Researchers have made a tiny modulator that can covert electrical signals over a bandwidth of nearly 1 THz into optical signals via the formation and modulation of surface plasmon polaritons. [Image: Johannes Grewer / Polariton Technologies]
Scientists in Switzerland have demonstrated a high-bandwidth electro-optic modulator that relies on what are known as surface plasmon polaritons (Optica, doi: 10.1364/OPTICA.544016). They say that the device, which exhibits an efficient, almost flat response across a frequency range of nearly 1 THz, could, with suitable modifications, be used in 6G (sixth-generation) mobile networks as well as for sensing and medical imaging.
Building the next generation
6G wireless networks are being designed to enable communication using terahertz radiation—electromagnetic waves with frequencies between 0.1 and 10 THz, or 100 and 10,000 GHz. These very high frequencies will enable data rates as high as terabits per second, while also being fairly robust to atmospheric turbulence and inclement weather such as fog or snow.
Among the challenges in building 6G networks are receiving the high-frequency signals and then converting these to optical signals in a wireless base station. Traditionally, high-frequency signals could be received by an antenna and then converted to the electrical domain before being converted to optical frequencies. However, for THz signals this involves conversion at a bandwidth that is at the limit of electronics' capabilities.
In 2015, Juerg Leuthold at ETH Zurich and colleagues in Switzerland and the University of Washington, USA, showed instead how to carry out a direct conversion between THz waves and an optical signal. They did so by exploiting surface plasmon polaritons—electromagnetic waves that travel along interfaces between metals and dielectric materials.
The researchers’ setup involved a plasmonic modulator integrated with an antenna onto a photonic chip. The idea was to create a surface plasmon polariton by feeding a laser beam along two metal interfaces and then modulating the polariton's phase by applying a terahertz signal to the antenna. When converting the plasmon polaritons back to the optical domain, they found that the laser beam carried the information from the THz electrical signal. However, at the time their demonstration was limited to a bandwidth of just 60 GHz.
Increased bandwidth
Having since increased this figure to 300 GHz and more recently to 500 GHz, in the latest work Leuthold and coworkers reach nearly 1 THz of bandwidth. They achieved this using a modified plasmonic modulator built on a photonic integrated circuit by ETH spinoff company Polariton Technologies.
The device contains two parallel phase shifters, each consisting of a 10- to 15-µm-long, 100-nm-wide slit between pieces of gold mounted on a silicon dioxide substrate and filled with a nonlinear organic electro-optic material. The phase shifters are connected by contact pads that feed in electrical radiofrequency signals, transforming the slit into a waveguide for surface plasmon polaritons.
To put their modulator to the test, the researchers fed an infrared laser beam onto the chip via a grating coupler and silicon waveguide. They then encoded the resulting surface plasmon polariton by using electrical mixers to generate sinusoidal THz signals across a broad range of frequencies. Finally, they converted the polaritons back into a laser beam and used an optical spectrum analyser to measure the laser's power spectrum—using that to gauge the modulation efficiency at each frequency.
They found that for the slightly larger version they could meet this requirement over a bandwidth of 880 GHz, while for the smaller one they achieved 997 GHz.
The team carried out the experiment with two versions of the device, each with slightly different-sized contact pads, to establish in each case over what range of frequencies the response would drop at most by 50% (the so-called 3 dB operation point). They found that for the slightly larger version they could meet this requirement over a bandwidth of 880 GHz, while for the smaller one they achieved 997 GHz.
Potential improvements and lower losses
The researchers also worked out how they might improve on these results by modeling the modulator and terahertz source and comparing their simulation with the experimental data. They reckon that by using a monolithic design to remove the pads—which add unwanted capacitance—they could extend the device's bandwidth to 1.9 THz. They also say that by integrating a THz antenna directly on to the chip they should be able to better match the electrical and optical modulations and so potentially reach a bandwidth of 2.4 THz.
Leuthold says that he and his colleagues were able to achieve the record bandwidth by combining plasmonics with the electro-optical material's fast nonlinear response. He points out that plasmonic modulators suffer higher losses than photonic modulators, but he maintains that the fundamental plasmonic losses in their experiment are quite low—at around 5.6 dB. He adds that they have previously shown how a resonant configuration can reduce these losses to below 2 dB.
Beyond 6G networks, Leuthold and colleagues believe that such high-bandwidth devices might be used to ferry optical data within and between high-performance computing centers. Other potential applications, they say, include medical imaging, material spectroscopy and baggage scanning.