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Layered Structured Materials and Nanotechnology for Photodetection
Published in Tuan Anh Nguyen, Ram K. Gupta, Nanotechnology for Light Pollution Reduction, 2023
Felipe M. de Souza, Magdalene Asare, Ram K. Gupta
The diffraction efficiency can be defined as the ratio between the diffracted beam’s intensity by the incident beam. Through that, a quantitative measurement from either brightness or color change can be obtained, which correlates with the analyte’s concentration. Based on that, different optic sensors can be designed according to the property that changes during the recording such as surface or volume relief, reflection or transmission, and amplitude or phase change [6]. Usually, there could be at least three components to design a photosensor: substrate, recording media, and embedded nanoparticles. The substrate should be transparent; hence it should be neither birefringent, optically active nor anisotropic as the latter could lead to a decrease in diffraction efficiency. In addition, it should present enough mechanical strength to support the photosensitive material that can be deposited through several variations of CVD methods, for instance. In that sense, when a photosensor is used in liquid-state systems the substrate should be stable against shrinking and swelling cycles to avoid detachment of photoactive material. To prevent this process a unimolecular sublayer can be placed underlying the substrate before the coating of active material.
Polymeric Materials as a Holographic Recording Medium
Published in Asit Baran Samui, Smart Polymers, 2022
Asit Baran Samui, Alips Srivastava
By dispersing liquid crystal droplets in a polymer matrix, polymer dispersed liquid crystals (PDLCs) can be made. By applying an electric field to the composite films, switching can be done between the opaque and the transparent state. Holographic polymer-dispersed liquid crystal films, containing liquid crystals and a photoreactive prepolymer, can be exposed to a coherent interference pattern generated by a laser.82 Upon holographic exposure (irradiation with a laser beam), there is the onset of a counter-diffusion process which results in the movement of the liquid crystals to the “dark” regions and the monomer to the “light” regions of the interference pattern. This is followed by polymerization of the monomer so that the periodic structure of alternating liquid crystal-rich and polymer-rich zones is set. The main advantage of this technique is the realization of large area grating structures by a fast and single-step process. The liquid crystal-rich zones contain randomly oriented sub-micrometer droplets, whose size depends on the exposure time, laser beam intensity, concentration of liquid crystal, and the polymer, respectively. The diffraction efficiency of the gratings can be modulated by applying an electric field. The index modulation can be erased by applying a sufficiently strong electrical field so that the liquid crystal molecules and the material will be optically homogeneous, which is possible by matching the ordinary index of the liquid crystal with the polymer,
Polyimides in High-Performance Electronics Packaging and Optoelectronic Applications
Published in Malay K. Ghosh, K. L. Mittal, Polyimides Fundamentals and Applications, 2018
Holographic phase gratings can be recorded in Probimide 412 by exposure to UV light (363 nm). Exposure to the UV produces a refractive index change in the material due to cross-linking, which causes the density of the exposed polymer to increase. The effectiveness of the grating is given by its diffraction efficiency, which is the intensity ratio of a diffracted order to an undiffracted order (given in %). Figure 28 shows the diffraction efficiency (+1/0) (representing the ratio of the intensity of the +1 diffracted order to the intensity of the 0th undiffracted order) for the holographic grating produced in 50 µm thick Probimide 412. The efficiency was recorded with a HeNe laser. Figure 28 indicates that the pure phase grating recorded holographically in this material is quite stable and effective.
Fringe field effect free high-resolution display and photonic devices using deformed helix ferroelectric liquid crystal
Published in Liquid Crystals, 2021
Zhibo Sun, Zhengnan Yuan, Runxiao Shi, Hoi-Sing Kwok, Abhishek Kumar Srivastava
Later, we made diffraction grating using the same ITO electrode patterns for negative LC and DHFLCs. Figure 8(a,b,c) shows the diffraction intensity profiles of different LC cells. When the cell gap is 3 μm, the negative NLC shows no diffraction orders, which means the strong FFE distorts the grating profile (Figure 3(b)). For 1.5 μm thick cell, the negative NLC shows multiple diffraction orders with poor diffraction efficiency due to the improper retardation and the FFE (as shown in Figure 3(d)). Whereas for the DHFLC, it shows clear first order with pretty high diffraction efficiency. The diffraction profile and deflection angle match well with the theory, and further optimisation can be done to maximise the diffraction efficiency by providing proper phase retardation conditions for the grating. What’s more, we have also made the microdisplay prototype using the DHFLC and Figure 8(d) shows the microscopic image of the prototype. The patterned ITO electrode patterns on both substrates in the display cell are perpendicular to each other and are addressed by the direct-drive method. The ITO electrode width is 4 μm with the ITO electrode gap ranging from 1 μm to 50 μm. The ITO electrode gap of 1 μm is recognisable at 5 V in the direction of parallel to the helix. Whereas, for the perpendicular direction, the minimum recognisable ITO electrode gap is 3 μm at 5 V. Thus, it is clear that DHFLCs, with the current set of material parameters, can support the up to 2822 PPI display resolution, which can be further improved for a relatively smaller helix pitch.
Fast switching beam steering based on ferroelectric liquid crystal phase shutter and polarisation grating
Published in Liquid Crystals, 2019
Qi Guo, Lin Xu, Jiatong Sun, Xiaoqian Yang, Hongwei Liu, Kexin Yan, Huijie Zhao, V. G. Chigrinov, H. S. Kwok
The FLC sample is prepared in 3.3 μm-thick to act as a switchable half-wave plate (HWP) for 1064 nm. The tilt angle of smectic layer of employed FLC material is 22.05°, thus S1 state represents HWP with slow axis along x, and S2 represents HWP with slow axis along approximate 45°. The FLC switchable HWP followed with QWP placed at 45° are utilised to function as a circular polarisation selector. The electro-optical response of +1 and −1 order shows complementarity (Figure 3(b,c)) under square wave driving (Figure 3(a)). The diffraction efficiency is defined as the intensity of specific order divided by the sum of all orders including 0 and±1 orders. The diffraction patterns (Figure 3(d,e)) reveal that the ±1 orders are much larger than 0 order, and no higher order exist. The measured efficiency is of 95.7% corresponding to S1 active state of FLC, and of 94.1% corresponding to S2 active state of FLC. The slight decrease of is possibly due to that the helix cone angle 2θ = 44.1° is slightly mismatch to 45°.
High-diffraction-efficiency Fresnel lens based on annealed blue-phase liquid crystal–polymer composite
Published in Liquid Crystals, 2019
Hua-Yang Lin, Nejmettin Avci, Shug-June Hwang
The voltage-dependent diffraction efficiency of the proposed BPLCFL was also measured under different polarisation angles of the incident linearly polarised light, as shown in Figure 8. The diffraction efficiency is defined as η = (I − Io)/It, here I denotes the transmitted light intensity at the primary focal point, Io is the background noise and It is the total incident light intensity after passing through the sample. The experimental results demonstrate that the initial diffraction efficiency of the BPLC lens at the voltage off state is ~10%, which is due to the effective refractive index mismatch between the even and odd zones of the BPLCFL. When a voltage beyond the critical value of approximately 10 Vrms was applied, the LC directors in the odd rings begin to realign in the electric field direction, and the effective refractive index is then induced by the Kerr effect. Alternatively, the polymer in the even rings remained basically unaffected. When the external voltage increases, the phase difference between these two neighbouring zones increases, as does the optical diffraction efficiency. As the applied voltage increases to 200 Vrms, the diffraction efficiency nearly reaches the maximum value of 35.8%, which is not so far the theoretical limit. Because the reflection at two interfaces of glass–air happens as ~4%, the slightly lower optical diffraction efficiency of the proposed BPLCFL is mainly induced by the reflection at the two substrate–air interfaces and weak light scattering occurring at the interfaces between polymers and BPLCs. The high operation voltage of 200 V could cause electrode breakdown and severe hysteresis effect [26]. To lower operation voltage and enhance the performance of BPLCFL, better selection of polymer composition and proper control of the PIPS condition are critically required such as curing UV intensity and curing temperature and time, respectively.