Hyeon-Don Kim

Synthetic Fourier transform light scattering.

We present synthetic Fourier transform light scattering, a method for measuring extended angle-resolved light scattering (ARLS) from individual microscopic samples. By measuring the light fields scattered from the sample plane and numerically synthesizing them in Fourier space, the angle range of the ARLS patterns is extended up to twice the numerical aperture of the imaging system with unprecedented sensitivity and precision. Extended ARLS patterns of individual microscopic polystyrene beads, healthy human red blood cells (RBCs), and Plasmodium falciparum-parasitized RBCs are presented.

Ultrafast refractive index control of a terahertz graphene metamaterial

We present an ultrafast dynamics of THz graphene-metamaterial hybrid devices, where the refractive index and the conductivity are largely modulated by electrical and optical methods. Unprecedentedly large modulation of refractive index and the pump-induced effective negative conductivity are investigated by an ultrafast time-resolved optical-pump THz-probe spectroscopy with varying gate voltage.

Nondispersive optical activity of meshed helical metamaterials

Extreme optical properties can be realized by the strong resonant response of metamaterials consisting of subwavelength-scale metallic resonators. However, highly dispersive optical properties resulting from strong resonances have impeded the broadband operation required for frequency-independent optical components or devices. Here we demonstrate that strong, flat broadband optical activity with high transparency can be obtained with meshed helical metamaterials in which metallic helical structures are networked and arranged to have fourfold rotational symmetry around the propagation axis. This nondispersive optical activity originates from the Drude-like response as well as the fourfold rotational symmetry of the meshed helical metamaterials. The theoretical concept is validated in a microwave experiment in which flat broadband optical activity with a designed magnitude of 45° per layer of metamaterial is measured. The broadband capabilities of chiral metamaterials may provide opportunities in the design of various broadband optical systems and applications.

Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

Broadband Modulation of Terahertz Waves With Non-Resonant Graphene Meta-Devices

Single-layer graphene absorbs a small fraction of incident terahertz waves by intraband transition of Dirac fermions. The amounts of absorption, reflection, and transmission of terahertz waves depend on the doping level of graphene, i.e., the Fermi level, and they exhibit relatively weak frequency dependency at terahertz frequencies. By hybridizing gated single-layer graphene with a non-resonant meta-atom structure, we show that the effective surface conductivity of meta-atom hybridized graphene can be significantly enhanced, and large intensity modulation of transmitted terahertz waves can be achieved without sacrificing the broadband modulation feature of single-layer graphene. For a frequency insensitive response, the meta-atoms are designed so that their resonance is positioned outside the frequencies of interest. Exploiting the enhanced effective surface conductivity with a non-resonant feature, larger modulation was possible over broad operating frequency from 0.3 to 2.3 THz. We anticipate that this electrically controlled graphene meta-device may play an important role in the realization of practical terahertz modulators.

Fabrication of a uniform microlens array over a large area using self-aligned diffuser lithography (SADL)

We describe a simple and effective method to fabricate a uniform plastic microlens array (MLA) with high fill-factor over a large area utilizing self-aligned diffuser lithography (SADL). In order to make an intimate contact between the photomask and the positive photoresist during 3D diffuser lithography, which is crucial for obtaining a uniform MLA mold over a large area, we fabricated a self-aligned metal mask directly on top of the positive photoresist, eliminating any air gap between the metal mask and the underlying photoresist. After replication of the developed concave MLA mold onto the poly(dimethylsiloxane) (PDMS), a standard deviation of sag (height) of the MLA was observed by laser scanning confocal lithography. The standard deviation, which indicates uniformity, was reduced by as much as a factor of 6 by applying SADL compared with that obtained from conventional diffuser lithography. Using this method, we fabricated a 7 inch MLA sheet with excellent uniformity. The proposed method can be extensively applied for fabrication of large-size MLA sheets with plastic materials thanks to its simplicity and versatility.

Electrical access to critical coupling of circularly polarized waves in graphene chiral metamaterials

Electric control of coupling in hybrid graphene/metamaterial system enables strong selective modulation of light polarization. Active control of polarization states of electromagnetic waves is highly desirable because of its diverse applications in information processing, telecommunications, and spectroscopy. However, despite the recent advances using artificial materials, most active polarization control schemes require optical stimuli necessitating complex optical setups. We experimentally demonstrate an alternative—direct electrical tuning of the polarization state of terahertz waves. Combining a chiral metamaterial with a gated single-layer sheet of graphene, we show that transmission of a terahertz wave with one circular polarization can be electrically controlled without affecting that of the other circular polarization, leading to large-intensity modulation depths (>99%) with a low gate voltage. This effective control of polarization is made possible by the full accessibility of three coupling regimes, that is, underdamped, critically damped, and overdamped regimes by electrical control of the graphene properties.

Photoinduced Nonlinear Mixing of Terahertz Dipole Resonances in Graphene Metadevices

The plethora of nonlinear optical phenomena can offer an innovative route for developing subwavelength-scale optical components. Here, using graphene-integrated metadevices, nonlinear interaction between two electric dipole resonances is demonstrated by ultrafast terahertz spectroscopy.

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

Novel buried inverse-trapezoidal micropattern for dual-sided light extracting backlight unit

We devised a novel buried inverse-trapezoidal (BIT) micropattern that can enable light extracting to both front and back sides of the backlight unit (BLU). The proposed BLU comprised of only a single-sheet light-guide plate (LGP) having the BIT micropatterns only on the top surface of the LGP. The proposed BLU shows normal directional light emitting characteristics to both the front and back sides of the LGP and successfully acts as a planer light source for a dual-sided LCD. The proposed BLU has the potential to dramatically reduce the thickness, weight and cost of the dual-sided LCD thanks to its single-sheet nature.