Characterization of cobalt doped ZnSe and ZnS crystals as saturable absorbers for alexandrite lasers

Robert A. Sims a,b, John Kernal ♠a, Vladimir V. Fedorov a, and Sergey B. Mirov ∗a a) Department of Physics, University of Alabama at Birmingham, 310 Campbell Hall, 1300

University Blvd., Birmingham, Alabama 35294, USA b) Samford Univerisity, 800 Lakeshore Drive, Birmingham, AL 35229, USA

ABSTRACT

Cobalt doped ZnSe and ZnS crystals have been studied to determine their effectiveness for passive Q-switching for 700-800nm spectral range (Alexandrite laser). Samples were prepared using Bridgeman technique for single-step growth of Co doped crystals as well as after growth thermal diffusion of Co in undoped crystals. ZnS:Co:Cr crystals, which have been produced using the Bridgeman technique, show maximum initial absorption coefficients of 17 cm-1 at 725nm. Experimental results are reported on effective thermal diffusion of Co2+ in ZnSe and ZnS polycrystals and thermal diffusion constants of cobalt ions in ZnSe and ZnS are estimated. The nonlinear saturation properties of cobalt doped ZnSe and ZnS crystals have been investigated experimentally. The induced transparency measurements were performed using electro-optically Q-switched, alexandrite laser radiation at 731, 741, and 778 nm with a pulse duration of about 70 ns. The induced transmission measurements were analyzed using a four-level absorber model and the absorption cross sections have been estimated at both 731nm and 741nm to be 9.5× 10-18 cm2 and 8.2× 10-18 cm2, respectively. Absorption cross sections calculated from saturation measurements at 4A2→4T1(4P) transition are in agreement with results earlier reported for mid-infrared spectral region 4A2→4T2 of Co2+ ions. The described Co-doped crystals are very promising as passive Q-switches for alexandrite laser resonators. Co2+ centers feature high cross section of saturation and their absorption bands are nicely matched to the spectral emission of the tunable alexandrite laser. An efficient ZnS:Co:Cr passive Q-switching of the alexandrite laser cavity was realized with output energy of 15 mJ and 50 ns pulse duration. Keywords: Saturable Absorber, Passive Q-Switch, Alexandrite laser, ZnS:Co crystals, ZnSe:Co crystals

I. Introduction

Recent studies have demonstrated that Cobalt doped ZnS and ZnSe are effective passive Q switchers of the lasers operating in the near infrared 1300-1500nm region of the electromagnetic spectrum.1-5 The effectiveness of these materials inheres in their high absorption cross sections relative to the emission cross section of Er:glass at 1.54µm and their relatively long relaxation times (~200µs).4 The energy structure of Cobalt doped ZnS shows 4A2 to 4T1 absorption transition centered at 1600 nm and the 4A2 to 4T1(4P) transition corresponding to absorption band covering 700-800nm spectral range. Application of Co doped ZnS and ZnSe crystals as saturable absorbers in the region from 1300-1500nm has been probed multiple times while the 700-800nm range, where these crystals also show promise as saturable absorbers, has not been explored. The objective of this study was to synthesize and characterize Cobalt doped ZnS and ZnSe as saturable absorbing Q-switchers in the 700-800nm spectral range. This paper focuses on preparation as well as the spectroscopic, saturation, and Q-switching properties of ZnSe:Co2+ and ZnS:Co:Cr crystals.

II. Synthesis and characterization of materials

2.1. Synthesis

∗ mirov@uab.edu; phone 1 205 934-8088; fax 1 205 934-8042; http://www.phy.uab.edu/~mirov/

Solid State Lasers XV: Technology and Devices, edited by Hanna J. Hoffman, Ramesh K. Shori, Proc. of SPIE Vol. 6100, 61000U, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.646666

Proc. of SPIE Vol. 6100 61000U-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

Samples of Co doped ZnSe and ZnS crystals were prepared using Bridgeman technique for single-step growth of Co doped crystals as well as after growth thermal diffusion of Co in undoped crystals. ZnS:Co:Cr crystals were purchased commercially from Eagle Picher, Inc, who used the Bridgeman technique to synthesize the crystals. Crystals were doped with 100 ppm of both Co and Cr. ZnSe:Co2+ crystals were synthesized by means of after growth thermal diffusion. Host ZnSe crystals were obtained commercially from II-VI, Inc and Cobalt (II) Selenide from Alpha Aesar. Powder CoSe dopant was 99+% pure with a density of 7.650. Synthesis of ZnSe:Co2+ was achieved by inserting ZnSe crystals into separate quartz ampoules with CoSe powder then evacuating and sealing the ampoule under high-vacuum, 10-5 Torr. Sealed ampoules were then placed in a furnace to be annealed at 950°C for 48-100 hours. Once removed from the furnace and cooled, doped crystals were extracted from ampoules and polished. Remnants of CoSe powder attached to ZnSe crystal was first polished until crystal showed no visible sign of dopant. ZnS:Co:Cr crystals were cut from a bulk sample using a diamond saw and polished to remove surface scratches from blade. 2.2. Characterization Absorption characterization of Cobalt doped of ZnS and ZnSe was performed with Shimadzu UV-VIS-NIR 3101PC spectrophotometer. ZnS:Co:Cr spectrum was taken in the spectral range from 600-2000nm. The non-homogenous characteristics of thermal diffusion cause ZnSe:Co2+ to be examined with a more thorough process. The gradient of concentration of dopant in the host crystalline material was examined. The gradient was determined by initially polishing the crystal to allow transmission. Transmission spectrum was taken over the 500-2500nm spectral range. Crystals were then polished approximately 50-100µm by gradually decreasing the coarseness of sandpaper. This process was completed when the 700-800nm range had approximately 20% transmission. 2.3. Theoretical model Due to non-homogenous distribution of absorbing species a theoretical model must be used to determine the gradient of concentration. To establish these values, the equation

( ) ( ) ( )dxxIxkxdI −= (1) is used, where k is the absorption coefficient. Transmission is generally defined as

])(exp[2

10

dxxIIT

x

x∫−== κ , (2)

where the variable I0 is incident light intensity and I is light intensity after the crystal. The absorption coefficient is defined as

)()( xCx aσκ = (3) in relation to concentration, C(x), and absorption cross section σa. For diffusing species in the medium,6 Fick’s second diffusion equation can be written as

0),(),(2

2

=∂

∂−

∂∂

xtxCD

ttxC

, (4)

where t is the time and D the diffusion coefficient considered independent of the position (x) and the time (t). For the following initial boundary conditions:

Proc. of SPIE Vol. 6100 61000U-2

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

,),0(,0),()0,(

0CtCtCxC

==∞=

(5)

the solution of the above one-dimensional equation in a semi-infinite medium is given as

).2

(),( 0 DtxerfcCtxC = (6)

Combining Eq. (6) with Eq. (2) yields

dxDtxerfcCT

x

x a )2

(exp[ 2

10∫−= σ (7)

The absorbance is defined as

)(TLogA −= (8)

Therefore,

∫= 2

1

)2

()10(

10

x

x a dxDtxerfcC

LnA σ (9)

By representing the absorption cross section σa as a Gaussian function of the wavelength:

]))2ln(/

(4exp[)( 200 wa

λλσλσ

−−= , (10)

where λ0 is the center, w is the full-width at half-maximum (FWHM) and σ0 is the height of the Gaussian. Values of σ0 and C0 do not depend on the thickness x, and, hence, absorbance given in Eq. (9) can be expressed as a function of the wavelength:

∫−

−= 2

1

)2

(*]))2ln(/

(4exp[)10ln(

)( 2000 x

xdx

Dtxerfc

wC

Aλλσ

λ , (11)

where λ0 and w were obtained from the measured absorbance spectra, and σ0 was obtained from literature. Removing a layer of thickness by polishing reduces the amplitude of absorbance for the remaining crystal. This allows a second equation from Eq. (11) to be created by changing the bounds of integration. The x1 value becomes x’1-x1, where x’1 is the amount polished. Two equations and two unknowns now exist and by solving, D and C0 can be calculated.

III. Experimental procedures

3.1. Saturation To determine if a material possess the ability to operate as a saturable absorbing Q-switch, saturation measurements were taken to determine the nonlinear properties of cobalt doped ZnSe and ZnS crystals. The schematic experiental set-up is depicted in Figure 1. The measurements were performed using electro-optically Q-switched, alexandrite radiation at 731, 741, and 778nm wavelengths with approximate pulse duration of 70ns and repetition rate of 13 Hz. A Fresnel

Proc. of SPIE Vol. 6100 61000U-3

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

rotator-polarizing prism combination was used for attenuation of incident photon flux. A Molectron PM3 power detector connected to a Molectron EMP1000 controller was manually moved before the crystal and after the crystal to measure incident power (Pin) and output power (Pout), respectively. A 500mm focusing lens was used to increase photon flux on the crystal. Two experimental methods were used to determine the nonlinear saturation properties of Cobalt doped ZnS and ZnSe crystals. One experiment bypassed the Fresnel rotator and polarizing prism to allow full beam energy density to illuminate the crystal. The crystal was moved along the track to find the position where laser damage begins. In this experiment Pin was measured and the power meter was placed behind the crystal and moved closer to the focal point. The second experiment used this position to identify saturation effects. The Fresnel rotator and polarizing prism were aligned in the system and the Fresnel rotator was turned to allow maximum energy to illuminate the detector. This data was recorded then the power meter was placed behind the crystal and Pout was measured. The Fresnel rotator was then rotated to decrease the power by approximately 5 mJ step. This process was repeated until the Fresnel rotator reached minimum power.

3.2. Pump beam profile characterization Beam quality was determined using a knife edge technique to verify the Gaussian distribution of the alexandrite laser radiation. Experimentally a knife edge is inserted into the crystal holder mounted on a one dimensional optical micrometer. The knife edge was moved perpendicularly to the beam with approximately 100 µm step and power measurements were taken. The reduction in power forms a Gaussian distribution curve, which can be used to find the diameter of the beam. The Gaussian distribution of the beam spatial profile is defined as:

]exp[),( 20

22

0 ryxIyxI +−

= , (12)

where I0 is the initial incident power and r0 is the beam radius. Beam power is found by

∫ ∫∞

∞− ∞==

zdydxyxIzP ),()( , (13)

where z is the diameter of the beam. Power is converted into energy by dividing the repetition rate. The equation for energy is

∫ ∫∞

∞− ∞−

+−=

zdydx

ryxEzE ]exp[)( 2

0

22

0 (14)

Figure 1. Schematic diagram of experimental set-up for saturation measurements

Proc. of SPIE Vol. 6100 61000U-4

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

Solving this equation yields

∫ ∞−

−=

zdx

rxrEzE ]exp[)( 2

0

2

00 π , (15)

where E0 and r0 were found by fitting Eq. (15) to the experimental data. Figure 2 graphs the experimental data and the theoretical data to determine the diameter of the beam. 3.3. Passive Q-Switching of alexandrite laser cavity Schematic diagram of experimental set-up on passive Q-switching of alexandrite laser cavity is shown in Figure 3. To determine if the ZnS:Co:Cr crystal can be used as a saturable absorbing Q-switch the doped crystal was placed in the laser cavity at the Brewster angle to reduce losses. The laser cavity contained a flashlamp pumped alexandrite crystal operating at 13Hz, high reflector, an output coupler with 50% reflectivity, and a Lyot filter for wavelength tuning. Wavelength of alexandrite laser measured with a wavemeter was tuned to 754nm, close to the center of alexandrite laser gain profile.

∆d (mm)

-2 -1 0 1 2

∆P (m

W)

-2

0

2

4

6

8

10

12

14

Figure 2. Experimental (solid dots) characterization and theoretical fit with Eq. 15

(solid line) of the spatial profile of the pump beam.

Lamp

Alexandrite crystal

Filter Liot

RoutR=100%

Power Meter

Oscilloscope

Wavemeter

Photodiode

Co:ZnSLamp

Alexandrite crystal

Filter Liot

RoutR=100%Lamp

Alexandrite crystal

Filter Liot

RoutR=100%

Power Meter

Oscilloscope

Wavemeter

Photodiode

Co:ZnS

Figure 3. Schematic diagram of experimental set-up on passive Q-switching of alexandrite laser cavity

Proc. of SPIE Vol. 6100 61000U-5

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

IV. Experimental results 4.1. ZnS and ZnSe Absorption measurements and measurements of Co diffusion in ZnSe Figure 4 shows the overall absorption spectrum of ZnS crystal co-doped with cobalt and chromium ions and measured over 600-2000nm spectral interval. The absorption band centered around 725 nm is due to Co2+ 4A2→4T1(4P) transition and is a focus of our attention in this study. The absorption band of ZnS:Co:Cr crystals show a maximum absorption coefficient of 17 cm-1 at 725nm. Less intensive absorption band spanning from 1200 till 2200 nm is due to a superposition of Cr 2+ (5T2→5E ) and Co2+ (4A2→4T2) absorption bands. ZnSe:Co2+ absorption spectra measured for samples with gradually decreased thickness after polishing of only one side of the samples is depicted in Figure 5. The spectrum shows a gradual decrease in the absorption coefficient for thinner samples.

Wavelength(nm)

600 800 1000 1200 1400 1600 1800 2000

k (c

m-1

)

0

2

4

6

8

10

12

14

16

18

Figure 4. Absorption spectrum of ZnS:Co:Cr crystal

Wavelength (nm)

500 1000 1500 2000 2500

Abs

orpt

ion

Coe

ffice

nt k

(cm

-1)

0

5

10

15

20

25

A

B

C

D

Figure 5. Average absorption coefficient of ZnSe:Co2+ crystals of different thicknesses versus wavelength obtained after

subsequent polishing of one side of the sample: A - thickness 0.425 cm; B - thickness 0.398cm; C - thickness 0.377cm; D - thickness 0.365cm.

Proc. of SPIE Vol. 6100 61000U-6

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

Figure 6 (solid dots) shows the experimental depth profile of cobalt distribution in ZnSe crystal after thermal annealing of ZnSe at 950 C for 100 hours. Experimental points were obtained from Figure 5 under the assumption that the average coefficient of absorption of Co2+ over the crystal depth polished between two measurements (e.g. Figure 5 A &B) equals to the difference of coefficients of absorptions measured for initial and polished off samples. Figure 6 (solid curve) represents theoretical fit of experimental depth profile with Eq. 6. The best fit was obtained for D= 6×10-9 cm2/s and initial and final cobalt concentrations of 4.5×1019 and 1.12×1019 ions/cm3, respectively. 4.2 Saturation of ZnS:Co:Cr and ZnSe:Co Saturation measurements for ZnS:Co:Cr and ZnSe:Co were performed for different wavelengths (720-800 nm) of the pump laser using experimental set-up depicted in Figure 1. Crystal transmission as a function of a pump power density was determined using the equation

in

out

PP

T = (16)

Figure 7 shows the saturation measurements for ZnSe:Co2+ crystal. As one can see Figure 7 shows no evidence of saturation in ZnSe:Co2+ even for high, close for optical damage levels of pump power density for all the wavelengths over 720-800nm spectral range. We hypothesize that it might be due to a strong excited state absorption of ZnSe:Co2+ crystals.

x (cm)0.0 0.1 0.2 0.3 0.4 0.5

Con

cent

ratio

n (io

ns/c

m3 )

0.0

2.0e+18

4.0e+18

6.0e+18

8.0e+18

1.0e+19

1.2e+19

1.4e+19

1.6e+19

1.8e+19

Figure 6. Experimental (solid dots) depth profile and theoretical fit (Eq. 6, solid line) of Co

diffusion into ZnSe crystal

Photon Flux (mJ/cm2)0 200 400 600 800

Tran

smis

sion

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Figure 7. Transmission of ZnSe:Co versus alexandrite laser (741 nm) pump energy density

Proc. of SPIE Vol. 6100 61000U-7

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

To contrast these measurements Figures 8 and 9 (solid dots) show experimentally obtained effective saturation of absorption in ZnS:Co:Cr under incident alexandrite laser radiation with output wavelengths of 731 and 741nm, respectively. Absoption cross section of ZnS:Co:Cr in the spectral range 720-800 nm was determined by fitting experimental plots with the Frantz-Nodvik dependence:

}*]1[exp1ln{ 0TEE

T szz

s −+= , (17)

where E is the energy density incident on the absorber, T the transmission of the absorber for the given energy power E, T0 the incident transmission (for the low power of light), and Es the saturation energy:

σνhEs = , (18)

where σ is the actual laser beam cross section.

The best fit of experimental saturation curves with the Eq. 17 was obtained for cross sections of absorption of 9.5× 10-18 cm2 and 8.2× 10-18 cm2 for wavelengths 731 and 741 nm, respectively. These values are in a good agreement with the previous data for absorption cross sections in the 1300-1500nm spectral range.

4.3 Preliminary results on passive Q-switching of alexandrite laser with ZnS:Co:Cr

Q-switching experiments of alexandrite laser cavity with ZnS:Co:Cr crystal were performed as described in Figure 3. Figures 10 a) demonstrates temporal profile of the output radiation of alexandrite laser operating in free running regime without passive Q-switch in the cavity. As one can see the temporal profile of the output radiation features spike oscillations with overall FWHM of the pulse envelope of ~ 80µs. Figure 10 b) shows the temporal profile of individual spike with a pulse duration of ~300-400 ns. As shown in Figure 11 a) placing the ZnS:Co:Cr crystal in the alexandrite laser cavity is accompanied by formation of a giant pulse with pulse duration of ~ 50ns (see Figure 11 b)) and output energy of ~ 15 mJ. This constitutes about 5% of efficiency in Q-switch regime with respect to free running oscillation. This can be explained by a poor quality of the crystal surfaces (home polishing). Comparison of the output energies in

0

2

4

6

8

0

800 600 400 200 0

0.2

0.4

0.6

0.8

T

Energy density, mJ/cm2

Figure 9. Experimental (solid dots) saturation curve of ZnS:Co:Cr at 741 nm normalized to the crystal transmission at 800 nm and theoretical fit with Frantz-Nodvik equation (solid line).

0 100 200 300 400 500 600 700 800

Tran

smis

sion

0.0

0.2

0.4

0.6

0.8

1.0

Energy density, mJ/cm2 Figure 8. Experimental (solid dots) saturation curve of ZnS:Co:Cr at 731 nm normalized to the crystal transmission at 800 nm and theoretical fit with Frantz-Nodvik equation (solid line).

Proc. of SPIE Vol. 6100 61000U-8

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

Q-switch regime free running oscillation when losses of the same value as passive losses of ZnS:Co:Cr crystal were incorporated in the resonator provides about 80% of efficiency. This indicates that real Q-switch efficiency could be greatly increased with the use of a higher quality ZnS:Co crystal.

V. Conclusion

Co concentration depth profile and its coefficient of thermal diffusion in ZnSe were explored. . It has been demonstrated that cobalt doped ZnSe does not show saturation in the 700-800nm spectral range. On the contrary ZnS:Co shows effective saturation in the 700-800 nm spectral range. The measured absorption cross section of Co2+ in ZnS was ~ 10-17 cm2, which is about 3 orders of magnitude higher than cross section of emission of alexandrite crystal. It is a good indication of possibility of effective Q-switching of alexandrite laser with the ZnS;Co crystals. For the first time to our knowledge passive Q-switching of alexandrite laser cavity with ZnS:Co crystal was realized with output energy of 15 mJ and pulse duration of 50 ns.

time, us0.0 0.5 1.0 1.5 2.0 2.5

0.00

0.01

0.02

0.03

0.04Free Runing

time, us0 20 40 60 80 100

V

0.00

0.01

0.02

0.03

0.04

time, us0.0 0.5 1.0 1.5 2.0 2.5

0.00

0.01

0.02

0.03

0.04Free Runing

time, us0 20 40 60 80 100

V

0.00

0.01

0.02

0.03

0.04

Figure 10. Temporal profile of the output radiation (a) and individual spikes (b) of alexandrite laser operating in free running regime without passive Q-switch in the cavity

tim e, ns0 50 100 150 200 250 300 350 400

0

1

2

Q-Swich

time, us

0 20 40 60 80 100

V

0

1

2

3

tim e, ns0 50 100 150 200 250 300 350 400

0

1

2

Q-Swich

time, us

0 20 40 60 80 100

V

0

1

2

3

Figure 11. Temporal profiles of the giant ZnS:Co Q-switched pulse in microsecond (a) and nanosecond (b) time scales

Proc. of SPIE Vol. 6100 61000U-9

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms

Acknowledgements

We acknowledge support from the Nation Science Foundation (NSF)-Research Experience (REU)-site award to the University of Alabama at Birmingham (UAB) under Grant No. DMR-0243640. This work was also partially supported by NSF grants ECS-0424310 and EPS-0447675. ♠Tragically before this paper was complete on 10 June 2005, John Kernal died of a car accident.

REFERENCES

1. Tzong-Yow Tsai, Milton Birnbaum, “Co2+:ZnS and Co2+:ZnSe saturable absorber Q switches”, J. Applied Physics 87, 25-29 (2000).

2. Tzong-Yow Tsai, Milton Birnbaum, “Characteristics of Co2+:ZnS saturable absorber Q-switched neodymium laser at 1.3µm”, J. Applied Physics 89, 2006-2012(2001).

3. Z. Mierczyk, A, Majchrowski, I.V. Kityk, W. Gruhn,”ZnSe:Co2+-nonlinear optical absorber for giant-pulse eye-safe lasers”, Optics & Technology 35, 169-172(2002).

4. V.N. Philippov, A. V. Kir’yanov, S. Unger “Advanced configuration of Erbium Fiber Passively Q-Switched Laser With Co2+:ZnSe Crystal as Saturable Absorber” IEEE Photonics Technology Letters 16, 57-59(2004).

5. L. D. DeLoach, R.H. Page, G.D. Wilke, S.A. Payne, and W.F. Krupke, IEEE J. Quantum Electroncs.32, 885 (1996).

6. J.-O Ndap, K Chattopadhyay, O.O.Adetunji, D.E.Zelmon, A. Burger “Thermal diffusion of Cr2+ in bulk ZnSe”, J Crystal Growth 240, 176-184(2002).

Proc. of SPIE Vol. 6100 61000U-10

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms