Hyunhee Choi

Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell

Spatial tuning of lasing wavelength in a dye-doped cholesteric liquid crystal wedge cell is demonstrated. The wedge cell possesses a series of dislocation lines along the wedge direction. In the lasing operation, we find that the lasing wavelength is continuously tuned in the region between two dislocation lines, while the lasing wavelength jumps when crossing the dislocation lines. The observed one-dimensional spatial continuous tuning is attributed to the presence of a gradient in the cholesteric helical pitch, while the laser wavelength jumping originates from the pitch jump owing to a change in the number of half-turns of the cholesteric helix.

Holographically generated twisted nematic liquid crystal gratings

A reflection holographic method is introduced to fabricate an electro-optically tunable twisted nematic (TN) liquid crystal (LC) grating, forgoing the geometrical drawing. The photoisomerization process occurring on the LC alignment layers of an LC cell in the reflection holographic configuration gives a control over the twist angle, and the grating spacing is determined by the slant angle of reflection holographic configuration. The resulting diffraction grating is in a structure of a reverse TN LC, permitting a polarization-independent diffraction efficiency. The electro-optic tunability of the diffraction efficiency is also demonstrated.

Holographic inscription of helical wavefronts in a liquid crystal polarization grating

A space-varying polarization hologram (PH) grating is fabricated in a nematic liquid crystal (LC) cell with azo-side-chain polymer alignment layers. The polarization-sensitive photoisomerization property of azo-side-chain polymer is utilized to inscribe the PH in a LC cell. Both transmission and reflection holographic configurations are adopted in fabrication. The transmission PH made by the circular orthogonal polarizations exhibits the polarization-controlled Laguerre-Gaussian beam generation, while the reflection PH made by the linear orthogonal polarizations generates a variety of Laguerre-Gaussian beam, though not polarization controlled.

Effective medium analysis for optical control of laser tuning in a mixture of azo-nematics and cholesteric liquid crystal

The selective reflection band structure of a mixture composed of azo-nematics and cholesteric liquid crystal (azo-based CLC) is investigated as an effective single optical medium. Berremans 4×4 matrix method is adopted to analyze the photoisomerization-induced changes in the azo-based CLC by identifying the effective refractive indices as well as the helical pitch. In a laser dye-doped azo-based CLC, a stable reversible tuning of laser wavelengths at each domain of the two-dimensional laser array is demonstrated.

Optical properties of holographically generated twisted nematic liquid-crystal gratings

By use of a trans-cis photoisomerization process occurring in an azo-benzene side chain, various twisted nematic liquid-crystal (LC) gratings are fabricated in LC cells. Two counterpropagating orthogonally polarized beams produce a polarization modulation, which inscribes the replica polarization modulation inside LC cells holographically, providing twisted nematic LC gratings. Microstructure, diffraction properties, and electro-optic tunability of diffraction efficiency are examined experimentally. From the Fourier transformation of the Jones matrix of twisted nematic LC gratings, Stokes parameters of the zeroth and first diffraction orders are calculated and compared with the optical measurements, resulting in a good agreement.

Ladder operators formalism for optical angular momentum transfer and space-variant Pancharatnam-Berry phase

Ladder operators are introduced in SU(2) parameter space to analyze the space-variant Pancharatnam-Berry (PB) phase. The spin and orbital angular momentum transfers to a wave passing through the PB phase optical elements are understood in terms of the PB phase. Space-variant PB phase structures are identified in polarization grating and cholesteric liquid crystal, providing an explanation for the beam propagation direction alteration. A simultaneous occurrence of the wavefront shaping and the beam propagation direction alteration is demonstrated experimentally.

Ladder operators for optical angular momentum transfer

Ladder operators are introduced to analyze the Pancharatnam-Berry(PB) phase. Space-variant PB phase structures are identified in polarization grating and cholesteric liquid crystal. The wavefront shaping and the beam propagation direction alteration are demonstrated.

Optical manipulation of micro-particles with Laguerre-Gaussian mode beam

We have demonstrated an optical trapping and rotation of polystyrene microspheres with a Laguerre Gaussian mode beam. We constructed a Mach-Zehnder interferometer to obtain an interference pattern of Laguerre Gaussian mode and Gaussian mode. A micron-sized objects are trapped in the interference pattern, and by changing the optical path length we could control this pattern to rotate. Since the interference pattern rotates, the trapped particles also rotate.

Analysis of the Polarization Liquid Crystal Gratings

A reflection holographic method is introduced to fabricate an electro-optically tunable twisted nematic(TN) liquid crystal(LC) grating. The resulting diffraction grating is in a structure of a reverse TN LC. By theoretical analysis, we were able to identify that the diffraction efficiency is related only with the twisted angle and cell thickness and the polarization modulation described by Stokes parameters in various incident polarized beams.

Microstructure of a poled surface-relief grating and its electro-optic response

The microstructure of a poled surface-relief grating of an azobenzene side-chain polymer film is investigated. From the in situ monitoring of diffraction efficiency and second-harmonic generation, as well as dynamic-contact electrostatic force microscopy images, the point group symmetry of C2nu is identified. The polarization-dependent electro-optic measurement of the reflected diffraction order further confirms the optically biaxial structure of the poled surface-relief grating.