Abstract
Based on a reduction of nonlocal nonlinear Schrödinger equation in strongly nonlocal regime, the linear Schrödinger equation with parabolic potential, analytical results describing the evolution of dual Airy beam are presented. The results show that the dual Airy beam in strongly nonlocal medium exhibits a periodic focusing and defocusing behavior, and forms the interference fringes between the focusing and defocusing positions. The analytical results are verified by numerically solving nonlocal nonlinear Schrödinger equation and shown to be reasonable when the characteristic response width is broader than the width of the dual Airy beam. Furthermore, the characteristics of the interference fringes induced by the dual Airy beam are also investigated in detail, and can be used for the measurement of the system parameters. In addition, we propose a scheme to generate dual Airy beam in strongly nonlocal medium.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is well known that Schrödinger equation describing a free particle in quantum mechanics can support a nonspreading Airy wave packet solution [1]. Although several works on Airy beam have been reported [2, 3], it has not attracted much attention due to its infinite energy, which is unrealizable in experiment. A decade ago, finite energy Airy beam in optical system was proposed [4], and exhibited some novel properties, such as weak diffraction, self-acceleration and self-healing [5–10]. Due to the unique characteristics, the researches are further focused on its generation [11,12], propagation and interaction [13,14] in different systems. Finite energy Airy beam was also involved in plasma channel [15], particle clearing [16], spatiotemporal light bullet [17], atmospheric communication [18] and so on. In other physical settings, spatiotemporal wavepackets and self-accelerating wave were also investigated [19–25].
Among these investigations, the dynamics of Airy beam in optical systems has been widely concerned by researchers. Recently, the periodic inversion and phase transition of Airy beam in the medium with parabolic potential were firstly found [26]. Subsequently, Hermite–Gauss, Bessel–Gauss and finite energy Airy beams in such medium have been deeply studied, and the results show that the beams perform an automatic Fourier transform oscillating behaviour [27]. Meanwhile, the dynamics of two-dimensional circular Airy beam has been studied extensively, such as abruptly autofocusing property of blocked circular Airy beams [28], dual abruptly focus of modulated circular Airy beams [29], propagation dynamics of a circular Airy beam in a uniaxial crystal [30]. Very recently, the dynamics of circular Airy beam in parabolic potential has been also studied in depth, and the results show that the circular Airy beam exhibits a periodic abruptly autofocusing and autodefocusing behavior [31]. In fact, the parabolic potential can be realized in strongly nonlocal medium provided that the characteristic response length of the nonlocal medium is much broader than the beamwidth [32–35]. Inspired by the results of these researches, we will consider the evolution of one-dimensional dual Airy beam in strongly nonlocal medium, which is helpful for us to better understand the dynamics of circular Airy beam in parabolic potential due to the comparability between dual Airy beam and circular Airy beam.
In this article, we focus on the evolution of dual Airy beam in strongly nonlocal medium. The analytical and numerical results show that the dual Airy beam exhibits a periodic focusing and defocusing behavior, and forms the interference fringes between the focusing and defocusing positions. Moreover, we systematically investigate the characteristics of the interference fringes, and based on these properties, we propose a method for measuring the system parameters and a scheme to generate dual Airy beam in strongly nonlocal medium, which are rarely involved in the literatures.
The rest of this article is structured as follows. The model and its reductions are presented in the next section. In Sec. III, based on the linear Schrödinger equation with the parabolic potential, the analytical results describing the evolution of dual Airy beam are presented. The numerical verification in strongly nonlocal medium and the characteristics of the interference fringes are discussed in Sec. IV. Finally, the main results of the article are summarized in Sec. V.
2. The model and reductions
Let us consider the dynamics of beam propagation in a nonlocal nonlinear medium, which can be described by the nonlocal nonlinear Schrödinger equation [35]
where ψ(x, z) is the complex amplitude envelope of beam, x and z represent the transverse and longitudinal coordinates, respectively. k = ωn0/c is the wave number, n0 is the linear refractive index of the medium, and c is the light speed in vacuum. η is a constant related to the medium with η > 0 and η < 0 denoting the focusing and defocusing nonlinearity, respectively. Here, we only consider the case of η > 0, i.e., the focusing nonlinearity. Δn is the refractive index change induced by optical beam with intensity I(x, z) = |ψ(x, z)|2, which is given by where R(x) is the response function of nonlocal medium and is normalized to unit, i.e., , whose characteristic width determines the degree of nonlocality. For the strongly nonlocal medium, i.e., the characteristic width of the response function is much larger than that of the optical beam, Eq. (1) can be simplified as the well-known Snyder-Mitchell linear model [32,35] where γ2 = −ηRxx(0) with Rxx(0) < 0 is a constant related to the medium and is the power of the beam, which is conserved for Eq. (3). By introducing the normalized transformation and scaling the transverse width and propagation distance by ρ0 and , respectively, Eq. (3) can be transformed into the dimensionless form [26,27] with , where α2 = P/Pc with critical power , ρ0 is the normalized width. Equation (4) is the linear Schrödinger equation with the parabolic potential, where α stands for the depth of parabolic potential. Thus, we can obtain the dynamics of the beam in the strongly nonlocal medium by solving the linear Eq. (4).3. The analytical descriptions for the evolution of dual Airy beam
In this Section, we will exhibit the evolution behaviors of the beam governed by Eq. (4). Here, the initial beam is taken as dual Airy beam [36,37]
where represents the Airy function, x0 is the input transverse position of each branch of dual Airy beam, and a (0 < a < 1) is the truncation coefficient which ensures that the energy of Airy beam is finite. Here, the truncation coefficient is taken as a = 0.16 throughout. We performed the propagation dynamics of the dual Airy beam by solving numerically Eq. (4). Figure 1 presents its numerical evolutions for different α and x0. One can see that its evolution exhibits a periodic focusing and defocusing behavior, and forms the interference fringes between the focusing position and the defocusing position. Besides, it can be also found that for a fixed x0, with the increasing of α, the first focusing position, the first defocusing position and the distance between them are gradually decreased, as shown in Figs. 1(a) and 1(b). When α is fixed, increasing x0 will result in the increasing of the number of the interference fringes and the narrowing of the spacing of the interference fringes, as shown in Figs. 1(b) and 1(c).In the following, we try to explain the dynamic behavior by an analytical method. In fact, the solution for Eq. (4) with initial condition ϕ(x, 0) can be written as [26,27,38]
where b = α cot(αz)/2, K = αx csc(αz) and Thus, for a given initial state ϕ(x, 0), we can get the analytical solution for Eq. (4) by calculating the integral in Eq. (6). Note that the integral indeed is the Fourier transform of ϕ(x, 0) exp(ibx2), with K being the corresponding spatial frequency, which can be obtained by the convolution of the Fourier transforms of ϕ(x, 0) and exp(ibx2). Here, we need to calculate the Fourier transforms of the input beam (5) and exp(ibx2), respectively. But it is difficult to calculate directly the Fourier transform of the dual Airy beam (5), so we have to consider a linear superposition of two Airy beams, i.e., ϕ1(x, 0) = Ai(x + x0) exp[a(x + x0)] and ϕ2(x, 0) = Ai[−(x − x0)] exp[−a(x − x0)]. The numerical results show that when x0 is large enough, for example x0 ≥ 3, the intensity distribution and power of the dual Airy beam are in agreement with that of ϕ1(x, 0) + ϕ2(x, 0), as shown in Fig. 2. This means that using the linear superposition to replace the initial dual Airy beam is reasonable. Thus, we can study the dynamics of the dual Airy beam by its replacement ϕ1(x, z) + ϕ2(x, z), where ϕ1,2(x, z) is the solution corresponding the initial state ϕ1,2(x, 0).It is easy to give the Fourier transforms of ϕ1,2(x, 0) and G(x) = exp(ibx2) as
and respectively. Thus, employing the convolution property of Fourier transform, we have Substituting Eqs. (7) and (8) into Eq. (9) and employing the integral formula Ai(N − M2) exp[iM(2M2/3 − N)] [39], we can easily obtain the solution ϕ1,2(x, z) as follows From Eq. (10), one can find that the trajectories of the main lobe of the two Airy beams are of the form They are periodic function of z with the period T = 2π/α and z ≠ zm ≡ (2m + 1)T/4, m = 0, 1, 2, · · ·. In order to verify the above analytical result, the trajectories of the two main lobes given by Eq. (11) are shown by the green curves in Fig. 1. From them, one can see that the analytical result is consistent with the numerical result. Moreover, from Eq. (11), one can obtain their intersecting point at z axis as which represent the focusing and defocusing positions, as shown by the white and yellow dashed lines in Fig. 1. Here, n is a nonzero positive integer. When n = 1, and denote the first focusing and defocusing positions. From Eq. (12), we can get the range of the interference fringes along propagation direction Obviously, , and r are functions of α and x0, where the dependences of , and r on α and x0 are shown by the black curves in Fig. 3. From it, one can see that for a fixed x0, as α increases, , and r decrease. When α is fixed, with the increasing of x0, increases, while and r decrease. These results match very well with the numerical results shown by the red points in Fig. 3.Note that Eq. (10) is invalid at z = zm due to b = 0 in Eq. (8), so we need to discuss specially the intensity distribution at z = zm. In this case, we can directly employ Eq. (6) and obtain
where s = +1 (−1) when m is even (odd). From Eq. (14), one can see that the two Airy beams evolve into two Gaussian beams with opposite chirp at z = zm. After superposing, the intensity distribution is expressed as which describes the interference pattern at z = zm. Figure 4 depicts intensity distributions at z0 = T/4 for different α and x0. From it, one can see that when x0 is fixed, with the increasing of α, the central peak pc of interference pattern increases, while the spacing between the central fringe and its adjacent fringe d and the width of the envelope of the interference fringes W decrease, as shown in Figs. 4(a) and 4(b). For a fixed α, as x0 increases, the interference fringes become more closer, as shown in Figs. 4(b) and 4(c). The corresponding numerical results are also shown in Fig. 4, and are consistent with the analytical results given by Eq. (15). Also, we find that a semi-analytical dynamic solution for Eq. (4) can be obtained by the expansion of the input (5) over a truncated set of eigenmodes of the parabolic potential reported in [40], and the results are consistent with our results given by Eqs. (10) and (14).4. The numerical verifications for the analytical results
Until now, we have analytically described the dynamic behaviors of the dual Airy beam (5) by Eqs. (10) and (14). It should be pointed out that the above results are based on the Schrödinger equation with the parabolic potential, which is an approximation of Eq. (1) when the characteristic width of the response function is broader than the width of the beam.
To verify our results in strongly nonlocal medium, we consider directly Eq. (1) with the Gaussian response function
where ρm is the characteristic width of the response function. By the same normalized transformation as Eq. (4), Eq. (1) can be written as [35] where δ = ρm/ρ0. Note that δ is the rate of the characteristic response width ρm to the normalized width ρ0. To simulate the dynamic behavior in strongly nonlocal regime, we need to calculate the width ρ for the initial dual Airy beam (5), which is dependent on x0. Thus, the relative rate of the characteristic response width ρm to the beamwidth ρ can be expressed as Δ = ρm/ρ = (ρ0/ρ)δ. Figure 5 shows the evolutions of the dual Airy beam and the coreesponding intensity distribution at z = T/4 for different Δ at x0 = 5, where the width of the initial dual Airy beam ρ = 9.7749ρ0 as x0 = 5. We find that when Δ is large enough, the numerical results match very well with the analytical results given by Eqs. (11) and (15). This means that the approximate solution given by Eqs. (10) and (14) can be used to describe the dynamics of the dual Airy beam in strongly nonlocal medium, provided that the characteristic width is much broader than that of the initial beam, for example, Δ ≥ 15. Thus, we can investigate the dynamical properties of the dual Airy beam in strongly nonlocal medium by the analytical results in above section.In the following, we discuss the characteristics of the interference fringes induced by the dual Airy beam. Firstly, from Eq. (15), one can find that the maximum and minimum of the intensity at z = zm are reached at 2α3x3/3−2α(a2 − x0)x = ±2nπ and 2α3x3/3−2α(a2 − x0)x = ±(2n+1)π, respectively, where n = 0, 1, 2, · · ·. For simplicity, we only discuss the case of the maximum with n = 0 and 1, i.e., 2α3x3/3 − 2α(a2 − x0)x = 0 and 2π. From them, one can obtain x1 = 0 and x2 = [(3π + X0)/(2α3)]1/3 + [(3π − X0)/(2α3)]1/3, which correspond to the positions of the central fringe and the adjacent fringe in the interference pattern given by Eq. (15), where X0 = [9π2 + 4(x0 − a2)3]1/2. Thus, we can obtain the spacing between the central fringe and its adjacent fringe as
Also, from Eq. (15), the envelope of the interference fringes can be expressed as 2αe2a3/3e−2aα2x2/π. Its central peak and width are of the form and From Eqs. (17), (18) and (19), one can see that d depends on α and x0, and pc and W are independent of x0.Figure 6 presents the dependences of d, pc and W on α and x0, respectively. One can see that for a fixed x0, as α increases, the spacing between the central fringe and its adjacent fringe d and the width of the envelope W are decreasing, while the central peak pc increases, as shown in Figs. 6(a), 6(c) and 6(e). When α is fixed, with the increasing of x0, d decreases, while pc and W remain unchanged, as shown in Figs. 6(b), 6(d) and 6(f). These results agree with the numerical results shown by the red points in Fig. 6. These properties can be used for the measurement of the system parameter. For example, according to Fig. 6(a), we can obtain the system parameter α by measuring the spacing between the central fringe and its adjacent fringe d in the interference pattern at z = T/4.
Finally, we discuss the generation of the dual Airy beam in strongly nonlocal medium. Intuitively, if we take ϕ (x, 0) = ϕ1(x, zm) + ϕ2(x, zm) as an initial input, then the dual Airy beam can be obtained by ϕ(x, 3T/4) because of the periodic feature of the evolution dynamics. As an example, we consider an initial input
From Eq. (14), one can see that it is the superposition of two Gaussian beams with opposite linear and cubic chirp. Thus, from Eq. (6), we can obtain which are the sum of two Airy beams. As shown in above section, as x0 ≥ 3, it can be replaced by a dual Airy beam, i.e., The numerical simulation shows that, as predicted by Eq. (21), at z = 3T/4 the initial state (20) is transformed into the dual Airy beam that agrees with the standard dual Airy beam (5), as shown in Fig. 7. Thus, we can use the superposition of two Gaussian beams with opposite chirp to generate the dual Airy beam in strongly nonlocal medium.5. Conclusions
In summary, based on a reduction of nonlocal nonlinear Schrödinger equation in strongly nonlocal regime, the linear Scgrödinger equation with the parabolic potential, we presented the analytical description of the evolution of the dual Airy beam in strongly nonlocal medium. The results have shown that the evolution of the dual Airy beam in strongly nonlocal medium exhibits a periodic focusing and defocusing behavior, and forms the interference fringes between the focusing and defocusing positions. The results have been verified by numerically simulating nonlocal nonlinear Schrödinger equation, and have shown that they are reasonable when the characteristic width of the response function is broader than the width of the dual Airy beam. Furthermore, the characteristics of the interference fringes induced by the dual Airy beam were also investigated in detail, and are useful for the measurement of the system parameters. In addition, we proposed a scheme to generate the dual Airy beam in strongly nonlocal medium. On the strongly nonlocal medium, the related experiments have been reported around 2000 [41–43], so we speculate that the results may be realized experimentally.
Funding
National Natural Science Foundation of China (NSFC) (61475198, 11705108).
References
1. M. V. Berry and N. L. Balazs, “Nonspreading wave packets,” Am. J. Phys. 47, 264–267 (1979). [CrossRef]
2. J. Durnin, “Exact solutions for nondiffracting beams. I. The scalar theory,” J. Opt. Soc. Am. A 4, 651–653 (1987). [CrossRef]
3. J. Durnin, J. J. Miceli, and J. H. Eberly, “Diffraction-free beams,” Phys. Rev. Lett. 58, 1499–1501 (1987). [CrossRef] [PubMed]
4. G. A. Siviloglou, J. Broky, A. Dogariu, and D. N. Christodoulides, “Observation of accelerating Airy beams,” Phys. Rev. Lett. 99, 213901 (2007). [CrossRef]
5. G. A. Siviloglou and D. N. Christodoulides, “Accelerating finite energy Airy beams,” Opt. Lett. 32, 979–981 (2007). [CrossRef] [PubMed]
6. G. A. Siviloglou, J. Broky, A. Dogariu, and D. N. Christodoulides, “Ballistic dynamics of Airy beams,” Opt. Lett. 33, 207–209 (2008). [CrossRef] [PubMed]
7. J. Broky, G. A. Siviloglou, A. Dogariu, and D. N. Christodoulides, “Self-healing properties of optical Airy beams,” Opt. Express 16, 12880–12891 (2008). [CrossRef] [PubMed]
8. X. X. Chu, G. Q. Zhou, and R. P. Chen, “Analytical study of the self-healing property of Airy beams,” Phys. Rev. A 85, 013815 (2012). [CrossRef]
9. R. Cao, Y. Hua, C. J. Min, S. W. Zhu, and X. C. Yuan, “Self-healing optical pillar array,” Opt. Lett. 373540–3542 (2012). [CrossRef] [PubMed]
10. I. Dolev, I. Kaminer, A. Shapira, M. Segev, and A. Arie, “Experimental observation of self-accelerating beams in quadratic nonlinear medium,” Phys. Rev. Lett. 108, 113903 (2012). [CrossRef]
11. M. D. Cottrell, J. A. Davis, and T. M. Hazard, “Direct generation of accelerating Airy beams using a 3/2 phase-only pattern,” Opt. Lett. 34, 2634–2636 (2009). [CrossRef] [PubMed]
12. L. Li, T. Li, S. M. Wang, C. Zhang, and S. N. Zhu, “Plasmonic Airy beam generated by in-plane diffraction,” Phys. Rev. Lett. 107, 126804 (2011). [CrossRef] [PubMed]
13. N. K. Efremidis, “Airy trajectory engineering in dynamic linear index potentials,” Opt. Lett. 36, 3006–3008 (2011). [CrossRef] [PubMed]
14. Y. Q. Zhang, M. R. Belić, H. B. Zheng, H. X. Chen, C. B. Li, Y. Y. Li, and Y. P. Zhang, “Interactions of Airy beams, nonlinear accelerating beams, and induced solitons in Kerr and saturable nonlinear medium,” Opt. Express 22, 7160–7171 (2014). [CrossRef] [PubMed]
15. P. Polynkin, M. Kolesik, J. V. Moloney, G. A. Siviloglou, and D. N. Christodoulides, “Curved plasma channel generation using ultraintense Airy beams,” Science 324, 229–232 (2009). [CrossRef] [PubMed]
16. J. Baumgartl, M. Mazilu, and K. Dholakia, “Optically mediated particle clearing using Airy wavepackets,” Nat. Photon. 363, 675–678 (2008). [CrossRef]
17. D. Abdollahpour, S. Suntsov, D. G. Papazoglou, and S. Tzortzakis, “Spatiotemporal Airy light bullets in the linear and nonlinear regimes,” Phys. Rev. Lett. 105, 253901 (2010). [CrossRef]
18. C. Chen, H. M. Yang, M. Kavehrad, and Z. Zhou, “Propagation of radial Airy array beams through atmospheric turbulence,” Opt. Lasers Eng. 52, 106–114 (2014). [CrossRef]
19. B. A. Malomed, D. Mihalache, F. Wise, and L. Torner, “Spatiotemporal optical solitons,” J. Opt. B: Quantum Semiclassical Opt. 7, R53–R72 (2005). [CrossRef]
20. B. A. Malomed, L. Torner, F. Wise, and D. Mihalache, “On multidimensional solitons and their legacy in contemporary Atomic, Molecular and Optical physics,” J. Phys. B: At. Mol. Opt. Phys. 49, 170502 (2016). [CrossRef]
21. Y. I. Salamin, “Approximate fields of an ultra-short, tightly-focused, radially-polarized laser pulse in an under-dense plasma: a Bessel-Bessel light bullet,” Opt. Express 25, 28990–28999 (2017). [CrossRef]
22. D. Mihalache, “Multidimensional localized structures in optical and matter-wave media: a topical survey of recent literature,” Romanian Reports in Physics 69, 403 (2017).
23. Y. I. Salamin, “Fields of a Bessel-Bessel light bullet of arbitrary order in an under-dense plasma,” Sci. Reports 8, 11362 (2018). [CrossRef]
24. Y. V. Kartashov, G. E. Astrakharchik, B. A. Malomed, and L. Torner, “Frontiers in multidimensional self-traping of nonlinear fields and matter,” Nature Reviews Physics 1, 185–197 (2019). [CrossRef]
25. J. L. Qin, Z. X. Liang, B. A. Malomed, and G. J. Dong, “Tail-free self-accelerating solitons and vortices,” Phys. Rev. A 99, 023610 (2019). [CrossRef]
26. Y. Q. Zhang, M. R. Belić, L. Zhang, W. P. Zhong, D. Y. Zhu, R. M. Wang, and Y. P. Zhang, “Periodic inversion and phase transition of finite energy Airy beams in a medium with parabolic potential,” Opt. Express 23, 10467–10480 (2015). [CrossRef] [PubMed]
27. Y. Q. Zhang, X. Liu, M. R. Belić, W. P Zhong, M. S. Petrović, and Y. P. Zhang, “Automatic Fourier transform and self-Fourier beams due to parabolic potential,” Annals of Physics 363, 305–315 (2015). [CrossRef]
28. N. Li, Y. F. Jiang, K. K. Huang, and X. H. Lu, “Abruptly autofocusing property of blocked circular Airy beams,” Opt. Express 22, 22847–22853 (2014). [CrossRef] [PubMed]
29. J. G. Zhang and J. He, “Dual abruptly focus of modulated circular Airy beams,” IEEE Photon. J. 96500510 (2017).
30. G. L. Zheng, X. Q. Deng, S. X. Xu, and Q. Y. Wu, “Propagation dynamics of a circular Airy beam in a uniaxial crystal,” Applied Optics 56, 2444–2448 (2017). [CrossRef] [PubMed]
31. J. G. Zhang and X. S. Yang, “Periodic abruptly autofocusing and autodefocusing behavior of circular Airy beams in parabolic optical potentials,” Opt. Commun. 420, 163–167 (2018). [CrossRef]
32. A. W. Snyder and D. J. Mitchell, “Accessible solitons,” Science 276, 1538 (1997). [CrossRef]
33. Q. Guo, B. Luo, F. Yi, S. Chi, and Y. Xie, “Large phase shift of nonlocal optical spatial solitons,” Phys. Rev. E 69, 016602 (2004). [CrossRef]
34. Q. Guo, B. Luo, and S. Chi, “Optical beams in sub-strongly non-local nonlinear medium: A variational solution,” Opt. Commun. 259, 336–341 (2006). [CrossRef]
35. H. Zhang, L. Li, and S. Jia, “Pulsating behavior of an optical beam induced by initial phase-front curvature in strongly nonlocal medium,” Phys. Rev. A 76, 043833 (2007). [CrossRef]
36. S. Longhi, “Fractional Schrödinger equation in optics,” Opt. Lett. 40, 1117–1120 (2015). [CrossRef] [PubMed]
37. J. F. Liu, Y. Q. Zhang, H. Zhong, J. W. Zhang, R. Wang, M. R. Belić, and Y. P. Zhang, “Optical Bloch oscillations of a dual Airy Beam,” Ann. Phys. (Berlin) 2017, 1700307 (2017).
38. C. Bernardini, F. Gori, and M. Santarsiero, “Converting states of a particle under uniform or elastic forces into free particle states,” Eur. J. Phys. 16, 58–62 (1995). [CrossRef]
39. O. Vallée and M. Soares, Airy Functions and Applications to Physics (Imperial University, 2004). [CrossRef]
40. T. Bland, N. G. Parker, N. P. Proukakis, and B. A. Malomed, “Probing quasi-integrability of the Gross–Pitaevskii equation in a harmonic-oscillator potential,” J. Phys. B 51, 205303 (2018). [CrossRef]
41. C. Conti, M. Peccianti, and G. Assanto, “Observation of Optical Spatial Solitons in a Highly Nonlocal Medium,” Phys. Rev. Lett. 92, 113902 (2004). [CrossRef] [PubMed]
42. C. Rotschild, O. Cohen, O. Manela, M. Segev, and T. Carmon, “Solitons in Nonlinear Media with an Infinite Range of Nonlocality: First Observation of Coherent Elliptic Solitons and of Vortex-Ring Solitons,” Phys. Rev. Lett. 95, 213904 (2005). [CrossRef] [PubMed]
43. C. Rotschild, M. Segev, Z. Y. Xu, Y. V. Kartashov, and L. Torner, “Two-dimensional multipole solitons in nonlocal nonlinear media,” Opt. Lett. 31, 3312–3314 (2006). [CrossRef] [PubMed]