Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power
Joachim Buldt, Michael Müller, Henning Stark, Cesar Jauregui, Jens Limpert
mmyjournal manuscript No. (will be inserted by the editor)
Fiber-laser driven gas-plasma based generation of THz radiation with50 mW average power
Joachim Buldt , Michael Mueller , Henning Stark , Cesar Jauregui , Jens Limpert , , Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-University Jena, Albert-Einstein-Str. 6, 07745Jena, Germany Helmholtz-Institute Jena, Fr¨obelstieg 3, 07743 Jena Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Str. 7, 07745 Jena, GermanyReceived: 9 August 2019 / Accepted: 21 November 2019 / Published online: 26 November 2019
Abstract
We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultra-fast fiber laser system. The applied scheme is a promisingapproach for scaling the THz average power but it hasbeen limited so far by the driving lasers to repetitionrates up to 1 kHz. Here we demonstrate recent results ofTHz generation operating at a two orders of magnitudehigher repetition rate. This results in a unprecedentedTHz average power of 50 mW. The development of com-pact, table-top THz sources with high repetition rateand high field strength is crucial for studying nonlin-ear responses of materials, particle acceleration or fasterdata acquisition in imaging and spectroscopy.
The THz spectral region has been attracting growinginterest over recent years thanks to a constantly increas-ing number of applications in many different fields. Ex-amples for industrial applications are: monitoring com-pounding processes [16], quality control by structuraland chemical analysis [29], quality inspection of foodproducts as well as inspection of plastic weld joints [16].In homeland security THz radiation is beneficial due tothe transparency of many materials which enable theidentification of hidden weapons by imaging [1] and ex-plosives by fingerprinting [29,11]. For medical diagnos-tics the low photon-energy of THz radiations allows forionization free morphological and compositional studies[11] as well as for in vitro imaging of multiple types oftissue as well as in-vivo imaging of epidermal tissue [33]. a r X i v : . [ phy s i c s . op ti c s ] F e b Joachim Buldt et al.
Scientific applications are particle acceleration [32,25,17,13,7,5] and the study of ultrafast material dynam-ics [15,35,28,20]. Of course, this is not an exhaustivelist and the development of more powerful, broadbandsources with high peak field strength enables even moreapplications and technologies in the THz spectral region.One way to generate THz radiation is to use near-infrared lasers and transform the radiation into the THzregion. Thus, the most straight-forward approach for av-erage power scaling of the THz radiation would be toincrease the average power of the driving laser source.However, individual limitations regarding the applica-ble laser parameters are given by the different frequencytransformation schemes themselves. One of these ap-proaches is inspired by electronics and employs photo-conductive antennas (PCA). Hereby a short laser-pulseacts as an ultrafast switch that generates free carriers ina semiconductor. These free carriers are accelerated to-wards electrodes placed on the semiconductor, thus giv-ing rise to a current that generates the THz radiation.Due to the materials employed in these antennas, theachievable THz power is limited by the damage thresh-old of the devices and until today the maximal reportedaverage power is 3 . . (cid:28) − ). Later, several schemes to enhance the ef-ficiency have been demonstrated [21,22,4,19]. Out ofthose, the two-color plasma approach [4] is one of themost popular ones nowadays due to its simple setup anda high achievable efficiency in the order of 10 − as wellas the large bandwidth and field strength. For this, alaser pulse is co-focused with its second harmonic, gen-erating an asymmetry in the electric field which causes amuch larger electron movement than the ponderomotivepotential. However, the highest average power of THz ra-diation generated using this scheme so far was 1 .
44 mW[26], with most experiments being limited by the av-erage power of the driving laser system. Most of theseexperiments have been powered by Ti:sapphire lasers,which usually show a rather low average power due tothe thermo-optical properties of the laser crystals. iber-laser driven gas-plasma based generation of THz radiation with 50 mW average power 3
In this contribution we present the latest results us-ing the two-color plasma scheme driven by a state-of-the-art, tabletop, high-average-power, ultrafast ytterbium-fiber chirped pulse amplification system (Yb:FCPA) [23,24].
Fig. 1
Experimental setup for the THz generation: Thepulses from the Yb:FCPA system are compressed with aHCF compression and focused with a 200 mm lens through a100 µ m thick BBO crystal into the gas. The laser light isdumped through a centered hole in the off-axis parabolicmirror that collimates the THz radiation. Residual light isblocked by a HRFZ-Si filter. For detection electro-opticalsampling is done in GaP with a sample of the driving pulse. The experimental setup is depicted in Figure 1. Thepulses were delivered by a 16-channel ytterbium-fiberchirped pulse amplification system (Yb:FCPA) [23,24]at 100 kHz repetition rate with 1 . µ mcore diameter and 1 m length filled with 1 . . µ m thick BBO crystalto a 40 µ m focal spot (measured at low power withoutplasma).From the simulation based on the photo-current model[18] depicted in Figure 2a the phase difference betweenthe fundamental and the second harmonic pulse is cru-cial for efficient THz generation in the two-color scheme.The propagation from the crystal to the focus throughthe gas allows to match the phase by adjusting the crys-tal position and taking advantage of the gas-dispersion.The fundamental and its second harmonic are focuseddown and generate a plasma that gives rise to the THzradiation. Afterwards, the THz beam is collected and Joachim Buldt et al.
Fig. 2
Simulation results based on the photo-current model[18]. (a) Efficiency in dependence of the phase difference be-tween the fundamental and second harmonic field. (b) Effi-ciency in dependence of the pulse duration, normalized to theefficiency of a 30 fs pulse. (c) THz spectrum simulated withthe pulse parameters of the experiment and a phase differenceof π/ collimated by a 50 . . Ω cm resis-tivity. For the detection of the THz radiation two optionsare available: In order to measure the average power ofthe THz radiation, an OAP can be inserted into the THzbeam path to focus it through an optical chopper andonto a calibrated thermal power meter. To minimize theinfluence of residual laser light on the power measure-ment three HRFZ-Si filters are inserted into the THzbeam path and the transmission of each one of them ismeasured to calculate the generated THz power. To fur-ther ensure that the measured power really is the gen-erated THz radiation each power measurement is vali-dated by making a comparison measurement with longpulses. The chirp control of the Yb:FCPA system al-lows to quickly change the pulse duration to over 1 ps.From the simulation in Figure 2b one can see that theconversion efficiency drops by three orders of magnitudewhen going from 30 fs to 1 ps pulses. The average powerreading with long pulses is subtracted from the one withshort ones to eliminate any influence from residual in-frared or stray light.As an addition to the power measurement the elec-trical field trace is measured by electro-optical sampling(EOS) [30]. Electro-optical sampling is done by superim-posing a small, delayed, circular polarized sample of thedriving 30 fs pulse with the THz pulse in a 1 mm thickgallium phosphate (GaP) crystal. Without the electricfield of the THz radiation the difference of the signals ofthe two photo-diodes behind a polarizer vanishes. Whenthe pulse delay is adjusted to overlap with the THz pulse, iber-laser driven gas-plasma based generation of THz radiation with 50 mW average power 5 the electric field of the THz radiation causes a phase dif-ference between the two polarization components of theprobe pulse which is detected by the photo-diodes. Byscanning the delay the temporal shape of the THz pulsecan be retrieved. EOS is a commonly used way to char-acterize THz radiation due to its simple setup, whichis also the reason for its application in this experiment.However it has to be noted, that it can not character-ize the full spectrum of the radiation generated in thiswork, where the focus is showing the average-power andrepetition rate scalability of the gas-plasma scheme. The results of the EOS measurements are depicted inFigure 3. The electrical field shows the single-cycle char-acteristic as it is expected from the applied generationscheme. The measured spectra corresponding to the field-traces extend up to about 5 THz, limited by the detec-tion bandwidth of the EOS measurement. Due to thelimited phase matching, decreasing nonlinear index to-wards higher frequencies as well as absorption of THz ra-diation in GaP [3,27], the detection can not retrieve thefull spectrum of the generated radiation. From the simu-lation depicted in Figure 2c, with the same pulse param-eters as in the experiment a spectrum extending to wellover 30 THz can be expected. For the characterization ofthe full spectrum of the generated THz radiation, the im-plementation of a broadband detection system such as
Fig. 3
Normalized electric fields measured by electro-opticalsampling as well as the spectra of the THz pulses retrievedby Fourier-transforming the EOS traces. As can be seen inthe simulation shown in Figure 2, the spectra are expected toextend to well over 30 THz, being limited here by the band-width of the GaP-crystal used for the EOS measurements. the air breakdown coherent detection (ABCD) scheme[6] is planned for the future.To evaluate the average THz power generated an ad-ditional measurement with a thermal powermeter wascarried out for each gas. The measured values and thecorresponding efficiencies of the THz generation pro-cess are shown in Table 1. The highest power of 50 mW
Joachim Buldt et al.
Table 1
Average power of the THz radiation measured withthe thermal powermeter as well as the calculated conversionefficiencies.
Gas Average Power Conv. Eff.
Helium 39 mW 3 . · − Nitrogen 47 mW 3 . · − Neon 50 mW 4 . · − Argon 39 mW 3 . · − was achieved using neon gas at an absolute pressure of1 bar which corresponds to an conversion efficiency of4 . · − . In conclusion, we have demonstrated the first step in thepower-scaling of broadband, plasma-based THz sourcesby generating an average power of 50 mW. For the futurea more precise temporal characterization of the THz ra-diation using an ABCD setup is planned as well as moresystematic investigations on the experimental parame-ters which might further increase the efficiency.For the results demonstrated here, only a fraction ofthe power delivered by our driving laser systems was re-quired. By using the full power, similar pulse parameterscould be achieved at over 1 MHz repetition rate. Withthe same efficiency demonstrated here, an average THzpower of 0 . Funding
This project has received funding from the EuropeanResearch Council (ERC) under the European Union’sHorizon 2020 research and innovation programme (grantagreement No. 835306).
Acknowledgement
J. Buldt acknowledges support from the IMPRS-PL withinthe international Ph.D. program.
Open Access
This article is distributed under the terms of the Cre-ative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/ ), which per-mits unrestricted use, distribution, and reproduction inany medium, provided you give appropriate credit to theoriginal author(s) and the source, provide a link to theCreative Commons license, and indicate if changes weremade.
References
1. R. Appleby and H. B. Wallace. Standoff detection ofweapons and contraband in the 100 GHz to 1 THz re-gion.
IEEE Transactions on Antennas and Propagation ,55(11):2944–2956, 2007.2. F. Blanchard, L. Razzari, H.-C. Bandulet, G. Sharma,R. Morandotti, J.-C. Kieffer, T. Ozaki, M. Reid, H. F.Tiedje, H. K. Haugen, , and F. A. Hegmann. Generationiber-laser driven gas-plasma based generation of THz radiation with 50 mW average power 7of 1.5 µ J single-cycle terahertz pulses by optical rectifica-tion from a large aperture ZnTe crystal.
Optics Express ,15(20):13212–13220, 2007.3. S. Casalbuoni, H. Schlarb, B. Schmidt, P. Schm¨user,B. Steffen, and A. Winter. Numerical studies on theelectro-optic detection of femtosecond electron bunches.
Physical Review Accelerators and Beams , 11:072802,2008.4. D. J. Cook and R. M. Hochstrasser. Intense terahertzpulses by four-wave rectification in air.
Optics Letters ,25(16):1210–1212, 2000.5. E. Curry, S. Fabbri, J. Maxson, P. Musumeci, andA. Gover. Meter-scale terahertz-driven acceleration ofa relativistic beam.
Physical Review Letters , 120:094801,2018.6. J. Dai, X. Xie, and X.-C. Zhang. Detection of broadbandterahertz waves with a laser-induced plasma in gases.
Physical Review Letters , 97:103903, 2006.7. A. Fallahi, M. Fakhari, A. Yahaghi, M. Arrieta, and F. X.K¨artner. Short electron bunch generation using single-cycle ultrafast electron guns.
Physical Review Accelera-tors and Beams , 19:081302, 2016.8. H. Hamster, A. Sullivan, S. Gordon, and R. W. Falcone.Short-pulse terahertz radiation from high-intensity-laser-produced plasmas.
Physical Review E , 49:671, 1994.9. H. Hamster, A. Sullivan, S. Gordon, W. White, andR. W. Falcone. Subpicosecond, electromagnetic pulsesfrom intense laser-plasma interaction.
Physical ReviewLetters , 71:2725, 1993.10. J. Hebling, A. G. Stepanov, G. Alm´asi, B. Bartal, andJ. Kuhl. Tunable thz pulse generation by optical rectifi- cation of ultrashort laser pulses with tilted pulse fronts.
Applied Physics B , 78(5):593–599, 2004.11. L. Ho, M. Pepper, and P. Taday. Signatures and finger-prints.
Nature Photonics , 2:541–543, 2008.12. S.-W. Huang, E. Granados, W. R. Huang, K.-H. Hong,L. E. Zapata, and F. X. K¨artner. High conversion ef-ficiency, high energy terahertz pulses by optical rectifi-cation in cryogenically cooled lithium niobate.
OpticsLetters , 38(5):796–798, 2013.13. W. R. Huang, A. Fallahi, X. Wu, H. Cankaya, A.-L. Cal-endron, K. Ravi, D. Zhang, E. A. Nanni, K.-H. Hong,and F. X. K¨artner. Terahertz-driven, all-optical electrongun.
Optica , 3(11):1209–1212, 2016.14. W. R. Huang, S.-W. Huang, E. Granados, K. Ravi, K.-H.Hong, L. E. Zapata, and F. X. K¨artner. Highly efficientterahertz pulse generation by optical rectification in sto-ichiometric and cryo-cooled congruent lithium niobate.
Journal of Modern Optics , 62(18):1486–1493, 2015.15. R. Huber. Terahertz collisions.
Nature , 483:545–546,2012.16. C. Jansen, S. Wietzke, O. Peters, M. Scheller, N. Vieweg,M. Salhi, N. Krumbholz, C. J¨ordens, T. Hochrein, andM. Koch. Terahertz imaging: applications and perspec-tives.
Applied Optics , 49(19):E48–E57, 2010.17. C. Kealhofer, W. Schneider, D. Ehberger, A. Ryabov,F. Krausz, and P. Baum. All-optical control and metrol-ogy of electron pulses.
Science , 352(6284):429–433, 2016.18. K.-Y. Kim. Generation of coherent terahertz radiationin ultrafast laser-gas interactions.
Physics of Plasma ,16:056706, 2009.19. M. Kreß, T. L¨offler, M. D. Thomson, R. D¨orner, H. Gim-pel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, Joachim Buldt et al.J. Ullrich, and H. G. Roskos. Determination of thecarrier-envelope phase of few-cycle laser pulses withterahertz-emission spectroscopy.
Nature Physics , 2:327–331, 2006.20. F. Langer, C. P. Schmid, S. Schlauderer, M. Gmitra,J. Fabian, P. Nagler, C. Sch¨uller, T. Korn, P. G. Hawkins,J. T. Steiner, U. Huttner, S. W. Koch, M. Kira, andR. Huber. Lightwave valleytronics in a monolayer oftungsten diselenide.
Nature , 557:76–80, 2018.21. T. L¨offler, F. Jacob, and H. G. Roskos. Generation ofterahertz pulses by photoionization if electrically biasedair.
Applied Physics Letters , 77:453, 2000.22. T. L¨offler and H. G. Roskos. Gas-pressure dependence ofterahertz-pulse generation in a laser-generated nitrogenplasma.
Journal of Applied Physics , 91:2611, 2002.23. M. M¨uller, M. Kienel, A. Klenke, T. Gottschall, E. Shes-taev, M. Pl¨otner, J. Limpert, and A. T¨unnermann. 1 kw1 mj eight-channel ultrafast fiber laser.
Optics Letters ,41(15):3439–3442, Aug 2016.24. M. M¨uller, A. Klenke, H. Stark, J. Buldt, T. Gottschall,A. T¨unnermann, and J. Limpert. 1.8-kw 16-channelultrafast fiber laser system.
Proc. SPIE 10512, FiberLasers XV: Technology and Systems , 1051208, 2018.25. E. A. Nanni, W. R. Huang, K.-H. Hong, K. Ravi, A. Fal-lahi, G. Moriena, R. J. D. Miller, and F. X. K¨artner.Terahertz-driven linear electron acceleration.
NatureCommunications , 6:8486, 2015.26. T. I. Oh, Y. J. Yoo, Y. S. You, and K. Y. Kim. Gener-ation of strong terahertz fields exceeding 8 MV/cm at 1kHz and real-time beam profiling.
Applied Physics Let-ters , 105:041103, 2014. 27. C. Paradis, J. Drs, N. Modsching, O. Razskazovskaya,F. Meyer, C. Kr¨ankel, C. J. Saraceno, V. J. Wittwer,and T. S¨udmeyer. Broadband terahertz pulse generationdriven by an ultrafast thin-disk laser oscillator.
OpticsExpress , 26(20):26377–26384, 2018.28. O. Schubert, M. Hohenleutner, F. Langer, B. Urbanek,C. Lange, U. Huttner, D. Golde, T. Meier, M. Kira,S. W. Koch, and R. Huber. Sub-cycle control of tera-hertz high-harmonic generation by dynamical bloch os-cillations.
Nature Photonics , 8:119–123, 2014.29. M. Tonouchi. Cutting-edge terahertz technology.
NaturePhotonics , 1:97–105, 2007.30. J. A. Valdmanis, G. Mourou, and C. W. Gabel. Picosec-ond electro-optic sampling system.
Applied Physics Let-ters , 41:211, 1982.31. C. Vicario, A. V. Ovchinnikov, S. I. Ashitkov, M. B.Agranat, V. E. Fortov, and C. P. Hauri. Generation of0.9-mJ THz pulses in DSTMS pumped by a Cr:Mg SiO laser. Optics Letters , 39(23):6632–6635, 2014.32. L. J. Wong, A. Fallahi, and F. X. K¨artner. Compact elec-tron acceleration and bunch compression in thz waveg-uides.
Optics Express , 21(8):9792–9806, 2013.33. X. Yang, X. Zhao, K. Yang, Y. Liu, Y. Liu, W. Fu,and Y. Luo. Biomedical applications of terahertzspectroscopy and imaging.
Trends in Biotechnology ,34(10):810–824, 2016.34. N. T. Yardimci, S.-H. Yang, C. W. Berry, and M. Jarrahi.High-power terahertz generation using large-area plas-monic photoconductive emitters.
IEEE Transactions onTerahertz Science and Technology , 5(2):223–229, 2015.35. B. Zaks, R. B. Liu, and M. S. Sherwin. Experimental ob-servation of electron-hole recollisions.