Full Polarization Control of Optical Planar Waveguides with Chiral

Article Cite This: ACS Photonics XXXX, XXX, XXX-XXX

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Full Polarization Control of Optical Planar Waveguides with Chiral Material Stéphan Guy,*,† Bruno Baguenard,† Amina Bensalah-Ledoux,† Dalila Hadiouche,‡ and Laure Guy§ †

Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, UMR 5306, Institut Lumière Matière, F-69622, Lyon, France Laboratoire de Génie de l’Environnement, Université A. Mira-Bejaïa, Route de Targa Ouzemour, 06000 Béjaïa, Algeria § Univ. Lyon, ENS Lyon, CNRS, Université Claude Bernard Lyon 1, UMR 5182, Laboratoire de Chimie, F69342, Lyon, France ‡

S Supporting Information *

ABSTRACT: Circularly polarized (CP) light is attracting growing interest in photonics, however it is not possible to use regular planar waveguides for the transmission of such CP light. While keeping the planar geometry, we conceived devices where the chirality of the propagation medium overcomes the planar symmetry. Thus, we report on the fabrication of chirowaveguides arising from the stacking of three layers of a new hybrid chiral organic modified silica (OrMoSil). A flexible strategy allows the control of the two main parameters impacting the ellipticity of the propagated waves. First, the high chirality of the transparent material is based on cheap and easy to access binaphthyl precursors simply shaped as films by dip-coating. Second, the refractive index (RI) contrast between the layers is finely tuned by TriEthOxySilane (TEOS) doping. The polarization of the two fundamental guided modes is measured on 2 cm long waveguides. We demonstrated that the polarization can be modulated from linear to nearly circular (80% ellipticity) depending on the RI contrast and the core thickness. These unprecedented achievements in the area of both optical materials and guided optics, open the way to fully integrated photonic devices dealing with CP light propagation. KEYWORDS: chirality, circular polarization, planar waveguides, TE/TM polarization, integrated photonics

C

been achieved by sophisticated techniques leading to a tilt of the planar geometry around the propagation axis.6,7 Waveguides can also be polarization independent, at least from a theoretical point of view, if phase matching between the TE and TM modes occurs. Numerous attempts based on anisotropic material or specially designed ridged waveguides11−14 have been reported aiming mainly to apply such waveguides as Faraday isolators. Nevertheless, these phase matching techniques are tedious since they require a highly accurate control of the waveguide structures over the whole propagation length. Consequently, alternative solutions without full phase-matching between the TE and TM modes are now investigated.15 Electromagnetic chirality is another way to break the planar invariance. In chiral media the constitutive equations write in the Drude-Born-Fedorov form:16

ircularly polarized (CP) light is the central information vector in many photonic technologies, including chiral biosensing, 3D-displays, or quantum optics.1−3 In “free-space” nonbirefringent bulky optical media, any polarization can propagate without deformation. Optical instruments using CP light are currently commercially available, such as circular dichroism spectrometers. Integrated version of those CP light optical devices would offer improved compactness and robustness compared to their free-space counterparts. In recent years, an integrated version of optical tools manipulating all the polarizations have been demonstrated.4−9 However, no CP guided-light connection between them is possible since the propagation of non linearly polarized guided light in planar waveguide is fundamentally forbidden. Indeed, in waveguides featuring a planar geometry, the polarization of propagated modes is ruled by the device’s symmetry resulting in two linearly polarized optical modes, the well-known TE and TM modes, schematized in Figure 1a and described in fundamental books.10 For channel waveguides, modes are hybrid in general, but practical realizations also deal with quasi TE and quasi TM modes. To free planar optics from the linear TE/TM polarizations requires to break the planar symmetry in three dimensions. Optical waveguides with non TE/TM linear polarizations have © XXXX American Chemical Society

D = ϵ(E + γ ∇ × E), B = μ(H + γ ∇ × H)

(1)

where ϵ and μ are the permittivity and the permeability and γ is the chirality parameter. γ is related to the circular birefringence (CB) and optical rotation (OR) by CB = 2k0n2γ and Received: August 30, 2017 Published: October 13, 2017 A

DOI: 10.1021/acsphotonics.7b00975 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the transverse polarizations in achiral (a) and chiral (b) planar waveguides.

Figure 2. Numerically calculated mode properties for symmetric planar chirowaveguides. Core RI is 1.633 and OR = 2°/mm. Working wavelength is λ = 633 nm. ΔRI is the refractive index mismatch between the guiding core and the cladding layers. (a) Transverse polarization ellipses as a function of the core thickness d and the refractive index mismatch ΔRI. (b) Profile of the electric field components for d = 1.5 μm and ΔRI = 5 × 10−3.

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k CB

OR = 02 , where k0 is the wavenumber in free space and n is the average refractive index. Chirowaveguides, as named by Engheta and Pellet in 1989,17 are waveguides with an isotropic chiral core. While the longitudinal (z) component of E and H are uncoupled in achiral waveguides, the ∇ × terms in eq 1 couple together these two fields in chirowaveguides. Thus, in chirowaveguides, neither TE nor TM modes can exist, and the modes feature transverse elliptical polarizations as schematized in Figure 2c.18−20 However, the transition from achiral linearly polarized modes to significant elliptically polarized modes requires extra conditions. Indeed, chirowaveguides are devices combining both CB via the core material and modal linear birefringence (LB) which denotes the difference between the effective indices of TE and TM modes. 10 The eigenpolarizations in chirowaveguides result then from the competition between these two birefringences leading to elliptical birefringence in general. But, as generally LB ≫ CB, significant results are difficult to obtain. Only two groups have published experimental results with a very small effect21 or without a systematical study of the device parameters.22 Quasi circular eigenpolarizations require to reduce the LB and increase CB. CB obviously increases with OR, and special materials need to be designed. LB can also be reduced by modulating the optogeometric parameters of the waveguides. It is well-known that the modal birefringence lowers with the guiding core thickness d. Moreover, it is bounded by the refractive index difference ΔRI between the guiding core and the surounding layers.10 Thus, the lower ΔRI, the lower LB. Our strategy was then to design chirowaveguides with small ΔRI combined with high core thickness in order to enhance the chiral behavior of the devices.

RESULTS AND DISCUSSION

Simulation, Device Requirement. For a given waveguide structure, the ellipticity of the propagating modes, defined as the ratio of the minor to major axes of the polarization ellipse, increases with OR.20,25 Therefore, we have first developed a transparent chiral material with optical rotation OR = 2°/mm at λ = 633 nm, 100× higher than the one of natural chiral molecules like sugars.23,24 The effect of the wavelength on material properties, here optical rotary dispersion and absorption, shows that OR and absorption decrease with the wavelength. Thus, short wavelengths are required for high chirality while long wavelengths are required for non lossy propagations. Based on our recorded optical rotary dispersion and absorption spectra,23 we choose to work in the red at the wavelength of HeNe laser (λ = 633 nm) as a compromise between significant OR and low losses. We have then computed the electromagnetic field in a planar chirowaveguide constituted of a high refractive index chiral material with OR = 2°/mm sandwiched between two achiral material of lower refractive index. We employed the Bohren’s decomposition and applied the boundary conditions at the interfaces as described in ref.20,25 The map of Figure 2a displays the calculated polarization ellipses of the transverse electric field for step-index planar chirowaveguides versus ΔRI and d. This map shows that the polarization of the guided modes can be tailored from purely achiral (linearly polarized, top left) to fully chiral (circularly polarized, bottom right) via any elliptically polarized states by modulating the core thickness d and ΔRI. More quantitatively, this simulation points out that our material, featuring OR about 2°/mm, requires refractive contrasts lower than ΔRI ≤ 1 × 10−2 to generate ellipticities higher than 0.1. B


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Figure 3. (a) Two enantiomers of the chiral molecular sol gel precursor. (b) Multilayer deposition: substrates (step 1) and upper claddings (step 3) are dip-coated from racemic sols doped with TEOS; cores (step 2) are dip-coated from enantiomerically pure sols doped with a lesser amount of TEOS.

Figure 4. Schematic of the experimental apparatus: A laser beam at a wavelength of 633 nm is sent through a polarizer (P1) followed by a BabinetSoleil compensator (BS). This beam is elliptically polarized with main axis in the x/y direction. The beam is launched into the chirowaveguide with the objective O1. The transmitted light is collected by the second objective (O2). A camera combined with a Stokes analyzer (quarter wave-plate QWP and polarizer P2) is used to image the propagating mode and to measure the output SOP.

three layers, achiral for the substrate and the upper cladding, chiral for the core, on a silicon wafer (Figure 3b). Prior to deposition, the silicon wafers substrates are immersed in a freshly prepared Piranha solution for 10 min then rinsed with filtrated water and methanol in order to wash and activate the surface for a good adhesion. Between each sol gel deposition, the annealed surface is washed with methanol to prepare the next deposition. The substrates and upper claddings are optically thick layers (>15 μm) deposited via dip-coating from highly concentrated racemic sols made from (±)-RSi(OEt)3 and doped with a larger amount of TEOS than the core layer, to lower their refractive indexes. Chiral cores are made from enantiopure sols with a low amount of TEOS. The typical annealing time is 72 h at 120 °C, a compromise between racemisation and material solidification. Finally, the multilayers assembly is cleaved (no polishing is necessary) to give ready-touse 2 × 5 cm2 waveguides. Light (λ = 633 nm) was launched and collected with microscope objectives. The measurements of the propagation losses with the scattered light technique were below the detection threshold, meaning that they are lower than 1 dB/cm.26 Due to the nonoptimized coupling and decoupling, the overall transmission of 2 cm long devices, varies between 20% and 30%.

For a typical chirowaveguide with an ellipticity of 0.22 (ΔRI = 5 × 10−3, d = 1.5 μm, OR = 2°/mm), we draw the electric field components profile in Figure 2b. It shows that the mode is hybrid and while the main x component is real, the y component is purely imaginary. The corresponding transverse polarization is then elliptical with x,y as main axes. Moreover, the electric field is poorly confined with 61% of the energy in the guiding core and the cutoff wavelength for multimode operation is calculated at λc = 382 nm. Films Making. Taking these features into account, we have developed a chiral hybrid silica glass easy to coat as thin film with a good control of its refractive index.23,24 For this, we conceived a chiral OrMoSil precursor possessing a bridged binaphtyl substituent linked to a polymerizable triethoxysilane group (+)-RSi(OEt)3 or (−)-RSi(OEt)3 presented in Figure 3a. While this precursor bears one chiral unit per silicon, it ends up, after hydrolysis/condensation, in a glass highly concentrated in chiral units (Figure 3b). Moreover, adding tetra-ethoxysilane TEOS in the deposition sol, strengthens the silica network and modulates the refractive index which is proportional to the relative ratio %TEOS of the number of TEOS over the chiral precursor molecules.24 Based on this method, planar chirowaveguides were successfully realized by a successive coating and annealing of C


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Figure 5. Intensity map of the guided mode imaged on the camera for the different samples. The intensity profile is plotted on the right of each map with the diameter measured at 1/e2 of the intensity maximum.

Figure 6. Poincaré sphere representation of the input (blue) and output (yellow) SOP before and after the chirowaveguide propagation. Fitted SOPs (black) correspond to the rotation transformation visualized with red circular arcs. The two eigenpolarization ellipses are indicated in inset for each sphere: (a) sample 1, high RI contrast results in an achiral propagation; (b, c) samples 2 and 3, similar thickness and RI but opposite core handedness results in modes with opposite handedness; (d−f) samples 4−6, low RI contrast and high thickness results in higher eigenmode ellipticities.

I135), and right circularly polarized preference (IRC−ILC) of light, respectively.27 The optical setup is schematized in Figure 4. It was built to scan the S2 = 0 meridian SOP at the input of the waveguides and to record the output SOP after the chirowaveguide interaction. The association of the polarizer P1 with main axis horizontal (x axis in Figure 3) and a Babinet−Soleil

Optical Setup. To fully characterize the polarization of the guided modes, we measured the state of polarization (SOP) of the output light when launching several different input SOPs. SOP are measured in term of Stokes vectors S0.3, where S0, S1, S2, and S3 denote total intensity, linearly polarized horizontal preference (I0−I90), linearly polarized plus 45° preference (I45− D


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Table 1. Material Properties are Given in the First Three Rowsa

a

Sample 1 has no upper cladding. RI contrast and core thickness are deduced from the optical properties of the guided mode (S3 and mode thickness). dcut‑off is the cut-off thickness for multimode propagation. Optical characterizations of the waveguides are given in the last three rows: guided mode’s waist diameter, the S3 Stokes parameter of the elliptical mode stretched along the TE direction (S1 > 0) and the corresponding polarization ellipses are drawn in the last row. Uncertainty on the S3 parameter, measured at different positions on a given sample, is about 0.05. Leftand right-handed ellipses are blue and red, respectively.

antipodal points.) Moreover, it could be clearly seen that the chirowaveguides cause a rotation of the input SOPs along the diameter axis defined by the polarization modes. This behavior is the signature of an elliptically birefringent device with two elliptically polarized modes (main axes along the x(TE)− y(TM) axes) represented by the intersection of the rotation axis with the sphere.27 For multimodal propagation, not shown here, we obtain more complicated features: TE/TM modes are not eigenpolarization either, but the trajectories on the Poincaré sphere do not follow simple circles and are difficult to interpret. Here, we will restrict our considerations to waveguides that are conventionally termed single-moded, i.e. only supporting two mutually orthogonally polarized, single-lobed guided modes and corresponding to simple transform on the Poincaré sphere. Impact of the Waveguide Parameters. In the following, we study the influence of the crucial fabrication parameters of the chirowaveguides. For this, we have elaborated the series of chirowaveguides presented in Table 1, which differ in their handedness and the layers compositions. The measured optical properties (SOP characterized by its S3 parameter and mode diameter 2ω), and samples characteristics (core thickness and ΔRI) for the differently elaborated waveguides are gathered in Table 1. All the waveguides are monomode, and the calculated cutoff thicknesses are given in the table. We use our chirowaveguide numerical simulations via a 2D fitting procedure, to extract the core thickness d and RI contrast ΔRI from the measured mode thickness and ellipticity. This measurement is of particular interest to control our devices while the structural characterization via other techniques is nearly impossible. Indeed, the three stacked layers are very similar one to the other’s in terms of composition and refractive index values. Hence, the optical images of the cleaved face did not show any interfaces, neither did the scanning electron microscopy nor the ellipsometry measurements that could not differentiate the small RI difference between the core layer, deeply buried in the structure, and the claddings. Additionally, m-line spectroscopy setup for RI and thickness measurements on single layer is not accurate for these soft organic films exhibiting deformation under the prism pressure. For the chirowaveguides 1−4, the guiding core has been annealed for 96 h, resulting in an OR of + or −2°/mm, depending on the enantiomer. For the two last samples 5 and 6, the upper cladding has been annealed for 72 h, that is, 48 h

compensator (BS) at 45° polarizes the incident light (HeNe laser λ = 633 nm) into an elliptically polarized light with the main axis horizontal corresponding to the S2 = 0 meridian on the Poincaré sphere. By adjusting the BS retardation, all the Stokes vectors (1, 1 − S33 , 0, S3) with S3 ∈ [−1:1] can be generated. The light is coupled/decoupled into the waveguide with different microscope objectives O1 (numerical aperture 0.18) and O2 (numerical aperture 0.4), respectively. The numerical aperture is optimized to maximize the light on the camera. The outcoming light was imaged on a 14 bit camera (Stingray 146B) 102 cm away from the O2 decoupling objective. A rotating quarter-wave plate (QWP) and a linear polarizer P2 are introduced into the pathway of the light, to measure the SOP after the waveguide. Mode Profile. The camera images allow to identify the guided modes via the intensity profile for the different samples (Figure 5). The fringes visible in the x direction come from some interferences in the Glan analyzer P2 in Figure 4. The profile of the guided modes shows a main lobe of 2ω width (taken at 1/e2 of the maximum). The magnification of the decoupling objective is calibrated by imaging Gaussian waist produced by different lenses onto the camera. Lower lobes with smaller intensity correspond to cladding modes propagating in the core and in the cladding layers. By measuring the surface area, we found that less than 5% of the energy is propagating in these cladding modes. Polarization Measurements. Our measurements show that the polarization rate after chirowaveguide interaction is higher than 0.95, that is S0 ∼ S12 + S22 + S32 . We will neglect the depolarization effect in the rest of this paper and represent the SOPs in terms of normalized Stokes parameters S1.3 on the relevant Poincaré sphere.27 This depolarization effect reflects the quality of the waveguides in term of homogeneities. Indeed, random scattering of light due to layer roughness or RI variations induces a loss of the polarizations along the propagation.28,29 The Poincaré sphere representation of the input/output SOPs is pictured in Figure 6 for samples 1−6 (3D-animated images are provided in the SM for all the samples). First, TE/ TM polarizations are not conserved during the propagation. Second, the polarization modes, found when input and output polarization coincide, are included in the S2 = 0 plane, but they intercept the sphere at different latitudes and are orthogonal. (On the Poincaré sphere, orthogonal states are represented by E


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CONCLUSION Our results, by combining highly chiral material and controlled ΔRI, constitute the first proof of concept for the simple fabrication of chirowaveguides, allowing the propagation of any polarization from elliptical to nearly circular, along a few centimeters with losses lower than 1 dB/cm. The combination of low ΔRI and high core thickness enhances the chiral nature of the devices. The approach applied here is a promising beginning for the use of molecular chiral material as optical devices. While being based on the flexible sol gel chemistry and the simple dip-coating process, we envision developments such as channel waveguides made by lithography techniques, or photonic chips that manipulate photon polarizations.

more than the other waveguides, which results in a reduction by a factor 2 of the OR of the core. Waveguide 1 does not have an upper cladding, which makes it an unsymmetrical waveguide where the contrast index between the core and the air, ΔRI = 0.63, is the highest value of the series. It propagates optical mode with thickness 2ω = 1.9 μm. Polarization results, reported on the Poincaré sphere are displayed on Figure 6a). Blue points represent the different polarizations launched into the waveguide. The polarizations measured at the output are represented by the yellow points. A red arrow connects each polarization couple (blue point to yellow point). For the TE input polarization (the blue point on the equator in the front of the view), the output is also TE: the two points merge on the sphere. Input TM also leads to output TM (at the back of the sphere). It means that linear TE and TM polarizations are eigenmodes. For the other input polarizations in blue, the measured output polarizations in yellow correspond to a rotation around the TE/TM axes. This is the signature of the beating between two modes of perpendicular polarizations (here TE and TM).27 Finally, this waveguide behaves as a regular nonchiral monomode waveguide with linear TE/TM eigenmodes. This example clearly highlights that not only the chirality of the core is necessary but also that the ΔRI is a key parameter to optimize for the propagation of elliptically polarized waves in planar waveguides. Waveguide 2 has been designed similarly as 3, except for the handedness of the chiral core; for 2, a left-handed sol has been used instead of a right-handed one for 3. Both guides exhibit similar properties in terms of ΔRI and d. The Poincaré representations show that neither TE nor TM are conserved after the waveguides propagation. The position where the blue and yellow points are superimposed corresponds to the eigenmodes. They are located out of the equatorial plane S2 = 0, S3 = ±0.19 and S2 = 0, S3 = ±0.25 for 2 and 3, respectively. These SOP are pointed out on the figure by their polarization ellipses. The eigenpolarizations define an axis drawn in red on the figure. All the output polarizations can be deduced from the corresponding input by a rotation thru this axis. This means that from a polarization point of view the waveguides are elliptical birefringent devices. These two chirowaveguides have almost the same mode ellipticity but opposite handedness as expected for two enantiomeric chirowaveguides. For samples 4−6, we have decreased the difference in TEOS codoping between the core and the cladding layers and adjusted the withdrawing speed during the dip coating process in order to increase the guiding layer tickness. The Poincaré representations, Figure 6d−f, clearly show that the chirowaveguides transform the input polarization according to rotations around axes out from the equatorial plane. This allows to unambiguously determine the polarization of the guided modes as elliptically polarized modes defined by the rotation axes. 5 and 6 waveguides present ΔRI of 3.3 × 10−3 and 2.3 × 10−3, respectively, and mode thickness similar to sample 4. For these samples, although the OR is halved, the low ΔRI allied with high mode thickness result in significant values of the S3 parameter, −0.46 and −0.8 for 5 and 6, respectively. The highest ellipticity of this study has been obtained with sample 4. This waveguide combines a low ΔRI = 2 × 10−3, the lowest of the series, with a high core thickness (1.62 μm) and OR = 2°/ mm. It allows the successful propagation of an elliptically polarized wave (2ω = 5.5 μm) with an S3 parameter close to 1 and an ellipticity of 80% .

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00975. Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 1. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG). Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 2. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG). Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 3. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG). Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 4. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG). Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 5. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG). Animation movies showing a 3D view of the measured polarization transformation induced by the chirowaveguides onto the Poincaré sphere for sample 6. Input polarizations are the blue points, output polarizations are the yellow points, and the fitted output polarizations are the black points (MPG).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stéphan Guy: 0000-0003-2145-0097 F


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Notes

(23) Guy, S.; Stoita-Crisan, A.; Bensalah-Ledoux, A.; Vautey, T.; Guy, L. Sol-gel chirowaveguides investigated via MLine technique. Opt. Mater. 2011, 34, 347−350. (24) Hadiouche, D.; Luyer, C. L.; Guy, L.; Bensalah-Ledoux, A.; Saoudi, S.; Khireddine, H.; Guy, S. Optimization of optical properties of high chiral planar waveguides obtained from a non-aqueous sol gel method. Opt. Mater. 2014, 36, 885−891. (25) Yokota, M.; Yamanaka, Y. Dispersion Relation and Field Distribution for a Chiral Slab Waveguide. IJMOT 2006, 1, 623. (26) Okamura, Y.; Yoshinaka, S.; Yamamoto, S. Measuring mode propagation losses of integrated optical waveguides: a simple method. Appl. Opt. 1983, 22, 3892−3894. (27) Huard, S. Polarization of Light; Wiley, 1997. (28) Kovalenko, A. V.; Kurashov, V. N.; Deriougina, N. I. Light depolarization in optical waveguides with irregular boundaries of dielectric layers. Proc. SPIE 1994, 2265, 364−373. (29) Garmire, E.; Honda, K. Depolarization in channel glass waveguides. J. Lightwave Technol. 1986, 4, 220−227.

The authors declare no competing financial interest.

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Full Polarization Control of Optical Planar Waveguides with Chiral

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