High-performance dichroic dye-doped flexible cholesteric polymer film optical filter for laser protection application

High-performance optical filters are quite useful in laser protection application that is critical in military and civilian fields for protection of human eyes and photonic devices. However, most existing technologies are often angle-dependent and complicated, which hinder the useful range of real-time application. Here, we report the development of a dichroic dye-doped flexible cholesteric polymer film optical filter with double-layer structure consisted of two films with opposite handedness for laser protection application. The fabricated films exhibit high and stable optical density (>4.6) in large angle range from 0 to 70°, indicating a good angle-independence. The proposed films show excellent stability in a broad temperature range from 100 °C to 100 °C, and high mechanical stress up to 9.8 × 105 Pa. As a laser protection, the optical filter can resist pulse pump energy up to 110 mJ/pulse, showing a high damage threshold. The flexible and free-standing film optical filters as laser protection device are integrated by simple fabrication, excellent stability to temperature and mechanical stress, angle-independence, high optical density, and good visibility simultaneously, showing dramatically application potential in laser protection and other flexible photonic devices. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.3710) Liquid crystals; (160.5470) Polymers; (310.6845) Thin film devices and applications. References and links 1. J. Kruger, M. Lenzner, S. Martin, M. Lenner, C. Spielmann, A. Fiedler, and W. Kautek, “Singleand multi-pulse femtosecond laser ablation of optical filter materials,” Appl. Surf. Sci. 208(1), 233–237 (2003). 2. M. Lenner and C. Spielmann, “Reliability of polycarbonate filters in the femtosecond regime,” Appl. Phys. B 78(6), 689–692 (2004). 3. E. Taracheva, S. Yulin, T. Feigl, and N. 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Introduction
Optical filters (OFs) have attracted a lot of interests for various applications such as laser protection and other photonic devices [1][2][3][4][5][6].For the laser protection application, earlier efforts on achieving high-performance optical filters with good visibility and high optical density include doping absorption materials [1][2][3][4] and deposition of reflection layers [5].However, two major disadvantages of current laser protection materials, angle-dependent protective behaviors and complex process of multilayer deposition under high temperature and vacuum [6], severely hinder its practical applications.Therefore, laser protection materials with angle-independent, simple fabrication procedure, good visibility, and high optical density are highly desirable [7].
Liquid crystals (LCs) have been explored as laser protection materials based on its extraordinary large optical nonlinearities, broadband birefringence, self-assembly ability and good tunability [8][9][10].Particularly, cholesteric liquid crystals (CLCs), which are usually formed by doping chiral dopants into nematic liquid crystals, possess self-assembly onedimensional (1D) chiral photonic crystal structure and can reflect the incident circularly polarized light with the same handedness due to Bragg reflection [11], indicating an excellent reflection material for laser protection application.On the other hand, dichroic dye, which are easily aligned by liquid crystal (LC) molecules, exhibit strongly absorption ability when the polarization direction of the incident light is parallel with the dichroic dye molecular long axis [12][13][14].Therefore, utilizing dichroic dyes as a functional component to be doped in CLCs will provide absorption material as well as reflection layer and is advantageous for absorbing light of different polarization directions.Based on this idea, W. Zhang et al reported a helical polymer structure using a multi-step "wash-out/refill" procedure, which was achieved by refilling a dye-doped in CLC with right-handedness helical structure into the polymer network with left-handedness helical structure.As a result, a film as laser protection device with angle-independence, good processibility, wavelength tunability, high optical density and good visibility has been fabricated [15].However, there are several drawbacks of aforementioned dichroic dye doped CLC film as laser protection application including quite complicated and time-consuming fabrication process of "wash-out/refill" step and low stability to environment temperature and mechanical stress due to existence of LC materials [16][17][18][19][20][21].
Herein, we demonstrate a dichroic dye-doped flexible cholesteric polymer film optical filter with double-layer structure consisted of two films with opposite handedness for laser protection application.One-step polymerization fabrication of cholesteric polymer film based on liquid reactive mesogens (LRMs) is used to replace the complicated "wash-out/refill" process.The proposed cholesteric polymer film optical filter demonstrates characteristics of simple fabrication, excellent stability to temperature and mechanical stress, angleindependence, high optical density, and good visibility simultaneously.In addition, the film is flexible and free-standing, which shows great application potentials in other flexible photonic devices.

Experiment
In our experiment, the pre-polymer mixture consisted of liquid reactive mesogens (LRMs, HRM1001-002, ne = 1.6600 and no = 1.5198, from HCCH), chiral dopant (S5011 and R5011, from HCCH), photo-initiator Darocur1173 (from Sigma-Aldrich), and dichroic dye (from Dalian University of Technology).respectively.The measured pitch (p) is 340 nm and 343 nm for RH and LH LCP film, respectively.The two films possess similar pitch thus similar photonic band structure that will lead to good overlap of photonic band gap when they are assembled together.For chiral liquid crystal polymer film, the central wavelength Ȝ of photonic band can be calculated according to Ȝ Q × p , where n is the average refractive index, and p is the pitch length.In our experiment, the average refractive index of chiral liquid crystal polymer film was 1.59 at 540 nm, which was measured by ellipsometer (iHR320, HORIBA).Therefore, the calculated central wavelength is 540.6 nm and 545.4 nm for RH and LH film, respectively, which is highly consistent with the measured results of 541 nm and 545 nm.The cross-sectional image of fabricated double-layer film is shown in Fig. 2(c), where the measured thickness of RH /&3 DQG /+ /&3 LV ȝP DQG ȝP UHVSHFWLYHO\ $V WKH 5+ DQG /+ ILOPV DUH fabricated by the same procedure, and the thickness difference is due to the tiny difference on the actual thickness of LC cells which is inevitable in experiment.The total thickness of double-OD\HU ILOP LV DURXQG ȝP Figure 2(d) shows the photograph of the double-layer film on glass substrate and flexible PET substrate, respectively.The film can be transparent and flexible, indicating a highly potential in applications of flexible optical filters and goggles.

Results and discussion
The optical properties of double-layer film fabricated in our experiment as an optical filter was measured and analyzed, including reflectance, absorptance, transmittance and optical density.The measurement was carried by an UV and visible spectrometer (UV-5200PC, from Shanghai Yuanxi). Figure 3(a) depicts the reflectance of RH (green curve) and LH (magenta curve) LCP film without dichroic dye and absorptance of pure dichroic dye (red curve) doped in ultra violet (UV) curable adhesive, NOA 81 (from Norland).In order to obtain filter worked at 532 nm, the concentration of the chiral dopant in LCP films with R-handed and Lhanded were carefully adjusted to ensure that the central wavelength of reflection spectra of two films with opposite handedness were completely superposed.For the RH and LH LCP film, the full width at half maximum (FWHM) of photonic band-gap observed in reflection spectra is in range from 521 nm to 561 nm, and from 525 nm to 565 nm, respectively.The corresponding central wavelength of photonic bandgap is 541 nm and 545 nm, with maximal reflectance of 46% and 48%, respectively.The absorption range of the dichroic dye covers from 400 nm to 580 nm, resulting in a reflective color of red.The double-layer film proposed in our experiment combines the dichroic dye with the liquid crystal polymer film with chiral structure, which leads to a completely optical block effect in range around 532 nm and becomes a good choice as an optical filter for green color.The optical setup for angle dependence and optical density measurement is demonstrated in Fig. 3(b).A semiconductor laser (MSL-FN-532-S, from Changchun New Industries Optoelectronics Technology), with wavelength of 532 nm and output power of 290 mW/cm 2 , was used as a light source.A neutral density filter was used as an attenuator to adjust the laser power.The fabricated double-layer film was used an optical filter sample.A power meter with resolution of 1 nW (S120VC, from Thorlabs) was used as a detector to detect the light power passing through the sample.As shown in the bottom of Fig. 3(b), the incident angle ș is the angle between the propagation direction of incident laser beam and normal line of OF sample surface.The incident angle is controlled by a rotator.
Figure 3(c) depicted transmission spectra of the OF, which is the double-layer LCP film doped with dichroic dye, for different incident angle.It can be seen that for the OF region with wavelength from 514 nm to 540 nm (blue color), the transmittance is kept almost zero while the incident angle ș varies from 0° to 70°.It is indicated that, the incident light falling in the OF region will be completely blocked or filtered by our OF based on double-layer LCP film doped with dichroic dye.The thickness of double-OD\HU ILOP 2) LV DURXQG ȝP ,W FDn be seen that, because the effective thickness of sample increases with the incident angle ș, leading to increase of scattering and thus reduced transmittance [22].For comparison purpose, the transmittance of pure dichroic dye doped NOA 81 with same thicNQHVV RI ȝP is depicted by the dash black curve.Without chiral structure of double-layer LCP film, the pure dichroic dye sample cannot completely block the light around 532 nm.The lowest transmittance is higher than 18%, indicating a poor optical filter worked at 532 nm.Therefore, the existence of double-layer LCP film with chiral structure is quite critical to proposed optical filter, which guarantee the transmittance in range from 514 nm to 540 nm is near to zero.It is noticed that, the laser protection material should have transmittance which is related to the input optical intensity [23].In our experiment, no decrease in transmittance was observed with the increase of incident light intensity.Therefore, the CLC film used in our experiments was actually a neutral-density filter which can be used in laser protection application.Figure 3(d) plots the optical density (OD), which is defined as the amount of light that attenuates through an object, of double-layer LCP film doped with dichroic dye as optical filter at different incident angle from 0° to 70°.The value of OD can be calculated according to Beer-Lambert law [24]: lg(1 / ) OD Trans , where the Trans represents the transmittance of our OF sample.The measurement is carried at wavelength of 532 nm, and the measured OD is in range from 4.6 to 5, which is quite high and applicable for optical filters or goggles application with typical OD from 3 to 5.1 [5,6,15].As the device rotates, the effective thickness (d = d0/FRVș, where d0 is thickness of film and ș is angle between the device normal and direction of the incident light) of the optical filter increases as the angle ș increases.For this reason, the scattering and absorption of the optical filter increases for incident light [15], ultimately resulting in a low transmittance of the device.Therefore, even the rotation of the device causes the move of reflection band [25], the device still possesses a good interception effect on the laser at wavelength of 532 nm in our experiment.The stabilities of proposed double-layer LRM film under external stimuli such as temperature, stress and strain are critical to its application as optical filter and other photonic devices.Figure 5

Fig. 1 .
Fig. 1.Schematic of fabrication process of dichroic dye-doped liquid crystal polymer film with double-layer structure.The left-handed (LH) and right-handed (RH) liquid reactive mesogens formed helix structures separately in a LC cell with anti-parallel rubbing alignment.Step i: the LH and RH LRM film cell is exposed under UV light and then stripped a substrate.Step ii: the LH and RH LCP films were assembled together to form a double-layer film.

Figure 1
Figure 1 depicts the fabrication process of dichroic dye-doped liquid crystal polymer film with double-layer structure.Firstly, left-handed (LH) and right-handed (RH) liquid reactive

Fig. 2 .
Fig. 2. (a) Cross sectional SEM image of right-handedness LCP film.(b) Cross sectional SEM image of left-handedness LCP film.(c) Cross sectional SEM image of double-layer film.(d) Double-layer film on glass and flexible PET substrate, respectively.The film can be transparent and flexible.

Figure 2 (
Figure 2(a)-2(b) show the cross-sectional image of RH (sampleA) and LH (sample B) liquid crystal polymer film observed under scanning electron microscopy (SEM), respectively.The measured pitch (p) is 340 nm and 343 nm for RH and LH LCP film, respectively.The two films possess similar pitch thus similar photonic band structure that will lead to good overlap of photonic band gap when they are assembled together.For chiral liquid crystal polymer film, the central wavelength Ȝ of photonic band can be calculated according to Ȝ Q × p , where n is the average refractive index, and p is the pitch length.In our experiment, the average refractive index of chiral liquid crystal polymer film was 1.59 at 540 nm, which was measured by ellipsometer (iHR320, HORIBA).Therefore, the calculated central wavelength is 540.6 nm and 545.4 nm for RH and LH film, respectively, which is highly consistent with the measured results of 541 nm and 545 nm.The cross-sectional image of fabricated double-layer film is shown in Fig.2(c), where the measured thickness of RH /&3 DQG /+ /&3 LV ȝP DQG ȝP UHVSHFWLYHO\ $V WKH 5+ DQG /+ ILOPV DUH fabricated by the same procedure, and the thickness difference is due to the tiny difference on the actual thickness of LC cells which is inevitable in experiment.The total thickness of double-OD\HU ILOP LV DURXQG ȝP Figure2(d)shows the photograph of the double-layer film on glass substrate and flexible PET substrate, respectively.The film can be transparent

Fig. 3 .
Fig. 3. (a) Reflectance of RH (green curve) and LH (magenta curve) LCP film without dichroic dye and absorptance of pure dichroic dye (red curve) doped in NOA 81.(b) Optical setup for OF measurement with different incident angle.(c) Transmittance of double-layer LCP film at different incident angle from 0° to 70°.The transmittance of pure dichroic dye doped NOA81 is presented for comparison purpose.(d) The optical density of double-layer LCP film at wavelength of 532 nm for incident angle from 0° to 70°.

Fig. 4 .
Fig. 4. (a) Optical density varies with thickness of film.(b) The dye concentration effects on optical density and transmittance.

Figure 4 (
Figure 4(a) shows the effect of sample thickness on optical density, where the red line plots the relationship of optical density at 0° incident angle and thickness of proposed R-L (RH and LH) double-layer LCP film doped with dichroic dye sample.It can be seen that with WKH LQFUHDVH RI WKLFNQHVV IURP ȝP WR ȝP WKH 2' RI VDPSOH LQFUHDVH IURP WR as well.For comparison purpose, two additional samples, one with R-R (RH and RH) doublelayer LCP film doped with dichroic dye (green line, at 0° incident angle) and another with R-L double-layer LCP film that only the LH layer was doped with dichroic dye (blue line, at 0° incident angle), were fabricated.The OD value for R-L double-layer LCP film doped with dirchroic dye was always higher than that of R-R double-layer (from, 1.5 to 3.6, and 1.3 times at 45 ȝP DQG 5-L double-OD\HU IURP WR WLPHV DW ȝP VDPSOHV DW GLIIHUHQW WKLFNQHVV IURP ȝP WR ȝP ,W LV QRWLFHG WKDW WKH FRQFHQWUDWLRQ RI GLFKURLF G\H LQ aforementioned three samples was fixed at 1 wt%.In addition, with the increase of concentration of doped dichroic dye on, the OD and transmittance of LCP film would LQFUHDVH DQG GHFUHDVH UHVSHFWLYHO\ )RU D IL[HG WKLFNQHVV RI ILOP ȝP WKH LQFUHDVHG G\H concentration (from 0.5 to 1.5 wt%, black curve) leaded to increase of optical density (from 1.3 to 3.4) and reduce of transmittance (from 95% to 82%, pink curve) of film, as shown in Fig. 4(b).The measurement was carried at wavelength of 633 nm.The stabilities of proposed double-layer LRM film under external stimuli such as temperature, stress and strain are critical to its application as optical filter and other photonic devices.Figure5(a)-5(c) plot the curves of optical density at different temperature, stress, and strain.The sample used here was the double-layer LRM film doped dichroic dye with FRQFHQWUDWLRQ RI ZW 7KH WKLFNQHVV ZDV ȝP $OO GDWD ZHUH PHDVXUHG DW LQFLGHQW angle.The temperature was controlled by a hot stage (MK2000, INSTEC).The stress data was collected by a pressure sensor (JHBM-M2-10Kg, JINRUO).The strain measurement was carried out by a stretch machine (QLW-5E, Qun Long).In Fig.5(a)-5(b), the OD was kept in range of 4.7~4.8when environment temperature varied from 100°C to 100°C, and mechanical stress increased up to 9.8*10 5 Pa, indicating a broad working temperature and excellent mechanical stress insensitivity.
Figure 4(a) shows the effect of sample thickness on optical density, where the red line plots the relationship of optical density at 0° incident angle and thickness of proposed R-L (RH and LH) double-layer LCP film doped with dichroic dye sample.It can be seen that with WKH LQFUHDVH RI WKLFNQHVV IURP ȝP WR ȝP WKH 2' RI VDPSOH LQFUHDVH IURP WR as well.For comparison purpose, two additional samples, one with R-R (RH and RH) doublelayer LCP film doped with dichroic dye (green line, at 0° incident angle) and another with R-L double-layer LCP film that only the LH layer was doped with dichroic dye (blue line, at 0° incident angle), were fabricated.The OD value for R-L double-layer LCP film doped with dirchroic dye was always higher than that of R-R double-layer (from, 1.5 to 3.6, and 1.3 times at 45 ȝP DQG 5-L double-OD\HU IURP WR WLPHV DW ȝP VDPSOHV DW GLIIHUHQW WKLFNQHVV IURP ȝP WR ȝP ,W LV QRWLFHG WKDW WKH FRQFHQWUDWLRQ RI GLFKURLF G\H LQ aforementioned three samples was fixed at 1 wt%.In addition, with the increase of concentration of doped dichroic dye on, the OD and transmittance of LCP film would LQFUHDVH DQG GHFUHDVH UHVSHFWLYHO\ )RU D IL[HG WKLFNQHVV RI ILOP ȝP WKH LQFUHDVHG G\H concentration (from 0.5 to 1.5 wt%, black curve) leaded to increase of optical density (from 1.3 to 3.4) and reduce of transmittance (from 95% to 82%, pink curve) of film, as shown in Fig. 4(b).The measurement was carried at wavelength of 633 nm.The stabilities of proposed double-layer LRM film under external stimuli such as temperature, stress and strain are critical to its application as optical filter and other photonic devices.Figure5(a)-5(c) plot the curves of optical density at different temperature, stress, and strain.The sample used here was the double-layer LRM film doped dichroic dye with FRQFHQWUDWLRQ RI ZW 7KH WKLFNQHVV ZDV ȝP $OO GDWD ZHUH PHDVXUHG DW LQFLGHQW angle.The temperature was controlled by a hot stage (MK2000, INSTEC).The stress data was collected by a pressure sensor (JHBM-M2-10Kg, JINRUO).The strain measurement was carried out by a stretch machine (QLW-5E, Qun Long).In Fig.5(a)-5(b), the OD was kept in range of 4.7~4.8when environment temperature varied from 100°C to 100°C, and mechanical stress increased up to 9.8*10 5 Pa, indicating a broad working temperature and excellent mechanical stress insensitivity.

Fig. 5 .
Fig. 5. (a) Temperatures and (b) stress effect on OD of proposed double-layer LRM film doped with dichroic dye.Measured optical density verse duration time at pulse energy of (c) 67 mJ/pulse and (d) 80 mJ/pulse.

Table 1 lists
the different compositions of samples used our experiment, where the sample A and B are doped with right-handedness and left-handedness chiral dopant R5011 and S5011, respectively.