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We report a comprehensive study of γ-irradiation on optical, electrical, and laser characteristics of pure and transition-metal doped single and polycrystalline ZnS and ZnSe. Polished pure, Cr-doped, and Ag, Au, Cu, Al, In, and Mn co-doped ZnS and ZnSe crystals after absorption and electro-conductivity characterization were γ-irradiated at doses of 1.28x108 rad at −3°C. Dynamic room temperature absorption studies, electro-conductivity measurements, and mid-IR lasing were performed after different exposition times of crystals at room temperature. Cr:ZnSe and Cr:ZnS lasers based on identical γ-irradiated and non-irradiated crystals featured very similar pump thresholds, slope efficiencies, and output powers. New fluorescence band spanning over 1.3-2.1 μm in the γ-irradiated Au:Cr:ZnS was attributed to 3A2→3T2(F) transition of Cr4+.
©2013 Optical Society of America
1. Introduction
There has been a long standing demand for compact and tunable middle infrared (mid-IR) laser sources operating at room temperature (RT). The wavelength range between 2 and 10 μm is especially important because many organic molecules have strong characteristic absorption lines in this “molecular fingerprint” wavelength range. Lasers operating in this range have a wide variety of applications such as high resolution molecular spectroscopy, non-invasive medical diagnostics, eye safe efficient laser surgery, industrial process control, laser radars, and many others. Mid-IR wavelengths are usually generated using nonlinear optical conversion techniques or with the use of high performance III-V semiconductor lasers [1–8]. Specifically, significant progress in III-V-Nitride based Quantum Well (QW) lasers (1.3-1.6 μm) [1,2], GaSb and dilute-nitride semiconductors type-II QW mid-IR lasers (1.24-6 μm) [3–6], and InP-based quantum cascade lasers [7,8] has been reported in the literature.
Impurity doped crystalline vibronic lasers, having a gain bandwidth up to 50% of central wavelength, constitute another promising route for broadly tunable mid-IR coherent sources. Among them transition metal (TM2+, e.g., Cr2+ or Fe2+) doped II-VI (e.g., ZnSe, ZnS, CdSe, CdS) crystals represent a relatively new class of solid state gain media with strong and ultra-broad absorption and emission bands in the mid-IR range of optical spectra. These crystals have been demonstrated by a few groups to lase at RT under optical excitation with high efficiency, broad (>1000 nm) tunability, and multi-watt output powers [9–15]. In spite of the fact that influence of ionizing radiation on defect formation and electrical properties of II-VI crystals has been a subject of intensive studies since 1950’s [16–25], the effects of ionizing radiation on laser performance of chromium doped crystals have not been documented. One of the goals of the current study was to compare laser properties of γ-irradiated and non-irradiated chromium doped II-VI gain elements and identify how resistant are these crystals to ionizing irradiation.
In addition to effective RT mid-IR lasing TM:II-VI media, as wide band semiconductors, hold potential for direct electrical excitation [26]. One of the possible approaches in fabrication of conductive II-VI crystals was to subject them to γ-irradiation and study whether shallow color centers enabling electrical conductivity could be formed in II-VI crystals. Hence, one of the goals of the research was to investigate formation and decay of color centers in doped and un-doped II-VI media as well as study electrical properties of γ-irradiated Cr:ZnS and Cr:ZnSe crystals.
One of the major problems in the realization of conductive TM:II-VI structures via traditional crystal doping with donor or acceptor impurity is the pairing of this impurity with laser active Cr2+ ions resulting in active ion valence change accompanied by a loss of p- or n- conductivity and increase of optical losses [27]. Hence, the possibility of breaking donor (acceptor) impurity—active ion pairs and gaining favorable characteristics for electrically pumped lasing by exposing the II-VI semiconductor materials to γ-irradiation was one of the motivations among others for conducting experiments on possible valence change of Cr2+ under γ-irradiation. In addition, γ-irradiation of Cr doped II-VI crystals could enable Cr ions in other valence states (e.g. Cr4+, Cr3+). These optical centers are also promising for near and mid-IR laser applications but their formation in II-VI crystals with concentration sufficient for lasing has not been documented in the literature.
2. Crystal preparation
In our experiments, we used single and polycrystalline ZnS and ZnSe undoped samples as well as samples doped with various metal impurities. Single crystals (In:Cr:ZnS; Cu:Cr:ZnS; Mn:Cr:ZnS; Ag:Cr:ZnS; Au:Cr:ZnS) were grown by Eagle-Pitcher Corp by a modified vertical Bridgeman technique with the doping level in the melt of 100ppm. Chemical vapor deposition (CVD) grown (II-VI, Inc) pure ZnSe and ZnS polycrystals were doped in two stages using post growth thermal diffusion technique at the Department of Physics, University of Alabama at Birmingham [14]. At the first stage, chromium was introduced into the ZnSe and ZnS crystalline hosts by thermal diffusion carried out in sealed ampoules under a pressure of 8x10−2 Torr and temperature of 1000°C over 5-20 days. In case of co-doped Al:Cr:ZnSe; Cu:Cr:ZnSe; and Ag:Cr:ZnSe samples Cr doped ZnS and ZnSe crystals were further sealed with aluminum, copper, and silver impurities in quartz ampoules under vacuum of 8x10−2 Torr and annealed for a week at the temperature of 1000°C.
Crystals were γ-irradiated with the use of Co60 source at −3°C for 16 days and additionally irradiated for 2 days at 10°C. The total irradiation dose was 1.28x108 Rad. All crystals were stored after irradiation in liquid nitrogen at −169°C. Spectra of all samples were taken with 3101 UV-V Shimadzu spectrophotometer at RT.
3. Spectroscopic characterization
3.1. Undoped ZnS, ZnSe crystals before and after γ-irradiation
Figure 1 shows dynamics of the change in absorption spectra of un-doped γ-irradiated ZnS and ZnSe polycrystals for different exposition times of crystal at RT after irradiation. Samples were stored at 77K between γ-irradiation and absorption characterization. To measure spectra they were heated to RT. The absorption spectrum of a ZnS crystal annealed for 10 min at RT (Fig. 1, curve i) features a broad absorption centered at 630 nm and extending up to 2500 nm. This absorption band probably consists of several overlapping bands with different degradation times at RT. Therefore, the maximum of the absorption peak is shifted to the 550 nm, after 15 min of keeping the sample at RT (curve ii). After 20 min of RT annealing, the majority of induced absorption is degraded and the resultant spectrum (curve iii) features a small, with respect to the spectrum of non-irradiated crystal (curve iv), but stable nonstructural absorption in the VIS– near IR spectral region.
Fig. 1 Absorption spectra of A) a undoped ZnS crystal before irradiation (dash-dot-dot line iv); after 5 minutes (solid line i), 15 minutes (dot line ii) and 20 minutes (dashed line iii) storage at RT after low temperature γ-irradiation with a dose 1.28x108 Rad; (B) ZnSe crystals before (dash-dot-dot line ii) and after 5 minutes (solid line i) storage at RT after low temperature γ-irradiation with a dose 1.28x108 Rad.
The absorption dynamics of undoped gamma-irradiated ZnSe crystals was similar to ZnS. The absorption spectra of undoped ZnSe crystals before and after gamma-irradiation are shown in Fig. 1B. As one can see, similarly to ZnS, the gamma-irradiation resulted in formation of nonstructural absorption in the visible and near IR spectral ranges.
One of the possible interpretations of results depicted in Fig. 1 can be as follows. γ-irradiation of undoped ZnS and ZnSe crystals results in appearance of a strong absorption covering the whole visible-mid-IR spectral range. Like in alkali-halides subjected to significant doses of γ-irradiation, these bands can be attributed to color centers accompanied by aggregation in the cation sublattice [28]. However, on the contrary to alkali halides (e.g. LiF crystals), the color centers in II-VI as well as metal aggregates are not stable at RT. After ~20 min of crystals annealing at RT the color center absorption and Zn aggregates scattering reduces significantly. As a result, after long-term (>30 min) exposure of γ-irradiated ZnS and ZnSe at RT the induced absorption is negligible, covers mainly visible-near IR region of optical spectrum and in principle should not affect laser characteristics of Cr:ZnS and Cr:ZnSe lasers operating over 2-3 μm spectral range.
3.2. Chromium doped ZnS, ZnSe crystals before and after γ-irradiation
Doping of II-VI semiconductors (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe) by chromium ions results in isovalent substitution of cations (such as Zn, Cd) by chromium. The 5D ground state of the Cr2+ is spilt into the triplet 5T2 (ground state) and the doublet 5E (first excited state) in the tetrahedral crystal field. The energy difference between the 5E state and the 5T2 state corresponds to the optical transition in the near-IR spectral region. Absorption spectrum at 5E ↔5T2 transition in Cr:ZnSe crystal is shown in the Fig. 2A. Peaks of 7.5 cm−1 and 6 cm−1 at 1700 nm represent absorption band of Cr2+. The Cr2+ concentration in the samples can be estimated as a ratio of the experimentally measured coefficients of absorption to the absorption cross section kabs/σabs at 1690 (1750 nm) in ZnS (ZnSe) crystals, respectively.
Fig. 2 (A) Absorption spectra of the Cr:ZnSe crystal before (i) irradiation and after 8 days storage at RT (ii). (B) Temporal dependence of the absorption coefficient at 1690 nm after irradiation (dash line show results after 8 day storage at RT).
In Cr:ZnS crystals, the Cr2+ concentration is practically unchanged after γ-irradiation. On the contrary to Cr:ZnS, as shown in Fig. 2, we observed a ~24% increase of the Cr2+ absorption coefficient in γ-irradiated Cr:ZnSe after 8 days of its storage at RT. These Cr:ZnSe polycrystals were fabricated using chromium thermal diffusion into the ZnSe host. In spite of the relatively large diffusion coefficient, Cr ions could be partially localized near grain boundaries in the polycrystalline host where they can be compensated to Cr + state by nearest (donor) defects. Gamma irradiation at low temperature could provide dissociation of these aggregates with additional formation of Cr2+ ions.
3.3. Co-doped Cr:ZnS, Cr:ZnSe crystals
ZnS and ZnSe crystals doping with TM impurities from group 11 of the periodic table (e.g. TM11: Cu, Ag, Au) results in acceptor-type cation substitution. One of the models to treat these centers is based on the (TM+)-h complex, where TM+ is an ion with a closed d10 shell and h is a hole localized near the TM+ ion [29]. Optical transitions in the visible spectral range are usually attributed to the defects localized at the dopant impurity. However, the authors of [30] attributed the excitation band between 380 nm and 480 nm in Ag:ZnS crystal to the 4d10→4d95s Ag+ ion transition. Similar UV transitions of Ag+ ions are well documented in alkali halide crystals (see, for example [31]).
Absorption spectra of Au:Cr:ZnS crystals are shown in Fig. 3. Spectra were taken at RT. Irradiated samples were taken from liquid nitrogen and heated before measurement. There are several overlapping bands in the visible and near IR spectral range in addition to the Cr2+ 5E ↔5T2 transition in Au:Cr:ZnS crystal before γ-irradiation (curve i, v). After γ-irradiation, the absorption bands of about 0.5 cm−1 with maxima at 600 and 740 nm are clearly seen in the curve ii. These bands are stable at room temperature and feature strong polarization dependence as depicted in Fig. 3B. We believe that these absorption bands could be due to the 3A2→3T1(F) transition of Cr4+ ions [32,33]. The strong polarization dependence of Cr4+ ions in the tetrahedral sites in Li2CaSiO4 was studied in [34] where absorption bands peaked at 620 nm and 720 nm were reported for two different polarizations. The weaker absorption band at the 3A2→3T2(F) transition of Cr4+ ions should be centered around 1000-1200 nm. The charge compensation of the Cr4+ could be achieved via formation of the (Au+)Zn-(Cr4+)Zn-(Au+)Zn aggregates. Figure 3C shows absorption spectra of the Au:Cr:ZnS crystal with a higher Au concentration. As one can see from the Fig. 3C, an increase of the Au concentration is accompanied with a 1-2 cm−1 growth of the absorption band centered at 890 nm.
Fig. 3 (A) and (C) Absorption spectra of the Au:Cr:ZnS with different Au concentration before (i, v) and after (ii, vi) irradiation, (B) polarized absorption spectra after irradiation for E//z (curve iv) and E⊥z (curve iii) light polarization.
The near IR band with 2 cm−1 coefficient of absorption was also observed in the Ag:Cr:ZnS crystal. However, the band was shifted to the IR spectral region with a maximum at ~940 nm (see Fig. 4A). This band was seen more clearly after γ-irradiation (Fig. 4A, curve ii). A change in the spectral bands was not observed in the Cu:Cr:ZnS crystals after annealing the crystal at RT (see Fig. 4B). However, the optical transition near 1.4μm was well identified as the 5E ↔5T2 transition of (Cu2+)Zn ions [35]. Therefore, near IR absorption in Au and Ag doped ZnS crystals could also result from 5T2 →5E transition of (Au)2+ and (Ag)2+ ions shifted to the shorter wavelength in comparison with Cu doped crystals. A more detailed study is required to clarify the nature of these bands.
Fig. 4 Absorption spectra of (A) Ag:Cr:ZnS and (B) Cu:Cr:ZnS crystal before (i, iii) and after (ii, iv) irradiation.
A near infrared tunable laser action at 3A2→3T2(F) transition of tetrahedral Cr4+ ions covering the 1400–1600 nm spectral range has been reported in several crystals [36]. Therefore, photoluminescence (PL) properties of Au:Cr:ZnS crystals were studied in 1-3μm spectral range. PL spectra of Au:Cr:ZnS and Cr:ZnS crystals measured under 532 nm excitation at RT are shown in Fig. 5. Spectra of irradiated samples were taken a long time after irradiation. As one can see, both crystals demonstrate luminescence spectra of Cr2+ ions at 5E ↔5T2 transition between 1.8 and 3.3 μm. The spectra were not corrected on spectral response of the registration system and a signal drop near 2.4 μm is mainly due to a decrease of the PbS detector sensitivity. In Au:Cr:ZnS crystal, an additional emission band, supposedly due to 3A2→3T2(F) transition of Cr4+, spanning over 1.3-2.1 μm, was detected. A peak was detected at ~1800 nm. The red-shift of the PL in ZnS crystal, in comparison with analogous transitions in YAG and forsterite crystals can be explained by smaller crystal field splitting in chalcogenide crystals. We didn’t observe any bands (except for Cr2+ transition) in Ag:Cr:ZnS and Cu:Cr:ZnS under green excitation at RT.
Fig. 5 (A) RT PL spectra of Au:Cr:ZnS (curve i) and Cr:ZnS (curve ii) under 532 nm excitation; (B) Difference between PL spectra of Au:Cr:ZnS and Cr:ZnS representing 3A2→3T2(F) emission spectrum of tetrahedral Cr4+ ions.
Contrary to Cu, Ag, and Au co-doped Cr:ZnS crystals, the In:Cr:ZnS samples did not reveal any absorption bands which could be associated with chromium ions in the irradiated and non-irradiated crystals. It can be explained similarly to Al co-doped crystals [27] by changing the valence state of Cr due to formation of In3+-Cr+ pairs.
4. Electrical characterization
γ-irradiation of II-VI crystal form color centers that can produce electrical carriers and affect conductivity of the material. Electrical conductivity of II-VI crystals could make them promising materials for mid-IR lasing under direct electrical excitation.
For electrical measurements indium contacts were made on the sample surfaces. The polished crystals were heated on a hot plate and indium was melted on the surface using a soldering iron. 5x5 mm faces were covered with indium. Smaller (5x1 mm) faces of samples were polished after this to remove residue of indium.
The conductivity of samples was studied using a Hewlett Packard 214B pulse generator. Square wave signals were sent to the load resistor and crystal connected in series. The input signal and voltage difference across the crystal were measured by a Tektronix TDS 744A oscilloscope. The measurements were taken by increasing the input voltage from zero to up to 50 volts with step equal to two volts. Resistance of changeable load resistor varied from 80 Ω to 200 kΩ. To preserve crystal from heating at the time of measurements, the crystal was fixed on water cooled copper base.
We tested In:Cr:ZnS, Cu:Cr:ZnS, Cu:Cr:ZnSe, Au:Cr:ZnS, Al:Cr:ZnS, Al:Cr:ZnSe, Mn:Cr:ZnS, Ag:Cr:ZnS, and Ag:Cr:ZnSe crystals. Only Cr: Al: ZnSe sample with thickness 0.15cm and area 0.17 cm2 was conductive. Its resistance was found to be 5.6 kΩ. Conductivity of Al:Cr:ZnSe before irradiation was 1.58x10−4(Ω cm)−1. After γ-irradiation the resistance dropped to 5.0 kΩ, and conductivity slightly increased to 1.76x10−4(Ωcm)−1. Color centers decay too fast to affect electrical conductivity of the crystals.
5. Laser characterization
We compared laser characteristics of the irradiated and non-irradiated Cr:ZnSe (14.1x8.3x4.4 mm for non-irradiated crystal and 8.3x7.3x4.4 mm for irradiated) and Cr:ZnS (2.0x4.8x4.4 mm for all samples) gain elements. Chromium concentrations in the studied samples were measured to be 2.38 x1018 and 5.3x1018 for Cr:ZnSe and Cr:ZnS crystals, respectively. Laser elements were irradiated to the total irradiation dose of 1.28x108 Rad. The experimental setup for the crystal characterizations is shown in Fig. 6.
Fig. 6 Experimental setup for laser characterization of Cr: ZnS and Cr: ZnSe samples before and after γ-irradiation.
To measure the lasing characteristics of Cr:ZnS and Cr:ZnSe, an IPG Photonics CW Er3+ fiber laser with wavelength 1.55 μm, was used as the optical pump source. A mechanical chopper operating at a rate of 70Hz was placed in the beam path to mitigate the thermal lensing in the crystal. To focus the pump beam onto the crystal, a lens with a focal length of 7.5 cm and 6cm for Cr:ZnSe and Cr:ZnS respectively, was placed in front of the input mirror M1. This mirror has 90% transmittance at pump wavelength and more than 99% reflectivity in the 2.1-2.6 um spectral range. The gain elements were mounted on a copper base attached with an aluminum radiator. Mirror M2 was used as an output coupler with reflectivity at oscillation wavelength equal to 70%.
Figure 7 demonstrates the best input-output dependencies of the irradiated and non-irradiated Cr:ZnS (Fig. 7A) and Cr:ZnSe (Fig. 7B) gain elements. The slope efficiencies of the irradiated Cr:ZnSe gain element (44%) was very close to the non-irradiated crystal (45%). However, the threshold for non-irradiated Cr:ZnSe was 3.3W, while it was 3.75W for the irradiated crystal. For Cr:ZnS the best slope efficiency was 16% for the non-irradiated sample and 15% for the irradiated sample. Laser threshold was found to be 3.3 W for the non-irradiated Cr:ZnS sample and 4.2W for the irradiated gain element, respectively. The spectral output was centered at 2450 nm and 2300 nm for Cr:ZnSe and Cr:ZnS lasers, respectively with full bandwidth at half maximum (FWHM) of ~30 nm.
Fig. 7 Output power of the Cr:ZnS (A) and Cr:ZnSe (B) lasers as a function of incident power for non-irradiated (solid symbols) and γ-irradiated (1.28x107 Rad) (open symbols) gain elements.
According to the simple laser model, both slope efficiency (ηsl) and laser threshold (Pth) depend on cavity losses. CW laser threshold is proportional to the total cavity losses Pth ~(Lp + La), where La = -ln(Rout) is active loss due to the output coupler and Lp is a passive loss. The laser slope efficiency ηsl is proportional to the ratio La/(La + Lp). In the case when active losses exceed passive losses, the slope efficiency is less sensitive to the cavity losses than the laser threshold. Using this model for the laser threshold, we estimated induced losses in the Cr:ZnS, Cr:ZnSe gain elements as 2-5% per crystal pass.
6. Conclusions
We report a comprehensive study of γ-irradiation on optical, electrical, and laser characteristics of pure and transition-metal doped single and polycrystalline ZnS and ZnSe. Polished pure, Cr-doped, and Ag, Au, Cu, Al, In, and Mn co-doped ZnS and ZnSe crystals after absorption and electro-conductivity characterization were γ-irradiated at a dose of 1.28x108 rad at −3°C. Dynamic RT absorption studies, electro-conductivity measurements, and mid-IR lasing were performed for different exposition times of crystals at RT.
Immediately after γ-irradiation, both ZnS and ZnSe crystals exhibit color-center formation accompanied by Zn metal aggregates and the appearance of a strong absorption covering the whole visible-mid-IR spectral range. After~20 min of crystal annealing at RT, color-center absorption and Zn aggregates scattering reduced significantly. Absorption behavior of Cr:ZnS and Cr:ZnSe crystals co-doped with Ag, Cu, Al, In, and Mn were similar to that of undoped crystals. The Au:Cr:ZnS crystal, in addition to 600 and 720 nm bands and Cr2+ 1690 nm absorption, featured an additional absorption peak at 890 nm which can be attributed to the 5T2 →5E transition of (Au)2+ ions. In the Au:Cr:ZnS crystal, an additional emission band, supposedly due to the 3A2→3T2(F) transition of Cr4+ and spanning over 1.3-2.1 μm, was detected. γ-irradiation did not change the conductivity of any of the crystals except Al:Cr:ZnSe, for which conductivity increased by 10%.
Cr:ZnSe and Cr:ZnS lasers based on identical γ-irradiated and non-irradiated crystals featured very similar pump thresholds, slope efficiencies, and output powers after 30 min of RT annealing.
Acknowledgments
The authors would like to acknowledge funding support from the Materials and Manufacturing Directorate of AFRL (Subcontract RSC09011, Prime Contract No. FA8650-06-D-5401/0013), and the National Science Foundation under grants DMR-0116098, EPS-0814103, and ECCS-0901376. The authors also would like to acknowledge National Aeronautics and Space Administration (NASA)-Alabama Space Grant Consortium Research Experiences for Undergraduates (REU) award to University of Alabama at Birmingham.
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