International audienceIn this paper, the recent progress on a new Ge-rich SiGe platform for mid-IR integrated photonics is presented. Low-loss spiral waveguides working over a broadband wavelength range are discussed, followed by a sensing proof-of-concept using a standalone photoresist with a known spectral absorption pattern. In addition, the development of new mid-IR interferometric devices for wavelength filtering and enhancement of the light-matter interaction are presented. Finally, efficient designs to exploit the third-order nonlinearities in these Ge-rich SiGe waveguides at mid-IR wavelengths are shown. The demonstration of these key building blocks will pave the way towards the implementation of new mid-IR photonic integrated systems with multiple functionalities

Alonso-Ramos, C

Ballabio, A

Chrastina, D

Frigerio, J.

Isella, G

Le Roux, X

Liu, Q

Marris-Morini, D

Ramirez, J.M

Vakarin, V

Vivien, L

English

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Ge-rich graded-index SiGe alloys: exploring a versatile
platform for mid-IR photonics
J.M Ramirez, Q Liu, V Vakarin, J. Frigerio, A Ballabio, D Chrastina, X Le
Roux, C Alonso-Ramos, G Isella, L Vivien, et al.
To cite this version:
J.M Ramirez, Q Liu, V Vakarin, J. Frigerio, A Ballabio, et al.. Ge-rich graded-index SiGe alloys:
exploring a versatile platform for mid-IR photonics. ICTON 2018, Jul 2018, Bucarest, Romania.
￿hal-02362444￿
Ge-rich graded-index SiGe alloys: exploring a versatile platform 
for mid-IR photonics 
J. M. Ramirez
1*
, Q. Liu
1
, V. Vakarin
1
, J. Frigerio
2
, A. Ballabio
2
, D. Chrastina
2
, X. Le Roux
1
, C. Alonso-
Ramos
1
, G. Isella
2
, L. Vivien
1
 and D. Marris-Morini
1
 
1
Centre de Nanosciences et de Nanotechnologies, Université Paris Sud, CNRS, Université Paris Saclay, 91405 
Orsay, France 
2
L-NESS, Dipartimento di Fisica, Politecnico di Milano, Polo di Como, Via Anzani 42, 22100 Como, Italy 
*e-mail: joan-manel.ramirez@u-psud.fr 
ABSTRACT 
In this paper, the recent progress on a new Ge-rich SiGe platform for mid-IR integrated photonics is 
presented. Low-loss spiral waveguides working over a broadband wavelength range are discussed, followed by a 
sensing proof-of-concept using a standalone photoresist with a known spectral absorption pattern. In addition, 
the development of new mid-IR interferometric devices for wavelength filtering and enhancement of the light-
matter interaction are presented. Finally, efficient designs to exploit the third-order nonlinearities in these Ge-
rich SiGe waveguides at mid-IR wavelengths are shown. The demonstration of these key building blocks will 
pave the way towards the implementation of new mid-IR photonic integrated systems with multiple 
functionalities.  
Keywords: Mid-IR integrated photonics, SiGe alloys, mid-IR spectroscopy, waveguides, interferometric optical 
devices, non-linear mid-IR photonics. 
1. INTRODUCTION 
Mid-IR integrated photonics is recently gaining attention due to the wide range of applications foreseen 
including chemical and biological sensing, thermal imaging, early medical diagnosis or secure communications 
[1]. Among others, the possibility to use the mature CMOS technology to develop novel photonic integrated 
circuits exploiting the unique molecular mid-IR absorption patterns of several chemical and biological 
substances rises as one of the driving forces that push this research field. This approach would enable ultra-
sensitive label-free photonic integrated sensors which could be integrated along with other components in a 
single photonic chip, hence opening the route towards multifunctional photonic systems. For that, however, a 
suitable platform capable of integrating several photonic components such as waveguides, Bragg filters, 
resonators or Mach-Zehnder interferometers should be implemented. Promising mid-IR integrated platforms to 
carry out this task stand for Ge-on Si, suspended Si or SiGe alloys with tailored Ge concentration, among others 
[2-4]. This latter approach is expected to offer high versatility and broadband transparency [5], leading to the 
development of important photonic building blocks such as low-loss waveguides and wideband Mach-Zehnder 
interferometers for the mid infrared [6, 7]. In spite of the progress achieved so far using SiGe alloys, there are 
still several open questions that require further exploration, such as the disclosure of the platform transparency 
bandwidth, the sensitivity background or the investigation of the nonlinear properties in the mid-IR, taking 
advantage from the absence of two-photon absorption at early mid-IR wavelengths [8].  
Thus, in this paper we will review our work on the development of a new Ge-rich SiGe platform for mid-IR 
photonics. Spiral waveguides showing broadband low-loss operation will be revisited, followed by the 
demonstration of a real sensing scenario based on molecular absorption within the evanescent field in 
waveguides cladded with a standalone photoresist. Additionally, the work done towards the demonstration of 
interferometric devices working at mid-IR wavelengths will also be shown. Finally, recent works on nonlinear 
active devices will be discussed, providing a comprehensive roadmap towards the exploitation of the third-order 
nonlinearity in Ge-rich SiGe systems to generate a supercontinuum at mid-IR wavelengths. 
2. BROADBAND LOW-LOSS GE-RICH SIGE WAVEGUIDES 
The development of photonic integrated structures in the mid-IR requires first a preliminary study to retrieve 
the best design parameters for efficient mode propagation over a wide range of wavelengths. In this regard, our 
design parameters have been optimized to cover a wavelength range from λ = 5.5 µm to 8.5 µm, since this is the 
available wavelength range in our experimental setup. Furthermore, the material constraints should also be 
considered, since Ge and Si have different lattice parameters. As a consequence, the growth of Ge-rich layers on 
Si substrates can result in the generation of a highly defective layer-substrate interface which is lossy and 
therefore limits the overall performance for photonic applications. In order to palliate this circumstance, a graded 
buffer layer is grown first, with a gradual increase of the Ge concentration in the growth direction to 
accommodate the lattice mismatch between Si and Ge, providing a linear increase of the vertical refractive 
index, as depicted in figure 1(a). Then, a Ge-rich SiGe layer is grown with a slightly higher Ge concentration, 
providing sufficient refractive index contrast to enable the confinement of an optical mode. By doing that, we 
obtain high quality layers with a low threading dislocation density (TDD) in the order of 3×10
6
 cm
-2
. Then, a 
partial etching of the SiGe stack is used to define the rib-like waveguide structures, as seen in the schematic 
waveguide cross-section of figure 1(b). In our case, we have defined 4 µm-wide waveguides with an equally 
etching depth of 4 µm since it provides a good trade-off between mode confinement and moderate sidewall 
roughness using the available fabrication facilities. A set of spiral waveguides with different lengths, i. e. L = 0.6 
cm / 4.2 cm / 6.1 cm / 8.3 cm, were fabricated to measure propagation losses by means of the cut-back technique 
in butt-coupling configuration [11]. Figure 1(c) shows a top microscope view of the fabricated spirals, along 
with an inset of a representative finite-difference-method (FDM) simulation of the propagating optical mode for 
an operating wavelength of λ = 5.8 µm and a TE polarization. The measured propagation loss characteristic can 
be observed in figure 1(d), showing a flat performance with losses comprised between 2-3 dB/cm for both 
polarizations. This performance demonstrates the potential of these waveguides to be used as fundamental blocks 
for the development of mid-IR photonic integrated circuits at these wavelengths.    
 
Figure 1: (a) Vertical refractive index profile of waveguides. (b) Waveguide cross-section. (c) Top view of fabricated 
waveguide spirals. Inset shows an FDM optical simulation of the propagating TE mode at λ = 5.8 µm. (d) Measured 
propagation loss characteristic over the available wavelength range from λ =5.5 µm to 8.5 µm for both TE (in black) and 
TM (in orange) polarizations. 
3. MID-IR SENSING USING AN ABSORPTIVE UPPER CLADDING 
The previous section has shown promising mid-IR transmission characteristics for Ge-rich SiGe spiral 
waveguides. Bearing in mind one of the main driving applications of these photonic integrated chips, which is 
the conception of new effective mid-IR sensing systems, a basic proof-of-concept was conceived to inspect the 
sensitivity of our waveguides to changes in the upper cladding [12]. For that, we spin-coated the spiral 
waveguides with a standard photoresist (S1818) and measured the changes in the transmission spectrum. Figure 
2 shows representative SEM images of the waveguides before (a) and after (b) covering them with the 
photoresist. As a particular example, figure 2(c) displays the comparison between the transmission of an air-
cladded (black line) and a photoresist-cladded (orange line) waveguide with a total length of L = 8.3 cm. As can 
be observed, several absorption patterns appear in the photoresist-cladded waveguide uniquely, which can be 
associated to different molecular absorption bands of the photoresist. To corroborate this fact, the absorption 
spectrum of the photoresist was retrieved from transmission measurements in spiral waveguides over a 
wavelength range from λ = 5.2 µm to 7 µm, obtaining a good correlation with the photoresist absorption 
spectrum retrieved when performing normal-incidence transmittance measurements in a standalone Si wafer 
spin-coated with the same photoresist. Figure 2(d) illustrates this comparison, showing a close correlation of the 
different absorption peaks. It should be noticed that the absorption spectra have been normalized and up-shifted 
for convenience.    
 
Figure 2. SEM image of the fabricated SiGe waveguide with an air cladding (a) and with an S1818 photoresist cladding (b). 
Insets show closer cross-section SEM views of the waveguide corresponding to the white-dashed line. (c) Mid-IR 
transmission for an air-cladded (in black) and a photoresist-cladded (in orange) spiral waveguide with a total length of L = 
8.3 cm. (d) Comparison of the retrieved absorption pattern of the S1818 photoresist obtained from transmission 
measurements in waveguides (in black) and the measured absorption spectrum of the photoresist obtained from surface 
illuminated experiments (in orange). 
4. INTERFEROMETRIC DEVICES: MACH-ZEHNDER INTERFEROMETERS  
Aside from waveguides, the demonstration of integrated optical components able to deliver interferometric 
patterns in the mid-IR is also a major requirement to implement complex photonic circuits with manifold 
functionalities. In particular, the demonstration of Mach-Zehnder interferometers is interesting to perform 
multiple wavelength filtering and eventually mid-IR Fourier transform integrated spectrometers. With this aim, 
we recently demonstrated asymmetric Mach-Zehnder interferometers working over a broadband wavelength 
range from λ = 5.5 µm to 8.5 µm for both polarizations, showing good tunability of the free spectral range and 
an extinction ration of at least 10 dB regardless of the operation wavelength range [7]. Figure 3(a) displays a top 
view of the fabricated devices, along with a SEM view of the multimode interferometer responsible for the 
power splitting and merging of the propagating optical mode. Figure 3(b) describes a representative spectrum of 
a Mach-Zehnder device containing a difference arm length of ΔL = 48 µm working all over the available 
wavelength range in the mid-IR. Thus, good prospects for the development of new Fourier transform integrated 
spectrometers are envisioned by combining these fundamental interferometric building blocks.    
 
 
Figure 3. (a) Top microscope view of the fabricated asymmetric Mach-Zehnder interferometers with a zoomed-in region 
showing an SEM image of the multimode interferometer. (b) Measured spectral response of a Mach-Zehnder device with ΔL 
= 48 µm.  
 
5. NEW WAVEGUIDE DESIGNS FOR SUPERCONTINUUM GENERATION IN THE MID-IR  
In addition to the above-described passive mid-IR structures, there is also an increasing interest in developing 
mid-IR active devices able to exploit the nonlinearities of the raw materials. In particular, Ge-rich SiGe alloys 
are good candidates for that purpose since they show remarkably high third-order nonlinearity compared to other 
CMOS-compatible materials such as silicon oxide, silicon nitride or silicon. Moreover, these nonlinear effects 
become more efficient in Ge-based materials at longer wavelengths due to the suppression of two-photon 
absorption from λ = 3.2 µm on. Consequently, any pumping signal with a wavelength centred beyond this value 
will potentially produce strong nonlinear effects in our Ge-rich SiGe platform provided that a proper waveguide 
dispersion engineering is accomplished. In order to fulfil this condition, an optimization of the waveguide 
geometry was carried out for a broad wavelength range from λ = 3 µm to 8 µm. In this case, instead of using the 
waveguides previously described in figure 1(b), we have employed a new waveguide cross-section that consists 
in a single SiGe graded buffer with a linear increase of the Ge concentration from Si to pure Ge in a total 
thickness of 6 µm. Thus, the optical mode is now confined in the graded buffer directly. Particularly, figure 4(a) 
shows the new waveguide cross-section with the dispersion engineered geometric parameters, having a 
waveguide width of W = 4 µm and an etching depth of D = 4 µm. These values were chosen based on a 
systematic study of the dispersion value as a function of both W and D for different wavelengths, as observed in 
the set of coloured panels on top of figure 4(b). It is worth to notice that by choosing the selected parameters 
(marked by a grey dot in each panel), flat anomalous dispersion enclosed in the low dispersion regime, i. e. D < 
100 ps/nm/km, is obtained over one octave spanning for both polarizations, as illustrated in figure 4(c). The 
origin of such broadband characteristic is believed to be due to the gradual mode-size accommodation that takes 
place in graded SiGe waveguides as illustrated at the bottom of figure 4(b). As seen, the mode is confined in the 
upper part of the waveguide corresponding to the Ge-rich area, and provides a gradual downwards expansion of 
the propagating optical mode for longer operating wavelengths, delivering a unique dispersion compensation at 
each wavelength that can be exploited to develop efficient supercontinuum light sources in the mid-IR range. 
Moreover, a promising third-order nonlinear parameter is anticipated using this approach [9].          
 
Figure 4. (a) Waveguide cross-section for the implementation of nonlinear devices. (b) (Top panels) Coloured maps showing 
the anomalous dispersion characteristic as a function of the waveguide geometry and the operating wavelength. (Bottom 
panels) Optical mode simulations as a function of the operating wavelength (TE polarization). (c) Calculated dispersion 
curves for a wavelength span of λ = 3-8 µm for both TE (black squares) and TM (red dots) polarizations.   
6. CONCLUSIONS 
In conclusion, we have shown the recent advances on the implementation of a new mid-IR photonic 
integrated platform based on Ge-rich SiGe alloys. Progress towards the demonstration of several fundamental 
building blocks having broadband operation in the mid-IR has been revisited, including waveguides, Mach-
Zehnder interferometers, and nonlinear waveguides for supercontinuum generation. Moreover, a representative 
proof-of-concept that exploits the evanescent-field absorption in SiGe waveguides to retrieve several mid-IR 
absorption patterns of a standalone photoresist has been performed, demonstrating the potential of this platform 
for mid-IR photonic chip-scale sensors.  
ACKNOWLEDGEMENTS 
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 
research and innovation program (grant agreement N°639107-INsPIRE). The fabrication of the devices was performed at the 
Plateforme de Micro-Nano-Technologie/C2N, which is partially funded by the “Conseil Général de l’Essonne”. This work 
was partly supported by the French RENATECH network.  
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[3] Z. Cheng, et al. “Mid-infrared suspended membrane waveguide and ring resonator on silicon-on-insulator,” IEEE 
photonics journal, 4(5), 1510 (2012). 
[4] M. Brun, et al. “Low loss SiGe graded index waveguides for mid-IR applications,” Opt. express, 22(1), 508 (2014). 
[5] J. M. Ramirez, et al. “Ge-rich SiGe waveguides for mid-infrared photonics,” Proc. Of SPIE 10108 (2017). 
[6] J. M. Ramirez, et al. “Low-loss Ge-rich Si 0.2 Ge 0.8 waveguides for mid-infrared photonics,” Opt. Lett. 42 (1), 105 
(2017). 
[7] V. Vakarin et al, “Ultra-wideband Ge-rich silicon germanium integrated Mach Zehnder interferometer for mid infrared 
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[8] F De Leonardis, et al. “Germanium-on-Silicon Waveguide Engineering for Third Harmonic Generation in the Mid-
Infrared,” J. Lightwav. Technol. 33 (24), 5103, 2015. 
[9] J. M. Ramirez et al, “Ge-rich graded-index Si1-xGex waveguides with broadband tight mode confinement and flat 
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Ge-rich graded-index SiGe alloys: exploring a versatile platform for mid-IR photonics

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Ge-rich graded-index SiGe alloys: exploring a versatile platform for mid-IR photonics

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