Some optical properties of Cr4+-doped crystals

Some optical properties of Cr4+-doped crystals

Optical Materials 13 (1999) 117±127 Some optical properties of Cr4‡-doped crystals B. Lipavsky a a,b,* , Y. Kalisky b, Z. Burshtein b, Y. Shimony c...

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Optical Materials 13 (1999) 117±127

Some optical properties of Cr4‡-doped crystals B. Lipavsky a

a,b,* ,

Y. Kalisky b, Z. Burshtein b, Y. Shimony c, S. Rotman

a

Department of Electrical and Computer Engineering, Ben Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel b Arava Laser laboratory, Rotem Industrial Park, Mishor Yamin 86800, Israel c Nuclear Research Center, Negev, P.O. Box 9001, Beer Sheva, Israel

Abstract We summarize brie¯y some of our past studies, and report of preliminary recent new results concerning the optical properties of Cr4‡ -doped crystals that are important for utilization as passive Q-switching devices in Nd:YAG laser systems. The host crystals involved are YAG, YSGG, GGG, LuAG and forsterite. Excited-state lifetimes of 4.0, 1.0 and 1.7 ls were measured by the Cr4‡ ¯uorescence decay (1.3±1.7 lm) following pulsed excitation at 1064 nm in YAG, YSGG and GGG, respectively. The ground- and excited-state absorption (ESA) cross-sections at k ˆ 1064 nm were estimated from transmission saturation measurements. For [Cr4‡ ,Mg2‡ ]:YAG the respective results were rgs ˆ …3:25  0:15†  10ÿ18 cm2 and res ˆ …6:25  0:5†  10ÿ19 cm2 . In the orthorhombic forsterite, the cross-sections were polarization dependent. We got rgs < …3:3  1†  10ÿ19 cm2 for Eka, rgs ˆ …23  2†  10ÿ19 cm2 and res ˆ …9:0  0:7†  10ÿ19 cm2 for Ekb, and rgs ˆ …16  1†  10ÿ19 cm2 and res ˆ …5:7  0:4†  10ÿ19 cm2 for Ekc. Polarised ESA spectra were measured between 680 and 960 nm using the pulsed pump/probe technique. At 750 nm we got rgs ˆ …110  10†  10ÿ19 cm2 and res ˆ …25  3†  10ÿ19 cm2 for Ekb. Passive Q-switching performance of a ¯ashlamp-pumped Nd:YAG laser using an intracavity [Cr4‡ ,Ca2‡ ]:GGG sample is also demonstrated. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Saturable absorption; Excited-state absorption; Fluorescence; Passive Q-switch; Chromium-doped materials

1. Introduction Until about a decade ago, passive laser Qswitching, especially for Nd:YAG lasers (k ˆ 1064 nm) was practical only in the low-power region. The reason was poor durability of the organicmaterial-based saturable absorbers then available [1]. The development of inorganic saturable absorbers initiated a breakthrough in this ®eld. The ®rst report in 1981 concerned LiF:Fÿ 2 color center crystals [2]. Later on, di€erent matrices containing Cr4‡ ions were developed [3±8]. The Cr4‡ ions provide the large absorption cross-section in the *

Corresponding author.

spectral region of interest (mainly near k ˆ 1064 nm), while the matrix provides the chemical stability and high-heat conductivity required for durability. Garnet crystal matrices are considered quite appealing due to their relatively high mechanical strength and heat conductivity. This family of crystals belongs to the Ia3d space-group (O10 h in Schoen¯ies notation). The general stoichiometric formula is C3 A2 D3 O12 , where C, A, and D denote di€erent lattice sites, each characterized by a speci®c site symmetry, which relates closely to its oxygen coordination (dodecahedral, octahedral and tetrahedral, respectively). The optically active Cr4‡ center occupies the tetrahedrally coordinated lattice site, which is, however, only of a D2d site symmetry. Still, for simplicity, this site is often

0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 2 0 - 8

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approximated as having an exact tetrahedral symmetry Td . The host materials reviewed and studied in the present work include the garnets (YAG), Y3 Sc2 Ga3 O12 (YSGG), Y3 Al5 O12 Gd3 Ga5 O12 (GGG), and Lu2 Al5 O12 (LuAG), and the olivine Mg2 SiO4 (forsterite). The latter crystal is orthorhombic, with lattice constants a ˆ 4.762  b ˆ 10.225 A  and c ˆ 5.994 A.  The crystal A, contains four formula units in the unit cell, of space-group Pnma (D16 2h in Schoen¯ies notation) [9]. Quadrivalent chromium doped crystals exhibit wide absorption (920±1100 nm) and emission (1300±1700 nm) bands, with excited-state lifetimes in the microsecond range. Apart from their potential application as passive Q-switching devices for lasers operating in their absorption range, these materials may also perform as tunable lasers at their emission range [10±14]. Our present paper emphasizes the importance of studying excited-state absorption (ESA) of saturable absorbers in order to assess their merits as a passive laser Q-switching devices. The energy-level schemes that are relevant for the garnet or olivine crystals are shown in Fig. 1, and will be discussed in Section 3. 2. Experimental The Cr4‡ -doped crystals used in our work were grown by the standard Czochralski technique. The Cr4‡ doped YAG, YSGG, and GGG samples were kindly supplied to us by the Union Carbide Corporation, and the forsterite by CREOL. Usually, co-doping with Ca2‡ is administered for charge compensation, as the original site in the crystal is rather inhabited by a trivalent ion. In one of our Cr4‡ :YAG samples, however, Mg2‡ was used for the same purpose [15], and the transmission saturation curves of that [Cr4‡ ,Mg2‡ ]:YAG crystals were measured for comparison. All samples used had the shape of a rectangular prism. Particularly, the Cr4‡ :forsterite had its sides parallel to the a, b, and c crystallographic axes, with dimensions ranging between 5 and 10 mm. The surfaces were polished to a high-optical quality. Polarized or non-polarized low-intensity transmission spectra were measured

Fig. 1. Schematic energy level diagrams for Cr4‡ in garnet (a), or in olivine (b) host materials. In the garnet, and an approximated Td site symmetry, 3 A2 is the ground state, 3 T2 is the excited state, 1 E is the metastable excited state. Second excited state is of an unspeci®ed symmetry. Transition from 3 T2 to 1 E is assumed to be faster than s or s . In the forsterite, the site occupied by the Cr4‡ is of a Cs symmetry. Only triplet states are shown, and the allowed transitions in the a, b or c polarizations. Note that optical absorption into the ®rst excited multiplet is forbidden in the a polarization.

for each sample using a JASCO UV/VIS/NIR model V-570 spectrophotometer. For transmission saturation measurements we used 1064 nm pulses from a Quanta-Ray GCR-12 Nd:YAG laser. It provided linearly polarized

B. Lipavsky et al. / Optical Materials 13 (1999) 117±127

pulses, of near TEM00 quality, of energy up to 350 mJ/pulse, and duration ranging between 20 and 50 ns (FWHM). When required, a sapphire bire¯ectant polarizing prism [16] was used to select the desired linear polarization component. The beam was focused by a lens of 50 cm focal length. The laser energy density impinging on the sample was varied by changing the laser output or by moving the sample along the propagation axis of the laser beam. The beam cross-section diameter at each point was measured by using the knife-edge method. The incident or transmitted beam energies were measured by an Ophir digital energy meter. For ESA spectroscopy, a pulsed pump/probe technique was used. To create the probe beam, a part of the Nd:YAG laser output was split o€. This part was frequency-doubled, then used to pump a Ti:sapphire laser, tunable between approximately 680 and 960 nm. The Ti:sapphire laser provided 1±30 mJ, 30±100 ns long (FWHM) pulses, of near TEM00 distribution. A sapphire bire¯ectant prism was used here as well to select the desired linear polarization component. This beam was reduced in energy to below 0.1 mJ by using beamsplitters and neutral-density ®lters, then folded to spatially overlap the 1064 nm beam incident on the sample. There was, however, a time delay of the probe Ti:sapphire laser pulse relative to the pump 1064 nm pulse, which varied between 30 and 100 ns. In the experiments conducted for Cr4‡ :forsterite, the 1064 nm beam itself was also polarized, with such polarization orientation and energy density which insured complete groundstate bleaching of the Cr4‡ ions for all polarization orientations of the probe Ti:sapphire beam. For ¯uorescence lifetime measurements, the same laser was used for excitation. The ¯uorescence was isolated by using a cut-o€ ®lter in front of a monochromator, which was also equipped with a fast germanium photodiode at the exit slit. The ¯uorescence decay curves were recorded using a Tektronix TDS-724A digital oscilloscope. The same system was used to measure the ¯uorescence spectrum by manual tuning of the monochromator. Regular and passively Q-switched Nd:YAG laser pulse shapes were measured by using a fast Silicon p.i.n. photodiode, with a risetime of less than 1.0 ns.

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3. Theoretical background Saturability of the optical absorption is the fundamental property that allows the use of any material as a passive laser Q-switch. Saturability of absorption means that the material optical transmittance increases with the light energy incidenting on the sample. It happens when the ground-state absorption cross-section rgs is large enough, and simultaneously the excited-state lifetime is long enough to enable considerable depletion of the ground state upon illumination. At the same time, the ESA cross-section res must be considerably smaller than that of the ground state. ESA is a cause for loss in the laser cavity, and thus limits the eciency of the Qswitching device. For that reason, the ratio c  rgs =res is considered to be an important ®gure of merit for the passive Q-switching application. The occurrence of ESA manifests itself in transmission versus laser pulse energy measurements when the saturated transmission never reaches a 100%. It is instructive to consider the saturation e€ect in relation to the Cr4‡ ion energy-level scheme. A possible scheme for the garnet crystals in the tetrahedral site notation is shown in Fig. 1(a). The ®rst photons in the incident pulse excite the Cr4‡ from its ground 3 A2 state to its excited 3 T2 state. It then crosses immediately to the metastable 1 E state [17]. The latter is apparently of a much lower absorption cross-section. The 1 E lifetime s is of the order of several microseconds; for example, 4.1 ls in Cr4‡ :YAG, 2.2 ls in Cr4‡ :GGG, 1.3 ls in Cr4‡ :YSGG [18]. Independent measurements of s are provided in this work, with slightly di€erent results. In Cr4‡ :forsterite, the excited-state lifetime is 3.0 ls [19]. The second excited state is of an unspeci®ed symmetry, and its lifetime s was found to be approximately 0.55 ns in Cr4‡ :YAG [20]. It should be noted, that the exact symmetry assignment of the metastable excited state is still uncertain; strong evidence suggests it to be a 1 E [17], as given in Fig. 1(a). Spectroscopic stress dependence studies suggest it rather to be the originally excited 3 T2 state [21]. The Stokes shift between absorption and emission in the latter case must then be

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accounted for by the excited-state interaction with lattice phonons. This interesting issue, while still open, bears no immediate consequence on the current subject matter. In Fig. 1(b) we show the energy level scheme for the Cr4‡ ion in forsterite [22]. The site occupied by the Cr4‡ is of a Cs symmetry. Only triplet states are shown, and the allowed transitions for the a, b or c polarizations are indicated. Note that optical absorption into the ®rst excited orbitally split multiplet of the 3 T2 is forbidden in the Eka polarization. In a recent publication issued by our laboratory [20] we have developed an approximate analytical solution for the energy transmission of a short laser pulse through a saturable absorber which exhibits ESA: Tlossy 

Eout TFN ÿ T0  T0 ‡ …Tmax ÿ T0 †; Ein 1 ÿ T0

…1†

4. Results and discussion 4.1. Fluorescence spectroscopy and lifetime Fluorescence versus time decays at room temperature of several Cr4‡ -doped crystals under excitation with the short (25 ns) Nd:YAG laser pulses are shown in Fig. 2. The lifetimes extracted from the decay curves are 4.0 ls for [Cr4‡ ,Mg2‡ ]:YAG, 1.0 ls for [Cr4‡ ,Ca2‡ ]:YSGG, and 1.7 ls for [Cr4‡ ,Ca2‡ ]:GGG. These values are similar to those reported by K uck et al. [18] (4.1, 1.3 and 2.2 ls, respectively). The emission spectra for the same crystals are shown in Fig. 3. Each exhibits a single structureless broad peak in the range 1.3±1.7 lm, with di€erences between di€erent hosts: The peaks are centered at approximately 1440 nm for YAG and GGG, and at 1550 nm for YSGG. All widths are approximately 200 nm

where Eout and Ein are the outgoing and incident total pulse energies, respectively, T0  exp…ÿN rgs L† is the small-signal limit of the energy transmission, with N being the Cr4‡ ion density and L the sample thickness, Tmax  exp…ÿN res L† is the high-energy (saturated) transmission limit, and TFN is the energy transmission under no-loss conditions (namely, assuming res ˆ 0), which is known as the Frantz±Nodvik expression [23]: TFN ˆ

  ÿ   1 ln 1 ‡ T0 exp rgs E0 ÿ 1 ; rgs E0

…2†

where E0 is the incident pulse photon density (units of photons per unit area), assumed to be constant. A straightforward approximate correction exists for the case of a transverse gaussian energy distribution, by which E0 in Eq. (2) may be replaced by half the peak photon density of the gaussian distribution, Ep 12 [20]. All above expressions are valid for a slow absorber, namely when the pulse duration is very short compared to the excited-state lifetime s, a condition satis®ed throughout all our experiments. A ®t of experimental transmission saturation curves to Eq. (1) allows to simultaneously obtain fairly accurate estimates of N, rgs and res .

Fig. 2. Fluorescence time decay of the 1.3±1.7 lm emission in [Cr4‡ ,Mg2‡ ]:YAG, [Cr4‡ ,Ca2‡ ]:YSGG, and [Cr4‡ ,Ca2‡ ]:GGG excited by 1064 nm 25 ns long pulses.

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Fig. 3. Room temperature emission spectra of [Cr4‡ ,Mg2‡ ]:YAG, [Cr4‡ ,Ca2‡ ]:YSGG, and [Cr4‡ ,Ca2‡ ]:GGG excited by 1064 nm 25 ns long Nd:YAG laser pulses.

(FWHM). In the work of K uck et al. [18], the peaks are at 1380 nm for YAG, 1440 nm for GGG, and 1560 nm for YSGG. The reason for the discrepancy between our result for YAG and that of K uck et al. is not yet clear, and will be further investigated in the future. 4.2. Transmission saturation curves Fig. 4 displays the measured transmission versus pulse energy through a [Cr4‡ ,Mg2‡ ]:YAG sample. The transmission values are corrected for Fresnel losses on both surfaces using the known refractive index of YAG at 1064 nm (1.82). The transmission starts at about T0 ˆ 68% in the lowenergy range, rises with increasing pulse energy, and saturates at about Tmax ˆ 96% in the highenergy range. A numerical ®t to Eq. (1) provides N ˆ …1:9  0:2†  1017 cmÿ3 , rgs ˆ …3:25  0:15† 10ÿ18 cm2 , and res ˆ …6:25  0:5†  10ÿ19 cm2 .

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Fig. 4. Energy transmission corrected for Fresnel re¯ection versus peak energy density of transversely gaussian light pulses 25 ns long, k ˆ 1064 nm, through a [Cr4‡ ,Mg2‡ ]:YAG sample, 6.2 mm thick.

These values are close to those estimated for [Cr4‡ ,Ca2‡ ]:YAG [20,24], as well as the ratio c  rgs =res , which is about 5 as compared to the values of 3.5 or 7 reported in Refs. [20,24], respectively. It appears thus that there is no signi®cant impact of the co-dopant (Mg2‡ or Ca2‡ ) on the optical absorption of the Cr4‡ ion in YAG, at least at 1064 nm. Similar measurements were performed for a Cr4‡ :forsterite crystal with the light electric ®eld aligned parallel to each of the crystallographic axes, and are shown in Fig. 5. In the particular case of the Eka polarization, the transmission results are noisy, and seem to be ¯at throughout the available energy density. Noting that the Eka con®guration constitutes a forbidden transition (Fig. 1(b)), the small absorption still obtained may be the result of slight misorientation of the sample,

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Fig. 5. Energy transmission corrected for Fresnel re¯ection versus peak energy density of linearly polarized, transversely gaussian light pulses, 25 ns long, k ˆ 1064 nm, through a Cr4‡ :forsterite sample. (a), (b) and (c) for the light polarization parallel to the a, b or c crystallographic axes, respectively.

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respectively. Three curves are shown for each case. The ground-state absorption cross-section is obtained by rgs ˆ a=N , where a is the polarized, lowintensity absorption coecient. The ``e€ective'' ESA cross-section is obtained by reff ˆ ÿ…1=NL† ln…T =T0 †, where T0 is the energy transmission of the probe Ti:sapphire laser beam through the sample without the Nd:YAG 1064 nm pulse excitation, and T is the same transmission with the Nd:YAG pulse on. The e€ective cross-sections are negative in the Ekb and Ekc cases, exhibiting a pronounced structure in the range 680±800 nm for Ekb. The e€ective cross-section is positive between 720 and 800 nm, and is practically zero between 800 and 950 nm in the Eka case. The cross-section absolute values are of the order of 80  10ÿ19 cm2 . The e€ective absorption is a result of both ESA and a reduction of ground-state absorption due to bleaching. Stimulated emission may be entirely neglected in this spectral region. Thus, the ESA cross-section is given by res ˆ reff ‡ rgs . Of special interest is Ekb case. In the 680±750 nm range, the ESA turns out to be considerably smaller than the ground-state absorption. Particularly at 750 nm, rgs ˆ …110  10† 10ÿ19 cm2 , res ˆ …25  3†  10ÿ19 cm2 , hence c  rgs =res  4, satisfying the necessary condition for passive Q-switching of lasers in this region. As to the spectroscopic aspects of the ESA measurements, the spectral range is too narrow to allow an unequivocal interpretation relating to the level scheme presented in Fig. 1(b).

or of an elliptic component in the light polarization state. This would also account for the excessive noise in the data. Thus, only an upper-limit estimate for the ground-state absorption crosssection may be extracted. In a summary, the polarization-dependent cross-sections were rgs < …3:3  1†  10ÿ19 cm2 for Eka, rgs ˆ …23  2†  10ÿ19 cm2 and res ˆ …9:0  0:7†  10ÿ19 cm2 for Ekb, and rgs ˆ …16  1†  10ÿ19 cm2 and res ˆ …5:7  0:4†  10ÿ19 cm2 for Ekc. The chromium ion concentration is estimated as N ˆ …4:0  0:5†  1017 cmÿ3 . The anisotropy obtained between the absorption cross-sections in the b and c polarizations is similar to that obtained through similar measurements by Kuleshov et al. [25], although our absolute values are about a factor of 3±4 greater than theirs. No estimate of the groundstate absorption is provided by the latter authors for the a polarization. Table 1 summarizes our present new results in comparison to those obtained previously in other studies [20,24]. It appears that [Cr4‡ ,Ca2‡ ]:LuAG displays the greatest value of the cross-sections' ration c, which makes it so far the most favorable material for passive Q-switching at k ˆ 1064 nm. 4.3. ESA spectra in Cr4‡ :forsterite The ESA spectra in the range 680±960 nm for polarized light in the a, b, and c orientations of Cr4‡ :forsterite are shown in Fig. 6(a), (b) and (c),

Table 1 Summary of measured density N, and absorption cross-section parameters rgs and res of some chromium-doped crystals Parameter

Units

[Cr4‡ ,Ca2‡ ]: [Cr4‡ ,Mg2‡ ]: [Cr4‡ ,Ca2‡ ]: YAGa YAGc GGGb

[Cr4‡ ,Ca2‡ ]: [Cr4‡ ,Ca2‡ ]: YSGGa LuAGa

Forsterite (E jja)c

Forsterite (Ekb)c

Forsterite (Ekc)c

N

1017 cmÿ3 10ÿ18 cm2 10ÿ19 cm2 ÿ

6.3

1.8 ‹ 0.2

0.71 ‹ 0.05

7.0

26

ÿ

4.1 ‹ 0.4

3.5 ‹ 0.4

3.2

3.25 ‹ 0.15

5.8 ‹ 0.5

4.6

1.1

<0.33 ‹ 0.1 2.3 ‹ 0.2

1.6 ‹ 0.1

4.5

6.25 ‹ 0.5

13 ‹ 2

4.0

0.45

ÿ

9.0 ‹ 0.7

5.7 ‹ 0.4

7.0, 3.5b

5.2 ‹ 0.4

4.5 ‹ 0.7

11.0

25

ÿ

2.6 ‹ 0.3

2.8 ‹ 0.3

rgs res c ˆ rgs /res a b c

Ref. [24]. Ref. [20]. Results of the present study.

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Fig. 6. Cross-section spectra for the ground-state (rgs ) and excited-state (res ) absorption, and the e€ective absorption cross section (reff ) of polarized light in a Cr4‡ :Mg2 SiO4 sample, for di€erent crystallographic orientations relative to the light electric ®eld, as indicated in the ®gures. Pump polarizations were parallel to c in (a), parallel to b in (b), and parallel to c in (c).

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Fig. 7. Oscilloscope traces of light intensity versus time dependence of a ¯ashlamp pumped Nd:YAG laser under (a) free-running conditions, and (b) when a Cr4‡ :GGG saturable absorber was inserted into the cavity. (c) Details of one Q-switched pulse.

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4.4. Passive Q-switching We have demonstrated passive Q-switching in a self-made, high-energy, ¯ashlamp-pumped Nd:YAG laser. Fig. 7(a) displays the oscilloscope traces of the laser pulse output under free-running conditions. The pulse energy is 110 mJ, and its entire duration is approximately 150 ls, similar to the duration of the pump ¯ash. The pulse exhibits the regular relaxation oscillation pattern. A polished [Cr4‡ ,Ca2‡ ]:GGG plate, 3.55 mm thick, was then inserted in the laser cavity, with its surface perpendicular to the laser axis, without any external cooling. The previously measured parameters of this sample [20] were N ˆ …7:15  0:5† 1016 cmÿ3 , rgs ˆ …58  5†  10ÿ19 cm2 , and res ˆ …13  2† 10ÿ19 cm2 . A train of well-isolated short pulses obtains, each of many times higher peak energies as compared those of the relaxation oscillation pulses ± Fig. 7(b). The separate pulses are approximately 150±200 ns long (FWHM), as shown in Fig. 7(c). The total train output energy was 40 mJ under the same pump conditions. A detailed theoretical simulation of passive Qswitching of a Nd:YAG laser using Cr4‡ -doped garnets is under current development in our laboratory. 5. Conclusions and summary We have presented some of our preliminary new results concerning our laboratory's on-going e€ort to develop improved materials for passive Qswitching of high-power Nd:YAG lasers. Our studies primarily involve the optical properties of Cr4‡ -doped crystals. The host crystals we have studied so far are YAG, YSGG, GGG, LuAG and forsterite. The methods used are ¯uorescence spectrum and lifetime measurements; transmission saturation of Nd:YAG laser pulses, pulsed pump/ probe ESA, and Q-switching performance of the various samples. All these methods are also re¯ected in this report, although the results are only of a preliminary nature. We summarized our previous results, and results of others, in comparison to the new ones. The latter are the following: Excited-state lifetimes of 4.0, 1.0 and 1.7 ls were

measured for the Cr4‡ through the ¯uorescence in the 1.3±1.7 lm spectral range in YAG, YSGG and GGG, respectively. Some ground- and ESA crosssections at k ˆ 1064 nm were estimated from transmission saturation measurements. For [Cr4‡ ,Mg2‡ ]:YAG the respective results rgs ˆ …3:25  0:15†  10ÿ18 cm2 and res ˆ …6:25  0:5† 10ÿ19 cm2 . In Cr4‡ :forsterite, the cross-sections were polarization dependent. We got rgs < …3:3  for Eka, rgs ˆ …23  2†  1†  10ÿ19 cm2 10ÿ19 cm2 and res ˆ …9:0  0:7†  10ÿ19 cm2 for Ekb, and rgs ˆ …16  1†  10ÿ19 cm2 and res ˆ …5:7  0:4†  10ÿ19 cm2 for Ekc. Polarized ESA spectra were measured between 680 and 960 nm using the pulsed pump/probe technique for the crystallographic a, b, and c orientations. For Ekb at 750 nm we got rgs ˆ …110  10†  10ÿ19 cm2 and res ˆ …25  3†  10ÿ19 cm2 , suggesting the possibility of future use of this con®guration for passive Q-switching of lasers near 750 nm. Passive Q-switching performance of a high-energy ¯ashlamp-pumped Nd:YAG laser using an intracavity Cr4‡ :GGG sample is demonstrated.

References [1] W. Kochner, Solid State Laser Engineering, 3rd ed., ch. 8, Springer, Berlin, Germany, 1992. [2] A.G. Kalintsev, A.A. Mak, L.N. Soms, A.I. Stepanov, A.A. Tarasov, Residual losses in passive shutters made from LiF crystals with color centers, Sov. Phys. Tech. Phys. 26 (1981) 1267. [3] I.J. Miller, A.J. Alcock, J.E. Bernard, Experimental Investigation of Cr4‡ :YAG as a passive Q-switch, OSA Proc. Adv. Solid State Lasers 13 (1992) 322. [4] P. Yankov, Cr4‡ :YAG Q-switching of Nd:host laser oscillators, J. Phys. D: Appl. Phys. 27 (1994) 1118. [5] I.V. Klimov, M.Yu. Nikol'skii, V.B. Tsvetkov, I.A. Scherbakov, Passive Q-switching of pulsed Nd3‡ lasers using YSGG:Cr4‡ crystal switches exhibiting phototropic properties, Sov. J. Quantum Electron. 22 (1992) 603. [6] Y. Shimony, Y. Kalisky, B.T.H. Chai, Cr4‡ :YAG saturable absorber as passive Q-switch for pulsed Nd:YAG laser, Opt. Mater. 4 (1995) 547. [7] Y. Shimony, Z. Burshtein, Y. Kalisky, Cr4‡ :YAG as passive Q-switch and Brewster plate in a pulsed Nd:YAG laser, IEEE J. Quantum Electron. 31 (1995) 1738. [8] Y. Shimony, Z. Burshtein, A. Ben-Amar Baranga, Y. Kalisky, M. Strauss, Repetitive Q-switching of a CW Nd:YAG laser using Cr4‡ :YAG saturable absorbers, IEEE J. Quantum Electron. 32 (1996) 305.

B. Lipavsky et al. / Optical Materials 13 (1999) 117±127 [9] J.D. Birle, G.V. Gibbs, P.B. Moore, J.V. Smith, Crystal structure of natural olivines, Amer. Mineral. 53 (1968) 807. [10] H. Eilers, W.M. Dennis, W.M. Yen, S. K uck, K. Peterman, G. Huber, W. Jia, Performance of a Cr:YAG laser, IEEE J. Quantum Electron. 29 (1993) 2508. [11] V. Petricevic, S.K. Gayen, R.R. Alfano, K. Yamagishi, H. Anzai, Y. Yamaguchi, Laser action in chromium-doped forsterite, Appl. Phys. Lett. 52 (1988) 1040. [12] V. Petricevic, S.K. Gayen, R.R. Alfano, Continuous-wave laser operation of chromium-doped forsterite, Opt. Lett. 14 (1989) 612. [13] T.J. Carrig, C.R. Pollock, Tunable, cw operation of a multiwatt forsterite laser, Opt. Lett. 16 (1991) 1662. [14] H.R. Verdun, L.M. Thomas, D.M. Andrauskas, T. McCollum, A. Pinto, Chromium-doped forsterite laser pumped with 1.06 lm radiation, Appl. Phys. Lett. 53 (1998) 2593. [15] Y. Kalisky, R. Moncorge, Y. Guyot, M. Kokta, Laser operation and Q-switching properties of [Cr4‡ ,Mg2‡ ]:YAG crystal, in: Advanced Solid-State Lasers, Technical Digest, Optical Society of America, Washington, DC, 1998, p. 276. [16] H. Lotem, U. Laor, Low-loss bire¯ectant (double-re¯ection) polarization prism, Appl. Opt. 25 (1986) 1271. [17] K.R. Ho€man, U. H ommerich, J.S. Jacobsen, W.M. Yen, On the emission and excitation spectrum of NIR laser center in Cr:YAG, J. Lumin. 52 (1992) 277. [18] S. K uck, K. Petermann, U. Pohlmann, G. Huber, Nearinfrared emission of Cr4‡ -doped garnets: Lifetimes, quantum eciencies, and emission cross sections, Phys. Rev. B 51 (1995) 17323.

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[19] V.P. Mikhailov, N.I. Zhavoronkov, N.V. Kuleshov, A.S. Avtukh, V.G. Shcherbitsky, B.I. Minkov, Saturation of visible absorption in chromium-doped silicates, Opt. Quantum Electron. 27 (1995) 767. [20] Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, M.R. Kokta, Excited-state absorption studies of Cr4‡ ions in several garnet host crystals, IEEE J. Quantum Electron. 34 (1998) 292. [21] W. Jia, H. Liu, Y. Wang, U. H ommerich, H. Eilers, K. Ho€man, W.M. Yen, Stress e€ects on the ¯uorescence spectra of tetravalent chromium in some crystalline hosts, J. Lumin. 59 (1994) 279. [22] N.V. Kuleshov, V.G. Shcherbitsky, V.P. Mikhailov, S. Hartung, T. Danger, S. K uck, K. Petermann, G. Huber, Excited-state absorption and stimulated emission measurements in Cr4‡ :forsterite, J. Lumin. 75 (1997) 319. [23] L.M. Frantz, J.S. Nodvik, Theory of pulse propagation in a laser ampli®er, J. Appl. Phys. 34 (1963) 2346. [24] Y. Kalisky, A. Ben-Amar Baranga, Y. Shimony, Z. Burshtein, S.A. Pollack, M.R. Kokta, Cr4‡ -doped garnets: their properties as nonlinear absorbers, Opt. Mater. 6 (1996) 275. [25] N.V. Kuleshov, V.G. Scherbitsky, V.P. Mikhailov, S. Hartung, S. K uck, K. Petermann, G. Huber, Near infrared and visible excited-state absorption, in: Cr4‡ :Forsterite, C.R. Pollock, W.R. Bosenberg (Eds.), OSA Trends in Optics and Photonics, vol. 10, Advanced Solid State Lasers, Optical Society of America, Washington, DC, 1997, p. 425.