1. Introduction
The ability of manufacturing metamaterials with prescribed and anomalous values of permittivity
ε and permeability
μ has triggered an intense research effort aimed at investigating novel regimes of linear electromagnetic propagation and suitable configurations have been devised for observing remarkable effects such as, for example, superlensing [
1
J. B. Pendry, “Negative Refraction Makes a Perfect Lens”, Phys. Rev. Lett.
85, 3966 (2000). [CrossRef]
[PubMed]
], optical cloaking [
2
J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling Electromagnetic Fields”, Science
312, 1780 (2006). [CrossRef]
[PubMed]
], guiding of nanometric optical beams [
3
J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, “Guiding of a one-dimensional optical beam with nanometer diameter”, Opt. Lett.
22, 475 (1997). [CrossRef]
[PubMed]
] and photonic circuits [
4
N. Engheta, “Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials”, Science
317, 1698 (2007). [CrossRef]
[PubMed]
]. In the nonlinear realm, the nonlinear properties of left-handed metamaterials have been investigated [
5
A. A. Zharov, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear Properties of Left-Handed Metamaterials”, Phys. Rev. Lett.
91, 037401 (2003). [CrossRef]
[PubMed]
] together with various soliton manifestations [
6
I. V. Shadrivov and Y. S. Kivshar, “Spatial solitons in nonlinear left-handed metamaterials”, J. Opt. A: Pure Appl. Opt.
7, 68 (2005). [CrossRef]
]. Propagation in metamaterials exhibiting cubic nonlinear response has also been considered [
7
Y. Liu, G. Bartal, D. A. Genov, and X. Zhang, “Subwavelength Discrete Solitons in Nonlinear Metamaterials”, Phys. Rev. Lett.
99, 153901 (2007). [CrossRef]
[PubMed]
] and, for ultra-short pulse nonlinear dynamics, it has been suggested that metamaterial linear property tailoring allows the observation of different nonlinear regimes [
8
M. Scalora, M. S. Syrchin, N. Akozbek, E. Y. Poliakov, G. D’Aguanno, N. Mattiucci, M. J. Bloemer, and A. M. Zheltikov, “Generalized Nonlinear Schrdinger Equation for Dispersive Susceptibility and Permeability: Application to Negative Index Materials”, Phys. Rev. Lett.
95, 013902 (2005). [CrossRef]
[PubMed]
].
In this Letter we show that a metamaterial with a very small linear dielectric constant and exhibiting a nonlinear cubic response is able to support nonlinear guided waves whose Poynting vector has the very peculiar property of being parallel and anti-parallel to the propagation direction in different transverse portion of the field. This novel phenomenology is a consequence of the fact that, since the metamaterial linear dielectric permittivity can be arbitrary small, the nonlinear contribution to the dielectric response can easily (i.e. at low intensities) be made comparable or greater than the linear part so that, the sign of the overall dielectric response can be different for different intensities. In the presence of an electromagnetic beam this implies that conditions can be found so that the effective dielectric response has different signs on the propagation axis and at its lateral sides. Therefore the transverse reversing of the power flow is understood since, for a monochromatic Transverse Magnetic (TM) field mainly propagating along a given direction, the Poynting vector globally lies along the same mean propagation direction and its sign coincides with that of the total effective dielectric constant. In order to discuss this effect on a feasible situation, we consider TM electromagnetic propagation in a defocusing nonlinear cubic metamaterial and we analytically obtain a class of nonlinear guided waves exhibiting the aforementioned transverse power flow reversing. It is remarkable that the power flow reversing effect can be observed even at very low intensities since it is a consequence of the interplay between the linear and nonlinear contributions to the dielectric response, the former being very small in the considered metamaterials and the latter being proportional to the intensity. The question naturally arises as to whether a medium exists or can be conceived where the range of electromagnetic intensities, for which its nonlinear response is purely cubic, is so large to produce a huge nonlinear response. At first one may reject this possibility since the cubic nonlinear response generally arises from a perturbative description of radiation-matter interaction so that the nonlinear polarization necessarily is a small correction to the linear part. However, exploiting the availability of metamaterials with somehow prescribed values of the dielectric permittivity, we propose that in a suitable sub-wavelength layered structure, consisting of alternating slabs of a metamaterial with negative dielectric constant and an optically active nonlinear cubic medium, the effective electromagnetic response is purely cubic in an intensity range where the nonlinear cubic term can exceed the linear contribution.
2. Nonlinear guided waves
Consider a monochromatic electromagnetic field (whose time variation is assumed to be e
-iωt
, where ω is the angular frequency) propagating through a nonlinear metamaterial characterized by the constitutive relations (holding for the field complex amplitudes)
where ε > 0 and μ > 0 are the linear permittivity and permeability, respectively, whereas χ > 0 and 0 < γ < 1 are the parameters characterizing the cubic defocusing nonlinear response. We focus our attention on transverse magnetic (TM) nonlinear guided waves propagating along the z- axis of the form
where
(
c is the speed of light in vacuum) are dimensionless spatial coordinates,
β is a real dimensionless propagation constant and
ux
and
uz
are dimensionless electric field components. Substituting the fields of Eqs. (
2) into Maxwell equations ∇ ×
E =
iω
B and ∇ ×
H = -
iω
D and using the constitutive relations of Eqs. (
1) we get
which is a system of ordinary differential equations fully characterizing the transverse profile of the considered nonlinear guided waves. Without loss of generality we consider solutions of Eqs. (
3) with definite parity where
ux
and
uz
are spatially even (
ux
(
ξ) =
ux
(-
ξ)) and odd (
uz
(
ξ) = -
uz
(-
ξ)), respectively and, as a consequence, we adopt the boundary conditions
ux
(0) =
u
x0,
uz
(0) = 0 and
ux
(+∞) =
u
x∞,
uz
(+∞) =
u
z∞. Since
ux
(
ξ) and
uz
(
ξ) have to asymptotically approach two constant values, their first and second derivative vanish for
ξ → +∞ so that, exploiting the boundary conditions, we require the right hand sides of Eqs. (
3) to vanish at
ux
=
u
x∞ and
uz
=
u
z∞. Therefore we obtain
and
from which we note that
u
2
x∞ < 1/2 is a necessary condition for the existence of the considered nonlinear waves. In order to prove their existence, we exploit the fact that the system of Eqs. (
3) is integrable [
9
A. Ciattoni, B. Crosignani, P. Di Porto, and A. Yariv, “Perfect optical solitons: spatial Kerr solitons as exact solutions of Maxwells equations”, J. Opt. Soc. Am. B
22, 1384 (2005). [CrossRef]
] since it admits the first integral
or, in other words, the relation
holds for any solution
ux
(
ξ),
uz
(
ξ) of Eqs. (
3). Evidently, after substituting the obtained
β into Eq. (
4),
F has a stationary point at (
u
x∞,
u
z∞) and the guided waves are represented by curves of constat
F in the plane (
ux
,
uz
) joining (
u
x0, 0) to the stationary point. Therefore, requiring that
F has a saddle point at (
u
x∞,
u
z∞) and exploiting the above necessary condition, we conclude that the considered nonlinear waves exist in the range
which is always not empty since
γ > 0. In addition the relation
F(
u
x0,0) =
F(
u
x∞,
u
z∞) yields the possible values of
ux
(0) =
u
x0 corresponding to the asymptotical value
ux
(+∞) =
u
x∞. In
Fig. 1(a) and
1(b) we plot the profiles of
ux
and
uz
corresponding to different values of
u
x∞, spanning the range of Eq. (
5), for
γ= 0.5 obtained by numerically integrating Eqs. (
3) with the above boundary conditions. The power flow carried by these waves is described by the time-average Poynting vector
which, exploiting Eqs.(
2) and the first of Eq. (
3), becomes
Fig. 1. Nonlinear guided waves transverse profile of
ux
(panel (a)) and of
uz
(panel (b)) at different values of
u
x∞ in the range of Eq. (
5), for
γ = 0.5. (c) Profiles of the
z- component of the Poynting vector (see Eq. (
6)) normalized with
corresponding to the fields reported in
Fig. 1(a) and
1(b). Each profile is characterized by an off-center positive part (black portion) and a central negative part (red portion). (d) Plot of the field
S/
S
0 (arrows) in the plane (
ξ,
ζ) corresponding to the nonlinear guided wave with
u
x∞ = 0.65 of
Fig. 1(a) and
1(b). The color is related to the local value of
Sz
/
S
0. Note the reversing of
S along the transverse
ξ axis.
i.e., for the considered waves, is purely along the
z- axis. In
Fig. 1(c) we plot the profiles of
Sz
evaluated for the fields reported in
Fig. 1(a) and
1(b), and we note that the sign of
Sz
is not constant along the transverse profiles, a region where
Sz
< 0 (red portion of the curves) existing around
ξ = 0. This reversing of the power flow along the transverse profile of the nonlinear guided waves is particularly evident from
Fig. 1(d) where we draw the vector field
S on the plane (
ξ,
ζ) for one of the fields of
Fig. 1(a) and
1(b). In order to physically grasp and discuss this unusual effect we recast Eq. (
6) in the form
where use of Eqs. (
2) and (
1) has been made, from which it is evident that the transverse power flow reversing is a consequence of the sign flipping of
Dx
along the wave profile while
Ex
does not change its sign. This implies that, regardless the absolute sign of the fields, the overall effective dielectric response undergoes a sign reversing due to the fact that, in the first of Eq. (
1), the nonlinear cubic term can be both smaller and greater than the linear part, depending on the local field strengths. It is worth noting that, although we have discussed this effect using the considered nonlinear guided waves admitting analytical treatment, the phenomenon is more general and holds for the wider class of TM fields of the form
E(
x,
z) =
eikz
[
Ex
(
x,
z)
ê
x
+
iEz
(
x,
z)
ê
z
] since, if ∣∂
z
Ex
∣ ≪
k∣
Ex
∣ and ∣∂
z
Ez
∣ ≪
k∣
Ez
∣ (i.e. the field manly propagates along the
z- axis) it is simple to obtain from Maxwell equations that
and the transverse power flow reversing can take place through the just discussed mechanism. We conclude that the predicted power flow reversing is a signature of the extreme nonlinear regime where the cubic nonlinear contribution to the medium polarizability is not a mere perturbation of the linear part. In this sense the medium behaves as a metamaterial whose character (positive or negative dielectric constant) locally depends on the field intensity. The discussed power flow reversing should be compared with the inhomogeneous power flow distribution occurring in linear photonic crystals since there it is associated to the fact that the variation of the index of refraction is comparable to average index of refraction. It is worth stressing that the discussed power flow reversing is very different from the effect that, in left handed metamaterials, the Poynting vector is antiparallel to the carrier wave vector which is a consequence of the fact that, in such media,
ε < 0 and
μ < 0 (with
n < 0). On the other hand, in our case,
μ > 0 and the sign of the power flow is not uniform being controlled through the field intensity. Note that such an extreme condition can be achieved when the field intensity ∣
E∣
2 is comparable or greater than
ε/
χ, so that, in standard materials where
ε is generally of the order of unity and
χ is very small (of the order of 10
-20m
2/V
2 in semiconductors [
10
R. W. Boyd, Nonlinear Optics (Academic Press, New York, 1994).
]), the required intensity is so large to rule out the whole discussed phenomenology. However, if a metamaterial is employed where
ε can be chosen to be much smaller than unity, the intensity threshold can be reduced to the point of making the extreme nonlinear regime accessible even for intensities much smaller than those employed in standard nonlinear optics experiments.
Fig. 2. Metamaterial layered structure able to support transverse power flow reversing of TM fields, consisting of alternating slabs of a negative permittivity dielectric (ND) and a nonlinear cubic medium (NL).
3. Nonlinear layered medium supporting the extreme nonlinear regime
Even though the use of a metamaterial (
ε ≪ 1) makes feasible intensities able to trigger the above linear-nonlinear competition, the main issue remains of finding a medium whose dielectric response is, in the considered intensity range, purely cubic. In fact, the first of Eqs. (
1) is a power series expansion of the constitutive relation
D =
D(
E) in the field strength
E and therefore, if the third order is comparable with the first one, one generally has to consider higher order terms. In order to show that the discussed extreme nonlinear regime can effectively be achieved, consider the metamaterial structure reported in
Fig. 2 consisting of alternate linear metamaterial and nonlinear medium layers, along the
y-axis, of thickness
d
1 and
d
2 respectively. The metamaterial is a negative dielectric (ND) whose constitutive relation is
D =
ε
0
ε
1
E (where
Re(
ε
1) < 0) whereas the nonlinear medium (NL) is characterized by the constitutive relation
D =
ε
0
ε
2
E -
ε
0
χ
2[(
E·
E
*)
E +
γ(
E·
E)
E
*], i.e. it is an isotropic defocusing (
Re
(
ε
2) > 0,
χ
2 > 0) Kerr medium. The ND medium is generally a metal so that, in order to compensate losses, we suppose that the NL medium is also an active medium and that
Im(
ε
2) can be tuned by adjusting the pumping efficiency (see example below) [
11
Note that the presented scheme can be improved by considering more than two basic layers constituents. This can simplify the identification of suitable active media (not coinciding with the nonlinear medium) to steer loss compensation.
]. The media relative permeability are [
μ
1 and
μ
2, respectively. If the spatial period
d =
d
1 +
d
2 is much smaller than the field vacuum wavelength 2
πc/
ω, the TM field propagating through the structure experiences the effective response described by Eqs. (
1) and characterized by the spatially averaged parameters
where f = d
1/(d
1+d
2) is the fraction of negative dielectric. From these relations, it is evident that suitable values of ε
1, f and Im(ε
2) can be chosen so that 0 < Re(ε) ≪ 1 and ∣Im(ε)∣ ≪ Re(ε) i.e. the overall medium effective response coincides with the one considered in present Letter. Most importantly, the medium is able to support the extreme nonlinear regime since if the field is such that ∣E∣2 ~ ε
2/χ, at the same time one has that ∣E∣2 ≪ ε
2/χ
2. Therefore the nonlinear medium layers (NL) are in the presence of a field for which their response is purely cubic and, as a consequence, the overall averaged structure response is purely cubic as well.
As a specific example, consider a TM field of wavelength
λ = 0.810
μm propagating through a layered metamaterial structure for which
ε
1 = -28.79800 + 1.55000
i,
μ
1 = 1 and
ε
2 = 10.90000 - 0.56750
i,
μ
2 = 1,
χ
2 = 6.56 × 10
-18
m
2/
V
2,
γ = 0.5. For the considered wavelength
λ, the chosen
ε
1 coincides with the silver permittivity [
12
E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, San Diego, 1998).
] whereas
Re(
ε
2) and
χ
2 are the linear and nonlinear parameters characterizing the AlGaAs [
13
M. Sheik-Bahae, D. C. Hutchings, D. J. Hagan, and E. W. Van Stryland, “Dispersion of Bound Electronic Nonlinear Refraction in Solids”, IEEE J. Quantum Electron.
27, 1296 (1991). [CrossRef]
]. Here we are exploiting the fact that AlGaAs optically amplifies the radiation at the chosen wavelength if the sample is pumped by ultra-violet light and, consequently, the above value of
Im(
ε
2) can be attained simply by adjusting the pump laser intensity [
7
Y. Liu, G. Bartal, D. A. Genov, and X. Zhang, “Subwavelength Discrete Solitons in Nonlinear Metamaterials”, Phys. Rev. Lett.
99, 153901 (2007). [CrossRef]
[PubMed]
,
14
S. A. Ramakrishna and J. B. Pendry, “Removal of absorption and increase in resolution in a near-field lens via optical gain”, Phys. Rev. B
67, 201101(R) (2003).
]. Choosing
f = 0.2754, from Eqs. (
8) we obtain the effective medium parameters
ε = 0.00235 + 0.00003
i,
μ = 1 and
χ = 4.76 × 10
-18m
2/
V
2. The absorption coefficient of the considered effective medium is
α = (4
π/
λ)
Im(∞
ε) ≃ 4×10
-3
μm
-1 so that the above power flow reversing effect can be observed for propagation distances up to the decay length 1/
α ≃ 208
μm (note that this decay length can be made larger by improving the balance between losses and gain). Consider now the nonlinear guided wave whose power flow is reported in
Fig. (2)b which is characterized by
u
x∞ = 0.65 and a transverse dimensionless width ∆
ξ ≃ 2 (∆
ξ also coincides with the width of the transverse portion of the field where the Poynting vector is antiparallel to the propagation direction). The physical width of the considered wave is
whereas the maximum of its normalized Poynting vector is
Sz
/
S
0 ≃ 0.1 (see
Fig. 2) so that the wave is characterized by the intensity
Sz
= 0.1
S
0 ≃ 0.3
MW/
cm
2. It is worth stressing that the considered micron-sized confined wave is observable with an intensity (~
MW/
cm
2) much smaller than that (~
GW/
cm
2) required for exciting a spatial soliton (of the same width and at the same wavelength) propagating through a AlGaAs sample [
15
Y. S. Kivshar and G. P. Agrawal, Optical Solitons (Academic Press, San Diego, 2003).
].