90° phase-matched up-conversion of CO2 laser radiation in AgGa0.86In0.14S2
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- Tanno, F. & Kato, K. Appl. Phys. B (2012) 109: 367. doi:10.1007/s00340-012-5210-7
The CO2 laser radiation at 10.5910–9.2714 μm was up-converted to the visible in the 90° phase-matched AgGa0.86In0.14S2 crystal by mixing with the output of the 0.3547 μm pumped BBO optical parametric oscillator at 25–120 °C. The new Sellmeier and thermo-optic dispersion formulas that reproduce these experimental results correctly as well as the previously published data [Banerjee et al. in Appl Phys B 87:101, (2007); Opt Commun 227:202 (2007)] for difference-frequency generation at 4.05–6.98 μm and second-harmonic generation at 5.2955 μm are presented.
In previous publications [1, 2], we have reported the Sellmeier and thermo-optic dispersion formulas for AgGa0.86In0.14S2 that provide a good reproduction for difference-frequency generation (DFG) at 4.05–6.98 μm and second-harmonic generation (SHG) at 5.2955 μm. However, a somewhat large discrepancy between theory and experiment was encountered for the 90° phase-matched up-conversion of the CO2 laser radiation achieved in this crystal. For instance, the experimentally observed OPO pump wavelengths for up-conversion of the 10.5910–9.2714 μm radiation are 14–17 nm shorter than the values given by the above-mentioned Sellmeier equations. In addition, a significant difference between theory and experiment was found for the temperature-dependent phase-matching conditions for this process. Thus, we have corrected these formulas so as to satisfy the new experimental results, and simultaneously fit the data points presented in [1, 2].
Here, we report the new experimental results on the 90° phase-matched up-conversion of the CO2 laser radiation in AgGa0.86In0.14S2 and the new Sellmeier and thermo-optic dispersion formulas for this crystal.
2 Experiments and discussions
The experiments were carried out with a step-tunable CW CO2 laser and a 0.3547 μm pumped BBO optical parametric oscillator (OPO) as the pump source. Both beams were combined with a ZnSe optical flat and collinearly incident on the refabricated, 7-mm-long, θ = 90° cut AgGa0.84In0.16S2 crystal mounted on the temperature controlled oven.
The OPO pump power was adjusted to 5–10 mJ at 10 Hz to avoid the surface damage of the AgGa0.84In0.16S2 crystal, and the CO2 laser power was adjusted to less than 20 mW to avoid local heating. The unfocused beam diameter was 4 mm for the former and 2 mm for the latter.
The pump and output wavelengths were measured by a 0.5-m spectrometer with an accuracy of less than 0.1 nm.
The measured acceptance angles and spectral phase-matching bandwidths at full-width at half-maximum (FWHM) are Δθint·ℓ½ = (2.3 ± 0.1) deg cm½ and Δλp·ℓ = (0.4 ± 0.1) nm cm, which agree well with the theoretical values of Δθint·ℓ½ = 2.35 deg cm½ and Δλp· ℓ = 0.39 nm cm for the CO2 laser wavelength of 10.5910 μm and Δθint·ℓ½ = 2.34 deg cm½ and Δλp·ℓ = 0.48 nm cm for the CO2 laser wavelength of 9.2714 μm.
Note that the ordinary and extraordinary refractive indices of AgGa1 − x In x S2 increase and the birefringence decrease as a function of In concentrations  as in the case of AgGa1 − x In x Se2 . While the Sellmeier equations of Banerjee et al. [1, 2] give the ordinary refractive indices that are smaller than those of pure AgGaS2  at wavelengths longer than 2.42 μm; in contrast, the new Sellmeier equations give the normal dispersion [n(AgGa0.86In0.14S2) > n(AgGaS2)] throughout the whole spectral range. In addition, this index formula correctly reproduces the 90° phase-matched DFG between the Ti:Al2O3 laser at 0.84274 μm and the Nd:YAG laser at 1.0642 μm as well as the phase-matching angle of θPM = 80.8° and 73.0° for SHG of the CO2 laser lines at 10.5910 [1, 2] and 10.2466 μm, respectively.
In order to clarify this inconsistency, we have once again measured DFG between the BBO/OPO and the Nd:YAG laser under the identical experimental conditions described in . The resulting tuning points (open circles) are shown in Figs. 3 and 4 together with the data points (triangles) taken from . As can be seen from these figures, our data points agree excellently with the theoretical values calculated with Eqs. (1) and (2). Thus, the data point shown in Fig. 3 of  is thought to be in error.
In summary, we have reported the 90° phase-matched up-conversion of the CO2 laser radiation at 9.2714–10.5910 μm in AgGa0.86In0.14S2. These data were used to construct the new Sellmeier and thermo-optic dispersion formulas that provide the excellent reproduction of these experimental results as well as the previously published data [1, 2] of DFG at 4.05–6.98 μm and SHG of the CO2 laser at 10.5910 μm. We believe that these Sellmeier and thermo-optic dispersion formulas are highly useful for predicting the temperature-dependent phase-matching conditions for frequency conversion in the AgGa1−x In x S2 (x ≦ 0.14) crystals when combined with our index and thermo-optic dispersion formulas for pure AgGaS2 .
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