Abstract
Abstract 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.
Highlights
To experimentally verify the proposed concept, we constructed a THz FP resonator consisting of two different types of mirrors: a temporal boundary mirror and a time-invariant mirror
We propose the use of a temporal boundary mirror in the construction of a FP resonator for the resonance-enhanced spectral funneling of an incident pulse
The input pulse is transformed into a transmitted pulse consisting of spectral components tightly confined to the modal frequencies of the FP resonator formed after the temporal boundary
Summary
By harnessing temporal degrees of freedom, time-variant photonic platforms have enabled diverse optical functionalities, such as spectral conversion [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28], nonreciprocal transmission [29,30,31], topologically nontrivial phases [32], synthetic dimensions [33,34], and beam steering in reflection and refraction [35,36,37]. A proper spatiotemporal design strategy is requested for the optimization of the resonance-enhanced spectral funneling process We show that this optimization task can be accomplished by adjusting the (spatial) length of the FP resonator and the time delay between the input pulse and the temporal boundary. While the energy conversion efficiency depends on the relative amount of spectral shift, we show that the efficiency can exceed 30% (for the relative amount of spectral shift of 0.26), which is orders of magnitude higher than the value achieved in our previous THz metasurface platform [24] and is comparable to the value observed in a THz waveguide system [16] All these experimental observations are verified by comparison with theoretical calculations
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