Intense broadband THz generation from femtosecond laser filamentation

Broadband and energetic terahertz （THz） pulses can be remotely generated in air through filamentation. We review such THz generation and detection in femtosecond Ti-sapphire laser induced remote filaments. New results are presented on the direct relationship between THz generation in a two color filament and induced N2 fluorescence through population trapping during molecular alignment and revival in air. This further supports the new technique of remote THz detection in air through the sensitive measurement of N2 fluorescence.

Cross-focusing of two chirped Gaussian laser beams and THz wave generation in plasmas of multi-ion species under a weakly relativistic and ponderomotive regime

The generation of terahertz(THz)waves via the beating of two high-intensity chirped Gaussian lasers in a multi-ion-species plasma is numerically studied by taking into account the weak relativistic and ponderomotive regime of interaction.The coupled differential equations for beamwidth parameters are extracted by introducing the dielectric function of such plasma and using WKB and paraxial ray approximations.The amplitude of THz radiation at beat frequency resulting from the nonlinear current density induced by the beat ponderomotive force of the cross-focusing of beams was obtained.The impacts of the chirp frequency parameter,initial laser intensity and initial ionic species density(specifically,the presence of singly and doubly charged ions)in the plasma on THz generation were discussed.Our numerical results reveal that THz radiation generation strongly depends on the chirp frequency parameter.A specific range of chirp frequencies exists for self-focusing as well as THz generation with a'turning point',where the THz emission reaches its maximum value.The results show that the strength of self-focusing and consequently the generated THz radiation are reduced by increasing the density of doubly charged ionic species in the plasma due to the suppression of the nonlinear effects.

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves(HSWs).The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity(RC)where low-order Stokes waves(LSWs)are converted to high-order Stokes waves again and again.The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation(OCDFG)where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally.Combined with the repeated and continuous frequency conversions,the optical-to-THz energy conversion efficiency(OTECE)exceeds 26%at 300 K and 43%at 100 K with pump intensities of 300 MW/cm^(2).

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.

HISTORY AND RECENT DEVELOPMENTS OF TERAHERTZ WAVE GENERATION

The unexplored terahertz (THz) region involves important phenomena of both fundamental and applied natures. Examples include phonon interactions, rotational transitions and intermolecular dynamics. Frequency tunable high power THz wave generation has been successfully achieved utilizing lattice resonance of LiNbO3 and GaP crystals, respectively. Semiconductor devices utilizing electron tunneling effect have also been developed.

Excitation-wavelength-dependent THz wave modulation via external bias electric field

A theoretical model was proposed to describe the effects of external bias electric field on terahertz(THz)generated in air plasma.The model predicted that for a plasma in a bias electric field,the amplification effect of the THz wave intensity increases with the increase of the excitation laser wavelength.We experimentally observed the relationship between the THz enhancement effect and the electric field strength at different wavelengths.Experimental results showed a good agreement with the model predictions.These results enhance our understanding of the physical mechanism by which femtosecond lasers excite air to generate THz and extend the practical applications of THz generation and modulation.

Enhancement and modulation of terahertz radiation by multi-color laser pulses

We study theoretically intense terahertz radiation from multi-color laser pulse with uncommon frequency ratios. Com- paring the two-color laser scheme, of which the uncommon frequency ratio should be set to be a specific value, we show that by using multi-color harmonic laser pulses as the first pump component, the lasers as the second pump component can be adjusted in a continuous frequency range. Moreover, these multi-color laser pulses can effectively modulate and enhance the terahertz radiation, and the terahertz yield increases with the increase of the wavelength of the uncommon pump com- ponent and is stable to the laser relative phase. Finally, we utilize the electron densities and velocities of ionization events to illustrate the physical mechanism of the intense terahertz generation.

Intense terahertz radiation: generation and application

Strong terahertz(THz)radiation provides a powerful tool to manipulate and control complex condensed matter systems.This review provides an overview of progress in the generation,detection,and applications of intense THz radiation.The tabletop intense THz sources based on Ti:sapphire laser are reviewed,including photoconductive antennas(PCAs),optical rectification sources,plasma-based THz sources,and some novel techniques for THz generations,such as topological insulators,spintronic materials,and metasurfaces.The coherent THz detection methods are summarized,and their limitations for intense THz detection are analyzed.Applications of intense THz radiation are introduced,including applications in spectroscopy detection,nonlinear effects,and switching of coherent magnons.The review is concluded with a short perspective on the generation and applications of intense THz radiation.

Terahertz aqueous photonics

Developing efficient and robust terahertz(THz)sources is of incessant interest in the THz community for their wide applications.With successive effort in past decades,numerous groups have achieved THz wave generation from solids,gases,and plasmas.However,liquid,especially liquid water has never been demonstrated as a THz source.One main reason leading the impediment is that water has strong absorption characteristics in the THz frequency regime.A thin water film under intense laser excitation was introduced as the THz source to mitigate the considerable loss of THz waves from the absorption.Laser-induced plasma formation associated with a ponderomotive forceinduced dipole model was proposed to explain the generation process.For the one-color excitation scheme,the water film generates a higher THz electric field than the air does under the identical experimental condition.Unlike the case of air,THz wave generation from liquid water prefers a sub-picosecond(200-800 fs)laser pulse rather than a femtosecond pulse(~50 fs).This observation results from the plasma generation process in water.For the two-color excitation scheme,the THz electric field is enhanced by one-order of magnitude in comparison with the one-color case.Meanwhile,coherent control of the THz field is achieved by adjusting the relative phase between the fundamental pulse and the second-harmonic pulse.To eliminate the total internal reflection of THz waves at the water-air interface of a water film,a water line produced by a syringe needle was used to emit THz waves.As expected,more THz radiation can be coupled out and detected.THz wave generation from other liquids were also tested.