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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 16 — Aug. 6, 2007
  • pp: 10381–10389
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Characterization of a nematic PALC at large oblique incidence angles

C. C. Tsai, H. C. Wei, C. H. Hsieh, L. P. Yu, C. R. Yu, H. S. Huang, and C. Chou  »View Author Affiliations


Optics Express, Vol. 15, Issue 16, pp. 10381-10389 (2007)
http://dx.doi.org/10.1364/OE.15.010381


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Abstract

Compared with conventional photometric methods of measuring cell parameters, including the cell gap and the pretilt angle of a nematic parallel-aligned liquid crystal (PALC) using multiple wavelengths at normal incidence, this research proposes the use of a phase-sensitive interferometric ellipsometer to determine cell parameters precisely based on a single wavelength at large oblique incidence angles. The advantage of this method is that it detects the phase difference using an optical heterodyne interferometer in which a common phase noise rejection mode is provided. Thus, there is a high signal-to-noise ratio (SNR) on the phase measurement. In addition, a range of large oblique incidence angles on the PALC is used so that a high sensitivity measurement of the cell parameters is obtained experimentally. During the measurements, the multiple reflections and spatial shifting effect of the emerging extraordinary ray (E-ray) and ordinary ray (O-ray) from the PALC at large oblique incidence angles are able to be reduced effectively by the use of retro-reflected geometry in the interferometer. The experimental results verify that the sensitivities for the cell gap and pretilt angle measurements are 0.3 nm and 0.01°, respectively.

© 2007 Optical Society of America

1. Introduction

Various techniques to evaluate liquid crystal (LC) cell parameters, such as the cell gap and pretilt angle techniques, have been demonstrated previously. These methods are based on intensity measurement by a conventional photometric ellipsometer, or polarimeter [1

1. S. V. Yablonskiĭ, K. Nakayama, S. Okazaki, M. Ozaki, K. Yoshino, S. P. Palto, M. Y. Baranovich, and A. S. Michailov, “Control of the bias tilt angles in nematic liquid crystals,” J. Appl. Phys. 85, 2556–2561 (1999). [CrossRef]

4

4. S. H. Lee, W. S. Park, G. D. Lee, K. Y. Han, T. H. Yoon, and J. C. Kin, “Low-cell-gap measurement by rotation of a wave retarder,” Jpn. J. Appl. Phys. 41, 379–383 (2002). [CrossRef]

]. Recently, a multiwavelength light source was introduced into the optical setup that can successfully extend the system’s capabilities to measure the two-dimensional (2-D) distribution of cell parameters [5

5. R. Simon and D. M. Nicholas, “An interferometric method of measuring tilt angles in aligned thin films of nematic liquid crystals,” J. Phys. D: Appl. Phys. 18, 1423–1430 (1985). [CrossRef]

7

7. M. Kawamura, Y. Goto, and S. Sato, “A two-dimensional pretilt angle distribution measurement of twisted nematic liquid crystal cells using Stokes parameters at plural wavelengths” Jpn. J. Appl. Phys. 43, 709–714 (2004). [CrossRef]

]; this involves a rotated polarizer and an analyzer that are arranged in conjunction with different compensators for different wavelengths. Kawamura et al. [7

7. M. Kawamura, Y. Goto, and S. Sato, “A two-dimensional pretilt angle distribution measurement of twisted nematic liquid crystal cells using Stokes parameters at plural wavelengths” Jpn. J. Appl. Phys. 43, 709–714 (2004). [CrossRef]

] used a broadband light source that was able to build up the 2-D pretilt angle and cell gap distributions of a twisted nematic LC (TNLC). Furthermore, Tang et al. [8

8. S. T. Tang and H. S. Kwok, “Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry,” J. Appl. Phys. 89, 80–85 (2001). [CrossRef]

] proposed a method that adopted spectroscopic ellipsometry for the measurement by the transmission of LC cell parameters. Another study by Ong [9

9. H. L. Ong, “Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incidence light,” J. Appl. Phys. 71, 140–144 (1992). [CrossRef]

] demonstrated measurement of the cell gap and the pretilt angle of a parallel-aligned LC (PALC) using oblique incidence; however, a compensator is required in this method in order to null the transmission of light intensity during the cell thickness measurement, while at the same time the variable oblique incidence angle is scanned so that the pretilt angle is obtained without using a compensator. As a result of this method, which uses a nulling technique for the transmission intensity, the detection sensitivity for the cell parameter measurement is improved. However, the insensitivity of the zero transmission intensity at the oblique incidence angle to the PALC causes some uncertainty in the cell parameter measurement and, in addition, the multiple reflections in the LC at small oblique incidence angles produce phase oscillation that affects the calculation of the cell parameters.

It is clear from the above that there are limitations to the measurement of cell parameters that result from adopting an oblique incidence technique. To circumvent this, an interferometric ellipsometer [10

10. C. H. Hsieh, “Linear birefringence parameters of an uncoated multiple-order wave plate with a phase-sensitive optical heterodyne ellipsometry,” M.S. thesis (Institute of Biophotonics, National Yang-Ming University, Taiwan2006).

] using a range of large oblique-incidence angles (30°≤ϕ≤50°) at a single wavelength to measure the cell parameters of a PALC is proposed and experimentally demonstrated in this study. This approach belongs to the phase-sensitive detection method and focuses on measuring the phase retardation between the P and S waves in the PALC. The setup involves a common-path configuration for the interferometric ellipsometer, and this allows a common-phase noise rejection mode to be implemented that is immune to environmental disturbance and laser frequency noise. This ensures that the phase detection is better stabilized and that there is a high signal-to-noise ratio (SNR) for the measurements. At the same time, the detection sensitivity for the cell parameters at large oblique incidence angles is enhanced significantly and the effects of multireflection and spatial shifting of emerging beams from the PALC are avoided effectively by setting up a retro-reflected geometry in this interferometer. As a result, precise measurements of the cell parameters of the PALC are possible, and the experimental verification was demonstrated. Overall, there are a number of significant advantages of this phase-sensitive interferometric ellipsometer method compared with the conventional photometric polarimeter approach.

2. Principle and experiments

As shown in Fig. 1, a polarized common-path optical heterodyne interferometer is set up [11

11. Y. C. Huang, C. Chou, L. Y. Chou, J. C. Shyu, and M. Chang, “Polarized optical heterodyne profilometer,” Jpn. J. Appl. Phys. 37, 351–354 (1998). [CrossRef]

]. The mirror M2 in the signal arm of the interferometer, which is able to reduce the spatial shifting of emerging beams, is arranged in order to retro-reflect the emerging beams of the extraordinary ray (E-ray) and ordinary ray (O-ray) from the PALC under test conditions. At the same time, the effects of the multiple reflections of the E-ray and the O-ray in the PALC at a large oblique incidence angle are reduced.

Fig. 1. Experimental setup: BS, beam splitter; AOM, acousto-optic modulator; M, mirror; A, polarizer; S, test sample on three-dimensional rotation stage; PBS, polarization beam splitter; Dp, Ds, photo detectors; BPF, band-pass filter; LIA, lock-in amplifier; DSC, digital stepping controller; PC, personal computer; PALC, parallel-aligned liquid crystal.

Figure 2 shows the ray-tracing diagram for the tested PALC in this experiment, where the retro-reflected geometry of the PALC at a large oblique incidence angle is also shown. A thick (5 mm) BK-7 glass plate is attached to the PALC in order to reduce the multireflection effect. Additionally, two thin BK-7 glass plates of refractive index n=1.516 and thickness t≅600µm are used to confine the liquid crystal molecules. At the same time, two indium tin oxide (ITO) electrodes (n≅1.82,d≅0.1µm) are built into the two BK-7 glass plates. The PALC molecules are introduced between the two ITO electrodes as shown in Fig. 3.

Fig. 2. Retro-reflected geometry of the PALC at a large oblique incidence angle. A thick BK7 glass plate is attached to the PA-LC in order to reduce multiple reflections of the laser beam.
Fig. 3. Nematic PALC device is equivalent to the phase retardation wave plate where the optical axis is pretilted at θpre to the surface.

Finally, the P-polarized heterodyne signal IPωt) is generated by the P1 wave from the reference beam, and the P2 wave from the signal beam and can be expressed by

Ip(Δωt)=2Ip1Ip2cos(Δωt+δp).
(1)

Similarly, the S-polarized heterodyne signal ISωt) is produced by the S1 and S2 waves coming from the reference and signal beams respectively:

Is(Δωt)=2Is1Is2cos(Δωt+δs),
(2)

2mπ+Γ=2πλ(Δn)d=2πλ[none(θpre)]d,
(3)

where 2+Γ is the untwisted phase retardation of the PALC (0°≤Γ≤360°), m is the order number, and ne(θpre) can be expressed by

ne(θpre)=nenone2sin2θpre+no2cos2θpre.
(4)

After normally rotating the PALC, the optical axis lies on the x-z plane until δ (o)(β=β min)=δ (o)=Γ so that Eq. (3) is satisfied. δ (o) is the phase retardation of the output polarization state and β is the rotation angle of the PALC. Figure 4 shows the ray tracing diagrams of the PALC at an oblique incidence angle in the different incidence planes. Figures 4(a) and 4(b) are scanned with respect to the variable oblique incidence angle in the y–z (tilted along x-axis) and x-z (tilted along y-axis) planes, respectively. Thus, the phase retardation at an oblique angle ϕtx tilted along the x-axis [see Fig. 4(a)] can be derived as

δtx(ϕtx)=22πλ(no2sin2ϕtxne2(θpre)sin2ϕtx)d.
(5)

If θpre≤3°, then ne(θpre)≅ne is satisfied. When no and ne are given, then δtx(ϕtx) becomes a function of the thickness d only. Thus, d can be obtained by using Eq. (5). Similarly, in Fig. 4(b), when the optical axis is perpendicular to the y-axis (lays on x-z plane), the phase retardation at ϕty along y-axis becomes

δty(ϕty)=22πλ[nosin2ϕtyno1(sinϕtyno)2nesin2ϕtyne1(sinϕtyne)2]d
=22πλ[no2sin2ϕtyne2sin2ϕty]d,
(6)

where

ne=neno{ne2sin2[θpresin1(sinϕtyne)]+no2cos2[θpresin1(sinϕtyne)]}12.
(7)

ne is the refractive index observed by E″-ray in Fig. 4(b). Then θpre can be calculated by using Eqs. (6) and (7) at a large incidence angle.

Fig. 4. Schematic diagram of the nematic PALC at oblique incidence: (a) in y-z plane (tilted along x-axis), (b) in x-z plane (tilted along y-axis).

In this experiment, a PALC with the refractive indices n o=1.489, n e=1.588 at 632.8 nm wavelength were used in the test. In Fig. 1, the PALC is placed between BS2 and M2 in order to compensate for spatial shifting and to avoid multiple reflections of the emerging E-ray and O-ray in the PALC. The oblique incidence angle is then scanned in a range from -50°≤ϕt≤-30° and a range from 30°≤ϕt≤50° at large incidence angles, both in the y-z plane (tilted along x-axis) and in the x-z plane (tilted along y-axis) separately. In order to offset the initial phase retardation at |ϕtx|=30°, which is due to the residual phase retardation of the optical components in the interferometer, δtx(-30°)≅0° and δtx(30°)≅0° are offset numerically. Then the phase retardation at the incidence angle in the range of -50°≤ϕtx≤-30° and 30°≤ϕtx≤50° are measured by use of a lock-in amplifier. Next, a thick BK-7 glass plate (5 mm) is attached to the PALC (see Fig. 2) in order to reduce the multiple reflections of the PALC at a large incidence angle. Figures 5(a) and 5(b) show the difference between the arrangements both with and without a thick glass plate attached to the PALC when a variable large incidence angle ϕtx is scanned in a range (30°≤ϕtx≤50°). It is obvious that the arrangement in Fig. 2 avoids multiple reflections.

Fig. 5. Phase retardation vs. oblique incidence angle: (a) without attaching the thick BK-7 glass plate, (b) after attaching the thick BK-7 glass plate.

Moreover, Fig. 6(a) shows the results of the phase retardations of δtx at ϕtx in the ranges of -50°≤ϕtx≤-30°(red color) and of 30°≤ϕtx≤50° (blue color), where δtx(30°) and δtx(-30°) were offset numerically. Both phase retardations δtx and ϕtx seem to virtually overlap. This is in contrast with δty, where there is a large deviation relative to the scanned angle ϕty in the range of -50°≤ϕty≤-30° (red color) and of 30°≤ϕty≤50° (blue color) tilted along y-axis as shown in Fig. 6(b). According to Eq. (4), if θpre is less than 3°, then ne(θpre)≅ne is satisfied. The cell gap of the tested PALC at d=4.156 µm is then calculated by use of the relationship {[δtx(50°)-δtx(30°)]+[δtx(-50°)-δtx(-30°)]}/2=-44.472° in Fig. 6(a) where δtx(50°)-δtx(30°)≅δtx(-50°)-δtx(-30°). Similarly, when the PALC is scanned along the y-axis in the range of -50°≤ϕty≤-30° and 30°≤ϕty≤50° independently, then the pretilted angle θpre is obtained by use of the relation [δty(50°)-δty(30°)]-[δty(-50°)-δty(-30°)]=15.648° in Fig. 6(b) where δty(30°) and δty(-30°) are offset numerically, too. The measured cell gap of d=4.156 µm and the pretilt angle of θpre=2.59° are comparable to the data d≅4000 nm and θpre≅1~2° for the PALC device that was obtained from the Chi-Mei Optoelectronics Co., Taiwan; this comparison confirms the experimental verification of this method at large incidence angles.

Fig. 6. Phase retardation vs. oblique incidence angle: (a) in y-z plane (tilted along x-axis), (b) in x-z plane (tilted along y-axis). The error bar of the measurement is smaller than the line width of red and blue lines.

In Fig. 7, the asymmetry of the phase retardation δty for -50°≤ϕty≤-30° and 30°≤ϕty≤50° is seen experimentally. This is due to the nonzero pretilt angle of the PALC, which is tilted along the y-axis. This is in contrast to the symmetry of the phase retardation δtx for -50°≤ϕtx≤-30° and 30°≤ϕtx≤50° using the same measurement. According to Eq. (6), a computer simulation based on the values θpre=2.59° and d=4.156 µm obtained in this experiment shows a good fit between the measured data by this method where the initial phase retardations at |ϕty|=30° are both compensated relative to the theoretical one. The deviations between the calculated and experimental data at 30°≤|ϕty|≤50° in Fig. 7 might be caused by the residual effect of multiple reflections in the PALC.

Fig. 7. Comparison of the experimental (-50°≤ϕty≤-30°, 30°≤ϕty≤50°) and theoretical (-50°≤ϕty≤50°) curves scanned in the x-z plane (y-axial tilt) where theoretical curve is calculated by (ne,no)=(1.588,1.489), d=4.156µm, and θpre=2.59° at 632.8nm wavelength. The error bar is smaller than the line width of the measurements (red line).

The conventional method used to characterize cell parameters of a nematic LC focuses on polarizer-sample-analyzer geometry by the photometric method. This is based on light intensity detection by which the phase retardation between the E-ray and O-ray are measured in terms of transmission intensity. There is limited sensitivity and dynamic range available for the intensity measurements used during cell parameters detection, and therefore we have proposed in this study to measure the cell parameters by phase-sensitive detection using a polarized common-path optical heterodyne interferometer. This not only enhances the sensitivity by the use of heterodyne signal detection and a wider dynamic range for phase measurement using a lock-in amplifier, but also provides larger oblique incidence angles on the PALC that are able to obtain high sensitivity for the phase retardation measurement. The result is high sensitivity for cell parameter detection compared to the conventional photometric method. In this setup, the multiple reflections and spatial shifting of the emerging E-ray and O-ray are avoided by the use of retro-reflected geometry in the interferometer. The experimental results demonstrate the utility of this method.

This method is also based on polarized common-path phase difference detection that enables the canceling of the common phase noise caused by both environmental disturbances and laser frequency noise. Therefore, a high SNR for the phase retardation detection is available. In this experiment, 0.3 °/hr for the phase stability was achieved, which implies a detection sensitivity of 0.3 nm for the cell thickness measurement of the PALC. Furthermore, 0.01°/min for the phase stability over a short testing period was obtained in these experiments, which implies 0.01° sensitivity for the pretilt angle detection according to Eq. (6) when using a lock-in amplifier. This contrasts with a pretilt angle measurement sensitivity of ±0.2° obtained by Kawamura et al. [7

7. M. Kawamura, Y. Goto, and S. Sato, “A two-dimensional pretilt angle distribution measurement of twisted nematic liquid crystal cells using Stokes parameters at plural wavelengths” Jpn. J. Appl. Phys. 43, 709–714 (2004). [CrossRef]

].

3. Discussion and conclusions

From the above results, it is clear that the proposed method of using a phase-sensitive polarized optical heterodyne interferometer at a large oblique incidence angle to characterize PALCs not only provides high sensitivity for the cell parameters measurements but also results in a wide dynamic range for the phase measurement compared to the conventional photometric method. It is based on a set of crossed polarizers that are used for optical geometry and intensity measurement. A large oblique incidence angle is scanned in this setup, and this is able to enhance the sensitivity of phase retardation measurement, while multiple reflections and the spatial shifting effect of the emerging beams in PALC are effectively avoided. Additionally, the scattering effect [13

13. L. C. Peng, C. Chou, C. W. Lyu, and J. C. Hsieh, “Zeeman laser-scanning confocal microscopy in turbid media,” Opt. Lett. 26, 349–351 (2001). [CrossRef]

] in the PALC can be reduced too, because the heterodyne efficiency is critical to the alignment between the signal and reference beams. The scattered P2 and S2 waves of the signal beam in the PALC provide lower contributions compared with the non-scattered P2 and S2 waves to the heterodyne signals during measurement. The result is that the scattered P2 and S2 waves are rejected owing to the low heterodyne efficiencies of the P and S polarized heterodyne signals. As a consequence, the cell parameters of the PALC are able to be measured precisely using the developed phase-sensitive optical heterodyne interferometer at large oblique incidence angles. In this experiment it was possible to measure the sensitivity for the cell gap detection to 0.3 nm and for the pretilt angle to be 0.01° when a range of large tilted oblique incidence angles (30°<|ϕt|<50) on the PALC was scanned. The multiple reflections and spatial shifting effect were avoided by setting up a correct retro-reflected geometry in the interferometer. Moreover, this method would also seem to show potential for 2-D cell parameter measurement if the amplitude of the heterodyne signal is detected using a CCD camera [14

14. M. Akiba, K. P. Chan, and N. Tanno, “Real-time, micrometer depth-resolved imaging by low-coherence reflectometry and a two-dimensional heterodyne detection technique,” Jpn. J. Appl. Phys. 39, L1194–L1196 (2000). [CrossRef]

]. As a result, to measure the phase and amplitude of the detected polarized heterodyne signal simultaneously in this setup, a 2-D highly sensitive characterization of the cell parameters of a PALC becomes applicable at a large oblique incidence angle.

Acknowledgment

We thank Chi-Mei Optoelectronics Co., Taiwan, for providing the nematic PALC sample for this research. The research was supported in part by the National Science Council of Taiwan through NSC 95-2221-E-010-015-MY3.

C. C. Tsai is with the Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei 106, Taiwan.

References and links

1.

S. V. Yablonskiĭ, K. Nakayama, S. Okazaki, M. Ozaki, K. Yoshino, S. P. Palto, M. Y. Baranovich, and A. S. Michailov, “Control of the bias tilt angles in nematic liquid crystals,” J. Appl. Phys. 85, 2556–2561 (1999). [CrossRef]

2.

A. Baba, F. Kaneko, K. Shinbo, K. Kato, S. Kobayashi, and T. Wakamatsu, “Evaluation of tilt angles of nematic liquid crystal molecules on polyimide Langmuir-Blodgett films using the attenuated total reflection measurement method,” Jpn. J. Appl. Phys. 37, 2581–2586 (1998). [CrossRef]

3.

Q. Lin, H. Zhu, Y. Tang, F. Yang, and H. Gao, “Accurate optical determination of liquid crystal tilt angle by the half-leaky guided mode technique,” Displays 21, 111–119 (2000). [CrossRef]

4.

S. H. Lee, W. S. Park, G. D. Lee, K. Y. Han, T. H. Yoon, and J. C. Kin, “Low-cell-gap measurement by rotation of a wave retarder,” Jpn. J. Appl. Phys. 41, 379–383 (2002). [CrossRef]

5.

R. Simon and D. M. Nicholas, “An interferometric method of measuring tilt angles in aligned thin films of nematic liquid crystals,” J. Phys. D: Appl. Phys. 18, 1423–1430 (1985). [CrossRef]

6.

M. Kawamura and S. Sato, “Measurements of cell thickness distributions in reflective liquid crystal cells using a two-dimensional stokes parameter method,” Jpn. J. Appl. Phys. 40, L621–624 (2001). [CrossRef]

7.

M. Kawamura, Y. Goto, and S. Sato, “A two-dimensional pretilt angle distribution measurement of twisted nematic liquid crystal cells using Stokes parameters at plural wavelengths” Jpn. J. Appl. Phys. 43, 709–714 (2004). [CrossRef]

8.

S. T. Tang and H. S. Kwok, “Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry,” J. Appl. Phys. 89, 80–85 (2001). [CrossRef]

9.

H. L. Ong, “Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incidence light,” J. Appl. Phys. 71, 140–144 (1992). [CrossRef]

10.

C. H. Hsieh, “Linear birefringence parameters of an uncoated multiple-order wave plate with a phase-sensitive optical heterodyne ellipsometry,” M.S. thesis (Institute of Biophotonics, National Yang-Ming University, Taiwan2006).

11.

Y. C. Huang, C. Chou, L. Y. Chou, J. C. Shyu, and M. Chang, “Polarized optical heterodyne profilometer,” Jpn. J. Appl. Phys. 37, 351–354 (1998). [CrossRef]

12.

C. C. Tsai, C. Chou, C. Y. Han, C. H. Hsieh, K. Y. Liao, and Y. F. Chao, “Determination of optical parameters of a twisted-nematic liquid crystal by phase-sensitive optical heterodyne interferometric ellipsometry,” Appl. Opt. 44, 7509–7514 (2005). [CrossRef] [PubMed]

13.

L. C. Peng, C. Chou, C. W. Lyu, and J. C. Hsieh, “Zeeman laser-scanning confocal microscopy in turbid media,” Opt. Lett. 26, 349–351 (2001). [CrossRef]

14.

M. Akiba, K. P. Chan, and N. Tanno, “Real-time, micrometer depth-resolved imaging by low-coherence reflectometry and a two-dimensional heterodyne detection technique,” Jpn. J. Appl. Phys. 39, L1194–L1196 (2000). [CrossRef]

OCIS Codes
(040.2840) Detectors : Heterodyne
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(120.5050) Instrumentation, measurement, and metrology : Phase measurement
(120.5410) Instrumentation, measurement, and metrology : Polarimetry

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: May 11, 2007
Revised Manuscript: July 16, 2007
Manuscript Accepted: July 18, 2007
Published: August 2, 2007

Citation
C. C. Tsai, H. C. Wei, C. H. Hsieh, L. P. Yu, C. R. Yu, H. S. Huang, and C. Chou, "Characterization of a nematic PALC at large oblique incidence angles," Opt. Express 15, 10381-10389 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-16-10381


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References

  1. S. V. Yablonskiĭ, K. Nakayama, S. Okazaki, M. Ozaki, K. Yoshino, S. P. Palto, M. Y. Baranovich, and A. S. Michailov, "Control of the bias tilt angles in nematic liquid crystals," J. Appl. Phys. 85, 2556-2561 (1999). [CrossRef]
  2. A. Baba, F. Kaneko, K. Shinbo, K. Kato, S. Kobayashi, and T. Wakamatsu, "Evaluation of tilt angles of nematic liquid crystal molecules on polyimide Langmuir-Blodgett films using the attenuated total reflection measurement method," Jpn. J. Appl. Phys. 37, 2581-2586 (1998). [CrossRef]
  3. Q. Lin, H. Zhu, Y. Tang, F. Yang, and H. Gao, "Accurate optical determination of liquid crystal tilt angle by the half-leaky guided mode technique," Displays 21, 111-119 (2000). [CrossRef]
  4. S. H. Lee, W. S. Park, G. D. Lee, K. Y. Han, T. H. Yoon, and J. C. Kin, "Low-cell-gap measurement by rotation of a wave retarder," Jpn. J. Appl. Phys. 41, 379-383 (2002). [CrossRef]
  5. R. Simon and D. M. Nicholas, "An interferometric method of measuring tilt angles in aligned thin films of nematic liquid crystals," J. Phys. D: Appl. Phys. 18, 1423-1430 (1985). [CrossRef]
  6. M. Kawamura and S. Sato, "Measurements of cell thickness distributions in reflective liquid crystal cells using a two-dimensional stokes parameter method," Jpn. J. Appl. Phys. 40, L621-624 (2001). [CrossRef]
  7. M. Kawamura, Y. Goto, and S. Sato, "A two-dimensional pretilt angle distribution measurement of twisted nematic liquid crystal cells using Stokes parameters at plural wavelengths" Jpn. J. Appl. Phys. 43, 709-714 (2004). [CrossRef]
  8. S. T. Tang and H. S. Kwok, "Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry," J. Appl. Phys. 89, 80-85 (2001). [CrossRef]
  9. H. L. Ong, "Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incidence light," J. Appl. Phys. 71, 140-144 (1992). [CrossRef]
  10. C. H. Hsieh, "Linear birefringence parameters of an uncoated multiple-order wave plate with a phase-sensitive optical heterodyne ellipsometry," M.S. thesis (Institute of Biophotonics, National Yang-Ming University, Taiwan 2006).
  11. Y. C. Huang, C. Chou, L. Y. Chou, J. C. Shyu, and M. Chang, "Polarized optical heterodyne profilometer," Jpn. J. Appl. Phys. 37, 351-354 (1998). [CrossRef]
  12. C. C. Tsai, C. Chou, C. Y. Han, C. H. Hsieh, K. Y. Liao, and Y. F. Chao, "Determination of optical parameters of a twisted-nematic liquid crystal by phase-sensitive optical heterodyne interferometric ellipsometry," Appl. Opt. 44, 7509-7514 (2005). [CrossRef] [PubMed]
  13. L. C. Peng, C. Chou, C. W. Lyu, and J. C. Hsieh, "Zeeman laser-scanning confocal microscopy in turbid media," Opt. Lett. 26, 349-351 (2001). [CrossRef]
  14. M. Akiba, K. P. Chan and N. Tanno, "Real-time, micrometer depth-resolved imaging by low-coherence reflectometry and a two-dimensional heterodyne detection technique," Jpn. J. Appl. Phys. 39, L1194-L1196 (2000). [CrossRef]

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