Left-Handed, or Negative Index, Materials

In the March of 2000, scientists at the University of California at San Diego announced their production of the first “Left-Handed Material.”1  These materials are exciting because of their unusual optical properties; in a left-handed material, light seems to propagate opposite the direction of energy flow.  This leads to a negative index of refraction and reversed Doppler shift for radiation!  The materials were called "left-handed", because the electric and magnetic fields propagate in a direction given by a left-hand rule:  put the thumb of your left hand in the direction of the electric field, the fingers of your left hand in the direction of the magnetic field, and your left palm will point in the direction of motion.  (Conventional materials follow a right-hand rule, as described in the Introduction to Waves and Electromagnetic Radiation module - same assignments of thumb, fingers, and palm but using the right hand.)  But "left-handed" is used to describe a different set of properties when applied to particles (see, for example, this page from NIST), so some2 prefer to identify these materials as "negative index" materials.

Snell’s law has long been used to calculate angles and refractive indices for refractive materials.  What we didn’t know until recently, however, is that these indices of refraction can be negative!  As light enters a left-handed material from a right-handed material (a material we are already familiar with like glass or air) the light will refract, but the refracted ray in a left-handed material will be a mirror image of the refracted ray we typically see in right-handed materials.  (See picture.)

Refraction is not the only effect where left-hand materials give an unexpected result.  Light from a source traveling toward a detector through normal (right-handed) materials appears “blue-shifted” because of the Doppler effect.  As the waves approach an object, they bunch up, decreasing the wavelength as measured by the object.  But in a left-handed material, light from a source traveling toward an object is red-shifted, appearing to have a longer wavelength!  For more information about the Doppler effect, refer to this summary from NASA's Imagine the Universe materials.

To this date, no one actually knows why the materials act the way they do.  All that is known is that the material has a negative electric permittivity and negative magnetic permeability, which could explain why the Doppler shift and Snell effect are inversed.  Unfortunately, no one has yet explained why the material’s permeability and permittivity would be negative.  The links below provide more information about this exciting new class of materials.  You might want to bookmark footnote 4; that page will be updated as more developments arise.