Everyone has experienced the Doppler effect. An ambulance's siren has a higher pitch as it approaches you, and a lower pitch when moving away. Now, imagine that the pitch goes down as the ambulance moves towards you—that is the inverse Doppler effect. Scientists have now demonstrated it in action using light; by shining an CO 2 laser through a negative-index material, they were able to demonstrate an inverse Doppler effect, in which the laser frequency went down as the material approached a detector.

What is a negative-index material (NIM)? Simply put, NIMs are substances that have a negative index of refraction. Normally, when light moves into a material with a different index of refraction, the light is bent into the material. If you were to trace it on paper, the slope of the line would change, but the sign of the slope would remain the same—provided the material has a positive index of refraction. If the material has a negative index of refraction, the traced light path would have the same change in slope but the opposite sign. It acts like it was reflected in the material as well as refracted.

To demonstrate a negative Doppler shift, the scientists used a NIM prism made from a lattice of silicon rods (0.2µm in radius) spaced 5µm apart. The size and spacing of the rods create a preferred direction of propagation of light in the crystal, and ultimately the index of refraction. In this case the lattice was designed to have a negative index of refraction for a CO 2 laser (10.6µm).

The CO 2 laser was sent through the prism and negatively refracted onto a detector. The prism was then moved at a uniform speed toward the detector. You can think of the point where the beam leaves the prism as the source of motion (until the beam leaves the prism it is propagating perpendicularly to the moving prism and is unaffected). The Doppler effect is a function of the source velocity and the index of refraction; in this case, since the index of refraction is negative, there should be a frequency shift to a lower value.

The scientists observed that if they moved the NIM prism towards the detector, the beam showed an inverse Doppler effect with respect to a reference laser. To further verify their results, they constructed a similar prism out of ZnSe, which has a positive index of refraction and repeated the experiment. This time, they saw the expected normal Doppler effect. All results were within 5 percent of theoretically values.

The inverse Doppler effect has been demonstrated at radio frequencies and with sound, but this is the first time it has been seen at optical frequencies. NIM materials have long been predicted to provide useful solutions in optics—NIM materials are designed for specific frequencies and can be used to as very specific bandpass filters, absorbers, or reflectors.

Nature Photonics, 2011. DOI: 10.1038/NPHOTON.2001.17 (About

DOIs).

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