**5. Phenomenological study of loss mechanisms**

#### **5.1. Structural and chemical characterization of the oxide**

To date, the performances of partially oxidized AlGaAs waveguides are mitigated by high propagation losses (~1 cm-1 in the infrared). As a comparison, typical PPLN waveguide loss‐ es are in the range of few 0.01 cm-1, enabling the fabrication of several cm-long devices. Since optical propagation losses play a major role in all guided-wave phase-matching schemes proposed so far in the AlGaAs platform, their reduction is a critical issue.

Promising results have already been obtained by decreasing the losses after optimization of the waveguides design and the oxidation parameters, but today, a better understanding of the propagation losses origins is necessary if we want to lower them even more. Indeed, many aspects of the oxidation process are still not clearly established, including the exact formation mechanisms and the fine chemical and structural properties of the oxide. There‐ fore a transmission electron microscopy characterization of oxidized layers has been carried out [45] to assess the quality of the material at microscopic scale.

#### *Chemical and structural characterization*

The fabrication of cross-section samples has been carried out with an ion slicer used to per‐ form the thinning of a waveguide oxidized guiding core. Based on high resolution transmis‐ sion electron microscopy pictures combined with power spectrum analysis and energy filtered transmission electron microscopy images, it has been shown that:


#### *Interface roughness*

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

proposed so far in the AlGaAs platform, their reduction is a critical issue.

out [45] to assess the quality of the material at microscopic scale.

filtered transmission electron microscopy images, it has been shown that:

*Chemical and structural characterization*

74 Optoelectronics - Advanced Materials and Devices

To date, the performances of partially oxidized AlGaAs waveguides are mitigated by high propagation losses (~1 cm-1 in the infrared). As a comparison, typical PPLN waveguide loss‐ es are in the range of few 0.01 cm-1, enabling the fabrication of several cm-long devices. Since optical propagation losses play a major role in all guided-wave phase-matching schemes

Promising results have already been obtained by decreasing the losses after optimization of the waveguides design and the oxidation parameters, but today, a better understanding of the propagation losses origins is necessary if we want to lower them even more. Indeed, many aspects of the oxidation process are still not clearly established, including the exact formation mechanisms and the fine chemical and structural properties of the oxide. There‐ fore a transmission electron microscopy characterization of oxidized layers has been carried

The fabrication of cross-section samples has been carried out with an ion slicer used to per‐ form the thinning of a waveguide oxidized guiding core. Based on high resolution transmis‐ sion electron microscopy pictures combined with power spectrum analysis and energy

**5. Phenomenological study of loss mechanisms**

**5.1. Structural and chemical characterization of the oxide**

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

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


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

For such step index waveguides with high index contrasts, optical modes are highly sensi‐ tive to any irregularities. We remind that the devices presented here are passive, and conse‐ quently the semiconductor alloys are chosen to be transparent for the wavelengths involved in the nonlinear process. Hence, assuming a good confinement by design, *i.e.* no substrate optical leakage, losses can only arise from scattering (rough sidewalls and layers interfaces, volume inhomogeneities) or absorption (defects).

( ) ( ) ( )

= - ++ - é ù ò ë û % (2)

 q

 b

Technological Challenges for Efficient AlGaAs Nonlinear Sources on Chip


% W = ò <sup>W</sup> (4)

is related to the autocorrelation function by

<sup>=</sup> å (5)

 qq

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

77

*<sup>k</sup> dn n r t R nk d*

Where φ(d) is the modal field evaluated at the waveguide core/cladding interface, normal‐

n1 and n2 are the core and cladding refractive indices respectively, k0 is the free-space wave vector, β is the modal propagation constant associated to the effective index neff, r and t are the respective reflection and transmission Fresnel coefficients and θ is the photon scattering angle with respect to the interface. The surface roughness of the waveguide walls is descri‐ bed by the spectral density function, which is obtained from the autocorrelation function

q

1 2 2 0 <sup>0</sup> ( ) 1 () () cos( ) <sup>4</sup> *<sup>r</sup>*

2 f

() 1 *x dx* +¥

*R R u i u du* ( ) ( )exp( ) +¥

half maximum of the Gaussian fit of the interface profile autocorrelation function.

This model requires two input parameters: the mean square deviation from a flat surface σ<sup>2</sup>

To adapt this model to our multilayer structures, we ideally separate the waveguide in sev‐ eral microstructures. In Figure 11 we also show a scheme of one of our device, for which the interfaces are bunched in groups of two, in order to mark off regions with refractive index n1 higher than the two adjacent ones n2. The core of each of these six microstructures, sand‐ wiched between AlOx layers and bounded by rough interfaces, is identified and numbered from 1 to 6. Its refractive index is the one of the appropriate alloy (GaAs or Al0.7Ga0.3As).

In order to infer a rough estimate of the optical losses for the fundamental optical TE0 mode, we compute α<sup>i</sup> (i=1,6) the losses of each microstructure, then we simply convert the equation (2) under the assumption that each contribution adds up incoherently with the others:

6

a

1 *tot i i*

At this point, we should stress that our adaptation is very simplistic and its derivation is on‐

 a=

, and the coherence length Lc of the interface profile corresponds to the half-width at


<sup>3</sup> <sup>2</sup> <sup>2</sup> 2 22 <sup>0</sup> <sup>2</sup>

p

*eff*

R(u) of the surface roughness via the Wiener-Khinchine theorem:

and the correlation length Lc. The parameter σ<sup>2</sup>

*n*

p

a f

ized following:

R(0)=σ<sup>2</sup>

ly qualitative.

Since the index contrast between AlOx and γ-Al2O3 is negligible, the polycrystalline grains are not expected to contribute significantly to scattering losses. Based on the previous obser‐ vations, our work hypothesis was then to ascribe the optical propagation losses of the guid‐ ed modes to scattering by the rough AlGaAs/AlOx boundaries.

#### **5.2. Modeling the scattering by rough interfaces**

The study of scattering losses in semiconductor waveguides is of great practical interest in the field of integrated optics, and since Marcuse's early work [46] a large amount of research has been done in this area. Most of the recent roughness studies in dielectric waveguides are now based on the convenient Lacey and Payne model [47,48], which offers the advantage to provide a semi-analytical expression for TE-mode losses, quite simply expressed in terms of fundamental waveguide parameters and statistics of the surface roughness.

**Figure 11.** (Left) Schematics of rough interfaces in a slab waveguide. (Right) Schematics of the adaptation to our mul‐ tilayer selectively oxidized waveguide.

The Lacey and Payne model deals with a three-layer slab core/cladding waveguide with rough interfaces as shown in Figure 11. It provides the scattering contribution to the propa‐ gation losses via the direct computation of the radiated far field, with the equivalent-current method [49]. Within this approach, it is possible to derive an expression for the exponential radiation loss coefficient due to scattering by the roughness, in a symmetric single mode waveguide of thickness 2d:

$$\alpha\_r = \phi^2(d)\left(n\_1^2 - n\_2^2\right)^2 \frac{k\_0^3}{4\pi n\_{\rm eff}} \int\_0^\pi \left[\left(1 + r(\theta)\right)^2 + t(\theta)^2\right] \tilde{R}\left(\beta - n\_2 k\_0 \cos(\theta)\right) d\theta \tag{2}$$

For such step index waveguides with high index contrasts, optical modes are highly sensi‐ tive to any irregularities. We remind that the devices presented here are passive, and conse‐ quently the semiconductor alloys are chosen to be transparent for the wavelengths involved in the nonlinear process. Hence, assuming a good confinement by design, *i.e.* no substrate optical leakage, losses can only arise from scattering (rough sidewalls and layers interfaces,

Since the index contrast between AlOx and γ-Al2O3 is negligible, the polycrystalline grains are not expected to contribute significantly to scattering losses. Based on the previous obser‐ vations, our work hypothesis was then to ascribe the optical propagation losses of the guid‐

The study of scattering losses in semiconductor waveguides is of great practical interest in the field of integrated optics, and since Marcuse's early work [46] a large amount of research has been done in this area. Most of the recent roughness studies in dielectric waveguides are now based on the convenient Lacey and Payne model [47,48], which offers the advantage to provide a semi-analytical expression for TE-mode losses, quite simply expressed in terms of

**Figure 11.** (Left) Schematics of rough interfaces in a slab waveguide. (Right) Schematics of the adaptation to our mul‐

The Lacey and Payne model deals with a three-layer slab core/cladding waveguide with rough interfaces as shown in Figure 11. It provides the scattering contribution to the propa‐ gation losses via the direct computation of the radiated far field, with the equivalent-current method [49]. Within this approach, it is possible to derive an expression for the exponential radiation loss coefficient due to scattering by the roughness, in a symmetric single mode

volume inhomogeneities) or absorption (defects).

76 Optoelectronics - Advanced Materials and Devices

**5.2. Modeling the scattering by rough interfaces**

tilayer selectively oxidized waveguide.

waveguide of thickness 2d:

ed modes to scattering by the rough AlGaAs/AlOx boundaries.

fundamental waveguide parameters and statistics of the surface roughness.

Where φ(d) is the modal field evaluated at the waveguide core/cladding interface, normal‐ ized following:

$$\int\_{-\infty}^{+\infty} \phi^2(\mathbf{x}) \, d\mathbf{x} = 1 \tag{3}$$

n1 and n2 are the core and cladding refractive indices respectively, k0 is the free-space wave vector, β is the modal propagation constant associated to the effective index neff, r and t are the respective reflection and transmission Fresnel coefficients and θ is the photon scattering angle with respect to the interface. The surface roughness of the waveguide walls is descri‐ bed by the spectral density function, which is obtained from the autocorrelation function R(u) of the surface roughness via the Wiener-Khinchine theorem:

$$\tilde{R}(\Omega) = \int\_{-\infty}^{+\infty} R(u) \exp\left(i\Omega u\right) du\tag{4}$$

This model requires two input parameters: the mean square deviation from a flat surface σ<sup>2</sup> and the correlation length Lc. The parameter σ<sup>2</sup> is related to the autocorrelation function by R(0)=σ<sup>2</sup> , and the coherence length Lc of the interface profile corresponds to the half-width at half maximum of the Gaussian fit of the interface profile autocorrelation function.

To adapt this model to our multilayer structures, we ideally separate the waveguide in sev‐ eral microstructures. In Figure 11 we also show a scheme of one of our device, for which the interfaces are bunched in groups of two, in order to mark off regions with refractive index n1 higher than the two adjacent ones n2. The core of each of these six microstructures, sand‐ wiched between AlOx layers and bounded by rough interfaces, is identified and numbered from 1 to 6. Its refractive index is the one of the appropriate alloy (GaAs or Al0.7Ga0.3As).

In order to infer a rough estimate of the optical losses for the fundamental optical TE0 mode, we compute α<sup>i</sup> (i=1,6) the losses of each microstructure, then we simply convert the equation (2) under the assumption that each contribution adds up incoherently with the others:

$$
\alpha\_{\rm tot} = \sum\_{i=1}^{6} \alpha\_i \tag{5}
$$

At this point, we should stress that our adaptation is very simplistic and its derivation is on‐ ly qualitative.

#### **5.3. Spectral study of propagation losses**

Among the several different ways to measure optical losses, the Fabry-Perot fringes meas‐ urement is the most suitable technique for low-loss waveguides [50]. Unlike the cut-back method, it has the advantage of not being destructive, and contrary to transmission meas‐ urement, it is independent of coupling and collection efficiencies estimations. In this frame, we can picture the waveguide as a resonator in which guided modes are reflected by the cleaved facets and travel in both directions. By scanning the wavelength of the input wave, we observe a multiple wave interference pattern at the output, namely the Airy function of the cavity. The contrast and finesse of the resonances are straightforwardly linked to the number of waves participating to the total interference, therefore to the reflection (estimated by FDTD simulations) and propagation losses experienced by the traveling modes.

For the longest wavelengths involved, losses are fairly low (around 1 cm-1 and less) and can be ascribed to the interfaces roughness that appears during oxidation. Indeed, the reasona‐ ble agreement between the experimental data (black dots) and the prediction band derived from our simple semi-analytical model (blue bands) confirms that, in this spectral range, losses are dominated by the scattering contribution in a Rayleigh-like regime. Note that this result is consistent with previous work achieved on an older generation of similar wave‐ guides [51], in which losses were probed by scattered light measurements, and they were

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79

For shorter wavelengths, our previous assumption is inadequate and a new loss mecha‐ nism must be considered. The very high losses and their exponential decay with wave‐ length suggest an Urbach's tail absorption, highlighting the fact that not only surface imperfections but also volume defects at the oxide interfaces play a critical role. As was already noticed several years ago [40], these additional losses are likely to be ascribed to arsenic antisites formed during oxidation at the oxide-semiconductor interface [52]. Trap‐ ped reaction products (*e.g.* elemental As and AsH3) could explain this excess of arsenic and the resulting EL2-like crystalline defects, with donor levels in the gap of AlGaAs. In‐ deed, the transition between the two regimes occurs at photon energy corresponding to 65 to 70% of the bandgap of the waveguide core material (respectively GaAs and

These levels of pump losses are currently the limiting factor for the efficiency of our Al‐ GaAs integrated devices. Nevertheless, as shown by existing research on quantum-well luminescence close to oxidized layers [52] hydrogenation can be effectively used to re‐ move absorbing species trapped at the oxide interfaces. Thermal annealing cycles are al‐ so expected to improve the crystallinity of spoiled materials. Moreover, a study on the spatial distribution of the mechanical strains arising from the volume shrinkage of oxi‐ dized thin AlAs layers showed that a critical overstrained zone appears at the merging point of the two counter-propagative oxidation fronts [53]. In our case, this could be an additional source of concern since the maximum intensity of the optical modes is at the confluence of the oxidation fronts. In order to circumvent such over-oxidation issue, new innovative designs are being tested (*e.g.* oxidation unidirectionaly launched from one side

Despite the fact that propagation losses have prevented the use of form birefringent phasematching scheme at its full potential, we are able to design and fabricate reasonably low-loss integrated waveguides. At the moment, aluminum oxide remains very promising for our photonic applications but its quality is not yet compatible with the most demanding nonlin‐ ear devices. Various investigations of its fine properties allowed us to better understand the loss mechanisms involved, and they gave us clues for their minimization. Improvement of the oxide quality is necessary for further significant progress, and will only occur after a specific technological development of the oxidation process, starting with the solutions

evoked above which are currently under exploration.

also found to decrease between 1.3 and 2.1 μm.

Al0.25Ga0.75As).

of the ridge).

In order to test our hypothesis and investigate the optical loss mechanisms, we carried out a study on their spectral dependence. The results are reported in Figure 12, for both the struc‐ tures presented previously. According to our phase-matching scheme, the data around λ<sup>P</sup> represent the TM00 losses, whereas the data at wavelength close to degeneracy represent the TE00 losses.

The data were obtained by using external cavity diode lasers tunable in the telecom band from 1.3 to 1.6 μm, and a slightly tunable DFB laser around 2.12 μm. However we resorted to transmission measurements of a CW Ti:Sapphire laser around 1 μm and below, where losses were found to be too high to be reliably inferred from Fabry-Perot fringes.

**Figure 12.** Propagation losses vs. Wavelength, fitted by decaying exponential (red curves) for short wavelengths, and inverse-power law (blue curves) for long wavelengths. The left (resp. right) figure corresponds to the OPO (resp. TTPS) structure.

Two different regimes are apparent: for wavelengths below ~1.1 μm, data are well fitted by decaying exponential curves (red lines), whereas their decays follow inverse power laws above ~1.1 μm.

For the longest wavelengths involved, losses are fairly low (around 1 cm-1 and less) and can be ascribed to the interfaces roughness that appears during oxidation. Indeed, the reasona‐ ble agreement between the experimental data (black dots) and the prediction band derived from our simple semi-analytical model (blue bands) confirms that, in this spectral range, losses are dominated by the scattering contribution in a Rayleigh-like regime. Note that this result is consistent with previous work achieved on an older generation of similar wave‐ guides [51], in which losses were probed by scattered light measurements, and they were also found to decrease between 1.3 and 2.1 μm.

**5.3. Spectral study of propagation losses**

78 Optoelectronics - Advanced Materials and Devices

TE00 losses.

structure.

above ~1.1 μm.

Among the several different ways to measure optical losses, the Fabry-Perot fringes meas‐ urement is the most suitable technique for low-loss waveguides [50]. Unlike the cut-back method, it has the advantage of not being destructive, and contrary to transmission meas‐ urement, it is independent of coupling and collection efficiencies estimations. In this frame, we can picture the waveguide as a resonator in which guided modes are reflected by the cleaved facets and travel in both directions. By scanning the wavelength of the input wave, we observe a multiple wave interference pattern at the output, namely the Airy function of the cavity. The contrast and finesse of the resonances are straightforwardly linked to the number of waves participating to the total interference, therefore to the reflection (estimated

by FDTD simulations) and propagation losses experienced by the traveling modes.

losses were found to be too high to be reliably inferred from Fabry-Perot fringes.

In order to test our hypothesis and investigate the optical loss mechanisms, we carried out a study on their spectral dependence. The results are reported in Figure 12, for both the struc‐ tures presented previously. According to our phase-matching scheme, the data around λ<sup>P</sup> represent the TM00 losses, whereas the data at wavelength close to degeneracy represent the

The data were obtained by using external cavity diode lasers tunable in the telecom band from 1.3 to 1.6 μm, and a slightly tunable DFB laser around 2.12 μm. However we resorted to transmission measurements of a CW Ti:Sapphire laser around 1 μm and below, where

**Figure 12.** Propagation losses vs. Wavelength, fitted by decaying exponential (red curves) for short wavelengths, and inverse-power law (blue curves) for long wavelengths. The left (resp. right) figure corresponds to the OPO (resp. TTPS)

Two different regimes are apparent: for wavelengths below ~1.1 μm, data are well fitted by decaying exponential curves (red lines), whereas their decays follow inverse power laws For shorter wavelengths, our previous assumption is inadequate and a new loss mecha‐ nism must be considered. The very high losses and their exponential decay with wave‐ length suggest an Urbach's tail absorption, highlighting the fact that not only surface imperfections but also volume defects at the oxide interfaces play a critical role. As was already noticed several years ago [40], these additional losses are likely to be ascribed to arsenic antisites formed during oxidation at the oxide-semiconductor interface [52]. Trap‐ ped reaction products (*e.g.* elemental As and AsH3) could explain this excess of arsenic and the resulting EL2-like crystalline defects, with donor levels in the gap of AlGaAs. In‐ deed, the transition between the two regimes occurs at photon energy corresponding to 65 to 70% of the bandgap of the waveguide core material (respectively GaAs and Al0.25Ga0.75As).

These levels of pump losses are currently the limiting factor for the efficiency of our Al‐ GaAs integrated devices. Nevertheless, as shown by existing research on quantum-well luminescence close to oxidized layers [52] hydrogenation can be effectively used to re‐ move absorbing species trapped at the oxide interfaces. Thermal annealing cycles are al‐ so expected to improve the crystallinity of spoiled materials. Moreover, a study on the spatial distribution of the mechanical strains arising from the volume shrinkage of oxi‐ dized thin AlAs layers showed that a critical overstrained zone appears at the merging point of the two counter-propagative oxidation fronts [53]. In our case, this could be an additional source of concern since the maximum intensity of the optical modes is at the confluence of the oxidation fronts. In order to circumvent such over-oxidation issue, new innovative designs are being tested (*e.g.* oxidation unidirectionaly launched from one side of the ridge).

Despite the fact that propagation losses have prevented the use of form birefringent phasematching scheme at its full potential, we are able to design and fabricate reasonably low-loss integrated waveguides. At the moment, aluminum oxide remains very promising for our photonic applications but its quality is not yet compatible with the most demanding nonlin‐ ear devices. Various investigations of its fine properties allowed us to better understand the loss mechanisms involved, and they gave us clues for their minimization. Improvement of the oxide quality is necessary for further significant progress, and will only occur after a specific technological development of the oxidation process, starting with the solutions evoked above which are currently under exploration.

#### **6. Implementation of an integrated cavity**

#### **6.1. Design and fabrication of the cavity mirrors**

A standard way to enhance nonlinear optical interactions consists in the insertion of the χ(2) parametric gain medium into an optical resonator. The large amount of energy resulting from the light confinement inside the cavity enables to trigger nonlinear processes with low‐ er input powers and larger efficiency. To this purpose, we have been focusing on the fabri‐ cation of an integrated resonant cavity. Indeed, regarding the realization of an OPO, the modest single-pass parametric gain in our waveguides makes the quality of the cavity mir‐ rors critically important towards the reach of the oscillation threshold. One should stress that, in a 2 mm-long cavity, the mirror losses αM=-ln(R1R2)/2L becomes of the order of the typical 1 cm-1 propagation losses as soon as their reflectivity reaches 82%.

As a trade-off between the pump power threshold and the spectral stability and tunability of the oscillator, we opted for a symmetric doubly resonant configuration (DROPO) with single pass of the pump, in which case the steady-state pump threshold at degeneracy PP th reads [54]:

$$P\_P^{th} = \frac{1}{\eta\_{nom}} \left( \frac{-\alpha\_p \ln \left( R\_{S,I} \exp \left( -\alpha\_{S,I} L \right) \right)}{2 \left( 1 - \exp \left( -\alpha\_p L / 2 \right) \right)} \right)^2 \tag{6}$$

Figure 13a shows an experimental FTIR spectrum of a 6 bi-layers dielectric stack deposited on silicon substrate along with data acquired by focusing lasers with different wavelengths below a waveguide facet. The fair agreement between the experimental sets of data acquired on both the silicon and the sample substrates shows that the adhesion of the coating is satis‐ factory. The cleanliness of the sample surface can be appreciated on the scanning electron microscopy pictures provided in Figures 13b-c, where the dotted line highlights the edge of

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81

**Figure 13.** a) Experimental FTIR reflectivity spectrum of a cavity mirror, measured on a silicon substrate (dashed blue line) and below waveguide facet (red squares). b) and c) Bird's views of a mirror deposited on a waveguide facet.

In order to quantitatively evaluate the coating optical quality, the modal reflectivity of the deposited mirrors was inferred from the study of the Fabry-Perot fringes. The Airy func‐ tions, presented in Figure 14 were acquired with a DFB laser diode slightly tunable around 2.12 μm injected in a waveguide respectively without and with one coating. From the modi‐ fication of the cavity finesse, increasing from 1.7 to 3.9, we can deduce a modal reflectivity of ~85% at this wavelength. The significant discrepancy from the designed nominal value of 98.5% can be explained by the simplicity of the design procedure that only considers plane waves perpendicularly impinging on an infinite surface mirror. Indeed, the effects of the guided-mode numerical aperture and the finite transverse dimensions of the mirror are not

Finally, SPDC experiment has been achieved in those integrated cavities. The TM-polarized pump beam of a CW Ti:Sapphire laser, tunable around 1 μm, was injected through a ×60 (0.85 N.A.) microscope objective into a 2-mm long waveguide with both facets coated. The device output was collected and collimated by an identical objective. Then it was passed through a germanium window to filter out the pump beam. Finally, the TE-polarized para‐ metric fluorescence signal around 2 μm was focused on a strained InGaAs photodiode and detected with a lock-in amplifier. In Figure 15 we have plotted the external signal power

the waveguide.

**6.2. Optical characterization**

taken into account.

With ηnorm the waveguide normalized conversion efficiency, αP the pump losses, αS,I the sig‐ nal and idler losses, L the waveguide length and RS,I the modal reflectivity at signal and id‐ ler wavelength. Consequently, for a 2 mm-long device, by plugging in our typical values of parametric gain and losses we can see that mirrors with reflectivity as high as 98.3% are re‐ quired to have a threshold power of 100 mW, which is the maximum power that we can safely couple into the waveguide. The fabrication of such a demanding integrated cavity is all the more challenging that the adhesion of coating is made tricky, due to AlAs layers shrinkage during oxidation [55] producing non-ideal facets with possible irregular surface and mechanical stress. The high quality of the sample surface and waveguide sidewalls must also be preserved since the slightest deterioration would lead to additional scattering losses.

The solution we adopted to place our nonlinear waveguide in a DROPO resonator is the fab‐ rication of integrated SiO2/TiO2 Bragg mirrors. The choice of these two materials has been dictated by the need to keep the thickness of the dielectric stack to a minimum thanks to their high-index contrast. The multilayer structures corresponding to the chosen cavity con‐ figuration are designed using commercial software [56] by implementing 1D transfer matrix algorithm along with optimization routines. They are then deposited by Ion beam Assisted Deposition (IAD) onto the cleaved facets of a sample, whose surface is protected from die‐ lectric material overspray with a photoresist film lifted afterwards.

Figure 13a shows an experimental FTIR spectrum of a 6 bi-layers dielectric stack deposited on silicon substrate along with data acquired by focusing lasers with different wavelengths below a waveguide facet. The fair agreement between the experimental sets of data acquired on both the silicon and the sample substrates shows that the adhesion of the coating is satis‐ factory. The cleanliness of the sample surface can be appreciated on the scanning electron microscopy pictures provided in Figures 13b-c, where the dotted line highlights the edge of the waveguide.

**Figure 13.** a) Experimental FTIR reflectivity spectrum of a cavity mirror, measured on a silicon substrate (dashed blue line) and below waveguide facet (red squares). b) and c) Bird's views of a mirror deposited on a waveguide facet.

#### **6.2. Optical characterization**

**6. Implementation of an integrated cavity**

A standard way to enhance nonlinear optical interactions consists in the insertion of the χ(2) parametric gain medium into an optical resonator. The large amount of energy resulting from the light confinement inside the cavity enables to trigger nonlinear processes with low‐ er input powers and larger efficiency. To this purpose, we have been focusing on the fabri‐ cation of an integrated resonant cavity. Indeed, regarding the realization of an OPO, the modest single-pass parametric gain in our waveguides makes the quality of the cavity mir‐ rors critically important towards the reach of the oscillation threshold. One should stress that, in a 2 mm-long cavity, the mirror losses αM=-ln(R1R2)/2L becomes of the order of the

As a trade-off between the pump power threshold and the spectral stability and tunability of the oscillator, we opted for a symmetric doubly resonant configuration (DROPO) with single pass of the pump, in which case the steady-state pump threshold at degeneracy PP th

, , ln exp 1

æ ö - - <sup>=</sup> ç ÷ - - è ø

*P SI S I th*

a

lectric material overspray with a photoresist film lifted afterwards.

*norm P*

( ( ))

*L*

 a

*R L*

a

2

(6)

( ( ))

2 1 exp 2

With ηnorm the waveguide normalized conversion efficiency, αP the pump losses, αS,I the sig‐ nal and idler losses, L the waveguide length and RS,I the modal reflectivity at signal and id‐ ler wavelength. Consequently, for a 2 mm-long device, by plugging in our typical values of parametric gain and losses we can see that mirrors with reflectivity as high as 98.3% are re‐ quired to have a threshold power of 100 mW, which is the maximum power that we can safely couple into the waveguide. The fabrication of such a demanding integrated cavity is all the more challenging that the adhesion of coating is made tricky, due to AlAs layers shrinkage during oxidation [55] producing non-ideal facets with possible irregular surface and mechanical stress. The high quality of the sample surface and waveguide sidewalls must also be preserved since the slightest deterioration would lead to additional scattering

The solution we adopted to place our nonlinear waveguide in a DROPO resonator is the fab‐ rication of integrated SiO2/TiO2 Bragg mirrors. The choice of these two materials has been dictated by the need to keep the thickness of the dielectric stack to a minimum thanks to their high-index contrast. The multilayer structures corresponding to the chosen cavity con‐ figuration are designed using commercial software [56] by implementing 1D transfer matrix algorithm along with optimization routines. They are then deposited by Ion beam Assisted Deposition (IAD) onto the cleaved facets of a sample, whose surface is protected from die‐

typical 1 cm-1 propagation losses as soon as their reflectivity reaches 82%.

**6.1. Design and fabrication of the cavity mirrors**

80 Optoelectronics - Advanced Materials and Devices

*P*

h

*P*

reads [54]:

losses.

In order to quantitatively evaluate the coating optical quality, the modal reflectivity of the deposited mirrors was inferred from the study of the Fabry-Perot fringes. The Airy func‐ tions, presented in Figure 14 were acquired with a DFB laser diode slightly tunable around 2.12 μm injected in a waveguide respectively without and with one coating. From the modi‐ fication of the cavity finesse, increasing from 1.7 to 3.9, we can deduce a modal reflectivity of ~85% at this wavelength. The significant discrepancy from the designed nominal value of 98.5% can be explained by the simplicity of the design procedure that only considers plane waves perpendicularly impinging on an infinite surface mirror. Indeed, the effects of the guided-mode numerical aperture and the finite transverse dimensions of the mirror are not taken into account.

Finally, SPDC experiment has been achieved in those integrated cavities. The TM-polarized pump beam of a CW Ti:Sapphire laser, tunable around 1 μm, was injected through a ×60 (0.85 N.A.) microscope objective into a 2-mm long waveguide with both facets coated. The device output was collected and collimated by an identical objective. Then it was passed through a germanium window to filter out the pump beam. Finally, the TE-polarized para‐ metric fluorescence signal around 2 μm was focused on a strained InGaAs photodiode and detected with a lock-in amplifier. In Figure 15 we have plotted the external signal power 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‐ GaAs-based nonlinear devices.

**7. Conclusion and perspectives**

their respective characteristics and performances.

degeneracy is typically between 4 and 5 μm.

schemes.

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

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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

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

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

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‐

**Ref. α(1.55 μm) [cm-1] α (775 nm) [cm-1] PSH[μW] ηnorm[%W-1cm-2] Δλ [nm] [57]** 7.8 *41* 0.023 6.8 × 10-3 *0.9* **[24]** *1.7* **2.2** *6.6* 92 **0.3 [38]** 5.3 70 10-4 **1250** 10 **[9] 1.1** 140 **267** *1120* 3

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

(resp. in italics) correspond to the best (resp. second best) value of each column.

quiring spectrally narrow and pure sources of telecom twin-photons.

chapter is part of the sustained research activity led on these two fronts.

**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.
