11/8/2022 0 Comments Reverse dispersio in light![]() ![]() Red lines trace the path of the beams, and the surface normals are shown in black. (b) A positive-index wedge, in contrast, will positively refract the same beam. Color represents intensity: red, highest blue, lowest. of a Snell’s law experiment, a negative-index wedge with ∊ = −1 and µ = −1 deflects an electromagnetic beam by a negative angle relative to the surface normal: The beam emerges on the same side of the surface normal as the incident beam. A negative-index material will refract light through a negative angle. ![]() In short, the fundamental electronic and magnetic processes that give rise to resonant phenomena in materials simply do not occur at the same frequencies, although no physical law would preclude such overlap.įigure 2. On the other hand, resonances in magnetic systems typically occur at much lower frequencies and usually tail off toward the THz and IR region. In existing materials, the resonances that give rise to electric polarizations typically occur at very high frequencies-in the optical for metals, and at least in the terahertz-to-IR region for semiconductors and insulators. These consequences can help answer our initial question as to why materials with ∊ and µ both negative are not readily found. Second, the usable bandwidth of negative materials will be relatively narrow compared with positive materials. First, negative material parameters will exhibit frequency dispersion: They will vary as a function of frequency. That negative material parameters occur near a resonance has two important consequences. If instead of electrons the material response were due to harmonically bound magnetic moments, then a negative magnetic response would exist. So large is this stored energy that even changing the sign of the applied electric field has little effect on the polarization near resonance! That is, as the frequency of the driving electric field is swept through the resonance, the polarization flips from in-phase to out-of-phase with the driving field, and the material exhibits a negative response. At frequencies near resonance, the induced polarization becomes very large, as is typical in resonance phenomena the large response represents accumulation of energy over many cycles, such that a considerable amount of energy is stored in the resonator (in this case, the medium) relative to the driving field. At frequencies far below ω 0, an applied electric field displaces the electrons from the positive cores and induces a polarization in the same direction as the applied field. The Drude-Lorentz model of a material is a good starting point, because it conceptually replaces the atoms and molecules of a real material by a set of harmonically bound electron oscillators resonant at some frequency ω 0. Why are there no materials with negative ∊ and µ? One first needs to understand what it means to have a negative ∊ or µ and how negative values occur in materials. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |