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74 4 Zero Detuning: Laser Hydrodynamics and Optical Vortices

In the di usive case, where aIm aRe, the vortex radius is given by

which is linearly proportional to the di raction/di usion ratio. In the purely di usive limit α tends to zero, corresponding to a uniform phase.

Note that, with the appropriate scalings (the parameter aIm/aRe used for asymptotic expansions plays the role of the g-factor of (4.2), as can be easily checked), these results agree with those obtained for the vortices of the LGL equation in the corresponding limits, thus justifying the use of (4.19) to evaluate the vortex parameters.

4.3 Vortex Interactions

A single, isolated vortex on a homogeneous background is stationary. If the spatial symmetry is broken, the vortex starts to move. The symmetry can be broken by a gradient of the background field on which the vortex is superimposed. For the study of vortex interactions, we assume the presence of two vortices; one vortex creates inhomogeneities, and the other vortex moves because of those inhomogeneities, and vice versa.

A vortex creates phase as well as amplitude inhomogeneities. Vortices interact predominantly as a result of phase inhomogeneities, since the amplitude inhomogeneities decay rapidly far away from the vortex core. The phase gradient has an angular component due to the helicity of the vortex, and also a radial component due to the vortex radiation, as discussed in the previous section. Therefore, we already know how the first vortex imposes phase inhomogeneities. To understand the vortex dynamics, one must find out how the second vortex responds to those phase inhomogeneities.

A mathematically rigorous derivation of the vortex–vortex interaction can be found in [5, 6]. Here we sketch a phenomenological theory, which is simpler and more transparent.

We assume that the second vortex is imposed on a tilted wave exp(ik ·r), and rewrite (4.19)1 with the ansatz A(r, t) = exp(ik · r) B(r, t) :

1For simplicity, we study the ordinary CGL equation. The main properties of the dynamics of vortices for the LGL equation are analogous, and will be discussed below in the present chapter.

The presence of the background tilted wave decreases the gain and shifts the frequency (the first term on the right-hand side in (4.25)), owing to the mismatch from the resonance (a tilted wave is at resonance in a laser with zero detuning). The remaining three terms on the right-hand side indicate that the presence of the background tilted wave does not alter the nonlinearity, the di raction or the di usion. Therefore the vortex solution (modified by a change of the gain) makes the right-hand side of (4.25) equal to zero. The terms on the left-side of the equation are responsible for the vortex motion.

The term 2aIm(k · )B implies a uniform translation of the vortex envelope B(r, t) with a velocity v = 2aImk. Indeed, the solution B(r, t) = Bv(r −2aImkt) makes the first two terms of the left-hand side of (4.25) equal to zero. The vortex envelope Bv(r, t) thus translates at a certain velocity, or, in other words, is advected by the photon flow. In general, not only a vortex but also an arbitrary inhomogeneity is advected by a background flow. Note that no assumption about the form of the perturbation B(r, t) has been made to calculate the advection.

The term with imaginary coe cient 2iaRe(k · )B, evaluated asymptotically close to the vortex core, can be rewritten as follows. The asymptotic form of Bv close to the vortex is Bv(r, t) = x+imy (m = ±1), and the following relation is valid:

where k is perpendicular to the gradient of the background tilted wave of wavevector k (it is rotated counterclockwise by 90◦). As a result, we obtain an asymptotic equation for the motion of the vortex envelope,

where m is a unit vector which is transverse to the (x, y) plane, and directed along the z axis (optical axis) for a vortex with positive topological charge, and in the opposite direction for a negative charge.

From this analysis, it follows that the vortices are advected by the mean flow, and move with the flow velocity. Di raction is responsible for the hydrodynamic advection. Owing to di usion, however, the vortices have a velocity component transverse to the flow. The hydrodynamic interpretation of this transverse velocity component of a vortex is the gyroscopic Magnus force: a vortex (and any rotating object in general) adquires a velocity component

76 4 Zero Detuning: Laser Hydrodynamics and Optical Vortices

transverse to the direction of the force acting on it. The Magnus force depends on the sense of rotation of a rotating body in hydrodynamics and, equivalently, on the topological charge of the optical vortex m in nonlinear optics (4.28). The magnitude of the Magnus force is proportional to the di usion coe cient.

A similar analysis performed on the LGL equation (4.2) reveals that the vortex motion induced by the phase gradient is described by

where the advection along k is the same as for the CGL equation, but the transverse motion is proportional to the third power of the wavenumber |k| of the background tilted wave, since not the usual di usion but super-di usion is present in the LGL equation.

In Fig. 4.6, results of numerical calculations based on (4.2) demonstrating vortex motion in a phase gradient are shown.

Fig. 4.6. Two vortices advected by a background flow, for g = 4. The phase gradient, visible from the phase pictures (right), is directed to the right. The time t between pictures is 40

The vortex velocity component directed along the phase gradient is independent of the vortex charge. The transverse (gliding) component depends on the charge: the positively charged vortex at the top left corner of Fig. 4.6 glides downwards, while the negatively charged vortex at the bottom right corner glides upwards.

Vortices gliding perpendicular to the background tilted wave diminish their average tilt. Indeed, below the positively charged vortex in Fig. 4.6, the tilt is larger than above the vortex: above the positively charged vortex there are three vertical interference fringes, while below there are four fringes. The zero-detuned (resonant) laser “prefers” a homogeneous distribution, because this corresponds to a minimum of the variational potential. Thus a vortex glides in such a way as to minimize the potential energy of the laser.

Fig. 4.7. Interaction of two vortices: (a) of the same charge; (b) of opposite charge at small separation; (c) of opposite charge at large separation

Once one knows how vortices create phase gradients and how they move because of phase gradients, one can analyze the interaction of vortices.

A system of two equally charged vortices is shown in Fig. 4.7a. The first vortex imposes an angular and a radial phase variation at the location of the second vortex, and vice versa, as shown by the dashed arrows. The induced velocity components are indicated by solid arrows. It follows that two positively charged vortices rotate one around another anticlockwise; if, on the other hand, both are negatively charged, they rotate clockwise. The vortices also repel one another, and the separation between them grows. This picture of the interaction remains qualitatively the same for di erent di usion– di raction ratios aRe/aIm for the CGL equation (4.19) or, equivalently, for di erent values of g for the LGL equation (4.2).

This also provides evidence that vortices of charge larger than one are never stable in broad-aperture lasers.

The behavior of two vortices with opposite charge is illustrated in Figs. 4.7b,c. It follows that a vortex pair translates in a direction perpendicular to the line connecting the vortex cores. The vortex separation can increase or decrease during the course of translation, depending on the diffusion/di raction ratio and on their initial separation. For a small initial separation, the vortices attract one another and eventually annihilate. For a su ciently large separation, the vortices repel one another. This follows from simple geometrical considerations, as illustrated in Figs. 4.7b,c: recall that the radial phase variation increases and the angular phase variation decreases monotonically with the vortex separation.

It is possible to evaluate analytically the critical separation of oppositely charged vortices for the CGL equation. At the critical radius, the relation aRe/aIm = Φr/Φϕ holds. The radial and angular phase variations are Φr = (α/r0) tanh(r/r0), and Φϕ = 1/r. Therefore, the critical separation rcr can

78 4 Zero Detuning: Laser Hydrodynamics and Optical Vortices

The critical vortex separation increases with increasing di usion, as (4.30) shows. In the strong-di usion limit, the equilibrium vortex separation can be expressed analytically, and is given by

A similar analysis for the LGL equation yields the result that the critical vortex separation is linearly proportional to g : rcr/r0 ≈ g .

In all cases, the more di usive the system is, the more dilute is the vortex gas at equilibrium. The equilibrium density of a vortex gas is thus proportional to n ≈ (r0/rcr)2 ≈ 1/g2.

The above scenario of vortex interaction is, however, valid only in the limit where di usion dominates. When di raction dominates, the shocks between neighboring vortices can significantly alter the picture discussed above. Figure 4.8. shows how the presence of shocks can strongly influence the vortex interaction.

Fig. 4.8. Two vortices interacting with a shock in between them, for g = 0.5. In each of the pictures, the upper vortex has a negative topological charge, and the lower has a positive topological charge. The time t between pictures is 40

The vortices in Fig. 4.8 are expected to propagate to the left, according to the analysis given above (Fig. 4.7); since the bottom vortex is of positive charge, it should drive the upper vortex to the left, and since the upper vortex is of negative charge, it should drive the bottom vortex to the left too. This occurs for a large di usion parameter g 1, as numerical integration of (4.2) shows. This is also the case in the initial stage of evolution for the