A new, all-optical technique for creating second-order nonlinear effects in materials that normally do not support them could lead to new options for creating these effects for optical computers, high-speed data processors, and bioimaging. A research group from Georgia Institute of Technology (Georgia Tech) developed the technique, using a red laser to create the nonlinear effects. For their experiment, the researchers created an array of tiny plasmonic gold triangles on the surface of a centrosymmetric titanium dioxide (TiO2) slab in their lab. They illuminated the TiO2/gold structure with a pulse of red laser light. The laser beam acted as an optical switch, breaking the crystal symmetry of the material. The laser pulse, when fired at the array of gold triangles on the TiO2 slab, excited electrons, and this excitation briefly doubled the frequency of a beam from a second laser as it bounced off the amorphous TiO2 slab. Georgia Tech researchers Kyu-Tae Lee and Mohammad Taghinejad demonstrate frequency doubling on a slab of titanium dioxide using a red laser to create nonlinear effects with tiny triangles of gold. The blue beam shows the frequency-doubled light and the green beam controls the hot-electron migration. Courtesy of Rob Felt, Georgia Tech. “The optical switch excites high-energy electrons inside the gold triangles, and some of the electrons migrate to the titanium dioxide from the triangles’ tips,” professor Wenshan Cai said. “Since the migration of electrons to the TiO2 slab primarily happens at the tips of [the] triangles, the electron migration is spatially an asymmetric process, fleetingly breaking the titanium dioxide crystal symmetry in an optical fashion.” The team observed the induced symmetry-breaking effect almost instantaneously after the red laser pulse was triggered. “Now that we can optically break the crystalline symmetry of traditionally linear materials such as amorphous titanium dioxide, a much wider range of optical materials can be adopted in the mainstream of micro- and nanotechnology applications such as high-speed optical data processors,” Cai said. The lifetime of the induced second-order nonlinearity depends on how fast the electrons can migrate back from the TiO2 to the gold triangles after the pulse disappears. In the experiment reported by the Georgia Tech team, the induced nonlinear effect lasted for a few picoseconds, which the researchers said is long enough for most applications where short pulses are used. A stable, continuous-wave laser could make the nonlinear effect last for as long as the laser is turned on. “The strength of the induced nonlinear response strongly depends on the number of electrons that can migrate from gold triangles to the titanium dioxide slab,” Cai said. “We can control the number of migrated electrons through the intensity of the red laser light. Increasing the intensity of the optical switch generates more electrons inside the gold triangles, and therefore sends more electrons into the TiO2 slab.” Diagram shows the process for breaking the inversion symmetry via hot-electron transfer. Courtesy of Georgia Tech. Additional research is needed to build on the team’s proof-of-concept, which showed that the crystal symmetry of centrosymmetric materials can be broken by optical means, via asymmetric electron migrations. “We still need to develop guidelines that tell us what combination of metal/semiconductor material platform should be used, what shape and dimension would maximize the strength of the induced second-order nonlinear effect, and what range of laser wavelength should be used for the switching light,” Cai said. Frequency doubling is one potential application for the new technique. “We believe that our findings not only provide varieties of opportunities in the field of nonlinear nanophotonics, but also will play a major role in the field of quantum electron tunneling,” Cai said. “Indeed, building upon the accumulated knowledge in this field, our group is devising new paradigms to employ the introduced symmetry-breaking technique as an optical probe for monitoring the quantum tunneling of electrons in hybrid material platforms. Nowadays, achieving this challenging goal is only possible with scanning tunneling microscopy (STM) techniques, which are very slow and show low yield and sensitivity.” The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.124.013901).
