Observation of transient reorientation of dye-doped liquid crystals by pumped attenuated total reflection

Lo, K. C.; Huang, Chien-Chih

doi:10.1063/1.2338561

Cited by 3 publications

(10 citation statements)

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“…Attenuated total reflection (ATR) is a useful effect that can be used to study the excitation of surface plasmons (SPs) [8]. This effect has been extensively investigated in liquid crystals (LCs) [9,10] is the penetration depth, and the penetration depth is about 100nm for the ATR system herein [11].The effect has been widely investigated in the many areas and liquid crystals (LCs) [12], because ATR measurement is highly sensitive to the varying of dielectric constants of the material-under-test nearby the surface of a layer with a thickness of around 100nm. However, the guide mode would disturb the reflectivity curve in the ATR experiment of the LC cell [13], because the guide mode results from the regularity of the NLC directors in the transversal plane and the gradient distribution of the refractive index with the depth reached by the evanescent wave [14].…”

Section: Theorymentioning

confidence: 99%

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Dynamics of the photoexictation of azo dye-doped liquid crystals probing by pumped attenuated total reflection

Huang

2007

SPIE Proceedings

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Kinetics of the photoexcitation of azo-dye doped liquid crystals (ADDLCs) is observed by the improved attenuated total reflection leading a pump light into a coupling prism, namely pumped ATR (PATR) experiment. The technique of ATR could be applied on an LC cell based on that the guide mode is overcome by the usage of an isotropic surface, which is also a usual way to observe the reaction involving the adsorption effect. The evanescent wave of the pump light induces the photoexcitation of azo dye at the limited effective depth, and the evanescent wave of the probe light detects the dynamic behavior in real time. The time response of the photoexcitation in ADDLCs in the pumped ATR experiment is less than 2ms and believed the reaction occur in the local region of 100 nm. This result is a witness of that the reaction in this local region is free of the influence of the elastic continuum of the whole LC cell. With the help of the local sensitivity and local disturbance, the detailed reactions during the photoexcitation process are clearly observed and the dynamics equation are expressed to illustrate the oscillation phenomena of LC directors as the pump light irradiates on this region.

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Section: Theorymentioning

confidence: 99%

“…Surface plasmas (SPs) are excited during ATR measurement [9][10][11][12] . Figure 1(a) schematically depicts Kretschmann configuration used in ATR measurements, and Figure 1(b) is the relation between a p-polarized electromagnetic wave and the ATR system.…”

Section: Theorymentioning

confidence: 99%

Dynamics of the photoexictation of azo dye-doped liquid crystals probing by pumped attenuated total reflection

Huang

2007

SPIE Proceedings

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“…The actual phenomena of the photoexcitation of ADDLCs near the irradiated surface were hidden behind the elastic continuum in a traditional pump-probe experiment [7,8,16,[18][19][20]. The fine evolution of the adsorbed layer and anchoring in the early stage should be time-dependent and only detectable in the local region without the influence of the elastic continuum of the entire LC cell [21,22]. However, related experiments on the dynamics at the surface region of the early-stage photoexcitation are lacking, even though the evolution of the dye-adsorbed layer is important for sequent photoalignment.…”

Section: Introductionmentioning

confidence: 99%

“…Attenuated total reflection (ATR) is a sensitive sampling technique for observing the motion of the LC director near the surface because the variation in the dielectric constant is probed by an evanescent wave, with a penetration depth of only a few hundred nanometers [23,24]. The pumped ATR (PATR) method was improved from ATR to detect the photoexcitation of ADDLCs in the local region by leading a pump light into a coupled prism [21,25]. The variation in the PATR intensity reveals the average reaction of LC directors near the surface at the photoexcitation process.…”

Section: Introductionmentioning

confidence: 99%

Competitive Dye Adsorption-Desorption on the Isotropic Surface at the Early Stage of the Photoexcitation of Azo Dye-Doped Liquid Crystals

Chen

et al. 2020

Crystals

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The instantaneous response of liquid crystal (LC) molecules near the dye-adsorbed layer of azo dye-doped LCs (ADDLCs) in early-stage photoexcitation was observed through the pumped-attenuated total reflection (PATR) method. As the sensing depth of the attenuated total reflection (ATR) method is in the order of hundreds of nm, in situ observation using the PATR method directly inspected the restructuring of the adsorbed layer in the early-stage photoexcitation, which demonstrated the competition between adsorption and desorption on the isotropic surface without the interference of the elastic continuum in the bulk of a cell. Analyzing the frequency of the oscillation-like PATR signal, which reflects the evolution of the orientation of the LC molecules, revealed that, depending on the dose of the pump light, the structural evolution of the adsorbed layer is fast at the beginning of photoalignment in ADDLCs and slows down with the accumulation of the adsorbed layer. A suitable intensity of the pump light in the early stage of the photoalignment process can improve the reform of the adsorbed layer to strengthen its anchoring.

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“…When ADDLC cells are exposed to certain wavelengths, azo-dye molecules on a surface diffuse, adsorb, and desorb sequentially because of photoisomerization. [1][2][3][4][5] This causes LC-structure reorientation by excited dye molecules. In related studies, the optical characteristics of ADDLC photoexcitation have been obtained on the basis of pump-probe twistnematic experiments.…”

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confidence: 99%

Dynamics of azo-dye-doped liquid crystals

Lo¹

2009

SPIE Newsroom

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Two novel approaches enable better insights into liquid-crystal photoalignment.Because of their potential importance for photoalignment, azodye-doped liquid crystals (ADDLCs) have been studied extensively over the past decade. Photoalignment of liquid-crystal (LC) molecules can avoid unnecessary contamination due to mechanical rubbing. When ADDLC cells are exposed to certain wavelengths, azo-dye molecules on a surface diffuse, adsorb, and desorb sequentially because of photoisomerization. [1][2][3][4][5] This causes LC-structure reorientation by excited dye molecules. In related studies, the optical characteristics of ADDLC photoexcitation have been obtained on the basis of pump-probe twistnematic experiments. 1, 2 To better understand the transient as well as the permanent behavior of ADDLCs, here we present two novel approaches to observe the reorientation of LC molecules on short 5 and long timescales, 6 respectively.The dye-adsorbed layer is formed during photoalignment. The dye molecules can interact with the surrounding LC molecules through intermolecular forces. As a result, the LC's director (i.e., the LC structure's preferred direction) is reoriented perpendicular to the pump field. 7 After irradiation of the sample for a sufficiently long period, a ripple structure is generated parallel to the pump field. 7 To explore the structural variation of the dye-adsorbed layer, we channeled unpolarized light into a traditional pump-probe twist-nematic experiment, (which is generally used for observing light scattering during formation of the dye-adsorbed layer). The transmitted probe light was scattered during photoexcitation because of the dye's adsorption. Because the ripple structure is formed gradually, it is regular, thus enhancing the dye's surface anchoring (see Figure 1). At the same time, Rayleigh scattering from isolated (or separated) dye-adsorbed grains changes to Mie scattering since the regular ripple structure has a similar spatial scale as the wavelength of the incident light. The increase in transmission intensity is more centralized for Mie than for Rayleigh scattering in the presence of the regular ripple structure. 8 After the ripple structure has formed, the dye molecules' adsorption increases the depth of the grooves, which in turn enhances the surface-anchoring coefficient. The Mie scattering does not vary significantly because the spacing of the ripple structure remains the same in the optical view. Our experimental results verify that the formation of the ripple structure correlates with light scattering induced by adsorption of dye molecules on the surface.Attenuated total-reflection (ATR) measurements are highly sensitive to variations in the dielectric constants close to the surface (approximately 100nm from the surface in LC systems) within the depth of the evanescent wave, which is not affected by the elastic continuum. 9 Our pumped-ATR experiments result in improved measurements compared to the traditional ATR approach. We channeled green pump light-accompanied by a red probe beam...

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Observation of transient reorientation of dye-doped liquid crystals by pumped attenuated total reflection

Cited by 3 publications

References 14 publications

Dynamics of the photoexictation of azo dye-doped liquid crystals probing by pumped attenuated total reflection

Dynamics of the photoexictation of azo dye-doped liquid crystals probing by pumped attenuated total reflection

Competitive Dye Adsorption-Desorption on the Isotropic Surface at the Early Stage of the Photoexcitation of Azo Dye-Doped Liquid Crystals

Dynamics of azo-dye-doped liquid crystals

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