  A temporal boundary refers to a specific time at which the properties of an
optical medium are abruptly changed. When light interacts with the temporal
boundary, its spectral content can be redistributed due to the breaking of
continuous time-translational symmetry of the medium where light resides. In
this work, we use this principle to demonstrate, at terahertz (THz)
frequencies, the resonance-enhanced spectral funneling of light coupled to a
Fabry-Perot resonator with a temporal boundary mirror. To produce a temporal
boundary effect, we abruptly increase the reflectance of a mirror constituting
the Fabry-Perot resonator and, correspondingly, its quality factor in a
step-like manner. The abrupt increase in the mirror reflectance leads to a
trimming of the coupled THz pulse that causes the pulse to broaden in the
spectral domain. Through this dynamic resonant process, the spectral contents
of the input THz pulse are redistributed into the modal frequencies of the
high-Q Fabry-Perot resonator formed after the temporal boundary. An energy
conversion efficiency of up to 33% was recorded for funneling into the
fundamental mode with a Fabry-Perot resonator exhibiting a sudden Q-factor
change from 4.8 to 48. We anticipate that the proposed resonance-enhanced
spectral funneling technique could be further utilized in the development of
efficient mechanically tunable narrowband terahertz sources for diverse
applications.
