Polarization-independent phase modulators enabled by two-photon polymerization

A double-layer polarization-independent liquid crystal (LC) phase modulator is demonstrated and characterized. Using two-photon polymerization process, an ultra-thin partition polymeric film to separate the LC cell into two layers with orthogonal alignment directions is fabricated. Such a thin partition film also helps to reduce the voltage shielding effect. Using a high birefringence and low viscosity LC mixture, 2π phase change at merely 10 V for a He-Ne laser beam is achieved, while keeping a fast response time. Simulation result agrees reasonably well with experimental data. © 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.3710) Liquid crystals; (260.5430) Polarization; (060.5060) Phase modulation; (220.4241) Nanostructure fabrication; (050.6875) Three-dimensional fabrication. References and links 1. H. Sasaki, K. Yamamoto, Y. Ichihashi, and T. 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Introduction
Phase-only spatial light modulators (SLMs) are widely used in holograms [1], laser pulse shaping [2], optical communications [3,4], laser beam steering [5][6][7], tunable-focus lens [8], and optical angular momentum control [9].Typically, to realize phase-only modulation, a linearly polarized light is required.For the above-mentioned applications, a polarizationindependent SLM is highly desirable.Currently, three types of liquid-crystal (LC) polarization-independent phase modulators have been developed.The first is based on optically isotropic media, such as polymer-stabilized blue phase LC (BPLC) [10][11][12] and nanoscale polymer-dispersed LC (nano-PDLC) [13][14][15].In the voltage-off state, the medium is optically isotropic, i.e. its refractive index is equal in all directions (n = ni).In presence of longitudinal electric field (E) (say in the z-axis), the effective refractive index sphere stretches along that direction, forming an optical axis.As the voltage changes, light traversing through the optical axis will experience a different effective refractive index (n o (E)).The advantages of such an isotropic-to-anisotropic transition are twofold: it is polarization independent, and its response time is fast (<1 ms).However, two challenges remain to be overcome before these Kerr media can find practical applications: 1) the required operation voltage is relatively high (>50 V; depending on the dielectric anisotropy of the employed LC), and 2) the effective refractive index change (n i -n o ) is only one-third of the LC birefringence (Δn = n e -n o ).
The second approach is called residual phase change [16][17][18].For instance, the phase of a 90° twisted nematic (TN) LC cell is polarization insensitive when the applied voltage is 3x greater than the threshold voltage [19].While the bulk of the LC is aligned vertically along the applied field, the planar tilt of the LC near the anchoring surface imparts a phase change.Fast response time can be achieved in these devices, but the major tradeoff is its relatively small remaining phase change (<1π).
The third approach is to use multiple birefringent layers which can compensate for each other's phase [20][21][22].A simple example is to stack two homogeneous LC layers with orthogonal alignment directions.In such a device, an effective refractive index ellipsoid is formed with the optical axis along stacking direction.This way, a phase change can be obtained by varying an electric field along the optical axis [21].The merit of this approach is that an adequate phase change can be readily obtained.However, the major shortcoming is that the partition layer may shield the applied voltage significantly, depending on its thickness.From practical application viewpoint, a polarization-independent phase modulator should exhibit a minimum 2π phase change, fast response time, and low operation voltage (<15 V).The maximum phase change (δ) of a transmissive polarization-independent mode is determined by the effective birefringence (Δneff), cell gap (d) and wavelength (λ) as: Meanwhile, the response time is governed by the rotational viscosity (γ 1 ) and splay elastic constant (K 11 ) of the employed LC as [23]: From Eqs. ( 1) and ( 2), a larger cell gap produces more phase change, but the tradeoff is slower response time.A polarization-independent, fast response and low operating voltage (< 15 V) phase modulator with 2π phase change will add significant technological value.
In this paper, a double-layer polarization-independent phase modulator enabled by twophoton polymerization (TPP) [24,25] is demonstrated.In recent years, TPP has been widely applied to generate three-dimensional structure with sub-micron resolution.Here, by utilizing this technique, a suspended ultra-thin polymer film is fabricated to separate the two LC layers between glass substrates, while generating orthogonal alignment directions for the top and bottom layers.Polymeric pillars are included to precisely control the cell gap and ensure both LC layers have the same thickness.In this double-layer structure, response time is not determined by the total cell gap, but by the individual layer thickness.Ideally, by introducing this partition layer the response time should be 4x faster than that of conventional LC cell [26].Simulation is performed to predict the device voltage-phase response while taking into consideration the voltage-shielding effect of the partition film.The simulation is verified through experiment, where a 3.3 by 3.3 mm 2 sample is fabricated and characterized.With the recently developed TPP process, the polarization-independent device exhibits a 2π phase change (λ = 633 nm) at 10 V with 8.85-ms relaxation time.This approach successfully eliminates the shortfalls of the aforementioned approaches and enables polarizationindependent phase modulators for practical applications.

Results and discussion
Figure 1(a) schematically shows the device architecture.To fabricate the proposed doublelayer structure, first the nano-groove alignment layer on the bottom glass substrate is patterned [27].Next, arrays of pillars to serve as supports for the partition layer are formed which is followed by polymerization of the partition layer written with orthogonal directions on either side.At the end of the TPP process, four quadrants of pillar arrays slightly outside of the functional area are added to control the total cell gap to approximately 5.75 µm (details in the Methods section).
Figures 1(b)-1(d) show the scanning electron microscopy (SEM) images of a sample with dimension of 360 by 360 µm 2 before filling the LC.The distance between pillars within the functional area is 13 µm and the size of each pillar is 1.8 by 1.8 µm 2 .The surrounding pillars are elongated to withstand the relaxation of the polymer film during TPP process [28].Individual writing sections are 90 by 90 µm 2 .Connecting bars are made to provide structural stability over the large-area pattern and periodic holes are designed to help with development and evaporation of solvents in order to prevent structural collapse.The size of surrounding pillars and connecting bars is set to be 3 by 9 µm 2 .Figure 1(d) demonstrates the function of the partition layer and shows the orthogonal alignment directions on the top and bottom sides of the partition, as indicated by the direction of the grooves.From SEM images the thickness of the bottom alignment layer and the partition layer are measured to be 0.1 μm and 1.4 μm, respectively.Subsequently, the thickness of each LC layer is estimated to be 2.12 µm from the total measured cell gap of 5.75 µm.It is noteworthy that, even though the partition layer is so thin, the structure is thermally stable as no noticeable changes are observed after heating the sample at 100 °C for two hours.To investigate the scale-up capability, a 3.3 by 3.3 mm 2 sample is fabricated.Figure 2 shows the polarized optical microscope (POM) images of the sample where the polarizer and analyzer are crossed.In Figs.2(a)-2(c), the bottom layer alignment of the sample is parallel, 45°, and perpendicular to the polarizer, respectively.The pillar arrays which control the total cell gap of the working area are shown in Fig. 2(d).It is worth noting that the bright spots in Figs.2(a)-2(c) indicate the location of pillars and that they offer orthogonal alignment to that of the top layer, as shown in Fig. 1(d).The LC molecules between the pillar and the rubbed superstrate will rotate 90° along the stack direction, forming a twisted-nematic (TN) region.In the voltage-off state, a TN cell transmits light and appears white between crossed polarizers [29].Also in Figs.2(a) and 2(c), some dimmed but not dark spots are observed.These spots correspond to periodic holes which are designed to help with structure development, as mentioned before.The LC molecules above the holes have no preferential orientation, leading to light scattering in some extent.However, the light scattering introduced by the holes is negligible as the size of the holes is small.Therefore, the actual working area is the dark area where nearly no light passing through the crossed polarizers.The aperture ratio of the fabricated structure is about 91%.By reducing the size of each pillar or further improving the stabilization structure, a larger working area is achievable.From Fig. 2(b), a small light leakage in some sites (dark blue color instead of black) is observed, indicating that the thickness of the two LC layers is not perfectly uniform across the entire working area.This non-uniformity could originate from the relaxation and lateral shrinkage of partition layer during polymerization process, therefore distorting the LC director.To investigate the electro-optic response, the device is characterized with a Mach-Zehnder interferometer.The sample with 3.3-by-3.3mm 2 working area is positioned at the center of one arm and a pinhole in front of the photodetector is used to select the region of the sample.By placing a linear polarizer after a unpolarized He-Ne laser (λ = 632.8nm), input polarization is well controlled.Figure 3(a) shows the measured and simulated voltagedependent phase change at different polarization states.The voltage-phase curves exhibit a small discrepancy for different polarizations, due to the slightly unequal thicknesses of the two LC layers.However, the overall response is polarization insensitive.Simulation result obtained by DIMOS software is shown by the red dashed curve which takes into account the voltage-shielding effect of polymer films.It agrees reasonably well with the experimental data.At 15 V, a phase change of 2.3 π is achieved at λ = 632.8nm.The relaxation time (100% to 10%) of the phase modulator is measured using an unpolarized He-Ne laser beam to be 8.85 ms at 23°C as can be seen in Fig. 3(b).
The present cell in the transmissive mode provides a phase change of 1.2π at λ = 1.06 µm which can be doubled to 2.4π in reflection mode, while maintaining a fast 8.85 ms response time.In order to achieve the full 2π phase change, the thickness of each layer needs to be 3.6 μm and the response time increases by 2.88 times based on Eq. ( 2).The concept can be extended to longer wavelength range by either slightly increasing the cell gap or operating in reflective mode.Another solution of working at longer wavelength is to build more layers.The advantages include more total phase change and keeping fast response time, but the tradeoffs are linearly increased operation voltage and potential structure stability issue.In experiment, it is noticed that larger pillars are preferred to maintain the stability of the structure during the TPP process when more layers are required.This will also result in smaller working area as the pillars are enlarged.

TPP processing
Polymer films and pillars were fabricated using a commercially available femtosecond laser lithography system (NanoScribe GmbH) [30].Dip-in mode was used with a x63, 1.4 numerical aperture (NA) oil immersion objective (Zeiss) and IP-Dip (NanoScribe GmbH) photoresist.After dropping IP-Dip on a one-side ITO-coated glass substrate, the objective was directly immersed into the photoresist.A 2D galvanometer scanner was utilized to steer the focal point of a 780-nm pulsed laser to expose the photoresist line-by-line.Grooves with 300 nm periodicity were established, providing strong anchoring force to LC molecules.After the writing, the sample was gently immersed into 1,2-Propanediol monomethyl ether acetate (PGMEA) solution for 20 min to remove the unexposed photoresist, and then it was placed in isopropyl alcohol (IPA) for 5 min to remove PGMEA.Last, to evaporate the IPA, the sample was held at 20 cm above a 200 °C hot plate until all the droplets were completely evaporated.

LC cell finishing
A single-side ITO-coated glass superstrate with alignment perpendicular to that of the substrate was adhered to the structured substrate using NOA 81, together with some 4.5 µm silica spacers to share some pressure on polymerized pillars which maintain the total cell gap in the functional area.Once UV cured, the test cell was filled with a LC mixture LCM-1660 (LC Matter, USA) under 10 Torr environments.This LC material has a relatively high birefringence (Δn = 0.38 at λ = 632.8nm, Δɛ = 16.01 at 1 kHz driving frequency, and γ1/K11 = 16.59 ms/μm 2 at room temperature) so that a large phase change could be obtained.

Fig. 1 .
Fig. 1.(a) Schematic of the proposed structure, and (b-d) SEM images of a 360 by 360 µm 2 sample.The series illustrate more details of the structure.The red arrows in (d) denote that the alignment on the top of the pillars and under the partition layer is perpendicular to the alignment on the top of the partition layer.Scale bars: 100 µm (b), 10 µm (c), and 1 µm (d).

Fig. 3 .
Fig. 3. (a) Simulated and measured voltage-phase responses of the two-layer LC cell under different input polarizations.Black dashed line denotes 2π phase change.(b) Measured