Hwi Kim

Three-dimensional display technologies of recent interest: principles, status, and issues [Invited]

Recent trends in three-dimensional (3D) display technologies are very interesting in that both old-fashioned and up-to-date technologies are being actively investigated together. The release of the first commercially successful 3D display product raised new research topics in stereoscopic display. Autostereoscopic display renders a ray field of a 3D image, whereas holography replicates a wave field of it. Many investigations have been conducted on the next candidates for commercial products to resolve existing limitations. Up-to-date see-through 3D display is a concept close to the ultimate goal of presenting seamless virtual images. Although it is still far from practical use, many efforts have been made to resolve issues such as occlusion problems.

High order plasmonic Bragg reflection in the metal-insulator-metal waveguide Bragg grating

High order plasmonic Bragg reflection in the metal-insulatormetal (MIM) waveguide Bragg grating (WBG) and its applications are proposed and demonstrated numerically. With the effective index method and the standard transfer matrix method, we reveal that there exist high order plasmonic Bragg reflections in MIM WBG and corresponding Bragg wavelengths can be obtained. Contrary to the high order Bragg wavelengths in the case of the conventional dielectric slab waveguide, the results of the MIM WBG exhibit red shifts of tens of nanometers. We also propose a method to design a MIM WBG to have high order plasmonic Bragg reflection at a desired wavelength. The MIM WBG operating in visible spectral regime, which requires quite accurate fabrication process with grating period of 100 to 200 nm for the fundamental Bragg reflection, can be implemented by using the higher order plasmonic Bragg reflection with grating period of 400 to 600 nm . It is shown that the higher order plasmonic Bragg reflection can be em ployed to implement a narrow reflection bandwidth as well. We also address the dependence of the filling factor upon the bandgap and discuss the quarter-wave stack condition and the second bandgap closing.

Wide viewing angle dynamic holographic stereogram with a curved array of spatial light modulators

A novel design of dynamic holographic stereogram with a curved array of spatial light modulators (SLMs) is proposed. In general, it is difficult to simultaneously achieve a wide viewing angle and an available width for the digital holographic display. Moreover, the wide viewing angle of a display system needs a large optical numerical aperture where the paraxial approximation fails, and thus an extremely large planar SLM is necessary in using previous methods. To solve this problem, our proposed display system is composed of a curved array of SLMs to obtain a large number of data points and reduce the spatial bandwidth in SLMs. In the curved array of SLMs, each SLM is individually transformed to display local angular spectra of object wave, which is based on a fundamental idea of holographic stereogram. To embody the dynamic holographic stereogram with SLMs, each SLM is effectively reformed for simplifying the optical structure and reducing the light power loss. In detail, spatially modulated wave is optically divided and transformed, as if each SLM were composed of three sub-SLMs. This design improves the scalability in viewing angle of holographic display and the loss of light power is significantly reduced. With this method, we can achieve the digital holographic display with 22.8 degrees viewing angle.

Synthesis and Dynamic Switching of Surface Plasmon Vortices with Plasmonic Vortex Lens

The generation of surface plasmon vortices with arbitrary higher order vortex topological charges with novel plasmonic vortex lens is experimentally demonstrated. It is shown that the polarization sensitivity of the plasmonic vortex lens can be utilized for the dynamic switching of the surface plasmon vortices with different topological charges. A simple algebraic rule related to the vortex topological charge change in the dynamic switching is formulated, and its proof is provided with theory and experiment. The synthesis and dynamic switching of higher order surface plasmon vortices have profound potential in optical trapping, optical data storage, and other related fields.

Mathematical modeling of triangle-mesh-modeled three-dimensional surface objects for digital holography

We develop a mathematical model of triangle-mesh-modeled three-dimensional (3D) surface objects for digital holography. The proposed mathematical model includes the analytic angular spectrum representation of image light fields emitted from 3D surface objects with occlusion and the computation method for the developed light field representation. Reconstruction of computer-generated holograms synthesized by using the developed model is demonstrated experimentally.

Off-axis directional beaming of optical field diffracted by a single subwavelength metal slit with asymmetric dielectric surface gratings

The authors propose a method for the off-axis directional beaming of optical field diffracted by a single subwavelength metal slit with dielectric surface gratings. In the proposed method, dielectric gratings are optimally designed to directionally couple the surface plasmon polariton modes induced by the metal slit into surrounding medium along a specific off-axis direction. Design of the gratings and the analysis are conducted based on the rigorous coupled wave analysis method. Their simulation shows the off-axis directional beaming of the oblique angle, 20.2°, with respect to the on axis by the proposed method. The beaming angle can be changed by adjusting the grating periods.

Optical beam focusing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings

A method for optical beam focusing by a single subwavelength metal slit surrounded by surface gratings is proposed. In our proposed method, the period of each surface grating is chirped so that the radiation fields of surface plasmon polaritons can be controlled to make a beam spot at the desired focal length. Through our proposed method, it is numerically shown that we can make a beam spot which is located at the several times of wavelength distance from the slit, and its focal length can be controlled.

Safe robot arm with safe joint mechanism using nonlinear spring system for collision safety

Collision safety between humans and robots has drawn much attention since service robots are increasingly being used in human environments. A safe robot arm based on passive compliance can usually provide faster and more reliable responses for dynamic collision than an active one involving sensors and actuators. Since both positioning accuracy and collision safety of the robot arm are equally important, a robot arm should have very low stiffness when subjected to a collision force greater than the injury tolerance, but should otherwise maintain very high stiffness. To implement these requirements, a novel safe joint mechanism (SJM-II) which has much smaller size and lighter weight than the previous model, is proposed in this research. The SJM-II has the advantage of nonlinear spring which is achieved using only passive mechanical elements such as linear springs and a double-slider mechanism. Various analyses and experiments on static and dynamic collisions show that stiffness of the SJM-II is kept very high against an external torque less than the predetermined threshold torque, but abruptly drops when the input torque exceeds this threshold, thereby guaranteeing positioning accuracy and collision safety. Furthermore, a robot arm with two SJM-IIs is verified to achieve collision safety in 2D space.

Holographic head-mounted display with RGB light emitting diode light source

A compact head-mounted holographic three-dimensional display with an RGB light-emitting diode (LED) light source is developed. Issues regarding full-color holographic image design and the quality associated with the use of an LED light source are investigated. The accommodation effect and background noise in the proposed system are discussed based on experimental observation.

Tunable directional beaming from subwavelength metal slits with metal-dielectric composite surface gratings.

A structure for surface-plasmon-polariton mediated directional off-axis beaming with a dynamic tunability of radiation direction is proposed. The key finding in this Letter is that properly designed metal-dielectric composite grating structures surrounding subwavelength metal slit produce a dynamically tunable off-axis beaming effect with variation of the grating permittivity value. The controllability of beam direction by the grating permittivity value opens a way to develop active plasmonic devices based on the beaming effect.