**2. Integrated frequency converters in the infrared**

#### **2.1. Key importance of the near and mid-infrared ranges**

Near and mid-infrared radiation corresponds to the region of the electromagnetic spectrum with wavelengths spanning between 1 and 20 μm. It contains two telecom windows (around 1.3 and 1.5 μm) as well as strong characteristics roto-vibrational lines of many molecules (pollutants, toxins, etc.) and two atmospheric transmission windows (3-5 μm and 8-13 μm), which makes it essential for civilian and military applications such as spectroscopy, material processing, molecular sensing, thermal imaging and defense.

The current state-of-the-art sources developed for these applications can be categorized in three main groups: solid-state and fiber lasers, semiconductor lasers, and parametric sour‐ ces. Although the former include a wide variety of well-known and established emitters, they still remain macroscopic objects falling out of the scope of on-chip integration, and their tunability is severely limited by the discrete energy transitions of the active media [11]. On the contrary, since their first demonstration respectively 50 and almost 20 years ago [12,13], laser diodes and quantum cascade lasers (QCLs) have greatly benefited from the flexibility enabled by the engineering of energy band structure and from clean-room fabrica‐ tion technologies. Yet, these two technological streams hardly overlap in the wavelength range around 3 μm. So far, room temperature operation has only been demonstrated for III-V antimonide laser diodes operating in continuous-wave (CW) regime up to 3.0 μm [14]; and for QCLs (either grown on InP substrate or in the III-V antimonide system) with emis‐ sion wavelength extended down to 3.1 μm in pulsed regime, and to 3.6 μm CW [15,16]. However, because of its controllability and re-growth constraints, commercial producers tend to avoid the use of antimony. That is why antimonide laser diodes are not yet standard 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to a mature and possibly Sb-free technology, are now finding commercial applications and increasingly replace the outdated lead-salt la‐ ser diodes.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‐ sion efficiency though [18].

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 and challenging goal.

#### **2.2. Integration of nonlinear devices**

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‐

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

Near and mid-infrared radiation corresponds to the region of the electromagnetic spectrum with wavelengths spanning between 1 and 20 μm. It contains two telecom windows (around 1.3 and 1.5 μm) as well as strong characteristics roto-vibrational lines of many molecules (pollutants, toxins, etc.) and two atmospheric transmission windows (3-5 μm and 8-13 μm), which makes it essential for civilian and military applications such as spectroscopy, material

The current state-of-the-art sources developed for these applications can be categorized in three main groups: solid-state and fiber lasers, semiconductor lasers, and parametric sour‐ ces. Although the former include a wide variety of well-known and established emitters, they still remain macroscopic objects falling out of the scope of on-chip integration, and their tunability is severely limited by the discrete energy transitions of the active media [11]. On the contrary, since their first demonstration respectively 50 and almost 20 years ago [12,13], laser diodes and quantum cascade lasers (QCLs) have greatly benefited from the flexibility enabled by the engineering of energy band structure and from clean-room fabrica‐ tion technologies. Yet, these two technological streams hardly overlap in the wavelength range around 3 μm. So far, room temperature operation has only been demonstrated for III-V antimonide laser diodes operating in continuous-wave (CW) regime up to 3.0 μm [14]; and for QCLs (either grown on InP substrate or in the III-V antimonide system) with emis‐ sion wavelength extended down to 3.1 μm in pulsed regime, and to 3.6 μm CW [15,16]. However, because of its controllability and re-growth constraints, commercial producers tend to avoid the use of antimony. That is why antimonide laser diodes are not yet standard

ous parametric down-conversion (SPDC) in AlGaAs-based waveguides [7,8].

60 Optoelectronics - Advanced Materials and Devices

fabrication of built-in cavity mirrors is described in the sixth section.

**2. Integrated frequency converters in the infrared**

**2.1. Key importance of the near and mid-infrared ranges**

processing, molecular sensing, thermal imaging and defense.

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 is not a trivial task, especially if the frequencies involved lie close to the material bandgap, where the dispersion is strong. Similarly to lithium niobate, the demonstration of QPM in bulk orientation-patterned GaAs (OP-GaAs) [24] enabled the demonstration of efficient tun‐ able infrared sources, including the first GaAs-based OPO in 2004 [25]. Regarding OP-GaAs waveguides, in addition to their complex fabrication process, their performances are limited by high optical losses due to scattering in the corrugated waveguide core, resulting in mod‐ est normalized conversion efficiencies of ~90 %W-1cm-2 [24]. Another approach, based on the engineering of modal dispersion, enabled the implementation of two additional phasematching strategies:

interactions between the visible and the mid-infrared are then prevented for the simple Al‐

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip

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

63

In 1990, the discovery of selective wet oxidation of Al-rich AlGaAs layers drastically broad‐ ened the potential of form-birefringent phase-matching, thanks to the density, homogeneity and stability of a new type of aluminum oxide [28]. This material exhibits nice optical proper‐ ties, such as a wide transparency window and a low refractive index of ~1.6, and is electrically insulating. A few engineering domains rapidly took advantage of these physical properties: electronics, with field effect transistors [29]; optics, with broadband Bragg mirrors [30]; and op‐ toelectronics, by combining optical and electrical confinement in vertical-cavity surface-emit‐

In the case dealt with in this chapter, the GaAs/AlOx system allows accessing birefringence of several 10-1, *i.e.* up to one order of magnitude higher than in a GaAs/AlAs heterostructure. This in turn enables to phase-match any nonlinear quadratic interaction with wavelengths spanning from the visible to the mid-infrared region [9,10,32]. Despite this substantial ad‐ vantage, this material is not yet completely mature for demanding photonics applications, and its fine understanding is a matter at the intersection of photonics and materials science.

To demonstrate the high potential of the form birefringence phase-matching scheme, we have implemented it into two multi-layered structures designed for the down-conversion of pumps with respective wavelength 1.06 μm and 0.775 μm. The first device is intended to perform as an OPO in the near and mid-infrared regions, while the second one is meant to

For each structure, the design's objective is to balance the combined material and waveguide dispersions with enough induced birefringence, taking into account the following criteria:

**•** the transverse dimensions of the waveguide should favor zero-order modes to maximize their nonlinear overlap integral and avoid injecting power into non phase-matched higher

**•** the aluminum fraction in the guiding core should be low to optimally exploit the χ(2) non‐ linearity of the material, which must also be transparent at every frequency involved in

**•** the aluminum fraction in the claddings should be high enough for a good confinement of the waves, but also guarantee the stability of the material, with no parasitic spontaneous

**•** the number of AlOx layers should be kept as small as possible as their second order sus‐ ceptibility is zero and their quality is expected to be worse than crystalline lattice-match‐

ting lasers (VCSELs) to improve their yield and ensure single-mode emission [31].

**3. Design and fabrication of the devices**

**3.1. Design guidelines of partially oxidized AlGaAs waveguides**

GaAs platform.

operate as a TTPS.

order modes,

oxidation,

ed AlGaAs.

the three-wave mixing,


The latter relies on optical heterostructures, in which thin low-index non-stoeichiometric AlOx layers are intertwined with AlGaAs layers, so to artificially induce the necessary bire‐ fringence to compensate for the chromatic dispersion [10]. For these two schemes, normal‐ ized conversion efficiencies of ~250 %W-1cm-2 and ~1000 %W-1cm-2 have been reported respectively, confirming that nonlinear integrated GaAs-based devices are a credible and promising alternative to standard LiNbO3.

#### **2.3. Form birefringence phase matching scheme**

Since AlGaAs is optically isotropic, the standard birefringent phase-matching scheme can‐ not be implemented. Nevertheless, in a guided-wave configuration, a small anisotropy ap‐ pears as the TE00 and TM00 solutions of the Maxwell equations experience different boundary conditions, hence leading to a non-zero birefringence |n(TE00)-n(TM00)|. The latter can then be tailored for fundamental, orthogonally polarized eigen modes. However this quantity is in general much smaller than the dispersion, so that this technique remains un‐ suitable to phase match any nonlinear interaction.

In order to boost this effect and artificially induce a significant amount of birefringence, one can pattern the waveguide core at sub-wavelength scale, by repeatedly breaking the refrac‐ tive index continuity with a two-material multilayer. The resulting metamaterial behaves as a macroscopic uniaxial crystal, whose birefringence is fully determined by the index contrast and the filling factors of the materials [27]. In particular, this so-called form birefringence phase-matching scheme has been developed in the AlGaAs platform during the late 90's at Thomson CSF laboratory (today Alcatel Thales III-V Lab) [26]. Thanks to the wide variety of index profile designs enabled by the dependence of refractive index with the aluminum fraction, the phase-matching condition can be engineered at will.

The first phase-matched interaction of this type dates back to the seventies, with the dou‐ bling of a CO2 laser emitting at 10.6 μm [27]. In that case, given the weak material dispersion in the mid-infrared (few 10-2) an AlAs/GaAs heterostructure suffices to meet the phasematching condition. However, since the material dispersion strongly increases when the fre‐ quencies of the interacting waves lie close to the bandgap of the material, nonlinear interactions between the visible and the mid-infrared are then prevented for the simple Al‐ GaAs platform.

In 1990, the discovery of selective wet oxidation of Al-rich AlGaAs layers drastically broad‐ ened the potential of form-birefringent phase-matching, thanks to the density, homogeneity and stability of a new type of aluminum oxide [28]. This material exhibits nice optical proper‐ ties, such as a wide transparency window and a low refractive index of ~1.6, and is electrically insulating. A few engineering domains rapidly took advantage of these physical properties: electronics, with field effect transistors [29]; optics, with broadband Bragg mirrors [30]; and op‐ toelectronics, by combining optical and electrical confinement in vertical-cavity surface-emit‐ ting lasers (VCSELs) to improve their yield and ensure single-mode emission [31].

In the case dealt with in this chapter, the GaAs/AlOx system allows accessing birefringence of several 10-1, *i.e.* up to one order of magnitude higher than in a GaAs/AlAs heterostructure. This in turn enables to phase-match any nonlinear quadratic interaction with wavelengths spanning from the visible to the mid-infrared region [9,10,32]. Despite this substantial ad‐ vantage, this material is not yet completely mature for demanding photonics applications, and its fine understanding is a matter at the intersection of photonics and materials science.
