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Efficient generation of vibrational stimulated Raman emission in the ultraviolet region using a femtosecond pump beam
Optics Communications 265 (2006) 603–606 www.elsevier.com/locate/optcom
Efficient generation of vibrational stimulated Raman emission in the ultraviolet region using a femtosecond pump beam Fumiaki Kira, Masahiro Matsuse, Shin-ichi Zaitsu, Totaro Imasaka
*
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan Received 9 August 2005; received in revised form 31 March 2006; accepted 31 March 2006
Abstract The third harmonic emission (261 nm, 30 lJ) of a femtosecond Ti:sapphire laser is focused into molecular hydrogen to generate vibrational stimulated Raman emission. High-order emission lines are efficiently generated by using a high-beam-quality nearly-transformlimited laser and high-pressure hydrogen (40 atm) as a Raman medium. 2006 Elsevier B.V. All rights reserved.
1. Introduction A femtosecond laser has been utilized in studies of ultrafast phenomena and dynamics. Numerous atoms and molecules, e.g., aromatic hydrocarbons, have absorption bands in the ultraviolet region. However, a widely tunable femtosecond laser has seldom been used in this spectral region. For example, Baum et al. have reported the generation of pulses as short as 7 fs by achromatic frequency doubling of a noncollinear optical parametric amplifier (NOPA; 450–1650 nm, 10–50 fs, some 100 nJ) [1]. On the other hand, Jailaubekov et al. have reported the generation of tunable 30-fs pulses (224–240 nm, >1 lJ) by means of phase-matched four-wave mixing (FWM) in an argon-filled hollow waveguide using an optical parametric amplifier (OPA, 1200–2200 nm) and a Ti:sapphire laser (800 lJ, 1 kHz, 110 fs, 800 nm) [2]. In addition, a traveling wave optical parametric amplifier of superfluorescence (TOPAS, 189 nm to 18 lm, 30 fs to 30 ps, 1–500 lJ) has already been commercially available. These advanced technologies are, however, complicated and is very expensive. Stimulated Raman scattering and subsequent four-wave Raman mixing have been employed for frequency conversion in the ultraviolet region, as well as in the visible and infrared *
Corresponding author. Tel.: +81 92 802 2883; fax: +81 92 802 2888. E-mail address: [email protected] (T. Imasaka).
0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.067
regions. In fact, a femtosecond excimer laser emitting at 248 nm has been used to generate high-order vibrational and rotational Raman lines [3]. In this study, a high-energy (10–20 mJ) nearly-transform-limited (Dt = 150–500 fs, Dk = 0.9 nm) pump beam is focused into hydrogen gas pressurized at 5–13 atm. A femtosecond Ti:sapphire laser has been proposed for use, to obtain a tunable laser. The second harmonic emission (390 nm) has already been employed for the generation of high-order vibrational and rotational lines [4]. In this work, a chirped femtosecond pulse (300 fs, 100 lJ) is focused into hydrogen gas pressurized to 70 atm. A variety of aromatic hydrocarbons, e.g., benzene and derivatives thereof, have absorption bands in the vicinity of 260 nm. Because of this, it would be desirable to generate a tunable femtosecond laser in the deep-ultraviolet region (200–350 nm). For this purpose, it is possible to use the third harmonic emission of a Ti:sapphire laser as a pump beam. Unfortunately, the pulse energy of the third harmonic emission is relatively small, especially for the 1-kHz Ti:sapphire laser that is in current use in spectroscopic applications. Therefore, it is necessary to conduct basic research aimed at improving the efficiency of generation of a stimulated Raman emission using an ultraviolet femtosecond laser. In this study, we report on the generation of high-order vibrational Raman lines in the deep-ultraviolet region using a simple and less expensive device, i.e., a Raman cell.
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F. Kira et al. / Optics Communications 265 (2006) 603–606
To improve the efficiency of generation of the vibrational Raman emission, an ultraviolet beam (261 nm, 30 lJ) generated by the third harmonic generation of a Ti:sapphire laser having a high-beam-quality (M2 = 1.2) and a nearly-transform-limited bandwidth (Dt · Dm = 0.52) was focused into hydrogen gas pressurized at 40 atm. This approach was found to efficiently generate first (293 nm) and second (334 nm) Stokes emissions. It should be noted that the first anti-Stokes Raman emission (236 nm) is also observed through four-wave Raman mixing thus allowing the conversion of the femtosecond laser emission to even shorter wavelengths. 2. Experimental The experimental apparatus used in the studies is shown in Fig. 1. A femtosecond Ti:sapphire laser (Thales, Concerto, k = 784 nm, Dk = 10 nm, Dt = 100 fs, 1.1 mJ, M2 = 1.2, 1 kHz) was used as the pump beam. The diameter of the laser beam was changed by a pair of lenses (L1, f = 500 mm; L2, f = 250 mm), to improve the efficiency of generating the third harmonic emission using a third harmonic generator (Spectra-Physics, HGS-T). In this instrument, the fundamental beam (783 nm) is passed through a KDP crystal (thickness, 1.5 mm; cutting angle, 44.5) for second harmonic generation. The fundamental and second harmonic beams are separated each other and combined after suitable delay of the fundamental beam. The aligned beam is passed through another KDP crystal (thickness, 0.4 mm; cutting angle, 67) for third harmonic generation. The ultraviolet beam (261 nm) was focused with a fused silica lens, L3, with a focal length of 1000 mm into molecular hydrogen in a Raman cell (Taiatsu Glass Co., 750 mm long) equipped with two fused silica windows (20 mm thick). The vessel of the Raman cell was tested at 100 atm and was used at a pressure below
M1
Ti:sapphire Laser L1
L2 M3
M2
f = 500 mm Fused Silica Plate
f = -250 mm L3
Hydrogen Gas Raman Cell
Optical Fiber
Third Harmonic Generator
M4
f = 1000 mm Personal Computer
Multichannel Spectrometer
Fig. 1. Experimental apparatus. Lenses, L1 and L2, are used to reduce the pump beam diameter, in order to improve the conversion efficiency of third harmonic generation. M1–M4, mirrors.
40 atm throughout this experiment. The beam transmitted from the Raman cell was partially reflected by a fused silica plate and was then introduced into an optical fiber interfaced with a multichannel spectrometer (Ocean Optics, USB2000, resolution 2.5 nm). The energy of the laser was measured by means of a joule meter (Coherent, J3-09). 3. Results and discussion Strongly transient stimulated Raman scattering occurs, when the duration of the pump pulse, sp, is much shorter than the dephasing time, T2. In other words, the nonlinear polarization does not reach steady state within the duration of the pump pulse. When the depletion of the pump energy is negligible and the gain is sufficiently high, the Stokes field can be expressed as ( 1=2 ) Z Z 8gR L sp I S I 0 exp Ipðz; tÞ dz dt ; ð1Þ T2 0 0 where I0 is the initial intensity of the Stokes emission, and gR is the steady-state Raman gain coefficient, L is the length of the Raman medium, and Ip(z, t) is the intensity of the pump pulse at the distance of z and the time of t [4,5]. The parameter, gR, is described by 32p3 c2 DN or gR ¼ 2 3 ; ð2Þ ns hxs DxR oX where c is the velocity of light, DN is the difference in the populations of the excited and ground states, ns is the refractive index, h is the Planck’s constant, xs is the frequency of the first Stokes emission, respectively, DxR is the Raman linewidth, and (or/oX) is the cross-section of Raman scattering [6]. The Raman cross-section is proportional to the quadruple power of the frequency of the Stokes emission. Therefore, the gain coefficient is proportional to the frequency of the Stokes emission, which permits stimulated Raman scattering to occur more efficiently in the ultraviolet region [7]. In fact, high-order Raman lines are efficiently generated in the near-ultraviolet region using the second harmonic emission (390 nm) of the femtosecond Ti:sapphire laser [4]. The spectrum measured for a laser beam transmitted from a Raman cell containing hydrogen gas at 40 atm of pressure is shown in Fig. 2. This result was obtained using the third harmonic emission generated by the nearly-transform-limited fundamental beam (784 nm, 107 fs, Dt · Dm = 0.52). High-order Stokes lines are efficiently generated, as shown in Fig. 2(a). It should be noted that anti-Stokes emission is observed in the spectrum at a wavelength of approximately 236 nm. The spectral response of the spectrometer (including the optical fiber, grating, and detector) is indicated as a broken line in Fig. 2. The ratio of the peak heights for the anti-Stokes and fundamental emissions was calculated to be ca. 10%. This suggests that this technique has potential advantages for the excitation of aromatic hydrocarbons to higher electronic states at shorter
F. Kira et al. / Optics Communications 265 (2006) 603–606
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Wavelength (nm)
Intensity (Arb. Units)
(b)
220
240
260
280
300
Wavelength (nm) Fig. 2. Emission spectra for the beam transmitted from the Raman cell. The pump pulse is nearly-transform-limited before (a) and after (b) the inlet window of the Raman cell. The broken line in the figure represents the spectral response of the spectrometer. Hydrogen pressure, 40 atm. The ordinate is expressed in arbitrary units, and the intensity of the peak in (b) is not necessarily larger than that in (a).
wavelengths. In the near-infrared region (784 nm), self phase modulation (SPM) is unavoidable and prevents the efficient generation of the Raman emission [5,8,9]. In contrast to previous studies, no appreciable SPM was observed, as shown in Fig. 2(a), which suggests the efficient generation of Raman emission at shorter wavelengths, as would be predicted from Eq. (2). The result shown in Fig. 2(b) was obtained using the third harmonic emission generated by a fundamental beam that was slightly negatively-chirped (130 fs). The relative intensity of the Stokes beam to that of the transmitted fundamental beam clearly increases, in comparison with the data in Fig. 2(a). This might be due to a shorter pulse width inside the Raman cell, providing a higher peak power for the efficient generation of stimulated Raman emission. Unfortunately, the pulse width of the third harmonic emission was not measured in this study, since an autocorrelator for the ultraviolet pulse was not available in our
laboratory. However, when the third harmonic emission is generated using a transform-limited Ti:sapphire laser in this study, the pulse width of the third harmonic emission is considered to be expanded to several hundred femtosecond with a positive chirp. On the other hand, the laser pulse of the third harmonic emission generated using a negatively-chirped fundamental beam can be compressed to a nearly-transform-limited pulse (<200 fs). It is attributed to the fact that the negative chirp is compensated for dispersion arising from a thick fused silica plate used as a Raman cell window. In fact, the GVD induced by the fused silica plate (thickness, 20 mm) at a wavelength of 261 nm is 4.1 · 1027 s2. This would provide a nearly-transform-limited pulse inside the Raman cell, when a negatively-chirped fundamental beam was used to generate the third harmonic emission for obtaining the result shown in Fig. 2(b). In addition, it should be noted that the energy of the third harmonic emission used in Fig. 2(b) was typically 17% lower than that used in the experiment of Fig. 2(a). This can be attributed to a lower peak power of the chirped pulse for the fundamental beam. However, such a chirped pulse can be transform-limited in the Raman cell, providing a higher efficiency in the generation of the Stokes beams (cf. Fig. 2(a) and (b)) [3,10]. This is a reason why a negatively-chirped fundamental pulse was employed in this study for the efficient generation of the Raman emission. Fig. 3 shows the dependence of the Stokes intensity on the pressure of hydrogen. Raman emission appears at 32 atm and rapidly increases, up to 40 atm. Thus, the use of high-pressure hydrogen to generate Raman emission with a femtosecond pump beam is highly desirable. This can be explained from Eq. (2) that the Raman gain coefficient is proportional to DN and to the pressure of hydrogen. This advantage is usually cancelled with a larger Raman linewidth induced by pressure broadening, when a transform-limited nanosecond pulse is employed in the experiment, as is recognized from Eq. (2). However, the
Intensity (Arb. Units)
Intensity (Arb. Units)
(a)
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0
30
32
34
36
38
40
42
Hydrogen Pressure (atm) Fig. 3. Dependence of the pulse energy of the first Stokes emission on the pressure of hydrogen in the Raman cell.
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linewidth of the femtosecond laser is 10 nm (several THz), which is much larger than the Raman linewidth of hydrogen for vibrational transition (several hundred MHz) [11,12]. Thus, the linewidth in Eq. (2) is independent of the pressure of hydrogen. For this reason, the Raman gain coefficient increases proportionally, and the Stokes intensity then increases exponentially, with the pressure of hydrogen. This suggests the existence of a threshold for the generation of the Stokes beam. This result, i.e., a rapid increase in Stokes intensity from 32 atm, is in reasonable agreement with the data obtained using high-pressure hydrogen (17–70 atm) and the second harmonic emission of the femtosecond Ti:sapphire laser (390 nm, 100 lJ) [4]. Thus, a further increase in hydrogen pressure should increase the efficiency of generation of a high-order Stokes emission. Further improvements in efficiency should be possible by using a laser with larger pulse energy, as predicted from Eq. (1). However, two-photon absorption arising from the fused silica used as a window material is not completely negligible under the present conditions. Therefore, it would be desirable to use a thinner window having smaller two-photon absorption (although it is not resistant to high pressure), or to expand the laser beam, to reduce the intensity at the position of the window to decrease two-photon absorption (although this would require a longer and wider Raman cell that is not resistant to high pressure). Accordingly, optimization is necessary in the design of a Raman cell for the efficient frequency conver-
sion of a femtosecond laser, especially in the ultraviolet region. Acknowledgements This work was supported by Grants-in-Aids for Scientific Research, for the 21st Century COE Program, ‘‘Functional Innovation of Molecular Informatics’’, from the Ministry of the Education, Culture, Science, Sports and Technology of Japan, and by financial support from the Ministry of the Environment of Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P. Baum, S. Lochbrunner, E. Riedle, Appl. Phys. B 79 (2004) 1027. A.E. Jailaubekov, S.E. Bradforth, Appl. Phys. Lett. 87 (2005) 021107. Y. Irie, T. Imasaka, Opt. Lett. 20 (1995) 2072. V. Krylov, O. Ollikainen, U.P. Wild, A. Rebane, V.G. Bespalov, D.I. Staselko, J. Opt. Soc. Am. B 15 (1998) 2910. R.L. Carman, F. Shimizu, C.S. Wang, N. Bloembergen, Phys. Rev. A 2 (1970) 60. D.C. Hanna, D.J. Pointer, D.J. Pratt, IEEE J. Quantum Electron. 22 (1986) 332. W.K. Bischel, M.J. Dyer, J. Opt. Soc. Am. B 3 (1986) 677. H. Kawano, Y. Hirakawa, T. Imasaka, Appl. Phys. B 65 (1997) 1. H. Otsuka, T. Uchimura, T. Imasaka, Opt. Lett. 29 (2004) 400. N. Zhavoronkov, F. Noack, V. Petrov, V.P. Kalosha, J. Herrmann, Opt. Lett. 26 (2001) 47. V.G. Cooper, A.D. May, E.H. Hara, H.F.P. Knapp, Can. J. Phys. 46 (1968) 2019. W.K. Bischel, M.J. Dyer, Phys. Rev. A 33 (1986) 3113.
Efficient generation of vibrational stimulated Raman emission in the ultraviolet region using a femtosecond pump beam Fumiaki Kira, Masahiro Matsuse, Shin-ichi Zaitsu, Totaro Imasaka
*
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan Received 9 August 2005; received in revised form 31 March 2006; accepted 31 March 2006
Abstract The third harmonic emission (261 nm, 30 lJ) of a femtosecond Ti:sapphire laser is focused into molecular hydrogen to generate vibrational stimulated Raman emission. High-order emission lines are efficiently generated by using a high-beam-quality nearly-transformlimited laser and high-pressure hydrogen (40 atm) as a Raman medium. 2006 Elsevier B.V. All rights reserved.
1. Introduction A femtosecond laser has been utilized in studies of ultrafast phenomena and dynamics. Numerous atoms and molecules, e.g., aromatic hydrocarbons, have absorption bands in the ultraviolet region. However, a widely tunable femtosecond laser has seldom been used in this spectral region. For example, Baum et al. have reported the generation of pulses as short as 7 fs by achromatic frequency doubling of a noncollinear optical parametric amplifier (NOPA; 450–1650 nm, 10–50 fs, some 100 nJ) [1]. On the other hand, Jailaubekov et al. have reported the generation of tunable 30-fs pulses (224–240 nm, >1 lJ) by means of phase-matched four-wave mixing (FWM) in an argon-filled hollow waveguide using an optical parametric amplifier (OPA, 1200–2200 nm) and a Ti:sapphire laser (800 lJ, 1 kHz, 110 fs, 800 nm) [2]. In addition, a traveling wave optical parametric amplifier of superfluorescence (TOPAS, 189 nm to 18 lm, 30 fs to 30 ps, 1–500 lJ) has already been commercially available. These advanced technologies are, however, complicated and is very expensive. Stimulated Raman scattering and subsequent four-wave Raman mixing have been employed for frequency conversion in the ultraviolet region, as well as in the visible and infrared *
Corresponding author. Tel.: +81 92 802 2883; fax: +81 92 802 2888. E-mail address: [email protected] (T. Imasaka).
0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.067
regions. In fact, a femtosecond excimer laser emitting at 248 nm has been used to generate high-order vibrational and rotational Raman lines [3]. In this study, a high-energy (10–20 mJ) nearly-transform-limited (Dt = 150–500 fs, Dk = 0.9 nm) pump beam is focused into hydrogen gas pressurized at 5–13 atm. A femtosecond Ti:sapphire laser has been proposed for use, to obtain a tunable laser. The second harmonic emission (390 nm) has already been employed for the generation of high-order vibrational and rotational lines [4]. In this work, a chirped femtosecond pulse (300 fs, 100 lJ) is focused into hydrogen gas pressurized to 70 atm. A variety of aromatic hydrocarbons, e.g., benzene and derivatives thereof, have absorption bands in the vicinity of 260 nm. Because of this, it would be desirable to generate a tunable femtosecond laser in the deep-ultraviolet region (200–350 nm). For this purpose, it is possible to use the third harmonic emission of a Ti:sapphire laser as a pump beam. Unfortunately, the pulse energy of the third harmonic emission is relatively small, especially for the 1-kHz Ti:sapphire laser that is in current use in spectroscopic applications. Therefore, it is necessary to conduct basic research aimed at improving the efficiency of generation of a stimulated Raman emission using an ultraviolet femtosecond laser. In this study, we report on the generation of high-order vibrational Raman lines in the deep-ultraviolet region using a simple and less expensive device, i.e., a Raman cell.
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F. Kira et al. / Optics Communications 265 (2006) 603–606
To improve the efficiency of generation of the vibrational Raman emission, an ultraviolet beam (261 nm, 30 lJ) generated by the third harmonic generation of a Ti:sapphire laser having a high-beam-quality (M2 = 1.2) and a nearly-transform-limited bandwidth (Dt · Dm = 0.52) was focused into hydrogen gas pressurized at 40 atm. This approach was found to efficiently generate first (293 nm) and second (334 nm) Stokes emissions. It should be noted that the first anti-Stokes Raman emission (236 nm) is also observed through four-wave Raman mixing thus allowing the conversion of the femtosecond laser emission to even shorter wavelengths. 2. Experimental The experimental apparatus used in the studies is shown in Fig. 1. A femtosecond Ti:sapphire laser (Thales, Concerto, k = 784 nm, Dk = 10 nm, Dt = 100 fs, 1.1 mJ, M2 = 1.2, 1 kHz) was used as the pump beam. The diameter of the laser beam was changed by a pair of lenses (L1, f = 500 mm; L2, f = 250 mm), to improve the efficiency of generating the third harmonic emission using a third harmonic generator (Spectra-Physics, HGS-T). In this instrument, the fundamental beam (783 nm) is passed through a KDP crystal (thickness, 1.5 mm; cutting angle, 44.5) for second harmonic generation. The fundamental and second harmonic beams are separated each other and combined after suitable delay of the fundamental beam. The aligned beam is passed through another KDP crystal (thickness, 0.4 mm; cutting angle, 67) for third harmonic generation. The ultraviolet beam (261 nm) was focused with a fused silica lens, L3, with a focal length of 1000 mm into molecular hydrogen in a Raman cell (Taiatsu Glass Co., 750 mm long) equipped with two fused silica windows (20 mm thick). The vessel of the Raman cell was tested at 100 atm and was used at a pressure below
M1
Ti:sapphire Laser L1
L2 M3
M2
f = 500 mm Fused Silica Plate
f = -250 mm L3
Hydrogen Gas Raman Cell
Optical Fiber
Third Harmonic Generator
M4
f = 1000 mm Personal Computer
Multichannel Spectrometer
Fig. 1. Experimental apparatus. Lenses, L1 and L2, are used to reduce the pump beam diameter, in order to improve the conversion efficiency of third harmonic generation. M1–M4, mirrors.
40 atm throughout this experiment. The beam transmitted from the Raman cell was partially reflected by a fused silica plate and was then introduced into an optical fiber interfaced with a multichannel spectrometer (Ocean Optics, USB2000, resolution 2.5 nm). The energy of the laser was measured by means of a joule meter (Coherent, J3-09). 3. Results and discussion Strongly transient stimulated Raman scattering occurs, when the duration of the pump pulse, sp, is much shorter than the dephasing time, T2. In other words, the nonlinear polarization does not reach steady state within the duration of the pump pulse. When the depletion of the pump energy is negligible and the gain is sufficiently high, the Stokes field can be expressed as ( 1=2 ) Z Z 8gR L sp I S I 0 exp Ipðz; tÞ dz dt ; ð1Þ T2 0 0 where I0 is the initial intensity of the Stokes emission, and gR is the steady-state Raman gain coefficient, L is the length of the Raman medium, and Ip(z, t) is the intensity of the pump pulse at the distance of z and the time of t [4,5]. The parameter, gR, is described by 32p3 c2 DN or gR ¼ 2 3 ; ð2Þ ns hxs DxR oX where c is the velocity of light, DN is the difference in the populations of the excited and ground states, ns is the refractive index, h is the Planck’s constant, xs is the frequency of the first Stokes emission, respectively, DxR is the Raman linewidth, and (or/oX) is the cross-section of Raman scattering [6]. The Raman cross-section is proportional to the quadruple power of the frequency of the Stokes emission. Therefore, the gain coefficient is proportional to the frequency of the Stokes emission, which permits stimulated Raman scattering to occur more efficiently in the ultraviolet region [7]. In fact, high-order Raman lines are efficiently generated in the near-ultraviolet region using the second harmonic emission (390 nm) of the femtosecond Ti:sapphire laser [4]. The spectrum measured for a laser beam transmitted from a Raman cell containing hydrogen gas at 40 atm of pressure is shown in Fig. 2. This result was obtained using the third harmonic emission generated by the nearly-transform-limited fundamental beam (784 nm, 107 fs, Dt · Dm = 0.52). High-order Stokes lines are efficiently generated, as shown in Fig. 2(a). It should be noted that anti-Stokes emission is observed in the spectrum at a wavelength of approximately 236 nm. The spectral response of the spectrometer (including the optical fiber, grating, and detector) is indicated as a broken line in Fig. 2. The ratio of the peak heights for the anti-Stokes and fundamental emissions was calculated to be ca. 10%. This suggests that this technique has potential advantages for the excitation of aromatic hydrocarbons to higher electronic states at shorter
F. Kira et al. / Optics Communications 265 (2006) 603–606
220
240
260
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300
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340
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320
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360
Wavelength (nm)
Intensity (Arb. Units)
(b)
220
240
260
280
300
Wavelength (nm) Fig. 2. Emission spectra for the beam transmitted from the Raman cell. The pump pulse is nearly-transform-limited before (a) and after (b) the inlet window of the Raman cell. The broken line in the figure represents the spectral response of the spectrometer. Hydrogen pressure, 40 atm. The ordinate is expressed in arbitrary units, and the intensity of the peak in (b) is not necessarily larger than that in (a).
wavelengths. In the near-infrared region (784 nm), self phase modulation (SPM) is unavoidable and prevents the efficient generation of the Raman emission [5,8,9]. In contrast to previous studies, no appreciable SPM was observed, as shown in Fig. 2(a), which suggests the efficient generation of Raman emission at shorter wavelengths, as would be predicted from Eq. (2). The result shown in Fig. 2(b) was obtained using the third harmonic emission generated by a fundamental beam that was slightly negatively-chirped (130 fs). The relative intensity of the Stokes beam to that of the transmitted fundamental beam clearly increases, in comparison with the data in Fig. 2(a). This might be due to a shorter pulse width inside the Raman cell, providing a higher peak power for the efficient generation of stimulated Raman emission. Unfortunately, the pulse width of the third harmonic emission was not measured in this study, since an autocorrelator for the ultraviolet pulse was not available in our
laboratory. However, when the third harmonic emission is generated using a transform-limited Ti:sapphire laser in this study, the pulse width of the third harmonic emission is considered to be expanded to several hundred femtosecond with a positive chirp. On the other hand, the laser pulse of the third harmonic emission generated using a negatively-chirped fundamental beam can be compressed to a nearly-transform-limited pulse (<200 fs). It is attributed to the fact that the negative chirp is compensated for dispersion arising from a thick fused silica plate used as a Raman cell window. In fact, the GVD induced by the fused silica plate (thickness, 20 mm) at a wavelength of 261 nm is 4.1 · 1027 s2. This would provide a nearly-transform-limited pulse inside the Raman cell, when a negatively-chirped fundamental beam was used to generate the third harmonic emission for obtaining the result shown in Fig. 2(b). In addition, it should be noted that the energy of the third harmonic emission used in Fig. 2(b) was typically 17% lower than that used in the experiment of Fig. 2(a). This can be attributed to a lower peak power of the chirped pulse for the fundamental beam. However, such a chirped pulse can be transform-limited in the Raman cell, providing a higher efficiency in the generation of the Stokes beams (cf. Fig. 2(a) and (b)) [3,10]. This is a reason why a negatively-chirped fundamental pulse was employed in this study for the efficient generation of the Raman emission. Fig. 3 shows the dependence of the Stokes intensity on the pressure of hydrogen. Raman emission appears at 32 atm and rapidly increases, up to 40 atm. Thus, the use of high-pressure hydrogen to generate Raman emission with a femtosecond pump beam is highly desirable. This can be explained from Eq. (2) that the Raman gain coefficient is proportional to DN and to the pressure of hydrogen. This advantage is usually cancelled with a larger Raman linewidth induced by pressure broadening, when a transform-limited nanosecond pulse is employed in the experiment, as is recognized from Eq. (2). However, the
Intensity (Arb. Units)
Intensity (Arb. Units)
(a)
605
0
30
32
34
36
38
40
42
Hydrogen Pressure (atm) Fig. 3. Dependence of the pulse energy of the first Stokes emission on the pressure of hydrogen in the Raman cell.
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linewidth of the femtosecond laser is 10 nm (several THz), which is much larger than the Raman linewidth of hydrogen for vibrational transition (several hundred MHz) [11,12]. Thus, the linewidth in Eq. (2) is independent of the pressure of hydrogen. For this reason, the Raman gain coefficient increases proportionally, and the Stokes intensity then increases exponentially, with the pressure of hydrogen. This suggests the existence of a threshold for the generation of the Stokes beam. This result, i.e., a rapid increase in Stokes intensity from 32 atm, is in reasonable agreement with the data obtained using high-pressure hydrogen (17–70 atm) and the second harmonic emission of the femtosecond Ti:sapphire laser (390 nm, 100 lJ) [4]. Thus, a further increase in hydrogen pressure should increase the efficiency of generation of a high-order Stokes emission. Further improvements in efficiency should be possible by using a laser with larger pulse energy, as predicted from Eq. (1). However, two-photon absorption arising from the fused silica used as a window material is not completely negligible under the present conditions. Therefore, it would be desirable to use a thinner window having smaller two-photon absorption (although it is not resistant to high pressure), or to expand the laser beam, to reduce the intensity at the position of the window to decrease two-photon absorption (although this would require a longer and wider Raman cell that is not resistant to high pressure). Accordingly, optimization is necessary in the design of a Raman cell for the efficient frequency conver-
sion of a femtosecond laser, especially in the ultraviolet region. Acknowledgements This work was supported by Grants-in-Aids for Scientific Research, for the 21st Century COE Program, ‘‘Functional Innovation of Molecular Informatics’’, from the Ministry of the Education, Culture, Science, Sports and Technology of Japan, and by financial support from the Ministry of the Environment of Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P. Baum, S. Lochbrunner, E. Riedle, Appl. Phys. B 79 (2004) 1027. A.E. Jailaubekov, S.E. Bradforth, Appl. Phys. Lett. 87 (2005) 021107. Y. Irie, T. Imasaka, Opt. Lett. 20 (1995) 2072. V. Krylov, O. Ollikainen, U.P. Wild, A. Rebane, V.G. Bespalov, D.I. Staselko, J. Opt. Soc. Am. B 15 (1998) 2910. R.L. Carman, F. Shimizu, C.S. Wang, N. Bloembergen, Phys. Rev. A 2 (1970) 60. D.C. Hanna, D.J. Pointer, D.J. Pratt, IEEE J. Quantum Electron. 22 (1986) 332. W.K. Bischel, M.J. Dyer, J. Opt. Soc. Am. B 3 (1986) 677. H. Kawano, Y. Hirakawa, T. Imasaka, Appl. Phys. B 65 (1997) 1. H. Otsuka, T. Uchimura, T. Imasaka, Opt. Lett. 29 (2004) 400. N. Zhavoronkov, F. Noack, V. Petrov, V.P. Kalosha, J. Herrmann, Opt. Lett. 26 (2001) 47. V.G. Cooper, A.D. May, E.H. Hara, H.F.P. Knapp, Can. J. Phys. 46 (1968) 2019. W.K. Bischel, M.J. Dyer, Phys. Rev. A 33 (1986) 3113.