Symmetry-Breaking Electron Dynamics Enable Ultrabroadband Optical-Field Sampling via Second-Harmonic Generation

TL;DR

This paper reveals that symmetry-breaking electron dynamics enable ultrabroadband optical-field sampling through second-harmonic generation, providing a new method for high-resolution waveform detection based on ionization-induced SHG signals.

Contribution

It uncovers the microscopic origin of SHG in strong-field ionization and demonstrates how electron dynamics can be harnessed for ultrabroadband optical-field sampling.

Findings

SHG signal arises from lifted half-cycle cancellation of photoelectron dipole emission.

SHG yield encodes the instantaneous electric field at ionization.

Detection bandwidth exceeds probe duration due to subcycle ionization gating.

Abstract

Optical-field sampling using second-harmonic generation (SHG) from strong-field ionization enables ultrabroadband terahertz detection, but the microscopic origin of the SHG signal and its ultrabroadband response have been unclear. Here we show that the target field lifts the half-cycle cancellation of photoelectron dipole emission, generating the SHG signal used for field sampling. Time-dependent Schrodinger-equation simulations, supported by classical-trajectory Monte Carlo analysis, demonstrate that the SHG yield directly encodes the instantaneous target electric field at the ionization time, enabling waveform retrieval by scanning the probe-target delay. Because the SHG response is gated by a subcycle ionization window rather than the probe envelope, the detection bandwidth can extend far beyond the probe duration. We further quantify practical constraints on retrieval, including intrinsic probe asymmetry and SHG back-action, providing a predictive framework to optimize sensitivity, temporal resolution, and fidelity through controlled electron dynamics.