Mid-IR Laser oscillation via energy transfer in the Co:Fe:ZnS/Se codoped

Transcription

1 Mid-IR Laser oscillation via energy transfer in the Co:Fe:ZnS/Se codoped crystals Jeremy Peppers, NoSoung Myoung, Vladimir V. Fedorov, Sergey. Mirov Center for Optical Sensors and Spectroscopies and the Department of Physics, University of labama at irmingham, CH 30, 300 University lvd., irmingham, L 329, US STRCT Room temperature iron doped II-VI lasers have demonstrated broad band tunability between 3. and 6 μm with efficiency ~0%. However, these lasers require pump sources with a wavelength ~3 μm which could be selected only from a few available. Cobalt ions in the II-VI materials have strong absorption bands at 2 T ( P) and 2 T ( F) transitions located at ~0.7 and. μm, respectively. number of different laser sources (including diode lasers) could be potentially used for cobalt excitation followed by energy transfer to iron ions. Here we report materials fabrication and study of energy transfer in Co:Fe:ZnS(ZnSe) crystals. Iron-cobalt co-doped samples were prepared using a two-stage post-growth thermal diffusion procedure with Fe concentrations of 8.-9x0 8 cm -3. Kinetics and photoluminescence spectra reveal energy transfer under cobalt excitation at 2 - T ( F) transition by.6 μm radiation and 2 - T ( P) transition by 0.7 μm radiation. nalysis shows effective energy transfer from T ( F), T 2 and T ( F) Co 2+ energy levels to T 2 excited level of Fe 2+ ions the first realization of Fe 2+ ions lasing at 3.6 μm and 3.9µm via Co-Fe energy transfer. Demonstrated effective Co 2+ Fe 2+ energy transfer process could result in utilization of a more convenient laser pump sources for the Fe 2+ :II-VI lasers. Keywords: Mid-IR lasers, Co-Fe Energy transfer, Fe:ZnS and Fe:ZnSe crystals, Transitional metal doped solid state lasers. INTRODUCTION Improvements in technology for gain media based on transition metal (TM) doped II-VI crystals have been driven by the growing demand for convenient room temperature middle infrared (mid-ir) lasers operating in the 2-6µm spectral range for application such as remote sensing, optical communication, and medical diagnostics. Room temperature gainswitched lasing has been employed to obtain pulse energies up in iron doped ZnSe crystals []. Fe 2+: II-VI lasers operating over the µm spectral range have already been subjected to considerable study [[2],[3],[][][6][7]]. One major barrier to the application of these lasers has been the lack of convenient pump sources. The peak absorption of Fe 2+ in II-VI crystals lies in the 3-µm range. The most common pump sources in this range are flashlamp pumped Er lasers operating near 3µm with repetition rate limited to only several tens of Hz. Energy transfer has previously been demonstrated for Cr-Fe [2], Co-Fe [8], and Ni-Fe [9] co-doped samples. Recently, first iron lasing under cobalt excitation at.6 µm was reported in [0]. Herein we report spectroscopic characterization and laser oscillation via Co-Fe energy transfer in Co:Fe:ZnSe and Co:Fe:ZnS crystals under cobalt excitation at 2 (F) T (P) (~ 760 nm) and 2 (F) T (F) (~60 nm) transitions. Figure contains a representation of the energy levels of Co 2+ and Fe 2+ in tetrahedral crystal fields such as those in ZnSe and ZnS. Cobalt ions in these II-VI crystals have two absorption bands of interest located at.6µm and another near 0.7 µm but differing slightly between ZnSe and ZnS. These bands correspond to Co 2+ transitions of 2 (F) T (F) and 2 (F) T (P), respectively. There are three likely avenues of energy transfer to the T 2 (D) level in Fe 2+ and enabling mid-ir lasing in the 3-6 µm spectral range. These excitations can be followed by cross-relaxation processes potentially providing quantum efficiency of Fe 2+ excitation in excess of 00%. Solid State Lasers XXI: Technology and Devices, edited by W. ndrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 823, SPIE CCC code: X/2/$8 doi: 0.7/ Proc. of SPIE Vol

2 2. SMPLE PREPRTION ND EXPERIMENTL METHOD Polycrystalline co-doped Co:Fe:ZnSe and Co:Fe:ZnS with dimensions of 0x0x.2 mm 3 were produced by controlled thermal diffusion of Co and Fe into bulk crystals. In our experiments we used Co:Fe:ZnS and Co:Fe:ZnSe with cobalt concentrations of 8x0 8 cm -3 and 6.x0 8 cm -3, respectively. Iron concentrations were correspondingly in the ranges of 0.-0x0 8 cm -3 and 8.-9x0 8 cm -3. Room temperature absorption measurements were performed with the use of Shimadzu UV-VIS-NIR-30PC and FTIR spectrophotometers. Kinetics and luminescence spectra were measured under two different excitation wavelengths. Excitation was accomplished with a Nd:YG laser (06nm) which was D 2 Raman shifted to.6µm. The Q-switched repetition rate of this laser was 0Hz and the pulse duration was 7ns. Excitation was also accomplished with a tunable lexandrite laser at 727nm for ZnS and 760nm for ZnSe. Q-switched repetition rate for this laser was 3Hz and the pulse duration was 200ns. The emission was measured by a computercontrolled cton Research SpectraPro-300 spectrometer and the signal was processed with the aid of a Tektronix TDS-380 (30 MHz bandwidth) digital averaging oscilloscope and a oxcar- Integrator SR20 (Stanford Research System). P 0.7μm T 3 T T F.μm absorption 3μm T 2 T 2 E ~3μm D Figure. Energy level diagram of Co 2+ and Fe 2+ ions and possible energy transfer channels in ZnS/ZnSe crystals 2 Co 2+ Fe EXPERIMENTL RESULTS ND DISCUSSION 3. bsorption The absorption spectra of the samples are depicted in Figure 2 over the near and mid-ir spectral ranges. Peak absorption coefficients for the 2 (F) T (F) (~.6µm) transition in Co:ZnS and Co:ZnSe were 6 and cm -, respectively. The peak absorption coefficients for the 2 (F) T (P) (~70nm) transition were on the order of 8 times larger for both cases. In the Co:Fe:ZnSe and Co:Fe:ZnS samples, iron absorption centered near 3µm is apparent. Cobalt emission overlaps with iron absorption and should allow for fast energy transfer in the co-doped crystals. Proc. of SPIE Vol

3 30 20 Fe 2+ iv) Fe=8.x0 8 cm -3 iii) Fe=2.x0 8 cm -3 ii) Fe=8.7x0 8 cm -3 i) Co=6.x0 8 cm -3 0 Co 2+ Fe 2+ v) Co=8x0 8 cm -3 vi) Fe=0.x0 8 cm -3 vii) Fe=7x0 8 cm -3 viii) Fe=0x0 8 cm -3 α (cm - ) 0 Co 2+ iv iii α (cm - ) viii vii ii 0 i v vi Figure 2. bsorption spectra of co-doped Fe:Co:ZnSe () and; Fe:Co:ZnS () samples; ) Co 2+ concentration was fixed at 6.x0 8 cm -3 for all samples; Fe concentration varies over 8.-9x0 8 cm -3 ; ) Co 2+ concentration was fixed at 8x0 8 cm -3 ; Fe concentration varies over 0.-0x0 8 cm Low temperature characterization Figure 3 shows photoluminescence spectra of Co:Fe:ZnSe and Co:ZnSe samples under direct cobalt excitation of the 2 (F) T (P) (~70nm) transition at K. In these experiments we used a crystal with Fe and Co concentrations of 9x0 8 cm -3 and 6.x0 8 cm -3, respectively. In the Co:ZnSe one can see the well-structured photoluminescence spectrum corresponding to the T 2 (F) 2 (F) spin-forbidden cobalt transition with luminescence maximum at 300 nm. In the Co:Fe:ZnSe co-doped samples, the photoluminescence shown in Figure 3.b was significantly shifted to longer wavelength and coincides with the earlier reported low temperature photoluminescence spectrum of the Fe 2+ ions. The maximum photoluminescence signal in the iron-cobalt co-doped samples was measured at 370 nm. lso, as one can see from this figure, the direct photoluminescence signal from cobalt was significantly quenched and was hardly detected in comparison with the iron photoluminescence signal. Co:Fe:ZnSe Co:ZnSe Figure 3. Mid-IR luminescence spectra of iron and cobalt co-doped ZnSe crystals at T=K under 760nm excitation Proc. of SPIE Vol

4 The results of low temperature kinetics measurements under 760nm excitation are depicted in Figure. Co:ZnSe samples exhibited PL kinetics with lifetimes of ~.ms. For Co:Fe:ZnSe the photoluminescence kinetics were of the significantly shorter decay time of ~0µs. This lifetime is very close to the low temperature luminescence lifetime in Fe:ZnSe crystals under direct optical excitation at the E T 2 transition []. This fact demonstrated that at low temperature the energy transfer rate from Co 2+ to Fe 2+ ions is even faster than the Fe 2+ lifetime. Therefore, this excitation mechanism could be utilized for the creation of population inversion and lasing of Fe 2+ under near IR excitation Co:ZnSe Co:Fe:ZnSe Time (ms) Time (μs) Figure. Low temperature (K) PL kinetics of Co 2+ in Co(6.x0 8 cm -3 ):ZnSe () and Fe 2+ in Co(6.x0 8 cm -3 ):Fe(9x0 8 cm -3 ):ZnSe () under 760nm cobalt excitation 3.3 Lasing via energy transfer in Co:Fe:ZnS and Co:Fe:ZnSe High concentration of co-dopants in Co:Fe:ZnSe and Co:Fe:ZnS crystals and fast energy transfer rate provided high gain, enabling lasing under 0.76 µm and.6 µm pumping without the need for an external cavity. The Co:Fe:ZnSe crystal used for these experiments had cobalt concentration of 6.x0 8 cm -3 and iron concentration of 9x0 8 cm -3, while the Co:Fe:ZnS crystal used had cobalt concentration of 8x0 8 cm -3 and iron concentration of 0x0 8 cm -3. Two experiments were performed with each crystal to demonstrate laser oscillation due to Co-Fe energy transfer. In the first experiments pump radiation of the alexandrite laser was slightly focused to a spot diameter of ~2mm and the Co:Fe:ZnSe crystal was pumped at 760nm. The crystal was contained in a close-cycle refrigerator to adjust temperature from K to 300K. Luminescence spectra and kinetics were measured over a range of pump energies to show threshold behavior, the results of which can be seen in Figure and Figure 6. Feedback in the crystal was provided by Fresnel reflections of ~8% from each of the polished facets. The results of Co:Fe:ZnSe lasing experiments are depicted in Figure. Figure demonstrates the PL spectrum of the Co:Fe:ZnSe crystal for different pump energies. s one can see, at low pump energy the measured PL spectra were typical for luminescence of iron ions. The dependence of the PL spectrum profile on the pump energy demonstrated a threshold behavior accompanied by appearance of the stimulated emission band around 3.9 µm. The laser threshold was measured to be 9 mj. Proc. of SPIE Vol

5 0.0 6mJ 2mJ 23mJ log(signal), a.u mJ 6mJ 2mJ time, μs Figure. 760nm excitation: ) Mid-IR luminescence spectra of Co:Fe:ZnSe crystal for different pump energies at T=K, ) Co:Fe:ZnSe crystal PL kinetics for different pump energies below and above laser threshold. The change of the luminescence profile above laser threshold was accompanied by a shortening of PL kinetics as shown in Figure. For low pump energy one can see low temperature Fe 2+ kinetics with luminescence rise time ~0. μs followed by exponential decay. bove laser threshold, the kinetics exhibit a typical laser spike with ~00 ns pulse duration followed by below threshold luminescence kinetics. The threshold dependence of emission, significant line narrowing, and shortening of lifetime are clear evidence of Co:Fe:ZnSe lasing via Co Fe energy transfer mechanism. Lasing via Co Fe energy transfer Co:Fe:ZnS crystal was also demonstrated under 2 (F) T (F) cobalt excitation at at.6 μm. In these experiments the D 2 Raman shifted Nd:YG pump laser was lightly focused to a spot diameter of ~3mm. Figure 6 shows threshold behavior of the spectrum narrowing and kinetic shortening. The observed behavior was very close to the Co:Fe:ZnSe lasing discussed above. However, the spectrum of the transition ions in the ZnS host is shifted to shorter wavelengths due to a stronger crystal field in comparison with ZnSe. The stimulated emission band for Co:Fe:ZnS in Figure 6 can be seen centered at about 3.60 µm. Laser threshold was measured to be 6mJ. 3.6 mj (). mj (2) 7 mj (3) mj () Signal (a.u.) 0. mj ().6 mj () Wavelength (nm) time, μs Figure 6..6µm excitation: ) Mid-IR luminescence spectra of Co:Fe:ZnS crystal at different pump energies at T=K, ) Co:Fe:ZnS crystal PL kinetics for different pump energies below (i) and above (ii) laser threshold. Narrowing of luminescence above lasing threshold was accompanied by the shortening of PL kinetics shown in Figure 6. t low pump energy Fe 2+ kinetics with rise time ~ µs followed by exponential decay can be observed. bove threshold, there is a laser peak with pulse duration ~200 ns followed by low temperature luminescence kinetics matching that below threshold. Proc. of SPIE Vol

6 . CONCLUSIONS Energy transfer to Fe 2+ ( T 2 ) under.6µm and ~0.7µm excitation of the Co 2+ 2 (F) T (F) and 2 (F) T (P) transitions in ZnSe and ZnS were studied. Lasing of Fe 2+ was demonstrated with two new pump sources via energy transfer from Co 2+ to Fe 2+. Laser oscillation at 3.6µm was demonstrated at low temperature (-2K) in Co:Fe:ZnS under.6µm excitation. Laser oscillation at 3.9µm was demonstrated at K in Co:Fe:ZnSe under 760nm excitation. Future optimization of co-dopant concentration will be necessary to obtain lasing at room temperature. CKNOWLEDGEMENTS The authors would like to acknowledge funding support from National Science Foundation (NSF) EPS-0803, ECCS , and Kirtland F/U contract F9-0-C-02/P The work reported here partially involves intellectual property developed at the University of labama at irmingham. This intellectual property has been licensed to the IPG Photonics Corporation. The U coauthors declare competing financial interests. REFERENCES [] S. Mirov, V. Fedorov, I. Moskalev, D. Martyshkin, and C. Kim, Progress in Cr2+ and Fe2+ Doped Mid-IR Laser Materials, Laser Photon. Rev., 2- (200). [2] V. V. Fedorov, S.. Mirov,. Gallian, D. V. adikov, M. P. Frolov, Yu. V. Korostelin, V. I. Kozlovsky,. I. Landman, Yu. P. Podmar kov, V.. kimov, and.. Voronov, μm Tunable Solid State Lasers based on Fe2+-doped ZnSe Crystals Operating at Low and Room Temperatures, IEEE J. Quantum Electron. 2, (2006). [3] J. J. dams, C. ibeau, R. H. Page, D. M. Krol, L. H. Furu, and S.. Payne,.0.-μm lasing of Fe:ZnSe below 80 K, a new mid-infrared laser material, Opt. Lett. 2, (999). [].. Voronov, V. I. Kozlovsky, Yu. V. Korostelin,. I. Landman, Yu. P. Podmar kov, Ya. K. Skasyrsky, and M. P. Frolov, IEEE J. Quantum Electron. 38, 3 (2008). [] J. Kernal, V. V. Fedorov,. Gallian, S.. Mirov, and V. V. adikov, μm gain-switched lasing of Fe:ZnSe at room temperature, Opt. Express 3, (200). [6] M. E. Doroshenko, H. Jelínková, P. Koranda, J. Sulc, T. T. asiev, V. V. Osiko, V. K. Komar,. S. Gerasimenko, V. M.Puzikov, V. V. adikov, and D. V. adikov, Tunable mid-infrared laser properties of Cr2+:ZnMgSe and Fe2+:ZnSe crystals, Laser Phys. Lett. 7, 38 (200) [7] NoSoung Myoung, Dmitri V. Martyshkin, Vladimir V. Fedorov, and Sergey. Mirov, "Energy scaling of.3 μm room temperature Fe:ZnSe laser," Opt. Lett. 36, 9-96 (20). [8] Ming Luo, N.C. Giles, Utpal N. Roy, Yunlong Cui, and rnold urger, Energy transfer between Co2+ and Fe2+ ions in diffusion-doped ZnSe J. ppl. Phys 98, (200). [9]. Karipidou, H. Nelkowski and G. Roussos, Near infrared luminescence of Ni and Fe doped ZnSe Crystals, J. Crystal Growth, 9, (932). [0] NoSoung Myoung, Dmitri V. Martyshkin, Vladimir V. Fedorov, Sergey. Mirov, Mid-IR Lasing of Iron- Cobalt co-doped ZnS(Se) crystals via Co-Fe energy transfer, J. Luminescence, in press doi:0.06/j.jlumin [] NoSoung Myoung, Vladimir V. Fedorov, Sergey. Mirov, and Lowell E. Wenger, Temperature and concentration quenching of mid-ir photoluminescence in iron doped ZnSe and ZnS laser crystals, J. Luminescence 32, (202). Proc. of SPIE Vol