Study of Liquid Crystal Phase Modulators with Fast Optical Response and Low Light Scattering Realized via Holographic Exposure Process with a He-Ne Laser

  • 簡 均祐

Student thesis: Doctoral Thesis

Abstract

Liquid crystal (LC) materials have been widely applied in various electro-optical devices include spatial light modulators LC lenses displays and beam steering Among them a homogeneous alignment LC (HALC) cell is usually adopted in phase modulator to provide maximum phase retardation (δ) using the equation δ = 2πΔnd/λ where Δn is LC birefringence d is cell gap and λ is incident wavelength HALC phase modulator can be used at any wavelength of interest for instance visible light or infrared light In general response time include rising time and falling time are importantly physical parameters for performance of LC devices However the falling time is usually limited by the LC rotational viscosity cell gap and LC elastic constant which mainly degraded the response time Polymer network LC (PNLC) is easily to demonstrate a fast falling time wherein the molecules of an anisotropic monomer possessing an LC building block structure are doped with LCs and processed with UV exposure Thereafter the generated polymer networks provide constraints to LCs to effectively speed up the falling time during the relaxation of LC molecular reorientations However light scattering occurs due to the refractive index mismatch of polymer networks and LC micro-domains In addition HALC phase modulators are polarization-dependent it is well known that polarization-independent LC devices in most optical systems usually demonstrate various advantages including operation convenience In this dissertation which consists of three achievements which covers LC phase modulators with a goal to address the issue on light scattering in PNLC cells while achieving superior performance include sub-millisecond response time and/or polarization-independent The degree of light scattering can be reduced by using much smaller LC micro-domains the dimensions should be reduced to lower than 200 nm for scattering-free in visible regime Given that holographic exposure can spatially generate very fine and directionally periodic interference patterns with a pitch of a few hundred nanometers We use the interference pattern produced by a low-power He-Ne laser to process the sample for reduction of LC domain dimension A novel photoinitiator and co-initiator successfully excite the photopolymerization of anisotropic monomer (RM257) under the red light irradiation As a result the holographic polymer network formed in LC phase modulators via a He-Ne laser in the first work demonstrates ultra-fast optically response and low light scattering These advantages are mainly caused by the small LC domains and uniform polymer network when processing LC cells via holographic exposure to a He-Ne laser The use of this method to fabricate LC cells as phase modulators results in a rising time of 21 μs and falling time of 49 μs under 2π phase modulation at room temperature The predicted fast optical response can be achieved when operating devices at high temperatures In addition the proposed PNLC cells are used at infrared light wavelength (1550 nm) The percentages of ingredients in the LC mixture filled in PNLC cells underwent an optimization for reduction of applied voltage Moreover the effect of relative ratio between of co-polymer (NVP) and RM257 was investigated As a result the fabricated phase modulators with co-polymer dopant also maintained well homogeneous LC alignments and optical-scattering-free characteristics Furthermore NVP dopant successfully reduced the operating voltages from 95 Vrms to 79 Vrms to prevent polymer network deformation when electrically operating with higher voltages The fabricated infrared phase modulators had a sub-millisecond response time that is rising time of 0 88 ms and falling time of 0 40 ms In the third work the holographic exposure processes was used to treat chiral/RM257 dopant liquid crystal cells and the cell was controlled in the isotropic phase during the exposure As a result small LC domains with random director distributions were obtained to show novel optical isotropy that is polymer-stabilized isotropic liquid crystal cells The PSILC cell applied in phase modulators showed unique performances including low light scattering polarization-independence and fast optical response Furthermore an extra fluoro-surfactant dopant in cells showed that the phase modulators retained their performance but with considerable reduction of operating voltages from 180 Vrms to 100 Vrms The rising time and falling time are respectively 0 11 ms and 1 32 ms when switching electrically between 0 and 100 Vrms
Date of Award2018 Jul 26
Original languageEnglish
SupervisorChia-Rong Sheu (Supervisor)

Cite this

Study of Liquid Crystal Phase Modulators with Fast Optical Response and Low Light Scattering Realized via Holographic Exposure Process with a He-Ne Laser
均祐, 簡. (Author). 2018 Jul 26

Student thesis: Doctoral Thesis