ultrafast two-laser pump-probe measurement using temporally incoherent lights

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Ultrafast twolaser pumpprobe measurement using temporally incoherent lights Fujio Minami and Atsushi Hasegawa Citation: Applied Physics Letters 54, 978 (1989); doi: 10.1063/1.100754 View online: http://dx.doi.org/10.1063/1.100754 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Two-tint pump-probe measurements using a femtosecond laser oscillator and sharp-edged optical filters Rev. Sci. Instrum. 79, 114901 (2008); 10.1063/1.3020759 Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser Appl. Phys. Lett. 93, 151106 (2008); 10.1063/1.2998648 Pump–probe Faraday rotation magnetometer using two diode lasers Rev. Sci. Instrum. 76, 056105 (2005); 10.1063/1.1912688 Measurement of silicon surface recombination velocity using ultrafast pump–probe reflectivity in the near infrared J. Appl. Phys. 88, 6954 (2000); 10.1063/1.1316047 Simulation of ultrafast dynamics and pump–probe spectroscopy using classical trajectories J. Chem. Phys. 104, 6919 (1996); 10.1063/1.471407 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.114.34.22 On: Tue, 02 Dec 2014 11:30:03

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Page 1: Ultrafast two-laser pump-probe measurement using temporally incoherent lights

Ultrafast twolaser pumpprobe measurement using temporally incoherent lightsFujio Minami and Atsushi Hasegawa Citation: Applied Physics Letters 54, 978 (1989); doi: 10.1063/1.100754 View online: http://dx.doi.org/10.1063/1.100754 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Two-tint pump-probe measurements using a femtosecond laser oscillator and sharp-edged optical filters Rev. Sci. Instrum. 79, 114901 (2008); 10.1063/1.3020759 Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser Appl. Phys. Lett. 93, 151106 (2008); 10.1063/1.2998648 Pump–probe Faraday rotation magnetometer using two diode lasers Rev. Sci. Instrum. 76, 056105 (2005); 10.1063/1.1912688 Measurement of silicon surface recombination velocity using ultrafast pump–probe reflectivity in the near infrared J. Appl. Phys. 88, 6954 (2000); 10.1063/1.1316047 Simulation of ultrafast dynamics and pump–probe spectroscopy using classical trajectories J. Chem. Phys. 104, 6919 (1996); 10.1063/1.471407

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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Page 2: Ultrafast two-laser pump-probe measurement using temporally incoherent lights

Ultrafast twoaiaser pumpwprobe measurement using temporally incoherent lights

Fujio Minami and Atsushi Hasegawa Research Institute of Applied Electricity, lIokkaido University, Sapporo 060, Japan

(Received 31 October 1988; accepted for publication 3 January 1989)

We propose a new type of pump-probe method to investigate picosecond and femtosecond processes. This method utilizes the intensity correlation between two different temporally incoherent lights. It is demonstrated theoretically that the time resolution is determined by the correlation time independently of the pump and probe pulse widths. The validity of the theoretical consideration is confirmed by lifetime measurements for excited dye molecules.

Recent advances in ultrafast transient spectroscopy have permitted research of the dynamical behavior of many physical systems with time resolution ofless than 1 ps. Most work has been made by using ultrashort optical pulses. In particular, the pump-probe technique using femtosecond pulses has been recognized as the most powerful method and is applied to investigations in the time scale of vibrational periods in condense media, carrier scattering times in semi­conductors, and chemical reactions and structural dynamics involved in phase transitions. I Another promising method for the investigation of rapid processes is the use of temporal­ly incoherent light with a short correlation time. This type of experiment has been shown to be capable of obtaining relax­ation information in the femtosecond regime, especially by way of transient four-wave mixing. 2

.3 Further, it has recently

been demonstrated that the measurement of the population relaxation time is possible by using the optical sampling technique in which the incoherent light is employed as a gating pulse.4

•5 The pump-probe measurement using tem­

porally incoherent light has been also applied to the study of population relaxation. 6 However, the experiment was made by the one-laser technique in which the pump and the probe pulses have the same frequency and only the absorption change at the pumping frequency is monitored. Consequent­ly, information on changes at other frequencies could not be obtained. We have devised a new pump-probe technique which avoids this problem and permits measurement, with picosecond or femtosecond resolution, of the characteristic time in many systems. The technique utilizes the intensity correlation between dye and pump laser pulses. This mea­surement scheme has thdlexihility of permitting the use of many different pumping and probing wavelengths providing detailed information on ultrafast relaxation processes. In this letter, we describe the principle of this method and verify this principle by applying it to lifetime measurements of ex­cited dye molecules.

I t is known that a broad dye laser pulse has an amplitude substructure and the intensity fluctuations in the pulse are highly correlated with those in a pump pulse, 7,8 although the phase correlation does not exist between the pulses. We have investigated the intensity correlation between the dye and pump pulses, by measuring the cross-correlation function which is defined as

(1)

where II (I) and 12 (I) are the time-varying intensities of the pump and dye laser pulses. A block diagram of the experi­mental setup is shown in Fig. 1. The laser system consists of a frequency-doubled Q-switched Nd:YAG laser (532 nm) and a rhodamine B dye laser. The repetition rate and pulse width of both laser outputs were 20 Hz and ~ 7 ns. The dye laser beam ( - 595 nm) was passed through a variable delay before being mixed in a nonlinear crystal, beta barium borate (,B-BBO), with the 532 nm beam to form sum frequency. The sum frequency light was analyzed in a spectrometer and detected by a photomultiplier. The signal averaging was made by a boxcar integrator. Measurement ofthe integrated intensity of the mixing signal versus relative delay between two pulses results in a direct measurement of G12 ( r). The obtained cross-correlation trace G!2( 'I) is displayed in Fig. 2. It is found from the figure that GI2 (r) can be expressed as follows:

(2)

where G7z is a slowly varying part with a width of the laser pulse duration tp (-7 ns) and G f2 is the "noise burst" with a length of the fluctuation correlation time 'Ie (-50 ps). G 1;2 (7) is almost constant in this delay range and makes a flat background. The ratio of G f2 (0) to G?2 (0) is fairly large (~ 1.8: I). This ratio and the width of the noise burst 7c

are dependent on the pump laser power. With decreasing pump power, the ratio G i\ (O)/G Y2 (0) becomes smaller and 'Ie longer. When the pump power increases, 'Ie ap­proaches the value of the fluctuation correlation time of the pump laser pulses (~50 ps). This suggests that 'Ie might become smaller when using pump pulses with faster intensi-

Q-sw YAG laser

SHG'"

B.S.

~_6_~_~~~d~y~~~-+-+~ ______ ,

~~. (Rhodamine B)

FIG. 1. Instrumental block diagram: beamsplitter CBS), photomultiplier (PMT).

978 Appl. Phys. Lett. 54 (11),13 March 1989 0003-6951/89/110978-03$01.00 @ 1989 American Institute of Physics 978

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Page 3: Ultrafast two-laser pump-probe measurement using temporally incoherent lights

~ 2 c: :::l

.ci ..... 0

>-

,~ -~ <fl c: <!J .... C

G~,(O)

0 -100 Delay

FIG, 2. Cross-correlation trace between the pump (532 !lrn) and dye (595 nm) laser pulses.

ty fluctuations. Details of the pump power effect on rc will be published elsewhere.

We consider the signal (gain) in pump-probe experi­ments using two temporally incoherent lights which have a cross correlation as shown in Fig. 2. The case considered here is that the transmission of a delayed probe pulse with fast intensity fluctuations, frequency (uz, is modified by exci­tation of a transition by a pump pulse of frequency 0) 1 , whose intensity is also fluctuating. Coherent interaction between pump and probe is not taken into account. For excitation during time dt . the gain experienced at frequency ([) 2 is given by the expression

dg(t) = KIJ (t ')RCt - t ')dt " (3)

where R (t) is the response function of the gain medium to delta function excitation, I[ (t) is the time-varying intensity of the pump pulse, and K is a constant. The time dependence of the gain is found by integrating Eq. (3),

o 100 Delay Ti m eo

200 (p s )

300 J

FIG. 3. Change in probe energy vs time fo; DTC in water.

979 AppL Phys. Lett, Vol. 54, No, 11, i 3 March 1989

g(t)=Kf' dt'IJ(t')R(t-t'). (4)

In a small-signal gain regime, the change in probe beam in­tensity caused by excitation is written as LI2 (t - r)g(t), where L is the length of the interaction region, 12 (t - 'I) is the intensity ofthe probe beam, and T is the time delay ofthe probe pulse with respect to the pump. The change in trans­mitted energy 8E( r) is then given by

bEer} =L JOG oc dtI2 (t-r)g(t). (5)

Substituting Eq. (4) into the integration on the right-hand side of Eq. (5) and changing variable from t - t' to t' + 'I, one finds

8E( r) = LK foor dt 'R (t' + r) f~>C ~ dtl2 (t -+- t' )11 (t)

= LK r: dtR( r - t)G I2 (t)· (6)

Here we consider popUlation relaxation processes. The re­sponse function R(t) then has the exponential form R(t) = exp( - t Itf ), with the lifetime tf and Eq. (6) is written as

8E(r) = LK exp( -r) f' dt exp(~)GI2(t). (7) tf .,~ tf

In the case of rc ~tJ4,Jp, Eq. (7) reduces to

8E( r) = LK l tf G 1;2 (r)

+1"cGf2(O)exp( -Tltf )], for r>O. (8)

It is found from this equation that information on the life­time (the second term ofthe right-hand side) appears on the flat background formed by G 1:2 ( r). This means that the time resolution is determined by the cross-correlation time rc of the pump and probe pulses independently ofthe widths of both pulses. When using the pump and probe pulses with the correlation time shown in Fig. 2, therefore, the time reso­lution becomes ~ 50 ps. The ratio bet ween the peak and background heights is calculated to be reG f2 (O):t;G(;2 (0).

To verify the above arguments, the temporal change in optical gain of DTC dye (3,3'-diethylthiacarbocyanine io­dide) in water was measured. The experimental setup is the same as that shown in Fig. 1, except that the nonlinear crys­tal is replaced by a sample. The 532 urn beam from the Nd:YAG laser was split into two beams. One was used to pump the dye laser and the other to excite the dye molecules from the ground state So to the first excited singlet state S] . The delayed dye laser output was employed to probe the time-varying induced changes in optical gain which follow the excitation. The change in the gain corresponds to that in the population of S] . Thus, the lifetime of S] is obtained by monitoring the decay of the gain experienced by the probe beam. Q In the experiment, the optical gain was measured at lower energies than the position of emission peak (- 580 urn). In Fig. 3, the change in transmitted energy of the probe pulse (~595 nm) is plotted for various relative time delays between the pump and probe pulses. Note that, in contrast to the case using the one-laser technique, (} the "coherent arti-

Po Minami and A. Hasegawa 979

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Page 4: Ultrafast two-laser pump-probe measurement using temporally incoherent lights

;3 (\

t'l I \ ~1' I ~ - I ~ C) I ~

o -100 0 100 200 300 400 Delay Time (ps)

FIG, 4, Fluorescence decay curve of DTC in water, The d~cay curve was obtained by using a mode-locked Ar + laser and 11 synchroscan streak cam­era,

fact" does not appear in the present experiment, because the pump and the probe have different frequencies. As predicted by Eq. (8), an exponential decay curve sits on a nearly flat background. The rise time of this decay is determined entire­ly by the time resolution in the present system (7c ~ 50 ps). From a log plot of the decay, the population relaxation time of 5\ can be estimated as-ISO ps, which is in agreement with the value (-160 ps) for the same solution obtained by conventional lifetime measurement using a streak camera in our laboratory (cr. Fig. 4). The ratio between the peak and

980 Appl. Phys, Lett., Vol. 54, No. 11,13 March i 989

background heights in the pump-probe gain trace is 1: ~ 1.6, which corresponds well to the prediction of Eq. (8) [ TeG f2 (O)/tfG ~2 (0) = ~O.6]. These results warrant the theoretical analysis we made above.

In summary, we have presented a new pump-probe technique with picosecond or femtosecond time resolution using two temporally incoherent lights. This scheme can clearly be extended to the case of two independent, simulta­neously pumped dye lasers. Many different pumping and probing wavelengths make this approach invaluable for sev­eral applications in femtosecond spectroscopy.

We acknowledge excellent discussions with Dr. S. Asaka and Professor K.lnoue, We also thank K. Watanabe for his technical assistance.

'See, for example, Ultrafast Phenomena V, edited by G, R Fleming and A. E. Siegman (Springer, Berlin, 1986),

2S, Asaka, H. Nakatsuka, M. Fujiwara, and M. Matsuoka, Phys, Rev. A 29, 2286 (1984).

'N. Morita and T. Yajima, Phys. Rev. A 30,2525 (1984). 's. Asaka and Ko Watanabe, Ultrafast Phenomena VI, edited by T. Yajima, K. Yosniwara, C. B. Harris, and S. Shionoya (Springer, Berlin, 1988), p. 375.

'H. Nakatsuka, Y. Katashima, K. Inouye, and R. Yano, Ultrafast Phenom­ena VI, edited by T. Yajima, K. Yoshiwara, C. B. Harris, and S. Shionoya (Springer, Berlin, 1988), p. 381.

"M. Tomita and M. Matsuoka, J. Opt. Soc. Am. B 3, 563 (1986), 7J. Kluge, D. Wiechert. and D. von deT Linde, Opt. Commun, 51, 271 (1984).

'J. Aaviksoo, A, Anijalg, A. Freiberg, and K. Timpmann. App!. Phys. B 37, 213 (1985).

oW. To Barnes and F. E. Lytle, App!. Phys. Lett. 34, 509 (1979).

F, Minami and A. Hasegawa 980

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