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Tunable picosecond pulses below 200 nm by external frequency conversion of cw modelocked Ti:Al2O3 laser radiation
OPTICS COMMUNICATIONS
Optics Communications 94 (1992) 369-372 North-Holland
Tunable picosecond pulses below 200 nm by external frequency conversion of cw modelocked Ti : A1203 laser radiation A. Nebel
and R. Beigang
Fachbereich Physik, Universittit Kaiserslautern, W-6750 Kaiserslautern, Germany
Received 6 July 1992
Sum frequency mixing of the fundamental and third harmonic radiation of a cw modelocked Ti : sapphire laser leads to tunable picosecond pulses down to a wavelength of 192 nm with average powers in the mW range.
Dye lasers, Ti : sapphire lasers and color center lasers have turned out to be reliable sources for picoand femtosecond pulses with substantial average and peak powers in the visible and near infrared spectral region [ l-31. However, in the uv and vuv spectral range no primary laser sources for the generation of tunable ultrashort pulses are available. This wavelength range is of potential interest for time resolved investigations in chemistry, biology or combustion diagnostics. Nonlinear frequency conversion in crystals is, in principle, a well suited method to generate tunable pulses in this wavelength range. In particular, frequency conversion of cw modelocked Ti : Al203 radiation is a very efficient way to generate tunable picosecond pulses in the deep blue, uv and even vuv spectral region. Using new nonlinear materials like &bariumborate (BBO) and lithiumtriborate (LBO) and utilizing different nonlinear processes the wavelength range from 205
dition the nonlinear coefficient de, decreases substantially when reaching the short wavelength limit and is exactly zero at 410 nm. Here we report on the generation of tunable picosecond pulses in a wavelength range which extends well below 200 nm using a different type of nonlinear interaction. In order to generate tunable pulses below 205 nm, sum frequency mixing of the fundamental and the third harmonic Ti : sapphire radiation has to be applied. Using BBO as the nonlinear crystal phasematched sum frequency mixing is possible down to 185 nm which corresponds almost to the short tuning end of the Ti:sapphire laser (720 nm). Figure 1 shows the phase matching curves for direct fourth harmonic and sum frequency generation in BBO together with the figure of merit (FOM) for both processes as defined by FOM~FM = (derr)‘l(n4wn3onw) FOMFHG
=
(4~)2/(naon2wn20)
,
(1)
.
(2)
de, is the effective nonlinear coefficient, ndo, nso, nzw and n, are the indices of refraction at the fourth harmonic, third harmonic, second harmonic and fundamental frequency, respectively. Unfortunately, the transparency limit of BBO at 190 nm as indicated in fig. 1 by the vertical dashed line restricts the sum frequency generation to wavelength A> 190 nm. This is still a major improvement in comparison to direct frequency doubling of the
0 1992 Elsevier Science Publishers B.V. All rights reserved.
369
Volume 94, number
i80
200
5
OPTICS
220
240
COMMUNICATIONS
December
1992
26;
h40, (nm)
Fig. 1. Phase matching curves for type-1 fourth harmonic generation and sum frequency mixing in BBO (solid lines) and figures of merit for both processes (dashed lines). The two vertical lines indicate the transparency limit of BBO and the limit of phase matched fourth harmonic generation, respectively. RIG: fourth harmonic generation, SFM: sum frequency mixing, 8: phase match angle.
second harmonic radiation which is limited to wavelength a> 205 nm. It is also obvious from fig. 1 that in wavelength ranges where both processes are allowed the figure of merit is always higher for the sum frequency mixing compared to direct frequency doubling. Therefore this nonlinear process is preferable also for wavelength ;1>205 nm although the experimental realization is more complicated compared to direct frequency quadrupoling of the Ti: sapphire laser radiation. A schematic diagram of the total experimental set up is shown in fig. 2. The Ti: sapphire laser used in these experiments produced a maximum average power of 1.75 W at 790 nm (repetition range 82 MHz), the tuning range extended from 720 to 850 nm and the pulse length was about 1.4 ps. Assuming a sech2 pulse shape the time-bandwidth product of AvAr~0.32 indicates transform limited pulses. In the first step the cw modelocked Ti: sapphire laser radiation is frequency doubled in an LB0 crystal (6x6~8 mm3, 0=90”, @=30”). Although second harmonic generation is more efficient in BBO or lithiumiodate (LiI03) crystals [ 41, LB0 was preferred because of the much smaller walk-off angle a! of approximately 1.1 O, which results in a superior beam profile compared to BBO ((Y= 39” ) or LiI03 ((Y= 5” ). This near gaussian beam profile is advan370
Fig. 2. Schematic diagram of the frequency conversion system, SHG: second harmonic generation, THG: third harmonic generation, SFM: sum frequency mixing, D: compensation of the temporal delay caused by the difference in group velocity, P: adjustment of polarisation.
tageous for subsequent frequency conversion processes. Owing to the difference in group velocity the pulses at the fundamental and the second harmonic frequency are delayed relative to each other after transmission through the doubling crystal. For crystal lengths of 10 mm typical delays are in the order of 1 ps depending on the exact wavelength [4]. This delay is comparable to the pulse length and thus requires compensation. Before combining w and 2w at the mixing crystal to generate the third harmonic frequency the time delay is compensated in an optical delay line. As the mixing process is a type-I-process fundamental and second harmonic radiation have to be ordinary polarized waves. Therefore the polarization of the second harmonic radiation which is orthogonal to the polarization of the fundamental is rotated by 90” with a half-wave plate. With BBO as the mixing crystal (9x7x6.5 mm3, @=50°) average powers up to P3w= 150 mW were generated in a
Volume 94, number 5
1 December 1992
OPTICS COMMUNICATIONS
wavelength range from 240 nm to 285 nm as determined by the tuning range of our Ti: sapphire laser. The conversion efficiency of the tripling process as defined by ~30=P3wl(PwP2w)L’2
(3)
reached 30% taken into account the average power P, and P20 before the tripling crystal. The conversion efficiency as a function of the fundamental wavelength is shown in fig. 3 (open circles). It qualitatively follows the decrease of the effective nonlinear coefficient de, with decreasing wavelength. The temporal shape of the pulses at w and 2w was directly determined from autocorrelation measurements of o and 20. The pulse lengths of the second harmonic pulses was measured to be 1.1 ps which corresponds to a reduction factor of 1.4 compared to the pulse length at the fundamental frequency w. This reduction in pulse length due to the nonlinear process is in good agreement with theoretical predictions for a sech2 pulse shape where a reduction factor of 1.55 is expected. The pulse length at 30 was measured with a cross correlation between o and 30 in a 6.85 mm long BBO crystal. The measured pulse length of 1.8 ps can be explained qualitatively by the difference in group velocity between w and 20. During the mixing process pulses at o and 20 are spatially separated along the crystal and the temporal pulse shape at 30 is a convolution of the pulse shapes at w and 20. This value is an upper limit of the actual pulse length as both the mixing crystal and the crystal used for the
cross correlation measurement contribute to the width. After the mixing crystal for 3w (see fig. 2) the temporal overlap of w and 3w is again adjusted in an optical delay line before combining w and 30 in an BBO crystal. The polarization of the third harmonic radiation has also to be rotated by 90” for the type-1 mixing process in BBO. The dimensions of the BBO crystal were 8.5 x 8 X 6.85 mm3 with a phase match angle Q $ = 75 ‘. Typical results from the mixing process for wavelengths below 2 10 nm are shown in fig. 4. Maximum average powers of upto 10 mW were obtained. The improvement in average fourth harmonic power above 205 nm compared to direct frequency quadrupoling is in good agreement with the difference in the effective nonlinear coefficients for frequency quadrupoling and sum frequency generation (fig. 1). The shortest wavelength we obtained so far was 192.4 nm with average powers of approximately 1 mW. The strong absorption of the BBO crystal below 192 nm did not allow for generation of shorter wavelength with “usable” output power ( > 1 mW), although fourth harmonic generation was observable almost down to 190 nm. The conversion efficiency for the sum frequency generation of 4~0 is again defined as *
t/4w=p4wl(pwp3w)1~2
(4)
It reached a maximum value of ~~~=4% with average input powers of P30= 120 mW and P,= 570 mW. The quantitative behavior as a function of wavelength is shown in fig. 3 (solid circles). The decrease at shorter wavelength can be explained by the 11
I
,
rIII,,
, , , , , , , ,
~,
10 -
.
98$ L a
.
,3
.
760
I
780
I
,
I
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800
820
840
0
hTisa[nml
Fig. 3. Conversion efficiency as a function of fundamental wavelength for the sum frequency processes w+20= 30 (open circles) and o+ 3~~40~ (solid circles).
.
01
I
I
190
.*.
. ,
.
.
1 t
0.
.
.
5 42
*
.
.
6'.
3-
0
..-
.
I
I
195
*
,
,
200
,
,
,
I
205
,
,
,
,
1 210
h[nml
Fig. 4. Experimental data for generation of 40 using sum frequency mixing in BBO in the wavelength range from 190 nm to 210nm.
371
Volume 94, number
194.0
5
193.8
OPTICS
193.6
b-----
193.4
COMMUNICATIONS
J
193.2
lawb-4 Fig. 5. Spectral profile of a typical pulse at 193.5 nm.
decrease of the effective nonlinear coefficient. We did not determine directly the pulse length of the pulses at 4~. However, owing to the difference in group velocity between w and 30 it can be assumed that a similar process as in the case of the frequency tripling occurred which increases the pulse length compared to the original length at w and 3w. Using shorter crystals for frequency tripling and sum frequency mixing this effect can be reduced consid-
372
1 December
1992
erably. For a BBO crystal length around 2 mm the broadening effect becomes comparable to the pulse shortening caused by the nonlinear process so that pulse lengths comparable to the original pulse lengths can be expected. A typical spectrum of a pulse at 193.6 nm is shown in fig. 5. The clean spectrum indicates that no serious deterioriation of the spectral pulse shape occurred during the various nonlinear processes. It should be mentioned that these pulses are well suited for further amplification in an ArF excimer laser. In summary we have shown that sum frequency mixing of the fundamental and third harmonic radiation of a cw modelocked Ti : A&O, laser leads to tunable picosecond pulses below 200 nm with average powers up to 10 mW which corresponds to an efficiency of almost 4%. The shortest wavelength we obtained so far was 192.5 nm and was limited by the transparency range of our BBO crystal.
References [l] R.L. Fork, C.H. Brito-Cruz, P.C. Becker and C.V. Shank, Optics Lett. 12 (1987) 483. [2] D.E. Spence, P.N. Kean and W. Sibbett, Optics Lett. 16 (1991) 42. [3] See e.g. W. Gellermann, J. Phys. Chem. Solids 52 (1991) 249; and references therein. [4] A. Nebel and R. Beigang, Optics Lett. 16 ( 1991) 1729.
Optics Communications 94 (1992) 369-372 North-Holland
Tunable picosecond pulses below 200 nm by external frequency conversion of cw modelocked Ti : A1203 laser radiation A. Nebel
and R. Beigang
Fachbereich Physik, Universittit Kaiserslautern, W-6750 Kaiserslautern, Germany
Received 6 July 1992
Sum frequency mixing of the fundamental and third harmonic radiation of a cw modelocked Ti : sapphire laser leads to tunable picosecond pulses down to a wavelength of 192 nm with average powers in the mW range.
Dye lasers, Ti : sapphire lasers and color center lasers have turned out to be reliable sources for picoand femtosecond pulses with substantial average and peak powers in the visible and near infrared spectral region [ l-31. However, in the uv and vuv spectral range no primary laser sources for the generation of tunable ultrashort pulses are available. This wavelength range is of potential interest for time resolved investigations in chemistry, biology or combustion diagnostics. Nonlinear frequency conversion in crystals is, in principle, a well suited method to generate tunable pulses in this wavelength range. In particular, frequency conversion of cw modelocked Ti : Al203 radiation is a very efficient way to generate tunable picosecond pulses in the deep blue, uv and even vuv spectral region. Using new nonlinear materials like &bariumborate (BBO) and lithiumtriborate (LBO) and utilizing different nonlinear processes the wavelength range from 205
dition the nonlinear coefficient de, decreases substantially when reaching the short wavelength limit and is exactly zero at 410 nm. Here we report on the generation of tunable picosecond pulses in a wavelength range which extends well below 200 nm using a different type of nonlinear interaction. In order to generate tunable pulses below 205 nm, sum frequency mixing of the fundamental and the third harmonic Ti : sapphire radiation has to be applied. Using BBO as the nonlinear crystal phasematched sum frequency mixing is possible down to 185 nm which corresponds almost to the short tuning end of the Ti:sapphire laser (720 nm). Figure 1 shows the phase matching curves for direct fourth harmonic and sum frequency generation in BBO together with the figure of merit (FOM) for both processes as defined by FOM~FM = (derr)‘l(n4wn3onw) FOMFHG
=
(4~)2/(naon2wn20)
,
(1)
.
(2)
de, is the effective nonlinear coefficient, ndo, nso, nzw and n, are the indices of refraction at the fourth harmonic, third harmonic, second harmonic and fundamental frequency, respectively. Unfortunately, the transparency limit of BBO at 190 nm as indicated in fig. 1 by the vertical dashed line restricts the sum frequency generation to wavelength A> 190 nm. This is still a major improvement in comparison to direct frequency doubling of the
0 1992 Elsevier Science Publishers B.V. All rights reserved.
369
Volume 94, number
i80
200
5
OPTICS
220
240
COMMUNICATIONS
December
1992
26;
h40, (nm)
Fig. 1. Phase matching curves for type-1 fourth harmonic generation and sum frequency mixing in BBO (solid lines) and figures of merit for both processes (dashed lines). The two vertical lines indicate the transparency limit of BBO and the limit of phase matched fourth harmonic generation, respectively. RIG: fourth harmonic generation, SFM: sum frequency mixing, 8: phase match angle.
second harmonic radiation which is limited to wavelength a> 205 nm. It is also obvious from fig. 1 that in wavelength ranges where both processes are allowed the figure of merit is always higher for the sum frequency mixing compared to direct frequency doubling. Therefore this nonlinear process is preferable also for wavelength ;1>205 nm although the experimental realization is more complicated compared to direct frequency quadrupoling of the Ti: sapphire laser radiation. A schematic diagram of the total experimental set up is shown in fig. 2. The Ti: sapphire laser used in these experiments produced a maximum average power of 1.75 W at 790 nm (repetition range 82 MHz), the tuning range extended from 720 to 850 nm and the pulse length was about 1.4 ps. Assuming a sech2 pulse shape the time-bandwidth product of AvAr~0.32 indicates transform limited pulses. In the first step the cw modelocked Ti: sapphire laser radiation is frequency doubled in an LB0 crystal (6x6~8 mm3, 0=90”, @=30”). Although second harmonic generation is more efficient in BBO or lithiumiodate (LiI03) crystals [ 41, LB0 was preferred because of the much smaller walk-off angle a! of approximately 1.1 O, which results in a superior beam profile compared to BBO ((Y= 39” ) or LiI03 ((Y= 5” ). This near gaussian beam profile is advan370
Fig. 2. Schematic diagram of the frequency conversion system, SHG: second harmonic generation, THG: third harmonic generation, SFM: sum frequency mixing, D: compensation of the temporal delay caused by the difference in group velocity, P: adjustment of polarisation.
tageous for subsequent frequency conversion processes. Owing to the difference in group velocity the pulses at the fundamental and the second harmonic frequency are delayed relative to each other after transmission through the doubling crystal. For crystal lengths of 10 mm typical delays are in the order of 1 ps depending on the exact wavelength [4]. This delay is comparable to the pulse length and thus requires compensation. Before combining w and 2w at the mixing crystal to generate the third harmonic frequency the time delay is compensated in an optical delay line. As the mixing process is a type-I-process fundamental and second harmonic radiation have to be ordinary polarized waves. Therefore the polarization of the second harmonic radiation which is orthogonal to the polarization of the fundamental is rotated by 90” with a half-wave plate. With BBO as the mixing crystal (9x7x6.5 mm3, @=50°) average powers up to P3w= 150 mW were generated in a
Volume 94, number 5
1 December 1992
OPTICS COMMUNICATIONS
wavelength range from 240 nm to 285 nm as determined by the tuning range of our Ti: sapphire laser. The conversion efficiency of the tripling process as defined by ~30=P3wl(PwP2w)L’2
(3)
reached 30% taken into account the average power P, and P20 before the tripling crystal. The conversion efficiency as a function of the fundamental wavelength is shown in fig. 3 (open circles). It qualitatively follows the decrease of the effective nonlinear coefficient de, with decreasing wavelength. The temporal shape of the pulses at w and 2w was directly determined from autocorrelation measurements of o and 20. The pulse lengths of the second harmonic pulses was measured to be 1.1 ps which corresponds to a reduction factor of 1.4 compared to the pulse length at the fundamental frequency w. This reduction in pulse length due to the nonlinear process is in good agreement with theoretical predictions for a sech2 pulse shape where a reduction factor of 1.55 is expected. The pulse length at 30 was measured with a cross correlation between o and 30 in a 6.85 mm long BBO crystal. The measured pulse length of 1.8 ps can be explained qualitatively by the difference in group velocity between w and 20. During the mixing process pulses at o and 20 are spatially separated along the crystal and the temporal pulse shape at 30 is a convolution of the pulse shapes at w and 20. This value is an upper limit of the actual pulse length as both the mixing crystal and the crystal used for the
cross correlation measurement contribute to the width. After the mixing crystal for 3w (see fig. 2) the temporal overlap of w and 3w is again adjusted in an optical delay line before combining w and 30 in an BBO crystal. The polarization of the third harmonic radiation has also to be rotated by 90” for the type-1 mixing process in BBO. The dimensions of the BBO crystal were 8.5 x 8 X 6.85 mm3 with a phase match angle Q $ = 75 ‘. Typical results from the mixing process for wavelengths below 2 10 nm are shown in fig. 4. Maximum average powers of upto 10 mW were obtained. The improvement in average fourth harmonic power above 205 nm compared to direct frequency quadrupoling is in good agreement with the difference in the effective nonlinear coefficients for frequency quadrupoling and sum frequency generation (fig. 1). The shortest wavelength we obtained so far was 192.4 nm with average powers of approximately 1 mW. The strong absorption of the BBO crystal below 192 nm did not allow for generation of shorter wavelength with “usable” output power ( > 1 mW), although fourth harmonic generation was observable almost down to 190 nm. The conversion efficiency for the sum frequency generation of 4~0 is again defined as *
t/4w=p4wl(pwp3w)1~2
(4)
It reached a maximum value of ~~~=4% with average input powers of P30= 120 mW and P,= 570 mW. The quantitative behavior as a function of wavelength is shown in fig. 3 (solid circles). The decrease at shorter wavelength can be explained by the 11
I
,
rIII,,
, , , , , , , ,
~,
10 -
.
98$ L a
.
,3
.
760
I
780
I
,
I
I
800
820
840
0
hTisa[nml
Fig. 3. Conversion efficiency as a function of fundamental wavelength for the sum frequency processes w+20= 30 (open circles) and o+ 3~~40~ (solid circles).
.
01
I
I
190
.*.
. ,
.
.
1 t
0.
.
.
5 42
*
.
.
6'.
3-
0
..-
.
I
I
195
*
,
,
200
,
,
,
I
205
,
,
,
,
1 210
h[nml
Fig. 4. Experimental data for generation of 40 using sum frequency mixing in BBO in the wavelength range from 190 nm to 210nm.
371
Volume 94, number
194.0
5
193.8
OPTICS
193.6
b-----
193.4
COMMUNICATIONS
J
193.2
lawb-4 Fig. 5. Spectral profile of a typical pulse at 193.5 nm.
decrease of the effective nonlinear coefficient. We did not determine directly the pulse length of the pulses at 4~. However, owing to the difference in group velocity between w and 30 it can be assumed that a similar process as in the case of the frequency tripling occurred which increases the pulse length compared to the original length at w and 3w. Using shorter crystals for frequency tripling and sum frequency mixing this effect can be reduced consid-
372
1 December
1992
erably. For a BBO crystal length around 2 mm the broadening effect becomes comparable to the pulse shortening caused by the nonlinear process so that pulse lengths comparable to the original pulse lengths can be expected. A typical spectrum of a pulse at 193.6 nm is shown in fig. 5. The clean spectrum indicates that no serious deterioriation of the spectral pulse shape occurred during the various nonlinear processes. It should be mentioned that these pulses are well suited for further amplification in an ArF excimer laser. In summary we have shown that sum frequency mixing of the fundamental and third harmonic radiation of a cw modelocked Ti : A&O, laser leads to tunable picosecond pulses below 200 nm with average powers up to 10 mW which corresponds to an efficiency of almost 4%. The shortest wavelength we obtained so far was 192.5 nm and was limited by the transparency range of our BBO crystal.
References [l] R.L. Fork, C.H. Brito-Cruz, P.C. Becker and C.V. Shank, Optics Lett. 12 (1987) 483. [2] D.E. Spence, P.N. Kean and W. Sibbett, Optics Lett. 16 (1991) 42. [3] See e.g. W. Gellermann, J. Phys. Chem. Solids 52 (1991) 249; and references therein. [4] A. Nebel and R. Beigang, Optics Lett. 16 ( 1991) 1729.