broadband ultrafast spectroscopy using a photonic crystal fiber: application to the photophysics...

Broadband ultrafast spectroscopy usinga photonic crystal fiber: application tothe photophysics of malachite green

Jeremie Leonard1∗, Nhan Lecong1, Jean-Pierre Likforman1,Olivier Cregut1, Stefan Haacke1,

Pierre Viale2, Philippe Leproux2, Vincent Couderc2

1 Institut de Physique et Chimie des Materiaux de Strasbourg; UMR 7504 ULP-CNRS;23, rue du Loess ; BP 43 ; F-67034 STRASBOURG CEDEX 2 ; FRANCE

2 XLIM - Photonics group; UMR CNRS n°6172123, avenue Albert Thomas; F-87060 LIMOGES CEDEX ; FRANCE

∗Corresponding author: [email protected]

Abstract: Femtosecond single-pulsed supercontinua (SC) are producedin a short sub-cm piece of photonic crystal fiber. The SC span from 450 nmto more than 1.1 μm with 1-nJ energy injection. UV light down to 340 nmis observed with increased injection power. Using such a single-pulsedSC we implemented a compact transient absorption spectrometer withbroadband detection and 150-fs FWHM time resolution to monitor theultrafast dynamics of the electronic states of malachite green in ethanolexcited to the S2 state. The full spectral evolution is observed from 450 nmto 1050 nm, with high sensitivity and a signal-to-noise ratio as high as 1000.

© 2007 Optical Society of America

OCIS codes: (190.4370) Nonlinear optics, fibers; (320.7150) Ultrafast spectroscopy;(300.6390) Spectroscopy, molecular.

References and links1. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air silica microstructure optical

fibers with anomalous dispersion at 800nm,” Opt. Lett. 25, 25–27 (2000).2. A. Husakou and J. Herrmann, “Supercontinuum Generation of Higher-Order Solitons by Fission in Photonic

Crystal Fibers,” Phys. Rev. Lett. 87, 203,901 (2001).3. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys.

78, 1135–1184 (2006).4. L. Tartara, I. Cristiani, and V. Degiorgio, “Blue light and infrared continuum generation by soliton fission in a

microstructured fiber,” Appl. Phys. B 77, 307–311 (2003).5. J. M. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, R. Trebino, S. Coen, and R. S. Windeler, “Cross-

correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystalfiber: simulations and experiments,” Opt. Express 10, 1215–1221 (2002).

6. V. Nagarajan, E. Johnson, P. Schellenberg, W. Parson, and R. Windeler, “A compact versatile femtosecond spec-trometer,” Rev. Sci. Instrum. 73, 4145–4149 (2002).

7. M. Punke, F. Hoos, C. Karnutsch, U. Lemmer, N. Linder, and K. Streubel, “High-repetition-rate white-lightpump-probe spectroscopy with a tapered fiber,” Opt. Lett. 31, 1157–1159 (2006).

8. G. Oster and Y. Nishijima, “Fluorescence and Internal Rotation : Their Dependence on Viscosity of the Medium,”J. Amer. Chem. Soc. 78, 1581–1584 (1956).

9. M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, “Femtosecond study of S2 fluorescence in malachite green insolutions,” Chem. Phys. Lett. 290, 43–48 (1998).

10. Y. Kanematsu, H. Ozawa, I. Tanaka, and S. Kinoshita, “Femtosecond optical Kerr-gate measurement of fluores-ence spectra of dye solutions,” J. Lumin. 87-89, 917–919 (2000).

11. A. C. Bhasikuttan, A. V. Sapre, and T. Okada, “Ultrafast Relaxation Dynamics from the S2 State of MalachiteGreen Studied with Femtosecond Upconversion Spectroscopy,” J. Phys. Chem. A 107, 3030–3035 (2003).

#84849 - $15.00 USD Received 3 Jul 2007; revised 24 Aug 2007; accepted 2 Sep 2007; published 20 Nov 2007

(C) 2007 OSA 26 November 2007 / Vol. 15, No. 24 / OPTICS EXPRESS 16124

12. S. H. Cho, B. E. Bouma, E. P. Ippen, and J. G. Fujimoto, “Low-repetition-rate high-peak-power Kerr-lens mode-locked Ti:Al2O3 laser with a multiple-pass cavity,” Opt. Lett. 24, 417–419 (1999).

13. M. M. Martin, P. Plaza, and Y. H. Meyer, “Ultrafast conformational relaxation of triphenylmethane dyes: SpectralCharacterization,” J. Phys. Chem. 95, 9310–9314 (1992).

14. T. Robl and A. Seilmeier, “Ground-state recovery of electronically excited malachite green via transient vibra-tional heating,” Chem. Phys. Lett. 147, 544–550 (1988).

1. Introduction

In recent years, ultrabroad spectra of light have been produced by injecting nJ-energy femtosec-ond pulses obtained from conventional non-amplified mode-locked lasers into micro-structuredfibers [1]. These ”supercontinua” (SC) have since emerged as attractive light sources for avariety of applications such as optical frequency synthesis, imaging techniques, or ultrafastspectroscopy. For the latter application, not only the ultrabroad spectrum, but also the temporaldistribution of the supercontinuum is a critical parameter. The broadest spectra are obtainedwhen high-order soliton fission takes place during propagation in the anomalous dispersionregime [2, 3]. As a result, blue light can be produced in fibers longer than a few tens of cm[4], but the temporal structure of the SC shows multiple pulses extending over picoseconds intime [5]. Under these conditions sub-picosecond spectroscopy is impossible unless the SC isspectrally filtered and single wavelength detection is used, as was demonstrated in earlier workwith probe wavelengths > 550 nm [6, 7]. However, in order to elucidate the ultrafast dynamicsof molecules in solution, where inter-state transitions may spectrally overlap and shift, transientabsorption spectroscopy is best performed with broadband detection. This requires � 400-nmpumping light and single-pulsed probe spectra extending into the blue or better near-UV, forthe large majority of molecules of interest.

In this work, we perform ultra-broadband transient absorption spectroscopy of malachitegreen (MG), using a compact non-amplified Ti:sapphire oscillator, and SC generated in a pho-tonic crystal fiber (PCF). Remarkably broad spectra are produced in a sub-cm short piece ofthe PCF with sub-picosecond single pulse output. Pump-induced absorption changes (ΔA) ofthe probe beam are measured with a sensitivity in the range of 5× 10−5. Time-resolved spec-tra can be recorded over a 300-nm broad spectral range in a single experimental run, with a150-fs FWHM time resolution, and wavelengths as short as 450 nm are probed. Triphenyl-methane (TPM) dyes, of which MG is an example, relax non-radiatively from the S 1 statesupposedly through the rotation of one or several phenyl group(s), with a rate strongly depend-ing on solvent viscosity [8]. These have been thus described as model systems for radiationlesselectronic relaxation involving large-amplitude motion with no intramolecular potential barrier.To the best of our knowledge, only time-resolved fluorescence has been used so far to probe therelaxation dynamics of the S2 state [9, 10, 11]. Here we present results of transient absorptionspectroscopy of MG in ethanol, excited to the S2 state by a 425-nm pump pulse, and discusssome new findings pertaining to the ultrafast relaxation scenario of MG.

2. Supercontinuum generation and characterization

The laser source is a Ti:Sa oscillator (KML), pumped by a 5-W cw VERDI laser. It delivers 15-nJ, 45-fs pulses with a 27-MHz repetition rate, due to the implementation of a multipass cavity[12]. The central wavelength is adjusted at 850 nm. A few nJ of the pulse energy are used forinjection into a sub-cm short piece of a PCF, the cross section of which being displayed onFig. 1(b). The remaining part of the laser power is frequency doubled in a 2-mm-thick BBOcrystal, producing typically 1.5-nJ pulses at 425 nm, which is used as a pump beam for ourtransient spectroscopy measurements.

The PCF was produced at the XLIM laboratory by using the standard stack-and-draw



method. The triangular structure of holes with an average diameter d = 1.85 μm and spacingΛ = 2.6 μm yields an air filling fraction d/λ = 0.71. Two lateral bigger holes with diameters of3.3 μm and 3.6 μm make the core asymmetric and induce strong birefringence. At 850 nm, theeffective area of the fundamental mode is � 4.9 μm2 and four different modes LP01x, LP01y,LP11x and LP11y can be guided in the core. Their zero-dispersion wavelengths (ZDW’s) are827, 866, 757 and 764 nm respectively. With a fiber as short as 8 mm, and a 25-kW peak powerinjected onto the LP01 mode close to the ZDW, the SC extends typically from 450 nm to above1.1 μm (limited by the CCD-based detection) as displayed by the solid black line of Fig. 1(a).The location of maxima and minima in the spectrum can be tuned by rotating the incident(linear) laser light polarization. The output polarization is wavelength-dependent, a broadbandpolarizer is used for the spectroscopic application. The spectro-temporal distribution of the SCis investigated by time-gated non-degenerate two-photon absorption in a 0.1-mm thick ZnSplate, where the frequency-doubled pulse is used as the gating pulse. Fig. 1(c) shows, in false-color scale, the spectrally and temporally resolved change of transmission of the SC which isobserved for temporal coincidence of gate and SC pulses in ZnS. The SC appears to be a 600-nm-broad single pulse extending over � 300 fs in time. It is worth noting that the spectrumextends this far towards short wavelengths (450 nm) with such a short propagation distance inthe PCF. Numerical computations of the vectorial and modal phase matching conditions forfour-wave mixing (FWM) in the fiber indicate that FWM is not expected to transfer energy towavelengths shorter than 600 nm. Hence wavelengths arising around 500 nm seem to be dueto high-order soliton break-up and phase-matched dispersive wave generation [3]. However,most probably due to the short propagation length, the temporal spreading is small, and thesingle-pulsed SC output is preserved.

Under the above injection conditions the piece of fiber deteriorates rather quickly. However,setting a protection window on its front face drastically reduces the aging of the PCF, allowingus to use it as a stable light source for more than 6 months. It is to be noted that, by injectingabout 5 times more power at a different angle of incidence, we could observe a SC extendingin the UV range down to 340 nm (red dashed curve of Fig. 1-(a)). As suggested by the spatialoutput pattern, we believe the power was mainly coupled into a leaky mode. The time-frequencydistribution showed again a single-pulsed SC, with the UV part (340-400 nm) spreading overabout 700 fs in time. However, the remaining laser power available for producing the blue pumppulse was too low to perform pump-probe spectroscopy with this spectrum.

300 400 500 600 700 800 900 1000

Inte

nsity

[10

dB/d

iv]

[nm]0

500[fs]

(a)

(c)(b)

x

y

Fig. 1. (Color online) (a) Spectra obtained by coupling incident infrared light into the LP01transverse mode (solid black curve) and in a supposedly leaky mode (red dashed curve)of the fiber (length 8 mm). (b) Cross-sction of the fiber. (c) Temporal distribution of thespectrum displayed in (a) for the LP01 mode. The solid red curve is a polynomial fit to thespectro-temporal distribution which reveals the chirp of the SC.



3. Application to broadband transient absorption spectroscopy

The high-repetition-rate non-amplified laser system and the SC light source fulfill all require-ments for the implementation of a sensitive transient absorption spectrometer. An energy den-sity of 0.25 mJ/cm2 is obtained at 425 nm by focusing the pump beam with a parabolic mirrorof 25-mm effective focal length. With MG, this leads to an excitation probability of � 3%, anda maximum bleach signal ΔABleach �−0.025 at 620 nm, as shown in Fig. 2.

The high repetition rate of the laser system may cause multiple excitation of the moleculesor accumulation of population in long-lived states. In these cases a non-zero bleach signal ismeasured at negative pump-probe delays, which is in fact the absorption change induced bythe previous pump pulses. Fast circulation of the solution and a small pump beam diameterlimit this effect. In our case, the bleach signal measured for MG at large negative delays is inthe range of ΔA<0 � −0.007, which means that on an average less than 1% of the moleculesare not in their ground state when the next pump pulse arrives. Hence, after subtraction of thenegative-time signal, at least 99% of the remaining transient signal is due to MG moleculesinitially excited to the S2 state.

In addition, strong focusing of the pump in the solution creates local temperature and indexgradients which in turn produce an optical lens. The resulting pump-induced change of thecollection efficiency of the probe light simulates a change of absorbance. This effect was takencare of by a fast circulation of the sample and by a moderate focusing of the pump beam.

Further, intensity fluctuations in the probe beam are a critical issue for sensitive pump-probeexperiments. We note that, the level of noise in our PCF-based spectrometer is not higher thanthat observed with supercontinua produced by μJ pulses in Sapphire. Thus, no reference beamhas been implemented to correct for the probe fluctuations in our experiment. The SC spectrumis measured with a CCD camera (Roper Scientific, Spec10), at an acquisition rate of 800 spectraper second with exposure times of 0.4 ms. The pump beam is chopped at half the acquisitionfrequency. It can be easily shown that the noise on the raw signal for the absorption change is infact the relative shot-to-shot noise in the probe beam, where a “shot” means a background-freerecord of the probe spectrum measured over the 0.4 ms integration time of the camera. Thisshot-to-shot noise is typically in the range 0.5 to 1%, depending on the wavelength. Enhancednoise (up to 3%) is observed below 550 nm and around 830- 860 nm, due respectively to largerfluctuations in the supercontinuum at specific wavelengths, and to residual scattered light at thefundamental wavelength of the oscillator.

Typical transient absorption data for MG in ethanol excited to the S 2 state are displayed inFig. 2(b). Three experimental runs recorded in 1 hour and covering a 300-nm spectral band-width were merged to build Fig. 2(b). Each time point is an average of 2000 spectra. Theultimate sensitivity is typically in the range of 5× 10−5 as given by the standard deviationof the residuals to the fits displayed in Fig. 3 (see below). We emphasize that, this noise figureobtained with spectrally resolved detection is a very good result since it reaches that of a single-wavelength lock-in detection. At the intensity minima of the SC, the noise figure degrades, dueto the finite dynamical range of the CCD. Indeed, for broadband detection the SC is attenuatedwith neutral OD filters 2-3, in order not to saturate the detector at more intense wavelengths.The temporal response function of the spectrometer is derived from pump-probe experiment ondiphenyl-hexatriene in acetone. It is in the range of 150 fs FWHM.

4. Ultrafast spectroscopy of Malachite Green: Results and discussion

The transient absorption spectra displayed in Fig. 2(b) are obtained with pump and probe beamspolarized orthogonally. As the S0-S1 and S0-S2 transition dipole moments are orthogonal, thisallows maximizing the S0-S1 bleach signal, which we observe to be the dominant feature be-tween 540 nm and 670 nm. On the low-energy side of the bleach signal and immediately after



ΔA

a)

b)400

Fig. 2. (Color online) a) Stationary absorption spectrum of MG in ethanol. The arrow showsthe excitation wavelength of 425 nm. b) Temporally- and spectrally-resolved transient ab-sorption spectra (in false colors) of MG excited to the S2 state. These data are reconstructedfrom 3 different experimental runs with different central detection wavelengths and mergedat 540 nm and 850 nm. Here the PCF length is 6.5 mm.

excitation a positive band appears from 750 nm up to 1050 nm at least. This hitherto unob-served feature is attributed to excited-state absorption (ESA) from S 2. Similarly, in the blue ofthe bleach signal a weak instantaneous gain (hardly visible on Fig. 2(b) because of the colorcode, best seen at early times in Fig. 3(a)) is detected which we attribute to stimulated emission(SE) from S2. Both bands decay rapidly and are followed by a negative one at 650-950 nm,which has already been identified unambiguously as SE from S 1 by several authors. On thesame time scale, an ESA signal, spanning from below 450 nm up to 590-600 nm, rises andoverlaps with the bleach signal. This ESA band has previously been assigned to a superpositionof ESA from the S1 state and an intermediate Sx state [13]. Finally, a positive signal appears af-ter � 2 ps between 640 nm and 690 nm. This absorption band has also previously been observedand discussed in different TPM dyes.

0 1 10

-2

-1

0

1

2

0 1 10 0 1 10

a) 490 nm b) 690 nm c) 800 nm

Time delay (ps) Time delay (ps)Time delay (ps)

ΔA (

x103 )

Fig. 3. (Color online) Kinetic traces (in black) and their best fits to a biexponential curve(in red) at a) 490 nm, c) 800 nm, and to a triexponential curve at b) 680 nm. The time scaleis linear below 1 ps and logarithmic above. Data are extracted from Fig. 2(b), and averagedon a 10-nm window, that is over 4 adjacent columns of the 2D plot displayed in Fig. 2(b).The standard deviation of the residuals to the fit gives a typical noise level of 5×10−5.



Data analysis at wavelengths shorter than 520 nm and longer than 720 nm is done by fittinga sum of two exponential functions convoluted with a gaussian instrument response function.Excellent agreement is obtained as illustrated by Fig. 3(a) and Fig. 3(c). At wavelengths longerthan 900 nm, the decay of the ESA band from S 2 is almost pure and fitted with a single expo-nential decay with a 0.31±0.03 ps time constant. In the 720 to 850 nm range, the same value isobtained when modeling both the decay of S2 and rise of S1 with a unique time constant. In ad-dition, a second time constant of 0.60±0.10 ps is obtained for the decay of the SE signal fromS1. Yoshizawa et al. [9] performed time-resolved fluorescence spectroscopy on MG excited tothe S2 state in water. They measured 0.27± 0.05 ps for the fluorescence life time of S 2, and0.43±0.06 ps and 0.54±0.06 ps for the rise and decay of the fluorescence of S 1. Although thesolvent is different, the viscosity is virtually the same and our measurements in ethanol agreewell with Yoshizawa’s results obtained in water.

At wavelengths shorter than 520 nm, we measure a rise time of 0.46±0.08 ps for the ESA,which also coincides with Yoshizawa’s measurement for the rise of the fluorescence of S 1 inwater. The measured decay time of this ESA is wavelength-dependent and shifts from 1.1±0.1 ps around 520 nm up to 1.7± 0.3 ps around 460 nm. Based on results obtained for ethylviolet, Martin et al. [13] argued that the ESA band was due to absorption from S 1 and from adark Sx intermediate state, the latter dominating at longer times. In line with that, the longerdecay time observed at shorter wavelengths supports the existence of two states contributing tothis positive transient signal for MG as well.

Between 660 nm and 720 nm a third time constant is required to account for the ps-livedabsorption. It varies from 3.3± 0.5 ps at 720 nm to 6.2± 0.5 ps at 660 nm. A fitting curve isdisplayed in Fig. 3(b), along with the experimental kinetic trace at 690 nm. To our knowl-edge, this wavelength dependence of the absorption signal decay has never been reported.This observation strongly supports the assignment of the absorption band to a distribution ofvibrationally-excited ground state molecules, which narrows on a few-picosecond time scalewhile intermolecular thermalization occurs with the solvent thermal bath [14].

In the 530-660 nm range, the transient absorption signal is the superposition of bleach, ESAand SE. In this range, no reliable fit could be obtained with a sum of exponential functions,indicating that a more complex relaxation scenario from S 2 may apply [9]. A more detailedanalysis is out of the focus of this paper and will be presented elsewhere.

5. Conclusion

Using a non-amplified Ti:Sa oscillator, we produce pulses of white light extending down to450 nm and spanning over� 300 fs in time, in a sub-cm short piece of a PCF. The single-pulsedSC can be used directly as a probe beam for ultrafast transient spectroscopy with broadbanddetection and a 150-fs FWHM time resolution. High shot-to-shot stability and very sensitivepump-induced signal detection are demonstrated. We perform spectrally resolved transient ab-sorption spectroscopy of MG in ethanol, excited to the S2 state. This work illustrates that accu-rate broadband ultrafast spectroscopy can now be implemented with non-amplified Ti:Sapphirelaser sources. This bears many advantages not only for the study of molecules in solution, butalso for solid-state spectroscopic surveys. Preliminary data analysis shows excellent agreementwith previous observations, and new interesting insights arise from the broadband detection. Inparticular, the wavelength dependence of the transient absorption signal in the spectral range660-720 nm strongly supports the presence of a vibrationally “hot” ground state.

Acknowledgments

Many helpful suggestions from B. Zietz and J.-Y. Bigot are gratefully acknowledged. Theproject is funded by the CNRS and the Universite Louis Pasteur, Strasbourg (“BQR 2005”).



broadband ultrafast spectroscopy using a photonic crystal fiber: application to the photophysics...

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