A Switchable Cholesteric Phase Grating with a Low Operating Voltage

Round 1

Reviewer 1 Report

In this paper the authors discuss light diffraction from uniform lying helical structure. The subject exhibits current interest and deserves publication. However, in the present form the manuscript is written in a rather careless manner and raises a number of questions.
1. In lines 49-50 it is stated that the pitch was set as half of the cell gap, and this statement is repeated in lines 77-78. However, in Figs.1 and 5 cells with different ratio d/p are used. In Figs. 3, 4 and 6 it is not clear whether this condition applies. Please clarify.
2 The periodicity of the fingerprint structure should be distinguished from that of the undistorted cholesteric structure, because the homeotropic boundary condition can influence the former one. Therefore I find somewhat misleading that the x axis title in Fig. 3 is "Grating period" instead of half-pitch. By the way, the deviation of the two periods could be inferred from the experimental data.
3. I have doubts about the applicability of Eq. 3, providing the diffraction efficiency. This relation was originally derived by Kogelnik (The Bell System Technical Journal 48, p. 2909 - 2947, 1969.) However, the original work refers to an incoming beam for which the Bragg reflection condition is satisfied, which is not the case for the situation considered in the present paper. Furthermore, in Figs. 4 and 5, according to Eq. 3 the theoretical curve should oscillate between 0 and 100%, while the "good fit" with the experimental data is achieved with a peak value of 30 and 20% respectively. In addition, the data in Fig. 4 show no oscillation as a function of the cell gap in contrast with calculated curve, which is rather suspicious.
4. In Fig. 6 the authors state that the calculated curves "agree well with the measured diffraction efficiency". What do they mean by this?
The paper should not be published without serious revisions.

Author Response

Responses to reviewer’s comments

Manuscript ID: crystals-1070662

Thank you very much for your valuable comments regarding our manuscript.
In this letter, we would like to explain our revisions.

 

For Reviewer # 1

Q1) In lines 49-50 it is stated that the pitch was set as half of the cell gap, and this statement is repeated in lines 77-78. However, in Figs.1 and 5 cells with different ratio d/p are used. In Figs. 3, 4 and 6 it is not clear whether this condition applies. Please clarify.

Reply) The pitch for the ULH state (fingerprint state) can be set within a d/p range of 1-4, as shown in Figure 1. Therefore, we simply set the cell gap as twice the pitch to confirm the POM images, diffraction angles, diffraction efficiencies, and wavelength dependence in Figures 2, 3, 4, and 6, respectively. Since the diffraction efficiency is dependent on the cell gap (Figure 5), we have chosen the optimized cell gap in Figure 7. Following the reviewer’s comments, we added the pitch conditions for all of the figures in the revised manuscript.

Q2) The periodicity of the fingerprint structure should be distinguished from that of the undistorted cholesteric structure, because the homeotropic boundary condition can influence the former one. Therefore I find somewhat misleading that the x axis title in Fig. 3 is "Grating period" instead of half-pitch. By the way, the deviation of the two periods could be inferred from the experimental data. 

Reply) Without consideration of the boundary condition, the grating period is a half of the cholesteric pitch. However, the grating period measured with the POM image was twice the pitch. The homeotropic boundary condition affects the periodic structure of the fingerprint texture, as mentioned by the reviewer. Following the reviewer’s comment, we added comments on the grating period in the revised manuscript as follows: “The measured grating period Λ of the ULH cells were 2.5, 5, 10, and 20 μm, which are twice  the cholesteric pitches, respectively. Because the helix could be distorted by confining ChLCs between the two substrates treated to produce homeotropic boundary conditions [26].”

We measured the grating period of fabricated LC cells, and we used the ‘grating period’ instead of the half-pitch in the x-axis of Figure 3.

 

Q3) I have doubts about the applicability of Eq. 3, providing the diffraction efficiency. This relation was originally derived by Kogelnik (The Bell System Technical Journal 48, p. 2909 - 2947, 1969.) However, the original work refers to an incoming beam for which the Bragg reflection condition is satisfied, which is not the case for the situation considered in the present paper. Furthermore, in Figs. 4 and 5, according to Eq. 3 the theoretical curve should oscillate between 0 and 100%, while the "good fit" with the experimental data is achieved with a peak value of 30 and 20% respectively. In addition, the data in Fig. 4 show no oscillation as a function of the cell gap in contrast with calculated curve, which is rather suspicious.

Reply) We appreciate your valuable comments on our mistakes in Figures 4 and 5. The ‘calculated results’ in the original manuscript were obtained by fitting experimental results with the proportional relationship between the diffraction efficiency and the square of a sinusoidal function with a period of (π∆nd)/(λcosβ). We confirmed the tendency with the periodicity as a function of the cell gap while ignoring the loss and other factors that affect the efficiency.

Following the reviewer's comments, we replaced the equation for the diffraction efficiency with the proportional relationship. We revised the explanation on the diffraction in the revised manuscript as follows: “Figure 4 shows the change in first-order diffraction efficiency as a function of the cell gap. The diffraction efficiency is proportional to the square of a sinusoidal function with a period of (π∆nd)/(λcosβ) where λ and β are the wavelength and the incident angle of the probe beam, respectively; d is the thickness of the liquid crystal layer; ∆n is the refractive index difference in the grating [32]. As shown in Figure 1, the measured results (dots) show the same periodicity with the proportional expression.”

Q4) In Fig. 6 the authors state that the calculated curves "agree well with the measured diffraction efficiency". What do they mean by this? 

Reply) We appreciate your comments on our mistakes in figure 6. We missed the measured data in Figure 6. However, as mentioned by the reviewer in Q3, we fitted the measured diffraction efficiency with the proportional expression. We deleted the sentence on the comparison of the measured and calculated efficiency and revised Figure 6 in the revised manuscript.

 

Thank you again for your kind comments and valuable time.

Author Response File: Author Response.docx

Reviewer 2 Report

In this manuscript authors discuss the use of a uniform-lying-helix (ULH) cholesteric liquid crystal (CLC) cell for phase grating device applications. Authors report on the experimentally reached diffraction angle of 10° and efficiency of 30%.

 

Although I find this manuscript interesting in terms of a technical report, I see the lack of originality in it. I also see the lack of technical details and a room for the optimization of the citations in this manuscript.

 

I invite authors to answer following question which may improve the quality of the manuscript.

 

Authors claim on reaching the high diffraction efficiency but they do not say to which value they compare it, what is the record for today?

 

Authors speak about a rubbing technique as a key for the CLC cell preparation but do not discuss the rubbing process and do not describe the resulting anchoring after the rubbing: how the rubbing is done? the resulting anchoring is uniform or patterned? is it oblique, if so at which angle?

 

Authors provide at least ten different citation to lasing with LCs but do not cite the plenty of the articles on the CLC phase grating: see for example “Cholesteric gratings with field-controlled period”, Appl. Phys. Lett. 71, 3323 (1997); https://doi.org/10.1063/1.120325 and articles citing it. Could authors be more clear about which message they convey when citing articles and maybe avoid big blocks like 1-15 and separate them into groups for lasers, mirrors, modulators etc.?

 

Figure 1 apparently contains a lot of information but it is not clear how this information was obtained: what is the protocol for the transmittance measurement? Does it involve the polarization filtering as in POM? Why the transmittance decreases? Because of the scattering? What authors call as “number of pitches”? Is it d/p? Why the image at d/p=1 looks dark when the transmittance is high? What happens to the helix orientation once the chiral torque overcomes the homeotropic anchoring?

 

Is it expected that the period of the grating is twice bigger that the pitch of the CLC (lines 106-107)? If so, why?

 

Due to the fact that I see the lack of technical details, discussion and citations I cannot recommend the manuscript to be published in the present form.

Author Response

Responses to reviewer’s comments

Manuscript ID: crystals-1070662

Thank you very much for your valuable comments regarding our manuscript.
In this letter, we would like to explain our revisions.

 

For Reviewer # 2

Q1) Authors claim on reaching the high diffraction efficiency but they do not say to which value they compare it, what is the record for today?

Reply) We appreciate your valuable comments about the reported diffraction efficiency. As mentioned by the reviewer, we did not compare the diffraction efficiency of the fabricated LC cell with that of other grating devices reported recently. The diffraction efficiency of the fabricated LC cell does is not higher than other LC grating cells [R1, R2]. Following the reviewer's comment, we revised the sentences in ‘Abstract’ and ‘Introduction’ in the revised manuscript as follows:
“We found that the fabricated grating device can provide a large diffraction angle of 10° and low operating voltage of 3 V with a diffraction efficiency of 30%.”
“In this paper, we propose an LC phase grating device with a large diffraction angle and low operating voltage.”
“We confirmed that the fabricated grating device exhibited a large diffraction angle of 10°, a low operating voltage of 3 V, and a diffraction efficiency of 30%.”

[R1] Yan, J.; Li, Y.; Wu, S.-T. High-efficiency and fast-response tunable phase grating using a blue phase liquid crystal, Opt. Lett. 2011, 36, 1404-1406.
[R2] Chen, H.; Tan, G.; Huang, Y.; Weng, Y.; Choi, T.-H.; Yoon, T.-H.; Wu, S.-T. A low voltage liquid crystal phase grating with switchable diffraction angles, Sci. Rep. 2017, 7, 39923.

Q2) Authors speak about a rubbing technique as a key for the CLC cell preparation but do not discuss the rubbing process and do not describe the resulting anchoring after the rubbing: how the rubbing is done? the resulting anchoring is uniform or patterned? is it oblique, if so at which angle?

Reply) We appreciate your valuable comments about the rubbing process and anchoring conditions. Following the reviewer's comments, we add the rubbing condition in the revised manuscript as follows: “The rubbing condition was set as follows: the average values of the fiber diameter, the fiber length, and the fiber density were 15 μm, 2.5 mm, and 1040 cm⁻², respectively. The diameter of the rubbing roller and the rubbing depth was 60 mm, 0.25 mm, respectively. The rotation speed of the roller was 800 rpm and the substrate proceeding speed was 33 mm/s, respectively.”

However, we did not measure the anchoring conditions or pretilt angle of the fabricated LC cell because the rubbing process was used to provide orientation to LCs in the fingerprint state. According to references, the anchoring conditions of the LC cell obtained by rubbing the homeotropic alignment layer shows uniform surfaces with a pretilt angle of about 87.5° [R3, R4].

[R3] Lee, H.M; Chung, H.-K.; Park, H.-G.; Seo, D.-S. Vertically aligned liquid crystal mode on UV-cured reactive mesogen using imprinting method, Opt. Mater. Express 2015, 5, 44-50.
[R4] Hsu, C.-J.; Chen, B.-L.; Huang, C.-Y. Controlling liquid crystal pretilt angle with photocurable prepolymer and vertically aligned substrate, Opt. Express 2016, 24, 1463-1471.

Q3) Authors provide at least ten different citation to lasing with LCs but do not cite the plenty of the articles on the CLC phase grating: see for example “Cholesteric gratings with field-controlled period”, Appl. Phys. Lett. 71, 3323 (1997); https://doi.org/10.1063/1.120325 and articles citing it. Could authors be more clear about which message they convey when citing articles and maybe avoid big blocks like 1-15 and separate them into groups for lasers, mirrors, modulators etc.? 

Reply) We appreciate your valuable comments about the articles on the ChLC phase grating. Following the reviewer’s comments, we added references on the ChLC phase grating [17,18] and separated the citing articles [1-15] into groups for each application in the revised manuscript as follow: “Recently, interest in ChLCs has grown due to their possible non-display applications, such as beam deflectors [1], controlled mirrors [2], light shutters [3-7], laser elements [8- 13], and diffraction grating elements [14-18].”.

Q4) Figure 1 apparently contains a lot of information but it is not clear how this information was obtained: what is the protocol for the transmittance measurement? Does it involve the polarization filtering as in POM? Why the transmittance decreases? Because of the scattering? What authors call as “number of pitches”? Is it d/p? Why the image at d/p=1 looks dark when the transmittance is high? What happens to the helix orientation once the chiral torque overcomes the homeotropic anchoring? 

Reply) We appreciate your valuable comments on the experimental conditions for the data in figure 1. Following the reviewer’s comments, we added the method for measuring the transmittance and the reason for the decrease in the transmittance in the revised manuscript as follows: “The transmittance was measured as the ratio of the incident light intensity and the transmitted light without polarizers. The transmittance of the LC cell decreased with the increase of d/p because of the light scattering in the fabricated LC cell.”

We are sorry for confusing the reviewers with the word ‘number of pitches.’ Following the reviewer’s comments, we replaced ‘the number of pitches’ with ‘d/p’ in the revised manuscript. We used crossed polarizers for the POM images, as mentioned in the original manuscript. The POM image for d/p =1 looks dark because the LCs are aligned vertically. When the chiral torque overcomes homeotropic anchoring, the LCs twist and form a chiral structure [R5].

[R5] Baek, J.-M.; Oh, S.-W.; Kim, S.-H.; Yoon, T.-H. Fabrication of an initially-focal-conic cholesteric liquid crystal cell without polymer stabilization, Displays 2017, 52, 55-58

Q5) Is it expected that the period of the grating is twice bigger that the pitch of the CLC (lines 106-107)? If so, why?.

Reply) Without consideration of the boundary condition, the grating period is half of the cholesteric pitch. However, the measured grating period with the POM image was twice the pitch. Helix can be distorted by confining ChLCs between the two substrates treated to produce homeotropic boundary conditions [26]. These homeotropic boundary conditions may affect the increase in the cholesteric pitch. Following the reviewer’s comments, we added sentences on the grating period in the revised manuscript as follows: “The measured grating period Λ of the ULH cells were 2.5, 5, 10, and 20 μm, which are twice the cholesteric pitch, respectively. Because the helix could be distorted by confining ChLCs between the two substrates treated to produce homeotropic boundary conditions [26].”

[26] Oswald, P.; Baudry, J.; Pirkl, S. Static and dynamic properties of cholesteric fingers in electric field, Phys. Rep. 2000, 337, 67.

 

 

Thank you again for your kind comments and valuable time.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors responded adequately to the criticisms presented in my previous report.

Reviewer 2 Report

Authors answered my questions and I find the manuscript to be good for accepting in present form.