Researchers at Argonne National Laboratory have succeeded in tuning the luminescence from gold nanorods by adjusting their lengths. By varying length from 300 nanometers down to 50 nm while fixing the diameter at 30 nm, the team demonstrated photoluminescence at 400- to 700-nm wavelengths. Further enhancements could make it possible to use the optical emissions from gold nanorods as a light source inside a silicon chip, transducing light for optical network switches, routers and all-optical computers. Other researchers had shown that surface plasmons could influence luminescence emission spectra, but the Argonne group wanted to verify that the spectra of photoluminescent gold nanorods could be controlled by surface plasmon resonances. When excited by two-photon-induced photoluminescence, the nanorods exhibited two spectral resonances, corresponding to their longitudinal and transverse dimensions, demonstrating sensitivity to both excitation energy and polarization. Surface plasmons are collective oscillations of electrons at the boundary between a conductor and an insulator. Plasmons themselves are collections of electrons that meld with photons to form a new order of object, called a surface plasmon polariton, that can enhance transmission in certain optical bands, acting as optical "bandpass filters." The Argonne team examined the photoluminescence in gold by exciting nanorods of various aspect ratios with 120-picosecond pulses from a 785-nm-wavelength laser. The rods responded with two-photon-induced photoluminescence that the team showed was dependent on their size and shape, thus confirming that surface plasmons — likewise dependent on size and shape — had allowed them to control spectral emission.