Photochromic Composite for Random Lasing Based on Porous

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Photochromic composite for random lasing based on porous polypropylene infiltrated with azobenzene-containing liquid crystalline mixture Victor Lisinetskii, Alexander Ryabchun, Alexey Yu. Bobrovsky, and Sigurd Schrader ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08032 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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ACS Applied Materials & Interfaces

Photochromic composite for random lasing based on porous polypropylene infiltrated with azobenzene-containing liquid crystalline mixture

Victor Lisinetskii*1, Alexander Ryabchun2,3, Alexey Bobrovsky3, Sigurd Schrader1 1

Technical University of Applied Sciences Wildau, Hochschulring 1, 15745 Wildau, Germany

2

Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Potsdam-Golm,

Germany 3

Faculty of Chemistry, Moscow State University, Leninskie Gory, 119991 Moscow, Russia

ABSTRACT We report on a new low-cost and easily fabricated type of liquid crystalline polymer composites demonstrating low threshold random lasing, which can be used as a cheap and simple mirror-less laser source. The composite is based on mass-producible commercially available porous polypropylene (Celgard 2500) infiltrated with low-molar-mass liquid crystal material doped with Rhodamine 800 laser dye. Excitation with red nanosecond laser (630 nm) induces random lasing with the emission peak in NIR spectral range (804 nm) with noticeable degree of linear polarization. The possibility to control the lasing threshold and polarization of the output light with UV radiation through photo-switching of liquid crystal phase from nematic to isotropic is demonstrated. The photo-controllable phase switching is achieved by reversible E/Z isomerization of the azobenzene dopant introduced to the nematic host matrix. It is revealed that the isotropic state of liquid crystal provides more efficient random lasing with lower threshold due to significant scattering of the ordinary wave.

KEYWORDS: random lasing, porous polypropylene, liquid crystal, light scattering, liquid crystal alignment.

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INTRODUCTION Since decades attention of the scientific community is devoted to creation of new materials with unusual and useful properties, as well as to development of new applications of the existing materials. One of the challenging practical and fundamental problems of modern material sciences for optics and photonics is the creation of advanced high performance materials for solid state and soft matter organic lasers. Organic materials are mechanically flexible and can be applied to a large number of conventional substrates such as glass, plastic or metal foils.1 The ease of processing makes organic laser sources attractive for fabrication of low-cost spectroscopic and sensing systems.2-3 Further simplification of organic laser fabrication as well as the development of new cheap organic laser compounds is important from the point of view of mass production of low-cost laser sources and devices. Recently a simple single-step holographic inscription of in-plane distributed feedback lasers in azobenzene-containing materials was demonstrated.4-7 These lasers do not require any lithographic processes and they can easily be inscribed within ca. 1 s with the possibility to optically control their lasing wavelength.5 Another example of low-cost organic lasers is the organic vertical cavity surface emitting laser completely fabricated through the spin-coating process only in 1.5 hours8 with the possibility to tune the lasing wavelength by applying mechanical stress.9 At the same time mirror-less lasers, where no special structuring is required at all, are of great interest. These are random lasers, where the feedback for lasing is realized due to multiple scattering of light (so-called light diffusion). To this day, a wide range of random lasing materials has been demonstrated, starting from powdered laser crystals10-11 and semiconductors12 to suspensions of scattering microparticles in laser dye13-14 and porous networks of air etched into a solid glass or semiconductor crystal.15 Very broad angular emission of random lasers is ideal for display application,16 while the use of liquid crystal (LC) droplets as scattering centers enables to control of the laser emission via an external electric field.17 The temperature tuning of


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the random laser emission18 can be used for remote temperature sensing or for fabrication of temperature dependent color coatings.16 Major advantages of random lasers are their technological simplicity and low fabrication cost.16 In addition, they can be realized on flexible materials like paper,19 as coatings on surface of arbitrary shapes20 or even in human tissues,21 what could be used for tumour diagnostics. In this paper we present for the first time a new technologically attractive material for random lasing, which is based on an already mass-producible low-cost porous polymer, and can be easily and quickly fabricated. We combine the properties of the porous polymer acting as a host matrix with the optical properties of LC materials. Commercially available porous polypropylene (PP) film, which is mainly used as a membrane in lithium batteries, was chosen. The strongly pronounced porous structure of the PP films gives the possibility to successfully apply them in composite systems as matrix. This structure appears in the PP films after their stretching as a part of production process and results in a very strong ability of the porous structure to align liquid crystals.22-23 It means that the introduction of LC materials into the porous structure leads to the uniaxial orientation of molecules along the stretching direction of the polyolefin films. Thus, the stretched porous polyolefin films can be considered not only as containers for different compounds, but also fulfill the function of good aligning media for LC materials. Due to the high mismatch of refractive indexes of PP and liquid crystal such composites demonstrate considerable light scattering which can be used for random lasing. In the current contribution, we used a nematic mixture containing photoisomerizable azobenzene dopant as LC materials (Table 1). The main feature of this mixture is the reversible photoinduced isotropisation at room temperature. It is known that under the action of UV light azobenzene molecules (6DABU dopant in our case) undergo E→Z photo-isomerization (Figure 1a). The back process (Z→E isomerization) can be induced either by exposure to visible light or by thermal treatment. However, the Z-form of 6DABU is relatively stable due to methyl-substitutes and back reaction usually takes several days.24 It is noteworthy that the Z-isomer of azobenzene 3 ACS Paragon Plus Environment


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chromophore possesses a band shape leading to the disruption of mesophase and, as a consequence, to the isothermal phase transition25 from the nematic to isotropic phase (see Figure 1b). Such photoinduced phase switching is accompanied with considerable alteration of refractive index of the LC material. So, the influence of the change of LC refractive index on random lasing can be studied. The ability of this LC mixture to be introduced into a porous PP film by capillary forces makes fabrication of the composites simple and quick. In order to realize random lasing in the composites small amount of laser dye Rhodamine 800 (Table 1) was introduced into the mixture.

Figure 1. Schematic representations of E/Z isomerization of 6DABU dopant (a) and photoinduced isotropization of the aligned LC phase (b).

Thus, the main goal of this paper is to study random lasing phenomena in the new photochromic LC-polymer composite. Special attention is paid to the demonstration of the possibility of reversible phototuning of emission properties of the composite films by the UV-induced E-Z isomerization of the azobenzene-containing dopant.

EXPERIMENTAL PART Materials and sample fabrication: Commercially available porous polypropylene (PP) film (Product 2500, Celgard; porosity is 55%, refractive index is ca. 1.49) with 25 µm thickness was used as a polymer matrix for LC composite. Scanning electron microscope (SEM) measurements


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were carried out using a SEM Neon 40, Carl Zeiss. Sample was imaged with the following parameters: ETH− 3.00 kV; Aperture Size − 30 µm; Signal − SE2. The mixture containing the commercial nematic cyanobiphenyl derivative 5CB (Merck) and the azobenzene based compound 6DABU in a ratio 70:30 by weight was specially designed in order to obtain the system capable to isothermal phase transition under the action of UV light. The synthesis of the azobenzene dopant 6DABU is described in Ref.24. The mixture was prepared by dissolving the components in chloroform, followed by slow evaporation of the solvent and drying in vacuum. Phase separation did not take place during the preparation of the liquid mixture and finally it was completely homogeneous. The prepared mixture with a small amount of the laser dye Rhodamine 800 (1.5 wt. %, Exiton) was introduced to the porous PP film by capillary forces at room temperature. The prepared sample was examined with a polarizing optical microscope (POM) Axioplan 2, (Carl Zeiss). Table 1. Chemical structures of the used materials. Chemical structures

Name 5CB

C5H11

CN

nematic LC 6DABU C6H13O

N N

N

OOC

O

O(CH2)9CH CH2

N

+

Photoisomerizable dopant

ClO4-

Rhodamine 800 laser dye

CN

Devices characterization: The set-up for investigation of lasing properties in the fabricated random laser devices is presented in the Supporting Information (Figure S1). A wavelength tunable optic parametric oscillator (Continuum Surelite OPO Plus) pumped with the third 5 ACS Paragon Plus Environment


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harmonic of a Q-switched Nd:YAG pulsed laser (Surelite I, Continuum, Inc.) was used as a pump source. The pump pulse duration was 3.5 ns, and the repetition rate was 10 Hz. The pump energy was measured with a pyroelectric detector PEM 4 (energy meter LEM2020, Sensor- und Lasertechnik, Germany). The pump radiation was linearly polarized. The polarization direction was controlled with a half-wave phase plate. A lens with focal length of 220 mm formed the round pump spot on the sample with diameter of about 4 mm. The investigated sample was oriented so that the direction of the sample pores and hence of the LC director was vertical. The generated radiation was detected at the direction orthogonal to the sample surface. The generated spectra were recorded with a fiber coupled spectrometer Polytec Berlin AG (BRC642E) with the instrument function width of 3.5-4 nm and the spectral sampling interval of 1.36 nm. A cut-off glass filter was placed in front of the fiber end to eliminate transmitted pump radiation. A spectrometer (Jobin Yvon SPEX (Model 1681 B) Minimate 2, resolution 0.25 nm) coupled with a fiber and a CCD-array (WinCamD, Gentec-EO Inc.) was additionally used to measure output spectra with higher resolution. A Glan prism and a quarter-wave phase plate were used for inspection of polarization state of the generated radiation. As a UV light source for illumination of the film a Xenon lamp (LSE 140/160.25C LOTQuantumDesign GmbH) equipped with a color glass filter was used. The light intensity in the spectral region 300-400 nm was about 12.6 mWcm-2. The exposure time was 10 min.

RESULTS AND DISCUSSION Structure and optical properties of composite films Here we discuss the main structure features of the used materials. The SEM image of a pure PP film is presented in Figure 2a. It is clearly seen that the PP film possesses a continuous porous structure with elongated shape of the pores characterized by the size of about 60-70 nm along the short axis and of about 300-400 nm along the long axis. The SEM image of the cross-section of the porous PP film is presented in Figure S2. 6 ACS Paragon Plus Environment

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Figure 2. a) SEM images of the pure PP film surface; b) the schematic representation of the alignment of LC molecules introduced to the porous PP matrix. The considered porous structure can easily be filled with liquids by capillary forces. Such infiltration of the porous PP structure is very fast and usually takes several seconds. The introduction of LC materials to the PP films leads to high optical anisotropy of the composites caused by orientation of the LC molecules along fibrils (or stretching direction) as it is shown in Figure 2b. This orientation is clearly demonstrated by polarized optical investigations. It can be seen in Figure 3 that in crossed polarizers the lowest transmittance (Figure 3a) is observed when the LC director is parallel to the one of the polarizers while the maximal transmittance (Figure 3b) is observed when the LC director is aligned at 45° angle with respect to the polarizers directions. Such behavior is associated with uniaxial optical anisotropy of the composite film due to almost perfect alignment of the LC molecules. It should be also mentioned that the porous PP films possess a small birefringence itself due to stretching during the film production. However, this value is negligible compared to the birefringence of the LC materials.26 The LC material used in this work has much higher extraordinary refractive index in comparison to polypropylene. Under assumption that the ordinary (no) and extraordinary (ne) refractive indices of the 5CB/6DABU mixture are close to the indices of 5CB LC one can calculate the 7 ACS Paragon Plus Environment


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values of these indices using three Cauchy coefficients presented in Ref.27 to be 1.524 and 1.689, respectively. The mismatch of the refractive indices (1.49 for PP and 1.689 for the LC material) resulted in light scattering providing the feedback for random lasing.

Figure 3. POM images of the PP composite; a) the LC director (red doted arrow) is parallel to the direction of one of the two crossed polarizers; b) the LC director is oriented at 45° angle with respect to the polarizers directions. The prepared LC mixture contains the azobenzene dopant 6DABU having two absorbance peaks at 355 and 440 nm which correspond to the π-π* and n-π* transition, respectively (Figure 4a blue line and Figure S3). Due to this fact the laser dye Rhodamine 800 absorbing at red spectral region (630-700 nm) was chosen for lasing generation. On the one hand, the pumping with laser light of 630 nm wavelength doesn’t act on the 6DABU molecules but only excites the dye Rhodamine 800 causing luminescence. On the other hand, UV light affects only the 6DABU dopant inducing its photoisomerization. Thus, the presented composite is fabricated very quickly, is very cheap as compared even to commercially available LC cell with rubbed polyimide coatings, does not require any special effort for LC alignment and its state can be controlled with UV radiation. Moreover, the composite is flexible and can be used without any substrate.


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Figure 4. a) Absorbance (blue dotted line) and emission (black and red solid lines) spectra of the investigated sample: black line is the spontaneous emission spectrum under the lasing threshold; red line is the spectrum with the lasing; b) the lasing spectrum measured with a high resolution.

In the following sections the random lasing in the composite as well as the influence of the phase condition (photoinduced phase switching) of the LC material on the lasing characteristic will be considered and theoretical description will be provided.

Random lasing of composite films in LC state First we consider the random lasing properties of the composite sample before irradiation with UV light, when the LC molecules are unidirectionally oriented within the PP pores. Being pumped with the radiation of 630 nm wavelength, the film exhibited at low pump energy a luminescence with the spectrum presented in Figure 4a (black line). The orientation of the LC director is vertical unless other stated. The luminescence signal was sensitive to the polarization state of the pump radiation. The ratio of emission intensities at vertically and horizontally polarized pumping was 1.7. The emitted radiation was also polarized. The dichroic ratio of this radiation determined as the ratio of the vertically polarized component to the horizontally polarized one was measured to be also ca. 1.7 irrespective to the pump polarization. These facts indicate that the dye molecules were also oriented along the LC director.



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As the pump energy increased the narrow lasing line arose at the wavelength of ca. 804 nm as it is shown in Figures 4a (red line) and 4b. The dependence of the intensity of this line as well as of the radiation spectral bandwidth (full width at half maximum) on the pump energy density is presented in Figure 5a (black line, squares) for vertically polarized pumping. It can be seen, that the slight linear increase of the intensity, which corresponds to spontaneous emission, is further transformed to the linear increase with a significant slope (Figure 5b presents the fitting of the experimental data with two lines). This transformation is accomplished with the abrupt decrease of bandwidth of emission line which is typical for random lasing.13, 28-30

Figure 5. a) Dependencies of intensity of the line at 804 nm wavelength (solid lines, solid symbols) and the bandwidth of the emitted radiation (dashed lines, open symbols) on the pump energy density, presented for the sample in the LC state pumped with vertically (black line, squares) and horizontally (red line, circles) polarized radiation and for the sample in photoinduced isotropic state (green line, triangles) after 10 min illumination with UV radiation (see "Experimental part" for details); b) fitting of the data for the PP composite in the LC state at the vertically polarized pumping with two lines (blue lines). The lasing spectrum in Figure 4a was registered with a spectrometer of low resolution. Figure 4b presents the spectrum measured in the same conditions but with a spectrometer of higher resolution. The spectrum is smooth, no spikes can be seen. This is similar to the results presented 10 ACS Paragon Plus Environment

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elsewhere13-14, 18-19, 29, 31 and indicates that random lasing occurs with incoherent feedback32 in contrast to coherent feedback random lasing characterized by presence of spikes in spectrum.12, 16, 32-33

The random lasing was obtained also with horizontally polarized pumping. The corresponding evolution of intensity and spectrum bandwidth of the emitted radiation with increase of the pump energy density is presented in Figure 5a (red line, circles). It can be seen, that for this case the threshold (ca. 1.2 mJ/cm2) was significantly higher than in the case of vertically polarized pump radiation (ca. 0.8 mJ/cm2). This fact can be explained by anisotropy of the dye molecules oriented along the LC-director. The ratio of thresholds (150:100=1.5) corresponds to the above specified ratio of spontaneous emission at vertical and horizontal pumping.

Figure 6. Spectra of vertical and horizontal polarization components of the lasing emission for the composite in the LC state (a) and in the isotropic state (b).

The polarization of the output radiation was inspected using a polarizer and a quater-wave phase plate and was found to be partially linearly polarized (see details in Figure S4). The dichroic ratio of this radiation was 5.0 (typical emission spectra of these components registered at the same pump energy density are presented in Figure 6a), what is significantly higher than the ratio (1.7) for spontaneous emission. This effect can be explained by a strong scattering anisotropy. Indeed, the horizontally polarized wave in the film is the ordinary wave. The difference of refractive indices of PP material (1.49) and of LC (1.524) is small for this wave and hence the 11 ACS Paragon Plus Environment


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scattering is weak. The vertically polarized wave is oriented along the optical axis of LC (the extraordinary wave), the refractive index of LC for this wave is high (1.689), providing strong scattering. The scattering anisotropy of the sample was measured using light at wavelength of 800 nm. The transmittance of the film for vertically polarized light was 12%, while for horizontally polarized light it was equal to 57%. According to the Lambert-Beer law the transmittance (T) is related to the scattering cross-section ( σ ) as follows:

σ=

 1 T ln  2, Nl  T f 

(1)

where Tf is the transmittance of the air-to-film interface (Fresnel losses); attenuation of 800 nm radiation due to absorption was neglected (see Figure 4). Using Eq.(1) the ratio of scattering cross-sections of vertically and horizontally polarized waves was estimated to be ca. 4. The generation of light significantly polarized along the direction of the highest scattering (vertical) evidences that we deal with the random lasing but not with the amplified spontaneous emission (ASE). Indeed, the light scattering results only in losses for the case of ASE and is a definitely negative factor. So, ASE in the presence of such polarization dependent losses would provide output radiation polarized so that to minimize these losses, i.e. polarized horizontally. However, this is not the case in our experiments. Generation of mainly vertically polarized light means that this generation is maintained with the light scattering; the scattering is a positive factor for this process. Such light generation supported with multiple light scattering is the random lasing. The duration of the emitted radiation pulse was slightly larger than the duration of the pump pulse for the case of spontaneous emission and was equal to it when the lasing started (oscilloscope traces of the pulses are presented in Figure S5). To summarize, excitation of the obtained LC composite films with the nanosecond red laser radiation allowed for obtaining sharp random lasing emission peak having strong polarization along the stretching direction of the PP porous films. 12 ACS Paragon Plus Environment

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Random lasing of composite films in photoinduced isotropic state As it was mentioned above, a UV light exposure of the PP composite causes the isothermal photo-isotropization of the LCs embedded in the porous PP structure. After irradiation, the film demonstrated insensibility to polarization state of the pump radiation due to the isotropic state of LC. The transmittance of the sample at 800 nm was measured to be 40%. This means that the scattering cross-section for this case is two times lower than that for vertically polarized light in the case of the LC state. In spite of this scattering decrease the sample in isotropic state demonstrated sufficiently lower lasing threshold and higher efficiency (Figure 5a, green line, triangles). The threshold energy density was measured to be 0.4 mJ/cm2 (intensity of 0.13 MW/cm2). Being order of magnitude lower than the thresholds of the typical dye-doped random lasers,13-14,

19, 29

this value is only six times higher than the lasing threshold in Ag-

nanoparticles- and dye-doped cholesteric LC mixtures with quality factor comparable to conventional lasers.33 The light was inspected to be partially linearly polarized without presence of circular or elliptical polarization. However, in this case the horizontally polarized component of the generated light was stronger than the vertically polarized one and the dichroic ratio of the generated light was ca. 0.5 (see Figure 6b). Thus, the UV-irradiation enables to perform phototuning of the random lasing parameters, such as lasing threshold, intensity and degree of polarization of the emitted light. In particular, the UV-light-induced transition to the isotropic state enables to decrease the threshold energy density of lasing. It is noteworthy, that due to slow rate of thermal Z-E isomerization of azobenzene compound 6DABU, the photoinduced isotropic state was stable for 17 days at room temperature (Figure S6). First droplets of nematic phase appeared only at the 18th day of keeping the sample at room temperature (Figure S6c). It should be noted, that the backward Z to E isomerization of azobenzene dopant 6DABU and the caused formation of nematic phase is a fully reversible process without any fatigue and does not influence the composite stability at



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all.25, 34 It can be considerably accelerated either by moderate heating down to tens of minutes or by irradiation with visible light down to minutes.

Theoretical analysis of lasing properties of composite films The unobvious decrease of the lasing threshold for the sample in the isotropic state we associate with a significantly reduced part of light which is lost through the film sides. Indeed, the film thickness is only 25 µm and is comparable to the scattering mean free path, which was estimated from the above mentioned values for the sample transmission at 800 nm wavelength to vary from ca. 12.5 µm to 52 µm depending on the polarization direction. This means that a wave scattered in the direction towards the film side can with a significant probability meet this side. If the incidence angle of this wave is smaller than the angle of the total internal reflection (TIR), the wave will pass through the film side resulting in losses (see Figure S7). Thus we can estimate the losses as a part of light scattered in the directions for which TIR does not occur. Although such a scattered wave might be scattered again prior to incidence onto the film side, at least qualitatively this assumption allows for comparing of the losses in the films in LC- and isotropic states. Indeed, the propagation distance to the film side for the waves scattered at angles for which the TIR condition is not fulfilled is comparable or less than the scattering mean free path. For instance, for a wave scattered in the middle of the film this distance varies from 12.5 µm for normal incidence to ca. 19 µm for incidence at the critical angle of TIR. Thus at least qualitatively we can assume that the larger part of light is scattered towards the film sides at angles less that the critical angle of TIR, the higher are the losses. To calculate the light distribution after a scattering event and to estimate the losses the approach developed elsewhere35-37 was used. This approach considers the scattering of light by a spherical liquid crystal droplet dispersed in a host medium in the Rayleigh-Gans approximation35. Surely, pores of the PP film are far from spherical shape. Nevertheless, for qualitative consideration we


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approximated 3

a

pore

filled

with

LC

with

a

spherical

droplet

of

diameter

65 ⋅ 65 ⋅ 350 nm = 108 nm .

The used equations, variables, and the details of calculations are presented in the supporting information (section “Equations used for theoretical analysis”). The values of averaged crosssections for ordinary and extraordinary waves were calculated for the case of oriented LC to be

σ oe = σ eo =11×10-15 cm2,

σ ee =124×10-15 cm2,

σ oo =33×10-15 cm2. One can see that

scattering of the ordinary wave ( σ oo + σ eo ) is significantly weaker than that of the extraordinary wave ( σ ee + σ oe ). Also the extraordinary wave mainly scattered into the extraordinary wave (only σ oe

(σ

oe

+ σ ee ) =8% of scattered radiation is transformed into the

ordinary wave). These facts explain the emission of vertically polarized radiation observed in the random lasing for the sample in the LC state.

The calculated surfaces representing angular distribution of differential cross-sections

dσ ee and dΩ

dσ oo for light propagating along the y-axis ( θi = 90°, ϕ i = 90°) are presented in Figure 7a. Blue dΩ

surface shows the angles for which TIR does not occur. Also the surface cross-sections corresponding to the inclination angle θ s equal to 90o are presented at the bottom of Figure 7a. It can be seen, that for a large part of scattered light with vertical polarization (extraordinary wave, cyan color) the TIR does not occur, and the light is lost. Assuming losses as a portion of scattered light which propagation angles do not satisfy TIR condition one can calculate for θ s equal 90o the losses to be 34 %. One can see that the angular dependence of differential crosssection for the horizontally polarized light (ordinary wave, red line) has minima in directions corresponding to the losses. However the value of scattering cross-section for this wave is low and the feedback is not sufficient to provide lasing.



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Figure 7. Differential cross-sections calculated for vertically (cyan) and horizontally (red) polarized light for the LC (a) and isotropic (b) states. Incident light propagates along the y-axis ( θi = 90°, ϕ i = 90°). Blue color designates the angles for which the total internal reflection does not occur. At the bottom the cross-sections of these surfaces, which correspond to inclination angle θ s equal to 90° are presented. It can be seen from Figure 7a and Eq. (S12) that the condition of TIR and, hence, the losses depend also on the inclination angle of the scattered radiation. The further is this angle from 90°, the longer is the range of azimuth angles which satisfy the condition for TIR and hence the smaller are the losses. This dependence is shown in Figure 8 (black line). However, this decrease of losses is accomplished with the decrease of the scattering, as it is also demonstrated in Figure 8 (red line), because the deviation of the inclination angle from 90° results in a decrease of refractive index of the extraordinary wave. So, we can assume that the losses due to transmission through the film facets are about 34 %. A portion of light, which is scattered into the ordinary wave (8%), should be also treated as losses, because this wave does not participate in the lasing due to the weak feedback. So the total losses in the case of the unidirectional aligned LC state can be estimated to be 42 %.


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Figure 8. Dependencies of the losses (black line) and the scattering cross-section σ ee (red line) on the inclination angle θ i of incident radiation (azimuth angle ϕi is 90°) calculated for the LC state. In the case of the photoinduced isotropic state of the sample there is no LC-induced birefringence in the film. Nevertheless we designate a wave as an ordinary one, if its polarization is orthogonal to the plane, formed with the wavevector and the z-axis, and as an extraordinary one, if the polarization is parallel to this plane. For this case the values of scattering cross-section both for the ordinary and extraordinary waves are equal to ca. 80×10-15 cm2. This value is ca. two times smaller than that for the LC state, which corresponds to the experimental data. The calculated dependencies of differential cross-sections

dσ dσ and on the azimuth angle ( θ s d Ω ee d Ω oo

was 90°) for the light propagating along the y-axis ( θi = 90°, ϕ i = 90°) are shown in Figure 7b. Similarly to Figure 7a, the scattering differential cross-section of horizontally polarized light has a minimum for angles, which do not satisfy TIR. As a result the losses of this wave can be calculated to amount to ca. 9%. These losses as well as the losses of the extraordinary wave, which are equal to 34%, do not depend on inclination angle. So, in the case of LC in the isotropic state the scattering cross-section for both light polarizations is significant (only two times smaller than that of the extraordinary wave in an oriented LC 17 ACS Paragon Plus Environment


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droplet), while the losses of the horizontally polarized wave are lower. This decrease of the losses can explain the lower generation threshold for the case of the photoinduced isotropic phase. Also the low losses of the horizontally polarized wave explain the experimentally observed higher intensity of the horizontal component of generated light as compared to that of the vertical component. The generation of only horizontally polarized radiation is prevented by the fact that according to calculations ca. 25% of the ordinary (horizontally polarized) wave scatters into the extraordinary wave Thereby we can conclude that thin films which provide sufficient scattering not only of light polarized along the LC director (or the PP stretching direction) but also of orthogonally polarized light (the case of LC in the isotropic state) provide better random lasing. The feedback of both polarizations is preferable from the point of view of decrease of lasing threshold and efficiency. However, it should be noted that the feedback provided only for light of one polarization results in generation of a polarized output. This can be worth the increased threshold and decreased efficiency. Moreover this drawback of the LC-state of a film can be eliminated with the usage of a film with higher thickness sufficient to provide a feedback in the direction orthogonal to its sides. The light scattered in this direction would be scattered back prior to incidence to the sides.

CONCLUSION Novel type of composites for random lasing based on a porous PP film filled with nematic LC is proposed. The LC mixture based on 5CB nematic host doped with laser dye Rhodamine 800 and azobenzene derivative 6DABU, having an ability of E-Z isomerization leading to isothermal photoinduced phase transition from the nematic to isotropic phase is introduced into the PP matrix. As compared to other dye-doped media for random lasing the presented composite combines low cost, fabrication quickness and simplicity with a relatively low threshold energy density. Also it can provide linearly polarized laser output. The possibility to control the 18 ACS Paragon Plus Environment

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parameters of random lasing with UV exposure in the composite is shown. It was found that the threshold and efficiency of random lasing are better when the scattering feedback was provided not only for the extraordinary wave but also for the ordinary one. However, linear polarization of emitted light in this regime is almost lost. The developed composite represents a novel type of low-cost and easy processing materials for random lasing which can be applied in spectroscopy and different optical sensors as a laser light source.

Supporting Information Available: Figure S1 shows the experimental setup. In Figure S2 a SEM image of the cross section of a PP porous film is presented. Figure S3 demonstrates the change of absorbance spectrum of the 5CB:6DABU mixture after UV- and subsequent visible light exposure. Figure S4 shows the results of polarization state measurements for the output radiation. Figure S5 presents the oscilloscope traces of a pump pulse as well as of emission pulses below and above the lasing threshold. Figure S6 demonstrates POM images of the 5CB:6DABU mixture before and after UV irradiation as well as after thermal relaxation. Figure S7 presents the coordinate system used for theoretical analysis. In Figure S8 polarized attenuation spectra of a porous PP film filled with nematic LC doped with dichroic azobenzene dye are shown together with an estimation of the order parameter. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Victor Lisinetskii, phone: (+49) 3375508461).

ACKNOWLEDGEMENTS This research was supported by the Alexander von Humboldt Foundation, the Russian Foundation of Fundamental Research (13-03-00648, 13-03-12071 and 13-03-12456) and by the German Federal Ministry of Education and Research (BMBF) (03FH086PX2 “DELTA”). We 19 ACS Paragon Plus Environment


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also sincerely thank Dr. G. Stoychev (Leibniz Institute of Polymer Research, Dresden, Germany) for SEM measurements and Dr. M. Cigl (Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic) for the synthesis of dopant 6DABU.

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Photochromic Composite for Random Lasing Based on Porous

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