High energy terahertz emission from two-color laser ... - ulaval ULAVAL

APPLIED PHYSICS LETTERS 95, 131108 共2009兲

High energy terahertz emission from two-color laser-induced filamentation in air with pump pulse duration control Tie-Jun Wang,1,a兲 Yanping Chen,1 Claude Marceau,1 Francis Théberge,2 Marc Châteauneuf,2 Jacques Dubois,2 and See Leang Chin1 1

Département de physique and Centre d’Optique, Photonique et Laser (COPL), de génie physique et d’optique, Université Laval, Quebec, Quebec G1V 0A6, Canada 2 Defence Research and Development Canada-Valcartier, 2459 Pie-XI Boulevard North, Quebec, Quebec G3J 1X5, Canada

共Received 15 June 2009; accepted 11 September 2009; published online 30 September 2009兲 Two-color laser-induced femtosecond filamentation was employed to generate high energy terahertz emission in air with high energy pump. By controlling the pump pulse duration, more than four times enhancement in terahertz pulse energy has been obtained when compared with a Fourier transform-limited pump. Multiple filaments’ dynamics might be responsible for the terahertz enhancement. Superbroadband terahertz pulse with energy up to 2 ␮J was generated using loose focusing condition, while the maximum terahertz pulse energy in the frequency range below 5.5 THz was around 60 nJ. © 2009 American Institute of Physics. 关doi:10.1063/1.3242024兴 Generation of intense terahertz pulses with large bandwidth is an active area of current research on terahertz science. Such broadband and powerful terahertz pulse would provide a great tool for the enormous prospect on terahertz nonlinear optics and spectroscopy. Conventional terahertz emitters such as semiconductor antennas or optical rectification in nonlinear crystals have few drawbacks. For example, they have a limited achievable terahertz emission strength due to the damage threshold of materials and also a limited obtainable bandwidth caused by the input optical pulse duration, carrier dynamics, or phase-matching requirements. Terahertz pulses have the potential to combine safe-to-use high-resolution imaging and identification through spectroscopy, however, the terahertz pulse cannot propagate over a long distance in the atmosphere because of beam diffraction and the strong attenuation due to water vapor. Recently, an observation of terahertz radiation generation from laser-induced-filament in air was reported in 2007.1 The technique not only shows a good way to generate an intense near single-cycle terahertz pulse without the damage threshold limit at high intensities, but also provides an effective approach to produce terahertz pulse at a significant long distance. The techniques of filamentation biased by external electric field2,3 and bifilamentation4 of femtosecond laser pulses have been applied to enhance the terahertz emission. But terahertz pulse energy was still below nanojoule level. Intense terahertz generations by two-color pump have been reported under the tight focusing conditions 共5–20 cm focal length兲.5–10 High energy terahertz pulses as compared to the approaches cited above in Refs. 1–4 were generated and superbroadband terahertz spectrum up to 75 THz 共␭ = 4 ␮m兲 were reported.5 However, for remote generation of terahertz pulses, it will also be necessary to use focusing elements with small numerical aperture 共large f-number兲 to prevent local dielectric breakdown. Under such conditions, the formation of a long plasma filament is unavoidable. The technique to increase the terahertz pulse energy from longer filaa兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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ments in air still remains a field to be explored. In this letter, intense terahertz emission in air from femtosecond filamentation induced by the fundamental and the second harmonic of optical pulses is demonstrated under loose focusing condition. By controlling the pump pulse duration, more than four times enhancement in terahertz pulse energy has been observed as compared to the Fourier transform-limited 共FTL兲 pump pulse duration. Higher than 60 nJ has been generated in the frequency range below 5.5 THz. The laser used in this experiment was described elsewhere.11 The schematic experimental setup used for intense terahertz generation is shown in Fig. 1. The laser beam with a diameter of 25 mm 共1 / e2兲 was focused by an antireflection coated lens with 70 cm focal length. A 100 ␮m thick Type I BBO crystal was used for frequency doubling. The

FIG. 1. 共Color online兲 Schematic experimental setup for high energy terahertz generation and detection from two-color laser-induced filamentation in air. The inset 共a兲 depicts the images of the multiple filaments for the FTL, positively chirped 130 fs and negatively chirped 105 fs pumps, respectively. The typical white light beam pattern of multiple filaments is shown in the inset 共b兲.

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Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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FIG. 2. 共Color online兲 Pump pulse duration dependence of terahertz emission with different filters in air. The dashed blue line denotes the FTL pump pulse duration.

relative phase between the fundamental and its second harmonic pulses was controlled by adjusting the distance between the BBO crystal and the laser focal spot. Pump energy was monitored after the lens with a Gentec power/energy meter. A Si wafer with 400 ␮m thickness, single side polished, and 3 in. in diameter was used to filter out the pumps after the filamentation. A charge coupled device camera was used to image the filaments and also beam patterns after filamentation. Terahertz pulses were collected by an off-axis 4-in.-diameter gold-coated parabolic mirror and detected by a pyroelectric energy meter 共Coherent P4-45CC兲. This pyroelectric energy meter has a nearly flat response over a broad spectral range 共2 – 100 ␮m兲 and was calibrated with our 10 Hz, 800 nm pump beam.5 Another 500 ␮m thick Si wafer and 1.6 mm thick Teflon were used as a long-pass wavelength filters to verify the spectral components of the terahertz pulse. The Si and Teflon filters have transmission cutoffs at frequencies around 300 THz 共1 ␮m兲 and 5.5 THz 共55 ␮m兲,12 respectively. After optimizing the BBO phase-matching and relative phase position for strong terahertz emission by monitoring the pyroelectric signal on the oscilloscope, pulse duration dependence of terahertz pulse generation from the two-color laser-induced filamentation was investigated at a fixed pump energy of 23.5 mJ 共before damaging the BBO crystal兲 by simply controlling the separation of the two gratings inside the compressor. Pulse durations were monitored by a single shot autocorrelator as shown in Fig. 1. The terahertz signal generated in the filaments under this two-color scheme increases as the 共chirped兲 pulse duration increases when compared to that using a FTL pulse at the same pump pulse energy. The increase in terahertz pulse energy reaches a maximum at the chirped pulse durations of around 150 fs for both the positively and negatively chirped pulses and then decreases. The experimental results are depicted in Fig. 2. With two Si wafers as filters, more midinfrared spectral components 共superbroadband signal兲 can be recorded. Pump pulses chirped positively or negatively can contribute to more intense terahertz emission under our experimental conditions. The terahertz emissions produced with positive chirps were much stronger. More than four times enhancement in terahertz pulse energy has been obtained with positively chirped pump pulse when compared to a FTL pump pulse. In the next experiment a detailed investigation on the pump energy dependence of terahertz emission has been

Appl. Phys. Lett. 95, 131108 共2009兲

FIG. 3. 共Color online兲 Pump energy dependence of high energy terahertz generation with frequencies less than 5.5 THz in 共a兲 and with the entire superbroadband terahertz frequencies in 共b兲 under different pump pulse durations, respectively. “FTL” and “P” denote the FTL duration and the positively chirped pulse, respectively.

made under different pulse durations with positive chirp. In order to focus on the low terahertz frequency range, 0.4 mm thick Si wafer with single side polished, and 1.6 mm thick Teflon have been employed to filter out frequencies higher than 5.5 THz. In order to avoid the damage of the BBO crystal, relatively low pump energy 共⬍25 mJ兲 has been used in the experiments for FTL pulses. As shown in Fig. 3共a兲, terahertz pulse energy 共⬍5.5 THz兲 can be increased by a factor of 4 with positively chirped 150 fs pump when compared with the case of FTL pump. The maximum terahertz pulse energy which has been measured by the pyroelectric detector is up to 30 nJ before considering the transmission losses by the Si and Teflon filters. Pump energy dependence of terahertz emission for superbroadband frequency range5 is depicted in Fig. 3共b兲, where only a Si filter has been used. Around 1 ␮J of terahertz pulse energy for the superbroadband frequency has been recorded. A clear enhancement can be seen in Fig. 3 for the chirped pulses as compared to FTL pulses under high energy pump. Accounting for the Fresnel loss 共⬎50%兲 and scattering from single side polished Si wafer and Teflon, the maximum terahertz pulse energy were estimated to more than 60 nJ for frequencies below 5.5 THz and 2 ␮J for entire superbroadband terahertz frequency range, respectively, which correspond to terahertz pulse energy conversion efficiency higher than 10−6 and 5 ⫻ 10−5, respectively. This increase with the chirp of the pump pulse is universal, since using argon gas the same result is obtained, as shown in Fig. 4. The phenomenon could be interpreted by multiple filaments’ dynamics. For the laser parameters used in our experiments, the peak power of the laser pulse was higher than 500 GW, which is more than 50 times the critical

FIG. 4. 共Color online兲 Pump pulse duration dependence of terahertz emission with different filters in argon at fixed pump energy of 23.5 mJ. The dashed blue line denotes the FTL pump pulse duration.


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power for self-focusing in air,13 and thus multiple filaments occurred. The typical white light beam pattern of multiple filaments taken at 20 cm away from the filaments is shown as the inset 共b兲 of Fig. 1. The reason why there is an increase in terahertz pulse energy when the pulse is chirped might be because, at the same energy, the transform-limited pulse has a higher peak power. Thus, initial hot spots in the beam profile will self-focus earlier during the propagation, which have been confirmed by the images of the multiple filaments for the 23 mJ pumps with different pulse durations as shown in the inset 共a兲 of Fig. 1. That is to say, in the focusing geometry, the multiple filaments will occur closer to the focusing optic 共farther from the geometrical focus兲 in a zone of larger diameter. Note that the images cannot resolve the structures of multiple filaments. Filament competition would occur in this case because the filaments are “far” apart from each other and would reduce the effectiveness of generating strong filaments as we have found out a few years ago in our laboratory.11,14 With longer chirped pulses, the peak power being lower, multiple filaments occur nearer the geometrical focus where the diameter of the beam is smaller. This smaller diameter favors multiple filaments cooperation 共constructive interference兲 resulting in stronger filaments, and hence more terahertz signal.15 The reason positively chirped pulses give higher terahertz signals might be that negatively chirped pulses, after propagation, will be shorter than the positively chirped pulses. Thus, the filaments start earlier in the case of negatively chirped pulse giving rise to more destructive competition and less signal. When the chirp becomes even larger, the pulse duration becomes so long that the peak power decreases to the extent that the number of strong filaments will reduce, and thus, decrease the terahertz signal. In summary, high energy terahertz emissions 共⬎60 nJ/ pulse in the frequency range below 5.5 THz and 2 ␮J for the entire superbroadband兲 have been obtained from two-color laser-induced filamentation in air by controlling the pump pulse duration and using long focal length lens. More than four times energy enhancement has been observed by positively stretched high energy pump pulses. Multiple filaments competition and cooperation could be re-

sponsible for the enhancement mechanism. The results reported in the paper not only provide an effective approach to generate powerful terahertz emission from two-color laserinduced filamentation in air with high energy table-top laser system, but also give a better understanding on the physical mechanism of the enhancement effect, which could be applied in future terahertz high-field physics. This work was supported in part by NSERC, DRDC— Valcartier, Canada Research Chair, the Canada Foundation for Innovation, the Canadian Institute for Photonics Innovation, and le Fonds Québécois pour la Recherche sur la Nature et les Technologies. Fruitful discussions with Dr. K.Y. Kim and technical support from Mr. M. Martin are also acknowledged. 1

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