Abstract
Optical singularities manifesting at the center of vector vortex beams are unstable, since their topological charge is higher than the lowest value permitted by Maxwell’s equations. Inspired by conceptually similar phenomena occurring in the polarization pattern characterizing the skylight, we show how perturbations that break the symmetry of radially symmetric vector beams lead to the formation of a pair of fundamental and stable singularities, i.e. points of circular polarization. We prepare a superposition of a radial (or azimuthal) vector beam and a uniformly linearly polarized Gaussian beam; by varying the amplitudes of the two fields, we control the formation of pairs of these singular points and their spatial separation. We complete this study by applying the same analysis to vector vortex beams with higher topological charges, and by investigating the features that arise when increasing the intensity of the Gaussian term. Our results can find application in the context of singularimetry, where weak fields are measured by considering them as perturbations of unstable optical beams.
Introduction
Light beams showing an inhomogeneous polarization distribution, commonly referred to as vector beams (VBs), represent a precious resource in an increasing number of photonic applications^{1}: astronomy^{2}, microscopy^{3,4}, optomechanics^{5,6}, materials structuring^{7}, nanophotonics^{8,9} and quantum sciences^{10,11,12,13} are some remarkable examples. Uniformly polarized beams can be easily converted into such spatially structured fields by coupling the vectorial and the spatial degrees of freedom of light^{1,14}, as recently demonstrated in a variety of photonic architectures^{15,16,17,18,19,20,21,22,23,24}. The fine structure of VBs polarization may show several typologies of singular points^{25,26,27,28,29}, in close analogy to other inhomogeneous systems (fingerprint, tidal heights across the oceans, etc.). Here we consider those spatial regions where there is no preferred direction for the oscillations of the electric field^{25,26,29}, with the most relevant case being represented by the so called Cpoints, that is points where the polarization is circular. Their formation and dynamical evolution have been investigated in the complex polarization pattern characterizing several structured fields, such as for instance speckle fields^{30}, random superposition of vector waves^{26}, light passing through inhomogeneous anisotropic media^{18,31}, photonic crystals^{32,33}. Independently of the specific system, the electric field around a polarization singularity is oriented according to the value of the associated topological charge η; this is an integer or semiinteger number, defined as the angle described by the major axis of the local polarization ellipse (divided by 2π) when following a closed path around the Cpoint. Besides its connection with the surrounding polarization distribution^{34}, the value of this charge is particularly important in determining the singularity robustness, since only the lowest order Cpoints with η = ±1/2 are stable with respect to small deformations of the optical system^{25,33,35,36,37}. This is analogous to the case of highorder optical vortices in scalar fields, which have been observed to split into elementary vortices as soon as a tiny perturbation is introduced^{38,39,40,41,42,43}. The instability of higherorder polarization singularities, with the role of Cpoints played by points of unpolarized light, can be beautifully observed in the skylight polarization pattern, where, differently from the case of Cpoints, such singularities are loci where the light is fully unpolarized; in the sky, the original two singular points (for which η = 1), positioned at the Sun and the antiSun loci, split into four slightly displaced lowestorder singularities (with η = 1/2) because of the contribution from multiple Rayleigh scattering of sunlight in the atmosphere^{44}. Inspired by these phenomena, but in the context of fully polarized laser light, here we investigate the formation of lowest order Cpoints at the center of vector vortex beams (VVBs)^{45,46} in presence of a weak perturbing field. VVBs are a particular class of vector beams (radially and azimuthally polarized beams are remarkable examples) for which a polarization singularity and an optical vortex (phase singularity) are superimposed at the center of the beam. This happens because the spatial mode associated with each of the two opposite circular polarizations is that of a light helical mode. These peculiar optical spatial modes, described in terms of an integer number m, show a helical wavefront and carry a definite amount of orbital angular momentum (OAM)^{47}, equal to per photon. The field amplitude vanishes at the center of these beams, where the associated phase is not defined and an optical vortex with topological charge m appears. Since left and right circular components are both helical modes, in VVBs the total field is vanishing along the beam axis and its orientation is undefined. This peculiar polarization singularity is typically referred to as Vpoint^{36}; unlike the case of a Cpoint, here the instantaneous oscillation direction of the electric field is undefined (at any time). The lowest topological charge admitted for Vpoints is ±1, since these are singularities of a field of vectors (the instantaneous electric field), whereas Cpoints refer to a field of ellipses (the trajectory described by the vector in a temporal cycle). Here we show that a small perturbation changes the nature of the vector field characterizing pure VVBs, whose local polarization states acquire a tiny ellipticity. Since in such a field Maxwell’s equations allows for polarization singularities with a lower charge (Cpoints), even the lowest order Vpoint becomes unstable and unfolds into a pair of equally charged Cpoints^{36}. We investigate experimentally this mechanism by perturbing a radial and azimuthal VVB (η = 1) with a uniformly polarized beam, and complete the analysis with an example of higher order VVB (η = 2). This kind of perturbation acts as a coherent background, whose role has been investigated in the decay of optical vortices at the center of beams carrying OAM^{42}. Recently a similar study of Vpoint unfolding was proposed theoretically in ref. 48, although the analysis is focused on Vpoint and Cpoint dynamics during the beam propagation, rather than the instability of Vpoints. By controlling the amplitude of the two fields, we report the progressive formation of Cpoints (that originate from the central Vpoint), whose separation increases as the Gaussian term gets higher. Importantly, the polarization pattern modification is always accompanied by a deformation of the original intensity pattern. Interesting features arise when increasing the intensity of the perturbing term, in particular when this becomes equal or higher than the original VVB and the polarization pattern may lose its nontrivial topological features (at least in the region where almost all the field energy is enclosed).
Results
Vpoint instability at the center of a vector vortex beam
A VVB corresponds to the superposition of two (or more) different helical modes of light associated with orthogonal circular polarizations. Denoting as L〉 and R〉 states of left and right circular polarizations, respectively, a VVB can be written as
where c_{L} ≠ 0, c_{R} ≠ 0 are complex coefficients and the complex amplitude f(r, z)e^{imϕ} describes the field associated with a helical mode of order m, expressed in terms of the cylindrical coordinates (r, ϕ, z) with the z axis corresponding to the optical axis of the beam. While the phase factor e^{imϕ} gives rise to the typical helical wavefront, the function f(r, z) describes the radial distribution of the field. Here we have considered the simple case of two helical modes with opposite m; similar results hold for all other VVBs. Except the case m = 0, helical modes vanish along the optical axis (r = 0) where they show an optical vortex with charge m^{43}. Accordingly, in the case of an ideal VVB, a Vpoint with charge η = m is present at the center of the beam, at any plane transverse to the propagation direction. In order to show a possible mechanism that leads to the Vpoint transformation into lowest order singularities, let us consider the specific case m = 1 and c_{L} = c_{R} = 1, corresponding to a radially polarized beam. At a fixed transverse plane and very close to the beam center, that is at r much smaller than the typical beam dimensions, Eq. 1 has a simpler expression:
where A is a real constant defining the field intensity. We add to Eq. 2 a linearly polarized term with uniform amplitude ( and are real constants), whose electric field is oriented at an angle θ with respect to the horizontal direction. In the representation of circular polarizations, this perturbation can be written as ; when added to the original VVB, Eq. 2 becomes
Left and right Cpoints are located at positions (r_{L}, ϕ_{L}) and (r_{R}, ϕ_{R}) where the right and left circular components of the field are vanishing, respectively. It is straightforward to see that
In general ϕ_{L} ≠ ϕ_{R} and two Cpoints with opposite handedness but with equal charge η = 1/2 generate from the original Vpoint. Using the same approach it is possible to show that Vpoints of order m unfold into 2m Cpoints, with the sign of their charge being equal to that of the original singularity.
Generation and perturbation of a VVB using electrically tunable qplates
The results we discussed in the previous section can be easily simulated experimentally by exploiting the same approach reported in ref. 19 for the generation of pure VVBs. The preparation and the controlled alteration of a VVB is obtained by tuning the spinorbit interaction of a light beam in a qplate^{49,50}. This is an optical element made of a thin layer of liquid crystals, whose optic axes are arranged so as to form a singular inhomogeneous pattern. Besides the topological charge q, defining the rotation of the local liquid crystal axis around the singular point (divided by 2π), the action of this device is determined by its optical retardation δ. The value of the latter can be suitably adjusted by applying an external electric field, which allows for controlling the strength of the spinorbit interaction mediated by the plate^{51}. When passing through a qplate placed at the beam waist (this is the standard configuration that we adopt throughout the manuscript), a TEM_{00} Gaussian beam with uniform left or right circular polarization is transformed as follows^{52}:
where α_{0} is the angle of the liquid crystals optic axis at ϕ = 0. The previous equation shows that left and right circular polarizations are partially converted into helical modes of order ±2q, respectively, with the amount of converted light depending on the value of δ ( δ = π corresponds to a full conversion). Helical modes generated by a qplate are described by the socalled HyperGeometricGaussian modes (HyGG_{p,m}, which in our notation corresponds to the radial amplitude profile only of the mode, as the azimuthal phase factor is written explicitly)^{53}, corresponding to a specific class of light beams carrying OAM^{54}, analogously to LaguerreGauss or Bessel beams. Two indices (p, m) specify the mode properties, where m is associated with the OAM content while p determines the radial distribution of the field. It is worth noting that the same helical modes with HyGG radial structure are also generated by any optical device (spiral phase plates, pitchfork holograms, etc.) that suddenly imprints an azimuthal phase factor on the input field, with such device placed in the focal region of the beam. Hence, besides our specific setup, our analysis applies to all configurations in which VVBs are generated relying on this approach. Eq. 5 clearly shows that, if δ = π, a linearly polarized Gaussian beam is fully converted into a VVB, showing a Vpoint with charge η = 2q at its center. In particular, azimuthally and radially polarized beams are obtained when the input polarization is vertical (V) and horizontal (H), respectively, the plate charge is q = 1/2 and α_{0} = 0 (see Fig. 1):
where kets H〉 and V〉 represent H and V polarizations states. When changing the value of the retardation to , with ≪ π, a fraction of the input beam is added to the pure VVB:
A similar expression holds for an azimuthal VVB. In close analogy to Eq. 3, the latter equations show that a small variation of δ can be treated as a perturbation to the original VVB. In Fig. 1 we show a simulation of the polarization distribution of perturbed radial and azimuthal VBs (Eq. 7). In particular in the upper part of panels a,b we plot a 2D map of the orientation angle ψ of the local polarization ellipse, calculated in terms of the reduced Stokes parameters; here the two Cpoints are clearly visible as vortices of this scalar field (the ellipse orientation), with their separation changing with the value of δ.
Experimental results
To confirm the theoretical predictions discussed in the previous section, we implemented the setup shown in Fig. 2. The output of a Ti:Sa laser (wavelength λ = 800 nm) is coupled into a singlemode fiber (SMF), used as a spatial filter in order to produce a pure TEM_{00} Gaussian mode at the input of the setup. At the exit of the SMF, the beam (uniform) polarization is prepared into vertical or horizontal states by means of a linear polarizer followed by a halfwave plate (HWP). A qplate (q = 1/2) with optical retardation δ, whose value is controlled through a tunable electric field applied to the outer faces of the cell^{51}, transforms the beam into the VB reported in Eq. 7. In order to reconstruct the 2D polarization pattern in a transverse plane we implemented a pointbypoint polarization analysis, similar to that reported in ref. 19. For each beam configuration, on a CMOS camera (1280 × 1024 pixels) we recorded the intensity profile of the field components associated with {H, V}, {L, R} and diagonal and antidiagonal ({D, A}) polarization states. These components are selected by rotating suitably a set of waveplates, followed by a linear polarizer. By using a dedicated software, Stokes parameters are calculated pointbypoint according to the definitions S_{0} = I_{H} + I_{V}, S_{1} = I_{H} − I_{V}, S_{2} = I_{D} − I_{A}, S_{3} = I_{L} − I_{R}; here I_{j} represent the measured intensities of the six polarization components, with j ∈ {H, V, D, A, L, R}. To take into account small fluctuations of the beam position with respect to the camera the field intensities are averaged over arrays of 3 × 3 pixels. An imaging system made of a lens (focal length = 10 cm) followed by a microscope objective is used to determine the polarization pattern at different positions along the propagation axis z.
By introducing a tiny alteration of the qplate voltage with respect to the optimal condition δ = π, we investigated the instability of a Vpoint singularity that transforms into a pair of Cpoints. When δ = π, the polarization is linear in every point of the transverse plane and has a radial or azimuthal pattern, depending on the input polarization. If we introduce a small detuning, that is δ → π − 2ϵ, a uniform polarized Gaussian beam is added coherently to the original VVB. As previously discussed, 2η Cpoints are expected to form in place of the original singularity with topological charge η. In Fig. 1 we show the experimental results that confirm these predictions. In order to unveil the formation of Cpoints pairs, that occurs very close to the beam center, we used a single lens to image this small portion of the beam on the camera sensor. We imaged on the camera the beam at z = 0.22z_{R} and considered only the region r < 0.58w_{0}, where z_{R} and w_{0} are the Rayleigh range and the beam waist, respectively. The measured Stokes parameters are used to determine the polarization pattern and, as a consequence, the orientation angle ψ of the local polarization ellipses, calculated as . In good agreement with the theoretical predictions (see Fig. 1c,d), we observe the original Vpoint splitting into two Cpoints with opposite handedness, whose spatial separation grows as is increased, in agreement with Eq. 4. The topological charge of such Cpoints is 1/2, hence the total charge is conserved.
The same phenomenon can be observed for Vpoints with higher topological charge, obtained using qplates with q > 1/2. In Fig. 3 we plot the polarization pattern and the orientation angle of ψ measured for a qplate with q = 1 and α_{0} = 3π/4. Here a Vpoint with charge η = 2 is observed to split into four Cpoints. For each circular component, the central vortex has a charge ±2; as mentioned previously these are unstable and decay into two equally charged vortices^{40,42}; this process is much faster (with respect to a variation of δ) if compared to the Vpoint splitting discussed previously (see Fig. 1). Two pairs of Cpoint move away from the beam center as δ decreases, although in each pair the distance between the two singularities remains small so that they cannot be clearly resolved in our system.
By changing the voltage applied to the qplate we can tune the device retardation to any value in the range (0, 2π), thus we can also explore what happens when the Gaussian term becomes comparable to the VVB amplitude. In Fig. 4 we show the polarization and intensity patterns measured in the near field of the beam (compared with theoretical predictions), obtained when varying δ between 0 and π with steps of π/8. Theoretical simulations are added here for comparison. At a glance, when decreasing δ, Cpoints are observed to move away from the beam center and seem to disappear when δ < π/2. Accordingly, the topological features of the polarization pattern change abruptly when the amplitude of the Gaussian term becomes higher than the original VVB. This is not surprising, as we are exploring an intermediate regime between the extreme cases δ = 0 (a Gaussian beam with no polarization singularities) and δ = π (VVB beam with a η = 1 Vpoint), which have different topological features. However, as the VVB and the perturbing term diffract differently, these features are expected to change when the beam propagates, making the situation much more complex. Importantly, this behavior will depend also on the VVBs radial profile, here assumed to be that of HyGG modes; although the generalization to other types of helical modes is out of the scope of this work, in the Methods we briefly discuss the simple case in which the VVBs radial profile is that of lowest radialorder LaguerreGauss modes with p = 0. In this specific case indeed an analytical expression for the Cpoint positions can be found. For our specific configuration, we investigate the dynamical evolution of polarization singularities by considering the expression of the beam generated by a qplate (with α_{0} = 0) when shined with a H or V polarized Gaussian beam:
where the ±sign stands for H or V input polarization, respectively, and m = 2q. Dimensionless units are introduced here, where the distance from the qplate z and the radial coordinate r are normalized with respect to the Rayleigh range z_{R} and the waist w_{0} of the beam, respectively (ζ = z/z_{R}, ρ = r/w_{0}). Cpoint positions are calculated by solving the equations C_{L/R}(ρ, ϕ, ζ) = 0, where left and circular components C_{L/R} are those reported in Eq. 8 (see the Methods for details about the solution of such equations). In Fig. 5a we plot the position of lefthanded Cpoints as a function of the coordinate ζ, for the case m = 1 (similar features are obtained for higher values of m). For small values of ζ, multiple rings characterize the radial distribution of HyperGeometricGaussian modes and, as a result, many Cpoints may appear at given transverse plane. However, for any value of δ and ζ, there exist at least a lefthanded Cpoint (and its righthanded partner), although it might be positioned in the peripheral regions of the beam where the field intensity is negligible. For high values of ζ (far field), numerical simulations show that the distance between Cpoint positions and the beam axis is proportional to the coordinate ζ. In this configuration, we can evaluate if these singularities can be still considered within the beam; in particular, for different values of δ we can compute the fraction of the beam intensity contained in the circular region delimited by the Cpoint radial position. In Fig. 5b we plot the relative encircled intensity of the beam, which is observed to increase as the Gaussian contribution becomes stronger. When δ < π/8, for example, more than the 99% of the beam intensity is contained in the radius defined by the Cpoint position, i.e. the singularities can be considered as lying outside the beam. In the near field, instead, qualitative differences manifest depending on the qplate retardation being higher or lower than π/2. In particular, in the latter case the singularities move away at infinite distances, while in the former, L and R Cpoints merge at the beam center (forming a single Vpoint). These different features are a consequence of the irregular behavior of HyGG_{−m,m} modes in the limit ζ → 0. As discussed in the Methods indeed (see Eq. 13) Cpoints can form at the intersections between the Gaussian and HyGG_{−m,m} envelopes; when δ > π/2, these envelopes cross in the region of the central dip characterizing HyGG_{−m,m} modes, at a specific radial distance that depends on δ. However, since the dip width vanishes when approaching the near field, independently of δ all Cpoints merge at the center. When δ < π/2, singularities do not form in the HyGG dip but rather at the intersection of the tails of both Gaussian and HyGG modes; such intersections exist since the latter have a larger radial profile, for finite values of ζ. But as ζ → 0, the HyGGs envelope converges to that of the Gaussian beam, and clearly two Gaussian profiles with different amplitudes cannot have intersection points, so that all Cpoints are expelled from the beam. In the Methods, we repeat this analysis for the simpler case in which LaguerreGauss modes are taken in the place of HyGG modes. In such configuration indeed an expression for Cpoints position can be derived analytically.
As shown in Fig. 5a, VBs obtained when δ < π/2 may show Cpoints only after a definite value of ζ. We confirm experimentally this effect by investigating the dynamical formation of such singular points in the polarization pattern of a beam obtained when δ = 6.7π/16. In Fig. 6 we report the experimental data and the associated theoretical predictions. Although no Cpoints are observed in the near field (see Fig. 4), they appear as we increase the propagation distance ζ, in agreement with our previous discussion (see Fig. 5a). In particular, at ζ = 0.2 we can observe a double pair of singularities, as a consequence of the oscillatory behavior of the VVB amplitude profile.
Discussion
In this study we investigated the topological features of vector vortex beams and the robustness of the associated singularities when introducing a perturbation to the field. Polarization singularities manifesting at the center of such beams are unstable and transform into multiple Cpoints with equal topological charge ±1/2, the lowest order singularities of fully polarized light. Here we perturb a VVB by adding coherently a tunable amount of a linearly polarized Gaussian beam and demonstrate experimentally a possible mechanism that leads to the unfolding of the central singularity, in analogy to similar phenomena observed in the skylight polarization^{44} or in highorder optical vortices^{29}. On one hand, this realization provides a simple example of transformations between different polarization singularities^{35,36,48,55}; on the other, it allows for a detailed investigation of phenomena that may affect optical systems exploiting VVBs, for which the presence of the fundamental TEM_{00} mode can result from different types of misalignments^{38,39,41}, scattering^{42} or from turbulence in the propagation medium^{56}. Investigating the stability of VVBs can be of interest for all photonic applications involving these structured beams, since a modification of the intensity pattern always accompanies the singularity splitting. As discussed before, similar alterations of a VVB can occur as a consequence of experimental imperfections, and the beam distortion caused by the Vpoint splitting may play a role in a variety of applications, as for example in material shaping^{57}. Reversing the current approach, as in prospect it could be possible to tune the qplate optical retardation in order to compensate the effect of experimental imperfections and reduce such deformations of the beam profile^{58}. Finally, our results may find application in the context of singularimetry; weak fields can indeed be measured by letting them perturb unstable optical fields, and features of materials that have interacted with such beams can be extracted from the pattern formed by split singularities^{59}. In addition, since perturbations can be introduced by imperfections in the optical setup, the unfolding of the central Vpoints can be used to assess the quality of the VVB generation system^{42}.
Methods
Helical modes of light
LaguerreGaussian and HypergeometricGaussian beams^{53} represent specific cases of the socalled Circular Beams (CB)^{54}, a class of optical spatial modes characterized by the phase factor associated with the orbital angular momentum. As in the main text, we used adimensional cylindrical coordinates ρ = r/w_{0} and ζ = z/z_{R}, where w_{0} is the waist radius of the Gaussian envelope and z_{R} the Rayleygh range, respectively.
LaguerreGaussian LG_{p,m} modes have the well known expression:
where is the generalized Laguerre polinomial and p is a positive integer.
A qplate shined by a TEM_{00} mode generates the socalled HypergeometricGaussian modes^{52} (see Eq. 5):
where Γ(z) is the Euler Gamma function and F_{1}(a; b; z) is the confluent Hypergeometric function. Through the main text we used the notation HyGG_{p,m} when referring only to the radial distribution of the associated HyGG modes, not including the azimuthal phase factor e^{imϕ}.
Determination of Cpoint position
Here we give a detailed description of the derivation of the Cpoint position (see Fig. 5a). We recall here the general expression describing a beam generated by a qplate with α_{0} = 0 when shined by a H or V polarized Gaussian beam (already reported in Eq. 8):
where the plus or minus sign is for H and V input polarizations, respectively, ζ = z/z_{R} is the propagation distance normalized with respect to the Rayleigh range z_{R}, and m = 2q.
In our experiment the function f_{m} is given by Hypergeometric Gaussian mode HyGG_{−m,m}. It is worth noting that specific architectures allow using the qplate to generate helical modes with a different radial profile; as an example, recently a qplate placed inside a laser cavity has been exploited for the generation of high quality LaguerreGauss VBs^{22} with p = 0. For this reason, we consider here also the case f_{m} = LG_{0,m}. As we will show in the following, the Cpoints positions can be deduced analytically in this case.
The Cpoint position at a given δ and ζ can be obtained simply by solving the implicit equation C_{L,R}(ρ, ζ, ϕ; δ) = 0 (see Eq. 8). We limit ourselves to searching for the distance of Cpoints from the center, which can be found by solving the simplified equation C_{L,R}(ρ, ζ, ϕ; δ)^{2} = 0. Explicitly, this reads:
A solution for such equation exists only if the following condition holds
hence, by solving jointly Eq. 12, 13 we are left with an implicit equation for ρ as function of ζ. If needed, then the solution can be inserted into Eq. 12 to find the azimuthal coordinates of the singularities.
The case f_{m} = HyGG_{−m,m} can be solved only numerically. Some solutions are shown in Fig. 5a and discussed in the main text. Here we focus on the case f_{m} = LG_{0,m} where we can find an analytical expression for Cpoints positions as a function of ζ. Importantly, this kind of beams (VVBs whose radial distribution is that of LG modes with the lowest radial index p = 0) can also be generated experimentally, as for example by exploiting a qplate placed inside a laser cavity^{22}. In this case Eq. 13 reads:
It follows that the distance of Cpoints from the beam center ρ_{C}(ζ) is given by:
In order to evaluate if such singularities are contained in the beam or not, one can either apply the same approach used for the case of HyGG modes (see Fig. 5b), or directly compare the position of singular points with the beam radius. This is typically defined as the rootmeansquare σ_{rms} of the beam intensity in the transverse plane^{60}:
where I(ρ, ζ, ϕ) is the beam intensity. From Eq. 11 with f_{m} = LG_{0,m} we obtain:
In the far field, the corresponding beam divergence is given by θ_{rms} = σ_{rms}/ζ, in the limit ζ → ∞. The associated expression is simply:
By comparing Eq. 17 and Eq. 15 we can observe that the beam radius has the same functional form as the Cpoint position. The divergence of the latter is:
By comparing Eq. 18 and Eq. 19 it is possible to compute exactly whether Cpoints diverge more or less rapidly than the beam radius, for any value of δ. However, we observe that our analysis aims only at identifying qualitative features: quantitative results are ambiguous as multiple definitions of the beam divergence can be given (for instance the quantity is often used). In the near field, at odds with the HyGG case, Cpoints have a definite position given by Eq. 15 when ζ = 0. By comparing the latter with Eq. 17 it is possible to check if such singular regions are contained inside the beam. In conclusion, we point out that this analysis relying on the r.m.s. as a measure of the beam width cannot be done in the case of HyGG beams; indeed, HyGG_{−m,m}^{2} ∝ r^{−4} when r → ∞ ref. 53 and the r.m.s becomes infinite for any value of ζ.
Additional Information
How to cite this article: D’Errico, A. et al. Topological features of vector vortex beams perturbed with uniformly polarized light. Sci. Rep. 7, 40195; doi: 10.1038/srep40195 (2017).
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Acknowledgements
We thank Mark Dennis for preliminary reading of the manuscript and for providing useful comments. This work was supported by the European Research Council (ERC), under grant no. 694683 (PHOSPhOR).
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Author notes
 Maria Maffei
Present address: ICFOInstitut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Spain.
Affiliations
Dipartimento di Fisica, Università di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, Napoli, Italy
 Alessio D’Errico
 , Maria Maffei
 , Bruno Piccirillo
 , Corrado de Lisio
 , Filippo Cardano
 & Lorenzo Marrucci
CNRSPIN, Complesso Universitario di Monte Sant’Angelo, Napoli, Italy
 Corrado de Lisio
CNRISASI, Institute of Applied Science and Intelligent Systems, Via Campi Flegrei 34, Pozzuoli (NA), Italy
 Lorenzo Marrucci
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Contributions
A.D., M.M., F.C. and L.M. devised various aspects of the project and designed the experimental methodology. A.D., M.M. and F.C., with contributions from C.d.L., carried out the experiment and analyzed the data. B.P. prepared the qplates. A.D. and F.C. wrote the manuscript, with contributions from M.M. and L.M. All authors discussed the results and contributed to refining the manuscript.
Competing interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to Filippo Cardano.
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Further reading

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Formation of hybrid higherorder cylindrical vector beams using binary multisector phase plates
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Scientific Reports (2018)
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