**7. Conclusion and perspectives**

measured after the exit mirror as a function of the external pump powerfocused onto the en‐ trance mirror, while the pump wavelength was set at degeneracy. The nonlinear trend at high pump powers may be evidence that the nonlinear process tends to go beyond the lowgain regime. However, given the estimation of modal reflectivity, the oscillation threshold is currently out of reach. But beyond the demonstration of an integrated OPO, the scope of these technological results is very broad and could concern various types of integrated Al‐

**Figure 14.** Fabry-Perot fringes of bare waveguide (orange) and semi-cavity (green) acquired around 2.12 μm.

**Figure 15.** Generated output signal power vs. Input pump power (external values).The linear fit at low power (red

solid line) points out the superlinear trend at high power.

GaAs-based nonlinear devices.

82 Optoelectronics - Advanced Materials and Devices

In this chapter we have shown that the field of semiconductor sources would largely benefit from the development of integrated AlGaAs nonlinear devices. Indeed, the demonstrations of an integrated OPO or a TTPS, for example, would have a great impact on applications such as optical spectroscopy and quantum telecommunications. The work presented in this chapter is part of the sustained research activity led on these two fronts.

Different technological solutions have been realized to fulfill the phase-matching condition necessary for efficient AlGaAs-based frequency converters. Thus it is interesting to compare their respective characteristics and performances.

It should first be noted that our "OPO structure", designed for guided-wave parametric down-conversion of a 1.06 μm CW pump, cannot be straightforwardly be compared to any other AlGaAs device. Indeed, the closest existing research, which is on OP-GaAs OPOs, is not focusing at the moment on monolithic integration but rather on applications such as metrology or gas spectroscopy. Furthermore, because these setups have been operated in pulsed regime at first, the pump wavelength is fixed above 1.8 μm to avoid two-photon ab‐ sorption in GaAs. Consequently, these crystals are optically pumped around 2 μm, and their degeneracy is typically between 4 and 5 μm.

The "TTPS structure" however is topical, as confirmed by the continuous track record of de‐ sign and experimental results on 0.775-to-1.55 μm frequency conversion in semiconductor devices. An overview of the current state-of-the-art is given by Table 4, in which we report‐ ed the loss values, generated power, normalized conversion efficiency and spectral accept‐ ance of type I CW SHG experiment for several phase-matching schemes. The figures in bold (resp. in italics) correspond to the best (resp. second best) value of each column.

This provides us with a synoptic vision of the strength and weakness of respectively modal phase-matching [57], QPM [24] and form birefringence phase-matching [9,38]. It is then quite clear that, regarding conversion efficiency, form birefringence phase-matching com‐ pare favorably with respect to modal phase-matching and QPM. Moreover, low infrared losses and high generated powers are enabled by optimized design and fabrication process‐ es. The very high losses in the visible are caused by the presence of AlOx layers, and they are the current limiting factor of this phase-matching strategy. The resulting broadening of the χ(2) process spectral acceptance may in turn be an issue for experimental protocols re‐ quiring spectrally narrow and pure sources of telecom twin-photons.


**Table 4.** Summarize of the characteristics and performances of devices implementing different phase-matching schemes.

In conclusion, we have shown that, although the choice of AlGaAs/AlOx nonlinear wave‐ guides is relevant to fabricate highly efficient integrated frequency converters, losses remain the main bottleneck and prevent further breakthrough. Nevertheless, progress has been made by investigating the different loss mechanisms, and a specific technological develop‐ ment of the AlAs oxidation process is expected to reduce further optical losses. Finally, we have demonstrated the feasibility of monolithic integrated Fabry-Perot cavity, by depositing highly reflective dielectric mirrors on the waveguide facets.

[5] Roelkens, G., Liu, L., Liang, D., Jones, R., Fang, A., Koch, B., & Bowers, J. (2010). III-V/silicon photonics for on-chip and intra-chip optical interconnects. *Laser & Photonics*

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip

http://dx.doi.org/10.5772/52201

85

[7] Lanco, L., Ducci, S., Likforman, J. P., Marcadet, X., van Houwelingen, J. A. W., Zbin‐ den, H., Leo, G., & Berger, V. (2006). A semiconductor waveguide source of counter‐

[8] Horn, R., Abolghasem, P., Bijlani, B. J., Kang, D., Helmy, A. S., & Weihs, G. (2012).

[9] Savanier, M., Andronico, A., Lemaître, A., Galopin, E., Manquest, C., Favero, I., Duc‐ ci, S., & Leo, G. (2011). Large second-harmonic generation at 1.55 μm in oxidized Al‐

[10] Ravaro, M., Le Dû, M., Likforman, J. P., Ducci, S., Berger, V., Marcadet, X., & Leo, G. (2007). Estimation of parametric gain in GaAs/AlOx waveguides by fluorescence and second harmonic generation measurements. *Applied Physics Letters*, 91(19), 191110.

[12] Hall, R. N., Fenner, G. E., Kingsley, J. D., Soltys, T. J., & Carlson, R. O. (1962). Coher‐

[13] Faist, J., Capasso, F., Sivco, D. L., Sirtori, C., Hutchinson, A. L., & Cho, A. Y. (1994).

[14] Lehnhardt, T., Hümmer, M., Rößner, K., Müller, M., Höfling, S., & Forchel, A. (2008). Continuous wave single mode operation of GaInAsSb/GaSb quantum well lasers

[15] Laffaille, P., Moreno, J. C., Teissier, R., Bahriz, M., & Baranov, A. N. (2012). High tem‐ perature operation of short wavelength InAs-based quantum cascade lasers. *AIP Ad‐*

[16] Feng, X., Caneau, C., Le Blanc, H. P., Visovsky, N. J., Chaparala, S. C., Deichmann, O. D., Hughes, L. C., Chung-en, Z., Caffey, D. P., & Day, T. (2011). Room temperature CW operation of short wavelength quantum cascade lasers made of strain balanced GaxIn1-xAs/AlyIn1-yAs material on InP substrates. *IEEE Journal of Selected Topics in*

[18] Vizbaras, A., Anders, M., Katz, S., Grasse, C., Boehm, G., Meyer, R., Belkin, M. A., & Amann, M. C. (2011). Room-temperature λ ≈ 2.7 μm quantum cascade laser sources based on intracavity second-harmonic generation. *IEEE Journal of Quantum Electron‐*

[17] The Scott Partnership. (2010). Mid-infrared lasers. *Nature Photonics*, 4(8), 576.

Monolithic source of photon pairs. *Physical Review Letters*, 108(15), 153605.

[6] Sutherland, R. L. (2003). Handbook of nonlinear optics. *New York: Marcel Dekker*.

propagating twin photons. *Physical Review Letters*, 97(17), 173901.

[11] Siegman, A. E. (1986). Lasers, Mill Valley, Calif. *University Science Books*.

emitting beyond 3 μm. *Applied Physics Letters*, 92(18), 183508.

ent light emission from GaAs junctions. *Physical Review Letters*, 9(9), 366.

GaAs waveguides. *Optics Letters*, 36(15), 2955.

Quantum cascade laser. *Science*, 264(5158), 553.

*vances*, 2(2), 022119.

*ics*, 47(5), 691.

*Quantum Electronics*, 17(5), 1445.

*Reviews*, 4(6), 751.
