3D Display Laboratory

Cyclic structural transitions between trans- and cis-isomerizations due to hole burning - a tendency of azo benzene to align perpendicular to the light polarization - a mass migration of bulk material occurs on the surface of the azo benzene layer. This light-induced material migration is usually achieved by two-beam interference (spatially varying intensity & polarization) and forms a sinusoidal topology that diffracts the incident light. Various optical elements can be realized based on the height and periodicity of the topology.

Our group and collaborators have created a material that reflects light of a specific and electrically controllable color. A structure of periodic dielectric slabs can influence EM wave propagation when the wavelength is similar to the pattern’s periodicity. This principle is the basis of photonic band gap materials, which can block the transmission of light of a specific color. Our team collaboration with Prof. Seung Hee Lee (Jeonbuk National University) produced a liquid-crystal-based photonic band gap material that reflects spectrally pure visible light. These band gaps are narrow bandwidths and can be electrically tunable across visible spectra. This system can be utilized as a tunable color filter integrated into optical microscopes and photonic devices.

The LC is a self-assembly of anisotropic rod-like molecules that does not require high-technology and sophisticated instruments for fabrication. And the exciting property of LC is that they exhibit a photonic effect due to the anisotropic permittivity at optical frequencies. In our study, we have tried to combine these LCs with photonic structures. Therefore, we can have a photonic crystal with the LC advantages, such as nanoscale self-assembly & easy tunability. We fabricated a single crystalline BPLC that exhibits a photonic band gap with narrow bandwidth and easy tunability. This system shows a better color gamut than sRGB by covering more than 85% of the NTSC color gamut. Taking advantage of this system, we proposed a high-color gamut display.

We propose a free-form and paper-like flexible LC diffractive film consisting of nano-phase separated LC droplets confined in a polymer matrix. It possesses an excellent optically isotropic nature, i.e. high transparency, and submillisecond response. Incorporating a small amount of elastomeric monomer into the acrylate monomers results in higher flexibility of fabricated films. The proposed film acts as a voltage-tunable phase grating with effective chromatic diffraction for an incident white light. In addition, this diffractive film exhibits excellent chemical stability against organic and inorganic solvents while having polarization-selective diffraction that can be widely implemented in wearable photonics.

An efficient 3D display device was demonstrated based on switchable micro-lenticular lens arrays (MLAs) utilizing phase modulation of the optically isotropic nano-PDLCs. The phase-separated LC exhibits an optically isotropic phase, and upon field application, the phase retardation induces due to the reorientation of LC directors encapsulated in the polymer network. As a result, the optical path follows the gradient refractive index, resulting in the realization of 3D image. Besides, the position of the focused beam in the MLA device can be changed depending on the polarization state of the incident beam. Therefore, a polarization-selective 2D and 3D switchable display can be produced.

Diffraction is a common phenomenon when a light beam encounters spatially periodic structures. This diffraction plays a crucial role in many optical devices. For conventional gratings, the diffraction intensity is usually higher for the zeroth order and spatial frequency is fixed. That would be the disadvantage of conventional grating. To overcome this, we propose an electrically tunable nano-PDLC diffraction grating based on IPS and FFS. As LC allows for manipulation of its refractive index, modulation of diffraction intensity and spatial frequency has been achieved. Owing to the nanoscale coherence length, our system very precisely follows the rectangular shape field profile and exhibits increased diffraction efficiency.