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Generation of picosecond VUV radiation by four-wave mixing of nanosecond and picosecond laser radiations
15 Febmary 1996
OPTICS
COMMUNICATIONS Optics Communications 124 (1996) 118-120
Generation of picosecond VUV radiation by four-wave mixing of nanosecond and picosecond laser radiations M. Berdaha, J.P. Visticot a, C. Dedonder-Lardeux
b, D. Solgadib, B. Soep b
a CEA. DSMLDRECAMLPAM, CE Saclay, 91191 Gif sur Yvette, France b Laboratoire de Photophysique Mokfculaire du CNRS Bat 213, Universite’ Paris Sud, 91405 Orsay, France
Received 19 September 1995; accepted 12 October 1995
Abstract We use the four-wave difference frequency mixing of a nanosecond laser (UV) and a picosecond laser (visible) in xenon gas to generate a picosecond VUV laser radiation. Since the visible laser has a spectral range between 450 nm and 800 nm, we can obtain a tunable VUV laser, from 148 nm to 173 nm. The time duration of the pulses generated depends (linearly in our case) on the shortest laser, which is the picosecond laser.
VUV powerful sources are of extreme interest for the diagnostics of molecular structure, as well as light sources for photoelectron spectroscopy. Picosecond and femtosecond VUV light sources will be used in the detection of transients and transition states in chemical reactions as photoelectron spectroscopy can probe the electronic structure of reaction complexes. The course of the reaction along the reaction coordinate should be best followed by the observation of the evolution of the electron distribution, i.e. the time resolved photoelectron spectra,. This calls for ultrashort VW lasers. The first experiments to generate VW radiation used third order nonlinear processes and were performed in the early seventies. Harris and co-workers [ 1,2] and later Hodgson and Al [ 31 generated respectively UV and coherent tunable VUV in metal vapors. Generally, the third order nonlinear susceptibility in metal vapor is about lo5 to IO6 times greater than that of He [ 11. However, because of the experimental difficulties of using metal vapors (especially mercury) rare gases became a natural choice. A.H. Kung reported in the mid-seventies the first generation of
tunable picosecond VW in xenon by four-wave mixing [43. We have generated by four-wave difference frequency mixing, intense (a few nanojoules) picosecond VW radiation in the 150-160 nm domain in xenon at a pressure of a few Torr. A nanosecond laser wl was tuned close to the two photon resonance in xenon at 249.6 nm, (6p[ l/2,0]) and VW was generated by difference frequency mixing 201 - w2 with a tunable picosecond laser 012 in the range of 500650 nm. That transition has a relative intensity of 10 (compared for instance to the 6p[ 3/2,2] where the relative intensity is 2)) which is the highest for xenon [ 51. The difficult step at 250 nm is made with the ns laser and the time controlling step is achieved by the visible picosecond laser. A similar experiment but offresonance, has been performed with a femtosecond excimer [ 61. Of course brute force picosecond frequency tripling can easily be achieved, but we feel that the present setup can obtain a very high yield in VUV with ultrashort time resolution. Sum frequency mixing, 2~1 +
0030-4018/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved SSDlOO30-4018(95)00628-l
119
M. Berdah et al./Optics Communications 124 (1996) 118-120
012, is foreseeable to make much harder VUV ( lOO105 nm), but in our case limited by the absorption of the windows. The originality of this work proceeds from the mixing of two laser radiations which are also different in pulse duration. The process 2wuv - myis = wvuv is thus a mixing of wuv (pulse duration: 6 ns) and @“is (pulse duration: 1 ps) . The picosecond source has already been described [ 71. It can deliver two picosecond pulses separately tunable. Typically, it compresses in a dye system the nanosecond output of a single mode YAG laser (Quantel) . This stage provides up to 400 PJ pulse energies in less than a picosecond, at a fixed wavelength of 600 nm. This pulse is focused in a water cell in order to generate a continuum . The resulting continuum is split in two beams which are separately filtered in frequency and reamplified. At the end, this laser generates powerful subpicosecond pulses at a rate of 10 Hz with output pulse energies of 300 PJ at @“is = 565 nm. The spectral range is from 450 nm to 800 nm. This allows us to obtain tunable VUV pulses between 173 nm and 148 nm. To avoid nanosecond VUV production, we eliminate the ASE by introducing a saturable absorber (Exciton saturable 598) before the last amplifier. The remaining ASE for each pulse is less than 1%. The green light (499.254 nm) from a dye laser (Lambda Physik LPD 3000) pumped by an excimer laser, is frequency doubled in a BBO crystal which provides UV radiation of wuv = 249.621 nm, where two photons are resonant with the 6p [ l/2,0] in xenon. The remaining green light is cut by a dichroic mirror. The filtered UV energy output is 270 ,xI. The UV (ns) and visible (ps) beams are spatially and temporally overlapped with the dichroic mirror in the 10 cm xenon cell, where pressure is set around 20 mbar. The xenon cell is closed by two MgF2 windows and coupled to a larger cell pumped down to 10m2 mbar which contains a monochromator. The detection system consists of a small Jobin-Yvon H20 monochromator and a Hamamatsu R14.59 photomultiplier model. Signals were processed using a Lecroy 93 10 oscilloscope and a PC AT386 for acquisition. Four-wave frequency mixing has been classically described [ 81 and the total generated power is given by:
Pvw
= 1.58 x 1O-4 k~1~h’2x2P;,P,z vuv
x F(bAk,
b/L, f/L,
k”/k’),
(1)
where kj is a wave vector (cm-‘), b the confocal parameter (cm), Pj the total power for each beam (Watt), N in atoms/cubic centimeters, x the nonlinear susceptibility in ESU per atom, ko the wave vector of the generated radiation in vacuum, kvuv the wave vector of the generated radiation in the nonlinear medium, f the position of the focus along the z axis, L the cell length, k’ = 2kl - k2, k” = 2kl+ kz, Ak the wave-vector mismatch defined by Ak = kvuv - k’. In the tight-focusing limit and in the ideal case k” = k’: F(bAk,0,0.5,1>
= dexp(-blhk]).
(2)
Resonance effects considerably enhance the intensity. Fig. 1 describes the combination of lasers sources and here the 5p6, 5p5-6p[ l/2,0] transition in xenon has been used in resonance for the first step (2 * wt ) . A frequency doubled nanosecond laser has been tuned to w 1, and wp is provided by a picosecond (or subpicosecond) dye laser. The global time response of the systemderivedfromEq. (l),isZwv(t) c( Zw~(t)~* lo2 ( t) , therefore the time duration of the VUV pulse Xe 5p”6p( l/2,0)
\
_-_----
I I I I
VW
w,=249.6nm
ns
t
picosecond
I ps 160.5“IT
I
I
Xe 5~’ ‘S, Fig. 1. Energy diagram of the four-wave difference frequency mixiig in xenon. The shaded region represents the tuning range of the visible ps radiation, therefore. the VUV tuning range.
120
M. Berdah et al./Optics
I,, 494.20
I,,
I,,
499,25
Communications
124 (1996) 118-120
,
499.30
excimer detuning (nm)
0.0
0.2
0.4
l(oJ
0.6
03
1.0
(relative intensity)
Fig. 2. Plot of the VUV output in xenon at 160.5 nm versus detuning of the excimer wt radiation frequency.
Fig. 3. Plot of the VUV as a function laser intensity at 02 = 565 nm.
should be determined by the shortest laser, here the picosecond laser ~2. VW difference frequency mixing generated by the two lasers WI (249.6nm) and w2 (565 nm) was observed through the monochromator at 160.5 nm. The WI laser has been tuned across the Xe transition and the resonance effect is distinctly observed in Fig. 2. The simulation in Fig. 2 of this resonance effect is given by Eq. (2). It has been modeled as in Ref. [ 8 ] using a confocal parameter of 5.5 x low3 cm which is very reasonable in our experimental conditions. The total energy measured at the peak of the resonance could be estimated at ca. 7 nJ. The detection system was calibrated with 5 ,zJ of 250 nm light and using the response curves of the Hamamatsu solar blind photomultiplier tube and the frequency response of the grating we could deduce the efficiency of the detection at 160.5 nm . The efficiency in VUV conversion has been represented in Fig. 3 as a function of Z(w2). The intensity deviates from linearity and the origin of this saturation is optical in essence since we checked for the linear response of the detection. The resulting intensity dependence is not far from linear in I( ~2). As 2wl+ w2 leads over the ionisation limit and as in Ref. [ 91, we suspect plasma formation as a possible cause for this deviation from linearity. Since the response curve does not deviate too much from linearity, the time duration of the VW should not deviate too much from the picosecond w2 pulse, which was determined by autocorrelation as 1 ps FWMH. The observed saturation process should induce a broadening of the VW pulse dura-
tion. We unfortunately have not been able to measure this broadening directly by a cross correlation of the pulse with the ps laser at 565 nm, but given the small saturation observed we believe that the increase in breadth should not exceed a factor of 2. The efficiency in conversion of the picosecond radiation was compared with that of a nanosecond source, here the green beam generated by the excimer laser at 499.254 nm, and was found to be similar. Therefore the maximum value of 7 nJ observed for the picosecond VUV represents quite a high efficiency ( 10e5 with respect to ~2).
of the relative picosecond
111SE. Harris and R.B. Miles, Appl. Phys. Lett. 19 (1971) 387. PI J.F. Young, G.C. Bjorklund, A.H. Kung, R.B. Miles and SE. Harris, Phys. Rev. Lett. 27 (1971) 1551. 131 R.T. Hodgson, P.I? Sorokin and J.J. Wynne, Phys. Rev. Lett. 32 (1974) 343. r41 A.H. Kung, Appl. Phys. L&t. 25 (1974) 653. I51 W. Kong, Doctoral thesis, University of Waterloo, Waterloo (Canada), 1993. [61 A. Tilnnermann, C. Momma, K. Mossavi, C. Windolph and B. Wellegehausen, IEEE J. Quantum Electron. 29 (1993) 1233. [71 Nguyen Dai Hung, P. Plaza, M. Martin and Y.H. Meyer, Appl. Optics 31 (1992) 7046. [81 G.C. Bjorklund, IEEE J. Quantum Electron. QE19 ( 1975) 287. r91 R. Hilbig and R. Wallenstein, IEEE I. Quantum Electron. QE-19 (1983) 194.
OPTICS
COMMUNICATIONS Optics Communications 124 (1996) 118-120
Generation of picosecond VUV radiation by four-wave mixing of nanosecond and picosecond laser radiations M. Berdaha, J.P. Visticot a, C. Dedonder-Lardeux
b, D. Solgadib, B. Soep b
a CEA. DSMLDRECAMLPAM, CE Saclay, 91191 Gif sur Yvette, France b Laboratoire de Photophysique Mokfculaire du CNRS Bat 213, Universite’ Paris Sud, 91405 Orsay, France
Received 19 September 1995; accepted 12 October 1995
Abstract We use the four-wave difference frequency mixing of a nanosecond laser (UV) and a picosecond laser (visible) in xenon gas to generate a picosecond VUV laser radiation. Since the visible laser has a spectral range between 450 nm and 800 nm, we can obtain a tunable VUV laser, from 148 nm to 173 nm. The time duration of the pulses generated depends (linearly in our case) on the shortest laser, which is the picosecond laser.
VUV powerful sources are of extreme interest for the diagnostics of molecular structure, as well as light sources for photoelectron spectroscopy. Picosecond and femtosecond VUV light sources will be used in the detection of transients and transition states in chemical reactions as photoelectron spectroscopy can probe the electronic structure of reaction complexes. The course of the reaction along the reaction coordinate should be best followed by the observation of the evolution of the electron distribution, i.e. the time resolved photoelectron spectra,. This calls for ultrashort VW lasers. The first experiments to generate VW radiation used third order nonlinear processes and were performed in the early seventies. Harris and co-workers [ 1,2] and later Hodgson and Al [ 31 generated respectively UV and coherent tunable VUV in metal vapors. Generally, the third order nonlinear susceptibility in metal vapor is about lo5 to IO6 times greater than that of He [ 11. However, because of the experimental difficulties of using metal vapors (especially mercury) rare gases became a natural choice. A.H. Kung reported in the mid-seventies the first generation of
tunable picosecond VW in xenon by four-wave mixing [43. We have generated by four-wave difference frequency mixing, intense (a few nanojoules) picosecond VW radiation in the 150-160 nm domain in xenon at a pressure of a few Torr. A nanosecond laser wl was tuned close to the two photon resonance in xenon at 249.6 nm, (6p[ l/2,0]) and VW was generated by difference frequency mixing 201 - w2 with a tunable picosecond laser 012 in the range of 500650 nm. That transition has a relative intensity of 10 (compared for instance to the 6p[ 3/2,2] where the relative intensity is 2)) which is the highest for xenon [ 51. The difficult step at 250 nm is made with the ns laser and the time controlling step is achieved by the visible picosecond laser. A similar experiment but offresonance, has been performed with a femtosecond excimer [ 61. Of course brute force picosecond frequency tripling can easily be achieved, but we feel that the present setup can obtain a very high yield in VUV with ultrashort time resolution. Sum frequency mixing, 2~1 +
0030-4018/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved SSDlOO30-4018(95)00628-l
119
M. Berdah et al./Optics Communications 124 (1996) 118-120
012, is foreseeable to make much harder VUV ( lOO105 nm), but in our case limited by the absorption of the windows. The originality of this work proceeds from the mixing of two laser radiations which are also different in pulse duration. The process 2wuv - myis = wvuv is thus a mixing of wuv (pulse duration: 6 ns) and @“is (pulse duration: 1 ps) . The picosecond source has already been described [ 71. It can deliver two picosecond pulses separately tunable. Typically, it compresses in a dye system the nanosecond output of a single mode YAG laser (Quantel) . This stage provides up to 400 PJ pulse energies in less than a picosecond, at a fixed wavelength of 600 nm. This pulse is focused in a water cell in order to generate a continuum . The resulting continuum is split in two beams which are separately filtered in frequency and reamplified. At the end, this laser generates powerful subpicosecond pulses at a rate of 10 Hz with output pulse energies of 300 PJ at @“is = 565 nm. The spectral range is from 450 nm to 800 nm. This allows us to obtain tunable VUV pulses between 173 nm and 148 nm. To avoid nanosecond VUV production, we eliminate the ASE by introducing a saturable absorber (Exciton saturable 598) before the last amplifier. The remaining ASE for each pulse is less than 1%. The green light (499.254 nm) from a dye laser (Lambda Physik LPD 3000) pumped by an excimer laser, is frequency doubled in a BBO crystal which provides UV radiation of wuv = 249.621 nm, where two photons are resonant with the 6p [ l/2,0] in xenon. The remaining green light is cut by a dichroic mirror. The filtered UV energy output is 270 ,xI. The UV (ns) and visible (ps) beams are spatially and temporally overlapped with the dichroic mirror in the 10 cm xenon cell, where pressure is set around 20 mbar. The xenon cell is closed by two MgF2 windows and coupled to a larger cell pumped down to 10m2 mbar which contains a monochromator. The detection system consists of a small Jobin-Yvon H20 monochromator and a Hamamatsu R14.59 photomultiplier model. Signals were processed using a Lecroy 93 10 oscilloscope and a PC AT386 for acquisition. Four-wave frequency mixing has been classically described [ 81 and the total generated power is given by:
Pvw
= 1.58 x 1O-4 k~1~h’2x2P;,P,z vuv
x F(bAk,
b/L, f/L,
k”/k’),
(1)
where kj is a wave vector (cm-‘), b the confocal parameter (cm), Pj the total power for each beam (Watt), N in atoms/cubic centimeters, x the nonlinear susceptibility in ESU per atom, ko the wave vector of the generated radiation in vacuum, kvuv the wave vector of the generated radiation in the nonlinear medium, f the position of the focus along the z axis, L the cell length, k’ = 2kl - k2, k” = 2kl+ kz, Ak the wave-vector mismatch defined by Ak = kvuv - k’. In the tight-focusing limit and in the ideal case k” = k’: F(bAk,0,0.5,1>
= dexp(-blhk]).
(2)
Resonance effects considerably enhance the intensity. Fig. 1 describes the combination of lasers sources and here the 5p6, 5p5-6p[ l/2,0] transition in xenon has been used in resonance for the first step (2 * wt ) . A frequency doubled nanosecond laser has been tuned to w 1, and wp is provided by a picosecond (or subpicosecond) dye laser. The global time response of the systemderivedfromEq. (l),isZwv(t) c( Zw~(t)~* lo2 ( t) , therefore the time duration of the VUV pulse Xe 5p”6p( l/2,0)
\
_-_----
I I I I
VW
w,=249.6nm
ns
t
picosecond
I ps 160.5“IT
I
I
Xe 5~’ ‘S, Fig. 1. Energy diagram of the four-wave difference frequency mixiig in xenon. The shaded region represents the tuning range of the visible ps radiation, therefore. the VUV tuning range.
120
M. Berdah et al./Optics
I,, 494.20
I,,
I,,
499,25
Communications
124 (1996) 118-120
,
499.30
excimer detuning (nm)
0.0
0.2
0.4
l(oJ
0.6
03
1.0
(relative intensity)
Fig. 2. Plot of the VUV output in xenon at 160.5 nm versus detuning of the excimer wt radiation frequency.
Fig. 3. Plot of the VUV as a function laser intensity at 02 = 565 nm.
should be determined by the shortest laser, here the picosecond laser ~2. VW difference frequency mixing generated by the two lasers WI (249.6nm) and w2 (565 nm) was observed through the monochromator at 160.5 nm. The WI laser has been tuned across the Xe transition and the resonance effect is distinctly observed in Fig. 2. The simulation in Fig. 2 of this resonance effect is given by Eq. (2). It has been modeled as in Ref. [ 8 ] using a confocal parameter of 5.5 x low3 cm which is very reasonable in our experimental conditions. The total energy measured at the peak of the resonance could be estimated at ca. 7 nJ. The detection system was calibrated with 5 ,zJ of 250 nm light and using the response curves of the Hamamatsu solar blind photomultiplier tube and the frequency response of the grating we could deduce the efficiency of the detection at 160.5 nm . The efficiency in VUV conversion has been represented in Fig. 3 as a function of Z(w2). The intensity deviates from linearity and the origin of this saturation is optical in essence since we checked for the linear response of the detection. The resulting intensity dependence is not far from linear in I( ~2). As 2wl+ w2 leads over the ionisation limit and as in Ref. [ 91, we suspect plasma formation as a possible cause for this deviation from linearity. Since the response curve does not deviate too much from linearity, the time duration of the VW should not deviate too much from the picosecond w2 pulse, which was determined by autocorrelation as 1 ps FWMH. The observed saturation process should induce a broadening of the VW pulse dura-
tion. We unfortunately have not been able to measure this broadening directly by a cross correlation of the pulse with the ps laser at 565 nm, but given the small saturation observed we believe that the increase in breadth should not exceed a factor of 2. The efficiency in conversion of the picosecond radiation was compared with that of a nanosecond source, here the green beam generated by the excimer laser at 499.254 nm, and was found to be similar. Therefore the maximum value of 7 nJ observed for the picosecond VUV represents quite a high efficiency ( 10e5 with respect to ~2).
of the relative picosecond
111SE. Harris and R.B. Miles, Appl. Phys. Lett. 19 (1971) 387. PI J.F. Young, G.C. Bjorklund, A.H. Kung, R.B. Miles and SE. Harris, Phys. Rev. Lett. 27 (1971) 1551. 131 R.T. Hodgson, P.I? Sorokin and J.J. Wynne, Phys. Rev. Lett. 32 (1974) 343. r41 A.H. Kung, Appl. Phys. L&t. 25 (1974) 653. I51 W. Kong, Doctoral thesis, University of Waterloo, Waterloo (Canada), 1993. [61 A. Tilnnermann, C. Momma, K. Mossavi, C. Windolph and B. Wellegehausen, IEEE J. Quantum Electron. 29 (1993) 1233. [71 Nguyen Dai Hung, P. Plaza, M. Martin and Y.H. Meyer, Appl. Optics 31 (1992) 7046. [81 G.C. Bjorklund, IEEE J. Quantum Electron. QE19 ( 1975) 287. r91 R. Hilbig and R. Wallenstein, IEEE I. Quantum Electron. QE-19 (1983) 194.