Guided modes below the bulk plasma frequency

We present the results of calculations for a waveguide in which the dielectric constant ? decreases exponentially from a positive value at the surface to a negative value in the bulk, the guided waves propagating perpendicular to ▽?. Both TM and TE modes are found, each having some unusual properties as compared with modes in the same guides above the plasma frequency. These modes could be realized at 10.6 μm in suitably doped GaAs.