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Announcing the Final Examination of Fenglin Peng for the degree of Doctor of Philosophy in Optics and Photonics

Tuesday, April 4, 2017 2:00 PM to 3:00 PM
CREOL Room 103

Dissertation Title:

“High Performance Liquid Crystals for Displays and Spatial Light Modulators”

Abstract:

Liquid crystals (LCs) are an amazing class of soft materials which have been widely used in the visible, infrared (IR), millimeter wave, and terahertz spectral regions. Both amplitude modulation (e.g. displays) and phase modulation (e.g. spatial light modulators (SLMs) for adaptive optics and adaptive lens) have been investigated extensively.

Thin-film-transistor liquid crystal displays (TFT-LCDs) have become ubiquitous in our daily lives. Its widespread applications span from TVs, monitors, tablets, smartphones, augmented reality, virtual reality, to vehicle displays. LCD shows advantages in 1) high resolution, 2) long lifetime, 3) vivid colors using quantum dots backlight, and 4) high dynamic contrast ratio employing local dimming technology. However, LCD exhibits a serious problem, which is slow response time. Therefore, it is commonly perceived that LCD exhibits a more severe image blur than organic light emitting diode (OLED) displays. Indeed, the response time of LCD is ~100x slower than that of OLED. To evaluate image blurs, Motion Picture Response Time (MPRT) has been proposed to quantify the visual performance of a moving object. MPRT is jointly governed by three factors: the sample and hold effect of an active matrix display, motion pursuing, and human vision system. It is a complicated problem and is difficult to obtain analytical solution. In this thesis, we analyze the sample-and-hold effects and derive a simple equation to correlate MPRT with LC response time, TFT frame rate, and duty ratio. From our analytical equation, we find that as long as an LCD’s response time is less than 2 ms, its MPRT would be comparable to that of OLED at the same frame rate, even if the OLED’s response time is assumed to be zero. To further reduce MPRT, we could boost the frame rate to 144 Hz or reduce the duty ratio through backlight modulation. This discovery sheds new physical insights for LCDs to achieve CRT-like displays with negligible image blurs.

In addition to displays, LCs are widely employed in SLMs for modulating the phase and polarization of an incident light. This is because LCs possess high birefringence and relatively low absorption from the visible, IR, to terahertz regions. The useful applications include adaptive lens, adaptive optics, fiber-optic communication, antenna, and phase shifter. Fast response time is a common requirement for the abovementioned photonic devices. To achieve fast response time while maintaining 2p phase change, polymer-stabilized blue phase liquid crystal (BPLC) and polymer-network liquid crystal (PNLC) are promising candidates for the visible and IR SLMs, respectively. However, the operation voltage of present BPLC and PNLC devices is too high. To reduce operation voltage while keeping fast response time, we developed a new device configuration for BPLC SLM to work in the visible region. The new device structure allows the incident laser beam to traverse the BPLC layer four times before exiting the reflective SLM. As a result, the 2p phase change voltage is reduced to below 24V, which is the maximum attainable voltage for a high resolution liquid-crystal-on-silicon device. On the other hand, PNLC is a better candidate for the IR SLM because several high birefringence LC materials can be used. To reduce the operation voltage of a PNLC, we have investigated following three approaches: 1) developing large dielectric anisotropy (?e) and high birefringence (?n) LC materials, 2) optimizing polymer concentration, and 3) optimizing UV curing conditions.

In the visible and near IR regions, most LCs are highly transparent. However, to extend the electro-optic application of LCs into MWIR and LWIR, absorption loss becomes a critical issue. In the MWIR region, several fundamental molecular vibration bands and overtones exist, which contribute to high absorption loss. The absorbed light turns to heat and then alters the birefringence locally, which in turns causes spatially non-uniform phase modulation. To suppress the optical loss, we have taken following approaches: (1) Designing high birefringence to minimize the LC layer thickness; (2) Shifting the absorption bands outside the spectral region of interest by deuteration, fluorination, or chlorination; (3) Reducing the overtone absorption by using a short alkyl chain. As a result, we have developed several low loss and high birefringence chlorinated LCs for the first time. To achieve fast response time, we demonstrated a PNLC with 2p phase change at MWIR and response time less than 5 ms. Molecular tailoring strategies for extending liquid crystal SLM into long-wavelength infrared (LWIR) are also explored.

Major:  Optics and Photonics

 

Educational Career:

BS: 2012, Information Engineering, Zhejiang University

 

Dissertation Advisory Committee Members:

Dr. Shin-Tson Wu (Chair)

Dr. M. G. Moharam

Dr. Boris Zeldovich

Dr. Jiyu Fang

Approved for distribution by Dr. Shin-Tson Wu, Committee Chair, on March 15, 2017

The public is welcome to attend.

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