Investigation of vibrational characteristics in BBO crystals by femtosecond CARS

Investigation of vibrational characteristics in BBO crystals by femtosecond CARS

Optics & Laser Technology 44 (2012) 2049–2052 Contents lists available at SciVerse ScienceDirect Optics & Laser Technology journal homepage: www.els...

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Optics & Laser Technology 44 (2012) 2049–2052

Contents lists available at SciVerse ScienceDirect

Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Investigation of vibrational characteristics in BBO crystals by femtosecond CARS Yuanqin Xia n, Yang Zhao, Zi Wang, Sheng Zhang, Zhiwei Dong, Deying Chen, Zhonghua Zhang National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150080, China

a r t i c l e i n f o

abstract

Article history: Received 17 January 2012 Received in revised form 6 March 2012 Accepted 21 March 2012 Available online 10 April 2012

Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) is utilized to study the ultrafast vibrational dynamics in BBO crystals at room temperature. Time-resolved two-beam and three-beam CARS are detected. The vibrational dephasing time is analyzed and the changes of vibrational mode intensities with the polarization of pump pulses are observed. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Femtosecond laser pulses Time-resolved CARS BBO vibrational dephasing

1. Introduction Coherent anti-Stokes Raman spectroscopy (CARS) has many advantages such as high spatial and temporal resolutions, high sensitivity, etc. Femtosecond time-resolved CARS is a powerful spectroscopic tool for studying the vibrational dynamics of high frequency Raman modes in time domain since the 1980s for some decades [1,2]. This technique has been widely used in different samples, such as polydiacetylene [3,4], porphyrin [5,6], toluene [7], methanol–water [8], ethanol [9], and dipicolinic acid [10–12]. Recently some scientists used the femtosecond time-resolved CARS to study the coherent vibrational process in crystals [13,14]. Liu et al. [15] gained as broad as 12,000 cm  1 CARS signals using two crossing femtosecond laser pulses. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. BBO crystals are the abbreviation of b phase barium metaborate crystals and have a wide transmission range, a remarkable nonlinear coefficient, a high damage threshold, etc. They are often used for frequency doubling or in optical parametric amplifiers. The research of BBO crystals focuses on the mechanical and optical properties, and its applications in laser generation; however, research for the ultrafast dynamics process in BBO crystals is relatively rare. In this paper, two-beam CARS is used to get duration of the laser beams, and then three-beam CARS is used to study the coherent vibrational dynamics process of BBO crystals. The intensities

n

Corresponding author. Tel./ fax: þ86 451 86412753. E-mail address: [email protected] (Y. Xia).

0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.03.028

of vibrational modes are controlled by using the polarization of pump pulses.

2. Experimental The experimental setup used for the time-resolved CARS is shown in Fig.1. The pulses from a commercial femtosecond laser system Ti:sapphire regenerative amplifier (MicraþLegend, Coherent) at 800 nm (2.5 mJ, 1 kHz and 40 fs) were divided into two parts by a 1:1 beam splitter. One beam was used as the Stokes beam (k3) and the other was used to pump an optical parametric amplifier (OPA, TOPAS, Light Conversion). The output of OPA was split into two parts by a 1:1 beam splitter to obtain the pump and the probe beams (k1 and k2). The three beams were attenuated with power of 5–10 mW by a variable neutral-density (ND) filter. The generation of CARS requires spatial and temporal overlap of the pulses in the samples. The relative timing among the different beams was varied by computer controlled delay stages (Jump Star, mmt32-150 with resolution of 0.625 mm). The three beams were aligned parallel to each other, and overlapped in the foldedBOXCARS beam geometry (4 mm spot diameter, square with 10 mm sides). The three beams were focused on a 0.1-mm-thick BBO (y ¼29.51) using a 250 mm focal lens at room temperature. This folded-BOXCARS beam geometry ensures that the CARS signal propagates in a direction different from those of the incoming beams and can therefore be background-free collected. The CARS signal was filtered by a spatial filter (a pinhole) and were detected by two methods. First, it was spectrally dispersed in a monochromator (Omni-l500, Zolix) and detected by a photomultiplier tube (PMT) with a lock-in amplifier (SR830,

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200 150 100 50 0 Fig. 1. Femtosecond time-resolved CARS experimental device.

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Fig. 3. Dependence of two-beam CARS signal on pump delay of BBO crystals (670 nm pump, 800 nm Stokes).

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Fig. 2. Time-resolved two-beam CARS of BBO crystals (675 nm pump, 800 nm Stokes).

Stanford Research Systems). Second, it was directly recorded with a fiber optic spectrometer (HR4000, Ocean Optics).

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Fig. 4. Time-resolved three-beam CARS spectrum of BBO crystals (675 nm pump, 800 nm Stokes).

3. Results and discussion 1.0

The time-resolved two-beam CARS of BBO crystal (675 nm pump, 800 nm stokes) is shown in Fig. 2. The signal is collected in 2k3  k1 direction. There are two islands in the contour plot which can be seen clearly in Fig. 2. We can see that two laser beams first stimulate the vibrational mode of 3100 cm  1 and then after 40 fs stimulate the vibrational mode of 2750 cm  1 from Fig. 2. Raman vibrational modes can be distinguished in time domain by using pulse chirp characteristics. Furthermore the duration [intensity full width at half maximum (FWHM)] of pump and Stokes beams can be got from timeresolved two-beam CARS. Rocha-Mendoza et al. [16] clearly show that CARS response for Gaussian pulses pexpð4t 20 =4:14t2 Þ=t2 , where t0 is the time delay and t is FWHM of the Gaussian pulses. The dependence of two-beam CARS signal on pump delay of BBO crystal (670 nm pump, 800 nm stokes) is shown in Fig. 3. Fitting the dependence of CARS signal on time delay using CARS response pexpð4t 20 =4:14t2 Þ=t2 , we can get FWHM of pump and Stokes beams as 42 71.2 fs. This result is similar to FWHM of the legend laser pulse. 3.2. Three-beam CARS The time-resolved three-beam CARS of BBO crystal (675 nm pump, 800 nm Stokes) is shown in Fig. 4. The signal is collected in k1 þk2  k3direction.

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Fig. 5. CARS of BBO crystals under pump pulse of different polarizations (675 nm pump, 800 nm Stokes).

Three-beam CARS is different from two-beam CARS. Only the vibrational mode of 3100 cm  1 appears clearly in three-beam CARS. The vibrational mode of 3700 cm  1 is weaker. The vibrational mode of 3700 cm  1 is not detected in the two-beam CARS, which may be due to different chirps between the pump and the probe beams. The polarization of pump pulses will influence the intensity of the vibrational mode. The CARS of BBO crystals under pump pulses of different polarizations is shown in Fig. 5. The degree is polarization difference between the pump and Stokes pulses. From Fig. 5 we can see that the polarization of pump pulses will

Y. Xia et al. / Optics & Laser Technology 44 (2012) 2049–2052

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Fig. 6. Dependence of three-beam CARS signal on probe delay of BBO crystals: (a) 670 nm pump, 800 nm Stokes and (b) 690 nm pump, 800 nm Stokes.

influence the ratio of intensity of vibrational modes. The reason is that polarizability tensor influences vibration modes. Ney et al. [17] properly obtained the vibration modes of BBO crystals by taking account of the light polarization. The Raman lines are different in different crystal geometries. So we can enhance different vibrational modes by the polarization of pump pulses or crystal geometries. Du et al. [18] present a theoretical description of using the femtosecond time-resolved CARS technique to study coherent phonon dynamics in crystals. The formula of time-integrated CARS signal for the case of two excited vibrational modes (Eq. (9) in Ref. [18]) is similar to function (1) in Ref. [8]. The formula is suitable for fitting CARS signals of BBO crystals. The frequency difference Do of the vibrational modes and their respective dephasing times can be obtained from the quantum beating signal. The dependence of three-beam CARS signal on probe delay of BBO crystal (a: 670 nm pump, 800 nm Stokes; b: 690 nm pump, 800 nm Stokes) is shown in Fig. 6. There is no quantum beating signal in Fig. 6a, which means that only one Raman mode is excited by pump and Stokes beams. However, there are two vibrational modes at time delay of zero in Fig. 4. This is ascribed to the vibrational mode of 3700 cm  1 which is much weaker than the vibrational mode of 3100 cm  1. Fitting the dependence of CARS signal on time delay (Fig. 6a), we can get dephasing time for inhomogeneous broadening as 116 712 fs. We observe the quantum beating between the two modes in Fig. 6b. The quantum beating signal is due to the different vibrational modes that are excited simultaneously because of the large spectral bandwidth of the femtosecond laser pulses and the close gap of neighboring Raman levels. In order to eliminate the strong DFWM signal in studying the quantum beating, we analyze the data of the time delay only larger than 60 fs. There are two methods to process the data. First is Fast Fourier Transform (FFT). The corresponding beating period in the time domain is approximately 112 fs. The frequency difference from the FFT is 308 cm  1. Second is biexponential fitting. The oscillations can be fit also with function (1) in Ref. [8]. The fitting results are T1 ¼130730 fs, T2 ¼ 9777 fs, and Do ¼49.571.8 mrad/fs (26379.5 cm  1). The corresponding dephasing times for the two vibrational modes are determined to be 130730 fs and 9777 fs. There is a big gap for Do between the two methods. The reason is that less period was detected due to a very strong DFWM signal [19] and the number of vibrational modes that are excited simultaneously was larger than two.

4. Conclusion In this article, through a femtosecond time-resolved CARS experimental platform, the ultrafast dynamics process of BBO

crystals was studied. The ratio of intensity of vibrational modes can be changed by the polarization of pump pulses or crystal geometries. When pump pulse are at 690 nm, there are oscillations in three-beam CARS. The frequency differences between two vibrational modes are 263 79.5 cm  1. This work has proved that femtosecond time-resolved CARS is a powerful tool to investigate ultrafast vibrational modes dynamics in thin crystals.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant nos. 11174068, 60878018, 10774033), the Fundamental Research Funds for the Central Universities (Grant no. HIT.KLOF.2010033) and the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT. NSRIF.2009011).

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