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Optical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, we study theoretically, and experimentally a chiral metamaterial structure composed of pairs of vertical U-shape resonators of "twisted" arms, and we reveal that it demonstrates large pure optical activity (i.e. optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25 (deg) for an unmatched bandwidth of 1 THz (relative bandwidth 80 %), from a 130 um-thickness structure, while theoretical optimizations show that the rotation can reach 45 (deg). The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which provides also simple optimization rules for the enhancement of its chiral response. The proposed chiral structures allow easy fabrication via direct laser writing and electroless metal plating, making them suitable candidates for polarization control applications., Comment: 17 pages, 7 figures

Interaction of intense terahertz (THz) waves with graphene based modulation devices holds great potential for the optoelectronic applications of the future. Here, we present a thin film graphene-based THz perfect absorbing device whose absorption and phase characteristics can be modulated through THz self-actions in the subps time scale. The device consists of a single-layer graphene placed on an ionic liquid substrate, back-plated by a metallic back-reflector, with the graphene doping level mediated through electrostatic gating. We experimentally record an absorption modulation of more than 3 orders of magnitude from the initial perfect absorption state, when the device is illuminated with THz field strengths in the range of 102 to 654 kV/cm. Furthermore, an absolute phase modulation of 130° is recorded. Detailed theoretical analysis indicates that the origin of the THz nonlinear response is the THz-induced heating of the graphene’s carriers that leads to a reduction of its conductivity and, consequently, to reduced absorption of the THz radiation. Our analysis also maps the temporal dynamics of the THz-induced temperature elevation of the graphene’s carriers, illustrating the ultrafast, subps nature of the overall process. These results can find applications in future dynamically controlled flat optics and spatiotemporal shaping of intense THz electric fields.

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems., Powerful terahertz pulses are generated during the nonlinear propagation of ultrashort laser pulses in gases. Here, the authors demonstrate efficient sub-cycle THz pulse generation by using two-color midinfrared femtosecond laser filaments in ambient air.

We experimentally demonstrate that 2D Airy wave packets can produce intense curved two-color filaments that emit terahertz (THz) radiation with unique characteristics. Due to the curvature of the plasma channel, THz waves, emitted from different longitudinal regions of the plasma, propagate in different directions resulting in non-concentric THz cones in the far-field. These cones have different cone angles and polarization which we attribute to the way the two-color 2D Airy driving fields are produced in the nonlinear crystal and then propagate to form the curved plasma filament.

We demonstrate controllable spectral shaping of supercontinuum radiation through intense femtosecond laser filamentation in ethanol. Neural networks are shown to predict robust results even under the presence of high-order nonlinearities and noise.

We demonstrate that thermal effects limiting femtosecond laser filamentation in gases can be addressed using a simple gas replenishment method. This enables high average power filamentation applications like THz sources.

We report the spectral shaping of supercontinuum generation in liquids by employing properly engineered Bessel beams coupled with artificial neural networks. We demonstrate that given a custom spectrum, neural networks are capable of outputting the experimental parameters needed to generate it experimentally.

Κατά την τελευταία δεκαετία, χάρη στην τεράστια τεχνολογική ανάπτυξη στην φωτονική επιστήμη και την επιστήμη υλικών, η περιοχή συχνοτήτων τέραχερτζ (THz) του ηλεκτρομαγνητικού φάσματος γίνεται πλέον προσβάσιμη, ανοίγοντας νέους ορίζοντες σε διάφορα ερευνητικά πεδία, συμπεριλαμβανομένης της χημείας, της βιολογίας, της ιατρικής και της επιστήμης των υλικών. Η ακτινοβολία THz βρίσκεται στην περιοχή συχνοτήτων από 0.1 έως 30 ΤΗz. Οι συχνότητες αυτές αντιστοιχούν σε μήκη κύματος από 3 mm έως 10 μm, σε ενέργειες φωτονίων από 0.4 meV έως 120 meV και σε αντίστοιχη ακτινοβολία μελανού σώματος έχουν θερμοκρασίες από 4 Κ έως 1200 Κ. Η εργασία αυτή επικεντρώνεται στην παραγωγή ισχυρών, ευρυζωνικών παλμών ΤΗz μέσω διχρωματικής νηματοποίησης στον αέρα. Η πηγή αυτή βασίζεται στη χρήση ενισχυμένων παλμών λέιζερ όπου η εκπομπή ακτινοβολίας THz προέρχεται από το εγκάρσιο ρεύμα πλάσματος το οποίο σχηματίζεται από ένα ασύμμετρο διχρωματικό νήμα. Προτείνουμε ένα ολοκληρωμένο φυσικό μοντέλο που εξηγεί τον κωνικό χαρακτήρα της δέσμης THz που παράγεται από μια τέτοιου είδους πηγή, ενώ παράλληλα ερευνούμε πειραματικά την εγκυρότητά του ρυθμίζοντας κατάλληλα και με προκαθορισμένο τρόπο τις ιδιότητες της πηγής. Για τον πλήρη χαρακτηρισμό της πηγής μας, παρουσιάζουμε μια μέθοδο για την ανασύσταση ενός ευρέος φάσματος παλμού ΤΗz ο οποίος έχει έντονα αλλοιωθεί κατά την διάδοση του σε μέσα με έντονη απορρόφηση και φασματική διασπορά. Η προσέγγισή μας περιλαμβάνει μια πειραματική διαδικασία μαζί με έναν αλγόριθμο που μας δίνει πρόσβαση στην αλλοιωμένη κατά την διάδοση του παλμού πληροφορία.Πέρα από τον χαρακτηρισμό της πηγής, προτείνουμε καινοτόμους τρόπους ενίσχυσης της παραγωγής ακτινοβολίας THz. Αυτοί περιλαμβάνουν αρχικά, την χρήση δεσμών λέιζερ με ικανότητα αυτοεστίασης για την παραγωγή ισχυρών παλμών THz σε προκαθορισμένες και απομακρυσμένες θέσεις, αφού πρώτα επαληθεύσαμε πειραματικά και θεωρητικά την απαραίτητη γένεση δεύτερης αρμονικής από τέτοιου είδους δέσμες. Τα αποτελέσματά μας δείχνουν πως μια τέτοια πηγή παράγει παλμούς THz 5.3 φορές πιο ισχυρούς σε σχέση με μια αντίστοιχη πηγή που χρησιμοποιεί Γκαουσιανή δέσμη λέιζερ. Στη συνέχεια, μελετήσαμε για πρώτη φορά την γένεση ακτινοβολίας THz μέσω διχρωματικής νηματοποίησης σε διαφανή στερεά. Τέλος, εξετάσαμε την επίπτωση που έχει ο ρυθμός επαναληψιμότητας του λέιζερ σε μια πηγή THz διχρωματικής νηματοποίησης στον αέρα και παρουσιάζουμε ενίσχυση της παραγόμενης ακτινοβολίας κατά 70% σε υψηλότερους ρυθμούς επαναληψιμότητας.

Two-color laser filamentation constitute one of the most convenient and efficient sources of pulsed ultra-broadband terahertz (THz) radiation. In this approach, a Gaussian ultrashort laser pulse together with its second harmonic are focused into a gas medium, forming a filament, which emits THz radiation with extremely high THz electric and magnetic fields [1] . A major advantage of two-color filamentation is that one can tune the emitted THz radiation in terms of energy, spectrum, polarization and THz beam divergence by properly tailoring the filament itself [2] , [3] . One way to achieve this tailoring is with the use of robust exotic-wavepackets from the family of accelerating beams such as the ring-Airy and the 2D Airy beams. Two-color plasma channels induced by pre-engineered abruptly autofocusing ring-Airy beams have already been proposed in order to enhance the THz yield by 5.3 times compared to a Gaussian-beam-induced plasma [4] .

Over the last two decades, terahertz (THz) radiation has attracted a constantly growing attention and has been routinely applied in a variety of applications, such as medical diagnostics, industrial quality control, food inspection and many others [1] . A new era of applications, that of THz wireless communication opens up ahead revealing the high importance of the development of fast modulation devices operating at the THz part of the electromagnetic spectrum. Towards this direction many devices based on graphene have been proposed to modulate radiation at THz frequencies. For example, a perfect absorber device based on a cavity of monolayer graphene over a grounded dielectric that can be tuned by an electrostatic gate was demonstrated in the low THz regime [2] . Under the same concept it was experimentally shown that the THz absorption of such device can be modulated in an ultrafast way via an optical pump signal [3] .

Nowadays, one of the most advantageous techniques for table-top terahertz (THz) generation is the two-color laser filamentation in gases, where an ultrashort laser pulse together with its second harmonic are focused into a gas medium, forming an ionized plasma channel, called filament, which emits THz radiation [1] . Typically, laser sources operating at pulse repetition rates from few Hz to several kHz are used for this generation scheme. Though, in case of high pulse repetition rates the dynamics of the gas medium become especially important, since the plasma channels act as a thermal source which generates density hole structures whose characteristic lifetime exceeds the time interval between two consecutive laser pulses [2] . Here we demonstrate that the THz emission from two-color laser filaments in gases is strongly affected by the pulse repetition rate of the driving laser [3] .

Ultra-fast modulation of 3D THz LC resonators is presented with a 280 GHz frequency shift obtained in 200 fs. The overall modulation cycle of the device takes 2 ps, yet convoluted by the probing technique.

We present the experimental demonstration of a subpicosecond all-optical THz switch based on three-dimensional (3D) terahertz meta-atoms. Combining a special design of 3D meta-devices and the ultrafast dynamics of low temperature grown gallium arsenide, we can modulate the reflectance of the THz microcavities within 2.2 ps. The device enables a 280 GHz switch in resonance frequency within less than 200 fs. The switch back to the original resonance takes 800 fs. Experimental results show that the speed values are strongly convoluted by the THz probing field and, thus, the real switching times are even shorter, in the few 100 fs range.

We report on THz generation from curved filaments produced by 2D Airy wave packets. Due to the curvature of the plasma channel, non-concentric THz beams with different polarizations are generated.

We demonstrate an ultrafast self-induced terahertz absorption modulator operating at 2.3 THz. A modulation of 50 dB is observed in the absorption when the THz field strength increases from 145 to 654 kV/cm.

We report on the impact of laser pulse repetition rate on two-color filamentation based terahertz sources. A 50% decrease on the terahertz energy is observed when the repetition rate increases from 0.6 to 6 kHz.

We experimentally demonstrate that the terahertz (THz) emission from two-color laser filaments in gases is strongly affected by the pulse repetition rate of the driving laser. We show that at repetition rates above 100 Hz, propagation of every next laser pulse in the pulse train is altered by gas density depressions produced by the preceding laser pulses. As a result, plasma channels at higher repetition rates become shorter, leading to less efficient THz generation. In particular, we observe a 50% decrease in the emitted THz energy when the repetition rate increases from 6 Hz to 6 kHz.

We discuss the unique possibilities stemming from exploiting the exotic properties of artificial metasurfaces designed for operation in the THz regime. We mainly focus on graphene based metasurfaces acting as modulators for the THz regime. Graphene exhibits unique optical properties particularly in the THz spectrum, where it predominantly exhibits a Drude-like response. Here we present that ultrafast modulation response in a graphene thin film absorber which can be assessed with use of a broadband THz time-domain-spectroscopic system (THz-TDS) in an IR pump-THz probe configuration. The simple scheme of the absorber can be used as a platform for ultrafast flat optics and metasurfaces.

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here we demonstrate both theoretically and experimentally the realization of a novel THz source with high peak power performance based on two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm. Our theory predicts that under this scheme sub-cycle THz pulses with multi-millijoule energies and record conversion efficiencies can be produced. Besides, we elucidate the origin of this high efficiency, which is made up of several factors, including a novel mechanism where the harmonics produced by the mid-infrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation. In our experiments we verify the theoretical predictions by demonstrating ultrashort sub-cycle THz pulses with sub-millijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm-1. Moreover, we show that these intense THz fields can drive nonlinear effects in bulk semiconductors (ZnSe and ZnTe) in free space and at room temperature. Our numerical simulations indicate that the observed THz yield can be significantly upscaled by further optimizing the experimental setup leading to even higher field strengths. Such intense THz pulses enable extreme field science, including into other, relativistic phenomena.

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1–50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities—a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids—a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium., Developing simple and efficient table-top sources of intense terahertz radiation is an ongoing pursuit. Here, Dey et al. demonstrate broadband terahertz generation from laser filamentation in liquids with an order of magnitude higher energy than from conventional two-color filamentation in air.

Nonlinear (NL) interactions of strong THz electric and magnetic fields with matter are the next frontier in nonlinear optics. NL THz spectroscopy requires ultrashort/broadband THz pulses with high electric field strengths. Here, we report on THz generation from two-color laser filaments, driven by intense midIR pulses centered at $3.9 {\mu } \mathrm {m}$. We demonstrate generation of THz pulses with a spectral bandwidth of $\gt 15$ THz, extraordinary conversion efficiency of 2.34% and remarkably high energy of 0.185 mJ.

We present the experimental and theoretical study of an ultrafast, optically tunable graphene-based thin film absorption modulator for operation in the THz regime. An ultrafast near-IR pulse induces the generation of hot carriers in the graphene sheet, consequently, reducing its conductivity with a decay time of 2.79 ps. As a result, a modulation of 40% in the THz absorption at 2.17 THz is observed in the structure.

We demonstrate that intense free space THz fields can nonlinearly induce excitonic photoluminescence in room temperature semiconductors as well as cross-phase modulation in an electro-optic crystal.

We discuss the unique possibilities of graphene-based THz metasurfaces and we present the experimental and theoretical study of an ultrafast graphene-based thin film absorption modulator for operation in the THz regime. The stimulus is a near-IR pulse that generates hot carriers in the graphene metasurface reducing its conductivity.

Graphene, a two-dimensional material made of carbon atoms arranged in a honeycomb lattice, has attracted enormous attention since its experimental realization in 2004 [1] due to its unique mechanical, thermal, electrical and optical properties. Particularly in the THz spectrum, graphene predominantly exhibits a Drude-like response due to its easily generated and controlled free carriers and therefore is considered a suitable platform for dynamically tunable metasurface components [2]. Such an example, are the so called thin film absorbers that are structures capable of absorbing all power of incident electromagnetic waves. They are used as non-reflective covering layers for shielding against electromagnetic radiation and minimization of backscattering from electromagnetic large structures. Within the concept of metasurfaces, thin film absorbers, are usually implemented by placing a lossy material on the top of a perfectly conducting metallic plate [3]. Recently, an electrostatic gate-tunable perfect absorber based on a monolayer graphene over a grounded dielectric was demonstrated experimentally in the low THz regime [4]. Nevertheless, the dynamic control of the absorption properties of such devices in an ultrafast way still remains a challenge.

We report on THz generation driven by 3.9 µm pulses, via either optical rectification in organic crystals or in two-color plasma filaments. Outstanding THz conversion efficiency of more than 2% and bandwidth exceeding 15 THz are achieved.

We demonstrate both theoretically and experimentally that the harmonics from abruptly auto-focusing ring-Airy beams present a surprising property, they preserve the phase distribution of the fundamental beam. Consequently, this "phase memory" inherits to the harmonics the abrupt autofocusing behavior, while, under certain conditions, their foci coincide in space with the one of the fundamental. Experiments agree well with our theoretical estimates and detailed numerical calculations. Our findings open the way for the use of such beams and their harmonics in strong field science., Comment: 5 pages, 6 figures

Developing coherent intense broadband light sources is undoubtedly one of the key elements in pushing the frontiers of optical sciences, especially those of nonlinear terahertz (THz) optics. Plasma-based THz emitters excel in many aspects thanks to their broad radiation bandwidth, high output pulse energy and immunity to high power damage. Various excitation wavelengths and gases have been explored in the search for a stronger THz wave emission from laser-induced plasmas, whereas few attempts have been made with artificially modulated exotic wavepackets. Here we demonstrate that abruptly autofocusing-beam-induced air-plasma can give a 5.3-times enhanced THz wave pulse energy compared to normal Gaussian-beam-induced plasma under the same conditions. We believe this work will inspire a new direction for controlling THz radiation from laser-induced plasma using tools such as spatial light modulators to sculpt the excitation beams, and pave the way to THz remote spectroscopy, which is considered critical for both homeland security and environmental monitoring.

By using abruptly autofocusing beams, as opposed to the conventional Gaussian beams, to generate plasma in the ambient air, an enhancement of the terahertz radiation from the plasma in the low frequency region is observed.

We experimentally investigated the THz emission from two-color ring-Airy beam induced plasma in the ambient air. The results show that this exotic autofocusing beam tends to form an elongated weak filamentation with a main peak at the front and a ‘tail’ with certain oscillations following, which leads to a higher THz yield and a slightly narrower THz spectrum than the emission from a Gaussian beam plasma under the same circumstances.

We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from femtosecond two-color laser-induced air plasma filaments. We show, that, in contrast to other models, emission is always conical, resulting from phase matching of the radiation produced inside the filament combined with a partial back reflection of the generated terahertz field from the filament itself due to the frequency-dependent critical plasma density. The obtained conical angle varies from 2${}^{\ensuremath{\circ}}$ to 10${}^{\ensuremath{\circ}}$, depending on the plasma density distribution and filament length. Unlike previously proposed models, our model shows good agreement with our experiments as well as a wide range of experimental findings from the literature.

Gas-plasma strings from 2-color femtosecond laser filaments constitute one of the most convenient and promising sources of pulsed terahertz (THz) radiation. It is one of the approaches that allow one to achieve at the same time ultrabroadband THz spectra and very high amplitudes of THz electric fields. Such sources present major advantages for applications of sensing, spectroscopy, imaging, and nonlinear optics. In view of the numerous applications an understanding of the spatial distribution of the terahertz radiation, from this source, is crucial. One of the first works towards this direction [1], revealed a highly directional THz radiation while a more recent publication [2] proposes a donut-shape spatial pattern. The most recent results reported by Kim et.al. [3], propose an off-axis phase matched emission from the filament due to interference effect between the different point sources across the filament, resulting thus in a donut-shape profile. A simple theoretical model was also proposed in [3], which is though insufficient to explain all observations.

Purpose: Upon irradiation the active diacetylene layer of radiochromic films undergoes dose-dependent polymerization in the blue polydiacetylene form. Dose assessment is currently based on linear absorbance measurements. The scope of the present study was designed for the further understanding of the linear and, in particular, the nonlinear optical characteristics of such films and the utilization of these characteristics for devising alternative “reading” procedures. Methods: The nonlinear optical interactions of laser light with previously irradiated GafChromic® HD-810 and MD55-2 films were studied by z-scan techniques. The focused laser beam strikes on the film, which is located at various distances from the focal spot. The beam induces linear and nonlinear absorption and refraction. The measurement of the transmitted power (open-aperture z-scan) allows the study of the nonlinear absorption. On the other hand, the measurement of the axial region of the transmitted beam (closed-aperture z-scan) is related to both nonlinear absorption and refraction effects. A 10 mW, 633 nm, linearly polarized He/Ne laser was employed as the light source. The transmittance measurements were coupled with absorbance measurements carried out over the whole visible spectrum and at various reading temperatures, in an attempt to clarify the underlying mechanisms dictating the observed effects. These effects were incorporated to an open-aperture z-scan model, developed for the purpose of comparison to the corresponding experimental curves. Results: The transmittance data obtained by both open- and closed-aperturez-scan were found to be dose-dependent, thus allowing such transmittance techniques to be employed for dose assessment. Low power open-aperture z-scan measurements (sensitive to absorptive nonlinearities) revealed a stepwise two-photon excitation of the active layer, through an intermediate state. At higher laser intensities (and/or absorbed dose), the shapes of the z-scan curves were found to be more complicated. These shapes were attributed to a temperature-dependent (thermochromic) transition from the blue form to the conformer red form of the active material, a transition which appears to be irreversible at high intensities. This transition was incorporated into the open-aperture z-scan model which reproduced satisfactorily the corresponding experimental curves. Finally, the use of a chopped laser beam in the closed-aperture z-scan technique allowed artifact-free, high spatial resolution dose measurements using a laser beam of relatively high power. Conclusions: The model developed in the present study elucidates some of the effects related to film reading and the causes of some of the artifacts reported in the literature. Two-dimensional dose distributions can be assessed byz-scan; however, the merits of such a technique relative to those currently in use have to be further investigated.

We present an experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on an SU-8 dielectric substrate that is grounded by a metallic ground plate. The structure shows enhanced absorption originating from constructive interference of the impinging and reflected waves at the absorbing graphene sheet. The modulation of this absorption, which is demonstrated via a THz time-domain spectroscopy setup, is achieved by applying an optical pump signal, which modifies the conductivity of the graphene sheet. We report an ultrafast (on the order of few ps) absorption modulation on the order of 40% upon photoexcitation. Our results provide evidence that the optical pump excitation results in the degradation of the graphene THz conductivity, which is connected with the generation of hot carriers, the increase of the electronic temperature, and the dominant increase of the scattering rate over the carrier concentration as found in highly doped samples.

Despite the rapid development of terahertz (THz) science during the last two decades, the majority of available THz sources remains rather weak, limiting the interactions of THz radiation with matter mostly in the realm of linear optics. Development of powerful THz sources will open the way to many exciting applications spanning from switching and controlling of magnetic domains to THz-enhanced attosecond pulse generation [1,2].

We demonstrate, strong THz emission from two-color mid-infrared (3.9 μm) femtosecond laser filaments. The conversion efficiency approaches the percent level, at least one order of magnitude higher than the previously reported for plasma-based THz sources.

Intense, ultra-broadband terahertz (THz) pulses can drive major advances in ultrafast dynamics, nonlinear THz optics, and bio-material imaging [1,2]. Single and two-color filamentation of ultrashort laser pulses in gases have been extensively used for the generation of such pulses [3,4]. However, filament formation in gases is hindered by the need of high laser pulse energies, motivating the exploration of alternate generation mechanisms [5]. On the other hand, filamentation can be achieved more easily in alternative target materials such as transparent solids and liquids [6,7]. Though, THz emission through either single or two-color filamentation in such materials, has not been investigated yet. Here we demonstrate, for the first time, strong THz emission through single and two-color filamentation in liquids and in fused silica (SiO 2 ) respectively and a significant THz field enhancement when using high repetition rate laser sources.

Two-color filamenation in gases is known to produce intense and broadband THz radiation. There are two physical mechanisms responsible for the THz generation in this scheme: four-wave mixing and emission from the induced plasma currents. The case when the main and second harmonic are linearly polarized is well studied including the impact from each of the above mechanisms. However, for the cases when the two-color fields have complex polarization states the role of the four-wave mixing and plasma mechanisms in the formation of the THz polarization is still under-explored. Here we use both the four-wave mixing and photocurrent models in order to consider the THz generation by two-color fields with arbitrary polarizations. We show that under specific polarizations of the two-color field components it is possible to determine which of the mechanisms is responsible for the THz polarization formation.