Nikolay N. Rosanov

We numerically study a new effect in the reflection of a single-cycle femtosecond pulse from a gaseous medium. When the gas layer possesses an engineered exponential density profile, the reflected field forms a unipolar pulse. Crucially, the trailing edge of this pulse replicates an exponential decay. The effect is demonstrated via self-consistent modeling coupling Maxwell's equations with plasma kinetics. This provides a direct method to control the temporal shape of unipolar optical waveforms by tailoring the initial spatial density gradient of the target.

Unipolar light pulses with non-zero electric area enable ultrafast control of quantum systems, requiring tailored temporal shapes like rectangular pulses for efficient atomic manipulation. Such pulses induce unique phenomena, including dynamic microcavities at resonant transitions in multilevel media. Here, we compare microcavity dynamics in two- and three-level media under rectangular unipolar pulse excitation via numerical solutions of the density matrix and wave equations. Unlike Gaussian pulses, rectangular pulses exhibit starkly different dynamics in a three-level medium, disrupting self-induced transparency-like regimes. Key differences emerge when pulses behave as 2 π pulses, highlighting the impact of the third level on microcavity dynamics. These findings establish rectangular unipolar attosecond pulses as a powerful tool for attosecond-scale quantum control, revealing fundamental limits of few-level approximations in extreme nonlinear optics and opening pathways to coherent light–matter engineering in complex atomic systems.

We demonstrate a novel, to the best of our knowledge, mechanism for generating half-cycle unipolar light pulses through reflection of an intense single-cycle femtosecond laser pulse from a transient plasma layer. The leading pulse edge ionizes a thin medium layer, while the trailing edge reflects from the plasma boundary, producing subcycle unipolar waveforms. Our 1D Maxwell-plasma model reveals universal scaling laws for pulse shape versus plasma density and thickness, showing how thin layers yield isolated half-cycle pulses, while thicker layers produce rectangular waveforms. This all-optical approach enables attosecond field synthesis without complex phase-matching requirements, bridging classical electrodynamics and ultrafast optics for quantum control applications.