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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 13 — Jun. 25, 2007
  • pp: 8021–8028
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Nematicons across interfaces: anomalous refraction and reflection of solitons in liquid crystals

Marco Peccianti and Gaetano Assanto  »View Author Affiliations


Optics Express, Vol. 15, Issue 13, pp. 8021-8028 (2007)
http://dx.doi.org/10.1364/OE.15.008021


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Abstract

The robustness of nematicons, i. e. spatial solitons in nematic liquid crystals, can be exploited to implement counter-intuitive negative reflection and refraction schemes for optical signal manipulation at interfaces.

© 2007 Optical Society of America

1. Introduction

Refraction at the interface between two transparent dielectric media is at the basis of simple steering schemes in several optical devices.[1

1. M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Pergamon Press, London, 1959) [PubMed]

] Recently, the phenomenon of negative refraction (NR) in negative-index media has attracted a great deal of attention, both for realizing perfect matched lenses and for observing a reversal of the Doppler shift and Vavilov-Cerenkov radiation.[2–3

2. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett. 84, 4184–4187 (2000). [CrossRef] [PubMed]

] V. G. Veselago, in a pioneering theoretical work, illustrated the potential use of the so called meta-materials for NR at the interface between a normal medium -with both permittivity e and permeability μ being positive- and an abnormal left handed material -with both ε and μ being negative.[4

4. V. G. Veselago, “The electrodynamics of substances with simultaneously negative value of ε and μ,” Sov. Phys. Usp. 10, 509–514 (1968). [CrossRef]

] In such left-handed materials the wavevector k of a plane electromagnetic wave forms a left-handed triplet with the electric E and magnetic H field vectors, i. e. the Poynting vector S is antiparallel to k. More recently, Lindell et al. pointed out that NR can occur at the interface between uniaxials exhibiting just one negative diagonal component of the dielectric tensor.[5

5. V. Lindell, S. A. Tretyakov, K. I. Nikoskinen, and S. Ilvonen, “Bw media - media with negative parameters, capable of supporting backward waves,” Microwave Opt. Technol. Lett. 31, 129–131 (2001). [CrossRef]

]

From the inception of second-order nonlinear optics, double refraction and walk-off have been recognized to be fundamentally important in both phase-matching processes and parametric energy exchange between waves.[14

14. G. D. Boyd, A. Ashkin, J. M. Dziedzic, and D. A. Kleinman, “Second-harmonic generation of light with double refraction,” Phys. Rev. 137, 1305–1309 (1965). [CrossRef]

] It has been much less emphasized the relevance and potential role of optical anisotropy in cubic nonlinear optics, particularly with reference to light self-trapping into optical spatial solitons. The latter are self-localized -hence non-diffracting- optical beams which, through a nonlinear alteration of the refractive index distribution, give rise to light-induced waveguides with large potentials in novel generations of readdressable/reconfigurable optical networks.[15

15. Y. S. Kivshar and G. P. Agrawal, Optical Solitons (Academic, London, 2003).

] Among the variety of materials where spatial solitons have been investigated, liquid crystals in the nematic phase have been shown to be an excellent workbench for their study at milliwatt power levels and in conjunction with a spatially non local response, i. e., one which extends well beyond the region excited by the electromagnetic field. [16–20

16. M. Peccianti, G. Assanto, A. De Luca, C. Umeton, and I. C. Khoo, “Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells,”Appl. Phys. Lett. 77, 7–9 (2000). [CrossRef]

] Nematic liquid crystals (NLC), in fact, consist of elastically-linked elongated organic molecules which can reorient under the torque generated by an electric field on the induced dipoles. Thereby, owing to the morphologic anisotropy and optical birefringence of these positive uniaxial molecules, their angular reorientation results in a refractive index increase (a positive nonlinearity) for extraordinarily-polarized waves with an electric field vector coplanar to both the wave-vector k and the major molecular axis or director n̂ .[21

21. I. C. Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley, New York, 1995).

] NLC reorientational nonlinearity and non locality support the generation and propagation of stable bright spatial solitons in the two transverse dimensions (2D+1), self-confined light-beams also known as Nematicons. [22

22. G. Assanto, M. Peccianti, and C. Conti, “Nematicons: optical spatial solitons in nematic liquid crystals,” Opt. Photon. News 14, 44–48 (2003). [CrossRef]

] While NLC large birefringence and walk-off can give rise to negative refraction at an interface with glass, [23

23. O. P. Pishnyaka and O. D. Lavrentovich, “Electrically controlled negative refraction in a nematic liquid crystal,” Appl. Phys. Lett. 89, 251103 (2006). [CrossRef]

] voltage-controlled reorientation can also be effectively employed to steer nematicons in angle, thereby redirecting the corresponding all-optical waveguide by as much as seven degrees.[24

24. M. Peccianti, C. Conti, G. Assanto, A. De Luca, and C. Umeton, “Routing of highly anisotropic spatial solitons and modulational instability in liquid crystals,” Nature 432, 733–737 (2004). [CrossRef] [PubMed]

] Moreover, taking advantage of their intrinsic robustness and polarization conservation,[25

25. A. Alberucci, M. Peccianti, G. Assanto, G. Coschignano, A. De Luca, and C. Umeton, “Self-healing generation of spatial solitons in liquid crystals,” Opt. Lett. 30, 1381–1383 (2005). [CrossRef] [PubMed]

] nematicons can refract and even undergo total internal reflection at an interface between two NLC regions differing in index and walk-off, [26

26. M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, “Tunable refraction and reflection of self-confined light beams,” Nature Phys. 2, 737–741 (2006). [CrossRef]

] resulting in various novel phenomena such as non-specular reflection in angle and power-dependent Goos-Hänchen shift. [27–28

27. M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, “Nonspecular total internal reflection of spatial solitons at the interface between highly birefringent media,” Phys. Rev. Lett. 98, 113902 (2007) [CrossRef] [PubMed]

]

2. Nematicons and anomalous refraction

Let us consider an NLC cell as in Fig. 1, consisting of two parallel glass slides which confine a liquid crystal by capillarity. Defining a right-handed reference xyz, with x across the cell thickness and z along its length and perpendicular to the input interface, special treatments of the inner interfaces permit to anchor the molecules, defining a planar director (optic axis) distribution in the yz plane at an angle ρ=ρ 0 with respect to y. An external voltage can be applied through indium tin oxide electrodes deposited on the inner glass surfaces, biasing the NLC and determining the molecular reorientation in the bulk of the sample, as indicated in Fig. 1(b): the azimuth angle ξ=ξ(V) can be increased, thereby changing the refractive index experienced by a propagating e-wave at optical frequencies.

Fig. 1. Sketch of the NLC cell geometry: (a) 3D view from the top; (b) xy cross sectional view with the indication of the (transparent) electrodes for the application of an external (low frequency) voltage V.

A light-beam of arbitrary polarization, impinging the input interface in z=0 with wavevector kin at an angle Φi with respect to z, is subject to double-refraction: it splits in the ordinary (o) and extraordinary (e) eigenwaves of the positive uniaxial NLC with wave vectors ko (ko<z=Φo) and ke (ke<z=Φe), respectively. Figure 2(a) displays this case at zero bias V=0 V. The Poynting vector Se associated to the e-wave lies in the extraordinary principal plane yzke n̂) at the walk-off angle δ with respect to ke, i. e., between ke and n̂.

Fig. 2. (a) Double refraction at the isotropic-uniaxial input interface. In refraction the y component of the wavevector is conserved. (b) From the ISWN we can obtain the direction of the emerging o- and e-wavevectors ko and ke, respectively, and the associated Poynting vectors So and Se.

In geometric optics, refraction can be determined from the wave normals. Fixing the origin, permitted eigen-wavevectors are represented by plotting k = k 0 n(r̂)r̂ , with k 0 being the vacuum wavenumber and n(r̂) the refractive index of waves propagating along r̂. If θ is the angle between n̂ and r̂, it is n(r̂) = no=n for ordinarily polarized (“o-”) waves and n 2(r) = n 2 e(r̂) = ([cos(θ)/n ]2 +[sin(θ)/n ]2)-1 for extraordinary (“e-”) waves. Here we use the subscripts ⊥ and ∥ with reference to the NLC molecular axis n̂ Considering all possible r̂, in each medium k describes a two-sheet surface called inverse surface of wave normals (ISWN). [1

1. M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Pergamon Press, London, 1959) [PubMed]

] The latter is a sphere (ellipsoid) for the o-waves (e-waves). In Fig. 2(b), the intersection of the ISWN with plane yz is shown for the isotropic incidence medium (dashed circle, either air or glass: for simplicity we consider just one isotropic medium of index n v) and NLC (o: solid line circle, e: solid line ellipse). Following Snell's law, the refracted wavevectors ko and ke conserve the kin projection along y, defining the angles Φo and Φe.

The Poynting vectors So and Se are perpendicular to the ISWN corresponding to the points where ko and ke contact the two sheets, respectively, with koSo and keSe/(|ke|∙|Se|)=cos(δ), being δ = arctan(εasin2θ/(2ε +εa + εacos2θ)) the walk-off, ε = n 2 and εa = n 2 - n 2 the birefringence. Therefore, in this geometry, when δ exceeds Φe (as governed by the Snell's law) the e-wave undergoes negative refraction, as pictured in Fig. 2.

It is in line with the result above that the first observation of NR can be attributed to Bartholinus: for small angles of incidence Φi, e-wave refraction is substantially governed by the walk-off angle. Therefore, through a calcite crystal (n =1.6646, n =1.4890 at λ=514nm) it is easy to observe a negative displacement of the image carried by e-rays for a range of Φi

Figure 3 shows the experimental evidence of negative refraction in the NLC configuration of Fig. 2: a mixed polarization (o and e) 1.5+1.5 mW beam from a Nd:YAG laser (λ=1064nm) is injected at a small Φi, and kiny>0 in a cell filled with commercial E7 (n =1.6954 and n = 1.5038), and prepared with ρ0=60°. The image of light evolution in yz, acquired from scattered photons using a microscope and a CCD camera, displays a diffracting o-beam under normal refraction and a negatively refracted e-beam which, due to the self-focusing reorientational response, forms a spatial soliton with invariant cross-section. From the acquired images we could estimate Φo=2.7° , Φi=4.1° and δ≈5.8° which correspond to negative refraction of an e-wave.

Fig. 3. Example of double refraction in NLC: the o-wave (upper beam) undergoes positive refraction and diffracts. Owing to walk-off and all-optical reorientation, the self-trapped e-beam becomes a nematicon and propagates with negative refraction (towards y<0).

The effect of an external bias on the NLC can be appreciated by simple geometrical considerations. Naming θ0 the angle between ke and the projection of n̂ on yz, we can write:

cos(θ)=cos(θ0)cos(ξ)
(1)

and similarly

cos(δ)=cos(δyz)cos(δx)
(2)

with δyz being the angle between ke and the projection of Se on yz and δx the S azimuthal angle. Therefore:

tan2(δyz)=tan2(δ)tan2ξsin2θ0+1
(3)

and

Sexs=sin(δx)
Seys=sin(Φeδyz)
(4)

Figure 5 shows the experimentally acquired evolution of a 3 mW nematicon as it propagates in the plane yz while the external voltage is ramped from 0 to 2V and ξ approaches 90°. As walk-off δ decreases, so does its apparent value δyz in yz, until eventually the e-beam assumes an o-ray configuration and overlaps with the (weaker) ordinary portion of the excitation, the latter obeying Snell's law.

Fig. 4. Effects of external voltage V on the propagation of a nematicon. As the bias increases making ξ larger, (a) the electro-optic reorientation increases the extraordinary index, decreasing Φe; (b) walk-off reaches a maximum and then decreases towards zero; (c) x and y components of S are altered by anisotropic refraction.
Fig. 5. Effects of external voltage V (movie: steering.mov, 345KB) on the propagation of a nematicon: a voltage ramp is applied from V=0 V and reaches V=2 V after 25 s. The e-ray nematicon progressively approaches the o-beam (faint diffractive components at t=0 s, top left photograph) as ξ increases and , consequently, δ tends to zero. [Media 1]

3. Negative reflection

A straightforward configuration for the latter case is illustrated in Fig. 6, where the incident beam or spatial soliton reaches the surface of separation between NLC and a metal mirror with Poynting vector Si and wavevector ki pointing towards positive and negative y, respectively (see Fig. 6(b)). Based on the simple ISWN construction examined above, upon reflection the wavevector kR will point towards negative y and the reflected power will travel along SR with SRy<0 due to the small walk-off, thereby undergoing negative reflection. The experimental demonstration of this phenomenon with near-infrared light is underway.

Fig. 6. Scheme of negative reflection at an NLC-metal interface. In the presence of high birefringence, the director distribution can be arranged in order to obtain both incident and reflected Poynting vectors belonging to the same quadrant. (a) Basic configuration and (b) sketch of ISWN: the transverse wavevectors are conserved and the Poynting vectors belong to the same (incidence) quadrant.

Finally, Fig. 7 suggests the extension of negative reflection for implementing an optical resonator with an anisotropic nonlinear dielectric such as NLC. Under appropriate conditions, owing to negative reflection a self-confined wave packet (nematicon) undergoes resonance while propagating with energy flow at an angle δ with respect to the cavity mirror normals, as in Fig. 7 (b).

Fig. 7. Principle of operation of an optical resonator in an anisotropic medium.

Although a detailed analysis is well beyond the scopes of this paper, even in the presence of significant anisotropy spatial solitons can be expected to be stable in such a resonator filled with a (spatially and temporally) nonlocal nonlinear medium.

4. Conclusions

Negative refraction as well as negative reflection can be obtained in media with strictly positive refractive index and permeability, provided there is significant birefringence and walk-off. We reported negative refraction and its voltage-tuning in uniaxial nematic liquid crystals, where a giant non local and non-resonant nonlinearity can support the propagation of self-guided beams or spatial solitons. We also pointed out a variety of voltage controlled phenomena which can be envisioned in conjunction with such all-optically induced nematicon waveguides.

Acknowledgments

The authors thank M. Kaczmarek and A. Dyadyusha (ORC, Southampton). This work was supported in part by the Italian MIUR (PRIN 2005098337).

References and links

1.

M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Pergamon Press, London, 1959) [PubMed]

2.

D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett. 84, 4184–4187 (2000). [CrossRef] [PubMed]

3.

R. W. Ziolkowski and E. Heyman, “Wave propagation in media having negative permittivity and permeability,” Phys. Rev. E 64, 056625 (2001). [CrossRef]

4.

V. G. Veselago, “The electrodynamics of substances with simultaneously negative value of ε and μ,” Sov. Phys. Usp. 10, 509–514 (1968). [CrossRef]

5.

V. Lindell, S. A. Tretyakov, K. I. Nikoskinen, and S. Ilvonen, “Bw media - media with negative parameters, capable of supporting backward waves,” Microwave Opt. Technol. Lett. 31, 129–131 (2001). [CrossRef]

6.

Y. Zhang, B. Fluegel, and A. Mascarenhas, “Total Negative Refraction in Real Crystals for Ballistic Electrons and Light,” Phys. Rev. Lett. 91, 157404 (2003). [CrossRef] [PubMed]

7.

X. L Chen, Ming He, XinXiao DU, W. Y. Wang, and D. F. Zhang, “Negative refraction: An intrinsic property of uniaxial crystals,” Phys. Rev. 72, 113111 (2005). [CrossRef]

8.

Z. Liu, Z. Lin, and S. T. Chui, “Negative refraction and omnidirectional total transmission at a planar interface associated with a uniaxial medium,” Phys. Rev. B 69, 115402 (2004). [CrossRef]

9.

E. Bartholinus, Experimenta crystalli islandici disdiaclastici quibus mira & insolita refractio detegitur, (Hafniæ, Denmark1669).

10.

C. Huygens, Traitè de Lumiere (Acad. des Sciènce, Paris, 1690).

11.

H. LaFey, “The Vikings,” Nat. Geogr. 137, 528–530 (1970).

12.

C. Roslund and C. Beckman, “Disputing Viking navigation by polarized skylight,” Appl. Opt. 33, 4754–4755 (1994). [CrossRef] [PubMed]

13.

H. Motz and H. Kogelnik, “Electromagnetic radiation from sources embedded in an infinite anisotropic medium and the significance of the Poynting vector,” in “Electromagnetic Theory and Antennas,” Section B, pap. no. 14, Copenhagen, Denmark, June 25–30, 1962.

14.

G. D. Boyd, A. Ashkin, J. M. Dziedzic, and D. A. Kleinman, “Second-harmonic generation of light with double refraction,” Phys. Rev. 137, 1305–1309 (1965). [CrossRef]

15.

Y. S. Kivshar and G. P. Agrawal, Optical Solitons (Academic, London, 2003).

16.

M. Peccianti, G. Assanto, A. De Luca, C. Umeton, and I. C. Khoo, “Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells,”Appl. Phys. Lett. 77, 7–9 (2000). [CrossRef]

17.

M. Peccianti, K. A. Brzdakiewicz, and G. Assanto, “Nonlocal spatial soliton interactions in bulk nematic liquid crystals,” Opt. Lett. 27, 1460–1462 (2002). [CrossRef]

18.

G. Assanto and M. Peccianti, “Spatial solitons in nematic liquid crystals”, IEEE J. Quantum Electron. 39, 13–21 (2003). [CrossRef]

19.

C. Conti, M. Peccianti, and G. Assanto, “Route to nonlocality and observation of accessible solitons,” Phys. Rev. Lett. 91, 073901 (2003). [CrossRef] [PubMed]

20.

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]

21.

I. C. Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley, New York, 1995).

22.

G. Assanto, M. Peccianti, and C. Conti, “Nematicons: optical spatial solitons in nematic liquid crystals,” Opt. Photon. News 14, 44–48 (2003). [CrossRef]

23.

O. P. Pishnyaka and O. D. Lavrentovich, “Electrically controlled negative refraction in a nematic liquid crystal,” Appl. Phys. Lett. 89, 251103 (2006). [CrossRef]

24.

M. Peccianti, C. Conti, G. Assanto, A. De Luca, and C. Umeton, “Routing of highly anisotropic spatial solitons and modulational instability in liquid crystals,” Nature 432, 733–737 (2004). [CrossRef] [PubMed]

25.

A. Alberucci, M. Peccianti, G. Assanto, G. Coschignano, A. De Luca, and C. Umeton, “Self-healing generation of spatial solitons in liquid crystals,” Opt. Lett. 30, 1381–1383 (2005). [CrossRef] [PubMed]

26.

M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, “Tunable refraction and reflection of self-confined light beams,” Nature Phys. 2, 737–741 (2006). [CrossRef]

27.

M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, “Nonspecular total internal reflection of spatial solitons at the interface between highly birefringent media,” Phys. Rev. Lett. 98, 113902 (2007) [CrossRef] [PubMed]

28.

M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, “Nonlinear shift of spatial solitons at a graded dielectric interface,” Opt. Lett. 32, 271–273 (2007). [CrossRef] [PubMed]

29.

Handbook of Laser Science and Technology: Optical Materials, ed. M. J. Weber (CRC Press, New York, 1995).

OCIS Codes
(160.3710) Materials : Liquid crystals
(190.4400) Nonlinear optics : Nonlinear optics, materials

ToC Category:
Nonlinear Optics

History
Original Manuscript: May 2, 2007
Revised Manuscript: June 5, 2007
Manuscript Accepted: June 6, 2007
Published: June 12, 2007

Citation
Marco Peccianti and Gaetano Assanto, "Nematicons across interfaces: anomalous refraction and reflection of solitons in liquid Crystals," Opt. Express 15, 8021-8028 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-13-8021


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References

  1. M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Pergamon Press, London, 1959) [PubMed]
  2. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, "Composite medium with simultaneously negative permeability and permittivity," Phys. Rev. Lett. 84, 4184-4187 (2000). [CrossRef] [PubMed]
  3. R. W. Ziolkowski and E. Heyman, "Wave propagation in media having negative permittivity and permeability,' Phys. Rev. E 64, 056625 (2001). [CrossRef]
  4. V. G. Veselago, "The electrodynamics of substances with simultaneously negative value of ε and μ," Sov. Phys. Usp. 10, 509-514 (1968). [CrossRef]
  5. V. Lindell, S. A. Tretyakov, K. I. Nikoskinen, and S. Ilvonen, "Bw media - media with negative parameters, capable of supporting backward waves," Microwave Opt. Technol. Lett. 31, 129-131 (2001). [CrossRef]
  6. Y. Zhang, B. Fluegel, and A. Mascarenhas, "Total Negative Refraction in Real Crystals for Ballistic Electrons and Light," Phys. Rev. Lett. 91, 157404 (2003). [CrossRef] [PubMed]
  7. X. L. Chen, Ming He, YinXiao Du, W. Y. Wang, and D. F. Zhang, "Negative refraction: An intrinsic property of uniaxial crystals," Phys. Rev. 72, 113111 (2005). [CrossRef]
  8. Z. Liu, Z. Lin, and S. T. Chui, "Negative refraction and omnidirectional total transmission at a planar interface associated with a uniaxial medium," Phys. Rev. B 69, 115402 (2004). [CrossRef]
  9. E. Bartholinus, Experimenta crystalli islandici disdiaclastici quibus mira & insolita refractio detegitur, (Hafniæ, Denmark 1669).
  10. C. Huygens, Traitè de Lumiere (Acad. des Sciènce, Paris, 1690).
  11. H. LaFey, "The Vikings," Natl. Geogr. 137, 528-530 (1970).
  12. C. Roslund and C. Beckman, "Disputing Viking navigation by polarized skylight," Appl. Opt. 33, 4754-4755 (1994). [CrossRef] [PubMed]
  13. H. Motz and H. Kogelnik, "Electromagnetic radiation from sources embedded in an infinite anisotropic medium and the significance of the Poynting vector,’’ in "Electromagnetic Theory and Antennas," Section B, paper no. 14, Copenhagen, Denmark, June 25-30, 1962.
  14. G. D. Boyd, A. Ashkin, J. M. Dziedzic, and D. A. Kleinman, "Second-harmonic generation of light with double refraction," Phys. Rev. 137, 1305-1309 (1965). [CrossRef]
  15. Y. S. Kivshar and G. P. Agrawal, Optical Solitons (Academic, London, 2003).
  16. M. Peccianti, G. Assanto, A. De Luca, C. Umeton, and I. C. Khoo, "Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells,"Appl. Phys. Lett. 77, 7-9 (2000). [CrossRef]
  17. M. Peccianti, K. A. Brzdąkiewicz, and G. Assanto, "Nonlocal spatial soliton interactions in bulk nematic liquid crystals," Opt. Lett. 27, 1460-1462 (2002). [CrossRef]
  18. G. Assanto and M. Peccianti, "Spatial solitons in nematic liquid crystals," IEEE J. Quantum Electron. 39, 13-21 (2003). [CrossRef]
  19. C. Conti, M. Peccianti, and G. Assanto, "Route to nonlocality and observation of accessible solitons," Phys. Rev. Lett. 91, 073901 (2003). [CrossRef] [PubMed]
  20. 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]
  21. I. C. Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley, New York, 1995).
  22. G. Assanto, M. Peccianti, and C. Conti, "Nematicons: optical spatial solitons in nematic liquid crystals," Opt. Photonics. News 14, 44-48 (2003). [CrossRef]
  23. O. P. Pishnyaka and O. D. Lavrentovich, "Electrically controlled negative refraction in a nematic liquid crystal," Appl. Phys. Lett. 89, 251103 (2006). [CrossRef]
  24. M. Peccianti, C. Conti, G. Assanto, A. De Luca, and C. Umeton, "Routing of highly anisotropic spatial solitons and modulational instability in liquid crystals," Nature 432, 733-737 (2004). [CrossRef] [PubMed]
  25. A. Alberucci, M. Peccianti, G. Assanto, G. Coschignano, A. De Luca, and C. Umeton, "Self-healing generation of spatial solitons in liquid crystals," Opt. Lett. 30, 1381-1383 (2005). [CrossRef] [PubMed]
  26. M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, "Tunable refraction and reflection of self-confined light beams," Nat. Phys. 2, 737-741 (2006). [CrossRef]
  27. M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, "Nonspecular total internal reflection of spatial solitons at the interface between highly birefringent media," Phys. Rev. Lett. 98, 113902 (2007) [CrossRef] [PubMed]
  28. M. Peccianti, A. Dyadyusha, M. Kaczmarek, and G. Assanto, "Nonlinear shift of spatial solitons at a graded dielectric interface," Opt. Lett. 32, 271-273 (2007). [CrossRef] [PubMed]
  29. Handbook of Laser Science and Technology: Optical Materials, ed. M. J. Weber (CRC Press, New York, 1995).

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