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Passive compression of KrCl excimer laser pulses in naphthalene solutions
Volume 51, number 3
OPTICS COMMUNICATIONS
1 September 1984
PASSIVE COMPRESSION OF KrCI EXCIMER LASER PULSES IN NAPHTHALENE SOLUTIONS ~
Y.S. HUO *, J. GLINSKI, XJ. GU and R.F. CODE Department o f Physics and Erindale College, University o f Toronto, Toronto, Ontario, Canada MSS 1A 7 Received 20 March 1984
The width of KrC1 laser pulses has been compressed from 5.2 ns to less than 800 ps using naphthalene as the saturable absorber dye. It was found that the width of the compressed laser pulse decreased with both the input laser intensity and the concentration of naphthalene in the solution. The pulse shortening mechanism is attributed to excited state S 1-Sn transitions in naphthalene.
Short pulses of intense coherent ultraviolet radiation have extensive applications in nonlinear optics, laser spectroscopy, and solid state physics. Several authors have reported short pulse excimer lasers based on injection mode locking [1,2], active [3,4] and passive [5,6] mode locking techniques. A much simpler technique is passive compression on excimer laser pulses by the use of saturable absorber dyes placed outside the laser cavity. It is possible to ob. tain subnanosecond UV laser pulses in this way as has been recently reported for the shortening of XeF, XeC1, and KrF excimer laser pulses [7]. In this communication we shall describe the passive compression of a KrC1 laser pulse (), = 222 nm) from a full width at half maximum (FWI-IM) of ~5 ns to "0.8 ns by using naphthalene as a saturable absorber dye. Most commercially available laser dyes which absorb in the UV region of the spectrum have their peak absorption at wavelengths longer than 250 nm, and their absorption cross sections decrease rather rapidly at shorter wavelengths. Saturable absorber dyes suitable for pulse shortening should have both a fast absorption recovery time as well as a reasonably large absorption cross section [7]. Passive pulse compression occurs with these * Research supported in part by the Natural Sciences and Engineering Research Council of Canada. * On leave from the Shanghai Institute of Optics and Fine mechanics, Shanghai, China.
0 030-4018/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
saturable dyes because they have a higher transmission coefficient for the peak intensity region of the incident laser pulse. Naphthalene has a very large absorption cross section at 222 nm, but has a very long fluorescence lifetime ('-~100 ns). The S0-S 3 transition in naphthalene is therefore not suitable for use in pulse compression experiments. However, it will be shown later that the S1-S n excited state transitions in naphthalene have rather large cross sections at X = 222 nm, and also have very short upper state lifetimes. It is these transitions in naphthalene that are able to passively compress US laser pulses. The experimental set-up is shown in fig. 1. A commercial UV.preionized discharge KrC1 excimer laser
Ai
[KrCI LASER t
] rLi DYE CELL- ~ - - ----~-
HAMAMATSU PHOTODIODE
"-
I
Az L2
TEKTRONIX 4520A DIGITIZED SCOPE Fig. 1. Schematic of the experimentalset-up. A I , A~ are apertures, and L1, L2 are the planoconvex lenses.
181
Volume 51, number 3
OPTICS COMMUNICATIONS
(Lumonics Model TE-860-2) was used in this experi. ment. It produced 5 mJ in a pulse of ~5 ns duration. The beam size was 7 × 20 mm and had a divergence of 2.4 X 6 mrad. The focal lengths of the planoconvex lenses L I; L 2 weie 5 em aiiu 15 emrespeetively; and the distance between them was adjusted to be a little less than the sum of the focal lengths (as determined at 632.8 nm) of the two lenses. This preferentially collimated the 222 nm radiation onto the photodiode, The quartz cell of optical path 20 mm was positioned horizontally, and the thickness of the dye solution was usually 2 mm. When the input laser beam was focused by the lens L 1 onto the surface of the dye solution, the intensity at the quartz cell surface could be kept fairly well below the damage threshold. The combined response time of the temporal measurement system, composed of the Hamamatsu photodiode R1193 U-02 (200 ps) and the Tektronix Model 7912 AD Programmable Digitizer (bandwidth 500 MHz), was about Ins. The waveforms of the input laser pulse of 5 ns in duration and of the output pulse compressed by a 2 nlm-thick naphthalene-cyclohexane solution of concentration 6.5 × 10 - 4 M/1 are shown in figs. 2(a) and (b) respectively. It is obvious that both the leading edge and the trailing edge of the input pulse were compressed considerably. The zero time may not be the same on both oscillograms because of a jitter (+2 ns) between the discharge trigger and lasing action. The compressed pulse width was 800 ps after being deconvolved from the detection response time. When a colour Filter which had high transmission (T > 90%) in the spectroscopic range of wavelengths longer than 300 nm and had strong absorption ( T < 1%) at the KrC1 laser wavelength (222 nm)was put in front of the photodiode, there were no observable signals on the oscilloscope. Because the fluorescence of naphthalene is mainly in the spectroscopic range of 300-380 nm [8] (independent of the excitation wavelength), the signal observed could be neither fluorescence nor stimulated emission of naphthalene molecules under the irradiance of the input KrC1 laser pulse. It is stiown in fig. 3 and fig. 4 that the width of the compressed laser pulse decreases with both the input laser intensity and the concentration of naph. thalene in cyclohexane. In our experiment, the input laser intensity was changed by UV-grade metal Film neutral density filters. When the input intensity in:l
182
'
1 September 1984
i
....
i
i
11~
_z
IM
a: (a)
T
--~ 2ns ~ -
>" t.-
_z
Q:ta
b)
T
-~2ns ~ -
Fig. 2. Oscillograms showing (a) the incident pulse and (b) the compressed pulse. The incident light intensity was 150 MW/cm 2 and the dye concentration was 6.5 X 10 -4 M/I of naphthalene in cyclohexane. The horizontal scale is 2 ns/div.
creased from 15 MW/cm 2 to 150 MW/cm 2 for a naphthalene concentration of 6.5 X 10 - 4 M/1 in cyclohexane, the FWHM of the compressed laser pulse decreased from ~3 ns to 0.8 ns. It can be seen from fig. 4 that the FWHM decreased from 3 ns to 1 ns as the concentration of naphthalene increased from 1 X 10 -4 M/1 to 6.5 X 10 - 4 M/I while the input light intensity was kept constant at 120 MW/cm 2. When the concentration of naphthalene was zero, the pulse width was about 4 ns, which was less than that of the input pulse ( " 5 ns). Therefore, these experiments established that pure cyclohexane had some compressional influence on intense KrC1 laser pulses. (Caledon spectro-grade cyclohexane was used as the solvent and no further purification was carried out.) For an'optically thxcK Iast saturable absorber, me variation of the transmission T with the input light intensity I satisfies the following equation [9]
3
~J
I
C
0.2
01.4
RELATIVE
01.6
Oi.
8
i
i.O
INTENSITY
..,
I
Fig. 3. FWHM of the compressed pulses as a functiot~ of ~e. incident light intensity. The concentration of naphthaleim i~ cyclohexane was 6.5 × 10 -4 M/1 and the unattenuated inci-~ , dent light intensity was 150 MW/cm2 .
ln(To/T ) + (I/Is)(1 - T) = 0 ,
(1)
as long as the recovery time of the absorber is much shorter than the input pulse width. Here T O and I s are
"r IE~
2.
uJ _J
I
I
1 September 1984
OPTICS COMMUNICATIONS
Volume 51, number 3
I 2
I 3
I 4
i 5
I 6
CONCENTRATION [1(~ 4 M / L ]
Fig. 4. FWHM of the compressed pulses as a function of the naphthalene concentration in cyclohexane. The incident light intensity was ~ 120 MW/cm2 .
the small.signal transmission and the saturation intensity respectively. It can be derived from eq. (1) that the transmission T and the output light intensity increase with the input light intensity under the condition that T O < 1. Therefore, the part of the output laser pulse with highest intensity will correspond to the maximum intensity of the input laser pulse. If we neglect the time interval for light to pass through the saturable absorber (~10 -11 s for a 2 m m thickness of dye), the peaks of both the input and the output laser pulses will then appear at the same time t o . Suppose t that Ip and lp are the peak input intensity and its corresponding peak output intensity respectively, I w i and I w are respectively the intensities of the input laser pulse and of the output laser pulse at so chosen a time t w that Iw/I p = 1/2, and therefore At = 2(t w - to) is the FWHM of the output pulse. Let y = lw/lp; then it is obvious t h a t y > 1/2 corresponds to the case of pulse compression and the larger y is, the stronger the pulse is compressed. From eq. (1), we easily derive the following equations r 0 exp[x (1 - rp)] = T p , 1
(2) 1
y exp [x (y - 1 + ~-Tp)] = ~-,
(3)
where Tp = I'p/Ip is the transmission for the peak intensity, x = Ip/I s is the dimensionless peak input intensity in units o f I s. In eqs. (2) and (3), y and Tp can be considered as functions of T O and x. For the case of absorption, we have Tp < 1, and the solution of eq. (3) exists only in the region o f y > 1/2. Differentiating both sides of eqs. (2) and (3) with respect to x and T O respectively, we have ay/i~T o < O, and ay/ax > 0 under the condition that Tp ,< 1. These results are qualitatively consistent with the experimental results shown in fig. 3 and fig. 4. After absorbing a photon from the KrC1 laser pulse (222 nm), a naphthalene molecule transits from its ground state S O (1A) into its third excited singlet state 83(1Bb). The peak absorption of the S0-S 3 transition is at the wavelength of 220.7 nm, where the extinction coefficient is 1.17 X 105 1M - 1 cm -1 [8]. The molecules in the S 3 state are rapidly converted to the first excited singlet state SI(1Lb) through radiationless internal conversion [8,10], which proceeds at a rate of Kic " d 0 1 ° - 1 0 1 2 s - 1. Because the transition SI(1Lb) ~ S0(1A ) for naphthalene molecules is symmetry-forbidden, it has a fluorescence 183
Volume 51, number 3
OPTICS COMMUNICATIONS
lifetime of Tf ~ 100 ns [8], which is much longer than the input laser pulse width ("~5 ns). Both the leading edge and the trailing edge of the laser pulse can be compressed only under the condition that the recovery time of the satuable absorber is much less than the input laser pulse width [ 11 ]. Therefore the pulse shortening shown in fig. 2 could not be caused solely by the naphthalene S0--S 3 absorption, which has such a long recovery time via the S 1 state. Since the laser pulse width is much shorter than the lifetime of the S 1 state, it is poss~le to excite most of the naphthalene molecules from the So state to the S 1 state, so that subsequent absorptions are of the transient S 1 - S n type [8]. Kiryukhin et al. have measured the absorption spectrum of the naphthalene S 1 --Sn transitions in the spectroscopic range of 2 4 0 320 nm [12], and have shown that the S 1 - S n absorption strength sharply increases in the X < 260 nm region. In the present experiment, the energy of the KrCl laser pulse passing through diaphragm A 1 was about 0.7 m J, corresponding to a photon number of 7 × 1014. The number of naphthalene molecules in the region irradiated by the focused laser beam was about 2 X 1013. It is possible that the photons in the front edge of the laser pulse excited most of the naphthalene molecules from the S O to the S 1 state, and the following S 1 - Sn transitions caused the pulse shortening. The variation of the transmission with input light intensity is shown in fig. 5 for the 2 ram-thick naph-
t.C
'_o z
0.£
1 September 1984
thalene-ethanol solution of concentration of 5 × 10 - 4 M/1. Spectroscopic grade ethanol was used as the solvent instead of cyclohexane in this measurement, because the former showed less variation of transmission with respect to incident light intensity than the latter. Fig. 5 shows a typical curve for a two-level or a fast three-level steady-state absorption [9]. The saturation intensity and the cross section for the S 1 --S n absorption were determined to be 20 MW/ cm 2 and 1.7 X 10-16 cm 2 respectively by using the steady-state equation for a fast three-level system [9]. An upper-state lifetime of 2.7 X 1 0 - 1 0 s for the S 1-S n transitions could be deduced from I s = hv/or, where hv is the photon energy and r is the upperstate lifetime. Because the cross section for the S 1 - S n absorption is about half of that for the S 0 S 3 absorption, there is an especially strong absorption for photons from the early part of the leading edge of the laser pulse where most of the naphthalene molecules are found in the ground state S O. Naphthalene should therefore be effective in compressing a laser pulse with a slowly rising leading edge. In conclusion, we have obtained subnanosecond pulses from a KrC1 laser using naphthalene solutions as the saturable absorber. Because of the strong absorption necessary for significant pulse shortening, the intensity of the laser pulse is considerably attenuated ('~10-3). Of course, high power subnanosecond KrC1 laser pulses could be obtained by further amplification of these compressed pulses. We have proposed that the transition between the first excited singlet state and a higher excited state of naphthalene is responsible for the observed pulse shortening. Based on this model, we have obtained the saturation intensity, absorption cross-section, and the upperstate lifetime for the naphthalene S 1 - S n transitions at 222 nm.
Z
~ OY
Fig. 5. Peak transanission coeffieient versus input intensity of KrCI laser pulses passing through a 2 mm thick solution of naphthalene in ethanol (5 X 10--4 M/l). i84
References [1] G. Reksten, T. Varghese and D.J. Bradley, Appl. Phys. Lett. 38 (1981) 513. [2] T. Varghese, Appl. Phys. Lett. 40 (1982) 127. [3] C.P. Christensen, L.W. Braverman, W.H. Steier and C. Wittig, Appl. Phys. Lett. 29 (1976) 424. [4] G. Reksten, T. Varghese and W. M~gulis, Appl. Phys. Lett. 39(1981) 129.
Volume 51, number 3
OPTICS COMMUNICATIONS
[5] T. Efthimiopoulos, J. Banic and B.P. Stoicheff, Can. J. Phys. 57 (1979) 1437. [6] S. Watanabe, M. Watanabe and A. Endoh, Appl. Phys. Lett. 43 (1983) 533. [7] T. Varghese, Appl. Phys. Lett. 41 (1982) 684. [8] J.B. Birks, Photophysics of aromatic moleeules (WileyInterscience, London, 1970).
1 September 1984
[9] M. Hercher, Appl. Optics 6 (1967) 947. [10] C.R. Giuliano and L.D. Hess, IEEE J. Quantum Electron. QE-3 (1967) 358. [11] P.G. Kruykov and V.S. Letokhov, Soy. Phys. Usp. 12 (1970) 535. [12] Yu.I. Kiryukhin, Z.A. Sinitsyna and Kh.S. Bagdasaryan, Opt. Spectrosc. (USSR) 46 (1979) 517.
185
OPTICS COMMUNICATIONS
1 September 1984
PASSIVE COMPRESSION OF KrCI EXCIMER LASER PULSES IN NAPHTHALENE SOLUTIONS ~
Y.S. HUO *, J. GLINSKI, XJ. GU and R.F. CODE Department o f Physics and Erindale College, University o f Toronto, Toronto, Ontario, Canada MSS 1A 7 Received 20 March 1984
The width of KrC1 laser pulses has been compressed from 5.2 ns to less than 800 ps using naphthalene as the saturable absorber dye. It was found that the width of the compressed laser pulse decreased with both the input laser intensity and the concentration of naphthalene in the solution. The pulse shortening mechanism is attributed to excited state S 1-Sn transitions in naphthalene.
Short pulses of intense coherent ultraviolet radiation have extensive applications in nonlinear optics, laser spectroscopy, and solid state physics. Several authors have reported short pulse excimer lasers based on injection mode locking [1,2], active [3,4] and passive [5,6] mode locking techniques. A much simpler technique is passive compression on excimer laser pulses by the use of saturable absorber dyes placed outside the laser cavity. It is possible to ob. tain subnanosecond UV laser pulses in this way as has been recently reported for the shortening of XeF, XeC1, and KrF excimer laser pulses [7]. In this communication we shall describe the passive compression of a KrC1 laser pulse (), = 222 nm) from a full width at half maximum (FWI-IM) of ~5 ns to "0.8 ns by using naphthalene as a saturable absorber dye. Most commercially available laser dyes which absorb in the UV region of the spectrum have their peak absorption at wavelengths longer than 250 nm, and their absorption cross sections decrease rather rapidly at shorter wavelengths. Saturable absorber dyes suitable for pulse shortening should have both a fast absorption recovery time as well as a reasonably large absorption cross section [7]. Passive pulse compression occurs with these * Research supported in part by the Natural Sciences and Engineering Research Council of Canada. * On leave from the Shanghai Institute of Optics and Fine mechanics, Shanghai, China.
0 030-4018/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
saturable dyes because they have a higher transmission coefficient for the peak intensity region of the incident laser pulse. Naphthalene has a very large absorption cross section at 222 nm, but has a very long fluorescence lifetime ('-~100 ns). The S0-S 3 transition in naphthalene is therefore not suitable for use in pulse compression experiments. However, it will be shown later that the S1-S n excited state transitions in naphthalene have rather large cross sections at X = 222 nm, and also have very short upper state lifetimes. It is these transitions in naphthalene that are able to passively compress US laser pulses. The experimental set-up is shown in fig. 1. A commercial UV.preionized discharge KrC1 excimer laser
Ai
[KrCI LASER t
] rLi DYE CELL- ~ - - ----~-
HAMAMATSU PHOTODIODE
"-
I
Az L2
TEKTRONIX 4520A DIGITIZED SCOPE Fig. 1. Schematic of the experimentalset-up. A I , A~ are apertures, and L1, L2 are the planoconvex lenses.
181
Volume 51, number 3
OPTICS COMMUNICATIONS
(Lumonics Model TE-860-2) was used in this experi. ment. It produced 5 mJ in a pulse of ~5 ns duration. The beam size was 7 × 20 mm and had a divergence of 2.4 X 6 mrad. The focal lengths of the planoconvex lenses L I; L 2 weie 5 em aiiu 15 emrespeetively; and the distance between them was adjusted to be a little less than the sum of the focal lengths (as determined at 632.8 nm) of the two lenses. This preferentially collimated the 222 nm radiation onto the photodiode, The quartz cell of optical path 20 mm was positioned horizontally, and the thickness of the dye solution was usually 2 mm. When the input laser beam was focused by the lens L 1 onto the surface of the dye solution, the intensity at the quartz cell surface could be kept fairly well below the damage threshold. The combined response time of the temporal measurement system, composed of the Hamamatsu photodiode R1193 U-02 (200 ps) and the Tektronix Model 7912 AD Programmable Digitizer (bandwidth 500 MHz), was about Ins. The waveforms of the input laser pulse of 5 ns in duration and of the output pulse compressed by a 2 nlm-thick naphthalene-cyclohexane solution of concentration 6.5 × 10 - 4 M/1 are shown in figs. 2(a) and (b) respectively. It is obvious that both the leading edge and the trailing edge of the input pulse were compressed considerably. The zero time may not be the same on both oscillograms because of a jitter (+2 ns) between the discharge trigger and lasing action. The compressed pulse width was 800 ps after being deconvolved from the detection response time. When a colour Filter which had high transmission (T > 90%) in the spectroscopic range of wavelengths longer than 300 nm and had strong absorption ( T < 1%) at the KrC1 laser wavelength (222 nm)was put in front of the photodiode, there were no observable signals on the oscilloscope. Because the fluorescence of naphthalene is mainly in the spectroscopic range of 300-380 nm [8] (independent of the excitation wavelength), the signal observed could be neither fluorescence nor stimulated emission of naphthalene molecules under the irradiance of the input KrC1 laser pulse. It is stiown in fig. 3 and fig. 4 that the width of the compressed laser pulse decreases with both the input laser intensity and the concentration of naph. thalene in cyclohexane. In our experiment, the input laser intensity was changed by UV-grade metal Film neutral density filters. When the input intensity in:l
182
'
1 September 1984
i
....
i
i
11~
_z
IM
a: (a)
T
--~ 2ns ~ -
>" t.-
_z
Q:ta
b)
T
-~2ns ~ -
Fig. 2. Oscillograms showing (a) the incident pulse and (b) the compressed pulse. The incident light intensity was 150 MW/cm 2 and the dye concentration was 6.5 X 10 -4 M/I of naphthalene in cyclohexane. The horizontal scale is 2 ns/div.
creased from 15 MW/cm 2 to 150 MW/cm 2 for a naphthalene concentration of 6.5 X 10 - 4 M/1 in cyclohexane, the FWHM of the compressed laser pulse decreased from ~3 ns to 0.8 ns. It can be seen from fig. 4 that the FWHM decreased from 3 ns to 1 ns as the concentration of naphthalene increased from 1 X 10 -4 M/1 to 6.5 X 10 - 4 M/I while the input light intensity was kept constant at 120 MW/cm 2. When the concentration of naphthalene was zero, the pulse width was about 4 ns, which was less than that of the input pulse ( " 5 ns). Therefore, these experiments established that pure cyclohexane had some compressional influence on intense KrC1 laser pulses. (Caledon spectro-grade cyclohexane was used as the solvent and no further purification was carried out.) For an'optically thxcK Iast saturable absorber, me variation of the transmission T with the input light intensity I satisfies the following equation [9]
3
~J
I
C
0.2
01.4
RELATIVE
01.6
Oi.
8
i
i.O
INTENSITY
..,
I
Fig. 3. FWHM of the compressed pulses as a functiot~ of ~e. incident light intensity. The concentration of naphthaleim i~ cyclohexane was 6.5 × 10 -4 M/1 and the unattenuated inci-~ , dent light intensity was 150 MW/cm2 .
ln(To/T ) + (I/Is)(1 - T) = 0 ,
(1)
as long as the recovery time of the absorber is much shorter than the input pulse width. Here T O and I s are
"r IE~
2.
uJ _J
I
I
1 September 1984
OPTICS COMMUNICATIONS
Volume 51, number 3
I 2
I 3
I 4
i 5
I 6
CONCENTRATION [1(~ 4 M / L ]
Fig. 4. FWHM of the compressed pulses as a function of the naphthalene concentration in cyclohexane. The incident light intensity was ~ 120 MW/cm2 .
the small.signal transmission and the saturation intensity respectively. It can be derived from eq. (1) that the transmission T and the output light intensity increase with the input light intensity under the condition that T O < 1. Therefore, the part of the output laser pulse with highest intensity will correspond to the maximum intensity of the input laser pulse. If we neglect the time interval for light to pass through the saturable absorber (~10 -11 s for a 2 m m thickness of dye), the peaks of both the input and the output laser pulses will then appear at the same time t o . Suppose t that Ip and lp are the peak input intensity and its corresponding peak output intensity respectively, I w i and I w are respectively the intensities of the input laser pulse and of the output laser pulse at so chosen a time t w that Iw/I p = 1/2, and therefore At = 2(t w - to) is the FWHM of the output pulse. Let y = lw/lp; then it is obvious t h a t y > 1/2 corresponds to the case of pulse compression and the larger y is, the stronger the pulse is compressed. From eq. (1), we easily derive the following equations r 0 exp[x (1 - rp)] = T p , 1
(2) 1
y exp [x (y - 1 + ~-Tp)] = ~-,
(3)
where Tp = I'p/Ip is the transmission for the peak intensity, x = Ip/I s is the dimensionless peak input intensity in units o f I s. In eqs. (2) and (3), y and Tp can be considered as functions of T O and x. For the case of absorption, we have Tp < 1, and the solution of eq. (3) exists only in the region o f y > 1/2. Differentiating both sides of eqs. (2) and (3) with respect to x and T O respectively, we have ay/i~T o < O, and ay/ax > 0 under the condition that Tp ,< 1. These results are qualitatively consistent with the experimental results shown in fig. 3 and fig. 4. After absorbing a photon from the KrC1 laser pulse (222 nm), a naphthalene molecule transits from its ground state S O (1A) into its third excited singlet state 83(1Bb). The peak absorption of the S0-S 3 transition is at the wavelength of 220.7 nm, where the extinction coefficient is 1.17 X 105 1M - 1 cm -1 [8]. The molecules in the S 3 state are rapidly converted to the first excited singlet state SI(1Lb) through radiationless internal conversion [8,10], which proceeds at a rate of Kic " d 0 1 ° - 1 0 1 2 s - 1. Because the transition SI(1Lb) ~ S0(1A ) for naphthalene molecules is symmetry-forbidden, it has a fluorescence 183
Volume 51, number 3
OPTICS COMMUNICATIONS
lifetime of Tf ~ 100 ns [8], which is much longer than the input laser pulse width ("~5 ns). Both the leading edge and the trailing edge of the laser pulse can be compressed only under the condition that the recovery time of the satuable absorber is much less than the input laser pulse width [ 11 ]. Therefore the pulse shortening shown in fig. 2 could not be caused solely by the naphthalene S0--S 3 absorption, which has such a long recovery time via the S 1 state. Since the laser pulse width is much shorter than the lifetime of the S 1 state, it is poss~le to excite most of the naphthalene molecules from the So state to the S 1 state, so that subsequent absorptions are of the transient S 1 - S n type [8]. Kiryukhin et al. have measured the absorption spectrum of the naphthalene S 1 --Sn transitions in the spectroscopic range of 2 4 0 320 nm [12], and have shown that the S 1 - S n absorption strength sharply increases in the X < 260 nm region. In the present experiment, the energy of the KrCl laser pulse passing through diaphragm A 1 was about 0.7 m J, corresponding to a photon number of 7 × 1014. The number of naphthalene molecules in the region irradiated by the focused laser beam was about 2 X 1013. It is possible that the photons in the front edge of the laser pulse excited most of the naphthalene molecules from the S O to the S 1 state, and the following S 1 - Sn transitions caused the pulse shortening. The variation of the transmission with input light intensity is shown in fig. 5 for the 2 ram-thick naph-
t.C
'_o z
0.£
1 September 1984
thalene-ethanol solution of concentration of 5 × 10 - 4 M/1. Spectroscopic grade ethanol was used as the solvent instead of cyclohexane in this measurement, because the former showed less variation of transmission with respect to incident light intensity than the latter. Fig. 5 shows a typical curve for a two-level or a fast three-level steady-state absorption [9]. The saturation intensity and the cross section for the S 1 --S n absorption were determined to be 20 MW/ cm 2 and 1.7 X 10-16 cm 2 respectively by using the steady-state equation for a fast three-level system [9]. An upper-state lifetime of 2.7 X 1 0 - 1 0 s for the S 1-S n transitions could be deduced from I s = hv/or, where hv is the photon energy and r is the upperstate lifetime. Because the cross section for the S 1 - S n absorption is about half of that for the S 0 S 3 absorption, there is an especially strong absorption for photons from the early part of the leading edge of the laser pulse where most of the naphthalene molecules are found in the ground state S O. Naphthalene should therefore be effective in compressing a laser pulse with a slowly rising leading edge. In conclusion, we have obtained subnanosecond pulses from a KrC1 laser using naphthalene solutions as the saturable absorber. Because of the strong absorption necessary for significant pulse shortening, the intensity of the laser pulse is considerably attenuated ('~10-3). Of course, high power subnanosecond KrC1 laser pulses could be obtained by further amplification of these compressed pulses. We have proposed that the transition between the first excited singlet state and a higher excited state of naphthalene is responsible for the observed pulse shortening. Based on this model, we have obtained the saturation intensity, absorption cross-section, and the upperstate lifetime for the naphthalene S 1 - S n transitions at 222 nm.
Z
~ OY
Fig. 5. Peak transanission coeffieient versus input intensity of KrCI laser pulses passing through a 2 mm thick solution of naphthalene in ethanol (5 X 10--4 M/l). i84
References [1] G. Reksten, T. Varghese and D.J. Bradley, Appl. Phys. Lett. 38 (1981) 513. [2] T. Varghese, Appl. Phys. Lett. 40 (1982) 127. [3] C.P. Christensen, L.W. Braverman, W.H. Steier and C. Wittig, Appl. Phys. Lett. 29 (1976) 424. [4] G. Reksten, T. Varghese and W. M~gulis, Appl. Phys. Lett. 39(1981) 129.
Volume 51, number 3
OPTICS COMMUNICATIONS
[5] T. Efthimiopoulos, J. Banic and B.P. Stoicheff, Can. J. Phys. 57 (1979) 1437. [6] S. Watanabe, M. Watanabe and A. Endoh, Appl. Phys. Lett. 43 (1983) 533. [7] T. Varghese, Appl. Phys. Lett. 41 (1982) 684. [8] J.B. Birks, Photophysics of aromatic moleeules (WileyInterscience, London, 1970).
1 September 1984
[9] M. Hercher, Appl. Optics 6 (1967) 947. [10] C.R. Giuliano and L.D. Hess, IEEE J. Quantum Electron. QE-3 (1967) 358. [11] P.G. Kruykov and V.S. Letokhov, Soy. Phys. Usp. 12 (1970) 535. [12] Yu.I. Kiryukhin, Z.A. Sinitsyna and Kh.S. Bagdasaryan, Opt. Spectrosc. (USSR) 46 (1979) 517.
185