A Frequency Doubler Controlled by Light

A red laser creates nonlinear effects with tiny triangles of gold. The blue beam shows the frequency-doubled light and the green beam controls the hot-electron migration. (Source: R. Felt, Georgia Tech)

Researchers have demons­trated a new all-optical technique for creating robust second-order nonlinear effects in materials that don’t normally support them. Using a laser pulse fired at an array of gold triangles on a titanium dioxide slab, the researchers created excited electrons that briefly doubled the frequency of a beam from a second laser as it bounced off the amorphous titanium dioxide slab. By widening the range of optical materials useful for micro- and nanoscale opto­electronic applications, the work could give optical engineers new options for creating second-order nonlinear effects, which are important in such areas as optical computers, high-speed data processors and bioimaging safe for use in the human body.

“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,” said Wenshan Cai, a professor in the School of Electrical and Computer Engineering at the Georgia Institute of Tech­nology.

A majority of optical materials tend to have a symmetric crystal structure that limits their ability to create second-order nonlinear effects such as frequency doubling that have important techno­logical applications. Until now, this symmetry could only be inter­rupted by applying electrical signals or mechanical strain to the crystal. In the labora­tory, Cai and colla­borators Mohammad Taghine­jad, Zihao Xu, Kyu-Tae Lee and Tianquan Lian created an array of tiny plasmonic gold triangles on the surface of a centro­symmetric titanium dioxide slab. They then illu­minated the titanium dioxide/gold structure with a pulse of red laser light, which acted as an optical switch for breaking the crystal symmetry of the material. The amorphous titanium dioxide slab would not naturally support strong second-order nonlinear effects.

“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,” Cai explained. “Since the migration of electrons to the titanium dioxide slab primarily happens at the tips of triangles, the electron migra­tion is spatially an asym­metric process, fleetingly breaking the titanium dioxide crystal symmetry in an optical fashion.”

The induced symmetry breaking effect is observed almost instan­taneously after the red laser pulse is triggered, doubling the frequency of a second laser that is then bounced off the titanium dioxide containing the excited electrons. The lifetime of the induced second-order non­linearity generally depends on how fast electrons can migrate back from the titanium dioxide to the gold triangles after the dis­appearance of the pulse. In the case study reported by the researchers, the induced nonlinear effect lasted for a few pico­seconds, which the researchers say is enough for most appli­cations where short pulses are used. A stable continuous wave laser can make this effect last for as long as the laser is 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 added. “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 titanium dioxide slab.” Additional research will be needed to build on the proof of concept, which showed for the first time that the crystal symmetry of centro­symmetric materials can be broken by optical means, via asymmetric electron migrations.

“To approach the practical criteria detailed on the essence of our technique, 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 noted. Frequency doubling is just one potential application for the technique, he said.

“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 added. “Indeed, built 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.” (Source: Georgia Tech)

Reference: M. Taghinejad et al.: Transient Second-Order Nonlinear Media: Breaking the Spatial Symmetry in the Time Domain via Hot-Electron Transfer, Phys. Rev. Lett. 124, 013901 (2020); DOI: 10.1103/PhysRevLett.124.013901

Link: School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, USA

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