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Generation of ultrashort far-infrared pulses optically pumped with truncated hybrid 10 μm CO2-laser pulses
Optics Communications 87 ( 1992 ) 249-253 North-Holland
OPTICS COMMUN ICATIONS
Generation of ultrashort far-infrared pulses optically pumped with truncated hybrid 10 gm CO2-1aser pulses D.P. Scherrer, A.W. K~ilin, R. Kesselring and F.K. Kneubtihl Infrared Physics Laboratoo', Institute of Quantum Electronics, ETH, CH-8093 Zurich, Switzerland Received 3 June 1991; revised manuscript received 7 October 1991
We have investigated the generation of ultrashort far-infrared pulses by optical pumping of 496 jam CH3F, 385 jam D20, 373 jam CH3CN and 291 jam NH3 with pulses from a hybrid CO2 laser which are truncated within ~ 10 ps by a plasma shutter of new design. We observe smooth and reproducible single far-infrared nano- and subnano-second pulses which are among the shortest single pulses produced in this wavelength range. Thus, we demonstrate that our technique is suited for the generation of welldefined ultrashort single far-infrared pulses.
1. Introduction The generation of ultrashort coherent submillimeter wave pulses is of interest for the understanding of fundamental processes occuring on a short-time scale in far-infrared gas lasers, for applications in realtime spectroscopy of gaseous plasmas or the study of transient phenomena in high Tc superconductors. Far-infrared lasers are generally low-pressure narrow-band systems. Therefore, wideband short laser pulses in this wavelength range cannot be produced by conventional means. On the contrary, coherentemission processes such as superradiance [1-3] or Raman emission [4,5] have been demonstrated to be attractive alternative approaches to produce ultrashort far-infrared pulses [ 6 ]. In the process of superradiant emission the atoms are coupled together by their common radiation field, and thus decay cooperatively. For a number N of emitting atoms or molecules the pulse intensity is therefore proportional to N 2 and the pulse duration to 1/N. Since N can be large, this process offers the possibility of producing intense short pulses. In the case of Raman emission where no inversion between two levels is needed, the pulses can be shorter than the inverse linewidth of the transition if the pump pulse is broadband. Hence, the pulse duration is determined by the large of either the pump bandwidth or the laser linewidth.
Our experiments concern the generation of nanosecond superradiant as well as nano-and subnano-second Raman far-infrared pulses.
2. Experimental arrangement Our experimental arrangement is shown in fig. 1. It includes a grating-tunable hybrid TEA CO2 laser which emits single-mode pulses of 70 ns duration and up to 0.5 J energy. Its pulses are truncated by an improved adjustable plasma shutter [7] which contains a 1:1 telescope formed by two ZnSe lenses. A high-voltage discharge in the focus of the telescope which is driven by a laser triggered spark gap (LTSG) initiates the plasma breakdown. The cut-off by the plasma shutter occurs within about 10 ps which is confirmed by the production of 35 ps pulses with an OFID CO2 laser system based on this plasma shutter [ 8 ]. The truncated COz pulse passes through the farinfrared cell consisting of a glass tube of 26 mm diameter and 3.5 m length with a NaC1 Brewster entrance window and a quartz output window reflecting the COz infrared radiation. The far-infrared pulses are detected by a fast sensitive Schottky-barrier diode detector [9 ].
Volume 87, number 5,6
OPTICS COMMUNICATIONS
15 February 1992
f :l,~.s'::~ ,~ ,Vb u C;~" lrllet
t t e r"
CO2
lO flltfl H y b r i d
Single I,on~itudinal
i
[
!*l
Off Axis Focusing Mirror
Schottky
[]armer/ / ~ Tektvomx
Diode D e t e c t o r
4~/ k[--[IZ @ ~
I .
7'912Ht]
•
]
/" "
I
Pho 1o,, Deter:tot I)eleO' ~.me'
~>
",/
IJ~,~ [
] ]
Fig. 1. Experiment arrangement. 3. Results
We first investigated the strong 496 g m CH3F superradiant emission. Thus, we obtained well-defined single pulses as short as 2.2 ns at a gas pressure of 4.2 Torr with an estimated peak power of about 25 kW. The duration r of such pulses is comparable with the conventional lower limit zp = 8 ns Torr given by the inverse linewidth of the transition [6]. In a first approach we might suppose that only the pressure broadening influences the pulse duration. However, when decreasing the pressure we observe that the pulses become shorter than the limit zp mentioned above. As recently reported [ 10 ], our measurements at a gas pressure below 1 Torr show that the pulses are considerably shorter than this limit and also shorter than those obtained by Rosenberger and DeTemple [ 11 ] under similar experimental conditions. These results are explained by an equivalent inhomogeneous broadening in CH3F due to its Kmultiplet structure [ 10 ]. This implies that the p u m p pulse essentially determines the duration of the farinfrared pulse. This is agreement with the fact that our plasma shutter performs a cut-off within about l 0 ps which have to be compared with the 100 ps of the plasma shutter used by Rosenberger and De250
Temple. This results in a larger spectral width of our pump pulse due to the drastic increase of the imaginary part of the Fourier transform of the truncated pulse. In fig. 2a we show the measured truncated CO2 pump pulse and in fig. 2b the corresponding calculated Fourier spectrum. The limited temporal resolution of the detection system is responsible for the apparent relatively long cut-off time of the CO2 pulse of fig. 2a. As mentioned before, the truncation occurs effectively within about 10 ps as confirmed by O F I D experiments [8]. In figs. 3a, b we show 496 ~tm CH3F pulses obtained for two different pressure regimes. In fig. 3a the pulse width of 8 ns is well under the conventional pulse-width limit r p = 2 0 ns for a gas pressure of 400 mTorr given by the inverse linewidth of the transition. In fig. 3b we notice the small peaks a, b, c, d appearing after the 2,2 ns CH3F pulse. These are due to the interaction of different K emissions originating in the K-multiplet structure of the rotational energy levels of CH3F. Our theoretical analysis shows that for a pressure near 4 Tort the five first K transitions contribute to the observed pulse shape. With our system, we also investigated the emission of CH3CN, which is a molecule of low laser efficiency, and generated for the first time 373 gm
Volume 87, number 5,6
OPTICS COMMUNICATIONS
(a)
496 k t m - C H 3 F
[--
Z
Z
[..., Z
E'-
15 February 1992
/
(a)
~
20 ns p
~ 10 ps
z /
r<
,.A
,/
r~
/
\
/
20
',..~
20 ns / Div.
ns/div.
496 g m - C H 3 F Z
e.8
(b)
I'A
7 ----~! ~ 2 . 2 I i
i ,
<
a
ns C /"~
d
10 n s / d i v . Fig. 3. (a) 8 ns, 496 MmCH3F pulse. Pressure 400 m Torr. (b) 2.2 ns, 496 ~tm CH3F pulse. Pressure 4.2 Torr.
385 ~.m-D20 -0.5
-0.2~
e
Am (GHz) Fig. 2. (a) Truncated CO2-1aserpump pulse. (b) Fourier transformation of the truncated CO2 pump pulse.
1 ns/div. CH3CN s u p e r r a d i a n t emission [ 10]. In our arrangement CH3CN emits 373 ~tm only in the pressure range between 0.25 and 1.5 Torr. The shortest pulses p r o d u c e d with this molecule exhibit a d u r a t i o n o f 6 ns with a peak power o f 10 kW. Also for this molecule we find a pulse width noticably shorter than that d e t e r m i n e d by the inverse linewidth o f the transition for a pressure below 1 Torr [ 10]. In the following we p e r f o r m e d the same experiments with molecules which emit by a R a m a n process, D 2 0 at 385 g m and N H 3 at 291 g m wavelength. As m e n t i o n e d before, R a m a n emission is o f advantage because it does not require inversion. Thus, the pulse f o r m a t i o n and emission can take
:. . . . . .
Fig. 4. 1.25 ns, 385 ~tm D20 pulse. Pressure 2.4 Torr. place on a shorter time scale. Furthermore, far-infrared emission is obtained for each frequency component o f the p u m p pulse. Thus, we generated smooth and reproducible 385 Mm pulses as short as 1.25 ns and 17 k W peak power with D 2 0 at 2.4 Torr (fig. 4). The pulse are well shorter than the 3 ns o f the inverse linewidth o f the transition in this pressure range. In addition, we notice that no shortening o f the pulse duration occurs when increasing the pressure up to 10 Torr where in our experiment the D20 radiation stops. This demonstrates that only the 251
Volume 87, number 5.6
OPTICS COMMUNICATIONS
characteristics of the p u m p pulse determine the farinfrared pulse duration as soon as an o p t i m u m pressure is reached. The o p t i m u m pressure depends on the laser gas and experimental conditions. As shown in fig. 5a we also generated 291 g m pulses as short as 0.8 ns and a peak power of 20 kW with NH3 at 9 Torr. These pulses are slightly shorter than the 1.2 ns corresponding to the inverse linewidth of the transition in NH3 for this pressure [ 12 ]. We note that in contrast the case of D~O the o p t i m u m pressure for the shortest pulses is not reached. In fact, by increasing the pressure above 10 Tort we were able to produce single 770 ps pulses. The strong absorption above 10 Torr requires a higher pump intensity for the reproducible generation of such subnanosecond pulses, fig. 5b shows the truncated CO2 p u m p pulse and the resulting simultaneous NH3 emission on a c o m m o n time scale. The rise time of the 350 MHz two-channel LeCroy oscilloscope used for delay measurements explains the longer cut-off time of the truncated p u m p pulse and the longer duration of the far-infrared pulse than those measured separately
;;,-,
(a)
291 Bm-NH3
Z
z > F--. ~e
i )/
~
800 ps
1 ns/div.
] [ 7~__ T.,.r
)(hi
10.35 ~m C02
! Ill
::,t
_
I. 0 ',ns/~liv.
_
"
[::1 ::~1
...........
I
Fig. 5. (a) 0.8 ns, 291 p.m NH3 pulse. Pressure 9 Torr. (b) Truncated CO2 pump pulse (upper trace) and resulting 291 p_mNH3 emission (lower trace). NH3 pressure 9 Torr. 252
15 Februaw 1992
with the one-channel 750 MHz Tektronix oscilloscope. In all of these experiments, we find that the bandwidth of the pump pulse drastically increased by our fast truncation essentially determines the duration of the far-infrared emission. For the Raman emitting gases and for those with a K-multiplet structure at a low pressure, we always observe pulses shorter than the conventional limit r v defined by the inverse linewidth of the transition. Gases with a K-multiplet structure reach this limit for increasing pressure. This is in contrast to the situation of conventional far-infrared lasers optically pumped by 10 p.m CO2 lasers where the duration of the far-infrared emission corresponds to the pump-pulse duration. In table I compare our results with the shortest farinfrared pulses produced hitherto. Among these we find the 355 ps, 496 btm CH3F and 710 ps, 385 p.m D20 mode-locked pulses generated by synchronously mode-locked optical pumping with CO2 lasers [13,14]. We cite also the experiments with modelocked CO2 TEA laser optical pumping in a single pass configuration [ 15 ]. With this method pulses as short as 300 ps for 496 ~tm CH3F, less than 2 ns for 385 p.m D20 and between 2 and 5 ns for 66 p.m D20, 152 g m N H 3 , 447 BmCH3I, and 1207 gm~3CH3 F have been achieved. In a similar way, superradiant 286 gm CH3F pulses of about 1 ns have been produced by optical pumping with a mode-locked highpressure CO2 laser [ 16 ]. Optical pumping with a high pressure CO2 laser in the self mode-locking state resuited in 800 ps, 385 lam D20 pulses [ 17]. However, in these cases the far-infrared emissions consist of a train of different short pulses in contrast to our experiments which provide single well-defined short pulses. In addition, we may mention that the other efficient methods for generation of single short farinfrared pulses where optical switching element is either placed outside the laser system [ 18,19 ] or used for cavity dumping [20], fail to produce pulses as short as those generated by our system.
4. Conclusion
We have demonstrated that optical pumping with rapidly truncated CO2 laser pulse of superradiant or Raman emitting far-infrared molecular gases is a
Volume 87, number 5,6
15 February. i 992
OPTICS COMMUNICATIONS
Table 1 Shortest reported far-infrared pulses at wavelengths between 100 lain and 500 lain. 2 (~tm)
Molecule
CO2 pump line
Pump scheme
fwhm (ns)
Remarks
ref.
496
CH3F
9P20
447 385
CH31 D20
10PI 8 9R22
373 291
CH3CN NH3
10P20 10R06
286 231 193 152 151 148 119
CH3F CH3F CH3F NH3 NH3 NH3 CH3OH
9R20 9R30 10R32 10P32 10P32 9P34 9P36
Synchron. mode locked Mode locked single pass configuration Optical switching ext. res. Optical switching int. res. Truncated pulse Mode locked single pass configuration Synchron. mode locked Mode locked single pass configuration High pressure self mode locking Truncated pulse Truncated pulse Optical switching int. res. Truncated pulse High pressure mode locked Optical switching int. res. Synchron. mode locked Mode locked single pass configuration Synchron. mode locked Optical switching int. res. Optical switching ext. res.
0.35 0.3 5 10-20 2.2 2-5 0.71 <2 0.8 1.25 6 7 0.8 1 7 0.37 2-5 0.53 7 2.1
Pulse train Pulse train Single pulse Single pulse Single pulse Pulse train Pulse train Pulse train Pulse train Single pulse Single pulse Single pulse Single pulse Pulse train Single pulse Pulse train Pulse train Pulse train Single pulse Single pulse
[ 13,14] [ 15 ] [ 18 ] [ 20 ] this work [ 15 ] [ 13,14 ] [ 15 ] [ 17 ] this work this work [20] this work [ 16 ] [20 ] [ 13,14] [ 15 ] [13,14] [ 20 ] [ 19 ]
successful method to produce ultrashort far-infrared pulses. The resulting smooth and reproducible single pulses are suitable for many applications.
Acknowledgements T h i s s t u d y is s u p p o r t e d b y t h e S w i s s N a t i o n a l Science Foundation and ETH Zurich. We wish to thank J.S. B a k o s , B u d a p e s t a n d H . a n d J. W a l d m a n n ,
Low-
ell, M a s s . , U S A , f o r s t i m u l a t i n g d i s c u s s i o n s .
References [ 1 ] R.H. Dicke, Phys. Rev. 93 (1954) 99. [2] R. Bonifacio and L.A. Lugiato, Phys. Rev. A 11 (1975) 1507. [3] J.C. MacGillivray and M.S. Feld, Phys. Rev. A 23 ( 1981 ) 1334. [4] S.J. Petuchowski, A.T. Rosenberger and T.A. DeTemple, IEEE J. Quant. F:lectron. QE- 13 (1977) 476. [ 5 ] H.R. Fetterman, H.R. Schlossberg and J. Waldman, Optics Comm. 6 (1972) 156. [6] T.A. DeTemple, Infrared and millimeter waves, 1, ed. K.J. Button (Academic Press, New York, 1979) chap. 3.
[ 7] A.W. Kiilin, R. Kesselring, T.E. Kopiczynski, H.J. Sch6tzau and F.K. Kneubiihl, Digest, 15th Intern. Conf. on Infrared and millimeter waves, Orlando, Florida ( 1990 ) p. 655. [8] R. Kesselring, A.W. K~ilin and F.K. Kneubtihl, Digest, 15th Intern, Conf. on Infrared and millimeter waves, Orlando, Florida (1990) p. 475. [9] H.P. R6ser, E.J. Durwen, R. Wanenbach and G.V. Schultz, Int. J. Infrared and Millim. Waves 5 (1984) 301. [ 10] D.P. Scherrer, A.W. Kiilin, R. Kesselring and F.K. Kneubiihl, Appl. Phys. B 53 ( 1991 ) (in print). [ 11] A.T. Rosenberger and T.A. DeTemple, Phys. Rev. A 24 (1981) 868. [ 12] T.Y. Chang and J.D. McGee, Appl. Phys. Lett. 29 (1976) 725. [ 13 ] W. Lemley and A.V. Nurmikko, Appl. Phys. Len. 35 ( 1979 ) 33. [ 14] W. Lemley, A.V. Nurmikko and B.J. Clifton, Int. J. Infrared and Millim. Waves 1 (1980) 85, [15] A.T. Rosenberger, H.K. Chung and T.A. DeTemple, IEEE J. Quant. Electron. QE-20 (1984) 523. [16] P.T. Lang, W. Schatz, K.F. Renk, E.V. Beregulin, S.D. Ganichev and I.D. Yaroshetskii, Int. J. Infrared and Millim. Waves 11 (1990) 851. [17] P.T. Lang, W. Schatz and K.F. Renk, Optics Comm. 84 (1991) 29. [ 18 ] R.E.M. de Bekkcr, G.L.J.A. Rikken, T. Strutz and P. Wyder, Proc. fourth intern, conf. on infrared physics, Zurich, Switzerland (1988) p. 379. [ 19] H. Salzmann, T. Vogel and G. Dodel, Optics Comm. 47 (1983) 340. [20] R.E.M. de Bekker, L.M. Claessen and P. Wyder, J. Appl. Phys. 68 (1990) 3729. 253
OPTICS COMMUN ICATIONS
Generation of ultrashort far-infrared pulses optically pumped with truncated hybrid 10 gm CO2-1aser pulses D.P. Scherrer, A.W. K~ilin, R. Kesselring and F.K. Kneubtihl Infrared Physics Laboratoo', Institute of Quantum Electronics, ETH, CH-8093 Zurich, Switzerland Received 3 June 1991; revised manuscript received 7 October 1991
We have investigated the generation of ultrashort far-infrared pulses by optical pumping of 496 jam CH3F, 385 jam D20, 373 jam CH3CN and 291 jam NH3 with pulses from a hybrid CO2 laser which are truncated within ~ 10 ps by a plasma shutter of new design. We observe smooth and reproducible single far-infrared nano- and subnano-second pulses which are among the shortest single pulses produced in this wavelength range. Thus, we demonstrate that our technique is suited for the generation of welldefined ultrashort single far-infrared pulses.
1. Introduction The generation of ultrashort coherent submillimeter wave pulses is of interest for the understanding of fundamental processes occuring on a short-time scale in far-infrared gas lasers, for applications in realtime spectroscopy of gaseous plasmas or the study of transient phenomena in high Tc superconductors. Far-infrared lasers are generally low-pressure narrow-band systems. Therefore, wideband short laser pulses in this wavelength range cannot be produced by conventional means. On the contrary, coherentemission processes such as superradiance [1-3] or Raman emission [4,5] have been demonstrated to be attractive alternative approaches to produce ultrashort far-infrared pulses [ 6 ]. In the process of superradiant emission the atoms are coupled together by their common radiation field, and thus decay cooperatively. For a number N of emitting atoms or molecules the pulse intensity is therefore proportional to N 2 and the pulse duration to 1/N. Since N can be large, this process offers the possibility of producing intense short pulses. In the case of Raman emission where no inversion between two levels is needed, the pulses can be shorter than the inverse linewidth of the transition if the pump pulse is broadband. Hence, the pulse duration is determined by the large of either the pump bandwidth or the laser linewidth.
Our experiments concern the generation of nanosecond superradiant as well as nano-and subnano-second Raman far-infrared pulses.
2. Experimental arrangement Our experimental arrangement is shown in fig. 1. It includes a grating-tunable hybrid TEA CO2 laser which emits single-mode pulses of 70 ns duration and up to 0.5 J energy. Its pulses are truncated by an improved adjustable plasma shutter [7] which contains a 1:1 telescope formed by two ZnSe lenses. A high-voltage discharge in the focus of the telescope which is driven by a laser triggered spark gap (LTSG) initiates the plasma breakdown. The cut-off by the plasma shutter occurs within about 10 ps which is confirmed by the production of 35 ps pulses with an OFID CO2 laser system based on this plasma shutter [ 8 ]. The truncated COz pulse passes through the farinfrared cell consisting of a glass tube of 26 mm diameter and 3.5 m length with a NaC1 Brewster entrance window and a quartz output window reflecting the COz infrared radiation. The far-infrared pulses are detected by a fast sensitive Schottky-barrier diode detector [9 ].
Volume 87, number 5,6
OPTICS COMMUNICATIONS
15 February 1992
f :l,~.s'::~ ,~ ,Vb u C;~" lrllet
t t e r"
CO2
lO flltfl H y b r i d
Single I,on~itudinal
i
[
!*l
Off Axis Focusing Mirror
Schottky
[]armer/ / ~ Tektvomx
Diode D e t e c t o r
4~/ k[--[IZ @ ~
I .
7'912Ht]
•
]
/" "
I
Pho 1o,, Deter:tot I)eleO' ~.me'
~>
",/
IJ~,~ [
] ]
Fig. 1. Experiment arrangement. 3. Results
We first investigated the strong 496 g m CH3F superradiant emission. Thus, we obtained well-defined single pulses as short as 2.2 ns at a gas pressure of 4.2 Torr with an estimated peak power of about 25 kW. The duration r of such pulses is comparable with the conventional lower limit zp = 8 ns Torr given by the inverse linewidth of the transition [6]. In a first approach we might suppose that only the pressure broadening influences the pulse duration. However, when decreasing the pressure we observe that the pulses become shorter than the limit zp mentioned above. As recently reported [ 10 ], our measurements at a gas pressure below 1 Torr show that the pulses are considerably shorter than this limit and also shorter than those obtained by Rosenberger and DeTemple [ 11 ] under similar experimental conditions. These results are explained by an equivalent inhomogeneous broadening in CH3F due to its Kmultiplet structure [ 10 ]. This implies that the p u m p pulse essentially determines the duration of the farinfrared pulse. This is agreement with the fact that our plasma shutter performs a cut-off within about l 0 ps which have to be compared with the 100 ps of the plasma shutter used by Rosenberger and De250
Temple. This results in a larger spectral width of our pump pulse due to the drastic increase of the imaginary part of the Fourier transform of the truncated pulse. In fig. 2a we show the measured truncated CO2 pump pulse and in fig. 2b the corresponding calculated Fourier spectrum. The limited temporal resolution of the detection system is responsible for the apparent relatively long cut-off time of the CO2 pulse of fig. 2a. As mentioned before, the truncation occurs effectively within about 10 ps as confirmed by O F I D experiments [8]. In figs. 3a, b we show 496 ~tm CH3F pulses obtained for two different pressure regimes. In fig. 3a the pulse width of 8 ns is well under the conventional pulse-width limit r p = 2 0 ns for a gas pressure of 400 mTorr given by the inverse linewidth of the transition. In fig. 3b we notice the small peaks a, b, c, d appearing after the 2,2 ns CH3F pulse. These are due to the interaction of different K emissions originating in the K-multiplet structure of the rotational energy levels of CH3F. Our theoretical analysis shows that for a pressure near 4 Tort the five first K transitions contribute to the observed pulse shape. With our system, we also investigated the emission of CH3CN, which is a molecule of low laser efficiency, and generated for the first time 373 gm
Volume 87, number 5,6
OPTICS COMMUNICATIONS
(a)
496 k t m - C H 3 F
[--
Z
Z
[..., Z
E'-
15 February 1992
/
(a)
~
20 ns p
~ 10 ps
z /
r<
,.A
,/
r~
/
\
/
20
',..~
20 ns / Div.
ns/div.
496 g m - C H 3 F Z
e.8
(b)
I'A
7 ----~! ~ 2 . 2 I i
i ,
<
a
ns C /"~
d
10 n s / d i v . Fig. 3. (a) 8 ns, 496 MmCH3F pulse. Pressure 400 m Torr. (b) 2.2 ns, 496 ~tm CH3F pulse. Pressure 4.2 Torr.
385 ~.m-D20 -0.5
-0.2~
e
Am (GHz) Fig. 2. (a) Truncated CO2-1aserpump pulse. (b) Fourier transformation of the truncated CO2 pump pulse.
1 ns/div. CH3CN s u p e r r a d i a n t emission [ 10]. In our arrangement CH3CN emits 373 ~tm only in the pressure range between 0.25 and 1.5 Torr. The shortest pulses p r o d u c e d with this molecule exhibit a d u r a t i o n o f 6 ns with a peak power o f 10 kW. Also for this molecule we find a pulse width noticably shorter than that d e t e r m i n e d by the inverse linewidth o f the transition for a pressure below 1 Torr [ 10]. In the following we p e r f o r m e d the same experiments with molecules which emit by a R a m a n process, D 2 0 at 385 g m and N H 3 at 291 g m wavelength. As m e n t i o n e d before, R a m a n emission is o f advantage because it does not require inversion. Thus, the pulse f o r m a t i o n and emission can take
:. . . . . .
Fig. 4. 1.25 ns, 385 ~tm D20 pulse. Pressure 2.4 Torr. place on a shorter time scale. Furthermore, far-infrared emission is obtained for each frequency component o f the p u m p pulse. Thus, we generated smooth and reproducible 385 Mm pulses as short as 1.25 ns and 17 k W peak power with D 2 0 at 2.4 Torr (fig. 4). The pulse are well shorter than the 3 ns o f the inverse linewidth o f the transition in this pressure range. In addition, we notice that no shortening o f the pulse duration occurs when increasing the pressure up to 10 Torr where in our experiment the D20 radiation stops. This demonstrates that only the 251
Volume 87, number 5.6
OPTICS COMMUNICATIONS
characteristics of the p u m p pulse determine the farinfrared pulse duration as soon as an o p t i m u m pressure is reached. The o p t i m u m pressure depends on the laser gas and experimental conditions. As shown in fig. 5a we also generated 291 g m pulses as short as 0.8 ns and a peak power of 20 kW with NH3 at 9 Torr. These pulses are slightly shorter than the 1.2 ns corresponding to the inverse linewidth of the transition in NH3 for this pressure [ 12 ]. We note that in contrast the case of D~O the o p t i m u m pressure for the shortest pulses is not reached. In fact, by increasing the pressure above 10 Tort we were able to produce single 770 ps pulses. The strong absorption above 10 Torr requires a higher pump intensity for the reproducible generation of such subnanosecond pulses, fig. 5b shows the truncated CO2 p u m p pulse and the resulting simultaneous NH3 emission on a c o m m o n time scale. The rise time of the 350 MHz two-channel LeCroy oscilloscope used for delay measurements explains the longer cut-off time of the truncated p u m p pulse and the longer duration of the far-infrared pulse than those measured separately
;;,-,
(a)
291 Bm-NH3
Z
z > F--. ~e
i )/
~
800 ps
1 ns/div.
] [ 7~__ T.,.r
)(hi
10.35 ~m C02
! Ill
::,t
_
I. 0 ',ns/~liv.
_
"
[::1 ::~1
...........
I
Fig. 5. (a) 0.8 ns, 291 p.m NH3 pulse. Pressure 9 Torr. (b) Truncated CO2 pump pulse (upper trace) and resulting 291 p_mNH3 emission (lower trace). NH3 pressure 9 Torr. 252
15 Februaw 1992
with the one-channel 750 MHz Tektronix oscilloscope. In all of these experiments, we find that the bandwidth of the pump pulse drastically increased by our fast truncation essentially determines the duration of the far-infrared emission. For the Raman emitting gases and for those with a K-multiplet structure at a low pressure, we always observe pulses shorter than the conventional limit r v defined by the inverse linewidth of the transition. Gases with a K-multiplet structure reach this limit for increasing pressure. This is in contrast to the situation of conventional far-infrared lasers optically pumped by 10 p.m CO2 lasers where the duration of the far-infrared emission corresponds to the pump-pulse duration. In table I compare our results with the shortest farinfrared pulses produced hitherto. Among these we find the 355 ps, 496 btm CH3F and 710 ps, 385 p.m D20 mode-locked pulses generated by synchronously mode-locked optical pumping with CO2 lasers [13,14]. We cite also the experiments with modelocked CO2 TEA laser optical pumping in a single pass configuration [ 15 ]. With this method pulses as short as 300 ps for 496 ~tm CH3F, less than 2 ns for 385 p.m D20 and between 2 and 5 ns for 66 p.m D20, 152 g m N H 3 , 447 BmCH3I, and 1207 gm~3CH3 F have been achieved. In a similar way, superradiant 286 gm CH3F pulses of about 1 ns have been produced by optical pumping with a mode-locked highpressure CO2 laser [ 16 ]. Optical pumping with a high pressure CO2 laser in the self mode-locking state resuited in 800 ps, 385 lam D20 pulses [ 17]. However, in these cases the far-infrared emissions consist of a train of different short pulses in contrast to our experiments which provide single well-defined short pulses. In addition, we may mention that the other efficient methods for generation of single short farinfrared pulses where optical switching element is either placed outside the laser system [ 18,19 ] or used for cavity dumping [20], fail to produce pulses as short as those generated by our system.
4. Conclusion
We have demonstrated that optical pumping with rapidly truncated CO2 laser pulse of superradiant or Raman emitting far-infrared molecular gases is a
Volume 87, number 5,6
15 February. i 992
OPTICS COMMUNICATIONS
Table 1 Shortest reported far-infrared pulses at wavelengths between 100 lain and 500 lain. 2 (~tm)
Molecule
CO2 pump line
Pump scheme
fwhm (ns)
Remarks
ref.
496
CH3F
9P20
447 385
CH31 D20
10PI 8 9R22
373 291
CH3CN NH3
10P20 10R06
286 231 193 152 151 148 119
CH3F CH3F CH3F NH3 NH3 NH3 CH3OH
9R20 9R30 10R32 10P32 10P32 9P34 9P36
Synchron. mode locked Mode locked single pass configuration Optical switching ext. res. Optical switching int. res. Truncated pulse Mode locked single pass configuration Synchron. mode locked Mode locked single pass configuration High pressure self mode locking Truncated pulse Truncated pulse Optical switching int. res. Truncated pulse High pressure mode locked Optical switching int. res. Synchron. mode locked Mode locked single pass configuration Synchron. mode locked Optical switching int. res. Optical switching ext. res.
0.35 0.3 5 10-20 2.2 2-5 0.71 <2 0.8 1.25 6 7 0.8 1 7 0.37 2-5 0.53 7 2.1
Pulse train Pulse train Single pulse Single pulse Single pulse Pulse train Pulse train Pulse train Pulse train Single pulse Single pulse Single pulse Single pulse Pulse train Single pulse Pulse train Pulse train Pulse train Single pulse Single pulse
[ 13,14] [ 15 ] [ 18 ] [ 20 ] this work [ 15 ] [ 13,14 ] [ 15 ] [ 17 ] this work this work [20] this work [ 16 ] [20 ] [ 13,14] [ 15 ] [13,14] [ 20 ] [ 19 ]
successful method to produce ultrashort far-infrared pulses. The resulting smooth and reproducible single pulses are suitable for many applications.
Acknowledgements T h i s s t u d y is s u p p o r t e d b y t h e S w i s s N a t i o n a l Science Foundation and ETH Zurich. We wish to thank J.S. B a k o s , B u d a p e s t a n d H . a n d J. W a l d m a n n ,
Low-
ell, M a s s . , U S A , f o r s t i m u l a t i n g d i s c u s s i o n s .
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