The ability to synchronize multiple time events with high precision has allowed humans to accomplish increasingly complex tasks. For example, an orchestra conductor enables synchronization and control of dozens of musicians with tens of milliseconds of precision, optimizing one of the most universally acclaimed art forms. Accurate timekeeping is a part of everyday life. Almost every electronic device has a timekeeper inside. These timekeepers are so accurate that counting microseconds is easy, something that would have amazed inventors from the past.
Accurate timekeeping devices are required in several applications in time and frequency metrology, geodesy, quantum technologies, astronomy, as well as materials, nuclear, and particle physics. For example, GPS and telecommunications both require extremely high precision. GPS satellites must have very accurate timekeeping device to geolocate within few meters and telecommunications also need time precision for the smooth functioning and better performance.
In such scenario, electronic components-based timekeeping systems cannot measure the time between two-time events precisely below two-digit picosecond scale. Additionally, the use of highly expensive electronic components and temperature increase due to the heat generated within the system are other drawbacks, limiting its long-term use and performance. To address these issues, TIMEKEEPER is developing a novel, extremely high precision, low energy, all-optical timekeeping methodology and Timekeeper device with epsilon-near-zero (ENZ) metamaterials.
ENZ materials possess exceptional nonlinear properties within sub-wavelength propagation length where the real part of permittivity approaches zero and exhibit huge change in refractive index (Δn) at ultrafast time scale, when excited by femtosecond pulsed laser light. Intrinsic materials like Indium tin oxide (ITO) and Aluminum doped Zinc oxide (AZO) and Nitride films possess ENZ response in NIR and UV-VIS regions, respectively. While ENZ phenomena can also be designed at desired wavelengths in subwavelength metal/dielectric based multilayer structures known as hyperbolic metamaterials (HMMs). Moreover, designing metal sub-wavelength nanostructures arrays enhance the features of ENZ film by confining the electric field within the film and also reduce the power consumption.
When the metasurface is excited with femtosecond pulsed laser light, a change in the refractive index of the metasurface is induced within femtosecond time scale. Probe beams (signal beam) passing through such time varying medium demonstrate strong nonlinear effects like self-phase modulation (SPM) and cross-phase modulation (XPM). SPM is a nonlinear optical effect where the phase of a light wave is modulated by its own intensity as it propagates through a nonlinear medium. XPM, on the other hand, is a nonlinear optical effect where the phase of a light wave is modulated by the intensity of another co-propagating light wave. This change results in phase modulation of the probe beam, leading to a frequency shift due to cross-phase modulation (XPM). The phase modulation of the probe beam passing through the ENZ metasurface is directly related to the change in the refractive index of the metasurface. This clearly means that phase change is directly proportional to the change in the refractive index, happening at a femtosecond time scale.
