Stimulated Raman scattering of KrCl laser radiation in CH4

OPTICS COMMUNICATIONS

Volume 56, number 3

STIMULATED

RAMAN

E. ARMANDILLO

SCATTERING

‘, A. LUCHES,

OF KrCl LASER

V. NASSISI

and

1 December 1985

RADIATION

IN CH,

M.R. PERRONE

Drpurtinwnto dt Fisicu. Untwrsitir dt Lecce, Lecce, Ita&

Received

10 April 1985; revised manuscript

received

29 July 1985

Stimulated Raman scattering experiments in methane have been performed using an UV-preionized KrCl discharge laser (222 nm) as the pump. The effect of the pumping beam divergence on the photon conversion efficiency has been investigated. Photon conversion efficiencies of 45% to the first Stokes at 237 nm were measured at 35 atm of methane pressure. The results are compared with those obtained in hydrogen as Raman medium under the same experimental conditions.

1. Introduction At present, it is very interesting to have a great selection of UV laser sources for applications in areas such as photochemistry [l], lithography [2] and material transformation studies [3 1. Stimulated Raman scattering (SRS) is one of the most efficient methods for shifting the output of high-power rate-gas halide lasers to different UV wavelengths [4]. Recently SRS experiments in hydrogen were performed by Fulghum et al. [5] with a collimated beam from an e-beam pumped XeF laser operating at 353 nm. They used a collimated pump beam in order to minimize four-wave parametric processes and thus control sequential SRS into a desired Stokes order. Energy conversion efficiency higher than 4.5 percent to first Stokes (S1) at 414 nm were observed at 10 atm of Hz pressure. Carlsten et al. [6] attempted to limit the higher Stokes and the anti-stokes generation due to four-wave mixing processes by operating at high pressures and also by utilizing a low-angle pumping geometry. Thus, by focusing a diffraction limited XeCl(308 nm) laser beam with a 2 m lens into the center of an Hz cell, they observed an energy conversion of 77 percent to S, (353 mn) and the holdoff of S, (414 nm) at 97 atm of Hz pressure. In the present work a KrCl excimer laser (222 nm), excited in an UV-preionized transverse discharge, was ’ ENEA-CRE

Casaccia,

Rome, Italy.

0 030-4018/8.5/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

used to investigate SRS in CH,. The primary goal of the experiment was to investigate the effects of the divergence of the pump beam on the SRS conversion efficiency and then maximize energy conversion to the first Stokes S, (237 nm), but using single pass SRS. To study the effect of the spatial quality of the pump beam on the conversion efficiency, two different confocal unstable resonators with a magnification M = 6 and M = 20, respectively, were tested. To assess the quality of the output beam obtained from both the unstable cavities, far field pinhole energy transmission measurements were carried out. It was found that the unstable cavity with M = 6 gives an output beam with a divergence of -4 mrad, whereas the beam divergence is -0.9 mrad when the cavity with M = 20 is used. Both output beams were used to study the energy conversion efficiency to S, as a function of pumping energy and methane pressure. Quite different results were obtained with the two different configurations.

2. Experimental

apparatus and results

The experimental apparatus is schematically shown in fig. 1. Our K&l laser was previously described [7]. As said above, the external optical cavity consisted of a positive branch, confocal unstable resonator with a magnification of 6 or 20. The output coupler was an aluminum scraper mirror set at 45”. 207

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CONCAVE MIRROR

1 December 1985

Then

(1) where X, is the wavelength of the first Stokes in the medium, vs is the first Stokes frequency, N is the number density of CH, molecules, do/da (3 X

Fig. 1. Layout of the experimental apparatus.

Both cavities produced an output beam of (2 X 1) cm2 outer dimension with a centered obscuration. The laser pulse length was 10 ns (full width at half maximum), and its spectral width Aup was 100 cm-l. The pump beam is normally focused to achieve substantial conversion in single-pass Raman scattering experiments. Then, a lens L, was used to focus the pump beam into the center of a 50 cm Raman cell. The beam was then recollimated by the lens b and separated into the various frequency components by a prism. The energy measurements were made at each wavelength with a pyroelectric detector. As previously said, a primary interest of the present work was to maximize the energy conversion from the pumping beam to S, . To this end, the pump beam was focused with a 1 m lens (L1) into the methane cell. In fact, a low angle pumping geometry reduces the angles which lead to phase matching for four wave processes [8,9]. These last processes are generally responsible for the higher Stokes and antiStokes emissions. The amount of Stokes light.generated by SRS depends on the intensity of the pump beam (Ip), on the interaction length (0 and on the Raman gain of the medium (g,) [4]. By analysing gs, further informations about the conversion efficiency can be obtained. Recently, experimental studies [9] and theoretical treatments [lo] showed that the forward gain for a given Raman medium is independent of the laser bandwidth. Thus, even if we used a broad band pump (Avp = 100 cm-l), the usual expression for the SRS gain for a monochromatic pump laser [4] can be considered still valid under our experimental conditions. 208

1O-28 cm2 sr-1 [ll]) is the total spontaneous scattering cross section and Au is the spontaneous Raman linewidth. It was found that between 1 and 50 atm Au increases with CH, pressure and can be expressed (in cm-l) as Au = 0.32 + 0.012 PCH~ where PCH4 is the methane pressure in atm [ 121. Inserting our parameters in eq. (l), we find that gs grows monotonically withPCH4 over the whole range of used pressures. Hence, from the previous arguments, it comes out that for a fixed pumping geometry the SRS conversion efficiency should increase with the pump intensity and with the methane pressure up to 35 atm, which is the safe pressure limit of our Raman cell. Figs. 2 and 3 give the conversion efficiency to S, as a function of pumping energy Ei and methane pressure PcH,, respectively, when the KrCl laser is used with a confocal unstable resonator with a magnificationM= 6. Under these experimental conditions, the area of the focal spot was measured to be 2 mm2 from burn

05

I

Fig. 2. Energy conversion efficiency as a function of KrCl energy Ei, at PCH, = 35 atm. Eo is the output energy of S 1 and of the pumping beam P.

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1 December 1985

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M,20 lo-

0.5 t

0-

0.5

-

2 dS//+---

O-

10

-1..

20

30

Pcti4(atm

Fig. 3. Energy conversion efficiency versus CH4 pressure PCH, for a pumping beam Ei = 32 mJ. Eo is the output energy of S 1 and of the pumping beam P.

Fig. 4. Energy conversion efficiency versus CH4 pressure PCH,, for a pumping beam Ei = 20 mJ. Eo is the output energy of S1, Sz and of the pump P.

spots and aperture transmissions. Thus, the pump intensity Zp was 160 MW/cm2 for a pulse energy of 32 mJ. The graphs of figs. 2 and 3 show a maximum conversion efficiency to S1 of about 20%. Second Stokes Raman emission ST (255 nm) was also observed, although the conversion efficiency was estimated to be less than 1%. In contrast to what one expects from the theory [4], these graphs show that once saturation is reached, the overall conversion efficiency remains constant even if the pumping energy and methane pressure are increased. From both figures it comes out also that an appreciable SRS conversion begins only when the Raman gain per unit length

the external optical cavity of the pump laser consisted of an unstable resonator with a magnification of 20, much better experimental results were obtained. Under these new experimental conditions the laser beam divergence was -0.9 mrad and the area of the focal spot was estimated to be 0.75 mm2, which gives a pump intensity Zp = 300 MW/cm2 for a pulse energy of 23 mJ. Fig. 4 gives the SRS energy conversion to SI and S, as a function of methane pressure for a pumping beam Of Ei= 20 mJ. In agreement with what one expects from theory, we observe that the depletion of the pump beam increases with PCH4 at least up to 35 atm. The conversion efficiency to S, also increases withPcH,. The conversion efficiency to S,, which is of about 10 percent at 1.5 atm, reduces to less than 1 percent at 35 atm. The S2 generation can be due to SRS pumped by the first Stokes beam and to four-wave mixing with the pump and first Stokes beams driving the non linear polarization. However, the S2 generation can be initiated by the latter process since it, as all parametric processes, has no threshold but requires phase-matching [ 131. In fact, we observe that at PCH~ > 15 atm the S2 generation slows down because, by operating at higher pressures, the coherence length for the four-wave process decreases. It is important to point out that the slowing down of S2 at PCH~ > 15 atm is also affected by

G =gsZp reaches the value of 0.2 cm-l, whereas saturation starts for G = 0.3 cm-l. The saturation effect observed in figs. 2 and 3 was also observed by Sze et al. [9], when an XeCl laser (308 nm) was used to investigate SRS in CH,. Then, the preceding observations lead to suppose that, under these experimental conditions, the spatial quality of the pump beam does affect the maximum SRS conversion efficiency. Then, it could be possible to get a more efficient Raman conversion by improving the spatial quality of the laser beam. In fact, when

209

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Volume 56, number 3

1 December 1985

b/E’ hl=*o

10 M,20 ,

0 5~

ok____dL 0

0

10

-_

)

20

Ei(m

0

J!

4 at

Fig. 5. Energy conversion efficiency versus pumping energy hx, = 35 atm. E, is the output energy of S1 and of the pump P.

Fig. 6. Energy conversion efficiency versus pump energy Ei at 35 atm of H2 pressure. Eo is the output energy of S1, S2 and of the pump P.

the low angle pumping geometry (1 m focusing lens, L1) used to reduce the angles that lead to phasematching for four-wave processes. These last observations can also help to understand why the conversion efficiency to S, was always lower than 1% when the laser beam obtained with the unstable resonator with M = 6 was used as pump, considering also the low intensity of the first Stokes beam at P,-H~ < 15 atm (fig. 3). By using the same pump beam of fig. 4, the conversion efficiency to S, was also investigated as a function of the pumping beam energy Ei at PCH~ = 35 atm. The results are shown in fig. 5. We observe that the energy transferred to S, increases monotonically with Ei. A maximum energy conversion of 42%, which corresponds to a photon efficiency of 45%, was obtained at Ei = 23 mJ. The energy conversion to S, was always less than 1%. Then, these experimental conditions resulted very effective in reducing the four-wave mixing processes, which give rise to higher Stokes emissions and so limit the conversion efficiency to S, . To test how the preceding experimental results depend on the Raman medium, experiments were also performed with H,, which exhibits a very high Raman gain [ 111. The results obtained under the same experimental conditions of fig. 5 are shown on fig. 6.

Fig. 6 gives the energy conversion to S, (245 mn) and S, (272 nm) as a function of KrCl pumping energy at 35 atm of Hz pressure. In respect of the experimental results obtained with CH, (fig. 5), we observe a higher depleted pump power, a nearly constant energy conversion to S, of about 32% and a monotonical increase of the energy conversion to S, with pumping energy. These experimental results can be justified by the high value of the Raman gain in H,. In fact, at H, pressures higher than 10 atmg, = 1.5 X lo-* cm/W [ 111, whereas at PCH~ = 35 atm we haveg, = 2.5 X 1O-g cm/W. From fig. 6 it comes out, also, that, in comparison with what one observes in CH, (fig. 5), in H, the energy conversion to S, is strongly limited by the growth of S,, even if the coherence length [ 131 for the parametric processes, from which S, starts to build up, is shorter in H, than in CH, under our working conditions. In fact, the wave vector mismatch, Ak = k, ~ 2k,, + k,, can be calculated from the refractive index data given in refs. [ 141 and [15], and it has been found Ak = 20 cm-l in H, and Ak = 9 cm-l in CH,. Then, since the wave vector mismatch is about twice larger in H, than in CH,, the stronger ST generation observed in fig. 6 is principally due to the much higher value of the Raman gain in H, . Thus, it would be necessary to work at H, pressures much higher than that used with CH4 in order

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to slow down the four-wave mixing process and so the S, generation as it has been observed on several papers [6,8,16]. It is important to point out that in all the experimental conditions investigated in this work, we observed that the width of the Stokes lines was equal to the pumping linewidth, as predicted from the theory [lo] when Au, p Av.

References [l] G. Hancock and H. Zacharias, [2]

[3 ]

[4]

3. Conclusions [S]

It was found that, when methane is used as Raman medium, the divergence of the pump beam does substantially affect the maximum SRS conversion efficiency which can be reached and the energy conversion to different Stokes lines. This statement comes out clearly by comparing fig. 2 with fig. 5 and fig. 3 with fig. 4. Moreover, with a pump beam of -0.9 mrad divergence, it was shown that a 1 m focusing lens and 35 atm of methane were very effective in reducing higher Stokes and anti-Stokes emissions by four-wave mixing processes. In fact, with a pump energy of 23 mJ a photon conversion efficiency of 45% to S, was measured. A better spatial quality of the pump beam should allow a more efficient energy conversion to Sl Work is is progress on this line. Finally, it was observed that, even if H, is characterized by a higher Raman gain than CH, , the energy conversion to S, is strongly limited by the growth of S,. For our pumping geometry, we expect that H, pressures much higher than 35 atm would be necessary to reduce the four-wave mixing processes from which S, starts to build up.

1 December 1985

[6] [7] [S] [9] [lo] [ll]

[12] [13] 1141

[15] [16]

Chem. Phys. Lett. 82 (1981) 402. M.W. Geis, J.N. Randall, T.F. Deutsch, P.D. DeGraff, K.E. Krohn and L.A. Stern, Appl. Phys. Lett. 43 (1983) 74. Laser Processing and Diagnostics, ed. D. Bauerele, Springer Series in Chemical Physics, Vol. 39 (SpringerVerlag, Berlin, 1984). W. Kaiser and M. Maier, in: Laser handbook, eds. F.T. Arecchi and E.O. Schulz-Dubois (North-Holland, Amsterdam, 1972) p. 1077. S.F. Fulghum, D.W. Trainor, C. Duzy and H.A. Hyman, IEEE .I. Quantum Electron. QE-20 (1984) 218. J.L. Carlsten, J.M. Telle and R.G. Wenzel, Optics Lett. 9 (1984) 353. E. Armandillo, A. Luches, V. Nassisi and M.R. Perrone, Appl. Phys. Lett. 42 (1983) 860. D.W. Trainor, H.A. Hyman and R.M. Heinrichs, IEEE J. Quantum Electron. QE-18 (1982) 1929. T.R. Loree, R.C. Sze, D.L. Barker and P.B. Scott, IEEE J. Quantum Electron. QE-15 (1979) 337. M.G. Raymer and J. Mostowski,Phys. Rev. A24 (1981) 1980. W.K. Bischel and G. Black, Opt. Sci. Amer. Topical Meeting “Excimer Lasers”, Incline Village NV, 1983, paper TuB3. Y. Taira, K. Ide and H. Takuma, Chem. Phys. Lett. 91 (1982) 299. S.R.J. Brueck and H. Kildal, IEEE J. Quantum Electron. QE-18 (1982) 310. Londolt Bornstein II Band, Optische Kostanten (Springer Verlag) p. 6-882. Kindly provided by the Referee. AIP Handbook (McGraw-Hill,New York, 1972) p. 6110. T.R. Loree, R.C. Sze and D.L. Barker, Appl. Phys. Lett. 31 (1977) 37.

Acknowledgements Work supported Istruzione.

in part by Minister0 della Pubblica

211