Chiung-Sheng Wu

Mixed-mode twisted nematic liquid crystal cells for reflective displays

A mixed‐mode twisted nematic (MTN) liquid crystal cell is developed for both reflective‐mode projection and direct‐view displays. This MTN cell exhibits two times higher brightness and eliminates the parallax problem as commonly observed in reflective displays using a nominal transmissive TN cell. In the meantime, the MTN structure preserves advantageous characteristics of the TN structure such as low operation voltage and high contrast ratio. Confirming computer simulations using Jones matrix method agree with the experimental results well.

High‐speed liquid‐crystal modulators using transient nematic effect

The transient nematic effect, together with multiple pass technique and optimal temperature effect, was employed to demonstrate high‐speed liquid‐crystal modulators. As a result, nematic liquid‐crystal modulators with frame time less than 50 μs have been achieved in the visible spectral region.

Refractive index dispersions of liquid crystals

Refractive indices of two nematic liquid crystals, 5CB and 5PCH, were measured from the 400- to 800-nm region at various temperatures. Results fit the three-band model well. Through comparisons, the contributions of σ and π electrons to the refractive indices and birefringence of uniaxial liquid crystals are evaluated quantitatively. These physical insights are useful for tailoring liquid crystal molecules with proper refractive indices.

Small angle relaxation of highly deformed nematic liquid crystals

Transient phase response associated with a small angle relaxation from highly deformed nematic liquid crystal (LC) directors is analyzed experimentally and numerically. Qualitative agreement between computer simulations and experimental results is obtained. Based on these results, decay time of a fast LC modulator employing the transient nematic effect is derived. This decay time is found to be fast (with potential to achieve ∼50 μs), insensitive to LC thickness, but proportional to (λ/Δn0)2, λ being the wavelength and Δn0 the corresponding birefringence.

Rotational viscosity of nematic liquid crystals A critical examination of existing models

Abstract A simple electro-optical method was developed to characterize the temperature dependent viscoelastic coefficient of six nematic liquid crystals with different conjugation lengths. Results were then used to test seven most frequently used models for rotational viscosity (y1). Computer results show that some models fit certain specific liquid crystals but, in general, two models fit all the experimental data. These two models are y 1 = α2 S 2 exp[E/k(T - T 0)] and y 1 = (α1 S + α2 S 2) exp (E/kT), with α1, α2 E, and T 0 as adjustable parameters.

CHIRAL-HOMEOTROPIC LIQUID CRYSTAL CELLS FOR HIGH CONTRAST AND LOW VOLTAGE DISPLAYS

Computer simulations on the display characteristics of chiral-homeotropic liquid crystal cells are performed. This normally black display mode exhibits an excellent contrast ratio, nearly achromatic light transmission, and low operation voltage. The optimal twist angle for suppressing the wavelength and voltage dependencies ranges from 90° to 150°. Promising applications of this operation mode for both transmissive and reflective displays are discussed.

A three‐band model for liquid‐crystal birefringence dispersion

A three‐band model which consists of one σ→σ* and two major π→π* electronic transitions was developed to account for the birefringence dispersion of a uniaxial liquid crystal. By comparing theory with experimental results, the contribution of each band to the overall birefringence is evaluated. The effect of each band on birefringence dispersion depends strongly on the conjugation length of the molecule. This model reveals new physical insights for designing a liquid‐crystal molecule with desirable birefringence.

High Speed Nematic Liquid Crystal Modulators

Abstract Nematic liquid crystal modulators with fast response time, high contrast ratio and low operation voltage are demonstrated. The transient nematic effect, together with reflective mode operation, is responsible for the observed fast response times. The use of a tunable liquid crystal phase compensator results in excellent contrast at a reasonably low operation voltage.

Liquid-crystal-based switchable polarizers for sensor protection

Linear polarizers are generally employed in conjunction with advanced liquid-crystal filters for the protection of human eyes and optical sensors. For detection sensitivity under a no-threat condition to be maximized, the polarizer should remain in a clear state with a minimum insertion loss. When threats are present, it should be quickly switched to function as a linear polarizer with a high extinction ratio. Two types of switchable polarizer for sensor protection are demonstrated. The polarization conversion type exhibits a high optical efficiency in its clear state, a high extinction ratio in the linear polarizer state, and a fast switching speed, except that its field of view is limited to approximately ±10°. In contrast, an improved switchable dichroic polarizer functions effectively over a much wider field of view. However, its extinction ratio and optical efficiency in its clear state are lower than those of the polarization conversion type.

Reflective Direct-View and Projection Displays Using Twisted-Nematic Liquid Crystal Cells

Jones matrix method was used to optimize the twisted nematic liquid crystal cell parameters for reflective direct-view and projection displays. Twist angles ranging from 0 to 90° were investigated. From these analyses, two operation modes are found to have particularly important applications. The mixed-mode 90° twisted nematic cell exhibits an excellent contrast ratio for normally white direct-view and projection displays whereas the improved 45° twisted nematic cell shows an impressive light efficiency for normally black projection display.