Yannis Laplace and his team at \u00c9cole Polytechnique\u2019s Lab of Irradiated Solids, and colleagues from the Coll\u00e8ge de France and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), recently introduced all-optical photonic time crystals, in which the material\u2019s optical properties\u2014reflectivity and resonance frequency\u2014are dynamically modulated at picosecond timescales.<\/p>\n
Thanks to the HZDR\u2019s TELBE superradiant terahertz (THz) source, the team was able to drive their photonic crystal into a new regime of light-matter interaction within the terahertz range\u2014where light oscillates at frequencies on the order of 1 trillion times per second. Their work expands photonic crystals\u2019 reach from space to time, which may prove to be a gamechanger for ultrafast optical computing, new telecom systems, and possibly new types of THz lasers.<\/p>\n
\u201cDuring the past few years, my group has worked on plasmonic metamaterials within the THz frequency range as a way to develop functional devices to manipulate light and light-matter interactions,\u201d says Laplace, an assistant professor of physics. \u201cNotably, we showed that in equilibrium these systems are highly tunable with parameters such as temperature and magnetic field.\u201d<\/p>\n
Naturally, the next step for the team was to explore dynamical aspects like tunability in time when modulated with an intense THz light pulse. \u201cPhotonic time crystals are optical systems that must be modulated dynamically very strongly at ultrafast timescales,\u201d he says. \u201cWe already knew our system would respond nonlinearly to THz light. Our question was: How strong and fast, and would it be enough to reach the photonic time crystal regime of operation?\u201d<\/p>\n
Plasmonic metamaterial platform<\/p>\n
The team\u2019s platform for their photonic crystal is a plasmonic metamaterial\u2014an artificially constructed array of cavities for THz photons\u2014that harnesses surface plasmons, which are collective excitations of electrons at the surface of a semiconducting material.<\/p>\n
\u201cOur system consists of the semiconducting material indium antimonide (InSb), an insulating dielectric material, and a periodic structure of gold stripes on top,\u201d says Laplace. \u201cOverall, this structure acts as resonators for surface plasmons, which allows us to design their resonant frequencies and properties.\u201d<\/p>\n
For their experiment, the researchers used an intense multicycle THz light field to drive the plasmonic excitations within the cavities to a large amplitude, which effectively modulates their properties in time (such as their resonant frequency) through a nonlinear effect. \u201cThis, in turn, modulates the optical properties of the whole metamaterial in time,\u201d he says. \u201cAnd this modulation is sufficiently strong and fast enough to reach the photonic time crystal regime of operation.\u201d<\/p>\n
How does their method work? \u201cIf we shine a strong multicycle THz pulse onto our metamaterial and probe the resulting optical properties with another THz pulse, we can observe the system\u2019s optical properties oscillate in time (due to the drive),\u201d Laplace explains.<\/p>\n
These oscillations are so strong and fast it completely redefines the optical properties of the system. \u201cIt means we can harness them to convert the frequencies of photons fast and efficiently,\u201d he says. \u201cThe time-averaged optical properties of the structure are also modified, which represents a new way to engineer optical properties\u2014not by designing a structure in equilibrium as is usually done, but with a temporal approach.\u201d<\/p>\n
Right now, the researchers can use the photonic time crystal regime to decrease optical dissipation of their metamaterial. In the future, they\u2019d like to explore further frequency conversion, amplification, and lasing.<\/p>\n
An advance for photonics in two ways<\/p>\n
How is this work an advance for photonics? First, plasmonics. \u201cPlasmonics is great because it allows us to confine photons at spatial scales orders of magnitude smaller than the diffraction limit (the lower bound for photon confinement in free space), so it shows promise for making optical devices very compact,\u201d says Laplace. \u201cBut this gift comes at a price: Losses. Plasmons rely on the motion of electrons within solids and are subject to large dissipation\u2014photonic losses\u2014and this is a problem for many applications.\u201d<\/p>\n
His team showed these losses can be decreased substantially with a temporal drive (by a factor of 2 in their experiment), and they hope to further decrease them in the future. \u201cUltimately, the possibility to develop lossless plasmonic platforms would be a gamechanger for these technologies, because they can profit from the confinement they allow without losses as a drawback,\u201d he says.<\/p>\n
Second, signal processing and manipulation in photonics. \u201cSignal manipulation in photonics relies on nonlinear optical phenomena, such as the conversion of light from one color to another,\u201d says Laplace. \u201cIn photonic time crystals, these phenomena arise on the time scale of the light\u2019s temporal period itself\u2014it\u2019s a completely new regime of parameters for photonics. Photonic time crystals should allow nonlinear optical processes to be achieved much faster and will dramatically increase the rates for signal processing\/optical computing.\u201d<\/p>\n
Laplace and his team experienced several aha! moments, including one during beamtime at the HZDR TELBE facility, when they saw \u201cstrong and fast modulations\u201d coming out of their metamaterial that told them their initial intuition was correct.<\/p>\n
\u201cWe developed the theoretical model of our system with our collaborators and saw it matches our experimental observations very well,\u201d he says. \u201cBuilding on it, we could use it to distinguish between a photonic time crystal and a more common modulated optical system\u2014and ours was a photonic time crystal.\u201d<\/p>\n
As you can likely guess, this moment \u201cfelt like intellectual satisfaction,\u201d Laplace says. \u201cNot only were we seeing something new but we could also precisely explain it. We now use this model to screen a wide range of the parameter space of this system to guide us before we perform our time-consuming experiments.\u201d<\/p>\n
The next big challenge the researchers will take on is to further decrease the losses within this system with a dynamical drive and overcome the threshold for lasing, which corresponds to a central prediction of photonic time crystals.<\/p>\n
Direct potential applications of this work? They\u2019ll pertain to the THz frequency range\u2014frequencies located at the intersection between electronic and photonic technologies. \u201cThis range is underdeveloped compared to electrical and photonic technologies\u2014the famous THz gap\u2014and its development is timely,\u201d he says. \u201cWe foresee the development of amplifiers, frequency converters, ultrafast modulators, and maybe new types of lasers within this range.\u201d<\/p>\n
His team will continue to work on their system \u201cso THz applications can emerge as early as possible\u2014hopefully within the next few years,\u201d says Laplace. \u201cSimultaneously, from a more fundamental point of view, we\u2019re interested in pushing the boundaries of knowledge in time-modulated optical systems, which represent one of the most exciting frontiers in photonics at the moment, to develop next-gen concepts and devices in photonics.\u201d<\/p>\n
FURTHER READING<\/p>\n