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CW infrared laseraction of optically pumped Co2+:KZnF3
Volume 36, number 5
CW INFRARED
OPTICS COMMUNICATIONS
LASERACTION
1 March 1981
OF OPTICALLY PUMPED Co2+:KZnF3
W. KUNZEL, W. KNIERIM and U. DURR Physikal. Institut, Teil2, Universitlii Stuttgart, FRG Received 13 November 1980
The optical properties of KZnFs:Co*+ were investigated by means of absorption and emission spectroscopy in the visible and infrared spectral region. This system is suitable for laseraction between the lowest 4Tr(4F) -4Ta(4F) Co*+phonon sidebands. CW laseraction of more than 20 mW (85K) was observed at 1.95 pm when pumped with an Ar+-laser at 514 pm (1SW). The system is at least tunable between 2.05 pm to 1.85 pm.
Based on the early work of Johnson et al. [l-3] it has been demonstrated by Mooradian et al. [4-61 that solid state lasers on the basis of transition-metal impurities like Ni2+, Co2+ or V2+ in ionic crystals can be broadly tunable and can produce powerful coherent infrared radiation. The reason for the tunability is, that some of the electronic transitions of the mentioned impurities are strongly coupled to the lattice of the host crystal and exhibit in additiorrto magnetic dipole allowed zero-phonon transitions, broad vibronic sidebands in the absorption- and emission spectra. Powerful and tunable cw infrared laseraction between these phononassisted sidebands of the d-d electronic transitions of the transition-metal-impurities were reported in host crystals like MgO and crystals with rutile structure like MgF, and ZnF,. In this paper the investigations were extended to the laser-suitability of transition-metal-impurities in other host crystals like the perovskites (KznF3) and layer-compounds like K2ZnF4. In these crystals the site symmetry of the substitutional impurities is cubic (oh) or tetragonal (Dii) respectively. Because of the inversion symmetry at the impurity site the d-d-electric transitions are only allowed by perturbation (spin-orbit coupling, electron-phonon interaction). Consequently the absorption usually is not pronounced. This can be changed for example by electron-irradiation of the crystals [7]. This procedure leads to an increase in transition-metalimpurity absorption and emission and to additional new F-center induced absorptions and emissions in the near infrared. We are investigating the properties
0 030-4018/81/0000-0000/$02.50
0 North-Holland
‘A2(‘Fl I
‘T2 @I r 7 2 ’ z
K Zn Fg :Co”
‘Tj CLPl
f
: i
4
T.15K
.E
5
10 Wavenumber
15
20
25
Ix lo3 cm-‘1
Fig. 1. Absorption spectrum of the unperturbed Co*+ in KZnF3 at 15 K. Dotted line: 4bsorption spectrum after eleo tron irradiation at 100 K.
of the IR-laser levels of Coz+ in the host crystals mentioned above and the present paper reports first results of the cw IR-laser action of the system KZnF3 : Co2+ at 1.95 fim. Fig. 1 shows a typical low temperature spectrum of the unperturbed Co2+ ion m such crystals. The main absorptions are due to the ?Tk-4T2, transition within the 4F orbital groundstate and the 4T,,(4F)4T,.J4P) transition. Both absorptions are suitable to couple pump power of standard lasers into the sample. For the infrared 4T8(F)-4T2g(F) transition which also is the laser transition, bhe 1.32 pm cw Nd:YAG Publishing Company
383
Volume 36, number 5
OPTICS COMMUNICATIONS
1 March 1981
1
60 KZn F3 :Co**
T=lSK
AbsorDtion
... -I L
6 Wavenumber
[ x lo3 cm-‘]
Fig. 2. Low temperature absorption- and emission spectra of the 4T~g-4T~,(4F)
laser output can be used, for the 4F-4P pump-transition in the visible the 5 14 pm output of an argon laser is suitable. The investigation of the IR-emission spectra did not show considerable differences in the quantum efficiency with the two pumping methods. Fig. 2 shows the absorption and emission for the IR. laser levels in Co2+:KZnF3. The crystals which were grown by the Bridgman method contained about 1 mole % Co2+. The amount of unwanted impurities like Ni2+, Fe2+, Ca2” was less than low3 mole %. The emission structure at 1.6 pm is due to these Ni2+ impurities. The electronic properties of the d7-Co2+ ion in different crystal fields is well established [8,9]. The parameters like the cubic crystal field parameter Dq and the Racah parameters like B are deduced from the energy spacing of the levels. They can be compared with the known similar parameters for Co2+ in rutile and perovskite host crystals [lO,l l] (see table 1). A peculiarity of Co2+ in cubic symmetry (KZnF3, KMgF3) lies in the fact, that the splitting of the first excited 4T2(4F) laser level is smaller than expected when using the parameters of table 1. As discussed by Sturge et al. [ 111 this is due to a dynamic T-E Jahn Teller effect, which considerably reduces the effective 384
laser levels.
spin-orbit coupling of this state (the separation of the lowest 4T2,(F6-rF,) level is only about 6 cm-l). As can be seen from fig. 2 the IR-emission spectra consists of the sharp, magnetic dipole allowed zeroTable 1 Opticaldata of Co*+in KZnF3, KzknF4 and ZnF, (A = 528 cm-’ , C = 4497 cm-’ for all symmetries)
Dq
____~
Sym- 0-phonon lines (cm-‘) S ____ metry Absorption Emission --
B
KZnF3
oh
6 606 6 601 6 677
6601 6 606 5 648
-4
715
920
K2ZnF4
D4h
6 815 6 892 6 966
6 815 5 908 5 751
-4.4
740
930
ZnFz
Dzh
6510 6 536 6 598
6 510 6 536 6 357 5 715 5 421 5 253 5 107
~4.9
680
960
(cm-‘)(cm-‘)
1 March 1981
OPTICS COMMUNICATIONS
Volume 36, number 5
phonon transitions to the 4T,(T’6-I’8) groundstate levels and the broad phonon assisted side bands. The important values describing the optical spectra for different host crystals are collected in table 1 together with the Huang Rhys factor S, which is a measure for the average number of phonons coupled to these IRtransitions. This value was estimated from the relative intensity of the zero-phonon line and the phonon sideband [12]. It is clear from table 1 that apart from frequency-changes due to changes in the lattice constant of the unit cells the IR-emission of Co*+ in the different crystals does not differ qualitatively which is also true for the low-temperature fluorescent lifetime of the 4T2,(F) state of several milliseconds [4,1 I]. The fine-structure of the phonon sideband is due to the contribution of the low number phonon absorptions in the different crystals and will not be discussed any further [ 131. Nevertheless these processes may be responsible for the structure in the tuning curves of the laser output [4]. For investigating the laser potential of the system KZnF, :Co*+ a three mirror astigmatically compensated laser cavity was built fig. 3. This cavity is similar to that used for the F-center lasers [14,15]. The laser crystals (d = 2 mm) oriented under the Brewster angle $ with respect to the direction of the pump beam, were clamped on a gold plated copper cold finger of a cryostat which allowed temperatures at the crystal between 25 K and room temperature. For convenience in this first study the 4T,,(P) Co*+ level was optically
\
‘Pump
beam
Fig. 3. Three mirror-cavity used in the experiment (I = 30 cm, f= 2.5 cm, 20 = 27”, $J= Brewster angle).
?4
mW
KZnG:Co’+
“,j_--/,,,,J 0
50 Temperature
100
150
[K]
Fig. 4. Temperature dependence of the threshold pump power using the 5 14 pm line of an argon-pump-laser.
pumped with the 5 14 m argon line with powers more than 1.5 W. About 50% of the pump power is absorbed in the sample. The spot size of the pump beam was about 50 pm and comparable to the calculated spot size of the cavity mode. Fig. 4 shows the threshold absorbed pump power of a typical crystal as a function of temperature. The crystal temperature was assumed to be the same as the sample holder temperature which may be valid as long the absorbed power is low. In this experiment the reflectivity of the output coupling mirror (M3) was 99.7% at 1.95 ,um. The upper limit at about 1.5 W is set by the thermal conductance of the system. The planned use of the Nd:YAG pump at 1.32 pm (5 W) will decrease the latter problems [4] and probably shift the laser operation to higher temperatures. A typical picture of the output power as a function of the absorbed power can be seen in fig. 5. For these experiments an output coupling mirror with a reflectivity of 99.9% was used. Choosing different spots in our laser crystals leads to different efficiencies. This indicates that the microscopic properties of the crystals (doping, local stress) are not homogeneous. Up to now the best performance achieved was more than 20 mW (85 K) with an argon pump power of 1.5 W. Up till now the quantum efficiency of Co*+ in KZnF3 is about 10 times less than the values published for Co*+ in MgF2 [4]. This difference may be partly due to the temperature problem in the “laser volume” of the crystal and certainly can be minimized using the 385
OPTICS COMMUNICATIONS
Volume 36, number 5
K ZnF,
:Co2*
1
T:27K
1 March 1981
spin- and parity selection rules of the Codefect can be changed. This leads to new broad absorption and emission bands in the infrared spectral region as well in the UV. In our crystal, laser operation on the transition at 0.9 urn may be possible. The investigation of the laser suitability of these defect induced Co2+ transitions is now in progress. The authors thank Professor Pick for financial support and K. Bahr for growing the crystals.
00°
500
loo0
1500
Absorbed power [mW]
References
Fig.
5. Infrared output (1.95 rm) versus absorbed argon-laser pump power for two different temperatures.
[l] L.F. Johnson, H.J. Cuggenheim and R.A. Thomas, Phys. Rev. 149 (1966) 197. -~
Nd:YAG pump. In addition the argon pump laser could only be operated in multimode. In our latest experiments, with higher qualityKZnF3:Cocrystals and with a considerably reduced spot size of the argon pump laser the threshold at 85 K could be reduced below 150 mW and the cw IR-laser output increased to more than 20 mW only limited by the mentioned temperature problems. From these arguments an improvement of the experimental setup probably leads to a higher efficiency of these laser crystals. No substantial tunability of the 1.95 pm laser wavelength is observed by changing the temperature of the crystal. The investigation of the tunability using frequency selective elements in the laser cavity is in progress. First measurements demonstrated that the laser is at least tunable between 2.05 and 1.85 pm, with the short wavelength limit given by the mirrors available. Motivated by the work of Sibley et al. [7] the KZnF3:Co laser crystal was electron-irradiated at low temperature (1.5 MeV, 10 PA, 100 K, 40 min) (fig. 1). By this treatment of the laser-crystal F-center like defects are produced and the site-symmetry of the laser defect (Co2+) may be perturbated, in which case the
386
[21 L.F. Johnson, R.E. Dietz and H.J. Guggenheim, Phys. Rev. Lett. 11 (1963) 318. [31 L.F. Johnson, R.E. Dietz and H.J. Guggenheim, Appl.
Phys. Lett. 5 (1964) 21.
[41 P.F. Moulton, A. Mooradian, in: Laser spectroscopy IV, ed. H. Walther and K.W. Rothe (Springer 1979) p. 584. [51 P.F. Moulton, A. Mooradian and T.B. Reed, Opt. Lett. 3 (1978) 164. [61 P.F. Moulton and A. Mooradian, Appl. Phys. Lett. 35 (1979) 838. 171 K.H. Lee and W.A. Sibley, Phys. Rev. B12 (1975) 3392. 181 J.C. Eisenstein, J. Chem. Phys. 34 (1961) 1628. 191 J.S. Griffith, The theory of transition-metal ions, (Cambridge University Press, 1964). [lOI H. Kamimura and Y. Tanabe, J. Appl. Phys. 34 (1963) 1239. IllI M.D. Sturge, Phys. Rev. B8 (1973) 6; M.D. Sturge and H.J. Guggenheim, Phys. Rev. B4 (1971) 2092. iI21 D.B. Fitchen, in: Physics of color centers, ed. W.B. Fowler (Academic Press, 1968) p. 294. iI31 U. Dtirr and R. Weber, Sol. St. Comm. 14 (1974) 907. iI41 L.F. Mollenauer and D.H. Olsen, J. Appl. Phys. 46 (1975) 3109. 1151 H.W. Kogelnik, E.P. Ippen, A. Dienes and C.V. Shank, IEEE QE-8 (1972) 373.
CW INFRARED
OPTICS COMMUNICATIONS
LASERACTION
1 March 1981
OF OPTICALLY PUMPED Co2+:KZnF3
W. KUNZEL, W. KNIERIM and U. DURR Physikal. Institut, Teil2, Universitlii Stuttgart, FRG Received 13 November 1980
The optical properties of KZnFs:Co*+ were investigated by means of absorption and emission spectroscopy in the visible and infrared spectral region. This system is suitable for laseraction between the lowest 4Tr(4F) -4Ta(4F) Co*+phonon sidebands. CW laseraction of more than 20 mW (85K) was observed at 1.95 pm when pumped with an Ar+-laser at 514 pm (1SW). The system is at least tunable between 2.05 pm to 1.85 pm.
Based on the early work of Johnson et al. [l-3] it has been demonstrated by Mooradian et al. [4-61 that solid state lasers on the basis of transition-metal impurities like Ni2+, Co2+ or V2+ in ionic crystals can be broadly tunable and can produce powerful coherent infrared radiation. The reason for the tunability is, that some of the electronic transitions of the mentioned impurities are strongly coupled to the lattice of the host crystal and exhibit in additiorrto magnetic dipole allowed zero-phonon transitions, broad vibronic sidebands in the absorption- and emission spectra. Powerful and tunable cw infrared laseraction between these phononassisted sidebands of the d-d electronic transitions of the transition-metal-impurities were reported in host crystals like MgO and crystals with rutile structure like MgF, and ZnF,. In this paper the investigations were extended to the laser-suitability of transition-metal-impurities in other host crystals like the perovskites (KznF3) and layer-compounds like K2ZnF4. In these crystals the site symmetry of the substitutional impurities is cubic (oh) or tetragonal (Dii) respectively. Because of the inversion symmetry at the impurity site the d-d-electric transitions are only allowed by perturbation (spin-orbit coupling, electron-phonon interaction). Consequently the absorption usually is not pronounced. This can be changed for example by electron-irradiation of the crystals [7]. This procedure leads to an increase in transition-metalimpurity absorption and emission and to additional new F-center induced absorptions and emissions in the near infrared. We are investigating the properties
0 030-4018/81/0000-0000/$02.50
0 North-Holland
‘A2(‘Fl I
‘T2 @I r 7 2 ’ z
K Zn Fg :Co”
‘Tj CLPl
f
: i
4
T.15K
.E
5
10 Wavenumber
15
20
25
Ix lo3 cm-‘1
Fig. 1. Absorption spectrum of the unperturbed Co*+ in KZnF3 at 15 K. Dotted line: 4bsorption spectrum after eleo tron irradiation at 100 K.
of the IR-laser levels of Coz+ in the host crystals mentioned above and the present paper reports first results of the cw IR-laser action of the system KZnF3 : Co2+ at 1.95 fim. Fig. 1 shows a typical low temperature spectrum of the unperturbed Co2+ ion m such crystals. The main absorptions are due to the ?Tk-4T2, transition within the 4F orbital groundstate and the 4T,,(4F)4T,.J4P) transition. Both absorptions are suitable to couple pump power of standard lasers into the sample. For the infrared 4T8(F)-4T2g(F) transition which also is the laser transition, bhe 1.32 pm cw Nd:YAG Publishing Company
383
Volume 36, number 5
OPTICS COMMUNICATIONS
1 March 1981
1
60 KZn F3 :Co**
T=lSK
AbsorDtion
... -I L
6 Wavenumber
[ x lo3 cm-‘]
Fig. 2. Low temperature absorption- and emission spectra of the 4T~g-4T~,(4F)
laser output can be used, for the 4F-4P pump-transition in the visible the 5 14 pm output of an argon laser is suitable. The investigation of the IR-emission spectra did not show considerable differences in the quantum efficiency with the two pumping methods. Fig. 2 shows the absorption and emission for the IR. laser levels in Co2+:KZnF3. The crystals which were grown by the Bridgman method contained about 1 mole % Co2+. The amount of unwanted impurities like Ni2+, Fe2+, Ca2” was less than low3 mole %. The emission structure at 1.6 pm is due to these Ni2+ impurities. The electronic properties of the d7-Co2+ ion in different crystal fields is well established [8,9]. The parameters like the cubic crystal field parameter Dq and the Racah parameters like B are deduced from the energy spacing of the levels. They can be compared with the known similar parameters for Co2+ in rutile and perovskite host crystals [lO,l l] (see table 1). A peculiarity of Co2+ in cubic symmetry (KZnF3, KMgF3) lies in the fact, that the splitting of the first excited 4T2(4F) laser level is smaller than expected when using the parameters of table 1. As discussed by Sturge et al. [ 111 this is due to a dynamic T-E Jahn Teller effect, which considerably reduces the effective 384
laser levels.
spin-orbit coupling of this state (the separation of the lowest 4T2,(F6-rF,) level is only about 6 cm-l). As can be seen from fig. 2 the IR-emission spectra consists of the sharp, magnetic dipole allowed zeroTable 1 Opticaldata of Co*+in KZnF3, KzknF4 and ZnF, (A = 528 cm-’ , C = 4497 cm-’ for all symmetries)
Dq
____~
Sym- 0-phonon lines (cm-‘) S ____ metry Absorption Emission --
B
KZnF3
oh
6 606 6 601 6 677
6601 6 606 5 648
-4
715
920
K2ZnF4
D4h
6 815 6 892 6 966
6 815 5 908 5 751
-4.4
740
930
ZnFz
Dzh
6510 6 536 6 598
6 510 6 536 6 357 5 715 5 421 5 253 5 107
~4.9
680
960
(cm-‘)(cm-‘)
1 March 1981
OPTICS COMMUNICATIONS
Volume 36, number 5
phonon transitions to the 4T,(T’6-I’8) groundstate levels and the broad phonon assisted side bands. The important values describing the optical spectra for different host crystals are collected in table 1 together with the Huang Rhys factor S, which is a measure for the average number of phonons coupled to these IRtransitions. This value was estimated from the relative intensity of the zero-phonon line and the phonon sideband [12]. It is clear from table 1 that apart from frequency-changes due to changes in the lattice constant of the unit cells the IR-emission of Co*+ in the different crystals does not differ qualitatively which is also true for the low-temperature fluorescent lifetime of the 4T2,(F) state of several milliseconds [4,1 I]. The fine-structure of the phonon sideband is due to the contribution of the low number phonon absorptions in the different crystals and will not be discussed any further [ 131. Nevertheless these processes may be responsible for the structure in the tuning curves of the laser output [4]. For investigating the laser potential of the system KZnF, :Co*+ a three mirror astigmatically compensated laser cavity was built fig. 3. This cavity is similar to that used for the F-center lasers [14,15]. The laser crystals (d = 2 mm) oriented under the Brewster angle $ with respect to the direction of the pump beam, were clamped on a gold plated copper cold finger of a cryostat which allowed temperatures at the crystal between 25 K and room temperature. For convenience in this first study the 4T,,(P) Co*+ level was optically
\
‘Pump
beam
Fig. 3. Three mirror-cavity used in the experiment (I = 30 cm, f= 2.5 cm, 20 = 27”, $J= Brewster angle).
?4
mW
KZnG:Co’+
“,j_--/,,,,J 0
50 Temperature
100
150
[K]
Fig. 4. Temperature dependence of the threshold pump power using the 5 14 pm line of an argon-pump-laser.
pumped with the 5 14 m argon line with powers more than 1.5 W. About 50% of the pump power is absorbed in the sample. The spot size of the pump beam was about 50 pm and comparable to the calculated spot size of the cavity mode. Fig. 4 shows the threshold absorbed pump power of a typical crystal as a function of temperature. The crystal temperature was assumed to be the same as the sample holder temperature which may be valid as long the absorbed power is low. In this experiment the reflectivity of the output coupling mirror (M3) was 99.7% at 1.95 ,um. The upper limit at about 1.5 W is set by the thermal conductance of the system. The planned use of the Nd:YAG pump at 1.32 pm (5 W) will decrease the latter problems [4] and probably shift the laser operation to higher temperatures. A typical picture of the output power as a function of the absorbed power can be seen in fig. 5. For these experiments an output coupling mirror with a reflectivity of 99.9% was used. Choosing different spots in our laser crystals leads to different efficiencies. This indicates that the microscopic properties of the crystals (doping, local stress) are not homogeneous. Up to now the best performance achieved was more than 20 mW (85 K) with an argon pump power of 1.5 W. Up till now the quantum efficiency of Co*+ in KZnF3 is about 10 times less than the values published for Co*+ in MgF2 [4]. This difference may be partly due to the temperature problem in the “laser volume” of the crystal and certainly can be minimized using the 385
OPTICS COMMUNICATIONS
Volume 36, number 5
K ZnF,
:Co2*
1
T:27K
1 March 1981
spin- and parity selection rules of the Codefect can be changed. This leads to new broad absorption and emission bands in the infrared spectral region as well in the UV. In our crystal, laser operation on the transition at 0.9 urn may be possible. The investigation of the laser suitability of these defect induced Co2+ transitions is now in progress. The authors thank Professor Pick for financial support and K. Bahr for growing the crystals.
00°
500
loo0
1500
Absorbed power [mW]
References
Fig.
5. Infrared output (1.95 rm) versus absorbed argon-laser pump power for two different temperatures.
[l] L.F. Johnson, H.J. Cuggenheim and R.A. Thomas, Phys. Rev. 149 (1966) 197. -~
Nd:YAG pump. In addition the argon pump laser could only be operated in multimode. In our latest experiments, with higher qualityKZnF3:Cocrystals and with a considerably reduced spot size of the argon pump laser the threshold at 85 K could be reduced below 150 mW and the cw IR-laser output increased to more than 20 mW only limited by the mentioned temperature problems. From these arguments an improvement of the experimental setup probably leads to a higher efficiency of these laser crystals. No substantial tunability of the 1.95 pm laser wavelength is observed by changing the temperature of the crystal. The investigation of the tunability using frequency selective elements in the laser cavity is in progress. First measurements demonstrated that the laser is at least tunable between 2.05 and 1.85 pm, with the short wavelength limit given by the mirrors available. Motivated by the work of Sibley et al. [7] the KZnF3:Co laser crystal was electron-irradiated at low temperature (1.5 MeV, 10 PA, 100 K, 40 min) (fig. 1). By this treatment of the laser-crystal F-center like defects are produced and the site-symmetry of the laser defect (Co2+) may be perturbated, in which case the
386
[21 L.F. Johnson, R.E. Dietz and H.J. Guggenheim, Phys. Rev. Lett. 11 (1963) 318. [31 L.F. Johnson, R.E. Dietz and H.J. Guggenheim, Appl.
Phys. Lett. 5 (1964) 21.
[41 P.F. Moulton, A. Mooradian, in: Laser spectroscopy IV, ed. H. Walther and K.W. Rothe (Springer 1979) p. 584. [51 P.F. Moulton, A. Mooradian and T.B. Reed, Opt. Lett. 3 (1978) 164. [61 P.F. Moulton and A. Mooradian, Appl. Phys. Lett. 35 (1979) 838. 171 K.H. Lee and W.A. Sibley, Phys. Rev. B12 (1975) 3392. 181 J.C. Eisenstein, J. Chem. Phys. 34 (1961) 1628. 191 J.S. Griffith, The theory of transition-metal ions, (Cambridge University Press, 1964). [lOI H. Kamimura and Y. Tanabe, J. Appl. Phys. 34 (1963) 1239. IllI M.D. Sturge, Phys. Rev. B8 (1973) 6; M.D. Sturge and H.J. Guggenheim, Phys. Rev. B4 (1971) 2092. iI21 D.B. Fitchen, in: Physics of color centers, ed. W.B. Fowler (Academic Press, 1968) p. 294. iI31 U. Dtirr and R. Weber, Sol. St. Comm. 14 (1974) 907. iI41 L.F. Mollenauer and D.H. Olsen, J. Appl. Phys. 46 (1975) 3109. 1151 H.W. Kogelnik, E.P. Ippen, A. Dienes and C.V. Shank, IEEE QE-8 (1972) 373.