Liquid crystal (LC) based polarization modulators will be used for the first time in space instrumentation in the ESA/NASA Solar Orbiter mission. LC birefringence dependence on temperature and wavelength is key for the design of these instruments. A complete characterization of the optical properties of five different LC mixtures (ZLI-3700-000, BL006, MLC-6025-000, MLC-6610, and MLC-6053-000) in a temperature range between 20 and 75 °C and a range of wavelengths between 480 and 650 nm is presented in this paper. In addition, the effects of the variety of harsh environmental conditions during the space mission lifetime on the LC properties is reported. Based on the results shown in this work, the most promising LC material was selected to be on board the Solar Orbiter mission.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Liquid crystal (LC) molecules are used extensively for flat panel display and devices technology (mobile phones, notebooks, computers and televisions), tunable photonics and nonlinear optics. A highlighted area of the science where liquid crystals have a strong impact is polarimetric instruments where Liquid Crystal Variable Retarders (LCVRs) , are used as polarization modulators. These devices use their variable optical retardance to modulate the incoming light polarization and perform polarimetric measurements. The optical retardance of these LCVRs is:
For space applications, LCVRs become a powerful alternative to the traditional rotary polarizing optics since they reduce mass, volume and avoid the utilization of mechanisms . In this sense, they will be used for the first time in a space mission, the ESA/NASA Solar Orbiter mission, as polarization modulators to perform polarimetric measurements in two of the remote sensing instruments of the mission (Polarimetric Helioseismic Imager (PHI)  and Multi Element Telescope for Imaging and Spectroscopy (METIS) ). The dependence of birefringence of the liquid crystal molecules on temperature and wavelength is key for the design of these instruments.
In the Solar Orbiter mission, the spacecraft distance to the Sun will oscillate between 0.28 and 0.95 astronomical units, thus exposing the instruments to very different thermal environments. The expected operating temperatures range in these instruments is between 30 to 80 °C, making mandatory the determination of the refractive indices and birefringence of the liquid crystal molecules in this temperature range. This wide range of temperatures imposed different requirements to the liquid crystal molecules that finally were selected to be used for the Solar Orbiter mission. On one hand, the isotropic point of the LC molecules must be beyond 80 °C as the LCVR cells must work up this temperature. On the other hand, a lower dependence of the optical properties of the LC molecules on temperature is preferred in order to reduce the requirements imposed to the thermal stabilization system of the instrument and achieve a higher accuracy in the polarimetric measurements. Moreover, these instruments impose a retardance range requirement that usually is higher than 360 deg and must be accomplished at all the temperature operating range. A significant decrease of the birefringence on temperature could reduce the retardance range of the LCVRs preventing the performance of the polarimetric measurements. Other key parameter in the design of the polarimetric instruments is the operating wavelength. For instance, for the case of the instrument PHI the operating wavelength will be 617.3 nm that corresponds to one of the spectral lines of Fe I. Other typical spectral lines used in solar spectropolarimetry includes 525.02 nm  and 630.25 nm . Liquid crystal molecules manufacturers usually only provide information at the standard Na wavelength (589.3 nm) and 20 °C. Knowing the dispersion law of the LC molecules refractive indices beforehand can be very helpful in the design of the architecture of the LCVR cells that will be used in future polarimeters. Same case for the dependence of LC properties on temperature.
In addition, for achieving a successful space mission, the optical properties of liquid crystal molecules previously mentioned must be preserved during the mission lifetime. Any element onboard a spacecraft is subjected to a variety of harsh environmental space conditions including ionizing and non-ionizing radiation, vibrational and shock loads and thermal-vacuum. In this way, all the components must be previously tested at simulated space conditions in different campaigns in order to assure functionality of each component during the entire space mission. LCVRs composed of different liquid crystal molecules were analyzed under the environmental conditions expected in Solar Orbiter in the framework of ESA project Validation of LCVRs for the Solar Orbiter Polarisation. This project validated the LCVR technology for space in 2011 increasing the Technology Readiness Level (TRL) to TRL5 “Component Validation in Relevant Environment”. This was reported in . Based on the outcomes of this work, the most promising LCVR design was selected . Later, a qualification test campaign must be performed. In this case, the qualification test campaign is carried out on the same manufacturing batch as those LCVR cells that will be onboard the spacecraft. This assures that the LCVRs on board will show same tolerance to space environment than the LCVRs tested guarantying appropriate performance during the entire mission.
In this paper, a complete characterization of the extraordinary and ordinary refractive indices of different LC mixtures has been performed at a temperature range between 20 and 75 °C and a range of wavelengths between 480 and 650 nm using an Abbe refractometer. On one hand, extended Cauchy equations  have been used to fit the dependence of the LC refractive indices on wavelength. On the other hand, the four parameter model  has been used to fit the experimental data of the refractive indices at different temperatures. The derivatives of the extraordinary refractive index (ne), ordinary refractive index (no) and birefringence (Δn) are also shown in order to compare dependence of the LC mixtures on temperature. The crossover temperature defined as the temperature where dno/dT jumps from negative to positive has been determined for all the LC mixtures studied. From that temperature the dependence of birefringence on temperature increases noticeably, which can be adverse for the performance of the polarimetric instruments. In addition, the environmental tests carried out in order to validate and qualify this technology for space instrumentation are described in this work, showing some of the most significant results not published before.
The outcomes shown in this paper were key to select successfully the most promising liquid crystal mixture to be used in the LCVRs on board the Solar Orbiter mission.
For an anisotropic liquid crystal there are two principal refractive indices, ne and no, for the extraordinary and ordinary rays, respectively. Birefringence is defined as Δn = ne-no. LC refractive indices depends on molecular structure, wavelength and temperature. Several models have been found in the literature, which describe the wavelength and temperature dependence of the liquid crystal molecules. A brief description of the models used to fit the experimental data shown in this paper is presented in the following:
2.1. Temperature dependence
Vuks  made the assumption that the internal field in a crystal is the same in all directions. With this assumption, that was later validated experimentally , he derived the following equation for anisotropic media:9,12], LC average refractive index <n > decreases linearly with increasing temperature as follows: 13], the order parameter is defined as S=(1-T/Tc)β. Therefore, the dependence of the birefringence on temperature is defined by:
2.2. Wavelength dependence
Several models have been developed to describe the wavelength dependent LC refractive indices. The three band model  describes the physical origins of refractive indices of LC compounds and it allows quantitative assessment of each band’s contribution to the overall refractive indices. LC molecules presents three main electronic absorption transitions in the UV region. One σ→σ* transition (defined as the λ0 band, located in the vacuum UV region, λ0→120 nm), and two π→ π* transitions (defined as the λ1 and λ2), with λ2> λ1 in the nearby-ultraviolet region. Nevertheless, commercial LC mixtures consist of several compounds with different molecular structures in order to obtain the desired LC properties. The individual λi are different and would have many unknown parameters to describe properly the refractive indices of a LC mixture. Li and Wu  derived successfully the extended Cauchy equations starting from the Vuks equation in order to have equations that describe the wavelength and temperature refractive indices of liquid crystal compounds and mixtures. According to them, this model is particularly useful for LC mixtures where each LC constituent has a different chemical structure and where the three band model fails. Nevertheless, three band model shows more accuracy in the near-resonance region.
In this paper, extended Cauchy equations have been used in order to fit the experimental data obtained. LC mixture studied are commercial mixtures where the nature of each individual LC molecules is unknown. In addition, the measurements have been taken in the visible range (480-650 nm) beyond the resonant region where the Cauchy model results to be as accurate as the three band model . Based on the extended Cauchy equations, the refractive indices are expressed as:
We have measured the refractive indices of the following LC mixtures from Merck: MLC-6053-000, ZLI-3700-000, MLC-6025-000, MLC-6610, BL006, using a 60/HR Abbe refractometer at λ=480 nm, 525.02 nm, 560 nm, 589.3 nm, 617.3 nm, 632.8 nm and 650 nm. The light wavelength was selected using a Bentham monochromator type DMc150. For 589.3 nm the refractive indices have been measured in the temperatures 20, 30, 40, 50, 60, 70 and 75 °C, and for the rest of the wavelengths at 20, 40 and 60 °C. The temperature of the Abbe refractometer is controlled by a circulating temperature bath. For each refractive index, five measurements have been taken in order to determine the statistics. The measurement uncertainty of the of the refractive indices has been calculated taking into account the monochromator and temperature accuracy, and the statistics obtained from the measured angle in the Abbe refractometer. The accuracy for the refractive indices shown in this paper is 1·10−4. The LC molecules are aligned perpendicular (homeotropic alignment) to the main and secondary prism surfaces of the Abbe refractometer by coating these two surfaces with a solution of 0.294 wt % of surfactant hexadecyltrimethyl-ammonium bromide (HMAB) dissolved in methanol. Both ne and no are obtained through a polarizing eyepiece of the refractometer. All the LC molecules are nematic with positive dielectric anisotropy, except for MLC-6610, which presents negative dielectric anisotropy.
The optical retardance as a function of voltage of the LCVRs were measured at controlled temperature in a null ellipsometer using a HeNe laser and in a Woolam Variable Angle of Spectroscopic ellipsometer. The specific null ellipsometer configuration used is a PSCA (Polarizer-Sample-Compensator-Analyzer) arrangement in transmission.
The LCVRs ionizing radiation tolerance tests were done with γ-radiation in CIEMAT facilities (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas) in Madrid. A 60Co source was used inside a water pool at atmospheric pressure and room temperature. The sample holder was located inside the underwater casing at the adequate distance to fulfill with the specified dose rate (10 krads(Si)/h). The accumulated doses received by the LCVRs were homogeneous better than 90%. The cells were subjected to accumulated doses rates of 30, 60 and 100 krads(Si).
The UV radiation tolerance tests were carried out in INTA facilities (Instituto Nacional de Técnica Aeroespacial) using a 500 Watt Xe lamp (Newport-Oriol model 66921). Light from the lamp is roughly collimated with an adjustable single lens. After the lens, there is an IR liquid filter to prevent overheating of the irradiated sample. The spectrum lamp is calibrated to assure UV irradiation level at the Equivalent Solar Hour (ESH) units required. 1 ESH corresponds to 381.6 kJ/m2 in the UV spectral range from 220 to 400 nm.
The non-operational tests were carried out in INTA facilities using thermal-vacuum chambers. 8 cycles between -45 and 90 °C were carried out at a pressure lower than 10−6 mbar. The temperature rate of change was 2° C/min. The LCVR cells were kept at each temperature during 2 hours.
4. Results and discussion
The measured refractive indices for the LC mixtures studied are shown from Table 1 to Table 5. In some cases, refractive indices could not be measured as not enough accuracy was achieved. The four parameter model has been used to describe the temperature dependence of the LC mixtures using the values of refractive indices measured at 589.3 nm.
4.1 Temperature dependence
The temperature-dependent refractive indices have been obtained by the following two-step fitting process. The first step consists of using Eq. (5) to fit the average refractive index as a function of temperature to obtain parameters A and B. The second step is to use Eq. (6) to fit birefringence with regard to temperature obtaining parameters (Δn)o and β. Then these four parameters are introduced in Eq. (7) to calculate the refractive indices as a function of temperature and compare the calculated results with the measured ones.
This two-steps process is shown in Fig. 1. At left the temperature-dependent average refractive index of the different LC mixture is plotted as a function of temperature. The average refractive index decreases linearly as the temperature increases. At right, the temperature-dependent birefringence is represented versus temperature. In both figures, symbol markers correspond to experimental data measured by Abbe refractometer. Solid lines corresponds to fittings by using Eqs. (5) and (6). The fitting parameters (A, B, (Δn)o and β) for these four materials are listed in Table 6. The clearing point temperature is also included. The experimental data have been measured until 348.15 K that corresponds to the higher limit of temperature of the Abbe refractometer used. For LC mixtures MLC-6053-000 and MLC-6610 the isotropic points (according to LC manufacturer) are closed to this temperature. For this reason, Tc has been also left free during the fitting process for these LC mixtures, as model fitting will be more accurate in this nonlinear area of the curve, close to Tc. A noticeable decrease of the Root Mean Square Error (RMSE) has been found compared to the case of fitting using clearing points specified by LC manufacturer, as shown in Table 6. The rest of LC mixtures show clearing points much higher. Leaving free the five parameters could increase the correlation between fitting parameters.
The dependence of refractive indices on temperature have been calculated using the four parameters (A, B, (Δn)o and β), or five parameters for the case of MLC-6053-000 and MLC-6610 (A, B, (Δn)o, β and Tc) shown in Table 6. The refractive indices calculated are shown in Fig. 2 and Fig. 3. Symbol markers are experimental data measured using the Abbe refractometer and the solid lines the refractive indices calculated from the Eq. (7). In all the cases, very good agreement between experimental and data obtained from fitting has been found.
In polarimeters that use LCVRs, the temperature dependence of the LC birefringence is a key parameter as it is linearly proportional to the LCVR retardance (Eq. (1)). A lower birefringence dependence on temperature helps to reduce the requirement restrictions to the thermal stabilization system of the polarimetric instruments, specifically for the Solar Orbiter mission. Based on equations (7), the temperature derivatives for ne, no and Δn can be calculated and then obtained for the LC mixtures studied. The temperature derivatives for ne, no and Δn are:
Using the parameters contained in Table 6 obtained from the fittings, the derivatives of ne, no and Δn for all the LC mixtures studied, as well as the crossover temperatures, have been calculated and plotted in Fig. 4 and Fig. 5. The values of Tc and To are shown in Table 7.
It can be observed that the dependence on temperature for the birefringence and the refractive indices dramatically increases close to the clearing point. In addition, extraordinary refractive index shows a higher dependence on temperature than ordinary refractive index. This can be deduced from equations (9a) and (9b) because, as said previously, the temperature derivative of refractive ordinary index consists of a negative term (-B) and a positive term. Nevertheless, extraordinary refractive index show two negative terms.
The birefringence derivative of the LC mixtures is highly dependent on how close current temperature is to their clearing point. The LC mixtures studied show very different clearing points temperatures, ranging from 355 to 386 K. In order to compare the birefringence temperature derivative for each LC mixture, values of (dΔn/dT) at different temperatures with respect to each LC mixture clearing point has been calculated and shown in Table 8.
According to these values, BL006 shows the highest temperature dependence, which corresponds to the LC mixture with higher birefringence and MLC-6025-000 shows the lowest dependence on temperature, which corresponds to the LC mixture with lowest birefringence. Same behavior is found for the rest of the LC mixtures except for MLC-6610 that show a higher birefringence derivative than ZLI-3700-000 with a lower birefringence. Nevertheless, difference is low and MLC-6610 corresponds to the unique LC mixture that shows negative dielectric anisotropy. Therefore, as a general rule and for the LC mixtures studied, we find that LC mixtures with higher birefringence show higher dependence on temperature. According to equation (9c), birefringence dependence on temperature is related to the birefringence of the LCs in the crystalline state or at T = 0 K, i.e., (Δn)o, and β term. Nevertheless, β term is more similar for all the LC mixtures studied, around (0.20-0.25) and values of (Δn)o are found in the range between 0.12 and 0.39. Therefore, (Δn)o has a stronger contribution in dΔn/dT than β term, which explains the higher dependence on temperature found for LC mixtures with higher birefringence.
For polarimetric instruments where a minimization of the birefringence dependence on temperature of the LC material used is required, different strategies can be carried out. On one hand, the first strategy could be focused on selecting LC mixtures with low dependence on temperature. Nevertheless, according to the results found, this could mean having LC mixtures with lower birefringence, which could has adverse effects. A lower birefringence involves using a higher thickness for reaching the same retardance range in the LCVRs, which increases the response times of these devices. Additionally, it is very recommendable to select LC mixtures with high isotropic points beyond and far away the working temperature ranges of the instrument. In this way, the large dependence on temperature in the proximity of the clearing point is avoided.
Based on the results previously mentioned, the best candidates for a polarimeter are the LC mixtures MLC-6025-000 and ZLI-3700-000, as both show low dependence on temperature and high isotropic points. Nevertheless, as previously said, other parameters have to be taken into account. The liquid crystal mixture MLC-6025-000 has a low birefringence, which involves the use of a higher thickness and therefore slower response times. Therefore, the LC mixture ZLI-3700-000 was selected. This liquid crystal mixture shows a low dependence on temperature, high isotropic point and a medium birefringence, which accomplishes a compromise between having a lower dependence on temperature and faster response times. Moreover, apart from the temperature dependence properties, other features, such as robustness and good behavior in simulated space conditions were taken into account, which will be detailed in section 4.3.
4.2 Wavelength dependence
The refractive indices of all the LC mixtures measured in the wavelength range between [480-650 nm] have been fitted to the extended Cauchy model showing very good agreement between experimental and theory. Fittings for the LC mixture ZLI-3700-000 are shown in Fig. 6. The Cauchy parameters obtained for the measurements at 20, 40 and 60 °C are shown in Table 9, Table 10 and Table 11, respectively. In addition, fitting with three (A, B and C) and two (A and B) Cauchy parameters have been compared. For some cases of the ordinary refractive index (highlighted in bold in the tables) in the LC mixtures MLC-6025-000, MLC-6610 and ZLI-3700-000, the term Co shows negative values, meaning that they do not have physical meaning. In these cases, when this term is removed from the fitting, RMSE values are similar or even reduced, indicating that this term is very close to zero and Cauchy fittings can be reduced to two terms, Ao and Bo.
4.3 Liquid crystals at environmental space conditions
The space environmental conditions can produce changes and degradation of the optical properties of the liquid crystals reported in the previous sections. In order to implement the liquid crystal technology for space instrumentation, the optical properties of liquid crystal molecules must be studied and analyzed under this environment. In this sense, LCVRs composed of different liquid crystal molecules were analyzed under the environmental conditions expected in Solar Orbiter in the framework of ESA project Validation of LCVRs for the Solar Orbiter Polarization. Eight different types of liquid crystal cells were characterized and their performances were verified under simulated environmental space conditions . This project validated the LCVR technology for space in 2011, increasing its Technology Readiness Level (TRL) to TRL5. The main studied parameters of the liquid crystal cells included different LC mixtures, different glass substrates, different alignment layers and different configurations. They were subjected to ionizing and non-ionizing radiation, vibrational and shock loads, which are produced during the satellite launch, and thermal-vacuum conditions. To determine the effects of these conditions on the LCVRs, a variety of optical indicators were measured before and after space environmental tests. These optical indicators include response times, optical transmission, transmission wavefront error and optical retardance as a function of voltage, temperature, wavelength and angle of incidence.
From the results of these environmental tests, the best LCVR candidates were selected, which included the LC mixture ZLI-3700-000 and BL006 as the most promising LC materials. The measurements of the LC mixtures birefringence and its dependence on temperature shown in Section 4.2 of this paper selected the liquid crystal ZLI-3700-000 as more suitable for the Solar Orbiter polarization modulators due to its lower temperature-dependent behavior.
The LCVR design was refined taking into account lessons learnt during these tests. Subsequently, the definitive LCVR design , including definitive materials and architecture must be qualified for space through a qualification test campaign, which increases the LCVRs TRL to TRL8 entitling this technology to be ready for launch. A qualification test campaign is performed on same manufacturing batch than the flight LCVR cells, which are those that will be onboard the spacecraft. In this way, we assure that flight LCVRs will show same tolerance to space environment than the LCVRs tested.
A brief summary of the validation campaign reported in  and the most significant qualification test campaign results, not published before, are shown in the following:
Ionizing and non-ionizing radiation:
During validation tests campaign, the LCVRs ionizing radiation tolerance tests were done using γ-radiation at the irradiation doses of 8 krad(Si), 25 krad(Si), 50 krad(Si), 75 krad(Si) and 100 krad(Si). The LCVRs non-ionizing radiation was carried out using 60 MeV protons with the following fluences: 0.232·1011 p+/cm2, 0.928·1011 p+/cm2, 1.39·1011 p+/cm2 and 2.78·1011 p+/cm2. No significant changes were found in the LCVRs optical indicators for these tests , concluding that liquid crystal molecules and therefore their optical properties are not damaged because of the ionizing and non-ionizing radiation at the irradiation values specified. During qualification test campaign, the LCVRs ionizing radiation tolerance tests were repeated to assure tolerance of the LCVR final design. Four LCVR cells were subjected to accumulated doses of 30, 60 and 100 krads(Si). Measurements of the optical retardance as a function of voltage for one of the LCVR studied at these intermediate doses are shown in Fig. 7. The optical retardance vs. voltage measured after the different accumulated doses is shown in Fig. 7(a). The optical retardance variation with regard to initial measurement (before irradiation) is shown in Fig. 7(b). Small differences in retardance were found, which can be attributed to slight differences in the measurements conditions, such as temperature. It is concluded that liquid crystal molecules and therefore their optical properties are not damaged up to 100 krad of ionizing radiation.
UV radiation needs special consideration due to the organic nature of the liquid crystal molecules. UV radiation in space is severely more harmful due to the lack of Earth atmosphere. UV shields will be used in order to avoid that UV radiation reach the components in the instruments, including the LCVRs. Therefore, no significant UV radiation is expected to damage the liquid crystal during the mission. Nevertheless, some residual radiation could reach the components. This residual UV radiation during the entire mission was calculated to be 0.33 ESH (Equivalent Solar Hour) as the worst case. Using appropriate safety factors this value was increased up to 1.0 ESH.
During the validation test campaign, UV radiation tests were performed with a wide margin up to 1.5 ESH. Damage of the LCVRs were found to different extents depending on LC mixture [7,16]. This damage included changes in the optical retardance curve and response times. Nevertheless, in all the cases, the degradation effects observed did not involve the destruction of the liquid crystal cells and most of them kept fulfilling the requirements for the Solar Orbiter mission. Anyway, a flight calibration was recommended to implement during the Solar Orbiter mission to take into account the possible changes in the optical retardance curve
The definitive LCVR design was tested during the qualification test campaign at the same UV radiation level, finding similar results although less damage than expected if we compare to the validation test campaign. The damage found was a reduction in the optical retardance range (Fig. 8) and an increase of the LCVR response times. The optical retardance vs. voltage measured after the different accumulated UV doses is shown in Fig. 8(a). The optical retardance variation with regard to initial measurement (before irradiation) is shown in Fig. 8(b).
The satellite in space will be subjected to vacuum conditions and very different thermal environments. The LCVRs expected operating range is between 30 and 80 °C, which means that LCVRs must comply with their functionality at this whole range of temperature . Nevertheless, during the mission phases, more drastic temperatures are reached. LCVRs must survive these temperatures, which is known as the non-operational temperature range. The environmental tests are performed in this temperature range adding a margin of 10 °C up and down to assure reliability. This is known as the non-operational qualification temperature range.
During validation test campaign, this temperature range was defined between -40 to 70 °C testing the LCVR cells at these temperatures . Nevertheless, as the spacecraft design is more advanced, the updating of the instrument thermal model can redefine the temperature ranges. The definitive non-operational qualification temperature ranges were extended to a range from -45 to + 90 °C, because the expected non-operational temperature range is between -35 to + 80 °C. Therefore, during qualification test campaign four LCVR cells were subjected to eight cycles in the range between -45 to + 90 °C at vacuum conditions.
Measurements of the optical retardance as a function of voltage of one of the LCVR studied before and after the test are shown in Fig. 9. No noticeable changes were observed in the LCVR optical response for three of the four LCVR cells. Nevertheless, one of the LCVR cell suffered from a decrease of 2% of optical retardance and an increase in the wavefront error measurement. This is attributed to a plastic thermal-mechanical deformation of the cell substrates, which changed the thickness of the cavity along the clear aperture and therefore the optical retardance exhibited by the cell. The increase observed in the wavefront error was a consequence of an increase of the power that for the case of PHI instrument can be corrected by its refocusing mechanism. Nevertheless, this environmental test was repeated in the flight polarization modulators, which include the flight LCVR cells, and in a more reduced temperature range at complete instrument level. In both cases, no changes in their properties were observed. Then, this was considered a punctual failure and the technology qualified for the non-operational thermo-vacuum conditions of Solar Orbiter.
In a space mission, the worst unexpected scenario could occur. For this reason, additionally we subjected one of the LCVRs cells with liquid crystal mixture ZLI-3700-000 at higher temperatures than its isotropic point (105 °C) to verify if the LCVR functional performance is recovered. Beyond this critical point, liquid crystal molecules became an isotropic liquid inside the LCVR cell. The most catastrophic effect would be a failure in the adhesion points of the glass substrates that would produce leakage of the liquid crystal mixture outside the cell.
The retardance vs. voltage at different temperatures for LCVR is shown in Fig. 10. As temperature is increased, the optical retardance is reduced as shown in Fig. 10(a) and as expected due to the reduction of the LC ZLI-3700-000 birefringence. Exceeding the isotropic temperature, at 115 °C, no optical retardance is exhibited by the LCVR cell as the liquid crystal became in an isotropic liquid material. When temperature is reduced up to 30 °C, the retardance vs. voltage curve is recovered with some changes as observed in Fig. 10(b). These changes could be attributed to some realignment of the molecules after reaching this critical temperature. Nevertheless, the LCVR comply with the functional requirements after this test, which assure the performance of the LCVR cells beyond its isotropic point (105 °C).
In this paper, a complete characterization of the extraordinary and ordinary refractive indices of the nematic LC mixtures ZLI-3700-000, BL006, MLC-6025-000, MLC-6610 and MLC-6053-000 has been performed at a temperature range between 20 and 75 °C and a range of wavelengths between 480 and 650 nm. The four parameter model has been used to fit the experimental data of the refractive indices at different temperatures. Parameters A, B, (Δn)o and β have been obtained for all the LC mixtures studied. In addition, derivatives of Δn, ne and no vs. temperature have been calculated and analyzed. On the other hand, extended Cauchy equations have been used in order to fit the dispersion law of the extraordinary and ordinary refractive indices. In both cases, excellent agreements between theory and experiment have been found. Based on these optical parameters, the LC mixture ZLI-3700-000 is the most promising material for the LC polarization modulators for the Solar Orbiter mission. This nematic LC mixture shows a low dependence on temperature, a high isotropic point and a medium birefringence, which accomplishes a compromise to comply with the requirements of a polarimeter that will be aboard of a space mission.
In addition, a description of the environmental tests campaigns at simulated space conditions is shown. Based on these results, the LCVR technology has been validated and qualified for space, which includes the LC mixture ZLI-3700-000 as the selected LC material for the Solar Orbiter mission.
Ministerio de Economía y Competitividad (MINECO) (ESP2016-77548-C5-4-R).
The authors would like to express their gratitude to Manuel Silva-López, Leire Ayuso Angulo and the rest of INTA team for their technical support.
The authors declare that there are no conflicts of interest related to this article.
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