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Physica Scripta

Towards nonreciprocal pulse dynamics in a time-varying medium

Piyali Biswas, Somnath Ghosh

Published Feb 1, 2023• 2 citations
Publisher

Abstract

Abstract We demonstrate the nonreciprocal propagation of optical pulses in a linear dispersive medium with a time-varying refractive index. The change of refractive index with time breaks the time-translation symmetry of the spatially homogeneous medium compelling it to be inherently nonreciprocal. A time-reversal asymmetric field ratio of the counter-propagating optical pulses along with a frequency shift leading to spectral nonreciprocity in such dynamic media have been observed. The findings have emerged a new avenue to realize nonreciprocal light flow in dynamically tuned integration-feasible photonic systems.

DOI
10.1088/1402-4896/acb7d3

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Recent experiments have demonstrated that it is possible to alter the dispersion of a medium without significantly altering its absorption or refractive index and that this may be done while a wave propagates through the medium. This possibility opens up a new set of potential experiments to the field of nonstationary optics. We consider the basic kinetics of waves propagating through a medium whose group and phase velocities are a function of position and time. We compare the dynamics of waves propagating through two homogeneous media, one a nondispersive medium with a time dependent phase velocity, and one a dispersive medium with a time dependent group velocity and show that new dynamic effects accompany new kinetic ones.
Abstract Optical nonreciprocity provides a firm basis for direction-dependent control of light transport. We propose a scheme to manipulate two counter-propagating light pulses based on dynamically controlled dual-channel optical nonreciprocity originating from the Doppler effect. This scheme is implemented in a four-level tripod-type thermal atomic medium driven by two counter-propagating control fields. Symmetric or asymmetric propagation and storage scenarios for the two probe pulses can be established by adjusting the detunings and intensities of the control fields. In particular, complete separation of the forward and backward transmission windows enables the realization of a frequency-tunable bidirectional optical isolator. Doppler shifts make the tripod system behave as two weakly coupled three-level Λ-type channels associated mainly with the forward and backward probe fields. Our proposal may have potential applications in designing direction-dependent all-optical devices for manipulating multiple optical pulses.
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Nonlinearly generated refractive index fronts in waveguides can change the signal frequency and wavenumber leading to the indirect transitions. Here, we discuss how dynamic light stopping and pulse time reversal can be implemented in dispersive waveguides via these transitions. Our scheme, in contrast to previous concepts of light stopping and time reversal, is not limited by a strict phase matching condition and does not require local index variations, thus, can manipulate broadband signals in a single step process. The presented results are experimentally feasible using existing photonic waveguides technologies.