Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

Abstract

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at ω2/ω1 =1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of ω2/ω1 =1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.Terahertz (THz) waves have broad applications in THz spectroscopy [1,2] and THz-field matter interactions [3,4]. These applications can potentially benefit from powerful THz radiation sources with various parameters via different laser-plasma-based schemes [5][6][7][8][9]. For example, MV/cm-scale THz radiation with either linear [5,10,11] or elliptical polarization [12][13][14][15] can be generated from gas plasma. THz radiation of near mJ can be produced via relativistic laser interaction with solid plasma [8,[16][17][18]. Among these schemes, the two-color laser scheme [5] has been studied most widely [19][20][21][22][23][24][25][26][27][28] because it can provide high-efficiency tabletop broadband sources. Generally, an 800nm pump laser pulse passes through a frequency-doubling crystal to generate a second-harmonic pulse and then the two pulses are mixed to produce gas plasma. Up to now, the frequency ratio of the two-color pulses has been always taken as ω 2 /ω 1 =1:2 in experiments, although the fundamentalpulse wavelength longer than 800nm was adopted in recent experiments to enhance the THz strength [29][30][31] and the second-harmonic-pulse frequency was detuned to yield ultra-broadband radiation [32]. Since 2013 a few theoretical reports [33][34][35] have predicted that the twocolor scheme could be extended to uncommon frequency ratios such as ω 2 /ω 1 =1:4, 2:3, but these predictions have not yet been verified experimentally.In this Letter, we present the first experimental demonstration of THz generation with uncommon frequency ratios. With the ω 1 -laser wavelength fixed at 800nm and 400nm, respectively, a scan of the ω 2 -laser wavelength from 1200nm to 1600nm shows that the THz energies have three resonantlike peaks located near ω 2 /ω 1 = 1:4, 1:2, and 2:3. The energies at these peaks are at the same order. Beyond the previous predictions [33][34][35], we find that the THz polarization can be adjusted by rotating the ω 2 -pulse polarization and however, the polarization adjustment becomes inefficient by rotating the ω 1 -pulse polarization. In this Letter we define the ω 1 pulse as the higher-frequency one. These observations agree with our particle-in...

Spin-polarization effects of an ultrarelativistic electron beam in an ultraintense two-color laser pulse

“…Such a laser field cannot generate more spin-up or spin-down electrons via nonlinear Compton scattering, as observed in Fig. 2(h vector potential [33,34] and radiation reaction [55]. Besides, it is shown in Figs.…”

Intense terahertz radiation: generation and application

“…By precisely controlling the relative phase between the ω and 2ω pulses and the polarization of the 2ω beam, a peak THz electric field exceeding 60 kV/cm can be routinely obtained in ambient air with a total excitation energy of approximately 500 µJ. However, this theory cannot explain THz radiation obtained via two-color filamentation with uncommon frequency ratios [67]. Furthermore, the generated THz radiation strongly saturates with increasing pump laser energy.…”

Quantum-mechanical simulations of low-frequency current excitation during ionization of many-electron atoms by intense laser pulses

“…These include the excitation of a lowfrequency current (with frequencies much lower than the optical one) in a plasma created by a laser pulse. The interest to this phenomenon is associated with the possibility of its use for the generation of coherent and broadband radiation in the terahertz and midinfrared ranges and possibility of tuning the spectrum of generated radiation [1][2][3][4][5][6][7][8][9][10][11]. The low-frequency current density is excited effitiently under the asymmetry of ionizing pulses [1][2][3][4][5][6][7][8][9][10] or ionized molecules [11].…”

“…The interest to this phenomenon is associated with the possibility of its use for the generation of coherent and broadband radiation in the terahertz and midinfrared ranges and possibility of tuning the spectrum of generated radiation [1][2][3][4][5][6][7][8][9][10][11]. The low-frequency current density is excited effitiently under the asymmetry of ionizing pulses [1][2][3][4][5][6][7][8][9][10] or ionized molecules [11]. Among the various methods of generating low-frequency current, the most wellknown one is based on the use of two-color ionizing laser pulses [1][2][3][4][5][6][7][8].…”

“…The low-frequency current density is excited effitiently under the asymmetry of ionizing pulses [1][2][3][4][5][6][7][8][9][10] or ionized molecules [11]. Among the various methods of generating low-frequency current, the most wellknown one is based on the use of two-color ionizing laser pulses [1][2][3][4][5][6][7][8]. The low-frequency current is generated at some combination frequency of twocomponent laser pulse for a whole set of ratios between component frequencies [5][6][7][8].…”