Development of a multiplex fast-scan system for ultrafast time-resolved spectroscopy

Atsushi Yabushita, Yu-Hsien Lee, and Takayoshi Kobayashi

Citation: Review of Scientific Instruments 81, 063110 (2010); doi: 10.1063/1.3455809 View online: http://dx.doi.org/10.1063/1.3455809

View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/81/6?ver=pdfcov Published by the AIP Publishing

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Development of a multiplex fast-scan system for ultrafast

time-resolved spectroscopy

Atsushi Yabushita,1Yu-Hsien Lee,1and Takayoshi Kobayashi1,2 1

Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan

2

Department of Applied Physics and Chemistry and Institute for Laser Science,

University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan;

International Cooperative Research Project (ICORP), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan; and Institute of Laser Engineering,

Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971, Japan

共Received 1 February 2010; accepted 28 May 2010; published online 24 June 2010兲

A fast-scan method was developed to obtain time-resolved signals with femtosecond resolution over a picosecond range on the fly and in real time. Traditional fast-scan methods collect data at each probe wavelength one by one, which is time consuming and thus not possible for the study of photofragile materials. In this work, we have developed a system that performs fast scans with multiplex detection. Ultrafast time-resolved spectroscopy was demonstrated using the newly developed system. Femtosecond laser pulses have been used for pump-probe studies of ultrafast processes in various materials, and both electronic relaxation and vibrational dynamics have been studied. However, experiments have been limited in sensitivity and reliability because they are affected by the long-term instability of the ultrashort laser pulses and by the fragility of the samples. The instability of the sources hinders precise determination of electronic decay dynamics and introduces systematic errors. The fragility of the samples reduces their amount or concentration, and can lead to contamination of the materials even if they were pure before the measurement. These effects make it difficult to obtain reproducible and reliable experimental data. In the present work, we have developed a fast-scan pump-probe spectroscopic system that can complete a set of measurements in less than 2 min. Quantitative estimates of the signal reproducibility demonstrate that these measurements provide higher reproducibility and reliability than conventional measurements. © 2010 American Institute of Physics. 关doi:10.1063/1.3455809兴

I. INTRODUCTION

Time-resolved studies have been performed using a scanning optical delay line with a motorized stage. They have elucidated the ultrafast dynamics of various materials. In such studies, a lock-in amplifier共LIA兲 is typically used to improve the signal quality. But a LIA has several problems, including base-band detection and 1/ f noise, the time re-quired for data acquisition, and modulation of the laser flu-ence incident on the sample. Fast-scan acquisition of the sig-nals could solve these problems by collecting data on the fly and in real time.1 However, spectroscopic studies require measurement at each probe wavelength. Such iterative mea-surements are time consuming, and laser damage accumu-lates on the sample. As a result, the sample condition at the last probe wavelength can be considerably different from that at the first probe wavelength. To overcome that drawback, we have developed a new fast-scan system with multiplex detection. As an example of the application of this method, we use it to perform femtosecond time-resolved spectros-copy.

Ultrafast spectroscopy is a powerful method for investi-gating photochemical, photophysical, and photobiological processes. Photochemistry of various reactions has been

studied, including isomerization, proton transfer, and elec-tron transfer. Photophysical processes have been measured for carrier dynamics and nonlinear excitations. Photobiologi-cal processes have been clarified in vision and photosynthesis.2–4 Ultrafast spectroscopy also has various important applications to photosensors,5–7 ultrafast optical switches,8–10and ultrafast optical memories.11–13

Following the development of femtosecond lasers, ul-trafast spectroscopy blossomed14–17 and expanded into areas such as plasma diagnosis and in situ probes for laser manu-facturing. Subsequent development of sub-10-fs laser pulses18–23 elucidated the ultrafast dynamics24–28 of elec-tronic relaxations and molecular vibrations. After the devel-opment of the noncollinear optical parametric amplifier 共NOPA兲,29,30

ultrafast spectroscopy could be performed across wide spectral regions because its structure is broad-band and smooth. The resulting observation of real-time mo-lecular vibrations has provided information about changes in molecular structure during relaxation processes such as inter-nal conversion and intersystem crossing, and chemical reac-tions including cis-trans and trans-cis isomerization, proton transfer, and oxidation.28,31–37

However, it has been difficult to obtain reproducible re-sults for molecular vibrational modes across long time scales

REVIEW OF SCIENTIFIC INSTRUMENTS 81, 063110共2010兲

0034-6748/2010/81共6兲/063110/10/$30.00 81, 063110-1 © 2010 American Institute of Physics

of several hundred femtoseconds. The difficulty stems mainly from共i兲 accumulated damage to the samples and 共ii兲 long-term instabilities in the laser intensity.

In conventional ultrafast pump-probe measurements, the delay time is scanned in fine steps共less than several femto-seconds兲 across a long time span 共up to a few tens of pico-seconds兲 in order to observe molecular vibrational signals along with the electronic dynamics. The relaxation and dy-namics of molecular vibrational modes can be studied by observing changes in the instantaneous frequencies of the modes associated with corresponding electronic states. Therefore it is necessary to have both a long time range to follow the reactions and a sufficiently short time step to re-solve the molecular vibrations whose frequencies change during the electronic processes. One complete scan can take 1 h or more. If the sample is chemically sensitive to radia-tion, it can be damaged during a single scan. The accumula-tion of damage, which can be significant when the measure-ment time is long, makes it difficult to observe the intrinsic dynamics of a sample.

Molecular vibrational periods are on the order of several tens of femtoseconds, observed as a modulation of the⌬A trace in a pump-probe measurement during accumulation of about 100 laser shots at each delay-time step. The intensity of the ultrashort laser pulses, used for generating high order nonlinear effects, is affected by small fluctuations in the tem-perature, humidity, and airflow around the system. These small disturbances cause instabilities in the laser intensity, which make it difficult to observe the ⌬A modulation, be-cause both modulations have roughly the same period of several minutes.

In the present work, we have developed a fast-scan pump-probe system with a multichannel lock-in amplifier 共MLA兲 for multiplex detection. This new method, which we dub “fast-scan pump-probe spectroscopy,” improves the re-producibility and reliability of the experimental data. For comparison, conventional pump-probe measurements are first performed for a conjugate polymer film. The data are found to be limited in sensitivity and reliability because they are affected by instabilities in the light source and by the fragility of the samples. When the pulse duration is kept below 10 fs, which is shorter than typical molecular vibra-tional periods, it is difficult to maintain the long term stabil-ity of the laser, hindering determination of the decay dynam-ics of the electronic states and introducing systematic errors. The fragility of the samples reduces the amount and concen-tration of the samples and can cause contamination of the materials. These effects make it difficult to obtain reproduc-ible and reliable data. But the experimental data are strik-ingly improved by using the fast-scan pump-probe spectro-scopic system.

II. EXPERIMENT A. NOPA

We built a NOPA to generate visible laser light whose spectral width is broad enough to generate sub-10-fs pulses for ultrafast time-resolved spectroscopy. Details of the NOPA are presented in Appendix A. The smooth spectral

shape of its output makes it ideal for spectroscopy. The pump source of the NOPA consists of a regenerative chirped pulse amplifier共Legend-USP-HE from Coherent兲 seeded with a Ti: sapphire laser oscillator共Micra 10 from Coherent兲. The am-plifier generated femtosecond pulses whose duration, central wavelength, repetition rate, and average power were 40 fs, 800 nm, 5 kHz, and 500 mW, respectively. Each pulse from the regenerative amplifier was separated into two pulses by a beam sampler. The larger intensity pulse was used for second harmonic generation 共SHG兲 centered at 400 nm which pumped the NOPA. The other pulse 共with ten times weaker intensity兲 was focused onto a sapphire plate to generate white light by self-phase modulation, to be used as a seed beam for the NOPA. The resulting broadband visible output spectrum plotted in Fig.1extends from 530共18 868 cm−1兲 to 740 nm共13 514 cm−1_{兲 with constant phase. A}

beamsplit-ter separated the pump and probe pulses, whose intensities were adjusted using a variable neutral density filter. The en-ergies of the pump and probe pulses were 1 and 0.2 nJ, respectively. Using a beam compressor consisting of a dif-fraction grating telescopic dispersion line, multilayer dielec-tric chirped mirrors, and a computer-controlled flexible mir-ror, the broadband visible laser pulse was compressed to a 10 fs duration before it arrived at the sample surface. The probe pulse was dispersed by a polychromator 共SP2300i from Princeton Instruments兲 into a fiber bundle, the other end of which was separated into 128 fibers connected to avalanche photodiodes共APDs兲. The time-resolved transmittance differ-ence⌬T of these 128 probe wavelengths was simultaneously detected. The signals were then sent to a MLA with a high signal-to-noise ratio.

B. Mechanical stage for conventional pump-probe measurements

1. Step-scan method

To evaluate our new technique, we compared it to a pump-probe signal obtained using a conventional method. A signal was collected by recording ⌬A while scanning the delay time between the pump and probe pulses across a range of ⫺300 to 1500 fs in 0.6 fs steps. For analysis pur-poses, data with 3 fs steps were obtained by averaging sets of five data points together to suppress artifacts due to interfer-FIG. 1. Incident laser spectrum and absorption spectrum of a sample film.

ence between the pump and probe pulses. The stage used for the step scan共PFS-1020 from Sigma-Tech兲 had 10 nm 共1/15 fs兲 resolution with full closed-loop control. The positioning resolution was sufficient for ultrafast time-resolved spectros-copy because the time resolution was limited by the duration of the pump and probe pulses. However, the stage required about 600 ms to stabilize between movements, so that a single scan of the pump-probe measurement required at least 30 min.

C. Mechanical stage for novel pump-probe measurements: Fast-scan method

In our new method, the ⌬A signal is recorded while scanning the delay time rapidly in 500 steps across a range of 1790 fs. The fast-scan stage 共ScanDelay-15 from APE-Berlin兲 was controlled by an external voltage generated by a digital/analog converter共LPC-361316 from Interface兲, scan-ning from⫺0.2 to +0.8 V in 5 s, which is 360 times faster than the 30 min scans of the conventional method. At each delay point,⌬A was obtained in 10 ms, storing the data in the memory of a MLA. Average values were calculated for the data in sets of 24 scans to maintain a good signal-to-noise ratio. Therefore, the measurement time for a single run was 2 min共5⫻24 s兲. One can thereby avoid laser fluctuations hav-ing correlation times longer than that.

III. RESULTS AND DISCUSSION

The experimental setup, consisting of the NOPA, step-scan stage, and fast-step-scan stage is shown in Fig.2共a兲.

A. Calibration of the delay positions of the fast-scan stage

A 20-␮m-thick␤-barium borate crystal on a fused silica base plate was used to generate an autocorrelation signal from the broadband spectral width of the laser pulse. While scanning the fast-scan stage, the autocorrelation signal was monitored by a photomultiplier tube 共H9656–04 from Hamamatsu兲 connected to the last channel of the MLA sys-tem. As the step-scan stage moves, the autocorrelation signal on the fast-scan trace shifts with the same amount of delay. From the shift, the delay position of the fast-scan trace could be calibrated. The autocorrelation traces were obtained by changing the delay position of the step-scan stage in 50 fs steps. The peak positions of the autocorrelation signal are plotted in Fig. 2共b兲, giving a calibration curve showing a linear relationship between the delay time and data points of 3.58⫾0.01 fs/point.

B. Pump-probe measurements using the step-scan method

Pump-probe measurements of a conjugated polymer film were performed by the step-scan method. The preparation of the sample film is detailed in Appendix B. The absorbance change⌬A in the wavelength region from 515 to 753 nm in 2.5 nm steps was obtained by scanning the delay time from ⫺318 to 1482 fs in 0.6 fs steps 共forward scan兲. For each delay point,⌬A was obtained by accumulating for 0.6 s. The mean ⌬A spectra in 3 fs steps are shown in Fig. 3共a兲 in

two-dimensional共2D兲 form. The data in Fig.3共b兲are for the corresponding backward scan. The poor reproducibility be-tween Figs.3共a兲and3共b兲is indicative of sample damage and laser intensity instabilities.

The 2D traces give time-resolved⌬A spectra along their cross sections. The⌬A spectra at three delays 共147, 326, and 1221 fs兲 are plotted in Figs. 3共c兲 and3共d兲 for forward and backward scans, respectively. Damage accumulated in the sample causes scattering, which decreases ⌬A in panel 共d兲 compared to that in panel共c兲.

Time-resolved ⌬A traces at three probe wavelengths 共568, 575, and 616 nm兲 were also extracted from the 2D traces in Figs.3共a兲and3共b兲and plotted in panels共e兲 and 共f兲. In the forward scans, the time-resolved traces start around ⌬A=0 for negative delays. In contrast, there exists a large offset at negative delays for the backward scans. The offset is due to damage accumulated on the sample during the step-scan measurements.

Fourier power spectra of the⌬A traces reveal molecular vibrational modes of the sample. Figures 4共a兲and 4共b兲 are the 2D results for forward and backward scans, respectively. They show strong peaks around 1588 cm−1_{, corresponding}

to the most intense Raman mode 共see Fig. 2 of Ref. 38兲. However, they have low reproducibility between the forward and backward scans and are not identical in shape with Ra-man spectra because of the instability of the laser source. FIG. 2. 共Color online兲 共a兲 Experimental setup showing the NOPA: noncol-linear optical parametric amplifier, VND: variable neutral density filter, and APD: avalanche photodiode.共b兲 Calibration data for the fast-scan stage 共squares兲 and a linear fit to the data 共solid line兲.

063110-3 Yabushita, Lee, and Kobayashi Rev. Sci. Instrum. 81, 063110共2010兲

Fourier power spectra probed at 616 nm were extracted from Figs.4共a兲and 4共b兲 and plotted in panels 共c兲 and 共d兲. Irreproducibility of the step-scan data can be seen. Figures 4共e兲and 4共f兲 are Fourier power spectra of the most

promi-nent mode at 1588 cm−1_{, corresponding to the CvC}

stretching mode. They also lack reproducibility.

To study the dynamics of the molecular vibrational modes, a spectrographic analysis was performed. Spectro-FIG. 3.共Color online兲 Absorption changes obtained by the step-scan method for delay times of 共a兲 ⫺318 to 1482 fs and 共b兲 1482 to ⫺318 fs. Time-resolved ⌬A spectra at delays of 147, 326, and 1221 fs are plotted in 共c兲 and 共d兲, and ⌬A traces at 568, 575, and 616 nm are plotted in 共e兲 and 共f兲.

grams were obtained by a sliding-window Fourier transform using the Blackman window function,

S共␻,␶兲 =

冕

0 ⬁

S共t兲g共t −␶兲exp共− i␻t兲dt,

g共t兲 = 0.42 – 0.5 cos共2␲t/T兲 + 0.08 cos共4␲t/T兲,

where ␶ is the gate width, which we set to 600 fs,

corresponding to a full width at half maximum共FWHM兲 of 240 fs. The calculation of the spectrogram was performed after applying a high-pass filter to the signal because slow modulations of the signal共slower than tens of femtoseconds兲 arise from fluctuations in the laser power. For forward and backward scans, the results are shown in Figs.5共a兲and5共b兲, respectively, using a time-resolved⌬A trace probed at 616 nm. They again show poor reproducibility.

FIG. 4.共Color online兲 共a兲 and 共b兲 are Fourier power spectra of the ⌬A traces from Figs.3共a兲and3共b兲, respectively. Fourier power spectra at 616 nm are plotted in共c兲 and 共d兲. Fourier power spectra of the most prominent vibrational mode at 1588 cm−1_{are graphed in共e兲 and 共f兲.}

063110-5 Yabushita, Lee, and Kobayashi Rev. Sci. Instrum. 81, 063110共2010兲

C. Pump-probe measurements using the fast-scan method

After the preceding step-scan measurements, we moved the sample position to avoid the damaged area and began fast-scan measurements so that the experimental conditions remained identical. Twenty-four forward scans were obtained from⫺390 to 1396 fs. The mean of the 24 traces is plotted in Fig.6共a兲, and backward scanned data from 1396 to⫺390 fs are shown in Fig. 6共b兲. In contrast to the step-scan results, these data show high reproducibility.

Figures 6共c兲 and6共d兲 are time-resolved ⌬A spectra for the forward and backward scans, respectively, at delays of 147, 326, and 1221 fs. Time-resolved ⌬A traces probed at three wavelengths共568, 575, and 616 nm兲 are plotted for the forward and backward scans in Figs.6共e兲and 6共f兲, respec-tively. The improvement in the signal reproducibility was estimated by calculating a correlation coefficient between the time traces for the forward and backward scans from 150 to 1350 fs. Figure 6共g兲 shows the resulting correlation coeffi-cients for the step-scan and fast-scan measurements. One can see that the reproducibility is much higher in the fast-scan measurements than in the step-scan measurements.

Fourier power spectra of the ⌬A traces for the forward and backward scans were calculated and are plotted in Figs. 7共a兲and7共b兲, respectively. Similar to Figs.4共c兲and4共d兲,⌬A traces probed at 616 nm are plotted in Figs.7共c兲and7共d兲for forward and backward scans, respectively. They confirm the reproducibility of the Fourier power spectra and show good agreement with Raman spectra from previous work.38 Fig-ures 7共e兲 and 7共f兲 are Fourier power spectra of the most prominent vibrational mode at 1588 cm−1. Some small dif-ferences are evident at wavelengths longer than 640 nm. The small amplitude of⌬A in that spectral region 关see Figs. 6共a兲 and6共b兲兴 is probably the origin of the error.

Between 600 and 640 nm, where the Fourier power is high, the signal reproducibility was quantified by calculating the error in the determination of the power for molecular vibrational modes of 963, 1112, 1261, 1316, and 1587 cm−1_.

The relative difference in the Fourier power of a forward

scan共If兲 and a backward scan 共Ib兲 is 兩共If− Ib兲/共If+ Ib兲兩.

Fig-ures7共g兲 and7共h兲 shows the errors calculated for step-scan and fast-scan measurements, respectively. Evidently the Fou-rier power can be determined more precisely for the latter.

Using a Blackman gate with a FWHM of 240 fs, spec-trograms were calculated for the time-resolved traces ob-tained in the fast-scan measurement. The results are plotted in Figs.8共a兲and 8共b兲 for the forward and backward scans, respectively, and show good reproducibility. Therefore, the fast-scan method is preferable not only for the study of elec-tronic dynamics but also for the study of vibrational dynam-ics.

IV. SUMMARY

The fast-scan method obtains time-resolved signals with femtosecond resolution over a picosecond range on the fly and in real time. However, it is traditional in time-resolved spectroscopy to measure data at each probe wavelength one by one, which is time consuming and results in damage ac-cumulation on a sample.

In this paper we reported a new fast-scan system with multiplex detection. It is particularly useful for ultrafast spectroscopy with a fine time resolution. We assessed its ad-vantage over the traditional step-scan method by performing time-resolved spectroscopy using each method. The fast-scan pump-probe system can perform measurements within 2 min. The results show higher reproducibility and reliability than those obtained by the step-scan method.

ACKNOWLEDGMENTS

A.Y. was supported by the National Science Council of Taiwan 共NSC Grant No. 98-2112-M-009-001-MY3兲. A.Y. and T.K. received a grant from the Ministry of Education, Aim for the Top University Project共MOE ATU兲 at National Chiao Tung University共NCTU兲. We also thank the Interna-tional Cooperative Research Project共ICORP兲 program of the Japan Science and Technology Agency共JST兲.

FIG. 5. 共Color online兲 共a兲 and 共b兲 Spectrograms calculated from the time-resolved traces at 616 nm shown in Figs.3共e兲and3共f兲.

APPENDIX A: NOPA

The source laser is a regenerative chirped pulse amplifier 共Coherent Legend-USP兲 seeded with a fiber laser oscillator. The amplifier generates femtosecond pulses whose duration,

central wavelength, repetition rate, and average power are 40 fs, 800 nm, 5 kHz, and 2.5 W, respectively. The NOPA uses a phase-matching condition to generate amplified visible pulses with a broad spectral width of 200 THz. Angular dis-FIG. 6.共Color online兲 ⌬A traces obtained by the fast-scan method for delay times of 共a兲 ⫺390 to 1396 fs, and 共b兲 1396 to ⫺390 fs. Time-resolved ⌬A spectra at delays of 147, 326, and 1221 fs are plotted in共c兲 and 共d兲, and ⌬A traces at 568, 575, and 616 nm are plotted in 共e兲 and 共f兲. 共g兲 Correlation coefficient between the time traces of the forward and backward scans from 150 to 1350 fs for the step-scan and fast-scan measurements.

063110-7 Yabushita, Lee, and Kobayashi Rev. Sci. Instrum. 81, 063110共2010兲

persion of the pump beam共introduced by a Brewster prism兲 also contributes to broadening the spectral width of the pulses. The time duration of the pump pulses was stretched to decrease their peak intensity and to avoid damaging the optical components. A long duration of the pump pulses also helps reduce sensitivity to timing drifts, which improve the power stability of the broadband output.

The spectral width of the output pulses is large enough to generate sub-10-fs laser pulses. For time-resolved spec-troscopy, it is necessary to use ultrashort laser pulses to re-solve real-time molecular vibrations. Consequently the broadband visible laser pulses were compressed using two dielectric broadband chirped mirrors, a 300 lines/mm diffrac-tion grating in negative first order, and a 200 mm radius FIG. 7.共Color online兲 共a兲 and 共b兲 are Fourier power spectra of the ⌬A traces from Figs.6共a兲and6共b兲, respectively. Fourier power spectra at 616 nm are plotted in共c兲 and 共d兲. Fourier power spectra of the most prominent vibrational mode at 1588 cm−1_{are graphed in共e兲 and 共f兲. Errors in five vibrational mode} intensities for step-scan and fast-scan measurements are shown in共g兲 and 共h兲.

spherical mirror with a flexible mirror positioned in its focal plane. A horizontal grating angle was used for crude com-pression of the pulse width. To fine tune the pulse compres-sion, the voltages applied to the piezoelectric array behind the flexible mirror were adjusted over a tuning range of 6 ␮m. During the tuning process, the character of the ul-trashort pulses was monitored to guide the alignment proce-dure, using the SHG frequency-resolved optical gating 共FROG兲 technique. Feedback is provided to the piezoelectric array behind the flexible mirror. A translation stage共SIGMA TECH model STC-1020X兲, with a built-in interferometer and active position stabilization, is used to obtain a position accuracy of 10 nm. It was used in the FROG apparatus for both wide- and narrow-range delay scans. Two identical 2-␮m-thick pellicle beamsplitters, which have flat reflectiv-ity across the visible spectral range, were used to balance the dispersion in the two beams of the FROG apparatus. Both beams were focused onto a wedged ultrathin␤-BaB2O4

crys-tal for broadband SHG and were collimated behind it using a 200 mm off-axis parabolic mirror. The thickness of the wedge varied across the face of the crystal between 5 and 20 ␮m. By observing the spectral phase in the SHG FROG measurement, chirp compensation was performed by adjust-ing the 38 actuators of the flexible mirror共lined up in two rows that provide a clear aperture of 39⫻11 mm2_兲.

The broadband visible laser pulses were compressed to resolve real-time molecular vibrations whose frequencies are lower than 3000 cm−1_{. The setup of the SHG FROG}

mea-surement system is identical to that of the time-resolved pump-probe system used for the molecular vibrational analy-sis. Therefore, the SHG FROG setup was also used for the measurement of the time-resolved pump-probe signals. The thin beamsplitters used for the SHG FROG measurement divided the compressed ultrashort visible laser pulses into pump and probe beams.

APPENDIX B: SAMPLE FILM

A poly关2-methoxy-5-共20-ethyl-hexyloxy兲-p-phenylene vinylene兴 共MEH-PPV兲 film was used for pump-probe mea-surements. It was prepared by spin coating chloroform

solu-tions of MEH-PPV onto quartz plates to form 0.5– 1.0 ␮m thick films. An absorption spectrum of the MEH-PPV film is shown in Fig. 1, recorded using a scanning spectrophotom-eter 共UV-3101PC from Shimadzu兲. All measurements were performed at room temperature共295⫾1 K兲.

1_{M. J. Feldstein, P. Vohringer, and N. F. Scherer,J. Opt. Soc. Am. B12,} 1500共1995兲.

2_{F. Pellegrino, Opt. Eng. 22, 508共1983兲.}

3_{A. B. Rubin, L. B. Rubin, V. Z. Pashchenko, A. A. Kononenko, and B. A.} Gulyaev, Kvantovaya Elektron.共Kiev兲 7, 52 共1980兲.

4_{M. R. Wasielewski, P. A. Liddell, D. Barrett, T. A. Moore, and D. Gust,}

Nature共London兲322, 570共1986兲.

5_{A. Boyer, M. Dery, P. Selles, C. Arbour, and F. Boucher,Biosens.}

Bio-electron.10, 415共1995兲.

6_{K. J. Hellingwerf, J. Hendriks, and T. Gensch, J. Biol. Phys.28, 395} 共2002兲.

7_{Y. Imamoto and M. Kataoka, Photochem. Photobiol. 83, 40共2007兲.} 8_{G. Eichmann, Y. Li, and R. R. Alfano, Opt. Eng. 25, 91共1986兲.} 9_{D. J. Olego, R. Schachter, M. Viscogliosi, and L. A. Bunz,Appl. Phys.}

Lett.49, 719共1986兲.

10_{C. P. Singh, K. S. Bindra, B. Jain, and S. M. Oak,Opt. Commun.245, 407} 共2005兲.

11_{L. Kuhnert,Nature共London兲319, 393共1986兲.} 12_{S. Tazuke, Solid State Phys. 23, 61共1988兲.}

13_{N. E. Korolev, I. Y. Mokienko, A. E. Poletimov, and A. S. Shcheulin,}

Phys. Solid State127, 327共1991兲. 14_{A. H. Zewail,Science242, 1645共1988兲.}

15_{R. M. Bowman, M. Dantus, and A. H. Zewail,Chem. Phys. Lett.156, 131} 共1989兲.

16_{M. Gruebele, I. R. Sims, E. D. Potter, and A. H. Zewail,J. Chem. Phys.} 95, 7763共1991兲.

17_{M. Dantus and A. H. Zewail,Chem. Rev.104, 1717共2004兲.}

18_{A. Baltuska, Z. Wei, M. S. Pshenichnikov, and D. A. Wiersma,Opt. Lett.} 22, 102共1997兲.

19_{M. Nisoli, S. De Silvestri, O. Svelto, R. Szipocs, K. Ferencz, Ch.} Spielmann, S. Sartania, and F. Krausz,Opt. Lett.22, 522共1997兲. 20_{A. Shirakawa, I. Sakane, and T. Kobayashi,Opt. Lett.23, 1292共1998兲.} 21_{G. Cerullo, M. Nisoli, S. Stagira, and S. De Silvestri,Opt. Lett.23, 1283}

共1998兲.

22_{M. Adachi, K. Yamane, R. Morita, and M. Yamashita, Jpn. J. Appl. Phys.,} Part 1 44, 4123共2005兲.

23_{A. Baltuška, T. Fuji, and T. Kobayashi,Opt. Lett.27, 306共2002兲.} 24_{A. Shirakawa and T. Kobayashi,Appl. Phys. Lett.72, 147共1998兲.} 25_{A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi,Appl. Phys.}

Lett.74, 2268共1999兲.

26_{A. Baltuška, M. F. Emde, M. S. Pshenichnikov, and D. A. Wiersma,J.}

Phys. Chem. A103, 10065共1999兲.

FIG. 8.共Color online兲 共a兲 and 共b兲 Spectrograms calculated form the time-resolved ⌬A traces at 616 nm plotted in Figs.6共e兲and6共f兲.

063110-9 Yabushita, Lee, and Kobayashi Rev. Sci. Instrum. 81, 063110共2010兲

27_{G. Cerullo, G. Lanzani, M. Muccini, C. Taliani, and S. De Silvestri,Phys.}

Rev. Lett.83, 231共1999兲.

28_{R. A. G. Cinelli, V. Tozzini, V. Pellegrini, F. Beltram, G. Cerullo, M.} Zavelani-Rossi, S. D. Silvestri, M. Tyagi, and M. Giacca,Phys. Rev. Lett.

86, 3439共2001兲.

29_{W. T. Pollard, S.-Y. Lee, and R. A. Mathies, J. Chem. Phys.92, 4012} 共1990兲.

30_{T. Kobayashi, T. Saito, and H. Ohtani,Nature共London兲414, 531共2001兲.} 31_{T. Taneichi, T. Fuji, Y. Yuasa, and T. Kobayashi,Chem. Phys. Lett.394,}

377共2004兲.

32_{A. Colonna, A. Yabushita, I. Iwakura, and T. Kobayashi, Chem. Phys.} 341, 336共2007兲.

33_{A. Yabushita and T. Kobayashi,Biophys. J.96, 1447共2009兲.}

34_{T. Kobayashi, I. Iwakura, and A. Yabushita, New J. Phys. 10, 065016} 共2008兲.

35_{G. Cerullo, G. Lanzani, L. Pallaro, and S. De Silvestri,J. Mol. Struct.521,} 261共2000兲.

36_{T. Cimei, A. R. Bizzarri, G. Cerullo, S. De Silvestri, and S. Cannistraro,}

Biophys. Chem.106, 221共2003兲.

37_{A. Gambetta, C. Manzoni, E. Menna, M. Meneghetti, G. Cerullo, G.} Lanzani, S. Tretiak, A. Piryatinski, A. Saxena, R. L. Martin, and A. R. Bishop,Nat. Phys.2, 515共2006兲.

38_{T. P. Nguyen, S. H. Yang, P. Le Rendu, and H. Khan,Composites, Part A} 36, 515共2005兲.