Generation of terahertz radiation from ionizing two-color laser pulses in Ar filled metallic hollow waveguides

Dominance of plasma-induced modulation in terahertz generation from gas filament

In this paper, we revisit the fundamental mechanism responsible for terahertz generation from laser-induced plasma filament based on the photocurrent model by employing a blend of analytical calculation and numerical simulation. By using the frequency-decomposed finite-difference time-domain (FD-FDTD) method, the role of two-color field and photocurrent radiation in terahertz generation from plasma filament is visually separated, and the driving effect of photocurrent radiation is confirmed pretty significant within the process. Then, a pair of numerical experiments are taken to further analyze the driving effect of photocurrent radiation, and it is revealed that plasma-induced modulation to photocurrent radiation is actually the underlying physical mechanism of terahertz generation from plasma filament. Furthermore, a three-step diagram is introduced to reillustrate the overall physical process and provides a more comprehensive explanation. In addition, the mechanism of plasma-induced modulation to photocurrent radiation in terahertz generation is substantiated by taking theoretical prediction and numerical simulation of minimal filament length required for achieving stable backward terahertz emission, which directly confirms the validity and significance of plasma-induced modulation to photocurrent radiation in terahertz generation from laser-induced plasma filament.

Trajectory analysis for low-order harmonic generation in two-color strong laser fields

Focusing two-color laser fields in gas-phase medium produces ultrashort ultra-broadband low-order harmonics spanning from terahertz to extreme ultraviolet regime. The low-order harmonic generation can be explained by both macroscopic photocurrent model and microscopic strong field approximation theory. Here, we analytically build a bridge between the macroscopic and microscopic theories by means of the trajectory method, which manifests correspondences between macroscopic and microscopic theories. And we demonstrate the trajectory analysis to explain phase-dependent terahertz and third-harmonic generations, and contribute the phase-dependent yields and spectral shapes to the coherent superposition of electron trajectories released at distinct ionization instants, reflecting electron interfering with itself in radiation process. The trajectory method readily connects the low-order harmonics characteristics and behaviors of electron wavepacket, which has potential for reconstructing ultrafast electron dynamics by means of low-harmonics observations.

Unusual terahertz waveforms from a resonant medium controlled by diffractive optical elements

Up to now, full tunability of waveforms was possible only in electronics, up to radio-frequencies. Here we propose a new concept of producing few-cycle terahertz (THz) pulses with widely tunable waveforms. It is based on control of the phase delay between different parts of the THz wavefront using linear diffractive optical elements. Suitable subcycle THz wavefronts can be generated via coherent excitation of nonlinear low-frequency oscillators by few-cycle optical pulses. Using this approach it is possible to shape the electric field rather than the slow pulse envelope, obtaining, for instance, rectangular or triangular waveforms in the THz range. The method is upscalable to the optical range if the attosecond pump pulses are used.

Generation of Few- and Subcycle Radiation in Midinfrared-to-Deep-Ultraviolet Range During Plasma Production by Multicolor Femtosecond Pulses

Our closed-form analytical formulas and numerical calculations show that the plasma production by a two-color (or, more generally, multicolor) femtosecond pulse leads to generation of strong few- and subcycle radiation. The spectral composition of the radiation is defined by the numerous combination frequencies of the ionizing pulse. The radiation duration is equal to the ionization duration, which is much shorter than the multicolor pump. The phenomenon opens a new direct (without additional compression) way to create tunable sources of extremely short pulses with smooth envelopes and spectra in a broad range stretching from the midinfrared to the deep ultraviolet.

Boosting terahertz generation in laser-field ionized gases using a sawtooth wave shape

Broadband ultrashort terahertz (THz) pulses can be produced using plasma generation in a noble gas ionized by femtosecond two-color pulses. Here we demonstrate that, by using multiple-frequency laser pulses, one can obtain a waveform which optimizes the free electron trajectories in such a way that they acquire the largest drift velocity. This allows us to increase the THz conversion efficiency to 2%, an unprecedented performance for THz generation in gases. In addition to the analytical study of THz generation using a local current model, we perform comprehensive 3D simulations accounting for propagation effects which confirm this prediction. Our results show that THz conversion via tunnel ionization can be greatly improved with well-designed multicolor pulses.

Broadband field-resolved terahertz detection via laser induced air plasma with controlled optical bias

We report a robust method of coherent detection of broadband THz pulses using terahertz induced second-harmonic (TISH) generation in a laser induced air plasma together with a controlled second harmonic optical bias. We discuss a role of the bias field and its phase in the process of coherent detection. Phase-matching considerations subject to plasma dispersion are also examined.

Controlling the carrier-envelope phase of single-cycle mid-infrared pulses with two-color filamentation

Carrier-envelope phase (CEP) of single-cycle pulses generated through two-color filamentation has been investigated. We have observed a particular behavior of the phase: the phase of high-frequency components of the generated pulses changes continuously and linearly with the relative phase between the two-color input pulses, whereas the phase of the low-frequency components takes only two discrete values. The transition of the phase behavior has been clearly observed by using frequency-resolved optical gating capable of CEP determination. We have found out that such a phase behavior is a unique feature of single-cycle pulses generated with a passive CEP stabilization scheme.

Effect of two-color laser pulse intensity ratio on intense terahertz generation

We theoretically demonstrate the effect of the intensity ratio of the two-color laser field on the terahertz generation based on a transient photocurrent model.

Analytical model for THz emissions induced by laser-gas interaction

We develop a one-dimensional model of THz emissions induced by laser-driven, time-asymmetric ionization and current oscillations in a hydrogen gas. Our model highlights complex scalings of the THz fields with respect to the laser and gas parameters, in particular, a non-monotonic behavior against the laser parameters. Analytical expressions of the transmitted and reflected fields are presented, explaining the THz spectra observed in particle-in-cell and forward-pulse propagation codes. The backward-propagating THz wave is mainly driven by the electron current oscillations at the plasma frequency, and its resulting spectrum operates below the plasma frequency. The transmitted THz wave is emitted from both plasma current oscillations and photo-ionization. Their respective signal presents a contribution below and around the plasma frequency, plus a contribution at higher frequencies associated to the photo-induced current. The interplay between these two mechanisms relies on the ratio between the propagation length and the plasma skin depth.

3D numerical simulations of THz generation by two-color laser filaments

Terahertz (THz) radiation produced by the filamentation of two-color pulses over long distances in argon is numerically investigated using a comprehensive model in full space-time-resolved geometry. We show that the dominant physical mechanism for THz generation in the filamentation regime at clamping intensity is based on quasi-dc plasma currents. The calculated THz spectra for different pump pulse energies and pulse durations are in agreement with previously reported experimental observations. For the same pulse parameters, near-infrared pump pulses at 2  μm are shown to generate a more than 1 order of magnitude greater THz yield than pumps centered at 800 nm.

Directionality of terahertz emission from photoinduced gas plasmas

Forward and backward terahertz emission by ionizing two-color laser pulses in gas is investigated by means of a simple semianalytical model based on Jefimenko's equations and rigorous Maxwell simulations in one and two dimensions. We find the emission in the backward direction has a much smaller spectral bandwidth than in the forward direction and explain this by interference effects. Forward terahertz radiation is generated predominantly at the ionization front and is thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model.