Boosting Terahertz Generation in Laser-Field Ionized Gases Using a Sawtooth Wave Shape

Abstract

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. DOI: 10.1103/PhysRevLett.114.183901 PACS numbers: 42.65.Re, 32.80.Fb, 52.50.Jm Ultrashort pulses in the terahertz (THz) range (from ∼0.1 to ∼30 THz) are extremely important for various timeresolved studies in molecular physics, chemistry, material sciences, and security applications [1][2][3][4][5][6][7][8]. One of the major challenges in this field is the development of THz emitters producing high peak intensities. So far, besides conventional devices such as antennas, photoconductive switches, etc., two main techniques have been explored for producing subps THz pulses with energies in the microjoule range. The first method is based on optical rectification in second-order nonlinear crystals [9]. Pumped by multi-mJ single color pulses, this technique requiring phase matching can generate THz pulses with 10 μJ energy, but the bandwidth is limited to a few THz. Recently, large-sized organic crystals were used to deliver THz pulses with GV=m electric field strength, and a conversion efficiency of about 1% was demonstrated [10]. The second method is based on focusing a femtosecond pulse together with its second harmonic into a gas cell and create a plasma [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In such two-color pump setup, free electrons produced by tunnel ionization acquire a nonzero drift velocity and generate a quasi-dc current which is responsible for THz emission [19]. The mechanism underlying THz generation in gases is intrinsically related to the optically induced stepwise increase of the free electron density near the extrema of the ionizing optical field [15,16,22]. An easy explanation is that the Fourier transform of a steplike function in time has its maximum near the zero frequency. Besides, critical for the generated THz energy is a pronounced asymmetry in time of the pump wave shape with respect to the field extrema, which dictates the electron drift velocity. This gas-based scheme for THz generation provides higher breakdown threshold and broader spectral ranges than the method involving crystals [11][12][13][17][18][19][20]. THz pulses in gases with high field strength > 1 GV=m were simulated in [16]; however, the highest reported THz energies (5 μJ) correspond to conversion efficiencies of about 10 −4 only.In this Let...

THz field engineering in two-color femtosecond filaments using chirped and delayed laser pulses

“…Moreover, if the number of colors is augmented (here up to eight), the harmonics in  N e L get alternating signs and relative amplitudes, which coincide better and better with those of the pulse spectrum and thereby enhance even more the THz yield. This enhancement is consistent with that reported in [29]. This property can also be retrieved from implementing the amplitudes a n and phase values f n of a sawtooth wave field into the gain factor equation (12).…”

“…We retrieve the best SH phase angle f p = 2 2 for THz production by photocurrents [30], which induces a substantial increase inN e L at FH frequency, hence inR. Indeed, if we apply the approximation that ionization occurs at the maxima of E L 2 and neglect the envelope effects, Another example illustrates in figure 3(b) the spectrum of the electron density computed on the first harmonics of a sawtooth wave shape, known to supply the highest THz yields to date at fixed ionization level [29]. For such waveforms, equation (3) consists of a single pulse (k=0) involving an increasing number of harmonics with = a n 1 n and f p = -( ) 1 2 n n .…”

“…By 'optical model', it is meant that only the spectral contents of the input laser field and associated electron density at the optical frequencies are utilized to evaluate and justify the changes in the laserto-THz conversion efficiency. Instead of exploiting electron velocity fields at the ionization instants as in [7,8,29], we focus on the convolution product of  N e L and   E L around the main harmonics of the optical pulse. To start with, let us ignore the influence of the Gaussian pulse envelopes.…”

Multibeam synthesis of high-power subcycle field waveforms

“…When implemented in a two-color laser field experiment, this approach provides the increase in the high-harmonic yield by orders of magnitude [19][20][21] and offers a tool for an attosecond control of electron trajectories in high-harmonic generation [22], promising a technique for the generation of shorter attosecond pulses with a better controlled chirp. In a recent theoretical study [23], two-color fem- * E-mail to: zheltikov@physics.msu.ru tosecond pulses combined to a sawtooth field waveform have been shown to radically enhance the generation of ultrashort terahertz pulses. A pair of orthogonally polarized ultrashort laser pulses has been demonstrated to provide a powerful tool for steering ultrafast electron dynamics both in time and space, thus giving an access to the fundamental electron processes underlying lasermatter interactions [24][25][26][27].…”

Revealing plasma oscillation in THz spectrum from laser plasma of molecular jet

“…Broadband ultrashort terahertz (THz) pulses are generated from the laser induced plasma by two-color pulses, and it is under intensive studies owing to the potential to produce high intensity THz field and achieve the off-site detection in air [1][2][3][4][5][6][7]. From the plasma filament pumped by the two-color laser pulses, the directional photocurrents generate the enhanced supercontinuum at the THz wavelength range with the conversion efficiency two orders larger than that from single color intense laser field [8,9].…”