We report new experimental results on the phase-matching properties of AgGa0.86In0.14S2 for second-harmonic generation (SHG) and sum-frequency generation (SFG) of a Nd:YAG laser-pumped KTiOPO4 (KTP), AgGaS2 optical parametric oscillators (OPOs), and a CO2 laser in the 0.615−10.5910 µm spectral range. In addition, we present Sellmeier equations that provide a good reproduction of the present experimental results and the value of the nonlinear optical constant d36(AgGa0.86In0.14S2) = 13.4 pm/V measured by SHG method.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Owing to the extended IR transmission up to 14.3 µm (Fig. 1) and the decreased birefringence and thermo-optic constants (Δn = no – ne = 0.0451 and d(Δn)/dT = −4.732 × 10−6 °C−1 at 1.0642 µm) compared to those (12.4 µm, Δn = 0.0529 and d(Δn)/dT = −5.030 × 10−6 °C−1 at 1.0642 µm) of AgGaS2 [1–5], AgGa0.86In0.14S2 appears to be more efficient than AgGaS2 for generating mid-IR pulses when pumped by near-IR lasers such as a Nd:YAG laser and a Yb:YAG laser.
We have already reported the 90° phase-matched difference-frequency generation (DFG) in the 4.04–6.98 µm spectral range in this material by using a Nd:YAG laser and a tunable Ti:Al2O3 laser as pump sources [2,3]. We also reported the 90° phase-matched up-conversion of a CO2 laser at 9.2714–10.5910 µm to the visible range by mixing with a Nd:YAG laser-pumped β-BaB2O4 (BBO) OPO . However, no attempt was made to compare these data with those of AgGaS2. In order to assess the potential of this crystal against AgGaS2, we have further studied its phase-matching properties for SHG and SFG in the 0.8428 − 10.5910 µm spectral range by using a Nd:YAG laser-pumped KTP and AgGaS2 OPOs and a CO2 laser as pump sources. Therefore, we analyzed them by using the refined Sellmeier and thermo-optic dispersion formulas presented in . In addition, we measured the nonlinear optical constant of this crystal as d36 = 13.4 pm/V by the SHG method at λ1 = 1.9710 µm. These results strongly suggested that AgGa0.86In0.14S2 is more useful than AgGaS2 for generating mid-IR pulses above ∼8 µm by OPO and optical parametric amplifier (OPA).
2. Experiments and discussion
The 7 mm long, θ = 90° and ϕ = 45° cut AgGa0.86In0.14S2 crystal used in the present experiments is the same one that had been used for the previous experiments . This crystal was mounted on a temperature-controlled copper oven set on a Nikon stepmotor-driven rotation stage having an accuracy of ± 0.02° .
The temperature stability of the oven is ± 0.1°C. By using the signal and idler outputs of a Nd:YAG laser-pumped KTP OPO as a pump source, we once again measured the SHG and SFG cut-off wavelengths at 20°C. The shortest SHG wavelength was measured to be λ2 = 0.9855 µm (λ1 = 1.9710 µm). For SFG between the fundamental and second harmonic, the fundamental wavelength was λ1 = 2.8236 µm (λ2 = 1.4118 µm, λ3 = 0.9412 µm). Moreover, the 90°phase-matched SFG wavelength between a Nd:YAG laser and the idler output of a Nd:YAG laser-pumped AgGaS2 OPO was λ3 = 0.8428 µm (λ1 = 4.0511 µm, λ2 = 1.0642 µm). We also measured the temperature phase-matching bandwidths (ΔT·l) at full-width at half-maximum (FWHM) for SHG from the temperature variation of the phase-matching wavelengths by heating the crystal from 20 °C to 140 °C at 20 °C intervals, and those for SFG by measuring directly the output powers at the fixed wavelengths by heating the crystal from 20 °C to 40 °C at 2 °C intervals. These results are tabulated in Table 1 together with the data for SFG between a CO2 laser and a Nd:YAG laser-pumped BBO OPO at λ3 = 0.6164 µm  and the theoretical values calculated with the following refined Sellmeier equations that were constructed by using the refractive indices given by the single-pole Sellmeier equations presented in Ref. .
We also measured the nonlinear optical constant of this crystal from the ratio of SHG output powers observed in this crystal and a 5 mm long, θ = 58.2° and ϕ = 45° cut reference AgGaS2 crystal by pumping with a Nd:YAG laser-pumped KTP OPO tuned at 1.9710 µm. By neglecting the absorption at 1.9710 µm and 0.9855 µm and taking the Fresnel reflection losses at the crystal surfaces and the reference value of d36(AgGaS2) = 14.1 pm/V at a phase-matching angle of θpm = 62.5° for AgGaS2, we found d36(AgGa0.86In0.14S2) = (13.4 ± 0.7) pm/V.
After these experiments, we have cut the present crystal into θ = 39.9° and measured the phase-matching angles for SHG and SFG of a CO2 laser at 1.7652 − 10.5910 µm by using a waveguide CO2 laser (Coherent DEOS, Model EOM-10) as a pump source. Since the CO2 laser was operated at low power when measuring the phase-matching angles, the thermal lensing effect was not observed in this experiment.
The results are tabulated in Table 1 together with the temperature phase-matching bandwidths (ΔT·l) determined from the temperature variation of the phase-matching angles (Δθext/ΔT) observed by heating the crystal from 20 °C to 140 °C at 20 °C intervals and the acceptance angles (Δθext·l) given by Eq. (2). As shown in Table 1, the experimentally observed values agree well with the theoretical values given by Eqs. (1) and (2).
In the meantime, we calculated the phase-matching angles for type-1 and type-2 SHG of this crystal at 20 °C. The resulting tuning curves (solid lines) are shown in Fig. 2 together with our experimental points (open circles) measured at λ1 = 1.9710, 3.5303, 5.2955, and 10.5910 µm. For comparison, we also inserted the tuning curves (dashed lines) of AgGaS2 calculated with the following refined Sellmeier equations  that were transformed from the single-pole Sellmeier equations presented in Ref. .
In addition, the refractive indices of AgInS2 are obtained from the well-known relationship Eq. (4) in mixed crystals. From Eqs. (1), (3), and (4), it is found to be a positive uniaxial crystal below 1.127 µm.
Since AgGa0.86In0.14S2 is more transparent than AgGaS2 at wavelengths longer than ∼8 µm (α = 0.35 cm−1 at 10 µm for AgGa0.86In0.14S2 and α = 0.6 cm−1 at 10 µm for AgGaS2) and its nonlinear optical constant does not differ from d36 = 14.1 pm/V of AgGaS2, AgGa0.86In0.14S2 seems to be more efficient than AgGaS2 when used as a Nd:YAG laser-pumped OPO and OPA. The laser-induced damage threshold of AgGa0.86In0.14S2 is almost as high as that of AgGaS2 (>2.2J/cm2 at 9.2714 µm ). Thus, we calculated the type-1 and type-2 phase-matching angles for this process by using Eq. (1). The resulting tuning curve (dashed line) for type-2 OPO is shown in Fig. 4 together with the tuning curve (solid line) and the experimental points of Vodopyanov et al. for AgGaS2 OPO taken from Ref. . For comparison, we also inserted our tuning curve (dotted line) of AgGaS2 calculated with Eq. (3) into this figure. As can be seen from this figure, the tuning curve of AgGa0.86In0.14S2 shifted towards longer wavelengths at fixed phase-matching angles as expected from the decreased birefringence. The acceptance angle and temperature phase-matching bandwidth of AgGa0.86In0.14S2 for generating 10.50 µm pulses at θpm = 44° are calculated to be Δθext·l = 0.33 deg·cm and ΔT·l = 24.0 °C·cm, which are ∼10% larger than Δθext·l = 0.28 deg·cm and ΔT·l = 19.6 °C·cm of AgGaS2 at θpm = 38.4° (The Sellmeier equations of Roberts  for AgGaS2 give Δθext·l = 0.27 deg·cm at θpm = 38°). Thus, AgGa0.86In0.14S2 is thought to be more stable than AgGaS2 to generate mid-IR pulses up to 14.3 µm when pumped by a high-repetition-rate Nd:YAG laser operating in the picosecond and femtosecond regimes.
We have reported the phase-matching properties of AgGa0.86In0.14S2 for SHG and SFG in the 0.615 − 10.5910 µm spectral range, which are precisely reproduced by the Sellmeier and thermo-optic dispersion formulas presented in this paper. Since the nonlinear optical constant (d36 = 13.4 pm/V) of this crystal does not differ from that of AgGaS2 and it is more transparent than AgGaS2 at wavelengths longer than ∼ 8 µm, AgGa0.86In0.14S2 is thought to be more useful than AgGaS2 for synchronously pumped high-repetition-rate OPO and chirped-pulse high-repetition-rate OPA to generate mid-IR pulses up to 14.3 µm under Nd:YAG laser excitation.
The authors declare that there are no conflicts of interest related to this article.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
1. V. V. Badikov, I. N. Matveev, V. L. Panyutin, S. M. Pshenichnikov, A. E. Rozenson, S. V. Skrebneva, N. K. Trotsenko, and N. D. Ustinov, “Growth and optical properties of the AgGa1-xInxS2 system,” Sov. J. Quantum Electron. 10(10), 1302–1303 (1980). [CrossRef]
2. S. Banerjee, K. Miyata, K. Kato, N. Saito, and S. Wada, “90° phase-matched parametric frequency conversion in AgGa1-xInxS2,” Appl. Phys. B 87(1), 101–103 (2007). [CrossRef]
3. S. Banerjee, K. Miyata, and K. Kato, “Temperature phase-matching properties of mixed chalcopyrite AgGa1-xInxS2 crystal,” Opt. Commun. 277(1), 202–204 (2007). [CrossRef]
4. F. Tanno and K. Kato, “90° phase-matched up-conversion of CO2 laser radiation in AgGa0.86In0.14S2,” Appl. Phys. B 109(2), 367–369 (2012). [CrossRef]
5. K. Kato, T. Okamoto, S. Grechin, and N. Umemura, “New Sellmeier and thermo-optic dispersion formulas for AgGaS2,” Crystals 9(3), 129 (2019). [CrossRef]
6. K. Kato, K. Miyata, V. V. Badikov, and V. Petrov, “Thermo-optic dispersion formula for BaGa4Se7,” Appl. Opt. 57(11), 2935–2938 (2018). [CrossRef]
7. K. Kato, V. Petrov, and K. Miyata, “Accurate Sellmeier equations for AgGaS2 in the 0.565–10.6321 μm spectral range,” Proc. SPIE 11670, 11670–54 (2021).
8. W. Chen, J. Burie, and D. Boucher, “Midinfrared cw difference-frequency generation using a synchronous scanning technique for continuous tuning of the full spectral region from 4.7 to 6.5 μm,” Rev. Sci. Instrum. 67(10), 3411–3415 (1996). [CrossRef]
9. W. Jantz and P. Koidl, “Efficient up-conversion of 10.6-μm radiation into the green spectral range,” Appl. Phys. Lett. 31(2), 99–101 (1977). [CrossRef]
10. T. Itabe and J. L. Bufton, “Phase-matching measurements for 10-μm upconversion in AgGaS2,” Appl. Opt. 23(18), 3044–3047 (1984). [CrossRef]
11. P. Canarelli, Z. Benko, R. Curl, and F. K. Tittel, “Continuous-wave infrared laser spectrometer based on difference frequency generation in AgGaS2 for high-resolution spectroscopy,” J. Opt. Soc. Am. B 9(2), 197–202 (1992). [CrossRef]
12. A. Harasaki and K. Kato, “New data on the nonlinear optical constant, phase-matching, and optical damage of AgGaS2,” Jpn. J. Appl. Phys. 36(Part 1, No. 2), 700–703 (1997). [CrossRef]
13. K. L. Vodopyanov, J. P. Maffetone, I. Zwieback, and W. Ruderman, “AgGaS2 optical parametric oscillator continuously tunable from 3.9 to 11.3 μm,” Appl. Phys. Lett. 75(9), 1204–1206 (1999). [CrossRef]
14. D. A. Roberts, “Dispersion equations for nonlinear optical crystals: KDP, AgGaSe2, and AgGaS2,” Appl. Opt. 35(24), 4677–4688 (1996). [CrossRef]