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Laser alexandrite crystals grown by horizontal oriented crystallization technique
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Optical Materials 30 (2008) 1399–1404 www.elsevier.com/locate/optmat
Laser alexandrite crystals grown by horizontal oriented crystallization technique V.V. Gurov *, E.G. Tsvetkov, A.M. Yurkin Institute of Geology and Mineralogy, SB of the Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia Received 7 November 2006; received in revised form 7 August 2007; accepted 9 August 2007 Available online 4 October 2007
Abstract Comparative studies were performed for alexandrite crystals, Al2BeO4:Cr3+, employed in solid state lasers and grown by the horizontal oriented crystallization (HOC) technique and alexandrite crystals grown by the Czochralski (Cz) method. It was shown that the structural quality and possibilities of generation of stimulated emission HOC-crystals are similar to Cz-crystals, whereas their damage threshold is about three times higher. The obtained results and considerably lower cost price of HOC-alexandrite crystals prove their advantageous application in powerful laser systems, which require large laser rods with a higher resistance to laser beam. It is emphasized that application of HOC technique is promising for growth of laser crystals of other high-temperature oxide compounds. 2007 Elsevier B.V. All rights reserved. PACS: 42.70.H; 42.60.J; 61.72.y Keywords: Laser materials; Alexandrite crystals; Structural defects; Lasing characteristics
1. Introduction Among Cr3+-doped crystals used as active medium for tunable lasers, crystals of Cr-doped chrysoberyl, i.e., alexandrite, Al2BeO4:Cr3+, are of interest to solid laser manufacturing. They possess good thermal and mechanical properties, high magnitude of the emission cross-section (3.0 · 1019 cm2 at 300 K), long lifetime of the upper lasing level (260 · 106 s at 300 K), and good lasing efficiency in the region 0.7–0.82 lm, as well as possibility of both diode and lamp pumping [1–3]. Growing of crystals for technical use makes serious claim to a material of containers because it can be the source of impurities. To grow alexandrite crystals rather expensive iridium crucibles are usually used [4–6]. In this paper, we report results of the complex study of alexandrite crystals grown by horizontal oriented crystallization (HOC) [7] which is not traditional for growing laser *
Corresponding author. Tel.: +7 383 3326632. E-mail address: [email protected] (V.V. Gurov).
0925-3467/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.08.004
crystals. We used molybdenum boat-like containers made from thin molybdenum sheets. The main goal of this work was to compare studies alexandrite crystals grown in molybdenum containers by HOC and crystals obtained from iridium crucibles by Cz-technique and to estimate features and aspects of HOC method for growing laser crystals. 2. Experimental Al2BeO4 congruently melts at 1867 C [8] HOC- and Cz-alexandrite crystals used in this study were grown in the direction [1 0 0] from the melt, using beryllium, aluminum, and chromium oxides in stoichiometric proportions in the atmosphere of purified argon. HOC crystals (Fig. 1a) were grown using resistance heating. Axial gradient of temperature in crystallization zone was ranged from 3 to 5 C/mm. The boat with melt moved out of the heating zone at a rate of 1.5–3 mm/h. Thermal conditions or Cz-crystal growth provided by RF inductor were characterized by axial and radial
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V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
Fig. 1. Alexandrite crystals grown by HOC- (a) and Cz- (b) technique.
gradients of temperature in crystallization zone close to 8– 10 and 5–6 C/mm, respectively. Crystal (Fig. 1b) was rotated at 10–50 rpm and pulled out of at a rate of 1.5– 2 mm/h. Comparative parameters of growing processes are reported in Table 1. General procedures, conditions, and equipment for crystal growth were described in detail earlier in [5–7]. The striation structures of grown crystals were visualized by shadow pattern in interference–polarization optical schemes. Various types of inclusions were analyzed under the optical microscope NU-2 (Carl Zeiss) with a 1200-time magnification. For this purpose crystals were cut, and parallel-sided surfaces were subjected to standard optical treatment. The composition of inclusions was determined by the JXA-5A microprobe analyzer. The EPR and optical absorption spectra were measured using RE-1303 and SF20 spectrometers (Russia), respectively. Lasing characteristics of alexandrite crystals were studied in a free running mode. A laser rod (B4.5 · 50 mm
and concentration of Cr3+ 0.24 wt.%) was placed in a 40 cm long resonator. The radius of curvature of nontransmitting mirror in the resonator was 1.0 m with the transmittance coefficient T in the region 0.7–0.8 lm 0.1%; exit mirror was flat with T 10%. Optical pumping was performed with a flashlamp IFP-800. The pump pulse had a bell shape with half-height time about 0.3 ls. The wave-length tuning was obtained using an intracavity selective element for which a prism of TF-7 glass was employed. The energy of laser pulse was measured by a calorimetric detector IKT-1 N (Russia). Spectrograph DFS-452 (Russia) and photographic film INFRA-760 were used for registration of lasing spectra. Comparative measurements of damage threshold of alexandrite crystals grown by the HOC and Czochralski method were conducted using a single-mode ruby laser operating in a single-pulse mode with a pulse of 10 ns (M2 3–4). Focusing of laser emission (50 lm) in experimental samples (fragments of grown crystals with optically polished surfaces) was performed using a quartz lens with a focal distance of 20 mm. A step-by-step control of pulse energy was realized by a set of light filters and the moment of breakdown was registered visually by the emergence of a spark in the sample volume. The respective pulse energy was registered by the measuring device IKT-1N. As the caustic surface diameter for different energies of laser pulse remained the same, we estimated the laser damage threshold by the pulse energy at which the breakdown occurred with a probability of 1/2 when the beam is focused in separate parts of the sample. The estimated value of error of damage threshold measurements was 25%. 3. Results and discussion The dependence of emission energy Eout of the HOCalexandrite laser rod on pump energy Ep is given in Fig. 2. The generation power was about 10 kW/cm2, the efficiency was 0.5%. Rather high thresholds and low efficiency factor of emission are due to the fact that no optimi-
Table 1 Parameters of growing processes
Used equipment The electric power The container’s material Thermal gradients in the crystallization zone The pulling speed The rate of rotation Average crystals sizes Melt’s free surface/ height ratio The mass transfer in the melt The efficient distribution coefficient of chrome dope
Czochralski
HOC
The induction heating apparatus Donets-2 with crucible weight control and power stabilization 25 kW Iridium Axial – 8–10 C/mm Radial – 5–6 C/mm
The ohmic heating apparatus Sapphir-2m with tungsten heater and voltage stabilization 25 kW Molybdenum Axial – 3–5 C/mm
1.5–2 mm/h 10–50 rpm 40 · 120 · 25 mm 8–12 The complex result of the forced and free convections interaction 3–4
1.5–3 mm/h – 80 · 160 · 25 mm 20–30 Free convection 0.3
V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
Fig. 2. Energy output versus pump energy for Cz- (a) and HOCalexandrite (b) laser rods.
zation of lasing conditions was performed. In the same conditions parameters shown by the Cz-alexandrite laser were about 20–25% better. The visualized spectrum of the emission of HOC-alexandrite laser rod was a set of 0.1 nm wide lines with varying intensity grouped in the region 750 nm (Fig. 3b). The distance between these lines changed arbitrarily from 0.1 to 1.5 nm. The spectrum of laser emission of the Cz-alexandrite laser rod was virtually the same (Fig. 3a).
Fig. 3. Visualized laser emission spectra of Cz- (a) and HOC-alexandrite (b) laser rods: regime of free-running laser action, nonselective resonator, resolution 0.3 nm.
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When analyzing the lasing characteristics of the HOCalexandrite laser rod, an interesting feature was observed: on simultaneous deflection (5) of resonator mirrors from its axis, in one plane only (Fig. 4a), we managed to lower the threshold of emission by 20%, i.e., to magnitudes close to those for the Cz-alexandrite laser rod. However, on this tuning no complete filling of mode volume was achieved at high pumping levels. In the Cz-alexandrite laser rod this effect was not observed and the minimal emission threshold was always achieved with the mirrors oriented perpendicular to the resonator axis (Fig. 4b). We think that the revealed orientation features of laser emission are the result of variations of refractive indices, caused by growth striations in crystals and in laser rods made of them. These striations are a sequel to the nonuniform ingress of dopant in crystal because of fluctuations of real crystallization rate. In analyzed alexandrite crystals these heterogeneities occur as variously scaled systems of zones from 5 to 500 lm in thickness alternating with a period from 10 to n · 103 lm, following the shape of crystallization front during their formation (Fig. 5). Since the shape of crystallization front in the crystals grown by the HOC and Cz methods can considerably change during the growth, laser rods made of them may also contain disoriented systems of periodic growth striations (PGS). The value of refraction index might changed by the value from 104 to 103. It was established that in the studied HOC-alexandrite laser rod striations were oriented at an angle of 60 to its axis (Fig. 4a), whereas in the Cz-alexandrite laser rod were nearly perpendicular to its axis. Specificities of heating zones configurations of both methods and faceted growth of alexandrite result in different sectorial structures of crystals. In Cz process the crystallization front was perpendicular to the pulling direction, while in HOC it was sloping relative to the direction of crystal growth. As a result the main volume of Cz-crystals consists of (1 0 0) growth sector, whereas in HOC-crystals it is the growth sector of the facet (1 2 0) (Fig. 5a and b). There is one more difference between these methods in formation growth striations of alexandrite crystals. Due to specific hydrodynamics and mass transfer the efficient distribution coefficient of chromium dopant in HOC is about 0.3 [7], whereas in Cz-process it is about 3–4 [6]. Therefore due to the dopant absorption mechanism on the phase boundary [9] variations of refractive indices and gradients of dopant concentration along the crystal are less in the former process. It is obvious that for the laser emission such growth layers are an additional source of inactive losses of the energy due to Fresnel refraction. Its total value for the whole laser rod may reach a few percent. Because of the polarization of laser emission (E||b) and at Brewster angle for growth layers 45, the intensity of reflected beam in the HOC-alexandrite laser rod significantly changes only when the resonator is adjusted in particular plane. It is in this plane that the slope angle of growth layers approaches the Brewster angle, and dR/dH value is rather high. In the Cz-alex-
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V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
HOC 60
0
~ 5o
{
a
c
PGS
a
Czochralski
E||b
b
{
b
PGS
R (RPG)
Fig. 4. Orientation of mirrors relative to resonator axis, periodic growth striations (PGS), and crystallographic axes of HOC-alexandrite laser rod at minimum threshold energy of laser emission (a); orientation of the PGS in Cz-alexandrite laser rod relative to its crystallographic axes and resonator axis (b).
Cz- alexandrite
HOC-alexandrite Fig. 5. Typical systems of PGS in HOC- (a) and Cz-alexandrite (b) crystals.
andrite laser rod the slope angle of growth layers significantly differs from the Brewster angle, value of dR/dH of caustic is low and corresponding effect is absent (Fig. 6). Regular growth layers can also influence the emission spectrum, modulating it with the period Dk ¼ ðk2g =2nÞ D1 , where D is the period of a striation growth structure in a crystal, n is the average refractive index along the direction, kg = 2nD/m is the emission wavelength, m is the order of interference [10]. However, no effect of this kind was observed in the crystals studied, which seems to be related to the irregular occurrence of growth layers in laser rods. One of the most important parameter of laser crystals is their damage threshold. Experiments showed that the laser breakdown in HOC-alexandrite crystals occurs at pulse energy of ruby laser about three times higher than those of Cz-alexandrite crystals. Since the experimental condi-
0
20
40
60
80
θ (deg)
Fig. 6. Angle dependence of reflection index on periodic growth striations (PGS) in laser rods made of alexandrite crystals.
tions were similar, one can conclude that the damage threshold of HOC-alexandrite is about three times higher. The breakdown in HOC-alexandrite occurred strictly at one point, which was a focus of lens, whereas in Cz-alexandrite the distance from the lens to breaking point changed for different parts of experimental samples, and sometimes the breakdown had a tracking character. The assumption that this fact was related to the concentration of defects was proved by microscopic analysis of crystal samples. Cz-alexandrite appeared to contain a great
V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
number of opaque plate-like microcrystals measuring 65 lm, which were normally of regular geometric shape of triangles (Fig. 7a). Single bubbles and melt inclusions of irregular shape 610 lm in size were also present. It was established that the composition of these plate-like inclusions is similar to that of the material of container from which the crystals under study were grown [5–7]. Correspondingly, the estimated density of such iridium inclusions in Cz-alexandrite crystals was 102–103 mm3, whereas that of cubic microcrystalline molybdenum inclusions (Fig. 7b) in HOC-alexandrite crystals was two orders of magnitude less, i.e., 1 mm3. It is known that absorption of light occurs on opaque inclusions. If these defects occur in a powerful laser beam, a rapid local heating of crystal takes place. This might lead to a localized thermal explosion and, in case of a high density of inclusions, to a breakdown in the track of laser beam. Absorbing inclusions measuring 65 lm are now considered the main reason of laser destruction of crystals [11,12]. Since the probability of occurrence of metal inclusions in a focused laser beam in HOC-alexandrite crystals appeared to be low, the measured damage threshold in them is evidence of the resistance of crystal matrix itself. At the same time, the significantly lower damage threshold of Cz-alexandrite crystals is due to abundant metal inclusions which are responsible for induced breakdown, often of track type. The features of heat-mass exchange in the system in growing alexandrite crystals by the HOC method, which favor self-purification of melt from particles of container material [7], also promote removal of volatile uncontrolled impurities owing to their evaporation from the wide open surface of melt. Comparison of EPR and optical absorption spectra of Cz- and HOC-alexandrite crystals shows that the latter contain an order of magnitude less uncontrolled impurity of iron and, as a result, display much weaker absorption in the UV region of spectrum (Fig. 8a and b). Moreover, the lower content of iron in HOC-alexandrite crystals provides better luminescence characteristics in them and improves their lasing properties.
α, cm
1403
-1
4
3
2
b
1
a 200
300
λ,nm
Fig. 8. UV absorption spectra of alexandrite crystals grown by HOC (a) and Cz (b) methods.
4. Conclusions • Laser characteristics of HOC-alexandrite was measured in free running mode. The efficiency and the power of generation were 20–25% lower than parameters shown by Cz-alexandrite in the same conditions. The reason for this is a specific striation structure of the studied sample. This source of losses can be minimized using growth technic. • Specific hydrodynamics and a large free surface of the melt in HOC process promote its self-purification and invariable chromium distribution that permits to obtain alexandrite crystals of high optical quality The study showed that due to these factors damage threshold of HOC crystal is much more higher than grown by Cztechnique. • Relatively simple technological realizations of HOC growth method, more available and cheap material of containers, are also pluses on this technology. Therefore HOC method of crystal growth can be a good alternative to Cz-method especially for powerful laser systems, which need large laser rods with a higher damage threshold.
Acknowledgments Fig. 7. Inclusions of container material in alexandrite crystals: (a) microcrystal of Ir in Cz-crystal and (b) microcrystals of Mo in HOCcrystal.
The authors thank N.V. Kovalenko for his valuable assistance in most experimental works. They are also grateful to
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Dr. M.G. Serbulenko and E.G. Samoilova for taking part in studies of grown alexandrite crystals. References [1] H. Samelson, J.C. Walling, D.F. Heller, Proc. SPIE 335 (1982) 85. [2] J.C. Walling, Laser Focus 19 (1983) 213. [3] E.K. Belonogova, S.V. Shavkunov, in: Review on Electronics, Tunable Alexandrite Lasers, vol. 11, No 1, TsNII Elektronika, Moscow, 1985, pp. 1–32. [4] J.C. Walling, E.W. O’Dell, IEEE J. Quant. Electron. QE-16 (1980) 1302. [5] G.V. Bukin, V.N. Matrosov, V.P. Orekhova, et al., J. Cryst. Growth 52 (1981) 537.
[6] E.G. Tsvetkov, Institute Geology and Geophysics SB AS USSR, Ph.D. Thesis, Novosibirsk, Russia, 1982. [7] V.V. Gurov, E.G. Tsvetkov, A.G. Kirdyashkin, J. Cryst. Growth 256 (2003) 361. [8] V.V. Gurov, E.G. Tsvetkov, Inorg. Mater. 34 (1998) 719 (translated from Russian). [9] F.A. Kroger, The Chemistry of Imperfect Crystals, North-Holland, Amsterdam, 1964, 654 p. Chapter 1. [10] G.A. Skripko, A.P. Shkadarevich, in: A.A. Kaminskii (Ed.), Physics and Spectroscopy of Laser Crystals, Nauka, Moscow, 1986, pp. 257– 268 (in Russian). [11] S.I. Anisimov, B.I. Makshantsev, Fiz. Tverd. Tela 15 (1973) 1090 (in Russian). [12] Yu.K. Danileiko, A.A. Manenko, V.S. Nechitailo, Kvantovaya Elektron. 5 (1978) 194 (in Russian).
Optical Materials 30 (2008) 1399–1404 www.elsevier.com/locate/optmat
Laser alexandrite crystals grown by horizontal oriented crystallization technique V.V. Gurov *, E.G. Tsvetkov, A.M. Yurkin Institute of Geology and Mineralogy, SB of the Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia Received 7 November 2006; received in revised form 7 August 2007; accepted 9 August 2007 Available online 4 October 2007
Abstract Comparative studies were performed for alexandrite crystals, Al2BeO4:Cr3+, employed in solid state lasers and grown by the horizontal oriented crystallization (HOC) technique and alexandrite crystals grown by the Czochralski (Cz) method. It was shown that the structural quality and possibilities of generation of stimulated emission HOC-crystals are similar to Cz-crystals, whereas their damage threshold is about three times higher. The obtained results and considerably lower cost price of HOC-alexandrite crystals prove their advantageous application in powerful laser systems, which require large laser rods with a higher resistance to laser beam. It is emphasized that application of HOC technique is promising for growth of laser crystals of other high-temperature oxide compounds. 2007 Elsevier B.V. All rights reserved. PACS: 42.70.H; 42.60.J; 61.72.y Keywords: Laser materials; Alexandrite crystals; Structural defects; Lasing characteristics
1. Introduction Among Cr3+-doped crystals used as active medium for tunable lasers, crystals of Cr-doped chrysoberyl, i.e., alexandrite, Al2BeO4:Cr3+, are of interest to solid laser manufacturing. They possess good thermal and mechanical properties, high magnitude of the emission cross-section (3.0 · 1019 cm2 at 300 K), long lifetime of the upper lasing level (260 · 106 s at 300 K), and good lasing efficiency in the region 0.7–0.82 lm, as well as possibility of both diode and lamp pumping [1–3]. Growing of crystals for technical use makes serious claim to a material of containers because it can be the source of impurities. To grow alexandrite crystals rather expensive iridium crucibles are usually used [4–6]. In this paper, we report results of the complex study of alexandrite crystals grown by horizontal oriented crystallization (HOC) [7] which is not traditional for growing laser *
Corresponding author. Tel.: +7 383 3326632. E-mail address: [email protected] (V.V. Gurov).
0925-3467/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.08.004
crystals. We used molybdenum boat-like containers made from thin molybdenum sheets. The main goal of this work was to compare studies alexandrite crystals grown in molybdenum containers by HOC and crystals obtained from iridium crucibles by Cz-technique and to estimate features and aspects of HOC method for growing laser crystals. 2. Experimental Al2BeO4 congruently melts at 1867 C [8] HOC- and Cz-alexandrite crystals used in this study were grown in the direction [1 0 0] from the melt, using beryllium, aluminum, and chromium oxides in stoichiometric proportions in the atmosphere of purified argon. HOC crystals (Fig. 1a) were grown using resistance heating. Axial gradient of temperature in crystallization zone was ranged from 3 to 5 C/mm. The boat with melt moved out of the heating zone at a rate of 1.5–3 mm/h. Thermal conditions or Cz-crystal growth provided by RF inductor were characterized by axial and radial
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V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
Fig. 1. Alexandrite crystals grown by HOC- (a) and Cz- (b) technique.
gradients of temperature in crystallization zone close to 8– 10 and 5–6 C/mm, respectively. Crystal (Fig. 1b) was rotated at 10–50 rpm and pulled out of at a rate of 1.5– 2 mm/h. Comparative parameters of growing processes are reported in Table 1. General procedures, conditions, and equipment for crystal growth were described in detail earlier in [5–7]. The striation structures of grown crystals were visualized by shadow pattern in interference–polarization optical schemes. Various types of inclusions were analyzed under the optical microscope NU-2 (Carl Zeiss) with a 1200-time magnification. For this purpose crystals were cut, and parallel-sided surfaces were subjected to standard optical treatment. The composition of inclusions was determined by the JXA-5A microprobe analyzer. The EPR and optical absorption spectra were measured using RE-1303 and SF20 spectrometers (Russia), respectively. Lasing characteristics of alexandrite crystals were studied in a free running mode. A laser rod (B4.5 · 50 mm
and concentration of Cr3+ 0.24 wt.%) was placed in a 40 cm long resonator. The radius of curvature of nontransmitting mirror in the resonator was 1.0 m with the transmittance coefficient T in the region 0.7–0.8 lm 0.1%; exit mirror was flat with T 10%. Optical pumping was performed with a flashlamp IFP-800. The pump pulse had a bell shape with half-height time about 0.3 ls. The wave-length tuning was obtained using an intracavity selective element for which a prism of TF-7 glass was employed. The energy of laser pulse was measured by a calorimetric detector IKT-1 N (Russia). Spectrograph DFS-452 (Russia) and photographic film INFRA-760 were used for registration of lasing spectra. Comparative measurements of damage threshold of alexandrite crystals grown by the HOC and Czochralski method were conducted using a single-mode ruby laser operating in a single-pulse mode with a pulse of 10 ns (M2 3–4). Focusing of laser emission (50 lm) in experimental samples (fragments of grown crystals with optically polished surfaces) was performed using a quartz lens with a focal distance of 20 mm. A step-by-step control of pulse energy was realized by a set of light filters and the moment of breakdown was registered visually by the emergence of a spark in the sample volume. The respective pulse energy was registered by the measuring device IKT-1N. As the caustic surface diameter for different energies of laser pulse remained the same, we estimated the laser damage threshold by the pulse energy at which the breakdown occurred with a probability of 1/2 when the beam is focused in separate parts of the sample. The estimated value of error of damage threshold measurements was 25%. 3. Results and discussion The dependence of emission energy Eout of the HOCalexandrite laser rod on pump energy Ep is given in Fig. 2. The generation power was about 10 kW/cm2, the efficiency was 0.5%. Rather high thresholds and low efficiency factor of emission are due to the fact that no optimi-
Table 1 Parameters of growing processes
Used equipment The electric power The container’s material Thermal gradients in the crystallization zone The pulling speed The rate of rotation Average crystals sizes Melt’s free surface/ height ratio The mass transfer in the melt The efficient distribution coefficient of chrome dope
Czochralski
HOC
The induction heating apparatus Donets-2 with crucible weight control and power stabilization 25 kW Iridium Axial – 8–10 C/mm Radial – 5–6 C/mm
The ohmic heating apparatus Sapphir-2m with tungsten heater and voltage stabilization 25 kW Molybdenum Axial – 3–5 C/mm
1.5–2 mm/h 10–50 rpm 40 · 120 · 25 mm 8–12 The complex result of the forced and free convections interaction 3–4
1.5–3 mm/h – 80 · 160 · 25 mm 20–30 Free convection 0.3
V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
Fig. 2. Energy output versus pump energy for Cz- (a) and HOCalexandrite (b) laser rods.
zation of lasing conditions was performed. In the same conditions parameters shown by the Cz-alexandrite laser were about 20–25% better. The visualized spectrum of the emission of HOC-alexandrite laser rod was a set of 0.1 nm wide lines with varying intensity grouped in the region 750 nm (Fig. 3b). The distance between these lines changed arbitrarily from 0.1 to 1.5 nm. The spectrum of laser emission of the Cz-alexandrite laser rod was virtually the same (Fig. 3a).
Fig. 3. Visualized laser emission spectra of Cz- (a) and HOC-alexandrite (b) laser rods: regime of free-running laser action, nonselective resonator, resolution 0.3 nm.
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When analyzing the lasing characteristics of the HOCalexandrite laser rod, an interesting feature was observed: on simultaneous deflection (5) of resonator mirrors from its axis, in one plane only (Fig. 4a), we managed to lower the threshold of emission by 20%, i.e., to magnitudes close to those for the Cz-alexandrite laser rod. However, on this tuning no complete filling of mode volume was achieved at high pumping levels. In the Cz-alexandrite laser rod this effect was not observed and the minimal emission threshold was always achieved with the mirrors oriented perpendicular to the resonator axis (Fig. 4b). We think that the revealed orientation features of laser emission are the result of variations of refractive indices, caused by growth striations in crystals and in laser rods made of them. These striations are a sequel to the nonuniform ingress of dopant in crystal because of fluctuations of real crystallization rate. In analyzed alexandrite crystals these heterogeneities occur as variously scaled systems of zones from 5 to 500 lm in thickness alternating with a period from 10 to n · 103 lm, following the shape of crystallization front during their formation (Fig. 5). Since the shape of crystallization front in the crystals grown by the HOC and Cz methods can considerably change during the growth, laser rods made of them may also contain disoriented systems of periodic growth striations (PGS). The value of refraction index might changed by the value from 104 to 103. It was established that in the studied HOC-alexandrite laser rod striations were oriented at an angle of 60 to its axis (Fig. 4a), whereas in the Cz-alexandrite laser rod were nearly perpendicular to its axis. Specificities of heating zones configurations of both methods and faceted growth of alexandrite result in different sectorial structures of crystals. In Cz process the crystallization front was perpendicular to the pulling direction, while in HOC it was sloping relative to the direction of crystal growth. As a result the main volume of Cz-crystals consists of (1 0 0) growth sector, whereas in HOC-crystals it is the growth sector of the facet (1 2 0) (Fig. 5a and b). There is one more difference between these methods in formation growth striations of alexandrite crystals. Due to specific hydrodynamics and mass transfer the efficient distribution coefficient of chromium dopant in HOC is about 0.3 [7], whereas in Cz-process it is about 3–4 [6]. Therefore due to the dopant absorption mechanism on the phase boundary [9] variations of refractive indices and gradients of dopant concentration along the crystal are less in the former process. It is obvious that for the laser emission such growth layers are an additional source of inactive losses of the energy due to Fresnel refraction. Its total value for the whole laser rod may reach a few percent. Because of the polarization of laser emission (E||b) and at Brewster angle for growth layers 45, the intensity of reflected beam in the HOC-alexandrite laser rod significantly changes only when the resonator is adjusted in particular plane. It is in this plane that the slope angle of growth layers approaches the Brewster angle, and dR/dH value is rather high. In the Cz-alex-
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V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
HOC 60
0
~ 5o
{
a
c
PGS
a
Czochralski
E||b
b
{
b
PGS
R (RPG)
Fig. 4. Orientation of mirrors relative to resonator axis, periodic growth striations (PGS), and crystallographic axes of HOC-alexandrite laser rod at minimum threshold energy of laser emission (a); orientation of the PGS in Cz-alexandrite laser rod relative to its crystallographic axes and resonator axis (b).
Cz- alexandrite
HOC-alexandrite Fig. 5. Typical systems of PGS in HOC- (a) and Cz-alexandrite (b) crystals.
andrite laser rod the slope angle of growth layers significantly differs from the Brewster angle, value of dR/dH of caustic is low and corresponding effect is absent (Fig. 6). Regular growth layers can also influence the emission spectrum, modulating it with the period Dk ¼ ðk2g =2nÞ D1 , where D is the period of a striation growth structure in a crystal, n is the average refractive index along the direction, kg = 2nD/m is the emission wavelength, m is the order of interference [10]. However, no effect of this kind was observed in the crystals studied, which seems to be related to the irregular occurrence of growth layers in laser rods. One of the most important parameter of laser crystals is their damage threshold. Experiments showed that the laser breakdown in HOC-alexandrite crystals occurs at pulse energy of ruby laser about three times higher than those of Cz-alexandrite crystals. Since the experimental condi-
0
20
40
60
80
θ (deg)
Fig. 6. Angle dependence of reflection index on periodic growth striations (PGS) in laser rods made of alexandrite crystals.
tions were similar, one can conclude that the damage threshold of HOC-alexandrite is about three times higher. The breakdown in HOC-alexandrite occurred strictly at one point, which was a focus of lens, whereas in Cz-alexandrite the distance from the lens to breaking point changed for different parts of experimental samples, and sometimes the breakdown had a tracking character. The assumption that this fact was related to the concentration of defects was proved by microscopic analysis of crystal samples. Cz-alexandrite appeared to contain a great
V.V. Gurov et al. / Optical Materials 30 (2008) 1399–1404
number of opaque plate-like microcrystals measuring 65 lm, which were normally of regular geometric shape of triangles (Fig. 7a). Single bubbles and melt inclusions of irregular shape 610 lm in size were also present. It was established that the composition of these plate-like inclusions is similar to that of the material of container from which the crystals under study were grown [5–7]. Correspondingly, the estimated density of such iridium inclusions in Cz-alexandrite crystals was 102–103 mm3, whereas that of cubic microcrystalline molybdenum inclusions (Fig. 7b) in HOC-alexandrite crystals was two orders of magnitude less, i.e., 1 mm3. It is known that absorption of light occurs on opaque inclusions. If these defects occur in a powerful laser beam, a rapid local heating of crystal takes place. This might lead to a localized thermal explosion and, in case of a high density of inclusions, to a breakdown in the track of laser beam. Absorbing inclusions measuring 65 lm are now considered the main reason of laser destruction of crystals [11,12]. Since the probability of occurrence of metal inclusions in a focused laser beam in HOC-alexandrite crystals appeared to be low, the measured damage threshold in them is evidence of the resistance of crystal matrix itself. At the same time, the significantly lower damage threshold of Cz-alexandrite crystals is due to abundant metal inclusions which are responsible for induced breakdown, often of track type. The features of heat-mass exchange in the system in growing alexandrite crystals by the HOC method, which favor self-purification of melt from particles of container material [7], also promote removal of volatile uncontrolled impurities owing to their evaporation from the wide open surface of melt. Comparison of EPR and optical absorption spectra of Cz- and HOC-alexandrite crystals shows that the latter contain an order of magnitude less uncontrolled impurity of iron and, as a result, display much weaker absorption in the UV region of spectrum (Fig. 8a and b). Moreover, the lower content of iron in HOC-alexandrite crystals provides better luminescence characteristics in them and improves their lasing properties.
α, cm
1403
-1
4
3
2
b
1
a 200
300
λ,nm
Fig. 8. UV absorption spectra of alexandrite crystals grown by HOC (a) and Cz (b) methods.
4. Conclusions • Laser characteristics of HOC-alexandrite was measured in free running mode. The efficiency and the power of generation were 20–25% lower than parameters shown by Cz-alexandrite in the same conditions. The reason for this is a specific striation structure of the studied sample. This source of losses can be minimized using growth technic. • Specific hydrodynamics and a large free surface of the melt in HOC process promote its self-purification and invariable chromium distribution that permits to obtain alexandrite crystals of high optical quality The study showed that due to these factors damage threshold of HOC crystal is much more higher than grown by Cztechnique. • Relatively simple technological realizations of HOC growth method, more available and cheap material of containers, are also pluses on this technology. Therefore HOC method of crystal growth can be a good alternative to Cz-method especially for powerful laser systems, which need large laser rods with a higher damage threshold.
Acknowledgments Fig. 7. Inclusions of container material in alexandrite crystals: (a) microcrystal of Ir in Cz-crystal and (b) microcrystals of Mo in HOCcrystal.
The authors thank N.V. Kovalenko for his valuable assistance in most experimental works. They are also grateful to
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Dr. M.G. Serbulenko and E.G. Samoilova for taking part in studies of grown alexandrite crystals. References [1] H. Samelson, J.C. Walling, D.F. Heller, Proc. SPIE 335 (1982) 85. [2] J.C. Walling, Laser Focus 19 (1983) 213. [3] E.K. Belonogova, S.V. Shavkunov, in: Review on Electronics, Tunable Alexandrite Lasers, vol. 11, No 1, TsNII Elektronika, Moscow, 1985, pp. 1–32. [4] J.C. Walling, E.W. O’Dell, IEEE J. Quant. Electron. QE-16 (1980) 1302. [5] G.V. Bukin, V.N. Matrosov, V.P. Orekhova, et al., J. Cryst. Growth 52 (1981) 537.
[6] E.G. Tsvetkov, Institute Geology and Geophysics SB AS USSR, Ph.D. Thesis, Novosibirsk, Russia, 1982. [7] V.V. Gurov, E.G. Tsvetkov, A.G. Kirdyashkin, J. Cryst. Growth 256 (2003) 361. [8] V.V. Gurov, E.G. Tsvetkov, Inorg. Mater. 34 (1998) 719 (translated from Russian). [9] F.A. Kroger, The Chemistry of Imperfect Crystals, North-Holland, Amsterdam, 1964, 654 p. Chapter 1. [10] G.A. Skripko, A.P. Shkadarevich, in: A.A. Kaminskii (Ed.), Physics and Spectroscopy of Laser Crystals, Nauka, Moscow, 1986, pp. 257– 268 (in Russian). [11] S.I. Anisimov, B.I. Makshantsev, Fiz. Tverd. Tela 15 (1973) 1090 (in Russian). [12] Yu.K. Danileiko, A.A. Manenko, V.S. Nechitailo, Kvantovaya Elektron. 5 (1978) 194 (in Russian).