Quantum control of flying doughnut terahertz pulses

Abstract

The ability to manipulate the multiple properties of light diversifies light-matter interaction and light-driven applications. Here, using quantum control, we introduce an approach that enables the amplitude, sign, and even configuration of the generated light fields to be manipulated in an all-optical manner. Following this approach, we demonstrate the generation of "flying doughnut" terahertz (THz) pulses. We show that the single-cycle THz pulse radiated from the dynamic ring current has an electric field structure that is azimuthally polarized and that the space- and time-resolved magnetic field has a strong, isolated longitudinal component. We apply the flying doughnut pulse for a spectroscopic measurement of the water vapor in ambient air. Pulses such as these will serve as unique probes for spectroscopy, imaging, telecommunications, and magnetic materials.

Fig. 1.. Dynamic ring current radiates FD terahertz pulse.

(A) Illustration of FD pulse generation. Two azimuthally polarized vector pulses generate transient ring currents in GaAs. The radiation from the rapidly oscillating ring currents is a single-cycle THz pulse with toroidal topology, an FD pulse. (B) Spatio-vectorial distribution of ring current measured with a single pixel current detector. Combining the detected x and y components (fig. S2) of the current results in spatial mapping of the ring current. (C) Spatiotemporal structure of the radiated electric field (E_φ) simulated using a dynamic ring current density source. (D) Simulated magnetic field (B_z) map of the emitted THz pulse.

Fig. 2.. Terahertz emission from two-color injected currents in GaAs.

(A) Schematic of experimental setup for measuring terahertz radiation from transient currents excited in GaAs by synthesized two-color fields. GaAs, gallium arsenide substrate; Ge, germanium wafer; M₁ to M₃, metallic mirrors; L, focusing lens; L_THz1 and L_THz2, THz lenses; QWP, quarter-wave plate; WP, Wollaston prism; BS, pellicle beam splitter; ZnTe, zinc telluride crystal. a.u., arbitrary units. (B) THz field as a function of two-color phase (∆φ_ω,2ω) and probe time delay. Terahertz waveforms are recorded at different values two-color phase. Spatiotemporal maps of the measured THz electric field when (C) linearly polarized (D) azimuthally polarized and (E) radially polarized two-color pulses are applied.

Fig. 3.. Measurements of FD terahertz pulses.

Far-field spatial maps of the FD THz pulse (A) E_x (x, y) and (B) E_y (x, y), radiated from ring current. Spatial profiles are measured at the peak of the THz waveform. (C) Spatiotemporal structure of electric field of the FD THz pulse. Measured data are symmetrized for magnetic field calculation. (D) Space-time structure of the calculated longitudinal magnetic field (B_z) of the FD pulse. The magnetic field is calculated from the measured electric field data using Maxwell’s equations.

Fig. 4.. Spectroscopy using FD pulse.

(A) Measured electric field of FD pulses in humid air (RH 41%) condition. (B) Space-frequency representation of the measured electric field. (C) Calculated magnetic field (B_z) structure of the FD pulse in humid air. (D) Spatio-spectral map of the magnetic field B_z.