A Stressed Liquid Crystal (SLC) is proposed for application as a single panel retardance element in a Fourier transform (FT) spectrometer. Volume alignment in SLCs increase the maximum retardance and subsequent FT resolution by enabling longer path lengths through the liquid crystal material. Here, the relationship between transmission and shear for thick SLC cells is characterized and the spectral resolution using the SLC phase modulators in a single and double pass FT spectrometer system is quantified. For a 100 μm thick SLC, the resolution of a single frequency peak was observed at 60 nm full width half maximum.
©2009 Optical Society of America
Several technologies have been explored as alternatives to a Michelson interferometer as the phase modulating element in a FT spectrometer. These include machining of ultra compact scanning mirror interferometers using electrostatic actuators  and other micro-machined and etched moving mirror arrays . Alternatively, static FT devices have been constructed using static birefringent materials with spatially resolved detectors .
Electro-optic and birefringent properties of liquid crystal (LC) materials allow them to be used as variable retarders. The resolution in this configuration depends on the maximum retar-dance, or explicitly the product of birefringence and LC layer thickness. While conventional nematic LC electrically controlled birefringence (ECB) cells are one possible configuration, the thin layer of LC material in most ECB devices (<20 μm) translates to low maximum re-tardance values. Different LC technologies and optical configurations have been investigated to increase maximum retardance, such as double-pass systems , multiple stacked ECB retarders , and multiple stacked retardance switches using surface stabilized ferroelectric liquid crystals . Although adding multiple LC panels increases device retardance, it adds complexity to the voltage driving schemes to synchronize panel tuning, as well as decreases optical throughput with additional optical interfaces. Similarly, many pass thin cell systems experience loss at each interface, limiting throughput. In this disclosure, the application of a single panel liquid crystal/polymer composite to increase maximum retardance/resolution, is investigated.
Stressed liquid crystal (SLC) polymer devices are a class of polymer alignment controlled liquid crystals first formally reported by West and colleagues  as phase retarders and beam stearing devices and subsequently investigated by another group  as mechanically tunable gratings and lenses. These materials are low polymer concentration (<20%) composites, lower that a polymer dispersed LC (30–50% polymer) but higher than a polymer stabilized LC (<10% polymer), that use mechanical deformations to align LC. The polymer-induced alignment stabilizes LC alignment throughout the layer thickness, and decreases response times. While most applications benefit from the decreased SLC response time, SLC phase retarders in single panel FT spectrometers make use of the volume alignment of the LC. Mechanical polymer alignment of LC’s allow layer thicknesses an order of magnitude greater than surface aligned LC devices. The subsequent increase in resolution from increased retardance may allow the FT-SLC spectrometer to be a suitable configuration for a chip-sized microspectrometer.
2. Fabrication procedure
SLC’s are formed using a mixture of a Norland Optical Adhesive (NOA65), a low concentration monomer, and the commercially available nematic liquid crystal K15 (5CB). These materials are chosen as the refractive index is matched between polymer (NOA65 np = 1.52) and LC ordinary index (K15 n0 = 1.52, Δn = 0.175), minimizing optical scattering effects and maximizing throughput in thick LC layer. The mixture is deposited between two Indium Tin Oxide (ITO) coated glass substrates separated by 100 μm mylar spacers. The mixture is heated past the isotropic phase transition and polymerized using bulk UV exposure for 120 minutes, forming a low-density polymer matrix binding the nematic LC into domains around the polymer fibers. Following polymerization, the LC material is oriented with random director fields and the cells are highly scattering in the unsheared state. This scattering results from both the index mismatch between randomly aligned LC domains and the index mismatch between LC and polymer fibers. In depth studies have been conducted by West and colleagues on LC domain formation during shearing and the subsequent effects on scattering of the system , which is beyond the scope of the paper here.
A mechanical shear force is applied to the sample by fixing one substrate and displacing the other by 120% of the layer thickness, or ≈ 120μm. This is performed using an in-house assembled precision shearing device with a single dimension translation stage. Shearing causes the polymer fibers traversing the sample to stretch and deform along the shear axis, resulting in increased polymer and LC order. As the polymer is dispersed throughout the volume of the SLC mixture, the increased ordering occurs throughout the depth of the sample. The transmission of a 100 μm cell reaches its maximum of approximately 36% with shear displacement > 120 μm, as shown in Fig. 1. When applied as a phase modulating element, the sample is fixed in the final sheared state.
3. Single pass system
As proof of concept, a 632.8 nm Helium Neon laser input source at 5 mW was initially tested using an 85:15 mixture of the K15 and NOA65. Phase modulation is observed using a polarizer and analyzer oriented at 45° on opposing sides of the SLC cell. Retardance is modulated by applying a 10 kHz wave from 0–400 V in 1.5 V steps. Incremental changes in LC alignment from planar to homeotropic vary the effective birefringence and subsequent retardance of the SLC. Transmitted intensity as a function of voltage is shown in Fig. 2(a). The change in SLC retardance begins at ~ 20 V and continues to phase shift in a nonlinear fashion at low voltages (< 200V). Above 200 V, the phase shift becomes roughly linear with applied voltage until the LC is aligned in the homeotropic condition > 400 V. Because of the differing domains of operation, the voltage to phase shift calibration is established using a piecewise spline t between the position of each peak location, which corresponds to 632.8 nm of retardance. A plot of the intensity versus phase shift (interferogram) using the spline t is shown in Fig. 2(b). The fitting has forced the modulation to a linear periodic function which can be Fourier transformed without defects from the phase shift nonlinearity at low voltages. A simulated interferogram is also generated using the experimental parameters of 85% LC , Δn = 0.17 and LC layer thickness = 100 μm, as is shown in Fig. 2(c). This simulated interferogram depicts the case of idealized planar and homeotropic alignment conditions in a scattering free system.
The FT is calculated and power spectra plotted for both the experimental and theoretical in-terferograms as shown in Fig. 3. The FWHM of the laser peak as recorded by the experimental interferogram is 160 nm while the theoretical FWHM is 50 nm. Reduction in maximum phase delay in the experimental versus ideal model is a result of the imperfect alignment conditions, and accounts for the observed resolution difference. For most application purposes, this accuracy will not meet resolution demands. In order to increase the maximum retardance in these samples without increasing layer thickness further, a system of mirrors is implemented to create a double pass system , doubling the effective LC layer while maintaining drive voltages. This configuration is depicted in Fig. 4.
4. Double pass system
For the double pass system, a 100 μm SLC cell is fabricated as described above using an 80:20 mixture of K15 to NOA65. The relative polymer concentration has been increased in order to reduce delamination between the liquid crystal composite and the ITO surfaces of substrates observed in many of the 85:15 K15:NOA65 samples. The SLC cell is sheared by 120 μm and drive voltage applied from 0–500 V at 10 kHz. The input signal is a 20 mW Verdi solid-state laser with λ = 532 nm. The resulting interferogram is shown in Fig. 5(a). The additional increase in effective pathlength results in an increased maximum retardance from 4.5 μm to ≈6.0 μm using the double pass system. Concurrently, the increase in polymer concentration from 15% to 20% coupled with the increase in pathlength generates additional scattering in the system which manifests as both (1) reduced modulation in intensity over each period and (2) an average intensity pattern which varies significantly with applied voltage, explicitly the average intensity increases with increased LC alignment because no = np ≠ ne. This intensity can be corrected by measuring the intensity transmission through the sample without the second polarizer (analyzer), eliminating variation in intensity not attributed to the phase modulation. The voltage versus intensity function can be fit and subtracted from the interferogram to yield phase only intensity modulation, and then the interferogram normalized by the fit to adjust the modulation intensity. Additionally, as in the single pass system, the voltage/phase shift calibration is established using a piecewise spline t between the position of each peak location corresponding to 532 nm of retardance. The resulting intensity corrected interferogram is shown in Fig. 5(b) with the FT of this function shown in Fig. 5(c). The FWHM of the laser peak is 60 nm in the double pass system.
Here, retardance does not increase purely by a factor of two as a result of (1) the increased polymer concentration decreasing LC birefringence and (2) increased scattering limiting the amount of light passing through the sample, and thus the low voltage modulations in intensity cannot be observed (where scattering is at a maximum). The inset of Fig. 5 shows the resulting FT from the non-intensity corrected interferogram to highlight the need to correct the scattering induced intensity variation before processing. Effects of scattering that appear in the interfero-gram manifest as broad underlying background noise in FT spectra. In principal, scattering in an SLC can be minimized in several fashions including reducing polymer concentrations enabled through more highly controlled shearing processes and optimizing monomer/liquid crystal index matching such that the monomer is index matched to the mean of extraordinary/ordinary index and not just to one or the other. Additionally, part of the increase in resolution is due to the slightly higher frequency source used in the double pass system. A higher frequency source was used in the two-pass system because increased Verdi laser intensity overcomes scattering loss from multiple passes through the sample. Implementing the two-pass system in a 100 μm 80:20 K15: NOA65 SLC cell has, however, shown promise as a low-end single panel wavelength dicrimination element.
Constructing a modied ECB cell with low density polymer network stabilizing LC alignment enables the fabrication of high retardance phase modulators, and subsequent FT spectrometers using fewer panels than conventional ECB devices. A single wavelength source was tested through a single pass SLC system resulting in 160 nm resolution. A two-pass system was devised where light passes through the SLC twice, doubling its path length, without the need for a higher voltage input. The outcome is a 37.5 % increase in resolution, showing that a spectrometer using a single SLC panel is capable of 60 nm resolution. Further increases in SLC thickness while optimizing to remove scattering can improve resolution of this single panel device toward that of a low-end grating type device while maintaining the simplicity and advantages of an FT type spectrometer. The authors wish to acknowledge the NASA GSRP Fellowship (NNG05GL57H) and the Rhode Island Science & Technology Advisory Council.
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