Random Lasing Engineering in Poly-(9-9dioctylfluorene) Active

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Random Lasing Engineering in Poly-(9-9dioctylfluorene) Active Waveguides Deposited on Wrinkle Corrugated Surfaces Marco Anni, Dongjoon Rhee, and Won-Kyu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18187 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Applied Materials & Interfaces

Random Lasing Engineering in Poly-(9-9dioctylfluorene) Active Waveguides Deposited on Wrinkle Corrugated Surfaces Marco Anni ,∗,† Dongjoon Rhee,‡ and Won-Kyu Lee‡,¶ †Dipartimento di Matematica e Fisica ”Ennio De Giorgi”,Università del Salento, Via per Arnesano, 73100 Lecce, Italy ‡Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA ¶Current address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA E-mail: [email protected] Abstract This paper investigates the correlation between the random lasing properties of organic waveguides made by poly-(9-9dioctylfluorene) (PFO) thin films and the morphology of wrinkle corrugated substrates. The capability to individually control the wrinkles wavelength, shape and height allows us to separately investigate their role on the samples emission properties. We demonstrate that the main parameter determining the presence of coherent random lasing is the substrate roughness and that, contrary to what could be qualitatively expected, as the roughness increases coherent random lasing is progressively reduced. Coherent random lasing is observed only for a substrate roughness below 33 nm, while higher roughness leads to Amplified Spontaneous Emission (up to 70 nm) or to absence of light amplification in the film (above

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70 nm). We demonstrate that this result is due to a progressive reduction of the light amplification efficiency in the PFO film, evidencing that coherent random lasing can be obtained only with a right interplay between light amplification and scattering. Besides clarifying the basic aspects of random lasing in organic waveguides our work opens the way to the realization of organic random lasing with predictable emission properties, thanks to the high control level of the scattering properties of the wrinkle corrugated surfaces.

Keywords Random laser, multiple scattering, optical gain, polyfluorene, organic laser, wrinkle

1

Introduction

In the frame of developing novel light emitting materials for photonics and optoelectronic applications, organic conjugated molecules received large attention in the last decades, due to their interesting combination of easy processing typical of plastic materials and active properties typical of semiconductors. In particular it has been demonstrated that thin films of many classes of molecules, either oligomers or polymers, show optical gain and Amplified Spontaneous Emission (ASE) under strong enough pulsed optical pumping, thus proposing these materials as potential active materials in optically pumped lasers. A good description of the actual state of the art of the organic materials with optical gain and of the different possible cavity geometries for organic laser can be found in some recent review papers. 1–3 In the wide family of organic optically pumped lasers a unique and fascinating class is represented by the so called coherent random lasers, in which the lasing feedback is not due to multiple reflection in a well defined resonator, but it is instead to due to interference effects of light partially scattered by scattering centers in the active material. 4,5 Random lasers are very interesting not only for basic science studies of light propagation in disordered systems, 2


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but also for their possible applications as low costs laser sources, as no resonator is needed to obtain lasing. Moreover the dependence of the multimode laser emission on the local spatial distribution of the scattering centers and their intrinsic low emission coherence have been recently exploited to demonstrate that random lasers can be used as light sources for speckle free imaging, 6 as a cancer diagnostic tool in dye infiltrated human tissues, 7,8 or as a pH sensor. 9 The range of systems showing coherent random lasing is very broad and includes finely grained inorganic laser crystals, 10 semiconductors polycrystalline films 11 and nanopowders, 12 dye solution with scattering nanoparticles, 13 solid dye-scatterer blends 14 and organic thin films 15–17 and nanofibers. 18–21 To date a severe limitation in the understanding and optimization of coherent random lasing in organic materials comes from the nature of the scattering centers, necessary to provide the lasing feedback, that typically are unintentional morphological imperfections, like local thickness fluctuations, 15 nano holes on the film surface, 16,17 film roughness 22,23 and cracks along nanofibers. 18,19 The spontaneous formation of these morphological defects during the active film deposition does not allow a fine control of their geometry and distribution. This limitation leads to the frequent presence of coherent random lasing in films deposited to simply investigate light amplification during propagation in uniform organic waveguides 24–27 and to date prevented the random lasing optimization and the control ex-ante of their emission properties. A possible strategy to overcome this limit is the realization of random lasers positioning a uniform active layer on a non planar substrate containing a known distribution of scattering centers. This approach has been to date demonstrated successfully in few cases, like coherent random lasing in polymeric active waveguides deposited on rough plastic substrates 28 and, more recently, organic solutions 29 and perovskite nanocrystals films 30 deposited on wrinkle corrugated rough substrates. Despite these interesting preliminary results to date the understanding of the random

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lasing dependence on the corrugation of the substrate and of the conditions to be fulfilled in order to obtain coherent random lasing is still missing. In this work we investigate in details the correlation between the emission properties of an organic active waveguide showing optical gain and the shape, wavelength, and height of the scattering centers realized by wrinkle corrugations on the substrate surface. Wrinkle corrugated surfaces realized by controlled strain release are subject of a wide research activity, thanks to their capability to modify the electrical, mechanical and optical properties of a substrate without changing the bulk properties of the material 31 that allows the engineering of hydrophobicity 32–35 and the development of materials for flexible electronic devices 36 and for mechanical measurements. 37 The surface corrugation can be created over large surfaces with a high control of the wrinkles dimensionality, including one dimensional and two dimensional wrinkles with tunable wavelength and height, 38 of their geometry by lithographically driven wrinkle formation, 39 eventually on multiple scale length, including hierarchical structures in which 1D and 2D structures with different characteristic wavelength can be combined. 32 In this paper this fine control of the wrinkle morphological features has been exploited to realize corrugated substrates in which we individually change the shape, the size, the height and the average distance of the scattering centers thus allowing a systematical investigation of their correlation with the emission properties of an active organic waveguide. Poly-dioctyl-fluorene (PFO) thin films have been used as active layers, chosen as prototypical organic active layer, thanks to the PFO high optical gain, possible deposition from solutions and good film forming properties. 40–43 We demonstrate that nominally identical active films show coherent random lasing, Amplified Spontaneous Emission or even no light amplification depending on the wrinkle geometry. In particular we show that the main parameter determining the random lasing properties is the substrate surface roughness and that, contrary to what could be qualitatively expected, the roughness increase does not enhance random lasing even if it increases the light scattering. On the contrary we observe that coherent random lasing is observed

4


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only for low roughness (up to 33 nm), while a progressive transition to Amplified Spontaneous Emission (between 33 nm and 70 nm) and to the absence of light amplification (above 70 nm) is observed as the roughness increases. These results are ascribed to the progressive reduction of the light amplification when the roughness increases, evidencing that a right interplay between efficient scattering and efficient light amplification is fundamental to obtain coherent random lasing in organic thin films. Our results, besides improving the understanding of the physics of organic random lasers, open the way to the realization of organic random lasers with predictable emission properties that can improve the applicative prospects of these systems.

Figure 1: a: schematic representation of (from top to bottom) the wrinkle realization on PS, replica with PU and PFO deposition. b: 10µm × 10µm AFM image showing the typical morphology of a PU substrate with 1D wrinkles (top) and of the top surface of the PFO film deposited on the corrugated PU substrate (bottom).

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Experimental Methods

Nanowrinkled polyurethane (PU) substrates with controlled structural features were created by molding against polystyrene (PS) master patterns (see Figure 1 a). First, pre-strained PS sheets were treated with CHF3 plasma (flow rate: 25sccm, pressure: 20 Pa, and power: 70W) in a reactive ion etching instrument (Samco Inc., RIE-10NR) to form fluoropolymer skin layers with controlled thickness. 34,39 In particular a plasma treating time of 40 s and 80 s was used to form 20 nm and 40 nm skin layers, respectively. The plasma-treated PS films were heated in a convection oven at 130 ◦ C to relieve strain and generate wrinkles with 325 nm (40 s-treated) and 530 nm (80 s-treated) wavelengths (defined as the average distance between two adjacent peaks along the height profile). The orientation of the wrinkles was controlled through the direction of strain; uniaxial and biaxial strains resulted in 1D and 2D wrinkles, respectively. Different heating times were applied to control the amount of strain relief, thus changing the wrinkle amplitude. 34 The strain (ε) was determined from the change in dimensions of the PS substrates. For 1D wrinkles, the strain was defined as ε = (L0 − L)/L0 , where L0 and L are the distance between two lines drawn on the surface before and after shrinking the PS sheet. 2D strain was determined by measuring the area of a rectangle marked on the sample before and after fabricating wrinkles: ε = (A0 − A)/A0 , where A0 and A are the initial and final areas. For example, shrinking PS uniaxially for 45 s resulted in 1D strain of 0.3, whereas 1D strain of 0.7 was reached upon shrinking for 180 s. Liquid polyurethane (Norland) was then cast against the PS wrinkles, cured under ultraviolet light for 10 minutes, and stripped from the surface. The patterned PU substrate served as a template for structuring the active films. Twelve samples with different wrinkle dimensionality (1D and 2D), wavelength (325 nm and 530 nm) and strain (0.3, 0.5 and 0.7) have been realized. The sample name in the following will include the wrinkle dimensionality (1D or 2D), with wavelength as subscript and the strain as superscript (i. e. the sample with 1D wrinkles, 325 nm wavelength and 0.3 0.3 strain will be 1D325 ).

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The active films of poly-dioctyl-fluorene (PFO) have been deposited on the PU substrates by spin coating from a toluene solution with a concentration of 30mg/ml with an initial rotation at 500 rpm for 5 seconds followed by a faster rotation at 2500 rpm for 2 minutes . The average thickness has been determined indirectly from the sample absorbance as described in the Supporting Information. All the films have a thickness between 350 nm and 400 nm. Surface topography before and after depositing PFO layers were characterized by tappingmode atomic force microscopy (AFM) (Dimension FastScan, Brucker) at a scanning rate of 1 Hz. Height profiles from the images were used to determine the wavelength and amplitude of wrinkles and the surface roughness values. The samples have been excited by a Nitrogen laser, delivering 3 ns pulses at a wavelength of 337 nm, with a pulse energy up to 155 µJ. The laser has been focused by a cylindrical lens, thus obtaining a rectangular pump stripe of 5 mm length and 100 µm height. The photoluminescence, guided by the active film, has been collected from the sample edge, collected by a telescopic lens system, and dispersed by an Acton 750 spectrometer, coupled with a Peltier cooled Andor CCD. For all the samples we performed the following measurements: 1) PL spectra as a function of the excitation density in order to observe the eventual presence of Amplified Spontaneous Emission (ASE) or Random Lasing (RL). In the samples with 1D wrinkle we performed the measurement both with the pump stripe parallel and perpendicular to the wrinkle direction; 2) PL spectra at high excitation density at different times in a fixed sample position and at fixed excitation density; 3) PL spectra at high excitation density in different positions at fixed excitation density; 4) Propagation losses in the waveguides; 5) Atomic Force Microscopy (AFM) in order to investigate the surface morphology of both the PU substrate surface and of the top PFO surface.

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3 3.1

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Results Samples morphology

The samples morphology has been characterized by AFM imaging of both the polyurethane (PU) substrate surface and of the top PFO surface (see Fig. S2 and S3). As the light propagation in the active films is directional, along the pumped stripe direction, the emission properties of the samples are expected to be related to the substrate corrugation along specific directions, rather than by the average morphology along all the film surface. For this reason we determined, starting from the 10µm*10µm AFM images, the average value of the root mean square roughness and the average distance between consecutive surface peaks (wrinkles wavelength), by averaging over 25 line-scan profiles extracted along the whole AFM map, and thus representative of the average morphology. Given the presence of a clear preferential orientation the 1D structures have been analyzed both along the wrinkle main orientation (named parallel) and across the wrinkles main direction (named perpendicular). All the samples show a similar evolution as a function of the strain and of the corrugation expected periodicity (wavelength), and similar kind of differences are observed between the PU surface the PFO film top surface. In particular we observe that (see Fig. S4): 1) The wrinkle wavelength of the 2D wrinkles and of the 1D wrinkles across the wrinkle direction are comparable and depend only on the PS skin layer thickness. In particular 20 nm thick skin layer leads to the realization of wrinkles with a wavelength of 325 nm, while 40 nm thick skin layer leads to wrinkles with wavelength of 530 nm. 2) The root mean square roughness increases with the strain in all the samples. 3) All the 1 D wrinkle samples show clear height undulations not only along the direction perpendicular to the wrinkles, but also along the wrinkles. Along the wrinkles the wavelength is much larger, with respect to the perpendicular directions (typically about 3 times), while the roughness values are comparable (see Fig. S4 and S5). 4) The deposition of the PFO films leads to a general planarization of the substrate cor-

8


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rugations, evidenced not only by a clear decrease of the surface roughness, but also by an increase of the corrugation wavelength (see Fig. S4 and S6). The difference between the PU roughness and the PFO one increases with the strain in all the samples, but in a stronger way in the 325 nm samples. The wrinkle wavelength increase of the PFO surface is very evident both in the 2D samples and in the 1D ones across the wrinkles, while is not present in the 1D530 samples along the 0.3 wrinkles and in the 1D325 along the wrinkles. These samples are the ones with the maximum

wavelength values, all above 1.5 µm, that are probably too high to be effectively modified by the deposition of a 350-400 nm thick PFO film. The wavelength increase due to the PFO coating can be reasonably ascribed to a planarization of the smaller thickness fluctuations on the PU surface. The correlation between the wrinkle amplitude, the root mean square roughness, the strain and the skin layer thickness and the reproducibility of the wrinkles morphology are discussed in the Supporting Information.

3.2

Emission Properties

In the following we will discuss the light amplification properties of all the investigated samples. The capability to individually change the wrinkle shape, wavelength and roughness allows us to separately investigated their effect on the emission properties of the PFO active film, in particular: 1) The effect of the dimensionality will be investigated by comparing the samples with identical wrinkle wavelength and roughness (strain), but different dimensionality (1D or 2D) and different pumping geometry of the 1D wrinkles (parallel or perpendicular) starting from the samples with minimum wavelength and minimum roughness; 2) The effect of the roughness will be investigated by comparing samples with fixed wrinkle wavelength and different strain; 3) The effect of the wrinkle wavelength will be investigated by comparing the results of the 9



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325 nm wrinkles with the ones of the 530 nm ones. The main results obtained for all the samples are summarized in Table 1. 3.2.1

0.3 Wrinkle geometry effect in 1D − 2D325

The PL spectra of the 2D0.3 325 sample as a function of the excitation density, are reported in Fig. 2. The typical photoluminescence spectrum of the PFO glassy phase is observed at low excitation density with a 0-0 line at about 425 nm and two vibronic replica at about 450 nm and 480 nm. 41,44 A clear line narrowing, due to ASE 40 peaked at about 450 nm, is observed for excitation density above 0.59 mJcm−2 . The ASE threshold, determined as the minimum excitation density necessary to observe ASE in the spectra, is about 0.50 mJcm−2 . The ASE band shows a clear fine structure (see inset of Fig. 2), with narrow laser-like peaks at 449.0 nm and 449.7 nm with linewidth of about 0.4 nm. The fine structure is reproducible when the measurement is repeated in time, under fixed excitation conditions ,while it changes when the position of the excitation stripe is moved on the sample (at fixed excitation density) (see Fig. S9). These features are the typical signatures of coherent random lasing (RL) assisted by scattering in the active film. 15,16,28 The 1D0.3 325 sample with perpendicular pump shows a qualitatively similar PL excitation density dependence (see Fig. S10), with a structured ASE band evident in the spectra at high excitation density and ASE threshold of about 0.87 mJcm−2 . Also in this case the RL contribution to the spectra depends on the position of the pump stripe on the sample (see Fig. S10). Finally the 1D0.3 325 sample with parallel pump also show an ASE band, with a threshold of about 0.16 mJcm−2 , with a less evident RL structure (see Fig. S11). 3.2.2

Surface roughness effects in 1-2D325 samples

In order to investigate the effects of the bottom and top surface roughness on the samples random lasing properties we compared the emission properties of the 1-2D325 samples with increasing strain values (and thus increasing roughness).

10


105 PL Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.8 mJcm 2.3 mJcm 1.9 mJcm

104

1.4 mJcm

-2

-2

-2

-2

0.94 mJcm 0.74 mJcm 0.59 mJcm 0.47 mJcm

-2

-2

-2

1400 1200 1000 800 600 400 200

PL Intensity (arb. units)

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-2

446 448 450 452 454 Wavelength (nm)

103 102 420 430 440 450 460 470 480 490 500 Wavelength (nm)

0.3 Figure 2: Excitation density dependence of the PL spectra of the 2D325 sample. Inset: high −2 resolution spectrum at 0.94 mJcm , evidencing the ASE fine structure due to coherent random lasing.

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0.5 The PL spectra of the 2D325 sample as a function of the excitation density show (see

Fig.3), as the excitation density increases, a smooth ASE band with a threshold of about 8.0 mJcm−2 , but no narrow peaks due to RL are present. Similar results are observed for the 0.5 1D325 sample (see Fig. S12), that only show ASE, with a threshold of about 3.0 mJcm−2

and 2.0 mJcm−2 for perpendicular and parallel pump, respectively but again no RL peaks. When the roughness is further increased by increasing the strain to 0.7 also the ASE band disappears in the PL spectra of the 2D sample and of the 1D sample pumped along the direction perpendicular to the wrinkles (see Fig. S13). A clear ASE band is instead still 0.7 observed in the PL spectra of the 1D325 sample with parallel pump (see Fig. S13) with a

threshold of about 1.5 mJcm−2 . These results evidence that the roughness increase of both the substrates and the top surface counter-intuitively determines the disappearance of the scattering assisted random lasing process. 3.2.3

Wrinkle wavelength increase effects

As last step we investigated the effects of the wrinkle wavelength increase to about 530 nm. We remember that when the wrinkle wavelength increases also the roughness increases roughly of the same factor. Thus in these last samples the average distance between consecutive light scattering along the film increases (contributing to reduce the scattering) but the scattering cross section of the single scattering events increases (contributing to a scattering increase). These last samples thus show a smaller number of scattering events, but with higher individual scattering cross section, and are thus expected to have a total scattering cross section similar to the one of the corresponding 325 nm wrinkles. Despite this, the observed emission properties evidences clear differences between the film deposited on the 325 nm and on the 530 nm wrinkles. Starting from the samples with lower roughness (0.3 strain) we observe an ASE band for high enough excitation density, but without evidences of RL (see Figure S14) for both the 2D wrinkles and for the 1D wrinkles with both parallel

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and perpendicular pumping. When the roughness is increased by increasing the strain to 0.5 and 0.7 the ASE band disappears and only spontaneous emission is observed up to the maximum available excitation density (spectra not reported).

20000


15000

2.3 mJcm

-2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2

-2

-2

1400 1200 1000 800

444

446

448

450

452

Wavelength (nm)

10000 5000

420 430 440 450 460 470 480 490 500 Wavelength (nm) 0.5 Figure 3: Excitation density dependence of the PL spectra of samples 2D325 . The inset show the the ASE band at high spectral resolution, evidencing the absence of RL.

Table 1: : Observed emission properties (ASE, RL or absence of both) and eventual threshold as a function of the wrinkle dimensionality (1D or 2D), the strain and the pumping geometry (pump stripe parallel or perpendicular to the wrinkle direction). 1D

2D

Λ = 325nm Strain 0.3 0.5 0.7

// RL 0.16 mJcm−2 ASE 2.0 mJcm−2 ASE 1.5 mJcm−2

Λ = 530nm

⊥ RL 0.87 mJcm−2 ASE 3.0 mJcm−2 NO

// ASE 2.0 mJcm−2 NO

⊥ ASE 2.0 mJcm−2 NO

NO

NO

13


Λ = 325nm

Λ = 530nm

RL 0.50 mJcm−2 ASE 8.0 mJcm−2 NO

ASE 3.0 mJcm−2 NO NO


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Table 2: : propagation losses in cm−1 at 450 nm for all the samples. The error bar is the best fit one, and likely underestimate the variations due to local non homogeneities of the samples. 1D Strain 0.3 0.5 0.7

3.3

Λ = 325nm // ⊥ 13.0 ± 1.2 5.0 ± 0.7 10.4 ± 0.2 4.5 ± 0.1 14.0 ± 0.9 6.2 ± 0.2

Λ = 530nm // ⊥ 9.6 ± 0.2 3.43 ± 0.14 10.3 ± 0.3 3.4 ± 0.9 11.1 ± 0.8 3.2 ± 0.2

2D Λ = 325nm Λ = 530nm 15.3 ± 0.3 5.8 ± 0.3 4.2 ± 0.1

5.2 ± 0.3 5.0 ± 0.2 6.8 ± 0.3

Waveguide losses

In order to further understand the differences between the samples we also measured the waveguide propagation losses by moving the excitation stripe from the sample edge (see Table 2). In order to compare the obtained values it is important to consider that the samples are likely non completely homogeneous, thus the absolute value of the losses can depend not only on the wrinkle properties but also on the particular measured point. The obtained values allow to observe that: 1) The pumping geometry of the 1D wrinkles strongly affect the losses, that are larger for parallel pump than for perpendicular pump (between 2.3 and 3.5 times) at any common value of strain and wrinkle wavelength. 2) The losses of the 2D samples are always lower than the ones of the corresponding 1D sample with parallel pump, and almost always higher than the ones of the corresponding 1D 0.7 sample with perpendicular pump (only exception the 1D325 showing losses higher than the

corresponding 2D sample). 3) Roughness effect: The samples with the same wrinkle size and geometry but different strain (along common columns of Table 2) show roughly similar losses, suggesting the absence of a strong losses dependence on the roughness. 3) Wrinkle wavelength effect: the losses of the samples with 325 nm wrinkles are generally higher than the losses of the 530 nm ones (observed in 8/9 samples, only exception the 2D0.7 325 sample). 14


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A qualitative understanding of the losses dependence on the wrinkle morphology and the pump geometry can be obtained considering that in 1D samples with perpendicular pump the light is frequently scattered from the wrinkles every about one wavelength, but the scattered light mainly continues to propagate along the pump stripe (see Figure 4). Under parallel pumping the scattering rate is lower, as evidenced by the larger wavelength values, but the light is mainly scattered outside the pump stripe. The 2D sample should be an intermediate case between the previous two. This simple picture is also consistent with the RL strength in the 1 − 2D325 samples, that requires constructive interference between scattered light in the pumped region. The 1D sample with perpendicular stripe is the one with the maximum fraction of scattered light that remains in the pumped region, thus showing the strongest interference, and thus the strongest RL. The 2D sample shows decreased RL as part of the light is scattered outside the pumped stripe. Finally the 1D sample with parallel pumping shows the lowest RL, due to the lowest fraction of scattered lights that remains in the pumped stripe, and thus the lowest interference effects.

4 4.1

Discussion Correlation between the emission properties and the sample morphology

In order to rationalize the previous results we investigated the correlations between the observed presence of Random Lasing (RL), of Amplified Spontaneous Emission (ASE), or the absence of light amplification effects and the sample morphology and waveguide losses. In particular we investigated the eventual correlation between the observed emission properties and the sample roughness, wrinkles wavelength and waveguide losses.

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Figure 4: Schematic representation of the light propagation and scattering in a 1D sample with pump stripe perpendicular to the wrinkle direction (a) and parallel to the wrinkle direction (b) and in the 2D wrinkle samples (c). 16


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4.1.1

Surface roughness

The presence of RL or ASE is clearly correlated with the surface roughness (see Fig.5), both of the bottom PU substrate and of the top of the PFO film. In particular we observe that RL is present in the three samples with the lowest bottom (top) roughness, namely below 33 nm (below 5 nm), ASE is observed in the seven samples with bottom (top) roughness in the range 33-70 nm (5-12 nm) and no light amplification is observed in the eight samples with bottom (top) roughness above 70 nm (12 nm). It is interesting to observe that the observed correlation is the opposite of what could be guessed starting from the simple idea that random lasing simply requires optical gain and high enough scattering. Actually the scattering during light propagation along the pumped stripe reasonably increases with the increase of the roughness, thus ASE should be expected in samples with low roughness (light amplification but poor scattering), RL should be expected between a minimum roughness (providing a minimum for lasing feedback) and eventually a maximum value (determining losses higher than the cavity gain, and thus the lack of light amplification). 4.1.2

Wrinkles wavelength

The presence of RL or ASE does not show any correlation with the wrinkles wavelength (see Fig. S15). This is clearly evidenced by the presence of many sample with similar top or bottom wavelength, but different emission properties. 4.1.3

Losses

The emission properties do not show any correlation with the waveguide propagation losses (see Fig. S16). This is evidenced by the presence of samples with comparable losses but different emission properties. For example in the low losses range (below 7 cm−1 ) we find one sample showing RL, four samples showing ASE and six samples showing no amplification. In a similar way in the high loss range (above 9 cm−1 ) we have two samples showing RL, three samples showing ASE and three samples not showing light amplification. 17



100

120

RL

ASE

NO

80

100

60

80 60

40

40

20 1D530-07//

1D530-07

2D530-07

1D530-05//

1D530-05

2D530-05

1D325-07

2D325-07

1D530-03//

2D530-03

1D530-03

1D325-07//

1D325-05//

1D325-05

2D325-05

1D325-03//

1D325-03

20 2D325-03

Bottom roughness (nm)

140

Top roughness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

Figure 5: Emission properties dependence on the bottom and top roughness evidencing the progressive transition from RL to ASE and no amplification as the roughness increases. This result evidences that the propagation losses of the waveguide are mainly determined by the scattering outside the pumped stripe, while random lasing is expected to mainly depend on the scattering along the pump stripe.

5

Sample modeling

In order to determine the effect of the roughness on the waveguiding properties of the samples and to explain the observed correlation with the emission properties we calculated the electric field profile of the TE0 guided mode, that determines the ASE properties of uniform PFO waveguides. 44 We considered 3 different geometries, one for samples showing random lasing, one for samples showing ASE and one for samples not showing light amplification, assuming a common PFO thickness of 375 nm. The top and bottom roughness of each sample was fixed to the average value of the corresponding sample class, and the top and bottom rough sur18


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faces have been modeled within an effective medium approximation of the rough layers. The rough samples have been then modeled by four layers with uniform thickness corresponding to the top air-PFO surface, the PFO film, the rough PFO-substrate interface and the substrate. The details of the procedure followed to determine the structure of the sample in the

350 325 300 275 250 225 200 175 150 125 100 75 50 25 0

70

a)

top

60

uniform

Electric field (V/m)

Thickness (nm)

modeling are reported in the Supporting Information.

bottom

RL

ASE

air

b)

top surface uniform PFO bottom surface

50

substrate

40 30

NO

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10

RL

0 -200

NO

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10 RL

ASE

0

NO

RL

ASE

NO

Figure 6: a):Thickness of the top rough layer, of the uniform PFO layer and of the bottom rough layer of the sample showing RL, ASE or NO amplification. b): Electric field profile of the TE0 mode in the three samples. The x scale is the depth in the film, with negative values corresponding to the air top surface. The different line colors evidences the top rough surface, the uniform PFO layer, the bottom rough surface and the PU substrate. c): Mode field Confinement (MC) in each layer for the three investigated geometries. d):Pump to Mode Overlap (PMO) in each layer for the three investigated geometries. The obtained thicknesses of the top rough layer, the uniform PFO layer and the bottom rough layer (see Fig. 6 a) allow to quantify the effects of the roughness variation on the uniform layer thickness. In particular we observe that the sample showing RL has a thin top layer (about 15 nm), a high thickness of uniform PFO of about 290 nm, and a bottom PFOPU layer of about 115 nm. The first roughness increase, that results in the disappearance of RL but still allows ASE, leads to a uniform PFO thickness reduction to about 220 nm, an 19



Page 20 of 36

increase of the top layer thickness to about 37 nm, and a considerable increase of the bottom layer thickness to 206 nm. The sample not showing amplification is instead characterized by a much lower thickness of the uniform PFO layer (134 nm), and the highest thickness of both the top and bottom rough layers (114 nm and 345 nm, respectively). Overall this trend is consistent with the picture that the roughness increase results in a scattering increase, thus in an enhancement of the feedback mechanism for RL, but also in a light amplification decrease, due to the progressive reduction of the guided mode confinement in the uniform PFO layer, and to the lower gain of the rough layers (made by PFO alternated to a inert material). In order to further address this point we investigated the effects of the sample structure the electric field profile of the guided TE0 mode (see Fig. 6 b). In particular we calculated both the mode field confinement (MC) in each layer (the ratio between the integrated electric field in each layer, relative to the total electric field integral) and the Pump Mode Overlap (PMO), in order to address the non uniformity of the sample excitation, due to the strong PFO absorption of the pump laser. 44 The sample showing RL is characterized (see Fig. 6 c) by a high mode confinement in the uniform PFO layer (about 77%) and a small confinement both in the top rough layer (about 1.4%) and in the bottom rough layer (about 14%). The sample showing ASE is characterized by a lower mode confinement in the PFO layer (59%) and in an increased confinement both in the top rough layer (3.1%) and in the bottom rough layer (about 24%). The sample not showing amplification is instead characterized by a poor mode confinement in the PFO uniform layer (only 36%), while a large fraction of the mode is in the bottom rough layer (52%) and about 6.8% is in the top layer. When the non uniformity of the film pumping is considered, due to the strong PFO absorption, we can observe (see Fig. 6 d) that most of the pump laser is absorbed in the uniform PFO layer, while the bottom layer is weakly excited in all the three samples. Remembering that efficient light amplification requires both a high mode confinement in the PFO layer and a high overlap between the guided mode and the excited region in the active material, 44 the obtained results evidences that the roughness increase progressively

20


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worsen the light amplification properties due to the simultaneous reduction of both MC and PMO in the uniform PFO layer. We also observe that the roughness increase also increases MC and PMO both in the top and in the bottom layer, thus increasing the contribution of these layers to the total light amplification. However it is useful to underline that in these layers the gain cross section is lower than in the uniform PFO layer, as only 50% of the material in the rough layers shows gain, thus the contribution to the light amplification improvement of these layers is expected to be smaller than the decrease due to the MC and PMO reduction in the PFO layer. We thus conclude that the roughness increase progressively decreases the light amplification efficiency in the samples while increasing the total scattering cross section. As coherent random lasing requires the interplay between efficient amplification and scattering, the disappearance of RL first, and even of ASE when the roughness of the top and bottom layers are increased evidences that the scattering increase is not enough to compensate the waveguiding decrease, overall leading to a negative impact of the roughness increase on the random lasing properties of the samples. A further possible contribution can be related to the size dependence of the scattering cross section at different angles. 45 In particular small structures mainly scatter in backward and forward direction, thus still along the pump stripe, while larger structures instead lead to a higher total scattering, but also scattering along lateral directions that leads to light losses outside the pumped stripe.

6

Conclusions

In conclusion we systematically investigated, for the first time, the dependence of the random lasing properties of organic waveguides on the dimensionality, height and periodicity of scattering centers induced by wrinkle corrugation on the substrate. We demonstrate that in order to obtain efficient random lasing the right compromise between light amplification and scattering is necessary. In particular we show that, even if counterintuitive, random lasing assisted by surface scattering during light propagation requires small values of roughness.

21



Page 22 of 36

Our results allow to understand why RL is often observed in simple thin films deposited from solution on flat substrates, that are typically assumed to be uniform waveguides, that are instead characterized by a weak top surface corrugation due to local thicknesses fluctuation. Moreover the demonstration of coherent random lasing from organic waveguides deposited on wrinkle corrugated substrates, with suitable morphological properties, opens the way to the realization of organic waveguides with random lasing properties engineerable by finely tuning the wrinkle shape, height, size and the pumping geometry, and to the control ex ante of the samples emission properties.

7

Supporting Information

Estimate of the active layer thickness; AFM images of the sample morphology and values of the roughness and wrinkles wavelength; correlation between wrinkle morphology and realization parameters; correlation between roughness values of the bottom and top surfaces; wrinkles reproducibility; emission properties; correlation between emission properties, the wrinkles wavelength and the waveguide losses; Samples modeling.

8

Acknowledgments

The authors acknowledge Prof. Teri W. Odom for the possibility to exploit her know how on the wrinkle fabrication, necessary for the realization of the samples used in this experiment. Dongjoon Rhee and Dr Won-Kyu Lee acknowledge support from the Office of Naval Research (N00014-17-1-2482), the National Science Foundation (CMMI-1462633), and the Vannevar Bush Faculty Fellowship from the Department of Defense (N00014-17-1-3023). Wrinkled substrates were fabricated using Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is partially supported by Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the Materials Research Science and Engineering Center (DMR-1720139), the State of Illinois, and Northwestern University. Surface topogra22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phy of samples were characterized in Scanned Probe Imaging and Development (SPID) facility of Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which has received support from SHyNE Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the International Institute for Nanotechnology (IIN), the Keck Foundation, and the State of Illinois. Dongjoon Rhee kindly thanks financial support from the Jeongsong Cultural Foundation (Republic of Korea). Dr Won-Kyu Lee gratefully acknowledges support from the Ryan Fellowship and the IIN.

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(42) Anni, M.; Alemanno, E.; Cret´ı, A.; Ingrosso, C.; Panniello, A.; Striccoli, M.; Curri, M. L.; Lomascolo, M. Interplay between Amplified Spontaneous Emission, Förster Resonant Energy Transfer, and Self-Absorption in Hybrid Poly(9,9dioctylfluorene)-CdSe/ZnS Nanocrystal Thin Films. J. Phys. Chem. A 2010, 114, 2086–2090. (43) Martino, M.; Caricato, A. P.; Romano, F.; Tunno, T.; Valerini, D.; Anni, M.; Caruso, M. E.; Romano, A.; Verri, T. Pulsed Laser Deposition of Organic and Biological Materials. J. Mater. Sci.: Mater. Electron. 2009, 20, 435–440. (44) Anni, M.; Perulli, A.; Monti, G. Thickness Dependence of the Amplified Spontaneous Emission Threshold and Operational Stability in Poly(9,9-dioctylfluorene) Active Waveguides. J. Appl. Phys. 2012, 111, 093109. (45) van de Hulst, H. C. Light Scattering by Small Particles; Dover Publications Inc, 1957.

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Figure Captions Figure 1: a: schematic representation of (from top to bottom) the wrinkle realization on PS, replica with PU and PFO deposition. b: 10µm × 10µm AFM image showing the typical morphology of a PU substrate with 1D wrinkles (top) and of the top surface of the PFO film deposited on the corrugated PU substrate (bottom). 0.3 Figure 2: Excitation density dependence of the PL spectra of the 2D325 sample. Inset: high

resolution spectrum at 0.94 mJcm−2 , evidencing the ASE fine structure due to coherent random lasing. 0.5 Figure 3: Excitation density dependence of the PL spectra of samples 2D325 . The inset show

the the ASE band at high spectral resolution, evidencing the absence of RL. Figure 4: Schematic representation of the light propagation and scattering in a 1D sample with pump stripe perpendicular to the wrinkle direction (a) and parallel to the wrinkle direction (b) and in the 2D wrinkle samples (c). Figure 5: Emission properties dependence on the bottom and top roughness evidencing the progressive transition from RL to ASE and no amplification as the roughness increases. Figure 6: a):Thickness of the top rough layer, of the uniform PFO layer and of the bottom rough layer of the sample showing RL, ASE or NO amplification. b): Electric field profile of the TE0 mode in the three samples. The x scale is the depth in the film, with negative values corresponding to the air top surface. The different line colors evidences the top rough surface, the uniform PFO layer, the bottom rough surface and the PU substrate. c): Mode field Confinement (MC) in each layer for the three investigated geometries. d):Pump to Mode Overlap (PMO) in each layer for the three investigated geometries.

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Figures

M. Anni et al Figure 1

31




104

1.4 mJcm

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105 PL Intensity (arb. units)

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2D325-03 1D325-03 1D325-03// 2D325-05 1D325-05

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36


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Random Lasing Engineering in Poly-(9-9dioctylfluorene) Active

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