Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal,  R. Morandotti,

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.99.053802

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission could be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed light on the role of intense transient fields in the coherent emission from photoexcited gas molecules.

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Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal, and R. Morandotti

https://journals.aps.org/pra/accepted/8007bY2dMa71016661fa2ff421161a92659e17711

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission can be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed new light on the role of intense transient fields in the coherent emission from photoexcited gas molecules. One of the key phenomena accompanying the focusing of an intense laser pulse in air is the fluorescence from gas molecules.()()()..~[1–3]. Under appropriate excitation conditions, those molecules produce coherent radiation, which is appealing for standoff spectroscopy applications, especially when emitted in the opposite direction of the ionizing laser pulse..().()()..~[4,5]. Owing to nitrogen’s abundance in our atmosphere, one of the most investigated effects has been the ultraviolet (UV) forward emission from photoexcited molecular nitrogen ions, first described as lasing by Luo and co-workers in 2003..()…….()..~[6]. A number of experiments report narrowband coherent emission at 391 nm and 428 nm, corresponding to the transitions ${N_2^{+}(B}^2{\Sigma }_u^{+}(\nu =0))\to {N_2^{+}(X}^2{\Sigma }_g^{+}({\nu }^{\prime }=0,1\left)\right)$ (see also Fig. 1(c) for potential energy diagrams). Such observations show narrowband amplification at the frequencies corresponding to molecular transitions. Those are seeded, for instance, by harmonics of the near-infrared (NIR) pump pulse…()….(..)(()())(..)…….()..~[7–9], white light…..()…()..~[10], or self-seeded by the pump’s supercontinuum itself…..()()()..~[10–12]. Such reports have reinforced the idea that a lasing process is at the origin of the coherent emission. However, the mechanism responsible for the gain is still not fully understood, although it is likely that laser-driven couplings between electronic states in the ion..().().()……..(..)…..()..~[13,14] and rotational effects()..~[15 –21] play a role Here, we report our experimental observation and analysis of the effect of a strong terahertz (THz) electric field on the coherent emission from photoexcited nitrogen ion molecules, as outlined in Figs. 1(a) and 1(b). We show a correlation between the THz field amplitude and the coherent emission at both 391 nm and 428 nm wavelengths. We interpret our results as consequence of the THz electric field induced symmetry-breaking of the gas molecules. This is turn leads to a THz-controlled seeding of the coherent emission by means of the THz induced second-harmonic of the near-infrared pump pulse. In our investigations we used a single-cycle pulse at THz frequencies as a strong electric field. The THz transient was generated by the transverse photocurrents induced through gas ionization via a laser pulse at 1.8 $$m carrier combined with its second harmonic. 

Abstract-Giant self-induced transparency of intense few-cycle terahertz pulses in n-doped silicon

O.V. Chefonov, A.V. Ovchinnikov, S.A. Romashevskiy, X. Chai, T. Ozaki, A.B. Savel'ev, M.B. Agranat, V.E. Fortov

(Submitted on 10 Oct 2017)

https://arxiv.org/abs/1710.03583

The results of high-field terahertz transmission experiments on n-doped silicon (carrier concentration of 8.7×1016 cm−3) are presented. We use terahertz pulses with electric field strengths up to 3.1 MV cm−1 and a pulse duration of 700 fs. Huge transmittance enhancement of ∼90 times is observed with increasing of the terahertz electric field strengths within the range of 1.5--3.1 MV cm−1.

Abstract-Effects of environmental conditions on the ultrafast carrier dynamics in graphene revealed by terahertz spectroscopy

H. A. Hafez, X. Chai, Y. Sekine, M. Takamura, K. Oguri, I. Al-Naib, M. M. Dignam, H. Hibino, and T. Ozaki

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.165428

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

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Abstract-Terahertz emission from two-plasmon-decay induced transient currents in laser-solid interactions

G.-Q. Liao¹, Y.-T. Li^1,2,a), C. Li¹, S. Mondal³, H. A. Hafez³, M. A. Fareed³, T. Ozaki³, W.-M. Wang^1,2, Z.-M. Sheng^2,4,5,a) and J. Zhang^2,5
 http://scitation.aip.org/content/aip/journal/pop/23/1/10.1063/1.4939605?TRACK=RSS
a) Authors to whom correspondence should be addressed. Electronic addresses:ytli@iphy.ac.cn and zmsheng@sjtu.edu.cn
Phys. Plasmas 23, 013104 (2016); http://dx.doi.org/10.1063/1.4939605

We have studied the generation of terahertz (THz) radiation via the interaction of intense femtosecond laser pulses with solid targets at a small incidence angle. It is found that preplasma with a moderate density gradient can enhance the emission. We also observe saturation of the THz output with the driving laser energy. We find that THz emission is closely related to the 3/2 harmonics of the driving laser. Particle-in-cell simulations indicate that under the present experimental conditions, the THz emission could be attributed to the transientcurrents at the plasma-vacuum interface, mainly formed by the two-plasmon-decay instability.

Abstract-Nonlinear transmission of an intense terahertz field through monolayer graphene

H. A. Hafez¹, I. Al-Naib², K. Oguri³, Y. Sekine³, M. M. Dignam², A. Ibrahim¹,D. G. Cooke⁴, S. Tanaka⁵, F. Komori⁶, H. Hibino³ and T. Ozaki^1,a)

http://scitation.aip.org/content/aip/journal/adva/4/11/10.1063/1.4902096
1 INRS-EMT, Advanced Laser Light Source, INRS, Varennes, Québec J3X 1S2, Canada
2 Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, Ontario K7L 3N6, Canada
3 NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
4 Department of Physics, McGill University, Montréal, Québec, H3A 2T8, Canada
5 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan
6 Institute for Solid State Physics, University of Tokyo, Chiba, 277-8581, Japan
a) Author to whom correspondence should be addressed. Electronic mail: ozaki@emt.inrs.ca
AIP Advances 4, 117118 (2014); http://dx.doi.org/10.1063/1.4902096

We report nonlinear terahertz (THz) effects in monolayer graphene, giving rise to transmission enhancement of a single-cycle THz pulse when the incident THz peak electric field is increased. This transmission enhancement is attributed to reduced photoconductivity, due to saturation effects in the field-induced current and increased intraband scattering rates arising from transient heating of electrons. We have developed a tight-binding model of the response using the length gauge interaction Hamiltonian that provides good qualitative agreement. The model fully accounts for the nonlinear response arising from the linear dispersion energy spectrum in graphene. The results reveal a strong dependence of the scattering time on the THz field, which is at the heart of the observed nonlinear response.