CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts.

T Balčiūnas,D Dietze,A M Zheltikov,K Unterrainer,M Ivanov,G G Paulus,A Baltuška,T Rathje,S Haessler,O Smirnova,D Lorenc

doi:10.1364/oe.23.015278

Abstract

We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Highlights

This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation
Since the demonstration a decade earlier by Cook and Hochstrasser [1] of intense THz emission from air ionized with a two-colour (ω + 2ω) laser field, this phenomenon remains in the focus of attention
To High-order Harmonic Generation (HHG) where the mixing of the laser driving field with its second harmonic allows breaking the symmetry between the two quantum paths and yields generation of even harmonics [11], the THz emission can be considered an even (0thorder) harmonic

Summary

Introduction

Since the demonstration a decade earlier by Cook and Hochstrasser [1] of intense THz emission from air ionized with a two-colour (ω + 2ω) laser field, this phenomenon remains in the focus of attention. The THz-range covers a wealth of fundamental resonances (in molecules: vibrational and rotational resonances, in solids: phonon and plasmon resonances, impurity transitions), which opens a wide field of possibilities for fundamental material and device research [2, 3] as well as possible sensor applications, including the identification of atmospheric pollutants and use in food quality-control, atmospheric and astrophysical remote sensing, and (medical) imaging with unique contrast mechanisms [4] Another motivation is the interesting underlying plasma dynamics leading to the THz emission. We experimentally demonstrate control over the timing of sub-cycle optical field ionization relative to the sign and value of the vector potential of the pulse [14] This is achieved by fixing the carrier envelope phases (CEP) and detuning the frequencies of the two optical driving fields away from commensurability [15]. Due to the long wavelengths of the constituent colours (1030 nm and 1800–2060 nm), our scheme is a promising route for a highefficiency [19, 20], CEP-locked, broadband and widely tunable mid-IR/THz source

Quantum mechanical transient electron-current model for THz emission

Experimental implementation

Observation of directional electron current

Findings

Conclusion