**1. Introduction**

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Photonics puts at stake a wide variety of applications, from applied fields of physics, such as ultrafast all-optical signal processing [1] or pollutant monitoring [2], to more fundamental ones, *e.g.* quantum information [3], and its convergence with electronics at chip-scale level is one of today's great scientific and technological challenges. As a consequence, the full inte‐ gration of optoelectronics devices on existing developed platforms is expected to be the next technological leap, with major breakthroughs in telecommunications, industry and health. While the main building blocks of optical integrated circuitry have been reported in the standard SOI platform [4], coherent light sources still markedly lack to achieve this transi‐ tion of paradigm. To date, the hybrid conjunction of silicon photonics and direct-gap III-V compounds appears to be one of the most promising key technologies towards large-scale photonic integration and scalability [5]. In particular, such photonics platform could capital‐ ize advanced functionalities enabled by guided-wave quadratic nonlinear optics. Thus, the demonstration of the electrically pumped versions of an optical parametric oscillator (OPO) or of a telecom twin-photon source (TTPS) would have a great impact on applications re‐ quiring room-temperature operation and wide tunability.

In this context, the Aluminum Gallium Arsenide (AlGaAs) system is an ideal candidate for the nonlinear photonic design, because of its numerous advantages: high second order susceptibil‐ ity, wide transparency window, good thermal conductivity and monolithic integration.

In order to design efficient frequency converters, the key issue is to keep a constant phase rela‐ tion between the three interacting waves. In general, this is not a trivial task because of the phase-velocity mismatch induced by the material chromatic dispersion [6]. While few phase-

© 2013 Savanier et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Savanier et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

matching strategies have been investigated, an original modal phase-matching scheme based on Bragg reflector waveguides has been recently addressed, reviving the interest for spontane‐ ous parametric down-conversion (SPDC) in AlGaAs-based waveguides [7,8].

off-the-shelf products, whereas QCLs, thanks to a mature and possibly Sb-free technology, are now finding commercial applications and increasingly replace the outdated lead-salt la‐ ser diodes. Nevertheless, integrated semiconductor sources are still lacking around 3 μm, and apart from few QCL products (*e.g.* λ~3.3 μm by Daylight Solutions), solid state and non‐ linear optics-based sources represent the majority of commercially available sources [17]. Recently, intra-cavity second harmonic generation (SHG) has been reported in QCLs, ex‐ tending their emission range to wavelengths as small as 2.7 μm, at the price of poor conver‐

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip

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

61

Nonlinear optics, by means of difference frequency generation (DFG) and optical parametric oscillation, is a well-known alternative to cover the whole 1-10 μm span. The wide variety of spectral/temporal formats allowed by nonlinear χ(2) processes in transparent materials, en‐ dows parametric sources with a high level of flexibility. Moreover, SPDC is currently the most widely used process to generate quantum photon pairs, which have become one of the building blocks of quantum information. To date, room-temperature SPDC has been report‐ ed in passive AlGaAs waveguides designed to perform 0.775-to-1.55 μm down-conversion [7,8], while entanglement has been demonstrated in light emitting diodes only at cryogenic temperature [19]. Thus, the fabrication of an electrically-pumped version of such light source operating at room temperature in the telecom range also constitutes a high-potential

Fulfilling the phase-matching condition is crucial for efficient three-wave mixing. The classi‐ cal approach to cancel out the phase-velocity mismatch between the interacting waves is to rely on the birefringence of the nonlinear medium. The limited choice of suitable materials led to quasi-phase matching (QPM), well established in ferroelectric crystals, with a great impact on the fabrication of infrared parametric sources. QPM consists in a periodic inver‐ sion of nonlinearity along the propagation direction, minimizing the phase-mismatch to al‐ low the nonlinear interaction to build constructively. In this context, the development of bulk dielectric crystals like periodically-poled LiNbO3 (PPLN) has made them the work‐ horse materials of χ(2) optics. Besides, by implementing a guided-wave configuration in which the three optical modes are confined and can interact over several centimeters, nor‐ malized conversion efficiencies up to ~150 %W-1cm-2 have been demonstrated [20], yielding to the demonstration of compact and efficient photon pairs sources [21] and OPOs [22]. Nonetheless, such setups are composed of discrete optical components with critical align‐ ment and do not lend themselves to optoelectronic integration. That is why direct-gap semi‐ conductor compounds, provided that they have significant second-order nonlinearity, are an attractive platform for the coming years' photonics, thanks to mature nano-fabrication technology. Indeed they promise on-chip integration of both efficient frequency converters and laser pumps. Gallium arsenide (GaAs), or more generally the AlGaAs system, is partic‐ ularly interesting because it exhibits a huge second-order nonlinearity (d14~100 pm/V), a broad transparency window (from 0.9 to 17 μm), and a large variety of design and fabrica‐ tion solutions [23]. Because AlGaAs is neither birefringent nor ferroelectric, phase matching

sion efficiency though [18].

and challenging goal.

**2.2. Integration of nonlinear devices**

In this chapter we will focus on AlGaAs-based nonlinear waveguides in which phase-match‐ ing is achieved through form birefringence, artificially induced in optical heterostructures by selective oxidation of Al-rich layers into Aluminum Oxide (referred to as AlOx thereafter). De‐ spite recent technological improvement and promising performances for frequency conver‐ sion in the near [9] and mid-infrared regions [10], neither the OPO nor the TTPS has been demonstrated yet on chip, because of technological issues, mainly excessive propagation loss‐ es and absence of appropriate built-in cavity. In the second section we present the scientific con‐ text of this work, focusing on AlGaAs integrated nonlinear devices exploiting the so-called form birefringence phase-matching scheme. Section three is devoted to the design procedure and the optimization of the fabrication process of two types of partially oxidized waveguides, while their experimental performances are summarized in section four. A comprehensive study of the different loss mechanisms involved is presented in section five, and the design and fabrication of built-in cavity mirrors is described in the sixth section.
