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This work demonstrates a polarization-independent electrically tunable Fabry-Pérot (FP) filter that is based on polymer-stabilized blue phase liquid crystals (PSBPLCs). An external vertical electric field can be applied to modulate the effective refractive index of the PSBPLCs along the optical axis. Therefore, the wavelength-tuning property of the FP filter is completely independent of the polarization state of the incident light. The change in the birefringence in PSBPLCs is governed by Kerr effect-induced isotropic-to-anisotropic transition, and so the PSBPLCs based FP filter has a short response time. The measured tunability and free spectral range of the FP filter are 0.092 nm/ V and 16nm in the visible region, and 0.12nm/ V and 97nm in the NIR region, respectively, and the response time is in sub-millisecond range. The fast-responding polarization-independent electrically tunable FP filter has substantial potential for practical applications.
©2011 Optical Society of America
1. Introduction
Liquid crystal tunable filters have been widely used in various optical applications, including telecommunication [1], color generation [2], and biomedical optical imaging [3,4]. Among many filters, the liquid crystal Fabry-Perót (FP) filter, a simple optical resonator that is composed of two highly reflecting mirrors and separated by thin liquid crystal layer, is a promising optical device, which has been demonstrated by several researchers. However, because of the anisotropic properties of the liquid crystal, the performance of FP filter depends on polarization of input light, limiting its range of applications. To improve the polarization-dependency of the FP filter using nematic liquid crystal, numerous methods, including twisting the structure of the nematic liquid crystal film, and using an axially symmetrical configuration of the NLC film, have been proposed [5–7].
Blue phase liquid crystals (BPLCs) have attracted much interest in recent years. BPLCs exist in a temperature range between the isotropic and cholesteric phases [8]. They are classified as BPIII, BPII and BPI in order of declining temperature. BPIII is amorphous; BPII and BPI have simple cubic and body-centered cubic structures, respectively. Blue phase liquid crystals are considered to be essential components of next generation displays, with a number of favorable characteristics, such as absence of the need for a surface alignment layer and insensitivity to the cell gap, both of which simplify the fabrication processes and reduce cost. Another revolutionary characteristic of BPLCs is the ultra-fast response, which is in the sub-millisecond range. Unlike the typical orientation of the LC director, change in birefringence in BPLCs is governed by the Kerr effect-induced isotropic-to-anisotropic transition. Intrinsic structural defects allow BP to operate only across a narrow temperature range. Recently however, the temperature range of BP has been successfully extended beyond 60K by exploiting the polymer-stabilized effect [9–11].
Unlike a typical LC phase, the blue phase is optically isotropic because it has a symmetric structure. Under the influence of an electric field, birefringence is induced with the optic axis in the electric field direction owing to the local reorientation of the LC molecules [12, 13]. Therefore, the BP is suitable for use in polarization insensitive devices [14]. This work demonstrates a polarization-independent electrically tunable FP filter using polymer-stabilized blue phase liquid crystals and studies the tunable properties in the visible and near-infrared (NIR) region. The response time and polarization-dependence were also examined.
2. Preparation of sample and experimental setup
To prepare the sample of PSBPLCs, two positive nematic LCs, BL006 (Δn=0.286) and 5CB, were mixed with two UV-curable monomers, TMPTA and RM257, a high-HTP chiral agent, and the photoinitiator DMPAP in a ratio of 37.1:45:5.4:7.1:5:0.4. Following homogeneous mixing, the mixture was placed on a temperature-controlled stage and then use to fill an Al-coated empty cell in the isotropic state. The sample was cooled to 34°C (BPI) at a rate of 0.5°C/min, and then irradiated with ultraviolet light (0.8mW/cm2) for 30min to achieve phase separation, forming the PSBPLC, which existed over a wide temperature range, from 20°C to 58°C.
Figure 1(a) presents the configuration of the PSBPLCs based FP filter and the morphology of PSBPLCs under a crossed polarizing optical microscope in the reflection mode (R-POM). The cell gap in the sample is 3.8 μm and the reflectivity of the Al-coated mirrors for visible and NIR light is around 88% at the interface with air. Most of the platelets of PSBPLCs are blue. Figure 1(b) schematically depicts the experimental setup. To measure the electro-optical properties of the FP filter, a halogen lamp (HL-2000-FHSA-LL, Ocean Optics) with a spectrum between 360 nm and 2000nm was used as the light source and the FP filter characteristics were examined using an optical spectrum analyzer (USB 4000, Ocean Optics). The voltage that was applied to the FP device was amplified from function generator (33220A, Agilent).
Fig. 1 (a) Configuration of sample and morphology of PSBPLCs under crossed polarizing R-POM. (b) Experimental setup.
3. Results and discussions
The transmission (T) of a typical FP filter as a function of the wavelength of the incident light normal to the surface is given by
where τ and ρ are transmittance and reflectance of the mirrors, respectively. n and d are the refractive index and the thickness of the cavity. The parameters (τ, ρ, d) of the FP filter were fixed in this experiment. The tunability of the FP filter depends on the change of refractive index of the PSBPLCs. In the absence of an external field, the PSBPLCs were optically isotropic with an average refractive index of , where no and ne are the ordinary and extraordinary refractive indices of the LC molecules, respectively. Applying a vertical electric field induces birefringence of the PSBPLCs, with the optic axis normal to the substrate, that is independent of polarization for normally incident light. As the applied electric field is increased, the effective refractive index declines and the transmission spectrum of the FP filter is blue-shifted.To investigate the tunable properties of the Fabry-Pérot filter, the transmission spectra of unpolarized light in the visible and NIR region were obtained under various applied voltages. The spectral range in the visible region was from 500nm to 700nm, and included 11 transmission peaks, as shown in Fig 2(a) . As the applied voltage was increased, the transmission peaks shifted toward shorter wavelengths because the effective refractive index of the PSBPLCs was reduced; the transmission spectra were shifted by approximately 11nm at 120V. Figure 2(b) plots the wavelength of the transmission peak in the visible region as a function of the applied voltage. The tunability of the FP filter was about 0.092 nm/ V and its free spectral range (FSR), which depended on the cell gap and reflective index, was around 16 nm.
Fig. 2 Voltage-dependent transmittance of FP filter for unpolarized light in (a)visible region and (c) NIR region. Wavelengths of transmission peak as a function of applied voltage in (b) visible region and (d) NIR region.
Figure 2(c) presents the transmission spectra from 1350nm to 1550nm, covering the optical communication region. Like that of the visible region, the transmission spectra in the NIR region shifted toward short wavelengths as the applied voltage increased. When the applied voltage was 100V, the transmission spectra were shifted by 12nm. The FSR of the FP filter was 97 nm and its tunability was about 1.2 nm/ V in the NIR region, as shown in Fig. 2(d).
To confirm that the change in the ordinary refractive index of PSBPLCs caused a shift in the transmission peaks of the FP filter, the phase shift (δn) of the PSBPLCs in an electric field was measured. Another PSBPLC sample was fabricated to measure the phase shift of the PSBPLCs. The sample was sandwiched between two indium-tin-oxide glass substrates without reflecting mirrors with a cell gap of 4.5μm. The δn of the PSBPLCs cell was measured using a reflective Michelson interferometer, as depicted in Fig. 3(a) [15]. The phase shift can be calculated from the interference fringe on the CCD camera that is connected to the computer. Figure 3(b) plots the phase shift as a function of voltage for the PSBPLCs sample. As the applied electric field increased, the phase shift increased to an extent that depended on the strength of the external electric field. The continuous lines in Figs. 2(b) and 2(d) represent the theoretical wavelengths shift calculated by the measured reflective index change. The results confirm that the change in the effective refractive index of the PSBPLCs was responsible for the shift in the transmission peaks of the FP filter.
Fig. 3 (a) Experimental setup of Michelson interferometer to measure phase shift of BPLC sample. M: mirror, V: applied voltage, and BS: beam splitter. (b) Measured optical phase difference in BPLC sample against applied electric field.
To verify the polarization-independence of the FP filter, a polarizer was placed in front of the halogen lamp, and transmission spectra were obtained by rotating the polarizer. Figures 4(a) and 4(b) plot the normalized transmittance as a function of incident light polarization at 0V and 60V, and the observed transmitted wavelengths were 572.5nm and 569.3nm, respectively. When linearly polarized light with variable angles of incidence passed through the FP filter, the normalized transmittance was almost constant. Therefore, the experimental results confirm that the PSBPLCs FP filter is indeed polarization-independent under various applied voltages.
Fig. 4 Normalized transmittance as function of angle of polarization incident light for (a) 0V (=572.5nm) and (b) 60V (λ=569.3nm).
PSBP is known to respond rapidly. Figures 5(a) and 5(b) present the measured response times of the PSBP-FP filter under an applied voltage of 60V. The rise time rise is ~660μs and the decay time is ~750μs. Since electrostriction of PSBPLCs is restrained by the polymer network, local reorientation occurs upon application of the electric field, yielding a sub-millisecond response time.
Fig. 5 Response time of FP filter with applied voltages of 0 and 60V. (a) Rise time, (b) decay time.
Since resources in our laboratory were limited, many methods can yet be exploited to improve the performance of the proposed PSBPLC based FP filter. For example, the mirror can be replaced by a highly reflective dielectric mirror to improve the finesse of the filter [16]. The BPLCs material, which requires a high driving voltage, can be replaced by a new type of BPLCs material with a higher Kerr constant. A BPLCs material with a high anisotropic reflective index and a low rotational viscosity will enable the cell gap and response time to be reduced.
4. Conclusion
In summary, this work demonstrated a polarization-independent electrically tunable FP filter that is based on PSBPLCs. The effective refractive index of the PSBPLCs along the optical axis normal to the substrate changes under the influence of a vertical electric field. The wavelength-tuning property of the FP filter is completely independent of the polarization for normally incident light. The tunability and FSR of the PSBPLCs-FP filter are 0.092 nm/ V and 16nm in the visible region, and 0.12nm/ V and 97nm in the NIR region, respectively. The rise time and decay time are ~660μs ~750μs, respectively. The FP filter that is based on PSBPLCs, which has many advantages, such as polarization-independence and a short response time, has great potential for practical application.
Acknowledgments
The authors would like to thank the National Science Council of Taiwan, for financially supporting this research under Contract No. NSC 99-2119-M-110-006-MY3 and NSC 100-2628-E-110 −007 -MY3. Ted Knoy is appreciated for his editorial assistance.
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