Intermixing of Chirality and Local Structure in the Second Harmonic

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Intermixing of Chirality and Local Structure in the Second Harmonic Generation Response of Dibenzo[c]acridine Helicene-Like Molecule Thin Films Aurélie Bruyère, Laure Guy, Amina Bensalah-Ledoux, Stephan Guy, Pierre-Francois Brevet, and Emmanuel Benichou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07401 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 14, 2017

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The Journal of Physical Chemistry

Intermixing of Chirality and Local Structure in the Second

Harmonic

Generation

Response

of

Dibenzo[c]acridine Helicene-Like Molecule Thin Films

Aurélie Bruyèrea, Laure Guyb, Amina Bensalah-Ledouxa, Stephan Guya, Pierre-François Breveta, Emmanuel Benichou a* a

Univ Lyon, Université Claude Bernard Lyon, CNRS, Institut Lumière Matière, F-69622 Villeurbanne, France

b

Laboratoire de Chimie; Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon cedex 07 ENS Lyon, France

ACS Paragon Plus Environment

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ABSTRACT

Enantiopure and racemic films of dibenzo[c]acridine helicene-like molecules were elaborated by laser pulsed deposition (PLD) and their quadratic nonlinear optical properties determined by Second Harmonic Generation (SHG). The SHG intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles. For the enantiopure films, the polarization analysis highlights a non-linear optical signature due to the presence of chiral molecules. These films also exhibit a remarkable in-plane homogeneity. A close inspection also reveals the formation of domains, especially in the case of the racemic film. The underlying molecular structure of the films is also discussed. The combination of the SHG technique and optical microscopy provides a deeper insight into the film morphology. For the racemic film, differences in the local structure with respect to the enantiopure ones were shown and segregation in domains was observed.


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INTRODUCTION The role of chirality in the interaction between light and matter, both in naturel systems and photonic devices is a long-standing problem. It can be traced back to the XIXth century and the observation of Optical Rotation (OR) and Circular Dichroism (CD) in solutions of chiral molecules. One such famous example is the work conducted by Louis Pasteur on tartaric acid in 1848. OR and CD techniques have now become standard analytical tools in laboratories throughout the world and their use in molecular recognition1-3, asymmetric catalysis4-6 or enantioselective separation7-9 has been extensively discussed in the literature over the past few decades. During the last fifteen years, molecules belonging to the family of helicenes have attracted a great interest in this context due to their exceptional chiroptical activity provided by a helical molecular structure.10-12 For these molecular systems, the optical activities are reported in the literature as large specific OR.11 Chiral media formed by helicene-like molecules have therefore been designed, contributing to the development of new technologies. As such an example, optical planar waveguides or chiroptical waveguides as they are known have been proposed recently.13,14 For potential applications, it is however necessary to realize thin chiral molecular films. Different techniques are available like spin- or dip-coating and LangmuirBlodgett layer transfer. Pulsed Laser Ablation (PLD) has also been successfully proposed to make pure organic thin films because it does not require additional functionalization of the molecules. It allows organic chiral thin films to be grown without degradation too.11,15,16 The PLD technique benefits from the fact that it is instantaneous by nature. All heated material is ejected from the target and is quickly (i.e., faster than the racemization time) cooled down on the substrate. By controlling the laser fluence, PLD therefore allows the development of chiral thin films with high chiroptical properties and defined thickness.11,16 To characterize the optical


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properties of these thin films, OR, electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectroscopies can be used beside UV-visible absorption.11 It is also relevant to use nonlinear optics and more specifically Second Harmonic Generation (SHG) since the molecular chiral properties of helicene-like molecules based films inherently break the centrosymmetry. SHG has been indeed proven in the past to be an adequate tool to investigate the properties of molecular thin films down to the monolayer, including their chiral properties.12,17-20 The technique, based on the conversion of two photons at the fundamental frequency ω into one photon at the harmonic frequency 2ω, is intrinsically sensitive to the symmetry of the material under study.21,22 In the particular case of the chiral properties, the advantage over linear optical techniques relies on measurements performed on dark background. Indeed, whereas linear methods like CD require the determination of a small difference between two large signals, SHG entails the determination of chiral tensor elements that are null in absence of chirality. Like other optical methods, SHG is non-invasive and can be used to unravel both the molecular symmetry as well as the local molecular organization. 17,19 The objective of the present work is therefore to investigate the local structure of dibenzo[c]acridine thin films formed by the PLD technique. This molecule is characterized by a large specific OR. Considering its molecular structure, we can also expect a large nonlinear response. To quantify this response and to obtain information on the local structure, we employed the SHG technique in combination with CD measurements and optical microscopy. In the first part of this work, we show the differences in the non-linear response between the enantiopure films formed by either the (+)- or the (-)-enantiomer and the racemic film, that is a film produced by PLD from a target containing a racemic mixture of the (+)- and (-)enantiomers. Then, we focus the discussion on the local structure in these films. We show that,


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unlike the CD measurements, the SHG technique can bring out different molecular organizations in homochiral and racemic films. These differences are analyzed in terms of inhomogeneity and domain segregation, with a particular attention on the racemic films.

MATERIALS AND METHODS The structure of the dibenzo[c]acridine helicene-like molecular compounds used in this study is presented in Figure 1. The details of the chemical synthesis and the characterization can be found elsewhere.11

Figure 1: Structure of dibenzo[c]acridine helicene-like molecule The two (+)- and (-)-enantiomer and the racemic films were made by PLD16 using an ablating laser fluence of 20 mJ/cm2 and a Suprasil substrate located only a few centimeters away from the target. The (+)- or (-)-enantiopure films as well as the racemic one were obtained using a target formed by a compressed powder of the appropriate enantiomers or a racemic mixture according to the desired chiral properties of the film grown. Using a pulse-to-pulse deposition method, thin films with different thicknesses could be obtained. In the present study, films with thickness between 250 and 500 nm were used. The solid state UV-visible absorption and CD spectra of the films were recorded using a home-made setup based on Photo-Elastic Modulator and lock-in


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amplifier detection device.23 The beam dimension to probe the absorption and CD was approximatively (2x8) mm2. The nonlinear optical studies were then performed with a SHG setup already described in a previous work.19,20 Briefly, the fundamental beam generated by a femtosecond Ti-sapphire oscillator laser source consisting of 70 femtosecond pulses at a repetition rate of 80 MHz with a wavelength of 810 nm was focused with a 50 mm focal lens onto the sample film at an oblique incidence angle of 70°. In order to avoid sample damage, the average power of the laser was limited to 50 mW at most. The spot size is estimated to be approximately (5x10) µm2, significantly smaller than the spot size used in the CD measurements. The SHG light at 405 nm was collected in specular reflection, this configuration avoiding the problem of linear absorption in the films, especially at 405 nm. This fundamental input beam was linearly polarized and the input polarization angle γ was selected with a rotating half-wave plate. The angle γ=0 corresponds to a p-polarized fundamental beam whereas γ=90° corresponds to a s-polarized fundamental beam. An analyzer, placed in front of a spectrometer, was used to separate the S-, Pand 45-polarized SH intensities detected with a CCD camera placed at the exit of the spectrometer.

RESULTS AND DISCUSSION 1. Linear optical properties In a first step, the absorption and the CD spectra of the films were measured. CD spectra with good mirror-image relationships were obtained for (+)- and (-)-enantiomers based films, see Figure 2. No CD was detected for the racemic film although the UV-visible absorption spectrum was perfectly similar to those of the enantiopure films.


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By comparing the NMR spectra of re-dissolved films with those of the starting molecules, it was also checked that no molecular degradation occurred during the deposition process.11 Based on the strong similarity of the electronic and vibrational CD spectra of the deposited helicenelike molecules as thin films with those recorded for the corresponding solutions, the integrity of

Norm. Abs. (a.u.)

the chiral properties of the molecules in the deposited films was therefore confirmed.11

(+)-enantiomer film (-)-enantiomer film Racemic film

200

250

300

350

400

450

500

200

250

300

350

400

450

500

Norm. CD (a. u.)

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λ (nm)

Figure 2: Normalized UV-visible absorption and CD spectra of (+)-, (-)-enantiomers and racemic thin films obtained by PLD 2. SHG to probe the chirality of the film SHG measurements were then performed on these thin solid films. In order to bring out the chirality property, polarization resolved intensity were measured. First, we will focus the discussion on the S-Out polarization configuration. This choice is explained by the higher sensitivity to chirality of this configuration.19,20 S-Out polarization plots were obtained by


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rotating the input polarization angle from 0° to 360° and collecting the SH intensity for S-out polarization. These polarization plots were normalized by the maximum intensity in order to overcome differences in film thicknesses and are shown in Figure 3 for the three (+)-, (-)enantiomer and racemic films.

Figure 3: Normalized S-Out SHG intensity as a function of the input polarization for respectively A: (+)-enantiomer, B: (-)-enantiomer and C: racemic thin films. The solid lines correspond to the adjustments of the experimental data using equation Eq.(1). These intensity plots contain four maxima at 45°, 135°, 225° and 315° and can be analyzed in a first approximation by using the following expression of the S-Out intensity as a function of the input polarization angle γ: 19,20 ∝ | 2 |

(1)

where a and b are two coefficients. In the case of an achiral film, the b coefficient is vanishing and consequently, the S-out polarization plot has a four lobs pattern with equivalent intensities. This case corresponds to the racemic film, see Figure 3-C. At this stage, we can however observe small deviations with the fit at 135° and 315°. This will be discussed later. In the cases of the (+)- and (-)-enantiomer films, it is clearly necessary to introduce a non-vanishing b coefficient. For the (+)-enantiomer film, the S-polarization intensity is maximal for the incident polarization


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angles of 45° and 225° with lower values at 135° and 315° (Figure 3-A). The situation is reversed in the case of the (-)-enantiomer film (Figure 3-B). This result demonstrates the high sensitivity of the SHG technique to the chirality. 3. Structural information Then, to get further details about the molecular structure and orientation in the film, polarization resolved measurements were also performed for different output polarizations. As proceeded previously, these measurements were obtained by rotating the input polarization angle of the linearly polarized fundamental beam from 0 to 360° and collecting the SH intensity for three different output polarizations, namely the S-out, P-out and 45-out polarizations. These polarization plots, normalized by the p-in / P-out intensity, are shown in Figure 4 for the three (+)-, (-)-enantiomer and racemic films.


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Figure 4: Normalized SHG intensity as a function of the input polarization angle for the S- (red triangles), P- (blue circles) and 45- (green squares) output polarization configurations obtained for, respectively, the A: (+)-enantiomer, B: (-)-enantiomer and C: racemic thin films. The solid lines correspond to the adjustments of the experimental data using equations Eq.(2-a,b,c). For the racemic film, the chiral susceptibility tensor element was considered null. These plots were then analyzed introducing the susceptibility tensor elements and using the standard expression of the SHG intensity within the electric dipole approximation as a function of the input polarization angle γ19,20: ∝ sin 2 cos

(2a)

∝ ! cos " sin # sin 2 (2b) !" ∝ $

% ! & cos

" sin % # & sin 2

$

(2c)

The ai, i=1..7 parameters are coefficients depending on the linear optical indices of air and the thin films and the incidence angle. The optical refractive indices of the films were obtained by the m-line method23 and fixed throughout the analysis. We obtained these indices both at the fundamental and at the harmonic frequency ( ' ( 1.68 and ' ( 1.83).


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Only four non-vanishing and independent elements of the susceptibility tensor at the level of the electric dipole approximation are necessary to describe the polarization-resolved SHG intensity plots shown on Figure 4. Three components, namely ../ , /.. , and /// , are associated with an isotropic and achiral symmetry. The last component is the chiral element of the susceptibility tensor. The experiment was run under non-resonant SHG conditions since the electronic transitions occur at shorter wavelengths than the harmonic wavelength of 405 nm. Hence, these four tensor elements were considered real valued in a first stage, similarly to the optical indices. The adjustment procedure of the S- and P-out SHG intensity plots then led to the numerical values reported in Table 1 for the three thin films. Only relative magnitudes are obtained in this fitting procedure because no absolute measurements using a reference material were performed here. For this reason too, the absolute phase is unavailable in these experiments. The S- and P-polarized SHG intensity polar plots given in Figures 4 were adjusted remarkably well with Eq.2. The data are reported in Table 1. It is observed that the /// tensor element is the dominant one and was set to unity. It underlines the fact that the nonlinearity principally stems from the interface lying in the plane normal to the z laboratory axis, as expected. Indeed, the films are expected in plane isotropic and if the chiral properties are disregarded at first order, the breaking of the centrosymmetry occurs along the interface normal, the direction where one goes from air into the thin film across the interface. Table 1 Values of the different ratios of the susceptibility tensor elements extracted with Eq. 2 from the polarization plots presented in Figure 4 for the different thin films. 0112 /0222

0211 /0222

0132 /0222

(+)

4. 56 ± 4. 48

4. 9: ± 4. 48

4. 46 ± 4. 48

(-)

4. 5: ± 4. 48

4. ;9 ± 4. 48

−4. 4; ± 4. 48

4. :8 ± 4. 48

4. 9= ± 4. 48

-----------

racemic


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For the elements ../ , /.. , their relative values to /// are similar in the three cases, indicating similar structuration of the films. These tensor elements

and

are however not equal.

Although field gradients, namely contributions of order higher that the electric dipole one involving the discontinuity of the normal component of the electromagnetic field at the interface, can be invoked to explain this difference, it is also possible that it is due to the truly threedimensional molecular structure of the helicene-like molecules. In this case, the number of molecular hyperpolarizability tensor elements is not reduced to a single one and the passage from the molecular hyperpolarizability to the macroscopic susceptibility

is not straightforward.

It has been shown in the literature that it is possible to determine by molecular modeling calculations the dominant elements in the hyperpolarizability tensor > of helicene-like molecules.24,25 Depending on the size and/or the charge of the molecule, this dominant contribution can be radial or axial. 25 At this stage of our study, it is not possible to conclude on the nature of the dominant contribution. Also, the latter may strongly depend on the formation of supramolecular arrangements too. It is however possible, by reducing the non-linear response to the single dominant hyperpolarizability element defining the nature of the response, to extract the orientation of the molecular response axis. The orientational parameter D can be defined by the following expression:22 ? ( 〈 A〉 ( C

CDDD

DDD ECDFF

(3)

D was estimated to lie between 0.62 and 0.65, using the different values of the susceptibility tensor elements in Table 1. The minimum angle A between the normal to the interface and the

molecular response axis can then be evaluated to be A ( 37 ± 2 ° using a Dirac distribution. This model suffers from its simplicity as it cannot explain the breaking of the Kleinman symmetry described previously, namely the fact that ../ ≠ /.. . Then two cases can be devised.


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If the nature of the response is radial, then the main plane of the helicene makes an angle A ( 37 ± 2 ° with the surface normal. On the contrary, if the response is axial, this plane makes an angle J⁄2 − A ( 53 ± 2 ° with the surface normal. It is interesting to note that

columnar supramolecular arrangements will favor the axial nature of the response. In the fitting procedure, the intermediate 45° output harmonic polarization state was used to remove the sign ambiguity between the susceptibility tensor elements. However, in order to adjust correctly the 45-polarized SHG intensity plot, it was necessary to introduce a phase between the S- and the P-polarized SHG intensities. This feature clearly indicates that the linear polarization of the SHG intensity was not preserved during the frequency conversion process in these thin films. This behavior was observed for the three thin films but was not observed for the neat Suprasil substrate (data not shown). This elliptical polarization of the SHG harmonic beam has not been observed in previous studies related to chiral molecular monolayers deposited onto liquid interfaces.20,21 It is possible that this phase arises either from a linear birefringence or from a weak resonant contribution due to the proximity of the electronic resonance to the 405 nm harmonic wavelength. In this last case, the initial approximation where the susceptibility tensor elements are real cannot be taken strictly. Moreover, as already noticed in Figure 3, it is also interesting to observe in Figure 4 the deviation from the fourfold pattern of the S- polarized SHG intensity. In particular, the (+)- and (-)-enantiomer films exhibit an alternated pattern where truly isotropic achiral films would exhibit four equally intense peaks. For the (+)-enantiomer, the first and third peaks are more intense than the second and fourth ones whereas it is the reverse case for the (-)-enantiomer. It is straightforward to see from Eq.2 that this effect arises from the chiral susceptibility tensor element. As expected, there is a flip of the sign for this element between the two enantiomers,


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see Table 1. This difference is the signature of the Linear Dichroism in the SHG intensity (LDSHG), a phenomenon only observed in the non-linear response of chiral systems.25,26 In the case of the racemic film, the introduction of the chiral element is not necessary to adjust correctly the polarization plots, see Figure 4-C. However, as mentioned previously, the presence of this elements permit to improve the slightly lower SHG intensities at 135° and 315°. Nevertheless, this element is still very close to zero, within the error bars. 4. Heterogeneities and segregation in the film Different films with different thicknesses were prepared in this study. The polarization plots obtained for these different thin films were similar to those presented in Figure 4. Moreover, for each film, measurements were also performed at different positions of the laser spot in the films and similar polarization plots were obtained for the (+)- and (-)-enantiomer films revealing a remarkable homogeneity of the produced films. However, in some cases, the adjustment of the polarization plots was clearly more difficult. In Figure 5, we present polarization plots measured at two positions of the laser spot for another (-)-enantiomer film (namely film #2) with a thickness of 400 nm.


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Figure 5: Normalized SHG intensity as a function of the input polarization angle for the S(red triangles), P- (blue circles) and 45- (green squares) output polarization configurations obtained for a (-)-entantiomer film (film #2). These two plots were obtained for two positions of the laser spot on the film. It is clear in these figures that it was not possible in these conditions to adjust correctly the polarization plots with Eq.2 and consequently extract the values of the susceptibility tensor elements. Moreover, the shape of the S- P- and 45-polarized SHG intensity polar plots in Figure 5 for two distinct areas of the (-)-enantiomer film depend strongly on the position of the laser spot, demonstrating a strong inhomogeneity in this film. This striking feature clearly shows that this (-)-enantiomer film possesses a molecular organization dramatically different from that of the other films. In order to relate this local analysis of the structure and the chirality of the films to macroscopic features, linear optical microscopy images were recorded using white light. Optical microscopy images were recorded for different films.


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Figure 6: Microscopy images of A: (+)-enantiomer thin film, B: (-)-enantiomer thin film #2 and C: racemic thin films The image in Figure 6-A corresponds to the (+)-enantiomer film for which the polarization plot was presented in Figure 4-A. A similar image was obtained in the case of the (-)-enantiomer film presented in Figure 4-B. The image in Figure 6-B is related to the (-)-enantiomer film (film #2) with the corresponding polarization plot presented in Figure 5. And finally, the Figure 6-C is the image of the racemic film. These images clearly show that the three films tend to break up into domains of tens of microns size. The (+)- and the (-)-enantiomer films were the least


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susceptible to crystallization whereas the racemic one seems highly metastable and crystallizes extensively as shown in Figure 6-C. However, if some structural changes (micro-crystallization, segregation, degradation, …) occur in the enantiopure films, as it seems it is the case of the (-)enantiomer film #2 where the crystallization is intermediate, then the consequence on the polarization plots is dramatic as presented in Figure 5. For this film, no significant effects were observed in the CD spectra, at least at the beginning of the structural changes alteration process. This shows the high sensitivity of the SHG technique to detect modifications in the molecular film. This high sensitivity is also related to the smaller dimensions of the probing area (5x10) µm2 compared to the millimeter size of this area for the CD. Surprisingly, the presence of large crystallization domains in the racemic film seems to have small effects on the polarization plots, see Figure 4-C. The profile of the polarization plots is conserved for this film. In order to obtain more information on the nature of these domains, we performed polarization measurements at different positions of the laser spot in the racemic film. S-out polarization plots were then recorded at different areas of the racemic film. Three polarization plots are presented in Figure 7.

Figure 7: Normalized S-Out SHG intensity as a function of the input polarization for the racemic thin films and for different areas in the film. The solid lines correspond to the adjustments of the experimental data using equation Eq. 2-a.


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The Figure 7 shows that the deviation from the fourfold pattern of the S- polarized SHG intensity depends also very strongly on the position of the laser spot onto the racemic film. This indicates a structure more complicated than just a simple random distribution of the two enantiomers at the molecular level. Such a random organization at the molecular level would indeed prevent the appearance of any chiral contribution in the SHG intensity. A possible picture is that, rather, domains of specific chirality have formed and that under the laser beam, there is an excess of domains with the same chirality sign as in the pure (+)- or (-)-enantiomer films. Such a segregation in molecular monolayer composed by chiral molecules has already been reported in the literature.26 However, in this experiment, we cannot neglect the fact that anisotropy can contribute the non-vanishing values of the chiral elements.

27

Hence, another

possible picture is that the presence of anisotropic domains occurs in the film. Considering the PLD fabrication method for the films, this possibility remains unlikely. Experiments are currently performed in the laboratory in order to distinguish chiral contributions from in-plane anisotropy in the SHG intensity as demonstrated in the literature. 27,28

CONCLUSION In conclusion, we have shown above that dibenzo[c]acridine helicene-like thin films could be formed by PLD with a well-defined chiral property. The characterization of their quadratic nonlinear optical properties was performed by SHG in reflection. The chiral properties of the films are exhibited through the electric dipole susceptibility tensor element which changes sign between the two (+)- and (-)-enantiomer films, as expected. An analysis of the SHG intensity as a function of the input and output polarization configuration was realized in order to obtain structural information. This work revealed differences in the molecular organization between homochiral and racemic films. The SHG features together with microscopy images


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showed a remarkable in-plane homogeneity of the (+)- and (-)- enantiomer films. However, in some cases, domains of crystallization of these films were observed, the SHG technique being particularly efficient to detect this film aging process as compared to CD measurements. Surprisingly, the racemic film shows a different crystallization than the enantiopure ones, revealing that molecular domains with different chirality or anisotropy appear in the films.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial support of the French National Agency for Research under project DYNACHIR ANR12-BS04-0018-01. Notes Any additional relevant notes should be placed here.


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ACKNOWLEDGMENT The authors thank the Centre for Nano-Optics (NanOpTec) of the University Claude Bernard Lyon 1 and the financial support of the French National Agency for Research under project DYNACHIR ANR-12-BS04-0018-01.

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Intermixing of Chirality and Local Structure in the Second Harmonic

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