Ultrafast Optical Modulation by Virtual Interband Transitions

A new frontier in optics research has been opened by the recent developments in non-perturbative optical modulation in both time and space that creates temporal boundaries generating ``time-reflection'' and ``time-refraction'' of light in the medium. The resulting formation of a Photonic Time Crystal within the modulated optical material leads to a broad range new phenomena with a potential for practical applications, from non-resonant light amplification and tunable lasing, to the new regime of quantum light-matter interactions. However, the formation of the temporal boundary for light relies on optical modulation of the refractive index that is both strong and fast even on the time scale of a single optical cycle. Both of these two problems are extremely challenging even when addressed independently, leading to conflicting requirements for all existing methods of optical modulation. However, as we show in the present work, an alternative approach based on virtual interband transition excitation, solves this seemingly insurmountable problem. Being fundamentally dissipation-free, optical modulation by virtual excitation does not face the problem of heat accumulation and dissipation in the material, while the transient nature of the excited virtual population that modifies the material response only on the time scale of a single optical cycle, ensures that the resulting change in the refractive index is inherently ultrafast. Here we develop the theoretical description of the proposed modulation approach, and demonstrate that it can be readily implemented using already existing optical materials and technology.