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Pulsed laser operation of diffusion-doped Cr2+:ZnSe
15 August 1999
Optics Communications 167 Ž1999. 129–132 www.elsevier.comrlocateroptcom
Pulsed laser operation of diffusion-doped Cr 2q:ZnSe A.V. Podlipensky a,) , V.G. Shcherbitsky a , N.V. Kuleshov a , V.P. Mikhailov a , V.I. Levchenko b, V.N. Yakimovich b, L.I. Postnova b, V.I. Konstantinov b a
International Laser Center, Belarus State Politechnical Academy, Scorina AÕenue 65, Minsk 220027, Belarus b Institute of Solid State and Semiconductor Physics, NASB, P. Brouki st. 17, Minsk 220072, Belarus Received 23 February 1999; received in revised form 4 June 1999; accepted 6 June 1999
Abstract The laser properties of Cr 2q:ZnSe crystals obtained by diffusion doping method was investigated under pumping by the 1.6 mm output of a Nd:YAG laser-pumped BaŽNO 3 . 2 Raman laser. A slope efficiency of 45%, a laser threshold of 12 mJ and round-trip passive losses of around 9% were measured for the best samples. q 1999 Elsevier Science B.V. All rights reserved. PACS: 42.55.-f; 42.55.Rz; 42.60.Lh
1. Introduction Cr 2q-doped II–VI semiconductors are very attractive for the development of mid-infrared solid-state lasers. Applications of these lasers include medicine Žophthalmology, neurosurgery, urology., optical communications, laser remote sensing of the atmosphere, and spectroscopy. Recently, mid-infrared laser action near 2.5 mm was reported for Cr-doped ZnS, ZnSe, and Cd 0.85 Mn 0.15Te w1–4x. The laser active center in these materials was identified as Cr 2q ion incorporated into the tetrahedral site. Laser demonstrations with Cr 2q-doped zinc chalcogenides gave slope efficiencies up to 30% using a Co 2q:MgF2 laser pump source w2x. A tuning range of about 650 nm was obtained around 2500 nm with the prediction that it can be extended. Peak stimulated emis-
sion cross section and lifetime of laser level for Cr 2q:ZnSe were estimated to be 8 = 10y1 9 cm2 and 8 ms, respectively w1x. These values are highly comparable to that found in the case of Ti-sapphire, indicating the high potential of Cr 2q-doped II–VI semiconductors as mid-infrared laser materials. However, one of the main problems encountered with this kind of materials is their relatively high level of passive losses at laser wavelengths near 2500 nm. Therefore, both detailed spectroscopic studies and synthesis of materials by using various crystal-growth techniques are necessary to estimate the amount of intrinsic optical losses and reduce as much as possible what is due to extrinsic reasons, such as unwanted impurities and lattice defects. 2. Experimental results and discussion
) Corresponding author. Fax: q375-17-232-6286; e-mail: [email protected]
The present article is dedicated to the investigation of the laser performance of Cr 2q:ZnSe crystals
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 8 9 - 8
130
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
which were produced by diffusion doping. Cr-doped ZnSe single crystals were prepared by a two stage process described in Ref. w5x. In the first stage, undoped single crystals of high optical quality were grown by a sublimation traveling heater method ŽSTHM. without seeding. In the second stage, the ZnSe crystals were Cr-doped by the thermal diffusion method from a solid metal source. The distinguishing feature of this method from the early one employed by Page et al. w2x is that the diffusion proceeds from chromium films deposited upon crystals of ZnSe by magnetron sputtering. Diffusion was carried out in sealed ampoules under a pressure of 5 = 10y6 Torr and a temperature of 700–10008C over 1–10 days. The advantages of this method are the high quality of the host crystals and the ability to control the impurity doping level both by adjustment of the diffusion time and temperature and by variation of chromium film thickness. Polished samples of 8 = 8 mm2 in cross-section and 1–10 mm in thickness were prepared for the laser efficiency measurements. A chromium concentration in ZnSe crystals of 4 = 10 18 cmy3 to 3.4 = 10 19 cmy3 was determined from optical absorption measurements using data on absorption cross-section given by DeLoach et al. w1x. The absorption spectra of Cr 2q:ZnSe single crystals used in experiments are shown in Fig. 1. A broad mid-infrared absorption band centered at 1780 nm belongs to the 5 T2 y5 E transition of tetrahedrally coordinated Cr 2q Ž3d 4 . ions. Absorption coefficients at 1.78 mm varied from 3 to 30 cmy1 for samples with different chromium concentrations. The passive losses in the emission range near 2500 nm were estimated from the absorption spectra measurements to be less than 0.05 cmy1 for high quality crystals. The figure of merit ŽFOM s a 1.78 m m ra 2.5 m m ., defined as the ratio of peak absorption coefficient of Cr:ZnSe at the 1.78 mm and absorption coefficient at emission wavelengths Ž2.5 mm., was estimated to be as high as 90–180 Žreflection losses were subtracted. for the best crystals. The laser experiments were performed using the 1.598 mm output of a Nd:YAG laser-pumped BaŽNO 3 . 2 Raman laser with a pulse duration of 10 ns. A nearly hemispherical laser cavity was formed by a flat mirror of reflectivity 99.9% in the lasing region of 2.3–2.6 mm and by a concave output
Fig. 1. Absorption spectra of Cr-doped ZnSe samples with different concentration.
coupler with a curvature radius of 150 mm and a transmission of 1.3%, 5.6%, 12.6%, and 20% at laser wavelengths. The spacing between the mirrors was 142 mm. The crystals were not antireflection coated and were placed perpendicular to the optical axis of the laser cavity. The incident beam was focused upon the sample through the high reflecting mirror by a 32 cm focal-length lens. Long-pass filters were used to separate residual pump light from the Cr 2qlaser beam. The laser threshold was measured using a PbS detector. The results of measurements for samples with different chromium doping levels Žand different FOM. are summarized in Table 1. Input–output data for one of the best samples of Cr 2q:ZnSe Ž a 1.78 y1 , FOM s 170, and thickness 4 mm. m m s 8.4 cm using four different output couplers are shown in Fig. 2. The threshold of lasing with respect to absorbed pump energy was measured to be as low as 12–32 mJ for different output couplers. On the other hand, the laser threshold of Cr:ZnSe laser was estimated using the formula w2,6x Ep Ž th . s
p hn 4 s
=
Ž v l2 q vp2 . Ž C q Ld . Tpumprt
1 y exp Ž yTpumprt .
.
Ž 1.
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
131
Table 1 Laser performance of Cr:ZnSe Peak absorption coefficient Žcmy1 .
FOM Ž a 1.78m m ra 2.5m m .
Output coupling Ž%.
Laser threshold ŽmJ.
Slope efficiency Ž%.
8.4
170
11.5
47
14
34
1.3 5.6 12.6 20 5.6 12.6 5.6
12 17 23 32 94 116 200
7.7 22 25 45 5 7.9 - 1.5
Photon energy hn of 8.5 = 10y2 0 J corresponds to the peak emission wavelength of 2350 nm. The values of emission cross-section and lifetime s and t were 80 = 10y2 0 cm2 and 8 ms w2x, respectively. The pump and signal waist sizes v p and v l for the cavity configuration used in our laser experiments were approximately 200 mm. The total loss per round-trip C q Ld , including out-coupling C and passive losses Ld was set to 0.12. The pump pulse duration Tpump of 10 ns is much shorter than the laser upper-level lifetime t of 8 ms and equation Eq. Ž1. can be simplified to Ep Ž th . s
p hn 4 s
Ž v l2 q vp2 . Ž C q Ld . .
Ž 2.
Using the values given above we find Ep Žth. s 8 mJ. The measured threshold energy of 12 mJ for
Fig. 2. Input–output diagram of a Cr:ZnSe laser for a sample with a peak absorption coefficient of 8.4 cmy1 .
output coupler transmission of 1.3% is evidently close to the calculated value. The highest slope efficiency of 45% was obtained with the 20%-transmission output coupler. The quantum-defect limited value h 0 of the slope efficiency is l prlo s 66%, l p and lo being the pump and output wavelengths. A plot of inverse slope efficiency 1rhX versus inverse output coupling Cy1 in Fig. 3 allows extraction of the round-trip passive loss via the relation 1 1 Ld . Ž 3. X s X 1q h h0 C
ž
/
The fit gives a round-trip loss of 6.8%. On the other hand, the value of Ld can be obtained from a Findlay–Clay analyses of the threshold data for different output couplers w7x. The value of Ld was determined by this method to be approximately 11%,
Fig. 3. Inverse slope efficiency versus inverse output coupling for a sample with a peak absorption coefficient of 8.4 cmy1 .
132
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
which was in satisfactory agreement with the previous value. The small difference between the passive loss estimated from laser experiments and that determined from absorption spectra Ž a 2.5 m m s 0.05 cmy1 gave Ld s 4% for 4 mm crystal thickness. can be associated with the reflection losses on the crystal surfaces, which were not AR coated. Similar passive loss of around 9–11% and a slope efficiency of 35% were also obtained for samples with a peak absorption coefficient of 4–5.5 cmy1 and an FOM of about 95–110. The data of Table 1 indicate that passive losses increase and slope efficiency decreases with increasing chromium concentration. The possible explanations are clustering and off-stoichiometric defects in the lattice w2x. However, additional investigations are necessary in order to define the related processes accurately. In any case, the proposed method of crystal preparation allows Cr:ZnSe laser crystals of high optical quality to be obtained, which exhibit comparatively high laser efficiency and law passive loss Žslope efficiency up to 33% and roundtrip loss about 11–18% was reported by the different crystal growth method in Ref. w2x.. 3. Conclusion In summary, we have demonstrated the laser performance of Cr 2q:ZnSe crystals prepared by the
modified diffusion doping method under pulsed laser pumping. A slope efficiency as high as 45% and a laser threshold as low as 12 mJ with respect to absorbed pump energy were achieved. The round trip passive loss was estimated to be approximately 7–11% for the best samples. The laser efficiency of diffusion doped Cr 2q:ZnSe laser crystals is expected to be enhanced by using pump wavelength around the absorption band 1.8 mm peak and by further reduction of passive loss due to optimization of crystal quality and doping procedure.
References w1x L.D. DeLoach, R.H. Page, G.D. Wilke, S.A. Payne, W.F. Krupke, IEEE J. Quantum Electron. 32 Ž1996. 885. w2x R.H. Page, K.I. Schaffers, L.D. DeLoach, C.D. Wilke, F.D. Patel, J.B. Tassano, S.A. Payn, W.F. Krupke, K.T. Chen, A. Burger, IEEE J. Quantum Electron. 33 Ž1997. 609. w3x U. Hommerich, X. Wu, V.R. Davis, S.B. Triverdi, K. Grasza, R.J. Chen, S. Kutcher, Opt. Lett. 22 Ž1997. 1180. w4x J.T. Seo, U. Hommerich, S.B. Trivedi, R.J. Chen, S. Kutcher, Opt. Commun. 153 Ž1998. 267. w5x V.I. Levchenko, V.N. Yakimovich, L.I. Postnova, V.I. Konstantinov, V.P. Mikhailov, N.V. Kuleshov, J. Cryst. Growth 198–199 Ž1999. 980. w6x P.F. Moulton, IEEE J. Quantum Electron. 21 Ž1985. 1582. w7x D. Findlay, R.A. Clay, Phys. Lett. 20 Ž1966. 277.
Optics Communications 167 Ž1999. 129–132 www.elsevier.comrlocateroptcom
Pulsed laser operation of diffusion-doped Cr 2q:ZnSe A.V. Podlipensky a,) , V.G. Shcherbitsky a , N.V. Kuleshov a , V.P. Mikhailov a , V.I. Levchenko b, V.N. Yakimovich b, L.I. Postnova b, V.I. Konstantinov b a
International Laser Center, Belarus State Politechnical Academy, Scorina AÕenue 65, Minsk 220027, Belarus b Institute of Solid State and Semiconductor Physics, NASB, P. Brouki st. 17, Minsk 220072, Belarus Received 23 February 1999; received in revised form 4 June 1999; accepted 6 June 1999
Abstract The laser properties of Cr 2q:ZnSe crystals obtained by diffusion doping method was investigated under pumping by the 1.6 mm output of a Nd:YAG laser-pumped BaŽNO 3 . 2 Raman laser. A slope efficiency of 45%, a laser threshold of 12 mJ and round-trip passive losses of around 9% were measured for the best samples. q 1999 Elsevier Science B.V. All rights reserved. PACS: 42.55.-f; 42.55.Rz; 42.60.Lh
1. Introduction Cr 2q-doped II–VI semiconductors are very attractive for the development of mid-infrared solid-state lasers. Applications of these lasers include medicine Žophthalmology, neurosurgery, urology., optical communications, laser remote sensing of the atmosphere, and spectroscopy. Recently, mid-infrared laser action near 2.5 mm was reported for Cr-doped ZnS, ZnSe, and Cd 0.85 Mn 0.15Te w1–4x. The laser active center in these materials was identified as Cr 2q ion incorporated into the tetrahedral site. Laser demonstrations with Cr 2q-doped zinc chalcogenides gave slope efficiencies up to 30% using a Co 2q:MgF2 laser pump source w2x. A tuning range of about 650 nm was obtained around 2500 nm with the prediction that it can be extended. Peak stimulated emis-
sion cross section and lifetime of laser level for Cr 2q:ZnSe were estimated to be 8 = 10y1 9 cm2 and 8 ms, respectively w1x. These values are highly comparable to that found in the case of Ti-sapphire, indicating the high potential of Cr 2q-doped II–VI semiconductors as mid-infrared laser materials. However, one of the main problems encountered with this kind of materials is their relatively high level of passive losses at laser wavelengths near 2500 nm. Therefore, both detailed spectroscopic studies and synthesis of materials by using various crystal-growth techniques are necessary to estimate the amount of intrinsic optical losses and reduce as much as possible what is due to extrinsic reasons, such as unwanted impurities and lattice defects. 2. Experimental results and discussion
) Corresponding author. Fax: q375-17-232-6286; e-mail: [email protected]
The present article is dedicated to the investigation of the laser performance of Cr 2q:ZnSe crystals
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 8 9 - 8
130
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
which were produced by diffusion doping. Cr-doped ZnSe single crystals were prepared by a two stage process described in Ref. w5x. In the first stage, undoped single crystals of high optical quality were grown by a sublimation traveling heater method ŽSTHM. without seeding. In the second stage, the ZnSe crystals were Cr-doped by the thermal diffusion method from a solid metal source. The distinguishing feature of this method from the early one employed by Page et al. w2x is that the diffusion proceeds from chromium films deposited upon crystals of ZnSe by magnetron sputtering. Diffusion was carried out in sealed ampoules under a pressure of 5 = 10y6 Torr and a temperature of 700–10008C over 1–10 days. The advantages of this method are the high quality of the host crystals and the ability to control the impurity doping level both by adjustment of the diffusion time and temperature and by variation of chromium film thickness. Polished samples of 8 = 8 mm2 in cross-section and 1–10 mm in thickness were prepared for the laser efficiency measurements. A chromium concentration in ZnSe crystals of 4 = 10 18 cmy3 to 3.4 = 10 19 cmy3 was determined from optical absorption measurements using data on absorption cross-section given by DeLoach et al. w1x. The absorption spectra of Cr 2q:ZnSe single crystals used in experiments are shown in Fig. 1. A broad mid-infrared absorption band centered at 1780 nm belongs to the 5 T2 y5 E transition of tetrahedrally coordinated Cr 2q Ž3d 4 . ions. Absorption coefficients at 1.78 mm varied from 3 to 30 cmy1 for samples with different chromium concentrations. The passive losses in the emission range near 2500 nm were estimated from the absorption spectra measurements to be less than 0.05 cmy1 for high quality crystals. The figure of merit ŽFOM s a 1.78 m m ra 2.5 m m ., defined as the ratio of peak absorption coefficient of Cr:ZnSe at the 1.78 mm and absorption coefficient at emission wavelengths Ž2.5 mm., was estimated to be as high as 90–180 Žreflection losses were subtracted. for the best crystals. The laser experiments were performed using the 1.598 mm output of a Nd:YAG laser-pumped BaŽNO 3 . 2 Raman laser with a pulse duration of 10 ns. A nearly hemispherical laser cavity was formed by a flat mirror of reflectivity 99.9% in the lasing region of 2.3–2.6 mm and by a concave output
Fig. 1. Absorption spectra of Cr-doped ZnSe samples with different concentration.
coupler with a curvature radius of 150 mm and a transmission of 1.3%, 5.6%, 12.6%, and 20% at laser wavelengths. The spacing between the mirrors was 142 mm. The crystals were not antireflection coated and were placed perpendicular to the optical axis of the laser cavity. The incident beam was focused upon the sample through the high reflecting mirror by a 32 cm focal-length lens. Long-pass filters were used to separate residual pump light from the Cr 2qlaser beam. The laser threshold was measured using a PbS detector. The results of measurements for samples with different chromium doping levels Žand different FOM. are summarized in Table 1. Input–output data for one of the best samples of Cr 2q:ZnSe Ž a 1.78 y1 , FOM s 170, and thickness 4 mm. m m s 8.4 cm using four different output couplers are shown in Fig. 2. The threshold of lasing with respect to absorbed pump energy was measured to be as low as 12–32 mJ for different output couplers. On the other hand, the laser threshold of Cr:ZnSe laser was estimated using the formula w2,6x Ep Ž th . s
p hn 4 s
=
Ž v l2 q vp2 . Ž C q Ld . Tpumprt
1 y exp Ž yTpumprt .
.
Ž 1.
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
131
Table 1 Laser performance of Cr:ZnSe Peak absorption coefficient Žcmy1 .
FOM Ž a 1.78m m ra 2.5m m .
Output coupling Ž%.
Laser threshold ŽmJ.
Slope efficiency Ž%.
8.4
170
11.5
47
14
34
1.3 5.6 12.6 20 5.6 12.6 5.6
12 17 23 32 94 116 200
7.7 22 25 45 5 7.9 - 1.5
Photon energy hn of 8.5 = 10y2 0 J corresponds to the peak emission wavelength of 2350 nm. The values of emission cross-section and lifetime s and t were 80 = 10y2 0 cm2 and 8 ms w2x, respectively. The pump and signal waist sizes v p and v l for the cavity configuration used in our laser experiments were approximately 200 mm. The total loss per round-trip C q Ld , including out-coupling C and passive losses Ld was set to 0.12. The pump pulse duration Tpump of 10 ns is much shorter than the laser upper-level lifetime t of 8 ms and equation Eq. Ž1. can be simplified to Ep Ž th . s
p hn 4 s
Ž v l2 q vp2 . Ž C q Ld . .
Ž 2.
Using the values given above we find Ep Žth. s 8 mJ. The measured threshold energy of 12 mJ for
Fig. 2. Input–output diagram of a Cr:ZnSe laser for a sample with a peak absorption coefficient of 8.4 cmy1 .
output coupler transmission of 1.3% is evidently close to the calculated value. The highest slope efficiency of 45% was obtained with the 20%-transmission output coupler. The quantum-defect limited value h 0 of the slope efficiency is l prlo s 66%, l p and lo being the pump and output wavelengths. A plot of inverse slope efficiency 1rhX versus inverse output coupling Cy1 in Fig. 3 allows extraction of the round-trip passive loss via the relation 1 1 Ld . Ž 3. X s X 1q h h0 C
ž
/
The fit gives a round-trip loss of 6.8%. On the other hand, the value of Ld can be obtained from a Findlay–Clay analyses of the threshold data for different output couplers w7x. The value of Ld was determined by this method to be approximately 11%,
Fig. 3. Inverse slope efficiency versus inverse output coupling for a sample with a peak absorption coefficient of 8.4 cmy1 .
132
A.V. Podlipensky et al.r Optics Communications 167 (1999) 129–132
which was in satisfactory agreement with the previous value. The small difference between the passive loss estimated from laser experiments and that determined from absorption spectra Ž a 2.5 m m s 0.05 cmy1 gave Ld s 4% for 4 mm crystal thickness. can be associated with the reflection losses on the crystal surfaces, which were not AR coated. Similar passive loss of around 9–11% and a slope efficiency of 35% were also obtained for samples with a peak absorption coefficient of 4–5.5 cmy1 and an FOM of about 95–110. The data of Table 1 indicate that passive losses increase and slope efficiency decreases with increasing chromium concentration. The possible explanations are clustering and off-stoichiometric defects in the lattice w2x. However, additional investigations are necessary in order to define the related processes accurately. In any case, the proposed method of crystal preparation allows Cr:ZnSe laser crystals of high optical quality to be obtained, which exhibit comparatively high laser efficiency and law passive loss Žslope efficiency up to 33% and roundtrip loss about 11–18% was reported by the different crystal growth method in Ref. w2x.. 3. Conclusion In summary, we have demonstrated the laser performance of Cr 2q:ZnSe crystals prepared by the
modified diffusion doping method under pulsed laser pumping. A slope efficiency as high as 45% and a laser threshold as low as 12 mJ with respect to absorbed pump energy were achieved. The round trip passive loss was estimated to be approximately 7–11% for the best samples. The laser efficiency of diffusion doped Cr 2q:ZnSe laser crystals is expected to be enhanced by using pump wavelength around the absorption band 1.8 mm peak and by further reduction of passive loss due to optimization of crystal quality and doping procedure.
References w1x L.D. DeLoach, R.H. Page, G.D. Wilke, S.A. Payne, W.F. Krupke, IEEE J. Quantum Electron. 32 Ž1996. 885. w2x R.H. Page, K.I. Schaffers, L.D. DeLoach, C.D. Wilke, F.D. Patel, J.B. Tassano, S.A. Payn, W.F. Krupke, K.T. Chen, A. Burger, IEEE J. Quantum Electron. 33 Ž1997. 609. w3x U. Hommerich, X. Wu, V.R. Davis, S.B. Triverdi, K. Grasza, R.J. Chen, S. Kutcher, Opt. Lett. 22 Ž1997. 1180. w4x J.T. Seo, U. Hommerich, S.B. Trivedi, R.J. Chen, S. Kutcher, Opt. Commun. 153 Ž1998. 267. w5x V.I. Levchenko, V.N. Yakimovich, L.I. Postnova, V.I. Konstantinov, V.P. Mikhailov, N.V. Kuleshov, J. Cryst. Growth 198–199 Ž1999. 980. w6x P.F. Moulton, IEEE J. Quantum Electron. 21 Ž1985. 1582. w7x D. Findlay, R.A. Clay, Phys. Lett. 20 Ž1966. 277.