Researchers in France and Germany have created the first all-optical photonic time crystal, a nanostructure whose optical properties can be modulated at terahertz frequencies. The advance could enable ultrafast optical computers and amplifiers, frequency converters, and possibly new types of lasers operating in the terahertz range.
Photonic crystals are nanostructured materials with a refractive index that varies on a length scale comparable to the wavelength of light, producing a photonic bandgap. This gap affects how photons propagate through the material in a way that resembles how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In photonic crystals, light in certain wavelength ranges can pass through the material, but other wavelengths cannot.
Photonic time crystals are similar, except their properties vary periodically in time rather than in space. Instead of photonic bandgaps, they host momentum bandgaps. These gaps are special in that light waves whose momenta fall within them grow exponentially in time.
Photonic time crystals show promise for applications such as new types of tuneable lasers and ultrafast frequency converters, but controlling their photonic properties has proven very difficult. This is because these properties need to be modulated very strongly and on ultrashort timescales, on the order of the temporal period of the light itself. Producing such extremely fast modulations is a key goal for scientists working in a branch of highly nonlinear optics known as time-domain photonics.
Previously, efforts have paid off in the microwave frequency range using photonic time crystals containing electrical circuits, but all-optical systems have proven elusive. A team of physicists led by Yannis Laplace of the Ecole Polytechnique in France has now made a photonic time crystal with properties that can be optically modulated at terahertz frequencies.
The new photonic time crystal is made from a type of photonic crystal known as a plasmonic metamaterial: an artificially engineered nanostructure consisting of micron-sized cavities made of gold atop an insulating layer and a semiconductor material based on indium and antimony. The cavities trap photons between the gold and semiconductor layers, and surface plasmons, which are collective, coherent oscillations of conduction electrons that interact very strongly with light, then keep them on the surface of the semiconductor.
Working with scientists from the Collège de France and the Helmholtz-Zentrum Dresden-Rossendorf in Germany, the researchers applied pulses of terahertz light from the TELBE light source at the HZDR's ELBE accelerator to their structure. They found that they could use this intense multi-cycle terahertz light field to modulate the material's optical properties over timescales of just picoseconds.
This study builds on previous work in Laplace's group related to the development and study of tuneable plasmonic metamaterials in the terahertz range as a means to create functional devices to control light-matter interactions in this range. One of the central questions in the work was to determine if the temporal modulation of the metamaterial would be strong and fast enough.
By developing a theoretical model in collaboration with Marco Schiró and his team at the Collège de France that reproduced the experimental observations remarkably well, the researchers showed that the system presented the characteristic spectroscopic signatures expected for a photonic time crystal and provided an explanation for the behaviour of the photons therein.
The researchers, who detail their work in Nature, also found that the ultrafast temporal modulation reduced the dissipation of photons within the metamaterial by half, and they are now looking to reduce these optical losses even further. The terahertz frequency range lies at the junction between electronic and photonic technologies and is historically less developed than these two. With the terahertz photonic time crystals that are now achievable, the researchers hope they can soon develop devices like terahertz amplifiers, frequency converters, and maybe new types of terahertz lasers in this range, hence contributing to closing the so-called terahertz gap.