm gap slit antennas

Sunghwan Kim, Dasom Kim, Youjin Lee, Geon Lee, Jeeyoon Jeong, Dukhyung Lee, and Dai-Sik Kim

Optics Express 30(17), 30038-30046 (2022)

Fig. 1. FE-SEM images of the slit array with the 200 nm periodicity and the 5 nm-thick Al2O3 dielectric gap. (a) The first 100 nm-thick Au pattern with a 200 nm linewidth and a 400 nm pitch after the RIE process. (b-c) The slit array with the 200 nm periodicity and the 5 nm gap fabricated by the atomic layer lithography in (b) top and (c) cross-section. The 5 nm gap between the first and second Au was determined by the thickness of Al2O3 layer in the ALD process.

Abstract: Critical factors for terahertz polarizers include broadband operation, high transmittance, and a good extinction ratio. In this paper, using a 5 nm-wide metallic slit array with a 200 nm periodicity as a wire grid polarizer, we achieved over 95% transmittance with an average extinction ratio of 40 dB, over the entire spectrum as defined by the terahertz time-domain spectroscopy (0.4 ∼ 2 THz). Theoretical calculations revealed that the slit array can show 100% transmission up to 5 THz, and wider bandwidths with a higher cutoff frequency can be achieved by reducing the slit periodicity. These results provide a novel approach for achieving a broadband THz polarizer and open a new path for seamless integration of the polarizers with nanophotonic applications.

<6> Multifunctional Terahertz Transparency of a Thermally Oxidized Vanadium Metasurface over Insulator Metal Transition

Hyosim Yang,Dai-Sik Kim,Hyeong Seok Yun,Sunghwan Kim,Dukhyung Lee

Laser & Photonics Reviews 2200399 (2022)

Fabrication and characterization of VO2 metasurfaces. a) Schematics of fabrication steps: A vanadium slit array is patterned on a c-plane sapphire substrate by photolithography. After that, the vanadium slit array is annealed under optimized oxygen pressure and temperature. After the annealing process, the thickness of the film doubled. b–d) Scanning electron microscope b,c) top view and d) cross-sectional images of the VO2 metasurface. e) Change in conductivity of the VO2 metasurface and a bare VO2 film across the phase transition temperature. f) Raman spectra of the metasurface and the bare film measured at room temperature.

Abstract: Vanadium dioxide (VO₂) is one of the most promising materials for active metasurfaces due to the insulator-metal transition, urging the development of an etching-free patterning method and realization of multifunctionality in various spectral bands. Here, without etching, photolithography of vanadium metal followed by thermal oxidation achieve all-VO₂ slit array metasurfaces that can be exploited as a multifunctional terahertz (THz) transparent electrode. The metasurfaces retain approximately constant THz transparency over the phase transition while the electrical conductivity of the VO₂ lines changes about a thousand times, and near-infrared (NIR) diffraction is switched selectively. Numerical simulation shows that, during the phase transition, a decrease in THz transmission through the VO₂ lines is compensated for by funneling through the slits, which is especially efficient with a deep subwavelength period. On the contrary, at the NIR range, the optical path difference between the slits and the VO₂ lines is controlled according to the VO₂ phase, enabling switching between constructive and destructive interferences for a specific diffraction order. It is expected that the demonstrated patterning method and multifunctional THz transparency will promote VO₂-based metasurfaces, finding multispectral applications such as THz/NIR hybrid communication.

<5> Beyond-hot-spot absorption enhancement on top of terahertz nanotrenches

Jeong, Jeeyoon; Kim, Dai-Sik; Park, Hyeong-Ryeol

Nanophotonics 11(13), 3159-3167 (2022)

Abstract: Metallic nanogaps are being widely used for sensing applications, owing to their ability to confine and enhance electromagnetic field within the hot spots. Since the enhanced field does not confine itself perfectly within the gap, however, fringe fields well away from the gap are of potential use as well in real systems. Here, we extend the concept of near field absorption enhancement by quantitatively analyzing terahertz absorption behavior of water molecules outside the hot spots of sub-20 nm-wide, ∼100 μm-long nanotrenches. Contrary to point-gaps which show negligible field enhancement at distances larger than the gap width, our extended nanogap act as a line source, incorporating significant amount of absorption enhancement at much longer distances. We observe absorption enhancement factors of up to 3600 on top of a 5 nm-wide gap, and still well over 300 at 15 nm away. The finding is well supported by theoretical analyses including modal expansion calculations, Kirchhoff integral formalism and antenna theory. Our results provide means to quantitatively analyze light-matter interactions beyond the hot spot picture and enable application of nanogaps for sensitive surface analyses of various material systems.

<4> Effective terahertz shielding properties of extreme graphene-silver nanowire surfaces investigated by nanoprobing

Geon Lee,Sung Jun Kim,Yeeun Roh,Sang-Hun Lee,Dai-Sik Kim,Sang Woo Kim,Minah Seo

iScience 25, 104033 (2022)

Abstract:In the terahertz (THz) electromagnetic wave regime, which has recently received great attention in the fields of communication and security, shielding of THz waves is a significant issue. Therefore, carbon-based nanostructures or polymer–carbon nanocomposites have been widely explored. Herein, significantly enhanced THz shielding efficiency is reported for silver nanowires coated with reduced graphene oxide (rGO) and nanoscale THz metamaterials, as compared to the cases without nanoscale metamaterials. Using a nanoslot-patterned metamaterial with strong resonances at certain frequencies, THz transmission in intensity is enhanced up to three orders of magnitude. Enhanced transmission by nanopatterns substantially increases the shielding performance to the external THz waves, even for ultrathin films (several tens of nanometers) produced by a simple spray of rGO (a few nm of flakes) on a complex random nanowire network. Excellent shielding performance is presented and the shielding mechanism is investigated by the nanoprobing configuration at the same time.

<3> Spatially Controlled Fabrication of Surface-Enhanced Raman Scattering Hot Spots through Photoinduced Dewetting of Silver Thin Films

Jeeyoon Jeong, Hyun Woo Kim and Dai-Sik Kim

J. Phys. Chem. Lett. 13(13), 2969-2975 (2022)

Abstract: A well-designed narrow gap between noble metal nanostructures plays a prominent role in surface-enhanced Raman scattering (SERS) to concentrate electromagnetic fields at the local point, called a “hot spot”. However, SERS-active substrate fabrication remains a substantial hurdle due to the high process cost and the difficulty of engineering efficient plasmonic hot spots at the target area. In this study, we demonstrate a simple photolithographic method for generating ultrasensitive SERS hot spots at desired positions. The solid-state dewetting of a Ag thin film (thickness of ∼10 nm) using a continuous-wave laser (∼1 MW/cm²) generates a closely packed assembly of hemispherical Ag nanoislands. Some of these nanoislands provide substantial plasmonic-field enhancement that is sufficient for single-molecule detection and plasmon-catalyzed chemical reaction. Such hot spot structures can be patterned on the substrate with a spatial resolution of better than 1 μm. In integrated analytical devices, the patterned SERS hot spots can be used as position-specific chemical-sensing elements.

<2> Gaptronics: multilevel photonics applications spanning zero-nanometer limits

Jeeyoon Jeong, Hyun Woo Kim and Dai-Sik Kim

Nanophotonics 11(7), 1231-1260 (2022)

Abstract: With recent advances in nanofabrication technology, various metallic gap structures with gap widths reaching a few to sub-nanometer, and even ‘zero-nanometer’, have been realized. At such regime, metallic gaps not only exhibit strong electromagnetic field confinement and enhancement, but also incorporate various quantum phenomena in a macroscopic scale, finding applications in ultrasensitive detection using nanosystems, enhancement of light–matter interactions in low-dimensional materials, and ultralow-power manipulation of electromagnetic waves, etc. Therefore, moving beyond nanometer to ‘zero-nanometer’ can greatly diversify applications of metallic gaps and may open the field of dynamic ‘gaptronics.’ In this paper, an overview is given on wafer-scale metallic gap structures down to zero-nanometer gap width limit. Theoretical description of metallic gaps from sub-10 to zero-nanometer limit, various wafer-scale fabrication methods and their applications are presented. With such versatility and broadband applicability spanning visible to terahertz and even microwaves, the field of ‘gaptronics’ can be a central building block for photochemistry, quantum optical devices, and 5/6G communications.

<1> Perspective on future applications with lights concentrated in zero‑nanometer gaps

Hyunwoo Kim and Dai-Sik Kim

Journal of the Korean Physical Society 81, 525-531 (2022)

Abstract: Nanogap metallic structures is a useful platform for efficient manipulation of the light-matter interaction in extreme subwavelength scale. With the recent advances in fabrication technology, zero-nanometer scale (angstrom) gaps are now available with unprecedented control capability in gap widths, and with scalability in area and uniformity. In this paper, we provide perspectives on potential novel applications of zero-nanometer gaps. By reviewing the current research trends in two-dimensional material based optoelectronics, and novel lithography techniques, we explore the opportunities given to the zero-nanometer gap structures. Optoelectronic devices based on two-dimensional materials will have facile tuning capabilities in carrier excitation efficiency and working wavelengths when the zero-nanometer gaps are incorporated in the devices. In photolithography, patterning resolution can be reduced down to sub-nanometer. We anticipate that the perspectives described in the paper motivate the further researches in zero-nanometer gap applications opening the era of Gaptronics.