Abstract
Analytical propagation expression of an Airy beam in uniaxial crystals orthogonal to the optical axis is derived. The ballistic dynamics of an Airy beam in uniaxial crystals is also investigated. The Airy beam propagating in uniaxial crystals orthogonal to the optical axis mainly depends on the ratio of the extraordinary refractive index to the ordinary refractive index. As an example, the propagation of an Airy beam in the positive uniaxial crystals orthogonal to the optical axis is demonstrated. The acceleration of an Airy beam in the transversal direction along the optical axis is more rapidly than that in the other transversal direction. With increasing the ratio of the extraordinary refractive index to the ordinary refractive index, the acceleration of the Airy beam in the transversal direction along the optical axis speeds up and the acceleration of the Airy beam in the other transversal direction slows down. The Airy beam propagating in uniaxial crystals orthogonal to the optical axis follows a ballistic trajectory. The effective beam size of the Airy beam in the transversal direction along the optical axis is always larger than that in the other transversal direction.
©2012 Optical Society of America
1. Introduction
As the solution of the force-free Schrödinger equation, an Airy beam contains infinite energy and exhibits a nonspreading property in vacuum [1]. The free acceleration [2, 3], the self-healing properties [4], the evolution of the Poynting vector and angular momentum [5], the ballistic dynamics [6], and the beam propagation factor [7] of an Airy beam have been widely investigated. The methods of the Wigner distribution function and the geometrical optics have been used to interpret the intriguing features of an Airy beam, respectively [8, 9]. The Airy beam has vital applications in optical micromanipulation and all-optical switching [10, 11]. The propagation of an Airy beam through an ABCD optical system [12], in water [13], in a nonlinear medium [14, 15], and in turbulence [16] has been examined, respectively.
Besides in free space, in a nonlinear medium, and in turbulent atmosphere, laser beams also propagate in anisotropic medium such as uniaxial crystals, which is treated by solving Maxwell’s equations. Some applications, e. g., the design of the polarizer and the compensator, are involved in laser beams propagating in uniaxial crystals. The propagation of various kinds of laser beams in uniaxial crystals has been reported [17–23]. To the best of our knowledge, however, no literature has been reported on the propagation of Airy beams in uniaxial crystals. In the remainder of this paper, therefore, the paraxial propagation of Airy beams in uniaxial crystals orthogonal to the optical axis is to be investigated.
2. Propagation of Airy beams in uniaxial crystals orthogonal to the optical axis
In the Cartesian coordinate system, the z-axis is taken to be the propagation axis. The optical axis of the uniaxial crystal coincides with the x-axis. The input plane is z=0 and the observation plane is z. The ordinary and extraordinary refractive indices of the uniaxial crystal are no and ne, respectively. The relative dielectric tensor of the uniaxial crystal reads as
The Airy beam considered here is linearly polarized in the x-direction and is incident on a uniaxial crystal in the plane z=0. The Airy beam in the input plane z=0 takes the form as [2, 3]
where w0 is the transverse scale. Ai(⋅) is the Airy function. a is the modulation parameter. Within the framework of the paraxial approximation, the propagation of the Airy beam in uniaxial crystals orthogonal to the optical axis obeys the following equations [24, 25]: with the two-dimensional Fourier transform and being given bywhere j = x or y (hereafter). In the paraxial approximation, the longitudinal component of the field can be neglected as long as the transversal beam radius w0 is larger compared with the wavelength [26]. Equations (3) and (4) indicate that the x component of the optical field is only a superposition of extraordinary plane waves and the y component uniquely contains ordinary plane waves [24]. Equations (3) and (4) can also be rewritten as [24, 25] where is the wave number with λ being the optical wavelength. Inserting Eq. (2) into Eq. (6), we can obtainwithandbeing given byEquation (9) can be rewritten as
where . We note that the convolution of two functions,and, is defined by [27]where ⊗ denotes the convolution. Therefore, Eq. (11) can be expressed in the form of the convolution:where the auxiliary functionsandare given byThe convolution theorem of the Fourier transform has the following property [27]
where the auxiliary functionsandare the Fourier transform of and, respectively. and are given by [27]Therefore, Eq. (13) turns out to be
where e = ne/no is the ratio of the extraordinary refractive index to the ordinary refractive index. We recall that the Airy function can be written in terms of the integral representation:Therefore, Eq. (19) can be analytically expressed as
Similarly, Eq. (10) is found to be
As a result, the analytical paraxial propagation equations of an Airy beam in uniaxial crystals orthogonal to the optical axis yield
Under the condition of ne = no = 1, Eq. (23) reduces to the familiar propagation formula of Airy beams in free space.
As shown in Eq. (23), the Airy beam in uniaxial crystals orthogonal to the optical axis follows a ballistic trajectory in the x-z and y-z planes that are described by the following parabolas
The corresponding Newtonian equations describing theses ballistics read as
where gx and gy play the role of “gravity”. As e>1 holds for the positive uniaxial crystals, the displacement in the x-direction is always larger than that in the y-direction, and gx is always larger than gy. When ne = no = 1, Eqs. (25)-(28) reduce to the ballistic dynamics in free space, which are same as those in Ref [6]. The effective beam size of the Airy beam in the x- and y-directions of the observation plane is defined as [28]:where jC is the centre of gravity of the Airy beam in uniaxial crystals orthogonal to the optical axis and is given byAccording to Ref [6], the centres of gravity of the Airy beam in uniaxial crystals orthogonal to the optical axis yield
Equation (31) demonstrates again that the center of gravity in uniaxial crystals is propagating on a straight line. After lengthy integral, the effective beam sizes of the Airy beam in the x- and y-directions of the observation plane turn out to be
whereare the effective beam sizes in the input plane. zrx and zry represent the Rayleigh length in the x- and y-directions of the Airy beam in uniaxial crystals and are given by3. Numerical calculations and analyses
Here we mainly pay attention to the influence of the uniaxial crystals on the propagation of an Airy beam, and the uniaxial crystals considered here are positive. Calculation parameters are chosen as λ = 0.53μm, w0 = 100μm, a = 0.1, and no = 2.616. Figure 1 represents the contour graph of the normalized intensity distribution of an Airy beam propagating in the uniaxial crystals at several observation planes. The normalized intensity is given by where the subscript means taking the maximum value. The top and bottom rows denote e = 1.1 and e = 1.5, respectively. The two observation planes are z = 0.1z0 and z = 5z0. Upon propagation in the uniaxial crystals orthogonal to the optical axis, the acceleration of the Airy beam in the y-direction is far slower than that in the x-direction, which is caused by the anisotropic effect of the crystals. In free space, the Airy beam remains almost invariant up to a certain distance, and it accelerates in the same manner along the 45° axis in the x-y plane [3]. While in the uniaxial crystal, the Airy beam no longer accelerates along the 45° axis in the x-y plane. The angle along which the Airy beam accelerates in the x-y plane of the uniaxial crystal is equal to . With increasing the value of e, we can find that the acceleration of the Airy beam in the x-direction speeds up and the acceleration of the Airy beam in the y-direction slows down. When e = 1.1, θ = 34.33°. θ decreases with increasing the value of e.

Fig. 1 Contour graph of the normalized intensity distribution of an Airy beam propagating in the uniaxial crystals at several observation planes. The top and bottom rows denote e = 1.1 and e = 1.5, respectively. (a) and (c) z = 0.1z0. (b) and (d) z = 5z0.
Equation (23) denotes that the x and y variables can be separated from each other. To further reveal the accelerating properties of the Airy beam in uniaxial crystals, the normalized intensity distribution in x- and y-directions of an Airy beam propagating in the uniaxial crystals are shown in Figs. 2 and 3 . When propagating in the uniaxial crystals orthogonal to the optical axis, the Airy beam in the x- and y-directions accelerates, which is more evident than that in Fig. 1. With increasing the value of e, the acceleration of Airy beam in the x-direction quickens up, and the acceleration of Airy beam in the y-direction retards. The above exotic features only belong to the Airy beam. Equations (25) and (26) denote that the displacements in the x- and y-directions are proportional to the square of the axial propagation distance. When the observation plane is far from the input plane, therefore, the displacement deflection augments with increasing the value of e.

Fig. 2 The normalized intensity distribution in the x-direction of an Airy beam propagating in the uniaxial crystals.

Fig. 3 The normalized intensity distribution in the y-direction of an Airy beam propagating in the uniaxial crystals.
The contour graphs of the normalized intensity distribution of an Airy beam propagating in the uniaxial crystals at x-z and y-z planes are shown in Fig. 4 . The top and bottom rows correspond to e = 1.1 and e = 1.5, respectively. The Airy beam in uniaxial crystals orthogonal to the optical axis follows a ballistic trajectory. With increasing the value of e, gx increases and gy decreases, which is indicated by Eqs. (27) and (28). Therefore, the parabolic deflection of the normalized intensity in the x-z plane augments with increasing the value of e. However, the parabolic deflection of the normalized intensity in the y-z plane decreases with increasing the value of e, which is open and shut in Fig. 4. Comparison between the right and left parts of Fig. 4 denotes that the acceleration of the Airy beam in the x-direction is far faster than that in the y-direction under the same condition.

Fig. 4 Contour graph of the normalized intensity distribution of an Airy beam propagating in the uniaxial crystals at different observation sections. The top and bottom rows denote e = 1.1 and e = 1.5, respectively. (a) and (c) x-z plane. (b) and (d) y-z plane.
The effective beam sizes of an Airy beam propagating in the uniaxial crystals versus the propagation distance z are shown in Fig. 5 . With increasing the value of e, the effective beam size in the x-direction increases and the effective beam size in the y-direction decreases. The reason is that ne/no of the anisotropic crystal is larger than unity. The effective beam size in the x-direction is always larger than that in the y-direction at any given propagation distance.

Fig. 5 The effective beam sizes of an Airy beam propagating in the uniaxial crystals versus the propagation distance z.
4. Conclusions
Analytical propagation expression of the Airy beam in uniaxial crystals orthogonal to the optical axis is derived. The parabolic defection of the normalized intensity features and the corresponding Newtonian equations describing theses ballistics are presented, respectively. The analytical formulae of the effective beam sizes are also derived. The Airy beam propagating in uniaxial crystals orthogonal to the optical axis depends on the ordinary refractive index no, the ratio of the extraordinary refractive index to the ordinary refractive index e, the axial propagation distance z, and the beam parameters w0 and a. As an example, the influence of the positive uniaxial crystals on the propagation properties of an Airy beam is numerically investigated. Upon propagation in the uniaxial crystals, the Airy beam in the two transversal directions accelerates. Moreover, the acceleration of the Airy beam in the transversal direction orthogonal to the optical axis is far slower than that in the transversal direction along the optical axis. The Airy beam in uniaxial crystals orthogonal to the optical axis follows a ballistic trajectory. With increasing the value of e, the acceleration of the Airy beam in the transversal direction along the optical axis speeds up, and the acceleration of the Airy beam in the transversal direction orthogonal to the optical axis slows down. The dependence of the effective beam size of the Airy beam in uniaxial crystals on the value of e is also examined. With increasing the value of e, the effective beam size in the transversal direction along the optical axis increases and that in the other transversal direction orthogonal to the optical axis decreases. At any given propagation distance, the effective beam size in the transversal direction along the optical axis is always larger than that in the other transversal direction.
Acknowledgments
This research was supported by National Natural Science Foundations of China under Grant No. 10974179 and 61178016. The authors are indebted to the reviewer for valuable comments.
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