Germanium-on-silicon waveguides are designed, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the in-coupling efficiency. On-chip propagation along bends and straight sections up to 0.8 mm is demonstrated around λ = 6 μm

Baldassarre, Leonetta

Biagioni, Paolo

Frigerio, Jacopo

Gallacher, Kevin

Giliberti, Valeria

Isella, Giovanni

Ortolani, Michele

Paul, Douglas J.

Sorgi, Alessia

English

Archivio della ricerca- Università di Roma La Sapienza

Integrated germanium-on-silicon waveguides for mid-infrared photonic sensing chips

Kevin Gallacher

Alessia Sorgi

Valeria Giliberti

Jacopo Frigerio

Giovanni Isella

Paolo Biagioni

Douglas J. Paul

Michele Ortolani

Leonetta Baldassarre

Crossref

Enlighten: Publications

  
 
 
 
 
Gallacher, K., Sorgi, A., Giliberti, V., Frigerio, J., Isella, G., Biagioni, P., Paul, D. J. , 
Ortolani, M. and Baldassarre, L. (2017) Integrated Germanium-on-silicon Waveguides 
for Mid-infrared Photonic Sensing Chips. In: 2017 42nd International Conference on 
Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Cancun, Mexico, 27 Aug - 
01 Sep 2017, ISBN 9781509060504. 
 
   
There may be differences between this version and the published version. You are 
advised to consult the publisher’s version if you wish to cite from it. 
 
 
 
http://eprints.gla.ac.uk/152526/  
     
 
 
 
 
 
 
Deposited on: 27 November 2017 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
Abstract—Germanium-on-silicon waveguides are designed, 
fabricated and characterized with a novel near-field infrared 
spectroscopy technique that allows on-chip investigation of the 
in-coupling efficiency. On-chip propagation along bends and 
straight sections up to 0.8 mm is demonstrated around λ = 6 μm. 
I. INTRODUCTION  
 AVEGUIDES represent the key component of 
integrated silicon photonics. The dominant technology 
in the near-infrared (λ = 1.55 μm) is the silicon-on-
insulator (SOI) platform, where Si waveguides are realized on 
SiO2 surfaces grown on Si wafers, exploiting the strong 
refractive index contrast (nSi=3.4 and nSiO2=1.4-1.7). In the 
mid-IR range of interest for biosensing (λ > 5 μm), Si presents 
non-negligible losses due to impurity states and it cannot be 
used as the guiding layer, and SiO2 has huge losses due to 
optical phonon absorption and it cannot be used at all. 
Therefore, IR transparent materials have been investigated but 
their n is typically lower than that of Si so, for use as guiding 
layers, they have to be physically suspended [1].  
  Germanium is transparent and has nGe=4.0 > nSi and 
therefore it can be used for guiding through simple Ge-on-Si 
strip waveguides [2]. The main difficulty related to this 
emerging integrated photonics technology is to couple light 
from an external mid-IR laser source into the Ge waveguide, 
and not into the Si substrate. This has been recently attained 
e.g. by graded SixGe1-x virtual substrates [3].  In this work, we 
have fabricated pure Ge waveguides on Si wafers starting 
from Ge-on-Si films grown by chemical vapor deposition. We 
have etched grating couplers and tapers into the Ge layer to 
engineer the coupling at 70° incidence angle from a 
wavelength-tunable quantum cascade laser (QCL). Finally, we 
introduce photothermal expansion spectroscopy as the 
scanning local sensing tool of the out-coupled power at the 
waveguide open end.  
II. RESULTS 
Conventional characterization of on-chip waveguides with 
dimensions smaller than the wavelngth is typically carried out 
by moving an optical fiber against the waveguide open-end 
(butt coupling). Another approach is to tightly focus the free-
space radiation onto a taper, i.e. a planar structure initially 
wider than the actual waveguide, fabricated on the chip edge; 
the taper width then decreases with increasing distance from 
the chip edge towards the waveguide width. Both methods are 
difficult to implement in the mid-IR and with a high-index 
material such as Ge, due to the unfavorable ratio between 
waveguide height and free-space wavelength that produces 
high coupling losses. The coupling losses also intervene when 
the radiation is out-coupled towards the detector, located in 
the far-field of the waveguide end. To solve these issues, we 
propose a method based on near-field mid-IR microscopy, in 
which the detector is substituted by a near-field sensor made 
by the combination of a polymer microblock fabricated on-
chip by lithography in the same process as the waveguides that 
one wants to test, and an atomic force microscopy (AFM) 
probe. When the microblock receives radiation power at a 
wavelength where it efficiently absorbs mid-IR radiation, it 
will heat up and result in a photo-induced expansion that can 
be measured by monitoring the oscillations of the AFM 
cantilever probe. Also, the in-coupling is obtained by 
illumination at oblique incidence of a diffraction grating 
fabricated on the waveguide with a tightly focused mid-IR 
laser beam, so that different regions of the same chip can be 
used for in-coupling by simply raster-scanning the laser beam 
and/or the chip. In this way, each chip with specific epitaxial 
material properties can contain several hundreds of waveguide 
circuits. This technique is closely related to the  photo-thermal 
expansion near-field microscopy (AFM-IR) [4, 5], except that 
the radiation is not directly focused on the tip, but it reaches 
the tip through the waveguide. 
 
 
 
 
 
 
In the present work, for analyzing each on-chip Ge-on-Si 
Kevin Gallacher1, Alessia Sorgi2, Valeria Giliberti3, Jacopo Frigerio4, Giovanni Isella4,5,         
Paolo Biagioni4,5,  Douglas J. Paul1, Michele Ortolani2 and Leonetta Baldassarre2 
1School of Engineering, University of Glasgow, Glasgow, G12 8LT, U.K. 
2Department of Physics, Sapienza University of Rome, Rome, 00185 Italy 
3Center for Life Nanosciences, Istituto Italiano di Tecnologia, Rome, 00185 Italy 
4L-NESS, Dipartimento di Fisica del Politecnico di Milano, Como, 22100 Italy   
5Dipartimento di Fisica, Politecnico di Milano, Milano, 20133 Italy 
Integrated Germanium-on-Silicon Waveguides  
for Mid-Infrared Photonic Sensing Chips 
W
Fig.1  Near-field imaging at λ=5.8 μm of the radiation intensity on a Ge-
on-Si photonic chip featuring a waveguide bend (grey line), an 
incoupling grating with taper (elongated intensity spot, mostly blue) and 
an open-end waveguide (red spot at the waveguide end, where the near-
field probe is positioned). Inset: SEM image of the grating coupler. 
waveguide circuit, s-polarized radiation from an external-
cavity tunable QCL tuned in the 900-1900 cm-1 is focused 
onto the corresponding grating coupler, while the AFM probe 
was kept on an SU8 microblock (absorbing between 1650 and 
1750 cm-1) fabricated by electron-beam lithography at the 
open end of the specific waveguide, or of a different nearby 
waveguide to check cross-talking.  
Successful in-coupling is demonstrated by scanning the 
QCL spot on the chip surface (Figure 1). A very bright spot 
appears when the AFM probe and the SU8 block are directly 
illuminated by the fee-space radiation, as expected from the 
AFM-IR principle (top-right spot in Figure 1), but a second 
elongated spot appears also when the laser is focused onto the 
grating coupler, with the AFM tip still on the SU8 block. 
The spectral efficiency of the grating is measured by tuning 
the QCL wavelength (Figure 2). A bandwidth of about 30 cm-1  
to 50 cm-1 is obtained for different grating coupler geometries, 
in good agreement with electromagnetic simulations (not 
shown). A pattern of Fabry-Perot fringes is also clearly 
observed, due to back-reflections at the waveguide end and at 
the coupler-waveguide interface. Propagation losses around 5 
dB/cm are estimated from the fringe intensity, indicating that 
the Ge-on-Si technology is extremely promising for realizing 
integrated CMOS-compatible molecular sensors. 
 
 
 
III. SUMMARY 
Germanium waveguides on silicon wafers are characterised 
by near field imaging and spectroscopy and represent an 
exciting perspective for mid-infrared integrated silicon 
photonics sensing chips. A new photo-thermal probe for near-
field characterization of on-chip integrated mid-infrared 
waveguides is introduced, based on an IR-absorbing polymer 
block of sub-λ size with an atomic force microscopy probe 
that detects its thermal expansion. 
REFERENCES 
[1]. P. T. Lin, V. Singh, L. Kimerling, and A. M. Agarwal “Planar silicon 
nitride mid-infrared devices” Appl. Phys. Lett. 102, 251121 (2013). 
[2]. Y. C. Change et al. “Cocaine detection by a mid-infrared waveguide 
integrated with a microfluidic chip” Lab on Chip 12, 3020-3023 (2012) 
[3]. J. M. Ramirez et al. “Low-loss Ge-rich Si0.2Ge0.8 waveguides for mid-
infrared photonics” Opt. Lett. 42, 105-108 (2017). 
[4]. F. Lu, M. Jin and M. A. Belkin, “Tip-enhanced infrared nanospectroscopy 
via molecular expansion force detection” Nat. Phot., 8, 4, (2014). 
[5]. E. Calandrini et al., “Mapping the electromagnetic field confinement in 
the gap of germanium nanoantennas” Appl. Phys. Lett., 109, 121104 (2016). 
 
The research leading to these results has received funding 
from the European Union’s Seventh Framework Programme 
under grant agreement n°613055. 
Fig.2 Intensity spectra of the radiation out-coupled at the open end of 
three straight waveguides of different length. The presence of 
interference fringes indicates that the radiation travels several times 
along the waveguides and is reflected by the open end and by the 
grating coupler. The in-coupling efficiency spectrum of the grating 
(1670-1750 cm-1) is measured with the photo-thermal near-field probe 
while the QCL wavelength is tuned across the range shown in the plot. 


Integrated Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips

Enlighten

      Gallacher, K., Sorgi, A., Giliberti, V., Frigerio, J., Isella, G., Biagioni, P., Paul, D. J. , Ortolani, M. and Baldassarre, L. (2017) Integrated Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips. In: 2017 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Cancun, Mexico, 27 Aug - 01 Sep 2017, ISBN 9781509060504.     There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.    http://eprints.gla.ac.uk/152526/             Deposited on: 27 November 2017                       Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Abstract—Germanium-on-silicon waveguides are designed, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the in-coupling efficiency. On-chip propagation along bends and straight sections up to 0.8 mm is demonstrated around λ = 6 μm. I. INTRODUCTION   AVEGUIDES represent the key component of integrated silicon photonics. The dominant technology in the near-infrared (λ = 1.55 μm) is the silicon-on-insulator (SOI) platform, where Si waveguides are realized on SiO2 surfaces grown on Si wafers, exploiting the strong refractive index contrast (nSi=3.4 and nSiO2=1.4-1.7). In the mid-IR range of interest for biosensing (λ > 5 μm), Si presents non-negligible losses due to impurity states and it cannot be used as the guiding layer, and SiO2 has huge losses due to optical phonon absorption and it cannot be used at all. Therefore, IR transparent materials have been investigated but their n is typically lower than that of Si so, for use as guiding layers, they have to be physically suspended [1].    Germanium is transparent and has nGe=4.0 > nSi and therefore it can be used for guiding through simple Ge-on-Si strip waveguides [2]. The main difficulty related to this emerging integrated photonics technology is to couple light from an external mid-IR laser source into the Ge waveguide, and not into the Si substrate. This has been recently attained e.g. by graded SixGe1-x virtual substrates [3].  In this work, we have fabricated pure Ge waveguides on Si wafers starting from Ge-on-Si films grown by chemical vapor deposition. We have etched grating couplers and tapers into the Ge layer to engineer the coupling at 70° incidence angle from a wavelength-tunable quantum cascade laser (QCL). Finally, we introduce photothermal expansion spectroscopy as the scanning local sensing tool of the out-coupled power at the waveguide open end.  II. RESULTS Conventional characterization of on-chip waveguides with dimensions smaller than the wavelngth is typically carried out by moving an optical fiber against the waveguide open-end (butt coupling). Another approach is to tightly focus the free-space radiation onto a taper, i.e. a planar structure initially wider than the actual waveguide, fabricated on the chip edge; the taper width then decreases with increasing distance from the chip edge towards the waveguide width. Both methods are difficult to implement in the mid-IR and with a high-index material such as Ge, due to the unfavorable ratio between waveguide height and free-space wavelength that produces high coupling losses. The coupling losses also intervene when the radiation is out-coupled towards the detector, located in the far-field of the waveguide end. To solve these issues, we propose a method based on near-field mid-IR microscopy, in which the detector is substituted by a near-field sensor made by the combination of a polymer microblock fabricated on-chip by lithography in the same process as the waveguides that one wants to test, and an atomic force microscopy (AFM) probe. When the microblock receives radiation power at a wavelength where it efficiently absorbs mid-IR radiation, it will heat up and result in a photo-induced expansion that can be measured by monitoring the oscillations of the AFM cantilever probe. Also, the in-coupling is obtained by illumination at oblique incidence of a diffraction grating fabricated on the waveguide with a tightly focused mid-IR laser beam, so that different regions of the same chip can be used for in-coupling by simply raster-scanning the laser beam and/or the chip. In this way, each chip with specific epitaxial material properties can contain several hundreds of waveguide circuits. This technique is closely related to the  photo-thermal expansion near-field microscopy (AFM-IR) [4, 5], except that the radiation is not directly focused on the tip, but it reaches the tip through the waveguide.       In the present work, for analyzing each on-chip Ge-on-Si Kevin Gallacher1, Alessia Sorgi2, Valeria Giliberti3, Jacopo Frigerio4, Giovanni Isella4,5,         Paolo Biagioni4,5,  Douglas J. Paul1, Michele Ortolani2 and Leonetta Baldassarre2 1School of Engineering, University of Glasgow, Glasgow, G12 8LT, U.K. 2Department of Physics, Sapienza University of Rome, Rome, 00185 Italy 3Center for Life Nanosciences, Istituto Italiano di Tecnologia, Rome, 00185 Italy 4L-NESS, Dipartimento di Fisica del Politecnico di Milano, Como, 22100 Italy   5Dipartimento di Fisica, Politecnico di Milano, Milano, 20133 Italy Integrated Germanium-on-Silicon Waveguides  for Mid-Infrared Photonic Sensing Chips WFig.1  Near-field imaging at λ=5.8 μm of the radiation intensity on a Ge-on-Si photonic chip featuring a waveguide bend (grey line), an incoupling grating with taper (elongated intensity spot, mostly blue) and an open-end waveguide (red spot at the waveguide end, where the near-field probe is positioned). Inset: SEM image of the grating coupler. waveguide circuit, s-polarized radiation from an external-cavity tunable QCL tuned in the 900-1900 cm-1 is focused onto the corresponding grating coupler, while the AFM probe was kept on an SU8 microblock (absorbing between 1650 and 1750 cm-1) fabricated by electron-beam lithography at the open end of the specific waveguide, or of a different nearby waveguide to check cross-talking.  Successful in-coupling is demonstrated by scanning the QCL spot on the chip surface (Figure 1). A very bright spot appears when the AFM probe and the SU8 block are directly illuminated by the fee-space radiation, as expected from the AFM-IR principle (top-right spot in Figure 1), but a second elongated spot appears also when the laser is focused onto the grating coupler, with the AFM tip still on the SU8 block. The spectral efficiency of the grating is measured by tuning the QCL wavelength (Figure 2). A bandwidth of about 30 cm-1  to 50 cm-1 is obtained for different grating coupler geometries, in good agreement with electromagnetic simulations (not shown). A pattern of Fabry-Perot fringes is also clearly observed, due to back-reflections at the waveguide end and at the coupler-waveguide interface. Propagation losses around 5 dB/cm are estimated from the fringe intensity, indicating that the Ge-on-Si technology is extremely promising for realizing integrated CMOS-compatible molecular sensors.    III. SUMMARY Germanium waveguides on silicon wafers are characterised by near field imaging and spectroscopy and represent an exciting perspective for mid-infrared integrated silicon photonics sensing chips. A new photo-thermal probe for near-field characterization of on-chip integrated mid-infrared waveguides is introduced, based on an IR-absorbing polymer block of sub-λ size with an atomic force microscopy probe that detects its thermal expansion. REFERENCES [1]. P. T. Lin, V. Singh, L. Kimerling, and A. M. Agarwal “Planar silicon nitride mid-infrared devices” Appl. Phys. Lett. 102, 251121 (2013). [2]. Y. C. Change et al. “Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip” Lab on Chip 12, 3020-3023 (2012) [3]. J. M. Ramirez et al. “Low-loss Ge-rich Si0.2Ge0.8 waveguides for mid-infrared photonics” Opt. Lett. 42, 105-108 (2017). [4]. F. Lu, M. Jin and M. A. Belkin, “Tip-enhanced infrared nanospectroscopy via molecular expansion force detection” Nat. Phot., 8, 4, (2014). [5]. E. Calandrini et al., “Mapping the electromagnetic field confinement in the gap of germanium nanoantennas” Appl. Phys. Lett., 109, 121104 (2016).  The research leading to these results has received funding from the European Union’s Seventh Framework Programme under grant agreement n°613055. Fig.2 Intensity spectra of the radiation out-coupled at the open end of three straight waveguides of different length. The presence of interference fringes indicates that the radiation travels several times along the waveguides and is reflected by the open end and by the grating coupler. The in-coupling efficiency spectrum of the grating (1670-1750 cm-1) is measured with the photo-thermal near-field probe while the QCL wavelength is tuned across the range shown in the plot. 

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Integrated Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips

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