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# PHOTONIC GE-BASED PLATFORMS FOR MID-INFRARED APPLICATIONS

V. REBOUD<fnref n="1" />*, J. M. HARTMANN<fnref n="1" />, S. SERNA<fnref n="2" />,<fnref n="3" />, K. STOLL<fnref n="3" />, C. MONAT<fnref n="4" />, C. GRILLET<fnref n="4" />

<figure><img src="image_1.png" /></figure>

https://doi.org/10.1051/photon/202412552

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in medium, provided the original work is properly cited.

Silicon photonics, initially developed for data communications in the near infrared (NIR), has been extended over the last ten years to the mid-infrared (MIR) region. Efforts have been made to fully integrate MIR functions at chip level on Ge and Si planar substrates, to achieve greater reliability and higher performance at lower cost. The fields of application are vast, ranging from IR imaging of biological tissues to environmental monitoring, control of industrial processes, security and many more.

Spectroscopic systems operating in the Mid-infrared (e.g. the MIR- wavelength region between 3 and 15 µm) can track specific chemical and biological fingerprints for molecule identification and quantification. They are based on light absorption induced by the excitation of rotational and vibrational energy levels of target molecules. As the light is absorbed by the target molecule, and analysed spectrally (e.g via the spectral analyser), multiple absorption lines appear in the absorption spectrum, their linewidths and peaks depending on the surrounding

environment (gas/liquid phases, temperature, pressure etc.). The wavelengths and amplitudes of the absorption in the spectrum provide information about the detected species and their amounts. Figure 1 shows examples of molecules of interest accessible in the mid-infrared covering the field of environmental monitoring, defence and security, industrial process monitoring and medical diagnostics.

Our long-term vision is to create integrated photonic absorption sensors wherein the evanescent part of the propagating optical mode is absorbed by a medium/target molecule located outside in the vicinity of the

waveguide. One approach relies on the use of tunable light sources (or an array of light sources) with narrow linewidths, with wavelengths that are absorbed by the substances/analytes to be detected. The transmission is then measured with a detector. Another approach involves using a broadband and bright optical source called supercontinuum (SC) generated in Ge or SiGe waveguides and placing a spectrometer before the photodetector to measure transmission as a function of wavelength. We present here examples of Ge-based integrated photonic circuits used for environmental detection

<footnote n="1">¹Univ. Grenoble Alpes, CEA, LETI, 38054 Grenoble, France</footnote>

<footnote n="2">²Bridgewater State University, Physics Department, 131 Summer St., Bridgewater, MA 02324, USA</footnote>

<footnote n="3">³Materials Research Laboratory and Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States</footnote>

<footnote n="4">⁴INL, Institut des Nanotechnologies de Lyon, Lyon, France</footnote>

<footnote>*vincent.reboud@cea.fr</footnote>
and the working principles of super-
continuum light sources and integrated
spectrometers.

GE-BASED INTEGRATED PHOTONIC
CIRCUITS

Over the past 10 years, optoelectronic
components based on germanium (Ge)
have been developed to expand the po-
tential of silicon (Si) photonic circuits.
Photodetectors, modulators, and Ge-
on-Si lasers have been demonstrated
in the mid-infrared region. Ge's main
advantage lies in its large transparency
window, ranging from 1.8 to 14 µm in
wavelength and its CMOS compatibility.
Ge and SiGe alloys have quickly been
regarded as preferred materials for the
development of integrated photonic com-
ponents. Thick Ge and SiGe layers (up to
40% of Ge) are conventionally grown on
200 mm and 300 mm Si(001) wafers by
chemical vapor deposition in industrial
epitaxy cluster tools. Further details on
Ge and SiGe growth can be found in re-
ference [1]. SiGe or Ge on insulator (like
SiN) wafers can be fabricated from the
previous epitaxies. In such a case, two
wafers need to be bonded: the first ones
with Ge or SiGe epitaxial layers capped
with SiNx layers and thin SiO₂ layers, the
second ones being oxidized Si wafers.
After SiO₂ to SiO₂ bonding, the starting

substrates on which Ge or SiGe thick
layers were grown, are removed by grin-
ding followed by chemical mechanical
polishing steps. Polishing improves the
Ge crystalline quality by removing the
heavily dislocated interface between Si
and Ge. Waveguides are then patterned
by conventional photolithography and
deep reactive ion etching (RIE) down to
the Si substrate (Figure 2a and Figure 2b)
or the SiN layer (Figure 2c).

A set of passive functions, such as
wavelength multiplexers, couplers,
paperclips, spirals, Mach Zehnder inter-
ferometers and ring resonators can be
fabricated using conventional lithogra-
phic steps on epitaxial wafers. A review
on silicon-based mid-IR integrated pho-
tonics can be found in reference [1,2].
Such miniaturized integrated devices
offer higher accuracy, reliability, porta-
bility, and cost effectiveness compared
to existing systems, enabling their use
in new application fields like oil and gas
process monitoring, gas leak detection,
agri-food manufacturing monitoring,
and point-of-care and portable labora-
tory bioanalyses. On-chip absorption
sensing of parrafin oil has, for example,
been achieved with Ge/SiN spiral wave-
guides (Figure 2c and Figure 2d). The
propagation losses were evaluated for se-
veral wavelengths (5.9 µm, 6.7 µm) •••

<figure><img src="image_2.png" /><figcaption>**Figure 1.**
Examples of
molecules
of interest
accessible in
the mid-infrared
covering the field
of environmental
monitoring,
defence and
security,
industrial process
monitoring
and medical
diagnostic.
Reproduced from
Laser Photonics
Rev. 11, No. 2,
1700005 (2017).</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Electron microscopy image of a/cleaved SiGe 40% on Si waveguide, b/a Ge on Si waveguide and c/a Ge on SiN waveguide, d/SEM top view of a Ge spiral waveguide on SiN, e/Injection set-up of laser light at 5.9 µm and 6.7 µm into the spirals of a silicon photonics circuit.</figcaption></figure>

in so-called butt-coupling configurations (Figure 2e). An increase in propagation losses of 0.9 dB/cm in the presence of chemically inert paraffin oil was obtained at 6.7 µm corresponding to the maximum peak absorption of the paraffin liquid. Apart from the absorption peak at 6.7 µm, an increase less than 0.15 dB/cm in propagation losses at 5.9 µm was measured in the presence of the oil (the mode deconfinement due to the presence of the liquid explains, among other things, this increase of losses at 5.9 µm). Such approach could also be applied in the medical field to identify chemicals and quantify the concentration of liquids thanks to their characteristic infrared

absorption features. For example, more patients could benefit from absorption spectroscopy analysis to detect biomarkers of metabolic variations or inflammatory phenomena. These biomarkers provide valuable information for the early detection of cancer, avoiding false positives, improving prognosis and lowering treatment costs.

## GE-BASED LIGHT SOURCES

A key ingredient for MIR integrated spectroscopic systems is the light source. A lot of progress has been made with important research efforts on direct band gap lasers (as III-V or group-IV lasers), interband cascade lasers, quantum cascade lasers (QCLs), optical parametric oscillators (OPO) or fiber-based-lasers, to name a few. QCLs, probably the current MIR sources of choice, are already commercially available but often require low temperature operation and are costly. They otherwise have a restricted tunable spectral

<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) Illustration of supercontinuum generation in a SiGe waveguide with either normal or anomalous dispersion dynamically controlled by top cladding phase change material (credit Alberto Della Torre), (b) Simulated and (c) measured (b) SC spectra for All Normal Dispersion waveguide for various coupled average powers. The top red curve in (b) shows the SC spectra's calculated coherence at the highest (3.14 kW) peak power. The black arrows in (c) indicate the 10 dB and 30 dB limits of the spectrum. The measured coupled input and on-chip output average powers are given in the inset (c) and the estimated coupled peak powers in the inset (c). Reproduced from [4].</figcaption></figure>
range, so that a single QCL cannot cover
the entire molecular fingerprint. Instead
of multiplexing the output of a series of
fixed wavelength QCLs, a single bright
and broadband source that can access all
the wavelengths in parallel like a super-
continuum (SC) [3], seems an attractive
alternative. Supercontinuum generation
is usually achieved by broadening, via
cascaded nonlinear effects (Figure 3), the
spectrum of a mode-locked pump laser,
constituted of hundreds of thousands
closely spaced frequency lines (frequen-
cy comb). This comb structure results
in a supercontinuum that can cover a
spectral band of more than one octave
in the molecular fingerprint region. It is
therefore well suited for high-resolution
multi-species parallel molecular spec-
troscopy with high throughput.

Generally speaking, SC generation
can be achieved in either anomalous
dispersion (i.e. positive group velocity
dispersion) or normal dispersion (i.e. ne-
gative group velocity dispersion) regime
(Figure 3). The former is characterized by
a broad spectrum, which is, however, not
spectrally flat, and has low coherence.
Our group demonstrated such SC back in
2018, with the first SiGe waveguide-based
SC reaching the onset of Si absorption
(8.5 µm) that was broadband (1.4 octave
span) and bright (20 mW on-chip average
power with potentially 70% conversion
efficiency). The latter allows to generate
flat and low noise SC spectra but general-
ly to the detriment of the bandwidth. We
demonstrated these so-called ANDi SC
for the first time in the MIR in SiGe wave-
guides in the early 2020 [4]. Recently, we
leveraged the potential of integrated op-
tics for a precise dispersion control to go
a step further. We designed a multi stage
waveguide (anomalous dispersion sec-
tion followed by a varying width section
resulting in a varying dispersion from
anomalous to normal) to simultaneously
increase the bandwidth of the generated
SC and improve its spectral flatness. We
exploited such SC for a proof-of-principle
demonstration of multi-species gas (wa-
ter vapour and carbon dioxide) spectros-
copy in our laboratory environment [5].
We are currently exploring heteroge-
neous integration schemes with phase

ange material to dynamically control
the waveguide dispersion state and the-
refore the supercontinuum spectral and
temporal properties (Figure 3).

As previously stated, the high ma-
nufacturing cost of quantum cascade
lasers remains a significant hurdle in
their widespread adoption for chemical
sensing applications. However, the in-
tegration of Mid-Infrared sources onto
silicon substrates utilizing CMOS techno-
logy presents a promising way for mass
production at a low cost. Additionally,
leveraging a silicon-based fabrication
platform enables the simultaneous in-
tegration of QCL Mid-Infrared sources
with SiGe-based waveguides, facilita-
ting the development of fully integrated
optical sensors on a planar substrate.
Distributed-feedback QCL sources, ope-
rating at 7.4 µm and bonded onto 200 mm
Si wafers, were recently fabricated in the
CEA 200 mm CMOS pilot line (Figure 4a).
This advancement is a significant step
toward the realization of Mid-Infrared
spectrometers at the silicon chip level [6].

GE-BASED INTEGRATED
SPECTROMETERS AND DETECTION

Various integrated spectrometers based
on SiGe or Ge waveguides have been
evaluated to analyse the absorbed light
that was beforehand emitted by a super-
continuum. SiGe 40 % arrayed waveguide
gratings (AWG) on Si and Ge AWG on Si
have been demonstrated in the 3 to 8 µm
and the 8 to 12 µm ranges, respectively.
They were operational for both TE and
TM light polarizations. The working
principle of a demultiplexing AWG is
presented in Figure 4b: (1) waveguide
containing several wavelengths from
a supercontinuum or from an array
of lasers, (2) planar waveguide, (3) an
array of waveguides, (4, 5) a recombi-
nation guiding area, (6) several output
waveguides containing different wave-
lengths. Figure 4c shows transmission
losses and spectral characteristics of a
multiplexing device with a Ge core em-
bedded in thick Si₈₀Ge₂₀ layers. A channel
spacing of 2.96 ± 0.16 cm⁻¹ was reached
with a crosstalk of 7.4 dB in this particu-
lar AWG [3].

Photodetection within this wavelength
<figure><img src="image_5.png" /></figure>

range requires semiconductors having small, direct band gaps, typically less than 0.4 eV to be placed after the spectral analyser. Widely used mid-infrared (mid-IR) photodetector materials encompass mercury cadmium telluride (HgCdTe), III-V compounds like InAs and InSb and group-IV compounds such as GeSn alloys. Additionally, commercially available detectors use lead salts or lead chalcogenides such as PbSe, and PbS or PbTe [7]. Interband cascade detectors with III-V materials or quantum cascade detectors are among the device architectures currently under investigation. Finally, photoacoustic detectors are as well good candidates for MIR detection as they can be integrated onto planar substrates [2].

## CONCLUSION

Ge-based photonics is a solution widely explored today for the development of photonic systems in the mid-infrared. Common technological platforms for integrated and cost-effective mid-IR photonic components are essential for novel applications such as chemical sensing in liquid and gas phase and could have an important impact in the field of sensors for the general public. Currently, the research primarily focuses on demonstrating various aspects with elementary components. Ultimately, the integration of these different elements

<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Typical spectral power</figcaption></figure>

of a QCL array emitting at 7.4 µm bonded on 200 mm silicon wafer, reproduced from [6], (b) Principle of operation of an AWG, (c) Spectra measured for 67 outputs of the AWG, reproduced from [3].

will be key to fabricate complex systems. The upcoming challenges will focus on the integration of MIR light sources and photodetectors on photonic platforms. ●

## ACKNOWLEDGEMENTS

We would like to acknowledge the scientific collaborators who have been working with us, in particular: O. Lartigue, V. Mathieu, N. Bresson, B. Luther-Davies, S. Madden, D. J. Moss, A. Biegansky, S. Cueff, E. Postelnicu, Pr. A. Murthy Agarwal,

M. Perestjuk, A. Mitchell, T. Nguyen, A. Boes, A. Della Torre, R. Armand, M. Sinobad. We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), the Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Actions (grant ECLAUSion 801512, REDI 101034328), the European Union's Horizon 2020 LASTSTEP Project under the grant agreement ID: 101070208, the Agence Nationale de la Recherche (MIRSIOMB, ANR-17-CE24-0028; MIRthFUL, ANR-21-CE24-0005; PEPR OFCOC, ANR-22-PEEL-0005), the MIT MITSI program and the IPCEI Microelectronics and Connectivity supported by the French Public Authorities within the frame of France 2030.

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