Generation of 30 <em>μ</em>J single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification

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Generation of 30 μ J single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification

STEPANOV, Andrei G., et al.

Abstract

We report the generation of 30 μJ single-cycle terahertz pulses at 100 Hz repetition rate by phase-matched optical rectification in lithium niobate using 28 mJ femtosecond laser pulses.

The phase-matching condition is achieved by tilting the laser pulse intensity front. Temporal, spectral, and propagation properties of the generated terahertz pulses are presented. In addition, we discuss possibilities for further increasing the energy of single-cycle terahertz pulses obtained by optical rectification.

STEPANOV, Andrei G., et al . Generation of 30 μ J single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification. Optics Letters , 2008, vol. 33, no. 21, p. 2497-2499

DOI : 10.1364/OL.33.002497

Available at:

http://archive-ouverte.unige.ch/unige:37845

Disclaimer: layout of this document may differ from the published version.

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Generation of 30 ␮ J single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification

Andrei G. Stepanov,^1,*Luigi Bonacina,²Sergei V. Chekalin,¹and Jean-Pierre Wolf²

1Institute for Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow Region 142190, Russia

2GAP-Biophotonics, Université de Genève, Rue de l’École de Médecine 202, Genève CH-1211, Switzerland

*Corresponding author: [email protected] Received July 30, 2008; accepted August 27, 2008;

posted September 23, 2008 (Doc. ID 99626); published October 24, 2008

We report the generation of30␮J single-cycle terahertz pulses at 100 Hz repetition rate by phase-matched optical rectification in lithium niobate using28 mJ femtosecond laser pulses. The phase-matching condition is achieved by tilting the laser pulse intensity front. Temporal, spectral, and propagation properties of the generated terahertz pulses are presented. In addition, we discuss possibilities for further increasing the energy of single-cycle terahertz pulses obtained by optical rectification. © 2008 Optical Society of America

OCIS codes: 190.7110, 260.3090, 190.2620.

High-power terahertz radiation is very attractive for many scientific and industrial applications. In par- ticular, it may allow imaging and spectroscopy of re- mote large-scale objects for security issues. One of the most challenging scientific applications of high- power terahertz waves is nonlinear spectroscopy. It is well known that many fundamental resonances in molecules and solid-state materials lie in the terahertz frequency range. On the other hand, terahertz nonlinear spectroscopy can provide quantitative in- formation that is difficult to obtain by traditional spectroscopic techniques, such as direct insight into higher-order cohesive lattice forces [1]. Only a few papers on nonlinear spectroscopy in the terahertz range have been published so far. The lack of terahertz nonlinear spectroscopy measurements is prin- cipally related to the difficulty of realizing intense co- herent sources in this frequency range. Most of the previous studies on terahertz nonlinear spectroscopy have been performed using accelerator-based sources, which are able to provide single-cycle terahertz pulses with energies up to 100␮J [2]. However, these sources present the typical drawbacks associ- ated with large-scale facilities. A few table-top techniques for the generation of ultrashort terahertz pulses based on femtosecond lasers have been recently proposed and realized [3]. Usually the peak power of the terahertz radiation delivered via these schemes is significantly lower than that of accelerator-based sources. At the same time, terahertz generation techniques based on femtosecond lasers are rapidly developing today; for instance, very recently 3 orders of magnitude enhancement of the terahertz energy radiated from laser filaments in air has been demonstrated [4]. In 2002. Heblinget al.[5]

proposed a new velocity-matching technique based on femtosecond laser pulse front tilting for large-area terahertz generation via optical rectification. Succes- sively it was demonstrated that this technique provides the possibility of generating 10␮J pulses at 10 Hz repetition rate using 20 mJ, 400 fs laser pulses [6]. Here we report the results of recent experiments aimed at further scaling up the energy and repetition rate of single-cycle terahertz waves exploiting pulse

front tilting with a high-repetition-rate terawatt femtosecond laser. A terahertz pulse energy of 30␮^{J ob-} tained in the present experiments is, to our knowl- edge, one of the highest energies ever achieved for single-cycle terahertz pulses, except for the accelerator-based systems. Moreover, the repetition rate of 100 Hz allowed by the present scheme is significantly higher than that 共2.5 Hz兲 reported for 100␮J pulses from an accelerator-based source [2].

We also present the terahertz beam propagation characteristics aimed at designing optimal focusing geometries for the generated terahertz wave.

Figure 1 shows a schematic of the terahertz generation setup. As a pump source we used the Helvet- era terawatt laser platform at the University of Geneva, which provides 35 mJ, 50 fs laser pulses at 800 nm at a repetition rate of 100 Hz. The main dif- ference with our previous experimental setup [7,8]

consists of using a gold coated spherical mirror 共f

= 150 mm兲 for tilted laser pulse imaging instead of one or two achromatic lenses, a 1800 l / mm diffraction grating, and a larger-scale MgO : LiNbO₃crystal.

The laser spot on the crystal surface was 6 mm in diameter. A pyroelectric detector calibrated in the terahertz range by the manufacturer (MicroTech Instru- ments), identical to the one recently employed to measure the absolute energy of terahertz pulses generated in ZnTe by 48 mJ femtosecond laser pulses [9], was used for the measurements of the terahertz beam power. Applying 28 mJ/ 50 fs laser pulses we obtained terahertz pulses with energies as high as 30␮J. We observed that a maximal terahertz output

Fig. 1. (Color online) Schematic of the terahertz generation setup.

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was obtained with the shortest laser pulses (about 50 fs), and any increase of the laser pulse duration by moving of the grating compressor resulted in a decrease of the generated terahertz power.

The temporal profile of the terahertz field was re- corded using a standard electro-optic setup (Fig. 1) with a 0.5 mm ZnTe crystal as a sensor [10,11]. The terahertz beam was not focused on the sensor, which was set at a distance of 30 mm from the output surface of the LiNbO₃ crystal. The measured electro- optic signal as a function of delay time is presented in Fig. 2(a). The excellent signal-to-noise ratio illus- trates the pulse to pulse stability of the terahertz wave (typically less than 5%), since every measured point shown in Fig.2(a)corresponds to an individual laser shot. The amplitude spectrum of the electro- optic signal delivered from time domain data by fast Fourier transform is shown in Fig.2(b). Compared to previous spectra obtained by the same technique at the microjoule level [7,12] the fundamental band at 0.35 THz in Fig. 2(b) is narrower and redshifted.

These spectral features result from long propagation distances of the laser and terahertz radiation inside the lithium niobate crystal in our case. Propagation of the femtosecond laser pulse with a tilted intensity front is accompanied by time and space spreading of the pulse [13], which leads to a redshift and narrowing of the generated terahertz radiation. Moreover highly efficient terahertz generation induces a redshift and narrowing of the pumping laser spectrum [14], which also modifies the terahertz spectrum. Fi- nally, the terahertz absorption of lithium niobate in- creases with frequency. For this reason, the propagation inside the crystal leads to a decrease of the high frequency part of the terahertz spectrum. This effect was observed before in lithium tantalate [15]. Similar spectral narrowing and shifts have been detected for 20 mJ pump laser pulses [6]. Broader spectra could be achieved by forming an elliptical laser spot stretched along the vertical axis, which reduces the pump laser and terahertz beams propagation distance inside the crystal. The two broad peaks cen- tered around 1 and 1.55 THz in the spectrum presented in Fig.2(b)might be a manifestation of second and third harmonic generations of the fundamental terahertz wave at 0.5 THz in the lithium niobate crystal [12,16]. For a definitive assignment of these features, additional measurements are needed.

Spatial intensity profiles and propagation properties of the generated terahertz beam have been stud- ied by transverse scanning of the pyroelectric detector (active element size of 2⫻3 mm) at different distances from the lithium niobate output surface.

The vertical and horizontal terahertz beam intensity profiles at 25 and 120 mm from the crystal output surface are shown in Figs.3(a)and3(b). The squares represent the measured data points, and the curves show the corresponding Gaussian fits. According to our measurements the terahertz beam has a divergence of 81± 7 and 133± 10 mrad in the vertical and horizontal directions, respectively. There are several reasons that can explain why the horizontal divergence exceeds the vertical one. This effect can appear because the different spectral components of the generated broadband terahertz pulse propagate at different angles with respect to the laser beam direction owing to spectral dispersion in the lithium niobate crystal. The measured terahertz beam divergence significantly exceeds the diffraction limit (39 and 48 mrad for 0.35 THz with FWHM waist diameters of 10 and 8 mm, respectively). The insets in Figs. 3(a) and 3(b) show the terahertz beam diameters (FWHM) in the vertical and horizontal directions as a function of propagation distance; triangles corre- spond to terahertz beam diameters obtained by fitting the measured terahertz beam transverse intensity profiles, while solid curves represent the theoretical evaluation of radially symmetric Gauss- ian beams at 0.35 THz with waist diameters of 7.9 and 10.4 mm as evaluated from the laser spot size and terahertz beam diameter at a propagation distance of 25 mm. The dotted curves in the insets in Figs. 3(a) and 3(b) show the Gaussian FWHM for 0.35 THz obtained by fitting the measured terahertz beam diameters at 25 and 120 mm (the waist diam- Fig. 2. (a) Electro-optic signal and (b) corresponding fast

Fourier transform amplitude.

Fig. 3. Terahertz beam transverse intensity distributions in the (a) vertical and (b) horizontal directions at 25 and 120 mm from the crystal out surface. Insets, terahertz beam diameter as a function of propagation distance (see text for more details).

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eter and its position were used as fitting parameters).

Evaluated in such a way the terahertz beam waist diameters are notably less than laser spot diameters, and the position of the terahertz beam waist is shifted by about 45 mm before the terahertz output surface of the crystal. Such behavior can be explained by terahertz self-focusing in the lithium niobate crystal. In our previous experiments with submillijoule femtosecond laser pulses and relatively small lithium niobate crystals (the laser and terahertz pulse propagation distance below 2 mm) the generated terahertz beams exhibited near to diffraction-limited divergence [7]. In precedence terahertz-induced lensing for a near-infrared probe beam and its use for detec- tion of terahertz pulse temporal field profiles have been demonstrated in a 4-N,N-dimethylamino-4⬘^-N⬘^- methyl stilbazolium tosylate crystal [17]. However, the terahertz focusing observed in the present case can also be induced by the pumping laser radiation directly. The observed different vertical and horizontal divergences can result from orientation depen- dence of the nonlinear refractive index in the lithium niobate crystal; however, to be more conclusive on this point more detailed studies are needed.

By taking into account the transmission of the optical setup (74%), the reflection of the laser beam at the crystal surface (15%), and the reflection of the terahertz beam at the crystal–air interface (44%) we obtain a laser-to-terahertz energy conversion efficiency of 0.25%. Note that this lower estimate does not account for the strong terahertz absorption in the lithium niobate crystal. This energy conversion efficiency in the generation of a 0.35 THz wave using femtosecond laser pulses at 800 nm corresponds to a laser-to-terahertz photon-number conversion efficiency of 250%. The possibility of obtaining more than 100% photon-number conversion efficiency owing to cascaded nonlinearities has already been discussed theoretically [18–20]. Recently, a laser-to- terahertz photon number conversion efficiency of 180% has been reported [6] that was evaluated by analyzing the laser spectral modifications after terahertz generation [14]. We point out that there are a few possibilities for increasing further the terahertz generation efficiency as well as the terahertz pulse energy in our setup. We expect that optimal shaping of the beam profile will result in an increase of the terahertz generation efficiency. Moreover, forming an elliptical laser spot stretched along the vertical axis as discussed before gives an opportunity for a further increase of the laser pulse energy keeping the laser intensity below crystal damage, as it was suggested before for a line source terahertz generation geom- etry [21]. It was also demonstrated that the energy efficiency conversion can be increased at least by a factor of 3 by cryogenic cooling of the lithium niobate crystal [7] owing to decreasing its terahertz absorption. This suggests that table-top techniques utilizing terawatt femtosecond laser pulses allow one to gen- erate single-cycle terahertz pulses with energies as

high as was reached with accelerator-based terahertz sources.

In conclusion, we have demonstrated the generation of single-cycle terahertz pulses with an energy of 30␮J using 28 mJ femtosecond laser pulses. Propa- gating properties of the generated intense terahertz beam have also been characterized.

This work was supported in part by Russian Foun- dation for Basic Research (RFBR) (grant 07-02- 01471-a) and Foundation for Assistance to Small In- novative Enterprises (project 5663p/8129). A. G.

Stepanov is grateful to the European Science Foun- dation for financial support (Dyna grant 1976). L.

Bonacina acknowledges the support of Swiss State Secretariat for Education and Research in the frame- work of European Cooperation in the field of Scien- tific and Technical Research P18. The Helvetera laser platform was funded by the Swiss National Science Foundation (NSF), as well as the Boninchi and Schmideiny foundations.

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