(Nanowerk NewsThe index of refraction — the ratio of the speed of electromagnetic radiation in a medium to its speed in a vacuum — can be modulated fast enough to produce photonic time crystals (PTC) in the nearly visible part of the spectrum, a new study published in the journal Nanophotonics show (“Time refraction optics with single cycle modulation”). The study authors suggest that the ability to sustain the PTC in the optical domain could have profound implications for light science, enabling truly disruptive applications in the future.
PTC, a material in which the refractive index rises and falls rapidly in time, is the temporal equivalent of a photonic crystal in which the refractive index oscillates periodically in space causing, for example, the colors of precious minerals and insect wings.
A PTC is only stable if the index of refraction can be made to rise and fall with one cycle of the electromagnetic wave at the frequency in question so, not surprisingly, PTC has so far been observed at the lowest frequency of the electromagnetic spectrum: with radio waves.
In this new study, lead author Mordechai Segev of the Technion-Israel Institute of Technology, Haifa, Israel, with collaborators Vladimir Shalaev and AlexndraBoltasseva of Purdue University, Indiana, USA and their team, sent very short pulses (5-6 femtoseconds) of laser light. at a wavelength of 800 nanometers through a transparent conductive oxide material.
This causes a rapid shift in refractive index which is explored using a probe laser beam at slightly longer wavelengths (near infrared). The probe beam rapidly red-shifts (that is, increases its wavelength) and then blue-shifts (decreases wavelength) as the refractive index of the material relaxes back to its normal value.
The time required for each of these changes in refractive index is very small – less than 10 femtoseconds – and, therefore, one cycle is required to form a stable PTC.
“Electrons excited to high energies in crystals generally take more than ten times longer to return to their ground state, and many researchers think that the ultrafast relaxation we observe here is impossible,” says Segev. “We don’t yet understand exactly how that happened.”
Co-author Shalaev further points out that the ability to sustain the PTC in the optical domain, as demonstrated here, will “open a new chapter in light science and enable truly disruptive applications”. However, we know so little about what this is because physicists in the 1960’s knew about the possible applications of lasers.