Liquid Crystal Light Modulators: Revised Edition

The way towards the liquid crystal modulators’ design and fabrication is drawn up. The theoretical and practical aspects of material studies and implementations of tailored liquid crystals in liquid crystal modulators are discussed. The way subsequent chapters drive the reader through liquid crystal modulators’ peculiarities is shown. In Chapter 2, a new complementary method of determination of Frank elastic constants in nematic LCs is described. Next, a custom method of determination of material parameters ε┴, Δε, Δn, γ, Kii of working liquid crystal nematic mixtures is discussed. Chapter 4 describes the liquid crystal polarization switch implemented in the rangefinder of the space lander module. In Chapter 5, the design and operation of a liquid crystal spectral filter for air pollution detection are discussed. Finally, Chapter 6 presents an efficient liquid crystal shutter for automatic welding helmet (PIAP-PS automatic).

new method for quick and accurate measurements of splay (S), twist (T) and bend (B) elastic constants of nematic liquid crystals (NLC) is proposed. The main concept relies on utilizing an electric field only and on determining magnitudes of nematic elastic constants from threshold fields for Freedericksz transitions in a single, hybrid, In-Plane-Switched (IPS) cell. In Hybrid In-Plane-Switched (HIPS) cell, the deformations of an investigated LC are optically monitored while driven by three separated pairs of electrodes. Two of them are interdigitating ones. Due to the appropriate IPS electrodes and boundary conditions on them, the splay, twist and bend elastic constants can be measured without allaying the magnetic field. In this chapter, we describe the layout of HIPS measuring cells and the results of tests conducted on them using 5CB, 6CHBT (with Δε > 0) and DE (with Δε < 0).

In the last few years, the following Multicomponent Nematic Liquid Crystalline (MNLC) mixtures of medium and high optical anisotropy Δn have been developed in Military University of Technology (MUT) in Warsaw:

• W1898 (composed mainly of two and three alkylcyclohexylbenzene and alkylbicyclo-hexylbenzene isothiocyanates),

• W1820 (composed mainly of fluoro-substituted alkylbiphenylisothiocyanates),

• W1852 (composed mainly of fluoro-substituted isothiocyanates, alkylbiphenyls, alkylcyclo-hexylbiphenyls and alkylbicycloheksylobiphenyls),

• W1825 and W1791 (composed mainly of fluoro-substituted isothiocyanates, alkyltolanes and alkylphenyltolanes),

• W1865 (composed mainly of fluoro-substituted isothiocyanates, alkylphenyls and alkylbiphenyltolanes),

• W1115 (composed mainly of cyclohexanolcyanobenzenes, fluorobenzenes and hydrocarbons),

• W1795B (composed mainly of fluoro-terphenyls).

In this chapter, the material characteristics of the perpendicular and parallel εII(T) components of the permittivity tensors ε (T) at the frequency range of f є [10 Hz, 10 MHz]; bulk viscosity Γ(T); rotational viscosity γ(T), splay K11(T), twist K22(T), bend K33(T) and KTN(T) for transition from twisted to the nontwisted structure; ordinary no(λ) and extraordinary ne(λ) refractive indices for wavelength λ є [0.3μm, 1.6 μm] are described and discussed in the terms of applicability of MNLCs to Liquid Crystal electro-optical Devices (LCEOD).

Liquid Crystal Cell (LCC) for space-borne laser rangefinder for space mission applications was developed, manufactured and tested at the Military University of Technology (MUT)Warszawa, Poland, in cooperation with Vavilov State Optical Institute (Vavilov SOI), Petersburg, Russia. LCC operated at the positive TN mode (d=2.5 μm) tuned for the laser beam of a wavelength of λ=1064 nm. It switched the polarization plane of the laser beam at the beam’s energy density not smaller than 0.15 J/cm2 at the pulse duration about 8 ns. At the working aperture, not less than 15 mm the transmission T of LCC was not smaller than 95%. Switching-on and switching-off times were assessed for LCC driven with a voltage of an amplitude U=10 V. At the temperature range from 20°C to 40°C, measured switching-on and switching-off times were not larger than 0.7 ms and 7 ms, respectively. The LLCs developed at the MUT were tested at the Vavilov SOI under procedure dedicated to space equipment. LLCs were mounted in the laser rangefinder of the space lander serving while the “Phobos- Grunt” mission was launched on November 8th, 2011, in Kazakhstan.

Multicomponent Nematic Liquid Crystalline mixtures (MNLC) containing isothiocyanato tolane and isothiocyanato terphenyl liquid crystals have been developed at the MUT. Some of them exhibit both; high optical (Δn ≤ 0,45) and high dielectric (Δε ≤ 20) anisotropies and are characterized by relatively low viscosity γ. Appling the mentioned above MNLC mixtures (W1791, See Chapter 2) in HG (HomoGeneously aligned) cells with thicknesses d about 1 μm, 3 μm and 5μm, one can obtain the possibilities to develop first-order electrically tunable liquid crystal filter and threestage ETLCF (Electrically Tunable Liquid Crystal Filter). Specific spectral filters can find some acceptance in astronomy and remote sensing for the pollution monitoring. Distinct advantages of ETLCF over conventional tunable filters are the possibility of construction of LCFs with an extremely large clear aperture, low power consumption, and low addressing voltage.

• Due to the relatively high and electrically controlled optical anisotropies Δn(U) of MNLC and by variation of cell gaps d (1 μm, 3 μm and 5 μm) used, the LCF can select the desired wavelength λ(U) from VIS and NIR ranges.

• Due to the high dielectric anisotropy Δε, low viscosity γ and small cell gap d of HG cells, the LCF can achieve the response time lower than 1 ms.

In this chapter, we describe our efforts in obtaining optimization of the LCFs.

In this chapter, our efforts to obtain and optimize Liquid Crystal Shutter (LCS) for switching a light transmission in welding helmets have been described and discussed. Automatic protection devices in the welding helmet should have sufficiently short switching time and extremely low transmission, preventing the absorption of harmful doses of radiation regardless of light direction and its polarization. A short time of return to the initial off-state is required as well. LCS operating in welding helmets has a form of a removable module consisting of a stack of two liquid crystal cells and an electronic control system. In normal illumination conditions, at off-state, this module is transparent, which allows observation of welded objects. In the presence of an extremely intensive illumination, caused by an electric arc, the module rapidly reduces the transparency, with the intensity of the light reaching welder’s eyes. Application of automatic helmets increases welder’s safety and, by making both hands free, also increases their performance. According to the idea formulated above, we developed the LCS. The LCS consisted of two liquid crystal cells (of cell gap d = 6 μm) filled with a W1115 mixture (Δn = 0.08 at λ = 0.589 μm, see Chapter 3) placed between three crossed polarizers and a band-pass Dielectric-Metallic Interference Filter (DMIF). Our LCS fully meets the requirements to be applied in professional welding helmets with protection degree up to 13 N.