Laser performance of Cr doped ZnS - UAB

Laser performance of Cr2+ doped ZnS Keith Grahama, Sergey Mirov∗a, Vladimir Fedorova, Mary Ellen Zvanuta, Andranik Avanesovb, Valeri Badikov∗∗b, Boris Ignat'evb, Vladimir Panutinb, Galina Shevirdyaevab a Dept. of Physics, The Univ. of Alabama at Birmingham; bKuban State Univ. ABSTRACT Laser properties and spectroscopic characterization of diffusion doped Cr2+: ZnS crystals synthesized by chemical transport reaction from gas phase are reported. Lasing was realized with a threshold of 170 µJ and slope efficiency of 9.5 % with respect to the 1.5607 µm pump energy, in a hemispherical cavity. Low doped samples (3-4 cm-1 at 1.7 µm) of 1.7 mm thickness were utilized. The 1.5607 µm excitation was realized with a D2 Raman cell pumped in a backscattering geometry by the 1.064 µm radiation of the single frequency Nd:YAG laser. Maximum output energy reached 100 µJ. Lasing in the hemispherical cavity was achieved with output couplers R2.36µm=80 % and 90 % and radius of curvature 20 cm. Absorption cross section was estimated from spectroscopic measurements and was in a good agreement with saturation data (σabs.= 0.80x10-18 cm2) calculated with the modified Frantz-Nodvik equation for a four level slow absorber. Findlay Clay losses were found to be about 14%. Selective cavity experiments were performed in a hemispherical cavity with a CaF2 prism as the dispersive element. A tuning range of 2.05-2.40 µm was realized, limited by the spectral range of the output coupler of the selective laser cavity. Keywords: Tunable lasers, Cr2+: ZnS crystals, saturable absorber, II-IV chalcogenide crystals.

1. INTRODUCTION There is a growing demand for compact, room temperature and broadly tunable mid-infrared lasers for use in a variety of wavelength specific scientific, industrial and military applications. Lawrence Livermore National Laboratory has recently performed detailed spectroscopic studies of several II-IV chalcogenide hosts with different TM ions as potential mid-IR laser materials1, 2. In particular, chromium doped ZnS and ZnSe were found to be the most attractive between 2 and 3 µm because of a low level of non-radiative decay at 300K and weak excited state absorption. Laser demonstrations were successfully performed with Cr2+ doped ZnSe, CdSe, and Cd0.85Mn0.15Te crystals1-9. One of the main problems that were found with this kind of materials was their relatively high level of passive losses in the region of laser emission. Therefore, both detailed spectroscopic studies and synthesis of chalcogenide materials by different crystal growing techniques are necessary for their optimization. In this paper we investigate the spectroscopic properties and the laser performance of Cr2+: ZnS crystals synthesized by chemical transport reaction from gas phase and activated by diffusion doping-additive coloration method. The advantage of this approach is a high quality of the host crystals and ability to control the doping level by adjustment of the diffusion temperature and time. In addition to this, color centers formed in the crystal at the stage of additive coloration may further interact with chromium optical centers and change or enhance their properties10.

2. CRYSTAL PREPARATION The Cr2+: ZnS crystals were prepared by a two-stage method. At the first stage, undoped single crystals were synthesized by a chemical transport reaction from gas phase using iodine gas transport scheme in a quartz tube of 20mm diameter and 200mm length placed in a two heating zone furnace. Powder obtained by a joint ignition of initial components served as raw material. Temperatures in the zones of a raw material and crystallization were 1200oC and 1100oC respectively. I2 concentration was 2-5 mg/cm3. High optical quality unoriented ingots of hexagonal ZnS, which were typically ∅2x1cm3, ∗

contact [email protected]; phone (205) 934-8088; fax (205) 934-8042; http://lorentz.phy.uab.edu/~mirov/; Dept. of Physics, The Univ. of Alabama at Birmingham, 1300 University Blvd., Birmingham, AL 35244-1170, USA ∗∗ [email protected]; Kuban State Univ., 149 Stavropolskaya St., Krasnodar, 350040, Russia

Solid State Lasers X, Richard Scheps, Editor, Proceedings of SPIE Vol. 4267 (2001) © 2001 SPIE · 0277-786X/01/$15.00

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were cut and ground to slabs of 5x5x2 mm size. At the second stage introduction of chromium into the crystalline host was performed by thermal diffusion carried out in sealed ampoules under a pressure of 10-5torr and temperature of 1100oC over 10-20 days. The polished samples of 1-2 mm thickness and up to 5 mm in aperture were used for spectroscopic and laser measurements. The crystal used for saturation measurements had enhanced chromium concentration. Chromium films were deposited on the ZnS slabs by pulsed laser deposition method and further introduction of chromium performed by thermal diffusion in a sealed quartz ampoule at 10-5torr and temperature of 1000oC for 10 days. This increased the chromium concentration by approximately a factor of four. The crystal used in saturation experiments was then polished to a thickness of 2.5 mm.

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Absorption, luminescence and kinetics of fluorescence Absorption, emission and kinetics of fluorescence measurements were performed in a close-cycle cryostat in a temperature range 14-325K. For excitation either tungsten-halogen lamp or 1.5607 µm pulsed radiation from a D2-Raman-shifted Nd:YAG laser was utilized. Light detection was performed by either a 0.75m or 0.3m Acton Research “SpectraPro” spectrometers (purged with N2) in combination with thermo-electrically (TE) cooled PbS or liquid-nitrogen cooled InSb fast (0.7µs) detector. Data acquisitions for absorption and fluorescence experiments were performed with a lock-in–amplifier or boxcar averager – SpectraSense (Acton Research) controller combination, respectively. All spectra have been corrected for sensitivity and response with an Oriel calibration lamp. The absorption and emission spectra of Cr2+: ZnS crystals are plotted in Fig.1a. Fig.1a depicts room temperature spectra measured in cross section units of 10-18 cm2. Fig.1b demonstrates spectra measured at temperature 14K and plotted in arbitrary units. The room-temperature emission cross section σem and absorption cross section were evaluated according to the Einstein relations λ2 g (ν ) (1) σ em = 8πcn 2τ rad g1

∫ σ em dν = g ∫ σ abs dν ,

(2)

2

where λ is the emission wavelength, g(ν) –the normalized lineshape function (photon/sec), c -speed of light, n- reflective index, and τrad−the emission lifetime, g1, g2.- the ground and exited state degeneracies. The radiative lifetime was estimated as low temperature limit of fluorescence time dependence (see Fig.2 and 3).

emission

0.8 abs. 0.4

0.0 1000

b

T=300K

1.2

1500

2000 Wavelength, nm

2500

Absorption/Luminescence

-18 2 Cross section (10 cm )

a

T=14 K

abs. emission

1500

1900

2300

2700

Wavelength, nm

2+

Fig. 1 Cr : ZnS absorption and emission spectra: a) measured at 300K and plotted in cross section units, b) measured at 14K and plotted in arbitrary units.

Polarized and unpolarized spectra were practically identical and hence, the crystals were treated as optically isotropic. The room temperature absorption and emission spectra revealed strong absorption and emission bands at 1.4-2.0 µm and 1.6-2.8

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µm, respectively, featuring a strong overlapping in the 1.7-2.0 µm region. Analysis of Cr2+: ZnS oscillation dynamics, for different pump excitation wavelengths, was performed analogously to reference11. Due to a strong overlapping of absorption and emission bands and stimulated emission at the pump wavelength a relatively saturated concentration of Cr optical centers around 1.9 µm will constitute only 16 % of a total Cr population density in the crystal. This analysis reveals an advantage of a shorter wavelength excitation (relative saturated concentration is 87 % at 1.5607 µm) for reducing the effect of stimulated emission at the wavelength of excitation. Low temperature spectra reveal doublet of zero-phonon lines in absorption and emission attributed to “hexagonal” lines12. Emission lifetime measurements of diffusion doped Cr2+: ZnS crystals (shown in Fig. 2 and 3) are consistent with the findings of earlier researchers12, where a Cr lifetime of about 7-8 µs is reported to be nearly constant (7.1-5.4µs) with temperatures up to 300K. Our lifetime results contradict the behavior of Bridgman grown Cr2+: ZnS crystals1 featuring unusual lifetime-temperature behavior peaking around 300K. The room temperature quantum yield of fluorescence was estimated using the following relationship: τ τ η = em = 300 K (3) τ rad τ 14 K e0

signal

e-1 e-2 e-3

c

a

e-4

0

5

10

15

time, µs Fig. 2. Kinetics of fluorescence of Cr2+: ZnS crystal measured at RT (a) and 14K (b). For comparison temporal response of InSb detector is also presented (c). 7

6

Decay Time (msec)

5

4

3

2

1

0 0

50

100

150

200

250

300

350

Temperature (K)

Fig. 3. Temperature dependence of emission lifetime in Cr2+: ZnS measured at 2000 nm under 4.5 µs 1560 nm pulsed excitation and 0.7 µs registration response.


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3.2 Saturation One of the important potential applications of Cr2+: ZnS crystals, is the passive Q-switching of Er: glass lasers. In this study experiments on saturation of Cr2+: ZnS absorption were performed under 1.56 µm excitation. The radiation from a D2Raman-shifted YAG:Nd laser has pulse duration of 5ns duration, pulse energy of up to 20 mJ and repetition rate of 10Hz. For saturation experiments the 2.5mm thick Cr2+: ZnS crystal with initial transmission of T=0.43 at 1.56 µm was utilized. The pump radiation was focused on the sample by the 26.5 cm lens and the dependence of the crystal transmission as a function of pumping energy density was measured by means of the sample Z-scanning. Spatial energy distributions of the pump radiation were determined by a standard knife-edge method. The effective radii of the pumping beam were measured at the 0.5 level of maximum pump intensity of radiation. Fig 4 shows the dependence of the transmittance, normalized to Fresnel losses, versus pump energy density. 1.0

Transm ittance

0.9

0.8

0.7

0.6

1

10

Energy density (10

18

photon/cm -2 )

Fig.4. Saturation of ground state absorption in Cr2+: ZnS crystal. Solid curve is a result of calculation with Eq. (4).

As one can see, the active absorption changes more than 1.4 times under increasing of pump energy fluence from W=0.8x1018 to 6.7x1018 photon/cm2. Since the pump pulse duration (5 ns) is much shorter than the relaxation time of Cr2+: ZnS saturable absorber (4.5 µs) the saturation behavior was analyzed in terms of energy fluence with a modified FrantzNodvik equation for a four level slow absorber. According to this equation the crystal transmission depends on pump energy fluence, “W”, and absorption cross section as follows:

(

(

))

1 T = ln 1 + T0 e z − 1 , z

(4)

where z = Wσ ab , T0- initial crystal transition at W=0, and σab-absorption cross section (cm2). Equation (4) was solved numerically, and from the best fit to the experimental results (Fig.4, solid line), the value of σab(λ=1.56 µm) was estimated to be 0.5x1018 cm2. Taking into account the ratio of absorption at 1.56 µm and in the maximum of absorption band (λ=1.7 µm, see Figure 1) the peak absorption cross section was determined to be 0.8x1018 cm2, which is in a very good agreement with the value of cross section estimated in the current study from spectroscopic measurements. The estimated Cr2+ concentration in the crystal was 3.5x1018 cm-3. This satisfactory agreement of σab values determined from spectroscopic and saturation measurements indicates on negligible excited state absorption losses for Cr2+: ZnS at 1.56 µm and the wavelength of Er: glass laser oscillation (1.54 µm). Hence, Cr2+: ZnS crystals feature a relatively high cross section of absorption 0.5x1018 cm2 at 1.56 µm compared with 7x10–21 cm2 for Er: glass. This value is practically two times larger than 0.27x1018 cm2 cross section value for Cr2+: ZnSe measured in reference 13 and in conjunction with negligible excited state absorption losses reveal possible application of Cr2+: ZnS crystals as a promising saturable absorber for resonators of Er: glass lasers.

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All spectroscopic parameters of ZnS:Cr2+ gain medium calculated in the current study are summarized in Table 1 and compared with parameters presented in the mentioned above papers1, 12. Table 1. Parameters of the Cr2+: ZnS gain medium τrad (µs)

τem µs, 300K)

η=τem/τrad (300K)

∆νem, FWHM, cm-1 300K

σemmax 10-18 cm2, 300K

∆νabs, FWHM cm-1, 300K

∆νabs, FWHM cm-1, 14K

σabsmax , 10-18cm2 300 K

References

6.5 11 7.1-8.7

4.5 8 5.4-4.7

0.69 0.73 0.6-0.7

1043 1624

1.3 0.75

1220 1245

860

0.80 0.52

Current [1] [12]

3.3 Laser experiments 3.3.1 Non-selective laser cavity Laser experiments were performed using the 1.5607 µm output from a D2 Raman cell pumped in the backscattering geometry by the 1.064 µm radiation of the single frequency Nd:YAG laser. A schematic of the optical system and the laser cavity is depicted in Fig.5. An optical diode was placed before the Raman cell to prevent possible damage of Nd:YAG laser optics by amplified backscattered 1.06 µm radiation. Pump pulses from the Raman cell had pulse duration of 5 ns at FWHM; output energy reached 100 mJ and was continuously attenuated by a combination of a half-wave plate and a Glan prism. Amplitude stability of the pump pulses was about 5 %. The hemispherical cavity consisted of the input mirror deposited on the facet of the ZnS crystal and output mirror with 20 cm radius of curvature. Output mirrors had either 10-20 % transmission in the spectral region 2.05-2.5 µm, or 20-30 % transmission in the spectral region 1.95-2.5 µm. Both mirrors had their peak reflectivity at 2.360 µm. Length of the cavity was 18.5 cm. Pump radiation was focused on the crystal with a 26.5 cm lens placed 22.5 cm before the crystal providing a good match of the pump caustics and the cavity mode size (200 µm). Low doped samples (3-4 cm-1 at 1.7 µm) of 1.7 mm thickness were utilized. The second facet of the crystal was anti-reflection (AR) coated in the lasing region and was fully reflective at the wavelength of pumping, providing a double pass pumping scheme. A Ge filter was used to separate residual pump light from the Cr2+: ZnS laser beam. 1064 nm Nd:YAG Laser

D 2 – Raman Cell

Optical Diode

1560.7

nm

Variable Attenuator Power Meter

BS

HR

PbS

Sample AR Monochromator Output Coupler Ge Filter Power Meter

Filters

2000 - 2700 nm

Fig.5. Schematic of the optical system and nonselective laser cavity used for Cr2+: ZnS laser experiments.


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Fig. 6 shows laser output energy versus input pump energy. Room temperature laser operation was realized with a threshold of 170 µJ and slope efficiency of 9.5 % with respect to the pump energy. Laser had an output linewidth of approximately 90 nm (FWHM), centered at 2.24 µm and maximum output energy reached 100 µJ. Further increase of the pump energy resulted in optical damage of the input mirror. 0.12

Output energy (mJ)

0.10

0.08

0.06

0.04

0.02

0.00 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pump energy (mJ) Fig.6. A Cr2+:ZnS laser slope efficiency plot of the output energy versus the pump energy when 15% output coupler is used. The measured slope efficiency is 9.5%

The laser performance, of the diffusion doped Cr2+: ZnS crystals, is expected to be improved by optimization of crystal quality, doping technology and optimization of the output coupler. With the R2.360 µm=80 % mirror laser operation was obtained with a threshold of 250 µJ. This allowed a Findlay-Clay calculation of the losses within the cavity14. With the crystal length of 1.7 mm and σabs= 0.8x10-18 cm2 the losses in the cavity were calculated to be 14.7%. It is felt that this can also be improved by the optimization of the crystal preparation technique. 3.3.2 Selective laser cavity In the wavelength tuning experiment, a hemispherical cavity of the length 19.7 cm was utilized. Wavelength tuning was realized using a CaF2 Brewster prism as the dispersive element placed 5 cm from the output coupler. The focusing lens and crystal remained at their positions that were used in the nonselective cavity. The output coupler was the 20 cm, R2.360 µm= 90 % mirror that was used in the nonselective cavity. This arrangement provided a nice match of the cavity waist and pump beam spot (~200 µm) in the crystal. The pump source was operating at 1.5607 µm with the pulse energy of about 600 µJ and 5 ns pulse duration in a TEM00 mode. This pump energy was about three times larger than the threshold pump energy level. The Cr2+: ZnS laser output was directed through a CaF2 lens to a 0.3 m “SpectraPro” monochromator with a PbS detector for wavelength measurements. A continuous wavelength tuning over the 2.05-2.40 µm spectral region was realized. The output of the chromium laser oscillation (see Fig. 7) had a linewidth of approximately 30 nm (FWHM). The peak efficiency of the tunable output was centered at 2.25 µm. The tuning limits were due to coatings of the cavity optics and not by the emission spectrum of Cr2+:ZnS crystal. The use of proper broadband coatings could potentially increase the tuning range to 1.85-2.7 µm. The laser output linewidth could be further narrowed by means of Littrow, or Littman configured grating tuned cavity.

86


Laser efficiency, arb.un.

1.0 0.8 0.6 0.4 0.2 0.0 2100

2200

2300

2400

W avelength, nm 2+

Fig. 7. The Cr : ZnS laser tuning curve with CaF2 prism selector.

4.CONCLUSIONS Laser properties and spectroscopic characterization of diffusion doped Cr2+: ZnS crystals synthesized by chemical transport reaction from gas phase are reported. Lasing was realized with a threshold of 170 µJ and slope efficiency of 9.5 % with respect to the 1.5607 µm pump energy, in a hemispherical cavity. Low doped samples (3-4 cm-1 at 1.7 µm) of 1.7 mm thickness were utilized. The 1.5607 µm excitation was realized with a D2 Raman cell pumped in a backscattering geometry by the 1.064 µm radiation of the single frequency Nd:YAG laser. Maximum output energy reached 100 µJ. Lasing in the hemispherical cavity was achieved with output couplers R2.36µm=80 % and 90 % and radius of curvature 20 cm. Absorption cross section was estimated from spectroscopic measurements and was in a good agreement with saturation data (σabs.= 0.80x10-18 cm2) calculated with the modified Frantz-Nodvik equation for a four level slow absorber. Findlay Clay losses were found to be about 14%. Selective cavity experiments were performed in a hemispherical cavity with a CaF2 prism as the dispersive element. A tuning range of 2.05-2.40 µm was realized, limited by the spectral range of the output coupler of the selective laser cavity. In summary, we performed a detailed spectroscopic characterization of chemical transport reaction from gas phase grown and diffusion doped Cr2+:ZnS crystals and demonstrated their saturation properties and laser performance in nonselective and dispersive cavities under 1.560 µm excitation. Cr2+:ZnS crystals exhibiting high saturation and emission cross-sections (0.5x10-18 at 1.54 and 1.3x10-18 cm2 at 2.1 µm, respectively), low level (~70%) of non-radiative decay at 300K, weak excited state absorption, and the best (among II-IV chalcogenides) thermo-optical properties can be considered as promising candidates for passive Q-switching of the cavities of Er-glass lasers as well as for compact high power mid-IR sources pumped by InGaAs diodes. Other potential utilizations include lidar remote sensing, high resolution and ultra short time resolved spectroscopy and wavelength specific military applications. Future study will be focused on optimization of crystal preparation techniques, laser power scaling, and further understanding of physics of doped II-IV wideband semiconductor materials for realization of room temperature mid-IR lasing with direct diode or electrical excitation.


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REFERENCES 1.

2.

3. 4.

5.

6.

7.

8.

9. 10. 11. 12. 13.

14.

88

L. D. DeLoach, R.H. Page, G.D. Wilke, S. A. Payne, and W. F. Krupke, "Transition Metal-Doped Zinc Chalcogenides: Spectroscopy and Laser Demonstration of a New Class of Gain Media", IEEE J. Quantum Electron, 32, pp.885-895, 1996. R. H. Page, K. I. Schaffers, L. D. DeLoach, G. D. Wilke, F. D. Patel, J. B. Tassano, S. A. Payne, W. F. Krupke, K. T. Chen, A. Burger, "Cr2+-Doped Zinc Chalcogenides as Efficient, Widely Tunable Mid-Infrared Lasers", IEEE J. Quantum Electron, 33/4, pp. 609-617, 1997. G. J. Wagner, T. J. Carrig, R. H. Page, K. I. Schaffers, J. O. Ndap, X. Ma, A. Burger, "Continuous-wave broadly tunable Cr2+: ZnSe laser", Opt. Lett., 24, pp. 19-21, 1999. A. V. Podlipensky, V. G. Shcherbitsky, N. V. Kuleshov, V. I. Levchenko, V. N. Yakimovich, A. Diening, M. Mond, S. Kuck, G Huber, '1W continuous-wave laser generation and excited state absorption measurements in Cr2+: ZnSe", OSA Trends In Optics and Photonics, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds, Vol. 34, pp. 201-206, Optical Society of America, Washington, DC, 2000. I. T. Sorokina, E. Sorokin, A. D. Lieto, M. Tonelli, R. H. Page, K. I. Schaffers, "0.5W efficient broadly tunable continuous-wave Cr2+: ZnSe laser", OSA Trends In Optics and Photonics, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds, Vol. 34, pp. 188-193, Optical Society of America, Washington, DC, 2000. A. Sennaroglu, A. O. Konea, C. R. Pollock, "Power performance of a continuous-wave Cr2+: ZnSe laser at 2.47µm", OSA Trends In Optics and Photonics, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds, Vol. 34, pp. 240-245, Optical Society of America, Washington, DC, 2000. T. J. Carrig, G. J. Wagner, A. Sennaroglu, J. Y. Jeong, C. R. Pollock, "Acousto-optic mode-locking of a Cr2+: ZnSe laser", OSA Trends In Optics and Photonics, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds, Vol. 34, pp. 182-187, Optical Society of America, Washington, DC, 2000. J. McKay, D. Kraus, K. L. Schepler, "Optimization of Cr2+: CdSe for Efficient laser Operation", OSA Trends In Optics and Photonics, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds, Vol. 34, pp. 219-224, Optical Society of America, Washington, DC, 2000. U. Hommerich, X. Wu, V. R. Davis, S. B. Trivedi, K. Grasza, R. J. Chen, S. Kutcher, " Demonstration of roomtemperature laser action at 2.5 µm from Cr2+: Cd0.85Mn0.15Te", Opt. Lett., 22, pp. 1180-1182, 1977. K. Lee, W. A. Sibley, "Exchange Enhancement of Co2+ and Mn2+ Transitions due to Radiation Defects", Phys. Rev. B, 12, p.3392, 1975. T. T. Basiev, P. G. Zverev, A. G. Papashvili, V. V. Fedorov, "Temporal and spectral characteristics of a tunable LiF:F2** color center laser", Russian J. of Quantum Electronics, 24, pp. 591-595, 1997. G. Grebe and H. J. Schulz, "Luminescence of Cr2+ centers and related optical interactions involving crystal field levels of chromium ions in zinc sulfide", Z. Naturforsch, 29a, pp. 1803-1818, 1974. A.V.Podlipensky, V.G.Shcherbitsky, N.V.Kuleshov, V.P.Mikhailov, V.I.Levchenko, and V.N.Yakimovich, “Cr2+: ZnSe and Co2+: ZnSe saturable-absorber Q switches for 1.54-µm Er:glass lasers,” Opt. Lett., 24/14, pp.960-962, 1999. D. Findlay and R. A. Clay, “The Measurement of Internal Losses in 4-Level Lasers”, Phys. Lett., 20/3, pp.277-278, 1966.
