NIM-comment-2002

NIM-comment-2002

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Comment on \Wave Refraction in Negative-Index Materials: Always Positive andVery Inhomogeneous"Valanju, Walser and Valanju (VWV) [1] have shown moves away from k , !(k) can be expanded in a Taylor0that for a group consisting of two plane waves incident series to rst order in k k to a good approximation.0i(k r !(k )t0 0on the interface between a material of positive refractive This gives, E = E e g(r ctk =k ), where0 0 0R2 i(k k )R0index (PIM) and material of negative refractive index g(R) = d kf(k k )e . Inside the NIM,k and0(NIM), the group velocity refracts positively. Here we k in the argument of the exponent get replaced by k0 rshow that this is true only for the special two plane wave and k which are related to k and k by Snell’s law.r0 0case constructed by VWV, but for genericlocalized wave Then the wave packet once it enters the NIM is given bypackets, the group refraction is generically negative.0 i(k r !(k )tr0 r0E = E e g (r v t); (1)The sum of two plane waves of wavevector and fre- r r gr0quency (k ;! ) and (k ;! ), considered by VWV, can1 1 2 2 R2 iR[(k k )r k ]0 k ri(k r ! t)0 0 where g (R) = d kf(k k )e . Herer 0be written as 2e cos[(1=2)(k r !t)],k denotesk evaluated atk=k andv =r !(k )r0 r 0 gr k rwhere(k ;! )the averagewavevectorandfrequencyand r0 0evaluated at k =k . Thus, the refracted wave movesr r0(k;!)denotetheirdierences. Clearly,theargumentwith the group velocity v . Evaluation of Eq. (1) forgrof the cosine is constant ...

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Comment on “Wave Refraction in Negative-Index Materials:Always Positive and Very Inhomogeneous”
Valanju, Walser and Valanju (VWV) [1] have shown that for a group consisting of two plane waves incident on the interface between a material of positive refractive index (PIM) and material of negative refractive index (NIM), the group velocity refracts positively.Here we show that this is true only for the special two plane wave case constructed by VWV, but for generic localized wave packets, the group refraction is generically negative. The sum of two plane waves of wavevector and fre-quency (k1, ω1) and (k2, ω2), considered by VWV, can i(k0rω0t) be written as 2ecos[(1/2)(ΔkrΔωt)], where (k0, ω0) the average wavevector and frequency and (Δk,ΔωClearly, the argument) denote their differences. of the cosine is constant along planes, which propagate in time along the direction of their normal,Δk. We have carried out numerical simulations of wave packets incident on the PIM-NIM interface and for the 2-wave case arrive at conclusions similar to VWV. For arbitrary number of incident plane waves whosekvectors are all parallel, the group refraction is again positive.Note that in all these special cases the packet remains nonzero on infinite planes. Here we show that for any wave packet that is spatially localized, the group refraction isgenerically negative. For 3 (or more) waves whose wave vectors not aligned, the group refraction will be negative.For example, con-sider three wave vectors in PIM in thex-z-plane, whose magnitudes are,kδk,k,kδkand whose angles with thez-axis are,θδθ,θ,θ+δθ, respectively.The disper-2 22 22 22 2 sionk= (ωω)(ωω)/c(ωω) were used for p b0 NIM. The results are shown in Fig.1. Thewave packet refracts negatively, in obvious contrast to VWV. As we have seen, two plane waves result in a wave packet-like structure which is constant along planes; the addition of a third wave breaks the planes into localized wave packets which refract negatively. A packet constructed from a finite number of plane waves will always give a collection of propagating wave pulses, as seen in Fig.1. A wavepacket localized in one region of space, as occurs in all experimental situations, can only be constructed from a continuous distribution of wavevectors. Consider such a wave packet incident from R 2i(krω(k)t) outside the NIM,E=E0d kf(kk0)e, whereω(k) =ck. Iff(kk0) drops off rapidly ask
moves away fromk0,ω(k) can be expanded in a Taylor series to first order inkk0to a good approximation. i(k0rω(k0)t This gives,E=E0e g(rctk0/k0), where R 2i(kk0)R g(R) =d kf(kk0)ethe NIM,. Insidekand k0in the argument of the exponent get replaced bykr andkr0which are related tokandk0by Snell’s law. Then the wave packet once it enters the NIM is given by
0i(kr0rω(kr0)t e g(rvt),(1) Er=E0r gr R 2iR[(kk0)∙rkkr] wheregr(R) =d kf(kk0)e. Here kr0denoteskrevaluated atk=k0andvgr=rkrω(kr) evaluated atkr=kr0. Thus,the refracted wave moves with the group velocityvgrof Eq.. Evaluation(1) for a Gaussian wave packet shows that the incident packet gets distorted but the maximum of the packet moves at vgrthe case of an isotropic medium, considered by. For VWV [1], the group velocity is anti-parallel to the wave vector in the medium.Hence, the group velocity will be refracted the same way as the wavevector is, contrary to the claim of VWV [1]. Thus VWV’s statement that the “Group Refraction is always positive” is true only for the very special (and rare) wave packets constructed by them and is incorrect for more general wave packets that are spatially localized. This work was supported by the National Science Foundation, the Air Force Research Laboratories and the Department of Energy. W. T. Lu, J. B. Sokoloff and S. Sridhar Department of Physics, Northeastern University, 360 Huntington Avenue, Boston, MA 02115.
[1] P. M. Valanju, R. M. Walser and A. P. Valanju, Phys. Rev. Lett. 88, 187401 (2002).
FIG. 1:Negative refraction of 3 plane waves withk= 6.32, δk= 0.32,θ=π/4,δθ=π/60,ω0= 2,ωb= 8,ωp= 10, andc= 1.(Inset above) Wavevectors for the 3 plane waves in the PIM (left) and NIM (right).The thick arrows indicate the wave packet propagation direction.