At the Ferninfrarot Linac- und Test-Experiment (FLUTE) at the Karlsruhe Institute of Technology (KIT) a new and compact method for longitudinal diagnostics of ultrashort electron bunches is being developed. For this technique, which is based on THz streaking, strong electromagnetic pulses with frequencies around 240 GHz are required. Therefore, a setup for laser-generated THz radiation using tilted-pulse-front pumping in lithium niobate was designed, delivering up to 1 µJ of THz pulse energy with a conversion efficiency of up to 0.03 %. In this contribution we study the optimization of the THz beam transport and environment

Andrian, Ivan

Assmann, Ralph

Bisoffi, Giovanni

Bründermann, Erik

Fabris, Alessandro

McIntosh, Peter

Mueller, Anke-Susanne

Nabinger, Matthias

Nasse, Michael

Ollmann, Zoltan

Schaefer, Jens

Schmidt, André

Steinmann, Johannes

Vinicola, Giulia

Widmann, Christina

English

KITopen

CHARACTERIZATION AND OPTIMIZATION OF LASER-GENERATEDTHz BEAM FOR THz BASED STREAKINGM. Nabinger∗, M. J. Nasse, J. Schäfer, A. Schmidt, J. L. Steinmann, C. Widmann, E. Bründermannand A.-S. Müller, Karlsruhe Institute of Technology, Karlsruhe, GermanyZ. Ollmann, University of Berne, Berne, SwitzerlandAbstractAt the Ferninfrarot Linac- Und Test-Experiment (FLUTE)at the Karlsruhe Institute of Technology (KIT) a new andcompact method for longitudinal diagnostics of ultrashortelectron bunches is being developed. For this technique,which is based on THz streaking, strong electromagneticpulses with frequencies around 240 GHz are required. There-fore, a setup for laser-generated THz radiation using tilted-pulse-front pumping in lithium niobate (LiN) was designed,delivering up to 1 µJ of THz pulse energy with a conversionefficiency of up to 0.03 %. In this contribution we study theoptimization of the THz beam transport and environment.INTRODUCTIONThe demand for ultra-short electron bunches is increas-ing in accelerator facilities worldwide. For instance, free-electron laser experiments such as FLASH or SwissFELrequire bunch lengths down to the single femtosecond (fs)level [1, 2], which need to be reliably diagnosed, often atseveral positions along the accelerator. However, longitu-dinal diagnostics of particle distributions in this regime isa feasible, but still challenging task that currently requirescost- and space-intensive methods such as large GHz-driventransverse deflecting RF-cavities [3]. Therefore, at the linac-based test facility FLUTE at KIT a new, compact and morecost-efficient approach for longitudinal single-fs diagnosticsbased on THz streaking is being investigated - the Split-RingResonator Compact Transverse-Deflecting System (SRRCompact-TDS).SRR Compact-TDSThe general idea of the system is to push the conven-tionally used GHz to THz frequencies while using a smallantenna-based resonator as amplification for the electricalstreaking field after excitation by a strong THz pulse. As weuse one laser system for both electron and THz generation,electron bunch and THz pulse are intrinsically synchronized.For a detailed explanation of the working principle, that isshown in Fig. 1, we refer to [4].The temporal resolution 𝑅𝑧 of the SRR compact-TDS isinversely proportional to the integrated field strength insidethe gap and the frequency of the resonant THz field [5]. Asthe resonance frequency and field amplification factor arefixed by the geometry of the SRR, a decisive parameter for𝑅𝑧 to reach the single-fs level is the amplitude of the incidentTHz wave, or more general the THz pulse energy.∗ matthias.nabinger@kit.eduFigure 1: Working principle of the SRR Compact-TDS di-agnostics THz-based streaking experiment at FLUTE.Tilted-pulse-front PumpingOne of the most efficient methods for laser-based gener-ation of intense THz pulses by optical rectification is theTilted-Pulse-Front (TPF) pumping in LiN [6,7]. AlthoughLiN offers comparably high electro-optical coefficients, thedifference of the refractive index for pump and THz wave pre-vents a collinear matching of the phase velocities involved inthe process. Therefore, in the TPF technique, the laser wave-front is tilted to overcome the problem of phase mismatchbetween pump laser and emitted THz wave.Experimental SetupFor the experimental SRR Compact-TDS setup, a TPFTHz generation setup was designed and aligned on a portablemodule [4] as illustrated in Fig. 2. Here, the input infraredbeam wavefront is tilted via a diffraction grating and imagedinto the LiN crystal by a 4-𝑓-telescope system consisting oftwo cylindrical lenses. After generation, the divergent THzbeam is transported achromatically via flat metal mirrors andthree THz lenses. Once the THz radiation leaves the moduleas collimated beam, it then passes a z-cut quartz vacuumwindow and is finally focused by an off-axis parabolic mirrorto the interaction point. Only the last quarter of this beampath of approximately 1 m length is inside vacuum (Fig. 2c).Facing experimental difficulties (e.g. instabilities of thelaser and RF systems) most recent measurement campaignshave not revealed any obvious streaking effects so far [8].While new simulation studies investigate possible improve-ments of the system design [9], one other possible reasonfor the missing streaking observation might be an insuffi-cient THz pulse energy reaching the resonator. Thus, inthis contribution the THz transport and environment is stud-ied to increase the resulting streaking strength of the SRRCompact-TDS.Figure 2: a) Sketch and b) technical drawing of the TPF setup.c) Photograph of the module installed at the accelerator.TERAHERTZ CHARACTERIZATIONWith the TPF module a THz pulse energy of up to1.08(5) μJ with a pump pulse energy of 3.56(1) mJ couldbe achieved (Fig. 3, top). This corresponds to a conversionefficiency of 0.030(3) %. The measurement was done atroom temperature and with a bandwidth limited pump pulselength of 35 fs, using a pyroelectric power meter calibratedby the PTB (Physikalisch-Technische Bundesanstalt).In addition, we coarsely determined the laser-generatedTHz spectrum by using metal mesh THz bandpass filtersand measuring the relative share of transmitted THz power(Fig. 3, bottom). We used seven bandpass filters with centralfrequencies 𝜈max ranging from 0.14 THz to 0.8 THz. Forcomparison, the dashed line indicates the resonance fre-quency of the SRR Compact-TDS, which by design is lo-cated at 𝑓res = 240 GHz.Figure 3: Top: THz pulse energy and conversion efficiencyas a function of pump energy. Bottom: THz spectrum mea-sured using THz bandpass filters. The horizontal uncertaintybars correspond to the FWHM filter bandwidths. The dashedline indicates the resonance frequency of the used SRR.BEAM TRANSPORT OPTIMIZATIONIn order to increase the THz pulse energy coupled intothe resonator, the THz beam transport from crystal to theinteraction point was improved.Terahertz OpticsIn the previous design of the TPF module, ZEONEXwas chosen as THz lens material, because it exhibits hightransmittance and a similar refraction index both in the THzand the visible spectral range. This permits an alignment ofthe THz beam using a visible alignment laser. However, for a20 mm thick lens (𝑓 = 75 mm) a transmittance of only 81.8±1.4 % was measured using our TPF THz source, indicatingan absorption coefficient of ≈ 0.1 cm−1 (see Table 1).Table 1: Comparison of ZEONEX and UHMWPE for THzoptics. The absorption coefficient was derived by a transmit-tance measurement of 20 mm thick lenses.Material Refractive index Abs. coeff. (cm−1)ZEONEX 1.53 [10] ≈ 0.1UHMWPE 1.54 ± 0.01 [11] ≈ 0.04To increase the transmittance per THz lens we investigatedUHMWPE (ultra-high-molecular-weight polyethylene) asan alternative material. Figure 4 shows a THz time-domainspectroscopic measurement [12] of a 10 mm thick plano-parallel UHMWPE sample in the time domain and the re-sulting transmittance over frequency. Since the experimentwas conducted in air, the transmission spectrum shows dis-tinct absorption lines caused by water in the air, as well asa somewhat noisy signal for lower frequencies. Neverthe-less, it can be confirmed that UHMWPE shows very littletransmittance losses for the relevant frequency range of aFigure 4: Top: Amplitude in the time domain of a THz-TDSsignal without sample (orange) and with 10 mm UHMWPE(blue). Bottom: Transmittance in frequency domain.Figure 5: Left: Pictures of self-made UHMWPE (white)and commercial ZEONEX (transparent) THz lenses. Right:Optical enclosure with filtration system for the THz module.few hundred GHz. From the measured delay in time-domainan average refractive index of 𝑛 = 1.52 can be derived. Con-sidering uncertainties regarding the sample thickness andthe step size of the used delay stage, the value is in goodagreement to [11].Consequently, in contrast to the commercially acquiredZEONEX lenses, the UHMWPE THz lenses were self-madeby turning in the institute’s workshop. Figure 5 (left) showsa photograph of both lens types for comparison. The dimen-sions of the lenses were designed based on the commonlyknown lensmaker equation for a plano-convex lens. Aftermanufacturing we observed a transmittance of 91.9 ± 1.9 %for a 20 mm thick UHMWPE THz lens (𝑓 = 75 mm), i.e.about 10 % more than the comparable ZEONEX lens, result-ing in an absorption coefficient of about half (see Table 1).Also - as shown in Fig. 6 - the optics were tested as a1-to-1 replacement in the exisiting THz TPF module andthe THz beam intensity profile was measured in the focalpoint of the first 4-𝑓-telescope after the first two THz lenses(distance ≈ 35 cm from the crystal). For the measurements,the THz camera MICROXCAM-384i-THz (384×288 pixels,based on uncooled microbolometer focal plane array [13])was used. By directly comparing the two intensity profilesand marked 2D-gaussian contour lines (white dashed lines),one can state that the self-made UHMWPE lenses do notshow any significant horizontal or vertical shift, while pre-serving focal length (equal spot sizes within uncertainties)and transmitting more light.Figure 6: THz intensity profiles after the first two THz lensesseen by a THz camera. Left: ZEONEX; Right: UHMWPE.The false color scale applies to both plots. In addition, con-tour lines (white; 85 %, 62 % and 38 % of maximum inten-sity scale) of a 2D-gaussian fit function are shown.Figure 7: THz pulse energy (normalized to first measure-ment value) and relative humidity over time while using adrying system for removing moisture. The THz pulse energywas measured after a transport distance of ≈ 35 cm.Ambient ConditionsSince three quarters of the THz beam path is outside ofvacuum, we expect to have absorption of THz power bythe water vapor in the ambient air. To quantify the amountof absorption loss, we built an optical enclosure (Fig. 5,right) around the THz module and used a dust and humidityfiltration system [14] to decrease THz absorption by waterin air and measure its impact on the THz transport.Figure 7 shows the THz pulse energy normalized to thefirst measurement value as well as the relative humidity overtime when the filtration system was turned on. It can benoted that after only about 4 hours, the relative humidity inthe volume could be decreased by roughly 10 % resultingin a THz pulse energy gain of about 5 %. After filteringfor 24 h an increase in THz power of nearly 15 % could beobserved.CONCLUSION AND OUTLOOKIn conclusion, crucial improvements for the THz pulseenergy could be realized, which is a critical parameter forachieving single-fs resolution in the SRR Compact-TDS di-agnostics experiment at FLUTE. By changing the materialand manufacturing UHMWPE THz lenses, as well as en-closing and dehumidifying the THz TPF setup environment,the overall transport loss of THz radiation could be reducedsignificantly. Based on our measurements, at the SRR inter-action point a THz pulse energy of 188 % ± 40 % comparedto the previous setup can be expected. This results in a fac-tor of the order of two times the streaking strength, whichwill substantially facilitate the search for streaking in futureexperiments.ACKNOWLEDGEMENTSM. Nabinger acknowledges the support by the DFG-funded Doctoral School ”Karlsruhe School of Elemen-tary and Astroparticle Physics: Science and Technology”(KSETA), use of material by Thorlabs and funding by theBMBF ErUM-Pro project Compact TDS (FKZ 05K22VK3).REFERENCES[1] J. Roensch-Schulenburg et al., “Generation of Ultra-shortElectron Bunches at FLASH”, in Proc. FEL’12, Nara, Japan,Aug. 2012, paper THPD33, pp. 610–613.[2] V. Schlott et al., “Overview and Status of SwissFEL Diag-nostics”, in Proc. IBIC’15, Melbourne, Australia, Sep. 2015,pp. 12–16. doi:10.18429/JACoW-IBIC2015-MOBLA03[3] P. Krejcik, R. Akre, L. Bentson, and P. Emma, “A TransverseRF Deflecting Structure for Bunch Length and Phase SpaceDiagnostics”, in Proc. PAC’01, Chicago, IL, USA, Jun. 2001,paper WPAH116, pp. 2353–2355.[4] M. Nabinger et al., “Efficient terahertz generation by tilted-pulse-front pumping in lithium niobate for the split-ring res-onator experiment at FLUTE”, in Proc. IPAC’21, Campinas,Brazil, May 2021, pp. 4299–4301.doi:10.18429/JACoW-IPAC2021-THPAB251[5] M. Yan et al., “Design of a Time-resolved Electron Diag-nostics Using THz Fields Excited in a Split Ring Resonatorat FLUTE”, in Proc. IBIC’16, Barcelona, Spain, Sep. 2016,pp. 475–478. doi:10.18429/JACoW-IBIC2016-TUPG56[6] J. Hebling, K.-L. Yeh, M. Hoffmann, B. Bartal and K. A. Nel-son, “Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities”,J. Opt. Soc. Am. B, vol. 25, no. 7, pp. B6–B19, 2008.doi:10.1364/JOSAB.25.0000B6[7] X. Wu et al., “Half-percent terahertz generation efficiencyfrom cryogenically cooled lithium niobate pumped by Ti:sap-phire laser pulses”, arXiv preprint, 2016.doi:10.48550/arxiv.1601.06921[8] J. Schaefer et al., “Split-ring resonator experiments and dataanalysis at FLUTE”, presented at the IPAC’23, Venice, Italy,May 2023, paper THPL122, this conference.[9] S. Glukhov et al., “Simulations of the Compact Transverse-Deflecting System for Ultra-Short Electron Bunch Diagnos-tic”, presented at the IPAC’23, Venice, Italy, May 2023, paperTHPL192, this conference.[10] Tydex, https://www.tydexoptics.com/products/thz_optics/thz_materials/.[11] E. Bründermann, H.-W. Hübers and M. F. Kimmitt, “OpticalComponents”, in Terahertz Techniques, Berlin, Heidelberg,Germany: Springer, 2012, pp. 51–102.[12] Menlo Systems GmbH, TDS Spectrometer TERA K15with emitter TERA15-FC, https://www.menlosystems.com/.[13] L. Marchese et al., “Overcoming the challenges of activeTHz/MM-wave imaging: an optics perspective”, in Proc.SPIE 10639, Micro- and Nanotechnology Sensors, Systems,and Applications X, Orlando, FL, United States, May 2018,pp. 372–379. doi:10.1117/12.2305398[14] Thorlabs, Inc., PACU https://www.thorlabs.com/thorproduct.cfm?partnumber=PACU/.

Characterization and optimization of laser-generated THz beam for THz based streaking

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Characterization and optimization of laser-generated THz beam for THz based streaking

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