The ability of pellets made up of compressed iron iodate nanocrystals to frequency-double the whole visible spectrum is demonstrated. We suggest their use for complete characterization of intense ultrabroadband laser pulse

Bonacina, L.

Béjot, P.

Extermann, J.

Galez, C.

Le Dantec, R.

Mazingue, T.

Mugnier, Y.

Wolf, J.-P

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Appl Phys B (2009) 97: 537–540DOI 10.1007/s00340-009-3569-xAn inexpensive nonlinear medium for intense ultrabroadbandpulse characterizationJ. Extermann · P. Béjot · L. Bonacina · Y. Mugnier ·R. Le Dantec · T. Mazingue · C. Galez · J.-P. WolfReceived: 27 April 2009 / Published online: 15 May 2009© Springer-Verlag 2009Abstract The ability of pellets made up of compressed ironiodate nanocrystals to frequency-double the whole visiblespectrum is demonstrated. We suggest their use for completecharacterization of intense ultrabroadband laser pulses.PACS 42.65.Re · 42.65.Ky · 42.70.Mp · 78.67.Bf1 IntroductionThe interest in few- or single-cycle pulses (FCP) has in-creased in the last years particularly because of their im-plementation for attosecond pulse generation [1, 2]. FCPpulses, which can be prepared for example by spectralbroadening via (co-)filamentation in rare gases [3, 4], ex-hibit very broad (octave spanning) bandwidths, whichmakes their characterization highly challenging. Standardpulse diagnostic techniques [5, 6], in fact, are trimmed downby limited spectral acceptance, as they are based on non-linear frequency mixing in bulk crystals [7]. Ideally, oneshould posses a nonlinear medium capable of producing anintense nonlinear signal, fulfilling the phase-matching con-dition over a very broad spectral domain, and inducing noneor negligible temporal dispersion on the pulses to be mea-sured. Recently, we successfully used individual Fe(IO3)3nanocrystals as nonlinear medium for complete pulse char-acterization within a microscope focus [8]. In that case, weJ. Extermann () · P. Béjot · L. Bonacina · J.-P. WolfGAP-Biophotonics, Université de Genève, 20 rue de l’Ecolede Médecine, 1211 Geneva, Switzerlande-mail: jerome.extermann@unige.chY. Mugnier · R. Le Dantec · T. Mazingue · C. GalezSymme, Polytech’ Savoie, B.P. 80439, 74944 Annecy le Vieux,Franceshowed that, as the average crystal size is much smaller thanthe coherence length, no phase-matching constraints ap-ply to the frequency-mixing process, allowing for completespectral doubling of broadband femtosecond pulses. Due tothe large nonlinear coefficients of Fe(IO3)3 and the cost-effective synthesis of the nanocrystals [9], it seemed tempt-ing to employ them also for the routine characterization ofintense ultrashort and ultrabroad laser pulses. In the follow-ing, we demonstrate that pellets of compressed Fe(IO3)3nanocrystals can efficiently frequency-convert broadbandinfrared and ultraviolet femtosecond laser pulses throughvarious nonlinear processes, enabling their spectral and tem-poral characterization.2 Experimental2.1 Sample preparationTwo aqueous solutions of iron nitrate and iodic acid saltswith molar concentrations are mixed at room temperatureunder vigorous stirring. Nanosized powders are obtainedafter filtering and drying the yellow-green precipitate. Thegrain size, estimated by the broadening of X-ray diffractionpeaks, is around 30–40 nm which is consistent with HighResolution Scanning Electron Microscope (SEM) images(Fig. 1). Pellets of compressed nanocrystals (3 mm thicknessand 8 mm diameter) were obtained by exerting a 250 MPapressure on dried powder for 15 minutes. The surface rough-ness is found to be in the micrometer range.2.2 Optical set-upThe direct output of a Ti:Sapphire amplified laser system(from hereon labeled as R pulses: central wavelength λR =538 J. Extermann et al.Fig. 1 High resolution SEM image of iron iodate nanosized powdersFig. 2 Power dependance of the 200 nm signal as a function ofthe 400 nm input pulse power (experimental data points (squares)and its quadratic fit (line)). The collection solid angle corresponds to5 × 10−2 srd792 nm, bandwidth 30 nm, energy 2.5 mJ) are frequency-doubled in a 150 µm thick BBO crystal, providing a typ-ical output energy of 150–300 µJ. In a first experiment,the second harmonic pulses (B) are directly focused ontoa nanoparticle pellet by an aluminium-coated spherical mir-ror (f = 50 cm). The scattered nonlinear UV signal (UV)(see the power dependence in Fig. 2) is collected by a fused-silica lens-doublet and coupled into a monochromator with0.4 nm spectral resolution. The spectrally-resolved signalis finally detected by a solar blind photomultiplier (Hama-matsu, R166UH). Alternatively, the B pulses are tempo-rally characterized using a collinear interferometric setup:the pulse train is separated by a 2 µm thick 50/50 pelliclebeamsplitter and the relative delay between the two armsadjusted with a motorized delay stage (temporal resolution∼0.1 fs). In a third experiment, the B pulses are slightly fo-cused by a f = 1 m lens and injected collinearly with Rpulses in an Ar-filled cell, via co-filamentation the spec-tral bandwidth broadens up from the initial 6.3 to 30 nmFWHM. The B pulses are finally focused on the pellet forcharacterization using the generated UV-200 nm signal. Ina last experiment, both R and B are focused with adjustableFig. 3 (a) SHG spectrum of a R pulse performed on a single nanocrys-tal and (b) its autocorrelation [8]. (c) SHG spectrum of the B pulsegenerated on the pellet and (d) its autocorrelationdelay and polarization onto the nonlinear sample to measuretheir sum-frequency signal at 266 nm. All the measurementsaround 200 nm are performed in an argon saturated environ-ment to prevent oxygen absorption below 190 nm.3 ResultsSince Fe(IO3)3 does not exist in bulk form, in a previouswork we characterized its nonlinear optical response bystudying the properties of single nanocrystals [10], deter-mining a value of the order of several pm/V for its χ(2)ijktensor elements. Thanks to their small size (l = 20–150 nm)and large second order susceptibility, we succeeded infrequency-doubling 800 nm radiation on individual nano-crystals (see Fig. 3(a)). As reported in Fig. 3(b), the non-linear response of the nanoparticle was used to acquire apulse autocorrelation trace at the focal plane of a high nu-merical aperture objective [8]. Due to the geometry imposedby the microscopic setup, the interference fringes typical ofcollinear measurements were removed by an ad hoc cut-offfilter in the Fourier space [11].Since Fe(IO3)3 nanocrystals are smaller than the coher-ence length, the material can simply be considered as an en-semble of nonlinear radiating dipoles, providing completedoubling of an incident spectrum centered at any wavelengthindependently from any phase-matching consideration. Thisproperty is exemplified by the plot in Fig. 3(b), showing theresult of the spectral doubling of an input 6.3 nm broad Bpulse. As reported in Fig. 3(d), the corresponding autocor-relation trace indicates a B pulse duration of 81 fs.The above results demonstrate the ability of Fe(IO3)3to generate an easily detectable scattered SHG signal from aUV–Vis pulse focused onto the pellet. As seen in Fig. 2, evenAn inexpensive nonlinear medium for intense ultrabroadband pulse characterization 539Fig. 4 UV spectrum (at 200 nm) of a B broad source obtained byco-filamentation (dots). Original spectrum corresponding traces fora 400 nm pulse without broadening (squares) (data reported from(Fig. 3(c)))Fig. 5 Solid line: signal generated only by the IR beam. (a) Signal(squares) for UV and IR laser pulses temporally separated by 750 fs,(b) simultaneous and parallel (dotted line) (respectively perpendicular(circles)) B and R laser pulses. (c) Signature of the SFG signalthough the detection efficiencies was limited by the low nu-merical aperture of the collection lens, a clear background-free quadratic dependence is observed on the scattered200 nm signal. Within the scope of FCP characterization,we show now that the bandwidth increase does not limitthis capability, by repeating the measurement using as inputfilamentation-broadened B pulses with spectral bandwidthspanning from 350 to 440 nm. As illustrated in Fig. 4, theirdoubled spectrum extends from 175 to 220 nm fully match-ing the spectral range of the input pulse. The measured widthis 8 nm FWHM, which could theoretically be recompresseddown to 7 fs pulse duration.In order to accurately perform a FCP autocorrelation, thenonlinear material has to efficiently support a nonlinear po-larization P ∝ E2(t), where E(t) is the real electric field. Inthe Fourier reciprocal space, this nonlinear source term cor-responds to all the frequency-mixing pairs within the spec-trum. We show that Fe(IO3)3 pellets support sum-frequencygeneration (SFG). In particular, when launching simultane-ously R and B pulses onto the nanoparticles agglomerate,a delay-dependent signal centered at 264 nm was recordedboth for orthogonal and parallel polarization of the two in-put pulses (Fig. 5(b)). In Fig. 5 we summarize the results ofthese measurements: in (a) one can observe that, also in theabsence of the B pulse on the sample, a weak 264 nm signalis present, which is a signature of R pulse frequency tripling.By subtracting from the zero-delayed curve (Fig. 5(b)) thisthird harmonic component, we obtain the signature of SFGspectrum at 264 nm (about 7.3 nm FWHM) (Fig. 5(c)).4 ConclusionIn conclusion, we have successfully demonstrated thatFe(IO3)3 pellets can support both ultrabroadband SHG andbroadband SFG. Because the nonlinear material is used ina reflective configuration, no dispersion occurs, allowingits use for FCP measurement. Moreover, since the phase-matching condition does not apply because of the nano-metric dimensions of the doubling particles, this approachopens the way to the utilization of this material with ultra-broadband spectra in its whole transparency region span-ning up to 13 µm. Unlike nonlinear crystals which need acareful polishing and are quite expensive, this material isinexpensive enough to be used in adverse conditions suchas within a pulse core where the intensity can exceed thedamage threshold.Acknowledgements Work supported by the Swiss NSF (contracts200021-111688), Swiss SER in the framework of COST P18. We aregrateful to the Laboratory of Ultrafast Spectroscopy, EPF Lausanne,for the photomultiplier and to M. Moret for technical support.References1. E. Goulielmakis, M. Schultze, M. Hofstetter, V.S. Yakovlev,J. Gagnon, M. Uiberacker, A.L. Aquila, E.M. Gullikson, D.T.Attwood, R. Kienberger, F. Krausz, U. Kleineberg, Single-cyclenonlinear optics. Science 320(5883), 1614–1617 (2008)2. C.P. Hauri, W. Kornelis, F.W. Helbing, A. Heinrich, A. Coua-iron, A. Mysyrowicz, J. Biegert, U. Keller, Generation of intense,carrier-envelope phase-locked few-cycle laser pulses through fila-mentation. Appl. Phys. B, Lasers Opt. 79(6), 673–677 (2004)3. T. Fuji, T. Suzuki, Generation of sub-two-cycle mid-infraredpulses by four-wave mixing through filamentation in air. Opt. Lett.32(22), 3330–3332 (2007)4. E. Goulielmakis, B. Koehler, B. Reiter, M. Schultze, A.J. Verhoef,E.E. Serebryannikov, A.M. Zheltikov, F. Krausz, Ultrabroadband,coherent light source based on self-channeling of few-cycle pulsesin helium. Opt. Lett. 33(13), 1407–1410 (2008)5. R. Trebino, K.W. DeLong, D.N. Fittinghoff, J.N. Sweetser, M.A.Krumbugel, B.A. Richman, D.J. Kane, Measuring ultrashort laserpulses in the time-frequency domain using frequency-resolved op-tical gating. Rev. Sci. Instrum. 68(9), 3277–3295 (1997)6. C. Iaconis, I.A. Walmsley, Spectral phase interferometry for directelectric-field reconstruction of ultrashort optical pulses. Opt. Lett.23(10), 792–794 (1998)7. S. Akturk, C. D’Amico, A. Mysyrowicz, Measuring ultrashortpulses in the single-cycle regime using frequency-resolved opti-cal gating. J. Opt. Soc. Am. B 25(6), A63–A69 (2008)540 J. Extermann et al.8. J. Extermann, L. Bonacina, F. Courvoisier, D. Kiselev, Y. Mug-nier, R. Le Dantec, C. Galez, J.-P. Wolf, NanoFROG: FrequencyResolved Optical Gating by a nanometric object. Opt. Express16(14), 10405–10411 (2008)9. C. Galez, Y. Mugnier, J. Bouillot, Y. Lambert, R. Le Dantec, Syn-thesis and characterisation of Fe(IO3)3 nanosized powder. J. Al-loys Compd. 416(1–2), 261–264 (2006)10. L. Bonacina, Y. Mugnier, F. Courvoisier, R. Le Dantec, J. Exter-mann, Y. Lambert, V. Boutou, C. Galez, J.P. Wolf, Polar Fe(IO3)3nanocrystals as local probes for nonlinear microscopy. Appl.Phys. B, Lasers Opt. 87(3), 399–403 (2007)11. G. Stibenz, G. 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An inexpensive nonlinear medium for intense ultrabroadband pulse characterization

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An inexpensive nonlinear medium for intense ultrabroadband pulse characterization

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