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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 20 — Oct. 1, 2007
  • pp: 12699–12707
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Enhanced output power for phase-matched second-harmonic generation at 10.6 μm in a ZnGeP2 crystal

Yi Jiang and Yujie J. Ding  »View Author Affiliations


Optics Express, Vol. 15, Issue 20, pp. 12699-12707 (2007)
http://dx.doi.org/10.1364/OE.15.012699


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Abstract

Second-harmonic generation was phase-matched at the fundamental wavelength of 10.6 μm in an annealed ZnGeP2 crystal at room temperature. Our results demonstrated that the phase-matching angle was decreased with increasing the pump power. Such a unique dependence resulted in a significant enhancement on the second-harmonic output power. The highest average output power at 5.3 μm was 55 mW for an average pump power of 5.0 W, which corresponded to a conversion efficiency of 1.1%. Due to the laser-induced heating effect, the second-harmonic output power was increased by 65%. Such an efficient conversion was made possible also by using a short-pulse repetition-frequency-excited waveguide and single-longitudinal-mode CO2 laser as a fundamental beam.

© 2007 Optical Society of America

1. Introduction

A CO2 laser produces radiations at a number of wavelengths in the range of 9.2-11.4 μm. Second-harmonic generation (SHG) using the output of such a laser as a fundamental beam can extend the output wavelengths down to the range of 4.6-5.8 μm. Such a frequency conversion still plays an important role for the efficient generation of mid-IR frequencies since other approaches such as quantum-cascade lasers produce relatively low powers at the emission wavelengths below 5.4 μm at room temperature. To successfully achieve an efficient conversion from the CO2 laser frequencies, one needs a nonlinear medium with a large figure of merit. In the past, a series of nonlinear materials were studied such as GaSe [1

1. D. R. Suhre, N. B. Singh, V. Balakrishna, N. C. Fernelius, and F. K. Hopkins, “Improved crystal quality and harmonic generation in GaSe doped with indium,” Opt. Lett. 22, 775–777 (1997). [CrossRef] [PubMed]

], ZnGeP2 [2

2. G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, “Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal,” Sov. J. Quantum Electron. 19, 494–498 (1989). [CrossRef]

], CdGeAs2 [3

3. N. Menyuk, G. W. Iseler, and A. Mooradian, “High-efficiency high-average-power second-harmonic generation with CdGeAs2,” Appl. Phys. Lett. 29, 422–424 (1971). [CrossRef]

], AgGaS2 [4

4. A. Harasaki and K. Kato, “New data on the nonlinear optical constant, phase-matching, and optical damage of AgGaS2,” Jpn. J. Appl. Phys. 36, 700–703 (1997). [CrossRef]

], and AgGaSe2 [5

5. R. Eckardt, Y. Fan, R. Byer, R. Route, R. Feigeison, and J. Laan, “Efficient second harmonic generation of 10- μm radiation in AgGaSe2,” Appl. Phys. Lett. 47, 786–788 (1985). [CrossRef]

]. It is worth noting that AgGaSe2 has produced the highest conversion efficiency out of these four nonlinear-optical crystals for the same pump power [5

5. R. Eckardt, Y. Fan, R. Byer, R. Route, R. Feigeison, and J. Laan, “Efficient second harmonic generation of 10- μm radiation in AgGaSe2,” Appl. Phys. Lett. 47, 786–788 (1985). [CrossRef]

]. Among them, ZnGeP2 has a potential for SHG from CO2 lasers since it possesses a wide transparency range (0.74-12 μm) and a large nonlinear coefficient (d36 ≈ 75 pm/V). Therefore, such a crystal has been used for frequency-doubling CO2 laser radiations and implementing optical parametric oscillators [6

6. P. B. Phua, B. S. Tan, R. F. Wu, K. S. Lai, L. Chia, and E. Lau, “High-average-power mid-infrared ZnGeP2 optical parametric oscillators with a wavelength-dependent polarization rotator,” Opt. Lett. 31, 489–491 (2006). [CrossRef] [PubMed]

]. To date, the highest conversion efficiency for the SHG is 49% for converting the laser beam at 9.52 μm in a ZnGeP2 crystal by using an extremely high peak power of the fundamental beam [7

7. Yu. M. Andreev, V. Yu. Baranov, V. G. Voevodin, P. P. Geiko, A. I. Bribenyukov, S. V. Izyumov, S. M. Kozochkin, V. D. Pis’mennyi, Yu. A. Satov, and A. P. Strel’tsov, “Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse,” Sov. J. Quantum Electron. 17, 1435–1436 (1987). [CrossRef]

]. Due to the amount of the birefringence available for a ZnGeP2 crystal, most papers claimed that it is not possible to phase-match SHG at 10.6 μm from a ZnGeP2 crystal [2

2. G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, “Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal,” Sov. J. Quantum Electron. 19, 494–498 (1989). [CrossRef]

,8

8. G. D. Boyd and F. G. Stortz, “Linear and nonlinear optical properties of ZnGeP2 and CdSe,” Appl. Phys. Lett. 18, 301–303 (1971). [CrossRef]

,9

9. G. C. Bhar and G. C. Ghosh, “Temperature dependent phase-matched nonlinear optical devices using CdSe and ZnGeP2,” IEEE J. Quantum Electron. 16, 838–843 (1980). [CrossRef]

,10

10. F. Madarasz, J. Dimmock, D. Dietz, and K. Bachmann, “Sellmeier paprameters for ZnGeP2 and GaP,” J. Appl. Phys. 87, 1564–1565 (2000). [CrossRef]

,11

11. K. Kato, E. Takaoka, and N. Umemura, “New sellmeier and thermo-optic dispersion formulas for ZnGeP2,” CLEO 2003, paper CtuM17.

]. Although a few papers claimed that such a conversion process can be still phase-matched [12

12. K. Kato, “Second-harmonic and sum-frequency generation in ZnGeP2,” Appl. Opt. 36, 2506–2510 (1997). [CrossRef] [PubMed]

,13

13. S. Das, G. Bhar, S. Gangopadhyay, and C. Ghosh, “Linear and nonlinear optical properties of ZngeP2 crystal for infrared laser device applications: revisited,” Appl. Opt. 42, 4335–4340 (2003). [CrossRef] [PubMed]

], there have been no reports on the direct measurement of the conversion efficiency for such a SHG process. Even the SNLO softwares confirmed that it would not be possible to achieve phase-matched SHG at 10.6 μm in a ZnGeP2 crystal at room temperature. Since only a few crystals can be practically used to achieve SHG at 10.6 μm, the investigation of the phase-matching condition for SHG at 10.6 μm in a ZnGeP2 crystal is important.

In this paper we report our results of SHG at 10.6 μm from a CO2 laser in a ZnGeP2 crystal at room temperature. In terms of linear and nonlinear optical properties, this crystal is quite similar to those investigated previously [14

14. D. E. Zelmon, E. A. Hanning, and P. G. Schunemann, “Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide form 2 to 9 μm with implications for phase matching in optical frequency-conversion devices,” J. Opt. Soc. Am. B 18, 1307–1310 (2001). [CrossRef]

, 15

15. W. Shi, Y. J. Ding, and P. G. Schunemann, “Coherent terahertz waves based on difference-frequency generation in an annealed zinc-germanium phosphide crystal: improvements on tuning ranges and peak powers,” Opt. Commun. 233, 183–189 (2004). [CrossRef]

]. We have observed that as the pump power was increased the phase-matching angle was decreased. This was caused by the increase of the crystal temperature induced by the increased absorption of the crystal for the pump power. Consequently, the output power was significantly increased since the effective nonlinear coefficient for the SHG strongly depends on the phase-matching angle. Although quantum cascade lasers can be used to generate an output at 5.3 μm, frequency-doubling a CO2 laser radiation in a ZnGeP2 crystal can be used to produce a peak power as high as 127 W following our experimental result. Out of the total output power of 127 W, 50 W originate from the laser-induced heating effect, which is amounted to 65% increase in the output power. Moreover, such a high-power source does not require any cooling device.

2. Experimental details

An annealed ZnGeP2 crystal having dimensions of 14×15×20.6 mm3 was cut and polished along x axis. It was mounted on a rotational stage for rotating it around y axis with an angle resolution of 0.1°. A fundamental beam was propagating in the x-z plane with its polarization lying in the x-z plane. Such a configuration corresponds to an ee-o type-I phase-matching configuration, where e and o specify the extraordinary and ordinary polarizations for the fundamental and SH beams inside the ZnGeP2 crystal, respectively. A short-pulse repetition-frequency-excited CO2 waveguide laser was used as a pump beam. Such a laser system was lasing at a single longitudinal mode. Its pulse width was measured to be 12 ns; its repetition rate was adjustable from 0 to 100 kHz; its linewidth was about 100 MHz. For our SHG study, we set the repetition rate to 60 kHz. The average output power can be continuously increased from 0 W to 5 W by using a λ/2 plate and a polarizer. A short-pass filter was placed behind the ZnGeP2 crystal to cut off the transmitted (i.e. un-converted) 10.6 μm radiation. A photovoltaic detector was used to investigate the characteristics of the SHG. The SH output powers were measured by using a power meter.

3. Results and discussions

A transmission spectrum was first measured on the 14-mm-long ZnGeP2 crystal using a FTIR system within a wavelength range from 2 μm to 12 μm, see inset to Fig. 1. The transmittances reduced by the losses through the Fresnel reflections of the entrance and exit facets for the infrared beam were obtained by using the Sellmeier equation given in Ref. [16

16. G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, “Tunable parametric ZnGeP2 crystal oscillator,” Sov. J. Quantum Electron. 17, 860–861 (1987). [CrossRef]

]. One can see that the transmittances are between 0.536 and 0.552. Using these Fresnel reflections, a spectrum of the absorption coefficient can be obtained, see Fig. 1. In particular, the absorption coefficients were determined to be 0.76 cm-1 and 0.15 cm-1 at 10.6 μm and 5.3 μm, respectively. One can see that since the absorption coefficient for the fundamental beam is much higher, it should be used to determine the optimum length of the ZnGeP2 crystal when a SHG experiment is designed.

Fig. 1. Spectrum of absorption coefficient of ZnGeP2 crystal determined from measurements of transmittances within 2-12 μm using FTIR. Inset: transmission spectrum measured on 14-mm-long ZnGeP2 crystal; dashed horizontal line represent transmittances limited by Fresnel-reflection losses.

The output power for the SH beam was then measured as a function of external phase-matching angle under two different powers for the fundamental beam. Based on these measurements and indices of the refraction obtained from the Sellmeier equations [16

16. G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, “Tunable parametric ZnGeP2 crystal oscillator,” Sov. J. Quantum Electron. 17, 860–861 (1987). [CrossRef]

], the SH power was plotted vs. internal phase-matching angle formed between the propagation direction of the fundamental beam and optic axis (θ), see Figs. 2(a) and 2(b). According to our results, the internal phase-matching angle was 84.9°±0.03° at the input power of 300 mW. However, this angle was decreased to 82.1°±0.03° when the input power was increased to 5.0 W. On the other hand, since the peak fundamental and SH powers are quite important for the nonlinear frequency conversion, the pulse widths of the fundamental and SH beams were measured by us using a photovoltaic detector with a temporal resolution of better than 1 ns, see Fig. 3. One can see from Fig. 3 that the pulse widths of the fundamental and SH beams were measured to be 12 ns and 7.2 ns, respectively.

A series of the phase-matching angles were subsequently measured for different input powers of the fundamental beam from 150 mW to 5.0 W (Fig. 4). As one can see from Fig. 4, with increasing the input power, the external phase-matching angle was decreased from 74.1°±0.1° to 64.4°±0.1° whereas the internal phase-matching angle was decreased from 84.9°±0.03° to 82.1°±0.03°. Such intensity-dependent phase-matching angles have not been reported previously on ZnGeP2 crystals to the best of our knowledge.

Fig. 2. Phase-matching angles measured at input power of fundamental beam: (a) 300 mW and (b) 5.0 W. Dots correspond to experimental data; solid curves correspond to fitting to data by using sinc2 function.
Fig. 3. Temporal profiles of fundamental and SH pulses.

According to Fig. 1, a ZnGeP2 crystal has a relatively large absorption coefficient at 10.6 μm. Therefore, a significant amount of the input power for the fundamental beam was absorbed by the crystal, followed by the conversion of the absorbed energy into heat. Such a laser-induced heating effect in the crystal results in the increase of the temperature of the crystal. Based on Ref. [8

8. G. D. Boyd and F. G. Stortz, “Linear and nonlinear optical properties of ZnGeP2 and CdSe,” Appl. Phys. Lett. 18, 301–303 (1971). [CrossRef]

], the refractive indices of ZnGeP2 crystal strongly depend on the temperature of the crystal:

(dno/dT)10.6μm=15.40×105K1(dne/dT)10.6μm=16.84×105K1
(1a)
(dno/dT)5.3μm=14.49×105K1(dne/dT)5.3μm=15.42×105K1
(1b)

We have fitted the difference between the measured values of the phase-matching angles for the pump powers of 150 mW and 5 W (see Fig. 4) by using the Sellmeier equations from Ref. [12

12. K. Kato, “Second-harmonic and sum-frequency generation in ZnGeP2,” Appl. Opt. 36, 2506–2510 (1997). [CrossRef] [PubMed]

], see our discussion in Section 4 below. As a result, the temperature rise of the crystal was determined to be 51 K using Eqs. (1) above. Even though this temperature rise was within the temperature phase-matching bandwidth of 62 K deduced from Ref. [12

12. K. Kato, “Second-harmonic and sum-frequency generation in ZnGeP2,” Appl. Opt. 36, 2506–2510 (1997). [CrossRef] [PubMed]

], the temperature gradient along the crystal may affect the conversion efficiency. The corresponding increases in the indices of refraction for the fundamental and SH beams were determined to be in the range of 0.0072-0.0084.

Fig. 4. External (red dots) and internal (black squares) phase-matching angles versus input power of fundamental beam in the range from 150 mW to 5.0 W. Internal phase-matching angles were calculated using the Sellmeier equation taken from Ref. [10].

On the other hand, one can estimate the temperature rise due to the absorption of the fundamental beam by the crystal. Assuming no heat dissipation within each pulse of the fundamental beam, the maximum temperature rise for each pulse can be estimated by using the following expression:

(ΔT)max=α1P1T1τ1Cpρπw2
(2)

where C p is the heat capacity, P 1 is the peak power of each fundamental pulse, τ1 is the pulse width of each fundamental pulse, T 1 is the transmittance for the fundamental beam propagating through the entrance facet, α1 is the absorption coefficient at 10.6 μm, and w is the beam radius for the fundamental beam. For C p ≈ 464 J/kg∙K, ρ ≈ 4.12 g/cm3, P 1 ≈ 6.9 kW, τ1 ≈ 12 ns, α1 ≈ 0.76 cm-1, T 1 ≈ 0.74, and w ≈ 0.2 mm, (ΔT)max is estimated to be 0.019 K. This implies that in order to produce a temperature rise of 51 K the number of pulses required on the fundamental beam required is at least 2600. Based on such an estimate, heat dissipation has played an important role after such a large number of the laser pulses. A more accurate determination of the temperature distribution within the ZnGeP2 crystal will require an extensive effort on numerical modeling and experiments, which is beyond the scope of this paper.

For the type-I phase-matching configuration in a ZnGeP2 crystal, the phase-matching condition is given by ne ω = no , where ne ω and no are the extraordinary and ordinary refractive indices for the fundamental and SH waves, respectively, see Fig. 5. When the crystal is heated up by absorbing some portion of the fundamental beam, the extraordinary refractive index is increased by an amount larger than the ordinary refractive index. Such a difference in the increases of the refractive indices for two different polarizations results in the enhancement of the birefringence for the ZnGeP2 crystal, and therefore, the decrease of the phase-matching angle, see Fig. 5. We would like to emphasize that even without any laser-induced heating effect the SHG can be still phase-matched at an angle measured to be 84.9°±0.03°, see Fig. 2.

Fig. 5. Smaller circle and dashed ellipse represent ordinary and extraordinary refractive indices for SH and fundamental beams. After crystal is heated up by laser beam, smaller circle and ellipse are enlarged. As illustrated here, phase-matching angle is therefore reduced.

For each input power of the fundamental beam in the range of 1.0-5.0 W, a maximum SH output power was measured by adjusting the phase-matching angle θ, see Fig. 6. One can see from Fig. 6 that the SH output powers measured by us are higher than those predicted by a quadratic dependence. Indeed, at the highest input power of 5.0 W, the average SH output power reached 55 mW whereas the peak power was 127 W. This peak power was actually 50 W higher than the value predicted by using the quadratic dependence, which implies that this amount of the output power was added after the laser-induced heating effect. For the ee-o phase-matching configuration in a ZnGeP2 crystal, the effective second-order nonlinear coefficient is given by d eff = d 36sin(2θ). Based on such an expression, one can see that when the phase-matching angle of below and close to 90° is decreased, the effective second-order nonlinear coefficient is increased. Consequently, the SH output power is also increased. By incorporating the measured values of θ into our data, we have modified the quadratic dependence illustrated in Fig. 6. One can see from Fig. 6 that the modified dependence fits our experimental values quite well. According to our experimental result, the conversion efficiency for the input power of the fundamental beam to be 5.0 W was 1.1% (a normalized conversion efficiency of 110% MW-1∙cm-1).

It can be readily shown that the conversion efficiency for the SHG in terms of input and output peak powers can be calculated by using the following expression [17

17. V. G. Dmitriviev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals (Springer, New York, 1997), p. 50.

]:

η=P2P1=8π2deff2L2ε0c(neω)2no2ωλ22P1πw2T2T12eα1L[1exp(ΔαL/2)ΔαL/2]2
(3)

where P 2 is the SH peak power, L is the crystal length, λ2 is the wavelength of the SH beam, T 2 is the Fresnel transmittance for the SH beam, and α2 is the absorption coefficient for the SH beam. For L ≈ 14 mm, w ≈ 0.2 mm, α1 ≈ 0.76 cm-1, and α2 ≈ 0.15 cm-1, the conversion efficiency was calculated to be 1.9%, which in a very good agreement with our measured value.

Fig. 6. Dots - output SH powers for different input powers of fundamental beam. Solid line -quadratic dependence predicted by theory. Dashed line - dependence of output SH power on input power modified by using phase-matching angles measured by us.

4. Comparison with previous results

Figure 7 illustrates the dependences of the internal phase-matching angles on the temperatures of the crystal, which were calculated by using five different sets of the Sellmeier equations [10–13

10. F. Madarasz, J. Dimmock, D. Dietz, and K. Bachmann, “Sellmeier paprameters for ZnGeP2 and GaP,” J. Appl. Phys. 87, 1564–1565 (2000). [CrossRef]

,16

16. G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, “Tunable parametric ZnGeP2 crystal oscillator,” Sov. J. Quantum Electron. 17, 860–861 (1987). [CrossRef]

]. One can see from Fig. 7 that the phase-matching angles calculated by using the equations given in Ref. [12

12. K. Kato, “Second-harmonic and sum-frequency generation in ZnGeP2,” Appl. Opt. 36, 2506–2510 (1997). [CrossRef] [PubMed]

] are the closest to the angles measured by us.

As mentioned above, since the CO2 laser we used was lasing at the single longitudinal mode, it had a very narrow linewidth of ∼ 100 MHz (∼ 0.037 nm at 10.6 μm), which was much narrower than most of the CO2 lasers used for frequency doubling in ZnGeP2 crystals in the past. Second, as illustrated in Fig. 3, the temporal profile of our CO2 laser is nearly symmetric unlike a TEA CO2 laser system which typically has a temporal profile of a 100 ns-long spike followed by a 1-μs-long tail. Because of the superior performance of our CO2 laser, an efficient frequency conversion was achieved from the annealed ZnGeP2 crystal. As summarized above, the highest conversion efficiency achieved by us was 1.1% for a peak intensity of 5.5 MW/cm2 (a peak power of 6.9 kW) in a 14-mm-long ZnGeP2 crystal. In comparison, the conversion efficiency for a 15-mm-long GaSe crystal was measured to be about 3.0% under the same experimental conditions, which is a factor of 2.7 higher. Third, the phase-matching wavelength for the fundamental beam is now extended to 10.6 μm. According to Ref. [2

2. G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, “Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal,” Sov. J. Quantum Electron. 19, 494–498 (1989). [CrossRef]

], the conversion efficiency at 10.3 μm was 6.2% for the input power of 68 kW in a 42-mm-long ZnGeP2 crystal. Therefore, the normalized conversion efficiency obtained previously [2

2. G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, “Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal,” Sov. J. Quantum Electron. 19, 494–498 (1989). [CrossRef]

] was 22% MW-1∙cm-1. This value is a factor of 5 lower than the value measured in our experiment, i.e. 110% MW-1∙cm-1.

Fig. 7. Curves - internal phase-matching (PM) angle vs. temperature of the crystal (bottom x axis), calculated based on five different sets of the Sellmeier equations: red - Ref. [13]; black - Ref. [12]; green - Ref. [11]; blue - Ref. [16]; and cyan - Ref. [10]. Black dots correspond to our measurement of internal PM angle vs. pump power (top x axis).

At a much higher input power, i.e. 1 GW/cm2, (a peak power of 10 MW) the conversion efficiency was measured to be 49% in a 3-mm-long ZnGeP2 crystal at the fundamental wavelength of 9.52 μm [7

7. Yu. M. Andreev, V. Yu. Baranov, V. G. Voevodin, P. P. Geiko, A. I. Bribenyukov, S. V. Izyumov, S. M. Kozochkin, V. D. Pis’mennyi, Yu. A. Satov, and A. P. Strel’tsov, “Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse,” Sov. J. Quantum Electron. 17, 1435–1436 (1987). [CrossRef]

]. In order to compare this result with ours, we have deduced the conversion efficiency to be 0.24% for a 3-mm-long ZnGeP2 crystal from our result based on linear scaling with the crystal length. After considering the spatial depletion for the fundamental beam, we have estimated the conversion efficiency from our crystal to be 53% when the input power of the fundamental beam is increased from 6.9 kW to 10 MW. This estimated value is quite close to that measured in Ref. [7

7. Yu. M. Andreev, V. Yu. Baranov, V. G. Voevodin, P. P. Geiko, A. I. Bribenyukov, S. V. Izyumov, S. M. Kozochkin, V. D. Pis’mennyi, Yu. A. Satov, and A. P. Strel’tsov, “Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse,” Sov. J. Quantum Electron. 17, 1435–1436 (1987). [CrossRef]

]. Based on the two comparisons made above, one can see that the conversion efficiency for the SHG phase-matched at the fundamental wavelength of 10.6 μm achieved by us is quite high. Such a relatively high conversion efficiency is due to the advantage of the laser-induced heating effect which enhanced the conversion efficiency for the SHG at 10.6 μm.

5. Conclusion

We have observed the second-harmonic generation phase-matched at the fundamental wavelength of 10.6 μm at room temperature in a ZnGeP2 crystal. Our experimental result demonstrates that the phase-matching angle is decreased with increasing the input power of the fundamental beam. Such a decrease in the phase-matching angle is caused by heating of the crystal induced by the significant absorption of the fundamental laser beam. The decrease of the phase-matching angle has been used by us to enhance the SH output powers and conversion efficiencies. For an input power of the fundamental beam to be 5.0 W, the SH output power was measured to be 55 mW which corresponds to a peak power of 127 W at 5.3 μm. It is worth noting that 39% of the generated SH power was provided by the laser-induced heating effect. The highest conversion efficiency for the SHG was measured to be 1.1%. Although other crystals such as AgGaSe2 can be used to produce much higher conversion efficiencies for the SHG the CO2 laser at the emission wavelength of 10.6 μm, our results indicated that ZnGeP2 crystal can be also used to achieve phase-matched SHG at 10.6 μm.

In the future, we would like to investigate the possibility of using the increase of the indices of refraction evidenced in our experiment to spatially confine the fundamental and SH waves, similar to the increase of indices of refraction studied in periodically-poled LiNbO3 [18

18. X. Mu and Y. J. Ding, “Optical-parametric generation and oscillation in periodically-poled lithium niobate in the presence of strong two-photon absorption,” Opt. Commun. 242, 305–312 (2004). [CrossRef]

]. As discussed by us above, the highest increases of the indices of refraction are estimated to be in the range of 0.0072-0.0084 for the fundamental and SH beams. Therefore, the increases in the indices of refraction may be sufficient to confine the fundamental and SH beams. Such a waveguide confinement may result in a further enhancement on the SH power.

Our high-power mid-IR source has potential applications in remote sensing. For example, since NO molecules have an absorption peak around 5.2 μm, our mid-IR source can be used to identify and to perhaps detect such molecules. Using different emission lines from CO2 lasers, one can produce the SH outputs with the wavelengths in the range of 4.6-5.8 μm. Therefore, such a frequency conversion device may be useful to countermeasures.

Acknowledgments

We are indebted to P. G. Schunemann for supplying us with the ZnGeP2 crystal. This work has been supported by U.S. AFOSR.

References and links

1.

D. R. Suhre, N. B. Singh, V. Balakrishna, N. C. Fernelius, and F. K. Hopkins, “Improved crystal quality and harmonic generation in GaSe doped with indium,” Opt. Lett. 22, 775–777 (1997). [CrossRef] [PubMed]

2.

G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, “Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal,” Sov. J. Quantum Electron. 19, 494–498 (1989). [CrossRef]

3.

N. Menyuk, G. W. Iseler, and A. Mooradian, “High-efficiency high-average-power second-harmonic generation with CdGeAs2,” Appl. Phys. Lett. 29, 422–424 (1971). [CrossRef]

4.

A. Harasaki and K. Kato, “New data on the nonlinear optical constant, phase-matching, and optical damage of AgGaS2,” Jpn. J. Appl. Phys. 36, 700–703 (1997). [CrossRef]

5.

R. Eckardt, Y. Fan, R. Byer, R. Route, R. Feigeison, and J. Laan, “Efficient second harmonic generation of 10- μm radiation in AgGaSe2,” Appl. Phys. Lett. 47, 786–788 (1985). [CrossRef]

6.

P. B. Phua, B. S. Tan, R. F. Wu, K. S. Lai, L. Chia, and E. Lau, “High-average-power mid-infrared ZnGeP2 optical parametric oscillators with a wavelength-dependent polarization rotator,” Opt. Lett. 31, 489–491 (2006). [CrossRef] [PubMed]

7.

Yu. M. Andreev, V. Yu. Baranov, V. G. Voevodin, P. P. Geiko, A. I. Bribenyukov, S. V. Izyumov, S. M. Kozochkin, V. D. Pis’mennyi, Yu. A. Satov, and A. P. Strel’tsov, “Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse,” Sov. J. Quantum Electron. 17, 1435–1436 (1987). [CrossRef]

8.

G. D. Boyd and F. G. Stortz, “Linear and nonlinear optical properties of ZnGeP2 and CdSe,” Appl. Phys. Lett. 18, 301–303 (1971). [CrossRef]

9.

G. C. Bhar and G. C. Ghosh, “Temperature dependent phase-matched nonlinear optical devices using CdSe and ZnGeP2,” IEEE J. Quantum Electron. 16, 838–843 (1980). [CrossRef]

10.

F. Madarasz, J. Dimmock, D. Dietz, and K. Bachmann, “Sellmeier paprameters for ZnGeP2 and GaP,” J. Appl. Phys. 87, 1564–1565 (2000). [CrossRef]

11.

K. Kato, E. Takaoka, and N. Umemura, “New sellmeier and thermo-optic dispersion formulas for ZnGeP2,” CLEO 2003, paper CtuM17.

12.

K. Kato, “Second-harmonic and sum-frequency generation in ZnGeP2,” Appl. Opt. 36, 2506–2510 (1997). [CrossRef] [PubMed]

13.

S. Das, G. Bhar, S. Gangopadhyay, and C. Ghosh, “Linear and nonlinear optical properties of ZngeP2 crystal for infrared laser device applications: revisited,” Appl. Opt. 42, 4335–4340 (2003). [CrossRef] [PubMed]

14.

D. E. Zelmon, E. A. Hanning, and P. G. Schunemann, “Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide form 2 to 9 μm with implications for phase matching in optical frequency-conversion devices,” J. Opt. Soc. Am. B 18, 1307–1310 (2001). [CrossRef]

15.

W. Shi, Y. J. Ding, and P. G. Schunemann, “Coherent terahertz waves based on difference-frequency generation in an annealed zinc-germanium phosphide crystal: improvements on tuning ranges and peak powers,” Opt. Commun. 233, 183–189 (2004). [CrossRef]

16.

G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, “Tunable parametric ZnGeP2 crystal oscillator,” Sov. J. Quantum Electron. 17, 860–861 (1987). [CrossRef]

17.

V. G. Dmitriviev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals (Springer, New York, 1997), p. 50.

18.

X. Mu and Y. J. Ding, “Optical-parametric generation and oscillation in periodically-poled lithium niobate in the presence of strong two-photon absorption,” Opt. Commun. 242, 305–312 (2004). [CrossRef]

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4400) Nonlinear optics : Nonlinear optics, materials

ToC Category:
Nonlinear Optics

History
Original Manuscript: August 14, 2007
Revised Manuscript: September 6, 2007
Manuscript Accepted: September 16, 2007
Published: September 19, 2007

Citation
Yi Jiang and Yujie J. Ding, "Enhanced output power for phase-matched second-harmonic generation at 10.6 μm in a ZnGeP2 crystal," Opt. Express 15, 12699-12707 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-20-12699


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References

  1. D. R. Suhre, N. B. Singh, V. Balakrishna, N. C. Fernelius, and F. K. Hopkins, "Improved crystal quality and harmonic generation in GaSe doped with indium," Opt. Lett. 22, 775-777 (1997). [CrossRef] [PubMed]
  2. G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov, and N. I. Chapliev, "Efficient generation of the second harmonic of CO2 laser radition in a GaSe crystal," Sov. J. Quantum Electron. 19, 494-498 (1989). [CrossRef]
  3. N. Menyuk, G. W. Iseler, and A. Mooradian, "High-efficiency high-average-power second-harmonic generation with CdGeAs2," Appl. Phys. Lett. 29, 422-424 (1971). [CrossRef]
  4. A. Harasaki and K. Kato, "New data on the nonlinear optical constant, phase-matching, and optical damage of AgGaS2," Jpn. J. Appl. Phys. 36, 700-703 (1997). [CrossRef]
  5. R. Eckardt, Y. Fan, R. Byer, R. Route, R. Feigeison, and J. Laan, "Efficient second harmonic generation of 10- µm radiation in AgGaSe2," Appl. Phys. Lett. 47, 786-788 (1985). [CrossRef]
  6. P. B. Phua, B. S. Tan, R. F. Wu, K. S. Lai, L. Chia, and E. Lau, "High-average-power mid-infrared ZnGeP2 optical parametric oscillators with a wavelength-dependent polarization rotator," Opt. Lett. 31, 489-491 (2006). [CrossRef] [PubMed]
  7. Yu. M. Andreev, V. Yu. Baranov, V. G. Voevodin, P. P. Geiko, A. I. Bribenyukov, S. V. Izyumov, S. M. Kozochkin, V. D. Pis’mennyi, Yu. A. Satov, and A. P. Strel’tsov, "Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse," Sov. J. Quantum Electron. 17, 1435-1436 (1987). [CrossRef]
  8. G. D. Boyd and F. G. Stortz, "Linear and nonlinear optical properties of ZnGeP2 and CdSe," Appl. Phys. Lett. 18, 301-303 (1971). [CrossRef]
  9. G. C. Bhar and G. C. Ghosh, "Temperature dependent phase-matched nonlinear optical devices using CdSe and ZnGeP2," IEEE J. Quantum Electron. 16, 838-843 (1980). [CrossRef]
  10. F. Madarasz, J. Dimmock, D. Dietz, and K. Bachmann, "Sellmeier paprameters for ZnGeP2 and GaP," J. Appl. Phys. 87, 1564-1565 (2000). [CrossRef]
  11. K. Kato, E. Takaoka, and N. Umemura, "New sellmeier and thermo-optic dispersion formulas for ZnGeP2," CLEO 2003, paper CtuM17.
  12. K. Kato, "Second-harmonic and sum-frequency generation in ZnGeP2," Appl. Opt. 36, 2506-2510 (1997). [CrossRef] [PubMed]
  13. S. Das, G. Bhar, S. Gangopadhyay, and C. Ghosh, "Linear and nonlinear optical properties of ZngeP2 crystal for infrared laser device applications: revisited," Appl. Opt. 42, 4335-4340 (2003). [CrossRef] [PubMed]
  14. D. E. Zelmon, E. A. Hanning, and P. G. Schunemann, "Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide form 2 to 9 µm with implications for phase matching in optical frequency-conversion devices," J. Opt. Soc. Am. B 18, 1307-1310 (2001). [CrossRef]
  15. W. Shi, Y. J. Ding, and P. G. Schunemann, "Coherent terahertz waves based on difference-frequency generation in an annealed zinc-germanium phosphide crystal: improvements on tuning ranges and peak powers," Opt. Commun. 233, 183-189 (2004). [CrossRef]
  16. G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, "Tunable parametric ZnGeP2 crystal oscillator," Sov. J. Quantum Electron. 17, 860-861 (1987). [CrossRef]
  17. V. G. Dmitriviev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals (Springer, New York, 1997), p. 50.
  18. X. Mu and Y. J. Ding, "Optical-parametric generation and oscillation in periodically-poled lithium niobate in the presence of strong two-photon absorption," Opt. Commun. 242, 305-312 (2004). [CrossRef]

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