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

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 phase-

**1.** modal phase-matching, involving transverse modes of different order at the price of a

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

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‐

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

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

matching strategies:

worse nonlinear overlap integral [8]; and

**2.** form-birefringent phase matching.

62 Optoelectronics - Advanced Materials and Devices

promising alternative to standard LiNbO3.

**2.3. Form birefringence phase matching scheme**

suitable to phase match any nonlinear interaction.

fraction, the phase-matching condition can be engineered at will.

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

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 operate as a TTPS.

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:


Moreover, our choice is for type I phase-matching, rather than type II, as it requires a small‐ er amount of birefringence (and therefore of AlOx). According to this configuration and the non-zero elements of the second order susceptibility tensor of GaAs, quadratic interactions are only possible between a combination of two low frequency TE modes (*i.e.* with polariza‐ tion in the layers plane) and a higher frequency TM mode (*i.e.* with polarization perpendicu‐ lar to the layers plane).

For technological reasons, the thickness of the Al-rich layers to be oxidized must be com‐ prised between few nanometers and few microns. Indeed, the diffusion of oxidant species along very thin layers is impeded, and the overall mechanical stability of thick oxidized lay‐ ers is critical. However, this mechanical instability can be mitigated by adding a small amount of gallium to the thin AlAs layers [33]. As a result, we have chosen to deal with ~30 nm layers of Al0.98Ga0.02As (basically AlAs). Note that all the thicknesses of the oxidized lay‐ ers are fixed to be the same throughout the whole structure, so to optimally calibrate the oxi‐ dation process and benefit from the best experimental conditions.

**Figure 1.** Refractive index profile (black line), phase-matched TE0 (red line), and TM0 (blue line) modes at 2.12 μm and

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip

**Layer Composition Thickness (nm) Repetition**

× 4

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65

× 8

× 4

Al0.8Ga0.2As 166

Al0.98Ga0.02As 37.5

Al0.25Ga0.75As 166

Al0.98Ga0.02As 37.5

Al0.8Ga0.2As 166

Al0.98Ga0.02As 37.5

**Figure 2.** Refractive index profile (black line), phase-matched TE0 (red line), and TM0 (blue line) modes at 1.55 μm and

1.06 μm respectively, represented along the growth direction.

Cladding

Core

Cladding

Cap GaAs 30

Buffer Al0.8Ga0.2As 100 Substrate GaAs -

**Table 2.** Multilayer sequence for 0.775 μm to 1.55 μm frequency conversion.

0.775 μm respectively, represented along the growth direction.

While for the first "OPO structure", the fulfillment of the phase-matching condition requires the insertion of only five AlOx layers in the GaAs core, the situation differs in two aspects for the "TTPS structure":


In both cases, the thickness of the (Al)GaAs layers surrounded by oxide layers is set to tune the degeneracy wavelength. The two final designs are given in Tables 1 and 2, whereas the corresponding refractive index profiles and the intensity distributions of the phase-matched modes at degeneracy are shown in Figures 1 and 2.


**Table 1.** Multilayer sequence for 1.06 μm to 2.12 μm frequency conversion.

**Figure 1.** Refractive index profile (black line), phase-matched TE0 (red line), and TM0 (blue line) modes at 2.12 μm and 1.06 μm respectively, represented along the growth direction.


**Table 2.** Multilayer sequence for 0.775 μm to 1.55 μm frequency conversion.

Moreover, our choice is for type I phase-matching, rather than type II, as it requires a small‐ er amount of birefringence (and therefore of AlOx). According to this configuration and the non-zero elements of the second order susceptibility tensor of GaAs, quadratic interactions are only possible between a combination of two low frequency TE modes (*i.e.* with polariza‐ tion in the layers plane) and a higher frequency TM mode (*i.e.* with polarization perpendicu‐

For technological reasons, the thickness of the Al-rich layers to be oxidized must be com‐ prised between few nanometers and few microns. Indeed, the diffusion of oxidant species along very thin layers is impeded, and the overall mechanical stability of thick oxidized lay‐ ers is critical. However, this mechanical instability can be mitigated by adding a small amount of gallium to the thin AlAs layers [33]. As a result, we have chosen to deal with ~30 nm layers of Al0.98Ga0.02As (basically AlAs). Note that all the thicknesses of the oxidized lay‐ ers are fixed to be the same throughout the whole structure, so to optimally calibrate the oxi‐

While for the first "OPO structure", the fulfillment of the phase-matching condition requires the insertion of only five AlOx layers in the GaAs core, the situation differs in two aspects

**1.** the guiding core material must be switched to Al0.25Ga0.75As to ensure its transparency at

**2.** due to the higher dispersion generated by the proximity of the material bandgap, the number of AlOx layers must be significantly increased, leading to their insertion into

In both cases, the thickness of the (Al)GaAs layers surrounded by oxide layers is set to tune the degeneracy wavelength. The two final designs are given in Tables 1 and 2, whereas the corresponding refractive index profiles and the intensity distributions of the phase-matched

**Layer Composition Thickness (nm) Repetition**

× 4

Al0.98Ga0.02As 37.5 GaAs 273

Al0.98Ga0.02As 37.5

dation process and benefit from the best experimental conditions.

lar to the layers plane).

64 Optoelectronics - Advanced Materials and Devices

for the "TTPS structure":

0.775 μm, and

Core

the waveguide claddings as well.

modes at degeneracy are shown in Figures 1 and 2.

Cap GaAs 30 Cladding Al0.7Ga0.3As 1000

Cladding Al0.7Ga0.3As 1000 Buffer Al0.92Ga0.08As 1000 Substrate GaAs -

**Table 1.** Multilayer sequence for 1.06 μm to 2.12 μm frequency conversion.

**Figure 2.** Refractive index profile (black line), phase-matched TE0 (red line), and TM0 (blue line) modes at 1.55 μm and 0.775 μm respectively, represented along the growth direction.

### **3.2. Waveguide fabrication steps**

The main advantage of the form birefringence phase-matching scheme with respect to competing techniques is the inherently high nonlinear overlap integral between very look‐ alike fundamental modes. Thereby, nonlinear partially oxidized AlGaAs waveguides are expected to give interestingly high conversion efficiencies. However, their performances rely on the quality of their fabrication, for optical propagation losses heavily affect the phase-matching.

tion at the desired wavelength, whereas the latter is essential to keep the scattering losses to

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67

**Figure 3.** Scanning Electron Microscopy pictures of etched and oxidized waveguide facets (Left: "OPO Structure"–

A thin film of positive photoresist is spin coated on the wafer and binary ridge patterns are defined with standard photolithography technique. Waveguides are oriented along the [110]

susceptibility tensor and the {110} cleavage plane to define the waveguide facets. Typical 3μm × 3μm ridges, are then etched using the CH3COOH:HBr:K2Cr2O7 solution. The result‐ ing waveguide sidewalls are extremely smooth (roughness RMS value of 2 nm), which pre‐ vents the scattering of guided modes and favors the diffusion of the oxidant species during the oxidation step. The whole process is quite reproducible and easily homogeneous over

Samples are then cleaved in mm-long devices and the Al-rich layers are selectively oxidized. Oxidation is held in a quartz tube where samples are heated up to high temperatures in the 400°C-500°C range, in a wet atmosphere. The latter is obtained using a 2L/min nitrogen flux

Optical propagation losses clearly originate from the oxidation process, for they are much lower before this fabrication step (respectively around 0.1 and 0.4 cm-1 in the near infrared for the best waveguides of the reported structures). Since the former demonstration of AlAs oxidation, such process has undergone a considerable progress in the last two decades, lead‐ ing to reliable parameters for the kinetics of the reaction [34]. Thus, using the above men‐ tioned processing technique and oxidation apparatus, this fabrication step has been

The kinetics of the oxidation depends on several parameters: the thickness and composition of the Al-rich layers, the furnace temperature, the carrier gas flux, and the bubbler tempera‐

xyz element of the GaAs nonlinear

crystalline direction in order to exploit the maximum χ(2)

flowing through a water bubbler stabilized at 70°C.

**3.3. Optimization of the oxidation process**

carefully calibrated and optimized.

*Furnace calibration*

a minimum.

Right: "TTPS Structure").

several millimeters.

#### *Fabrication improvements*

Thanks to the well-developed clean-room techniques and equipments (*e.g.* epitaxial growth, lithography and cleaving) the AlGaAs platform is readily mature and compatible with the fabrication of high quality integrated devices. Furthermore, over the last fifteen years GaAs/ AlOx waveguides have significantly benefited from the efforts and technological improve‐ ments achieved in terms of etching and oxidation [34].

The first mid-infrared DFG results have been reported in RIE-etched double mesa wave‐ guides consisting in a 3 μm wide ridge (for lateral confinement of the guided modes) on top of a 100 μm wide mesa (to access the buried AlAs layers), with high propagation losses of ~2 cm-1 in the near infrared, and a 3 %W-1cm-2 normalized conversion efficiency [35]. This high attenuation coefficient was assumed to stem from


After optimization of the design, dry etching has then been replaced with H2SO4:H2O2:H2O (3:1:1) chemical wet etching to obtain smoother sidewalls. The resulting losses of oxidized samples were reduced to ~1.5 cm-1, and a normalized conversion efficiency of 1000 %W-1cm-2 was estimated through SPDC around 2 μm [36]. Then, always in the same group, the wave‐ guide geometry has been switched to single ridges (3μm × 3μm) to shorten the oxidation time to several minutes, hence low losses of ~0.7 cm-1 and a 1500 %W-1cm-2 normalized con‐ version efficiency [10]. However, the slight etchant selectivity led to peculiar saw-tooth transverse profiles with mediocre process uniformity. Finally, sample homogeneity has been improved by using the non selective CH3COOH:HBr:K2Cr2O7 etchant (1:1:1) [37].

Two scanning electron microscopy images of finalized samples are presented in Figure 3. Despite the poor control on the waveguide transverse profile, chemical etching is still rele‐ vant compared to dry etching techniques such as ICP-RIE, whose development in terms of sidewalls roughness and chemical state is not yet satisfactory for low-loss integrated fre‐ quency converters [38].

#### *Growth and processing*

The epitaxial structures reported in Tables 1 and 2 have been grown on semi-insulating (001) GaAs substrates by means of molecular beam epitaxy (MBE). This technique enables the growth of high purity layers with a sharp control of their compositions and thicknesses, with abrupt interfaces. The former accuracy is important to meet the phase-matching condi‐ tion at the desired wavelength, whereas the latter is essential to keep the scattering losses to a minimum.

**Figure 3.** Scanning Electron Microscopy pictures of etched and oxidized waveguide facets (Left: "OPO Structure"– Right: "TTPS Structure").

A thin film of positive photoresist is spin coated on the wafer and binary ridge patterns are defined with standard photolithography technique. Waveguides are oriented along the [110] crystalline direction in order to exploit the maximum χ(2) xyz element of the GaAs nonlinear susceptibility tensor and the {110} cleavage plane to define the waveguide facets. Typical 3μm × 3μm ridges, are then etched using the CH3COOH:HBr:K2Cr2O7 solution. The result‐ ing waveguide sidewalls are extremely smooth (roughness RMS value of 2 nm), which pre‐ vents the scattering of guided modes and favors the diffusion of the oxidant species during the oxidation step. The whole process is quite reproducible and easily homogeneous over several millimeters.

Samples are then cleaved in mm-long devices and the Al-rich layers are selectively oxidized. Oxidation is held in a quartz tube where samples are heated up to high temperatures in the 400°C-500°C range, in a wet atmosphere. The latter is obtained using a 2L/min nitrogen flux flowing through a water bubbler stabilized at 70°C.

#### **3.3. Optimization of the oxidation process**

Optical propagation losses clearly originate from the oxidation process, for they are much lower before this fabrication step (respectively around 0.1 and 0.4 cm-1 in the near infrared for the best waveguides of the reported structures). Since the former demonstration of AlAs oxidation, such process has undergone a considerable progress in the last two decades, lead‐ ing to reliable parameters for the kinetics of the reaction [34]. Thus, using the above men‐ tioned processing technique and oxidation apparatus, this fabrication step has been carefully calibrated and optimized.

#### *Furnace calibration*

**3.2. Waveguide fabrication steps**

66 Optoelectronics - Advanced Materials and Devices

phase-matching.

*Fabrication improvements*

quency converters [38]. *Growth and processing*

ments achieved in terms of etching and oxidation [34].

attenuation coefficient was assumed to stem from

The main advantage of the form birefringence phase-matching scheme with respect to competing techniques is the inherently high nonlinear overlap integral between very look‐ alike fundamental modes. Thereby, nonlinear partially oxidized AlGaAs waveguides are expected to give interestingly high conversion efficiencies. However, their performances rely on the quality of their fabrication, for optical propagation losses heavily affect the

Thanks to the well-developed clean-room techniques and equipments (*e.g.* epitaxial growth, lithography and cleaving) the AlGaAs platform is readily mature and compatible with the fabrication of high quality integrated devices. Furthermore, over the last fifteen years GaAs/ AlOx waveguides have significantly benefited from the efforts and technological improve‐

The first mid-infrared DFG results have been reported in RIE-etched double mesa wave‐ guides consisting in a 3 μm wide ridge (for lateral confinement of the guided modes) on top of a 100 μm wide mesa (to access the buried AlAs layers), with high propagation losses of ~2 cm-1 in the near infrared, and a 3 %W-1cm-2 normalized conversion efficiency [35]. This high

**1.** the poor quality of the etched sidewalls responsible for significant scattering losses and

After optimization of the design, dry etching has then been replaced with H2SO4:H2O2:H2O (3:1:1) chemical wet etching to obtain smoother sidewalls. The resulting losses of oxidized samples were reduced to ~1.5 cm-1, and a normalized conversion efficiency of 1000 %W-1cm-2 was estimated through SPDC around 2 μm [36]. Then, always in the same group, the wave‐ guide geometry has been switched to single ridges (3μm × 3μm) to shorten the oxidation time to several minutes, hence low losses of ~0.7 cm-1 and a 1500 %W-1cm-2 normalized con‐ version efficiency [10]. However, the slight etchant selectivity led to peculiar saw-tooth transverse profiles with mediocre process uniformity. Finally, sample homogeneity has been

Two scanning electron microscopy images of finalized samples are presented in Figure 3. Despite the poor control on the waveguide transverse profile, chemical etching is still rele‐ vant compared to dry etching techniques such as ICP-RIE, whose development in terms of sidewalls roughness and chemical state is not yet satisfactory for low-loss integrated fre‐

The epitaxial structures reported in Tables 1 and 2 have been grown on semi-insulating (001) GaAs substrates by means of molecular beam epitaxy (MBE). This technique enables the growth of high purity layers with a sharp control of their compositions and thicknesses, with abrupt interfaces. The former accuracy is important to meet the phase-matching condi‐

**2.** long oxidation times (1h20min) during which the oxide quality was degraded.

improved by using the non selective CH3COOH:HBr:K2Cr2O7 etchant (1:1:1) [37].

The kinetics of the oxidation depends on several parameters: the thickness and composition of the Al-rich layers, the furnace temperature, the carrier gas flux, and the bubbler tempera‐ ture [33]. We have chosen here to focus only upon temperature, the carrier gas flux and the bubbler temperature being fixed to have an excess of reactants in the wet atmosphere. Fig‐ ure 4 represents, for our grown AlAs layers, the log-log representation of lateral oxidation depth versus process duration, for different temperatures ranging from 400°C to 500°C. The linear progression of the oxide front with time is pointed out by the unitary slope of the dashed lines fitting the data, meaning that the process is reaction limited. From the y-inter‐ cept we can estimate the indicated oxidation rates, varying from fractions of microns to sev‐ eral microns per minute. Since the reaction is thermally-activated, they follow an Arrhenius law r = r0.exp(-Ea/kBT), where r is the oxidation rate (in μm/min), r0 the reaction constant (in μm/min), Ea the activation energy (in eV), kB the Boltzmann constant, and T the process tem‐ perature (in Kelvin). According to Figure 4 we obtain Ea = 1.7eV in fair agreement with the literature [33].

As we can see from the graph, this last condition is particularly difficult to meet at high temperature where losses are highly sensitive to any imprecision on the oxidation time. Consequently the set of parameters {14 min., 420°C} has been chosen as the best reasona‐

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69

**Figure 5.** Normalized losses vs. Oxidation time for three different process temperatures.

Nonlinear characterization of a frequency converter is achieved through three-wave mix‐ ing experiments for which the nonlinearity of the material couples fields of angular fre‐ quency ωi (i=1,2,3) such as ω1+ω2=ω3 [6]. Different setup configurations can be implemented depending on the modes injected at the guiding structure input, and sever‐ al figures of merit can be inferred. Second harmonic generation (ω+ω→2ω) is the simplest to achieve, as the output power of the generated beam scales quadratically with the input

inverse process (at least at degeneracy) that is SPDC (ω3→ω1+ω2) is more subtle to detect as the pump wave interacts with weak vacuum quantum fluctuations. It gives access to the device tuning curve and to the off-degeneracy conversion efficiency. Finally, seeded down-conversion (ω3-ω1→ω2) can alternatively be described as DFG or parametric amplifi‐ cation. Normalized parametric gain g/P1/2=ηnorm 1/2 is crucial when it comes to optical para‐

L2 ). The

power. It enables the estimation of normalized conversion efficiency ηnorm=P2ω/(P<sup>ω</sup> <sup>2</sup>

**4. Nonlinear optics experimental performances**

**4.1. Nonlinear optics characterization**

metric amplifiers (OPAs) and OPOs.

ble trade-off.

**Figure 4.** (Left) Lateral oxidation depth of 37.5nm thick Al0.98Ga0.02As layers vs. Time, for several temperatures; (Right) Arrhenius plot of the oxidation rate.

#### *Optimization of the oxidation parameters*

Knowing the width of our ridge waveguides, the previous calibration allows us to estimate the nominal process duration required to complete the oxidation (*i.e.* when the oxidation fronts merge at the centre of the structure). Systematic loss measurements have been per‐ formed on several waveguides oxidized at 410°C, 420°C and 430°C, with oxidation dura‐ tions around the estimated nominal values. In Figure 5 we report the average value and standard deviation of TE00 modal losses, conveniently measured around 1.55 μm. Data are normalized to the reference loss level obtained for a 14 minutes long oxidation at 420°C. Two conclusions can be drawn:


As we can see from the graph, this last condition is particularly difficult to meet at high temperature where losses are highly sensitive to any imprecision on the oxidation time. Consequently the set of parameters {14 min., 420°C} has been chosen as the best reasona‐ ble trade-off.

**Figure 5.** Normalized losses vs. Oxidation time for three different process temperatures.

#### **4. Nonlinear optics experimental performances**

#### **4.1. Nonlinear optics characterization**

ture [33]. We have chosen here to focus only upon temperature, the carrier gas flux and the bubbler temperature being fixed to have an excess of reactants in the wet atmosphere. Fig‐ ure 4 represents, for our grown AlAs layers, the log-log representation of lateral oxidation depth versus process duration, for different temperatures ranging from 400°C to 500°C. The linear progression of the oxide front with time is pointed out by the unitary slope of the dashed lines fitting the data, meaning that the process is reaction limited. From the y-inter‐ cept we can estimate the indicated oxidation rates, varying from fractions of microns to sev‐ eral microns per minute. Since the reaction is thermally-activated, they follow an Arrhenius law r = r0.exp(-Ea/kBT), where r is the oxidation rate (in μm/min), r0 the reaction constant (in μm/min), Ea the activation energy (in eV), kB the Boltzmann constant, and T the process tem‐ perature (in Kelvin). According to Figure 4 we obtain Ea = 1.7eV in fair agreement with the

**Figure 4.** (Left) Lateral oxidation depth of 37.5nm thick Al0.98Ga0.02As layers vs. Time, for several temperatures; (Right)

Knowing the width of our ridge waveguides, the previous calibration allows us to estimate the nominal process duration required to complete the oxidation (*i.e.* when the oxidation fronts merge at the centre of the structure). Systematic loss measurements have been per‐ formed on several waveguides oxidized at 410°C, 420°C and 430°C, with oxidation dura‐ tions around the estimated nominal values. In Figure 5 we report the average value and standard deviation of TE00 modal losses, conveniently measured around 1.55 μm. Data are normalized to the reference loss level obtained for a 14 minutes long oxidation at 420°C.

**2.** the process must be stopped at the exact moment when all the AlAs has been converted

**1.** the average loss level suffers from slow oxidation kinetics, and

literature [33].

68 Optoelectronics - Advanced Materials and Devices

Arrhenius plot of the oxidation rate.

Two conclusions can be drawn:

into oxide.

*Optimization of the oxidation parameters*

Nonlinear characterization of a frequency converter is achieved through three-wave mix‐ ing experiments for which the nonlinearity of the material couples fields of angular fre‐ quency ωi (i=1,2,3) such as ω1+ω2=ω3 [6]. Different setup configurations can be implemented depending on the modes injected at the guiding structure input, and sever‐ al figures of merit can be inferred. Second harmonic generation (ω+ω→2ω) is the simplest to achieve, as the output power of the generated beam scales quadratically with the input power. It enables the estimation of normalized conversion efficiency ηnorm=P2ω/(P<sup>ω</sup> <sup>2</sup> L2 ). The inverse process (at least at degeneracy) that is SPDC (ω3→ω1+ω2) is more subtle to detect as the pump wave interacts with weak vacuum quantum fluctuations. It gives access to the device tuning curve and to the off-degeneracy conversion efficiency. Finally, seeded down-conversion (ω3-ω1→ω2) can alternatively be described as DFG or parametric amplifi‐ cation. Normalized parametric gain g/P1/2=ηnorm 1/2 is crucial when it comes to optical para‐ metric amplifiers (OPAs) and OPOs.

Since the first experimental demonstration of mid-infrared DFG in form birefringent wave‐ guides in 1997 [35], the performances of such device have been significantly improved in terms of optical losses and conversion efficiency. Thereby, this phase-matching scheme has been successfully extended to visible and near infrared spectral ranges [9,10]. Concerning the first structure discussed here, designed for 1.06-to-2.12 μm parametric down-conversion, three-wave mixing experiments have been extensively carried out, and their results have al‐ ready been published. A normalized conversion efficiency of ηnorm = 1500 %W-1cm-2, corre‐ sponding to a normalized parametric gain of 3.9 cm-1W-1/2 at degeneracy, was estimated by parametric fluorescence measurements, and confirmed by SHG [10]. Direct parametric am‐ plification measurements have been performed, and a maximum single-pass parametric gain of 4.5% has been obtained with 30 mW in-coupled pump power [37]. This the highest value ever reported in semiconductor waveguide. Furthermore, tunability for signal and id‐ ler beams in the 1.7 – 2.7 μm range has been established. The only limitation to this span was shown to stem from the O-H absorption band around 3 μm, due to the presence of hy‐ droxide AlO(OH) [32]. Nevertheless, this band can be recovered, extending the tunability between 1.3 and 4.2 μm, after dehydroxylation of the sample (*i.e.* decomposing AlO(OH) in‐ to γ-Al2O3) by thermal annealing [39]. These main figures reflect the competitiveness of inte‐ grated GaAs-based frequency converter in the mid-infrared with respect to standard PPLN. However, the excessive propagation losses still prevent the use of cm-long waveguides in which the oscillation threshold would be at reach.

**Figure 6.** Second harmonic power vs. Fundamental harmonic wavelength. The photodiode signal (black dots) is theo‐ retically fitted (red dotted line), and the Lorentzian single-pass phase-matching curve is extracted (solid blue line).

A typical phase-matching spectrum is shown in Figure 6. It was obtained by monitoring the second harmonic output power while scanning the fundamental harmonic input wave‐ length at constant 9.3 mW power. The experimental data exhibit a smooth phase-matching resonance modulated by Fabry-Perot fringes and can be nicely fitted after [41]. This high fre‐ quency feature corresponds to the interference pattern arising from the multiple reflections of the pump beam on the waveguide facets. Once the cavity fringes are filtered, the singlepass phase-matching curve is extracted and fitted by a Lorentzian curve with a 2.9 nm full width at half maximum (FWHM). The phase-matching condition is met at 1544 nm instead of the designed nominal value of 1550 nm. This slight discrepancy is a combined effect of the small systematic deviation of our MBE reactor on the layer thicknesses (typically 1 to 2 percents), the shift of the phase-matching wavelength induced by the 2D transverse confine‐ ment whereas the design procedure is 1D, and the temperature dependence of the structure refractive indices [42]. The broadening of the envelope shape and its deviation from the ide‐ al *sinc* function are generally ascribed to waveguide inhomogeneities along propagation or to optical losses. In our case, however, the relatively large width and the high quality of the waveguide ridge suggest that we can assume waveguide invariance along propagation. Us‐ ing the Fabry-Perot fringes method, TE optical losses, αFH, were measured to be 0.4 cm-1 and 1.2 cm-1 in the 1.55 μm region, respectively before and after oxidation of the thin AlAs barri‐ ers. TM losses, αSH, were assessed *via* transmission measurement of a Ti:Sapphire laser at 775 nm, yielding a value of 151 cm-1. Consistently, losses of 140 cm-1 were inferred for the second harmonic from the FWHM of the lorentzian curve according to the textbook equation [6]:

( ) ( ) ( )

D+ - é ù ë û µ-+ ´ é ù ë û

sin sinh

2

*L*

*FH SH*

exp

 aa

h

2 2

( ) ( )

a

*k L L*

D +- é ù ë û

2 2 2 2 2

*FH SH*

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip

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71

 a

(1)

2 2 2

*k L L*

a

*FH SH*

 a

Shortly after the demonstration of SHG of a λ = 1.6 μm pump in AlGaAs/AlOx waveguides as early as 1998 [40], the interest in the telecom band faded because of propagation loss is‐ sues. During the following decade, up and down-conversion between 0.775 μm and the tele‐ com band remained out of reach for the form birefringent phase-matching scheme as the fabrication and oxidation steps were not fully optimized. In the following, we present our latest results obtained in the most recent generation of partially oxidized AlGaAs wave‐ guides. State-of-the-art conversion efficiency is estimated *via* SHG experiment, and off-de‐ generacy dispersion is assessed by DFG experiment. A tunability greater than 500 nm is reported.

#### **4.2. Second Harmonic Generation**

In the experiment we report here, a CW single mode external cavity laser diode, largely tunable in the telecom band, was used to generate the pump beam at the fundamental frequency. The laser beam was TE-polarized then injected into a 500 μm long waveguide by end-fire coupling through a ×60 (0.85 N.A.) microscope objective. The internal funda‐ mental harmonic input power was estimated using the in-coupling efficiency of 37% as‐ sessed by taking into account the objective transmission (75%), the 28% facet reflectivity calculated accurately by finite-difference time domain (FDTD) method, and the overlap integral between the laser beam and the guided mode (69%). The second harmonic radia‐ tion was collected at the output facet by a similar microscope objective, filtered by a TM polarizer to get rid of the pump beam then focused onto a silicon photodiode connected to a lock-in amplifier.

Since the first experimental demonstration of mid-infrared DFG in form birefringent wave‐ guides in 1997 [35], the performances of such device have been significantly improved in terms of optical losses and conversion efficiency. Thereby, this phase-matching scheme has been successfully extended to visible and near infrared spectral ranges [9,10]. Concerning the first structure discussed here, designed for 1.06-to-2.12 μm parametric down-conversion, three-wave mixing experiments have been extensively carried out, and their results have al‐ ready been published. A normalized conversion efficiency of ηnorm = 1500 %W-1cm-2, corre‐ sponding to a normalized parametric gain of 3.9 cm-1W-1/2 at degeneracy, was estimated by parametric fluorescence measurements, and confirmed by SHG [10]. Direct parametric am‐ plification measurements have been performed, and a maximum single-pass parametric gain of 4.5% has been obtained with 30 mW in-coupled pump power [37]. This the highest value ever reported in semiconductor waveguide. Furthermore, tunability for signal and id‐ ler beams in the 1.7 – 2.7 μm range has been established. The only limitation to this span was shown to stem from the O-H absorption band around 3 μm, due to the presence of hy‐ droxide AlO(OH) [32]. Nevertheless, this band can be recovered, extending the tunability between 1.3 and 4.2 μm, after dehydroxylation of the sample (*i.e.* decomposing AlO(OH) in‐ to γ-Al2O3) by thermal annealing [39]. These main figures reflect the competitiveness of inte‐ grated GaAs-based frequency converter in the mid-infrared with respect to standard PPLN. However, the excessive propagation losses still prevent the use of cm-long waveguides in

Shortly after the demonstration of SHG of a λ = 1.6 μm pump in AlGaAs/AlOx waveguides as early as 1998 [40], the interest in the telecom band faded because of propagation loss is‐ sues. During the following decade, up and down-conversion between 0.775 μm and the tele‐ com band remained out of reach for the form birefringent phase-matching scheme as the fabrication and oxidation steps were not fully optimized. In the following, we present our latest results obtained in the most recent generation of partially oxidized AlGaAs wave‐ guides. State-of-the-art conversion efficiency is estimated *via* SHG experiment, and off-de‐ generacy dispersion is assessed by DFG experiment. A tunability greater than 500 nm is

In the experiment we report here, a CW single mode external cavity laser diode, largely tunable in the telecom band, was used to generate the pump beam at the fundamental frequency. The laser beam was TE-polarized then injected into a 500 μm long waveguide by end-fire coupling through a ×60 (0.85 N.A.) microscope objective. The internal funda‐ mental harmonic input power was estimated using the in-coupling efficiency of 37% as‐ sessed by taking into account the objective transmission (75%), the 28% facet reflectivity calculated accurately by finite-difference time domain (FDTD) method, and the overlap integral between the laser beam and the guided mode (69%). The second harmonic radia‐ tion was collected at the output facet by a similar microscope objective, filtered by a TM polarizer to get rid of the pump beam then focused onto a silicon photodiode connected

which the oscillation threshold would be at reach.

70 Optoelectronics - Advanced Materials and Devices

reported.

**4.2. Second Harmonic Generation**

to a lock-in amplifier.

**Figure 6.** Second harmonic power vs. Fundamental harmonic wavelength. The photodiode signal (black dots) is theo‐ retically fitted (red dotted line), and the Lorentzian single-pass phase-matching curve is extracted (solid blue line).

A typical phase-matching spectrum is shown in Figure 6. It was obtained by monitoring the second harmonic output power while scanning the fundamental harmonic input wave‐ length at constant 9.3 mW power. The experimental data exhibit a smooth phase-matching resonance modulated by Fabry-Perot fringes and can be nicely fitted after [41]. This high fre‐ quency feature corresponds to the interference pattern arising from the multiple reflections of the pump beam on the waveguide facets. Once the cavity fringes are filtered, the singlepass phase-matching curve is extracted and fitted by a Lorentzian curve with a 2.9 nm full width at half maximum (FWHM). The phase-matching condition is met at 1544 nm instead of the designed nominal value of 1550 nm. This slight discrepancy is a combined effect of the small systematic deviation of our MBE reactor on the layer thicknesses (typically 1 to 2 percents), the shift of the phase-matching wavelength induced by the 2D transverse confine‐ ment whereas the design procedure is 1D, and the temperature dependence of the structure refractive indices [42]. The broadening of the envelope shape and its deviation from the ide‐ al *sinc* function are generally ascribed to waveguide inhomogeneities along propagation or to optical losses. In our case, however, the relatively large width and the high quality of the waveguide ridge suggest that we can assume waveguide invariance along propagation. Us‐ ing the Fabry-Perot fringes method, TE optical losses, αFH, were measured to be 0.4 cm-1 and 1.2 cm-1 in the 1.55 μm region, respectively before and after oxidation of the thin AlAs barri‐ ers. TM losses, αSH, were assessed *via* transmission measurement of a Ti:Sapphire laser at 775 nm, yielding a value of 151 cm-1. Consistently, losses of 140 cm-1 were inferred for the second harmonic from the FWHM of the lorentzian curve according to the textbook equation [6]:

$$\eta \propto \exp\left[-\left(a\_{\rm FH} + a\_{\rm SH}/2\right)L\right] \times \frac{\sin^2\left(\Delta k \, L/2\right) + \sinh^2\left[\left(a\_{\rm FH} - a\_{\rm SH}/2\right)L/2\right]}{\left(\Delta k \, L/2\right)^2 + \left[\left(a\_{\rm FH} - a\_{\rm SH}/2\right)L/2\right]^2} \tag{1}$$

The evolution of the second harmonic power with the fundamental harmonic power at de‐ generacy is given in log-log scale in Figure 7. The quadratic law is confirmed by the slope of the line fitting the experimental data, and a SHG efficiency of 2.8 %W-1 is estimated. The normalized conversion efficiency is found to be 1120 %W-1cm-2 *i.e.* comparable to the best value reported in AlGaAs waveguides [38], and which corresponds to a 3.3 cm-1W-1/2 nor‐ malized parametric gain at degeneracy. Furthermore, no sublinear deviation is observed for pump power up to 50 mW, whereas such an onset occurs at 10 mW and is ascribed to twophoton absorption in [43]. Finally a maximum second harmonic power of 267 μW is report‐ ed, only limited by our source power at fundamental harmonic. This is the highest value reported for integrated AlGaAs waveguides.

Typical spectra are shown in Figure 8, where the central peaks are unambiguously attribut‐ ed to the second order of the residual pump beam diffracted by the monochromator grating. They were obtained for three different pump wavelengths below degeneracy (λP = 773.2 nm), after adjustment of the seed wavelength to optimize the difference frequency power. All the (λP, λS, λDF) sets verify energy conservation at phase-matching and are reported in Figure 9 along with the simulated tuning curve, with a good agreement. The spread of the dark line corresponds to the spectral acceptance of the DFG around phase-matching and its width accounts for the losses experienced by the three fields. Regarding the tunability of the device, we can see that a shift of the pump wavelength as small as 5.2 nm requires a spectral separation for the seed and difference frequency waves of 570 nm. This is due to the strong dispersion in the vicinity of material bandgap [44], and it turns out to be a great advantage regarding the fabrication of integrated source tunable in the near infrared for instance.

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73

**Figure 8.** Normalized DFG spectra obtained for three different pump wavelengths (the seed peaks are clipped due to

The combined study of SHG and DFG allowed us to fully characterize our form birefringent partially oxidized AlGaAs waveguides in terms of tunability and parametric gain. While this device strongly benefited from technologic improvements in terms of design, etching and oxidation, its performances compare favorably with respect to other alternative phasematching approaches, but they are still limited by optical losses, which prevented so far the observation of SPDC. However, the reported figures are strongly motivating in view of the fabrication of a telecom source of quantum light. Indeed, for a 2 mm long waveguide, we theoretically estimate a parametric fluorescence efficiency of ηPF ~ 1.7×10-8 pairs/pump pho‐ ton, whereas a significant reduction of optical losses to ~1 cm-1 would lead to ηPF ~ 3×10-7

pairs/pump photon, *i.e.* comparable with cm-long PPLN waveguides values [3].

the lock-in finite dynamic range).

**Figure 7.** Log-log representation of second harmonic power vs. Fundamental harmonic power. The slope of the linear fit highlights the expected quadratic behavior.

#### **4.3. Difference Frequency Generation**

DFG was achieved on the same sample in order to investigate its performance out of degen‐ eracy. In our case, this three-wave mixing process required a TM-polarized pump mode at visible wavelength λP, interacting with a TE-polarized infrared seed at wavelength λS, so to generate a TE-polarized difference frequency beam whose wavelength λDF is set by energy conservation 1/λDF = 1/λP - 1/λS. The pump and seed beams were provided respectively by a linearly polarized CW Ti:Sapphire laser tunable around 775 nm, and an external cavity laser diode tunable between 1490 and 1600 nm. Both beams were combined using a 50/50 beam splitter then collinearly coupled in, and out of, a waveguide by two ×40 (0.65 N.A.) micro‐ scope objectives. Its collimated output was either visualized on a camera or passed through a spectrometer before being filtered by a TE polarizer then detected using a strained InGaAs photodiode and a lock-in amplifier.

Typical spectra are shown in Figure 8, where the central peaks are unambiguously attribut‐ ed to the second order of the residual pump beam diffracted by the monochromator grating. They were obtained for three different pump wavelengths below degeneracy (λP = 773.2 nm), after adjustment of the seed wavelength to optimize the difference frequency power. All the (λP, λS, λDF) sets verify energy conservation at phase-matching and are reported in Figure 9 along with the simulated tuning curve, with a good agreement. The spread of the dark line corresponds to the spectral acceptance of the DFG around phase-matching and its width accounts for the losses experienced by the three fields. Regarding the tunability of the device, we can see that a shift of the pump wavelength as small as 5.2 nm requires a spectral separation for the seed and difference frequency waves of 570 nm. This is due to the strong dispersion in the vicinity of material bandgap [44], and it turns out to be a great advantage regarding the fabrication of integrated source tunable in the near infrared for instance.

The evolution of the second harmonic power with the fundamental harmonic power at de‐ generacy is given in log-log scale in Figure 7. The quadratic law is confirmed by the slope of the line fitting the experimental data, and a SHG efficiency of 2.8 %W-1 is estimated. The normalized conversion efficiency is found to be 1120 %W-1cm-2 *i.e.* comparable to the best value reported in AlGaAs waveguides [38], and which corresponds to a 3.3 cm-1W-1/2 nor‐ malized parametric gain at degeneracy. Furthermore, no sublinear deviation is observed for pump power up to 50 mW, whereas such an onset occurs at 10 mW and is ascribed to twophoton absorption in [43]. Finally a maximum second harmonic power of 267 μW is report‐ ed, only limited by our source power at fundamental harmonic. This is the highest value

**Figure 7.** Log-log representation of second harmonic power vs. Fundamental harmonic power. The slope of the linear

DFG was achieved on the same sample in order to investigate its performance out of degen‐ eracy. In our case, this three-wave mixing process required a TM-polarized pump mode at visible wavelength λP, interacting with a TE-polarized infrared seed at wavelength λS, so to generate a TE-polarized difference frequency beam whose wavelength λDF is set by energy conservation 1/λDF = 1/λP - 1/λS. The pump and seed beams were provided respectively by a linearly polarized CW Ti:Sapphire laser tunable around 775 nm, and an external cavity laser diode tunable between 1490 and 1600 nm. Both beams were combined using a 50/50 beam splitter then collinearly coupled in, and out of, a waveguide by two ×40 (0.65 N.A.) micro‐ scope objectives. Its collimated output was either visualized on a camera or passed through a spectrometer before being filtered by a TE polarizer then detected using a strained InGaAs

reported for integrated AlGaAs waveguides.

72 Optoelectronics - Advanced Materials and Devices

fit highlights the expected quadratic behavior.

**4.3. Difference Frequency Generation**

photodiode and a lock-in amplifier.

**Figure 8.** Normalized DFG spectra obtained for three different pump wavelengths (the seed peaks are clipped due to the lock-in finite dynamic range).

The combined study of SHG and DFG allowed us to fully characterize our form birefringent partially oxidized AlGaAs waveguides in terms of tunability and parametric gain. While this device strongly benefited from technologic improvements in terms of design, etching and oxidation, its performances compare favorably with respect to other alternative phasematching approaches, but they are still limited by optical losses, which prevented so far the observation of SPDC. However, the reported figures are strongly motivating in view of the fabrication of a telecom source of quantum light. Indeed, for a 2 mm long waveguide, we theoretically estimate a parametric fluorescence efficiency of ηPF ~ 1.7×10-8 pairs/pump pho‐ ton, whereas a significant reduction of optical losses to ~1 cm-1 would lead to ηPF ~ 3×10-7 pairs/pump photon, *i.e.* comparable with cm-long PPLN waveguides values [3].

**•** GaAs surrounding AlOx becomes amorphous in the close vicinity of the oxidized layers

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75

**•** Residual oxidation of the neighboring GaAs and Al0.7Ga0.3As layers occurs through the in‐ terfaces, over 3 to 9 nm. The richer in aluminum the deeper an AlGaAs layer gets oxi‐

**•** AlOx layers are composed of γ−Al2O3 polycrystalline grains, with size between 10 and 20

Additionally, the internal interfaces were investigated by high-angle annular dark-field (HAADF) scanning transmission electron microscopy. Figure 10 shows two images of the same waveguide core area acquired before and after oxidation. Oxidation is obviously re‐ sponsible for the deterioration of the multilayer morphology, as rough interfaces are clearly visible afterwards, especially at the bottom interface. The acquisition of several of such pic‐ tures contiguous over about 700 nm allowed us to reconstruct the respective roughness pro‐ files. Gaussian fits of their autocorrelation functions have been performed in order to reliably extract their stochastic parameters: the RMS amplitude σ and the coherence length

**Figure 10.** HAADF images of an Al0.98Ga0.02As layer (in dark) embedded in GaAs (Top) and Al0.7Ga0.3As (Bottom), before

**Interface GaAs/Al0.98Ga0.02As Al0.7Ga0.3As/Al0.98Ga0.02As GaAs/AlOx Al0.7Ga0.3As/AlOx σ (nm)** 0.37 0.37 0.53 0.69 **Δσ (nm)** 0.08 0.08 0.08 0.06 **Lc (nm)** 4 4 53 30 **ΔLc (nm)** 0 0 14 7

**Table 3.** Measured RMS amplitudes and coherence lengths for two types of interface, before and after oxidation.

Lc. The results are summarized in Table 3 for the different types of interfaces.

**•** The oxidation of Al0.98Ga0.02As results in a 12% contraction of the layers thicknesses.

(~20 nm from the interface) and remains mono-crystalline beyond.

nm, embedded in an amorphous AlOx matrix.

dized.

*Interface roughness*

(Left) and after oxidation (Right).

**Figure 9.** Tuning curve: experimental data (colored dots) and theoretical prediction (grayscale).
