Modulating Light at Terahertz Frequencies

Illustration the structure and action of a nanopatterned plasmonic metasurface that modulates polarized light at terahertz frequencies. An ultrashort laser pulse excites cross-shaped plasmonic structures, which rotate the polarity of a second light pulse that arrives less one picosecond after the first. (Source: A. Assié)

U.S. and Italian engineers have demons­trated the first nano­photonic platform capable of manipulating polarized light 1 trillion times per second. “Polarized light can be used to encode bits of information, and we’ve shown it’s possible to modulate such light at terahertz frequencies,” said Rice University’s Ales­sandro Alabastri. “This could poten­tially be used in wireless communi­cations,” he said. “The higher the operating frequency of a signal, the faster it can transmit data. One terahertz is about 25 times higher than the operating frequencies of commer­cially available optical polarization switches.”

The research was a colla­boration between experi­mental and theoretical teams at Rice, the Polytechnic University of Milan (Politecnico) and the Italian Institute of Technology (IIT) in Genoa. This colla­boration started in the summer of 2017, when Andrea Schirato was a visiting scholar in the Rice lab of physicist. Each of the researchers work in nano­photonics, a fast-growing field that uses ultrasmall, engi­neered structures to manipulate light. Their idea for ultrafast polari­zation control was to capitalize on tiny, fleeting varia­tions in the generation of high-energy electrons in a plasmonic metasurface.

By varying the size, shape and makeup of metasurfaces with the embedded nano­particles and by arranging them in precise two-dimensional geometric patterns, engineers can craft meta­surfaces that split or redirect specific wavelengths of light with precision. “One thing that differen­tiates this from other approaches is our reliance on an intrin­sically ultrafast broadband mechanism that’s taking place in the plasmonic nanoparticles,” Alabastri said.

The Rice-Poli­tecnico-IIT team designed a metasurface that contained rows of cross-shaped gold nanoparticles. Each plasmonic cross was about 100 nanometers wide and resonated with a specific frequency of light that gave rise to an enhanced localized electro­magnetic field. Thanks to this plasmonic effect, the team’s metasurface was a platform for generating high-energy electrons. “When one laser light pulse hits a plasmonic nano­particle, it excites the free electrons within it, raising some to high-energy levels that are out of equilibrium,” Schirato said. “That means the electrons are uncom­fortable and eager to return to a more relaxed state. They return to an equilibrium in a very short time, less than one picosecond.”

Despite the symmetric arrangement of crosses in the meta­surface, the non­equilibrium state has asymmetric properties that disappear when the system returns to equilibrium. To exploit this ultrafast phenomenon for polari­zation control, the researchers used a two-laser setup. Experiments performed by Margherita Maiuri at Poli­tecnico’s ultrafast spectro­scopy labora­tories and confirmed by the team’s theo­retical predictions used an ultra­short pulse of light from one laser to excite the crosses, allowing them to modulate the polari­zation of light in a second pulse that arrived less than a picosecond after the first.

“The key point is that we could achieve the control of light with light itself, exploiting ultrafast electronic mechanisms peculiar of plasmonic meta­surfaces,” Alabastri said. “By properly designing our nano­structures, we have demonstrated a novel approach that will potentially allow us to optically transmit broadband information encoded in the polari­zation of light with unpre­cedented speed.” (Source: Rice U.)

Reference: A. Schirato et al.: Transient optical symmetry breaking for ultrafast broadband dichroism in plasmonic metasurfaces, Nat. Phot., online 19 October 2020; DOI: 10.1038/s41566-020-00702-w

Link: Dept. of Physics, Politecnico di Milano, Milan, Italy Laboratory for Nanophotonics, Rice University, Houston, USA

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