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Abstract
We design and demonstrate newcomers in the field of frequency down-converters based on quasi-phase matching: low-loss orientation-patterned gallium antimonide ridge waveguides suited to parametric conversion from pumping lasers around 2 µm to mid-infrared wavelengths around 4 µm. Thanks to careful control of the epitaxial growth conditions including low N-type doping and to an optimized dry etching process, losses as low as 0.7 dB/cm at 2 µm and 1.1 dB/cm at 4 µm are obtained.
© 2017 Optical Society of America
1. Introduction
In the field of nonlinear crystals with large second order susceptibilities suited to user-friendly frequency conversion based on Quasi-Phase Matching (QPM), the advent of Periodically Poled Lithium Niobate (PPLN) and related oxides has turned into a lasting commercial success. Orientation-Patterned semiconductors have been developed more recently for applications requiring longer wavelengths and often been compared to PPLN. For free-space beams of diameters roughly below one millimeter, the main practical difference lies in the transparency window of the various crystals. For numerous applications, bulk Orientation-Patterned gallium arsenide (OP-GaAs) can for example be considered as “the PPLN of the mid-infrared” from below 2 µm up to about 15 µm [1,2] and thick orientation-patterned growth techniques have been extended to other semiconductors [3,4].
In waveguide configurations, examples are scarcer. Since seminal experiments on frequency conversion for telecommunication applications in OP-GaAs/GaAlAs structures [5], propagation losses have been identified as an important limitation: one of the best results to date at those wavelengths is still superior to 6 dB/cm [6]. The problem could nevertheless be circumvented at longer wavelengths, leading to the first QPM waveguide optical parametric oscillator (OPO) pumped around 2 µm [7]. To the best of our knowledge, apart from recent OP-GaP/AlGaP structures [8], no other QPM waveguides have been fabricated.
A second-order susceptibility of 150 pm/V and thermal conductivity of 32 W/m.K both make GaSb a promising candidate for QPM nonlinear integrated optics. In this work, we have been investigating for the first time low-loss OP-GaSb/AlGaAsSb waveguides designed for parametric conversion from pumping wavelengths around 2 µm to the mid-infrared part of the spectrum well known for its spectroscopic applications.
Our approach is presented as follows. After recalling some relevant GaSb properties, paragraph 2 deals with the initial design choices and gives the simulation results that led to the selected waveguide design. Paragraph 3 presents the main fabrication steps and associated microscopic characterizations. It also describes the motivations for key iterations ending in the selection of low N-type doping. Paragraph 4 puts the emphasis on the obtained losses before the conclusion is drawn.
2. Waveguide design
The first difficulty encountered to model our waveguides was the lack of reliable refractive index values. In the absence of sufficiently precise data in the mid-infrared, we resorted to fabricate planar waveguide samples suited to M-lines measurements for various wavelengths, alloy compositions and doping levels. The results, published elsewhere [9], were fed into a commercial software with a finite difference mode solver (FIMMWAVE) to design OP-GaSb single mode waveguides optimized for mid-infrared generation close to degeneracy (i.e. around 4 µm) from a 2 µm pumping laser.
Figure 1(a) shows the selected epitaxial layer structure and an example of mode calculation. The ridge etching depth is low, around 500 nm, to reduce etching time and roughness. We used two confinement layers made of AlGaAsSb alloys, grown before the GaSb core. All layers are grown by Molecular Beam Epitaxy (MBE) and lattice-matched to GaSb. Together, they form a 2.1 µm-thick optical insulation layer between the 3.8 µm-thick core and the substrate. To reduce propagation losses, we chose a design where the energy propagates mostly below the ridge and barely sees the interface with air at the top of the structure (Fig. 1(b)). To study the limit between single mode and multi-mode propagation at 2 µm wavelength, 3 to 5 µm wide ridges were considered. The computed effective index values were used to find the QPM periods for the targeted interactions.
Figure 2 gives some examples of tuning curves, comparing the bulk and guided options (in a 4 µm-wide waveguide) for various pumping wavelengths.
Fig. 2 Tuning curve calculations in bulk (solid lines) and guided-wave (dashed lines) configurations for different pump wavelengths as a function of QPM period.
3. Fabrication steps
Figure 3 summarizes the main steps necessary to fabricate OP-GaSb waveguides. To facilitate our study, epitaxial regrowth was first performed on un-patterned substrates with a single [001] orientation, yielding what we call “reference waveguides” thereafter, before shifting to OP-template substrates to obtain OP-GaSb waveguides.
Initial reference waveguides, already reported in [10], had very satisfactory losses at 2 µm, as low as 0.7 dB/cm. Nevertheless, subsequent characterization gave a level of loss at 4 µm superior to 6 dB/cm, much too high for our applications. This was attributed to the residual P-type doping of the non-intentionally doped (nid) GaSb guiding layers. Thanks to careful control of growth conditions [11], we were able to ensure low N-type doping of our epitaxial layers, as illustrated in Fig. 4. New reference waveguides were thus fabricated with two different doping values for comparison of losses (see next paragraph).
Fig. 4 Net carrier-concentration determined from Hall measurements performed on GaSb:Te layers grown on semi-insulating GaAs substrates. The solid line shows the expected values in the absence of residual P-type doping. The experimental data show the potential to control low N-GaSb doping by changing the Te cell temperature to compensate for the residual doping.
In parallel to the study of reference waveguides, the growth of orientation-patterned GaSb-based layers was started from scratch and could not rely on any existing technology to fabricate OP-GaSb template substrates. We succeeded only very recently to develop on GaSb the first key steps recalled in Fig. 3, i.e. wafer bonding and substrate removal thanks to a sacrificial layer. The results presented in this paper are rather based on the already available OP-GaAs template technology. In order to check that hetero-epitaxial growth of GaSb/AlGaAsSb layer occurs while preserving the OP-GaAs template orientation-patterning, some samples were ion-beam etched for Transmission Electron Microscopy (TEM) characterizations. We also verified that the growth was conformal to the initial template corrugation and free from defects at the interface between opposite orientations (Fig. 5).
Fig. 5 TEM picture of the GaAs bonding interface and of the vertical wall between the two adjacent opposite crystal orientations (A GaAs buffer is grown before the GaSb-based layers).
For the final fabrication step, special attention was paid to etch smooth ridges. Residual differences in etching speed as a function of crystal orientation proved detrimental for all the tested chemical etchants. A dedicated Inductively Coupled Plasma (ICP) procedure was therefore developed. It gave reproducible results, characterized by Scanning Electron Microscope (SEM), shown in Fig. 6, and Atomic Force Microscopy (rms roughness around 1.5 nm on both orientations). Note that the same ICP etching conditions were used for the reference waveguides to enable rigorous comparisons.
4. Loss characterization
Transmission loss measurements in the waveguides were performed using the Fabry-Perot resonance technique [12], thanks to a 1996 nm DFB laser diode from Nanoplus GmbH. The waveguide loss coefficient is directly calculated as a function of the contrast of the fringes in transmitted power when the wavelength is changed, of the Fresnel reflectivity and of the length of the waveguide. This accurate measurement method is suitable for waveguides with moderate to low losses. Wavelength tuning is performed by modulating the laser diode current. At the uncoated cleaved GaSb facets, the Fresnel reflectivity values used for loss calculations are obtained from the computed effective indices of the fundamental modes.
The measurements have been repeated on several waveguides and samples. In the posted example (Fig. 7), they have been sorted in two categories after observation of the cleaved facets. For comparison, the best reported losses in OP-GaAs waveguides, to our knowledge, are 0.8 dB/cm at 1.55 µm, with typical values of 1-1.5 dB/cm [7]. Our novel OP-GaSb waveguides, with a lowest measured loss coefficient at 2 µm of 0.73 dB/cm and typical values of 1-3 dB/cm, thus compare favorably to the GaAs state-of-the-art. Note also that the best value measured in reference waveguides with the same doping level is around 0.5 dB/cm, emphasizing that the orientation-patterning adds only moderate losses to our design.
Fig. 7 Propagation losses at 2 µm in TE polarization for several 21-µm-period OP-GaSb waveguides of various ridge widths on the same 7.5 mm-long sample (P-type, 2.5 10−16 cm3).
Going to longer wavelengths, the source available for characterization was a homemade Quantum Cascade Laser (QCL) at 3.9 µm. Its spectral width was not suitable for Fabry-Perot experiments. We therefore used the cut-back technique. Figure 8 summarizes the results of the study of the influence of doping on losses outlined in Paragraph 3, clearly demonstrating the advantages of low N-type doping for mid-infrared generation. At the 2-µm pumping wavelength, the influence of low N-type doping is less pronounced: best values for the 6 x1015 cm−3 sample were around 1.5 dB/cm.
Fig. 8 Propagation losses at 4 µm in TE polarization for several reference GaSb waveguides of various ridge widths and residual doping levels.
5. Conclusion
Low-loss orientation-patterned gallium antimonide waveguides were designed and fabricated for the first time. First based on hetero-epitaxial MBE regrowth of antimonide layers on orientation-patterned GaAs, they will soon benefit from homo-epitaxy on novel OP-GaSb template substrates. Their excellent performance in terms of propagation losses, both at the selected 2 µm pumping wavelength and in the mid-infrared signal range around 4 µm, obtained thanks to the control of low N-type doping, makes them very promising for future wavelength converters for low cost spectroscopic applications.
Funding
“Investment for the future” program (EQUIPEX EXTRA, ANR 11-EQPX-0016), DGA/ANR (Great GaSby, ANR-13-ASTR-0034), Renatech network.
Acknowledgements
The authors thank M. Garcia and B. Simozrag for their involvement in waveguide processing.
References and links
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