Muhan Choi

This study systematically investigated an atmospheric pressure plasma reactor with a centered single pin electrode inside a dielectric tube for depositing the polyaniline (PANI) thin film based on the experimental case studies relative to variations in pin electrode configurations (cases I, II, and III), bluff-body heights, and argon (Ar) gas flow rates. In these cases, the intensified charge-coupled device and optical emission spectroscopy were analyzed to investigate the factors affecting intensive glow-like plasma generation for deposition with a large area. Compared to case I, the intense glow-like plasma of the cases II and III generated abundant reactive nitrogen species (RNSs) and excited argon radical species for fragmentation and recombination of PANI. In case III, the film thickness and deposition rate of the PANI thin film were about 450 nm and 7.5 nm/min, respectively. This increase may imply that the increase in the excited radical species contributes to the fragmentation and recombination due to the increase in RNSs and excited argon radicals during the atmospheric pressure (AP) plasma polymerization to obtain the PANI thin film. This intense glow-like plasma generated broadly by the AP plasma reactor can uniformly deposit the PANI thin film, which is confirmed by field emission-scanning electron microscopy and Fourier transform infrared spectroscopy.

Unlike the ideal circular whispering gallery cavities, those without mirror symmetry intrinsically support resonant modes exhibiting chirality which indicates an imbalance between clockwise and counterclockwise wave components. In extreme cases, nearly degenerate pairs of copropagating modes can be found around the chiral exceptional points (EPs) in parameter spaces. The chiral EPs have been studied in various schemes; however, most attention has been focused on the cases with piecewise constant or periodic refractive index profiles. In this Letter, we report the formation of a chiral EP in a gradient-index cavity designed by conformal transformation optics. Here, the mirror symmetry of the cavity is broken solely by its gradient index profile, and the parameter space is constructed with coordinate transformation parameters. We unveil the chirality, nonorthogonality, and complex-square-root topology near the chiral EP, which can be explained by the non-Hermitian model Hamiltonian.

The electrical properties of polycrystalline graphene grown by chemical vapor deposition (CVD) are determined by grain-related parameters—average grain size, single-crystalline grain sheet resistance, and grain boundary (GB) resistivity. However, extracting these parameters still remains challenging because of the difficulty in observing graphene GBs and decoupling the grain sheet resistance and GB resistivity. In this work, we developed an electrical characterization method that can extract the average grain size, single-crystalline grain sheet resistance, and GB resistivity simultaneously. We observed that the material property, graphene sheet resistance, could depend on the device dimension and developed an analytical resistance model based on the cumulative distribution function of the gamma distribution, explaining the effect of the GB density and distribution in the graphene channel. We applied this model to CVD-grown monolayer graphene by characterizing transmission-line model patterns and simultaneously extracted the average grain size (~5.95 μm), single-crystalline grain sheet resistance (~321 Ω/sq), and GB resistivity (~18.16 kΩ-μm) of the CVD-graphene layer. The extracted values agreed well with those obtained from scanning electron microscopy images of ultraviolet/ozone-treated GBs and the electrical characterization of graphene devices with sub-micrometer channel lengths.

Using the transformation cavity, a gradient index cavity designed by transformation optics, we propose a hybrid resonator system to extract unidirectional narrow-beam emission from high-Q whispering gallery modes by embedding a transformation cavity inside a deformed uniform index cavity that exhibits unidirectional narrow-beam emission. For effective mode coupling between the transformation cavity and enclosing cavity, the embedded transformation cavity is designed to have bidirectional evanescent emission, which enables most of the emission from the transformation cavity to be laterally incident on the rim of the enclosing deformed cavity. Consequently, ultrahigh-Q resonances of this system can provide a sharp free-space light output, which is difficult to achieve by embedding a homogeneous disk cavity instead of the transformation cavity.

It was reported that whispering gallery cavities designed by conformal transformation optics can support high-Q resonant modes with emission directionality. Intrinsically, these cavities have gradient index profiles implementing conformal mappings in physical space. In this paper, using the linear coordinate transformation, we propose another design scheme of whispering gallery cavities with (piecewise-) homogeneous, anisotropic index profile. We numerically show that so-designed cavities are also able to support high-Q whispering gallery modes with directional far-field emission patterns. We verify such characteristics by using a phase space representation (called the Poincaré Husimi function) of the intracavity wave function.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Circle-shaped transformation cavities of shifted center are the most simple transformation cavities in the sense that they have only spatial index variation without boundary shape deformation. For these cavities, we analyze the characteristics of conformal whispering gallery modes according to center-shift coordinate transformation. For this study, we use an alternative scheme for designing a transformation cavity without using a shrinking parameter that is needed to satisfy total internal reflection condition. It is observed that the isotropic emission of whispering gallery modes in the uniform index circular cavity changes to the bi-directional emission of conformal whispering gallery modes as the center-shift increases. Also, using a purity factor that quantitatively measures in RV space the degree of intactness for the internal wave pattern of resonant modes in transformation cavities compared with corresponding whispering gallery mode in uniform disk cavity, we show that the conformal whispering gallery modes in the circle-shaped transformation cavity has not only directionality but also high persistence for characteristics of whispering gallery modes.

Graphene-metal contact is crucial to fabricate high-performance graphene photodetectors since the external quantum efficiency (EQE) of the photodetector depends on the contact properties, and the influence of the contact properties is particularly dominant in short channel devices for high-speed applications. Moreover, junction properties between the channel graphene and graphene near the contact are also important to analyze the photoresponse because the built-in electric field in the junction determines the EQE of the photodetector. In this study, we investigated a relation between the photoresponse and the built-in electric field induced from the doping level difference in the junction between the channel graphene and graphene near the contact. The photoresponse could be enhanced with a high junction barrier height that is tuned by the doping level difference. In addition, we observed that the improved electrical characteristics of channel graphene do not guarantee the enhancement of the photoresponse characteristics of graphene photodetectors.

We investigated the effect of reduced graphene oxide (rGO) doping on the birefringence of 5CB liquid crystal (LC). The characteristics of the synthesized rGO and LC-rGO composite with different rGO concentrations were analyzed by atomic force microscopy, X-ray photoelectron spectroscopy, white light polarized microscopy, voltage-dependent transmission measurement, and differential scanning calorimetry. We found that doping LC with an appropriate concentration of rGO enhances the birefringence of the LC. This is mainly due to the improved anisotropy of polarizability, which stems from the high shape anisotropy of rGO. However, the aggregation of rGO reduces the birefringence by decreasing the anisotropy of polarizability as well as the order parameter. Our study shows the promising potential of LC-rGO for developing various electro-optic devices that offer improved electro-optic effects.

In the case of two-dimensional gradient index cavities designed by the conformal transformation optics, we propose a boundary integral equation method for the calculation of resonant mode functions by employing a fictitious space which is reciprocally equivalent to the physical space. Using the Green’s function of the interior region of the uniform index cavity in the fictitious space, resonant mode functions and their far-field distributions in the physical space can be obtained. As a verification, resonant modes in limaçon-shaped transformation cavities were calculated and mode patterns and far-field intensity distributions were compared with those of the same modes obtained from the finite element method.